AN ABSTRACT OF THE THESIS OF
Gary A. Smith for the degree of Doctor of Philosophy in Geology
presented on ,November 19,
1985.
Title: Stratigraphy, Sedimentology, and Petrology of Neogene Rocks in
the Deschutes Basin, Central Oregon: A Record of Continental
Margin Volcanism and its Influence on Fluvial Sedimentation in
an ArcAdjacent Basin.
a
Redacted for Privacy
Abstract approved:
Edward MiTaylor
Neogene rocks of the Deschutes basin include the middle Miocene
Columbia River Basalt Group and Simtustus Formation, and late Miocene
to early Pliocene Deschutes Formation. Assignment of Prineville
chemicaltype flows to the Grande Ronde Basalt of the Columbia River
Basalt Group is based upon correlation of these lavas from their type
area, through the Deschutes basin, and onto the Columbia Plateau where
they have been previously mapped as Grande Ronde Basalt.
Simtustus
Formation is a newly defined unit intercalated with and conformable
upon these basalts and is unconformably overlain by Deschutes
Formation.
Burial of mature topography by middle Miocene basalts raised local
base levels and initiated aggradation by lowgradient streams within
the basin represented by the tuffaceous sandstones and mudstones of the
Simtustus Formation.
These sediments are enriched in pyroclastic
constituents relative to contemporary Western Cascades volcanics
reflecting preferential incorporation of easily eroded and more
widespread pyroclastic debris in distal sedimentary sequences compared
to epiclastic contributions from lavas.
Following a 5 to 7 m.y. hiatus, aggradation was renewed at about
7.5 Ma when coarsegrained volcanogenic sediments, lava flows and
ignimbrites from the early High Cascades entered the basin for 2 m.y.
The proximal Deschutes Formation is primarily basalt and basaltic
andesite lava flows but andesite to rhyolite ignimbrites are the
primary volcanic constitutents in the sedimentarydominated section
farther east.
Deposition on a broad, eastwardtapering alluvial plain
was by debris flows, sheetfloods, and hyperconcentrated flood flows.
Episodic aggradation correlates to periods of sediment influx following
eruptions' of widespread pyroclastic debris and was separated by periods
of incision.
The abundance of basalts, combined with the paucity of hydrous
minerals and FeO and TiO
enrichment in intermediate lavas characterize
2
early High Cascade yolcanics as atypical for convergentmargin arcs.
These petrologic characteristics are consistent with highlevel
fractionation in an extensional regime.
Extension culminated in the
development of an intraarc graben which ended Deschutes Formation
deposition by structurally isolating the basin from the High Cascade
source area.
Intraarc extension may represent invasion of Basin and
Range tectonism into the Cascades, or may relate to platemargin
processes, particularly decreasing convergence rate and highly oblique
convergence vector.
Stratigraphy, Sedimentology, and Petrology of Neogene Rocks in the
Deschutes Basin, Central Oregon: A Record of ContinentalMargin
Volcanism and its Influence on Fluvial Sedimentation in an ArcAdjacent
Basin
by
Gary Allen Smith
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Doctor of Philosophy
Completed November 19, 1985
Commencement June 1986
APPROVED:
Redacted for Privacy
Associate Professor o
eology in charge of major
Redacted for Privacy
bai man of De art ent of Geology
Redacted for Privacy
Dean of
aduate
Date thesis is presented
Typed by Gary A. Smith
November 19, 1985
ACKNOWLEDGEMENTS
During the course of this project I have benefited from the
guidance, inspiration, and support of a number of individuals and
assistance from several institutions, agencies, and corporations.
It
is a pleasure to acknowledge these contributions, without which this
endeavor could not have been successfully completed.
Financial support for field work was provided by grants from the
Geological Society of America, the Sohio Field Research Fund at Oregon
State University, Sigma Xi, and a fellowship from Shell Oil Company.
Analytical expenses at Oregon State University were covered by a grant
from the Oregon Department of Geology and Mineral Industries and a
research grant from the National Science Foundation (EAR-8313986) to
Dr. Edward Taylor and myself.
The dissertation was written during the
tenure of a Laboratory Graduate Appointment from the Northwest College
and University Association for Science (University of Washington) under
contract DEAM06-76RL02225 with the U. S. Department of Energy.
Preparation of text figures and plates was supported by Rockwell
Hanford Operations (RHO), Basalt Waste Isolation Project (BWIP); the
efforts of Dr. Stephen Reidel (RHO) for arranging this support is
greatly appreciated.
Many aspects of this project required the assistance of others in
Steve Sans, Mark Darrach, Paul Cushing, Rose McKenney,
Allison Church, and Joan Givens availed themselves when this assistance
was most needed.
the field.
Most of the fieldwork was conducted on public land, however access
to critical exposures on private property was provided by numerous
people, particularly Miss Gladys Grant, Mr. Robert Beasely, Mr. Len
Cooper, and the directors of Recreation Properties Incorporated. My
special appreciation to the Confederated Tribes of the Warm Springs
Indian Reservation for permitting access to the reservation during the
summers of 1982 and 1983.
My appreciation to Portland General Electric for the courtesies
extended to me while studying the section at Round Butte Dam, for
providing camping privileges at Pelton Park, and for accomodating field
trip groups on weekends.
My understanding of the geology of the Deschutes basin and
adjacent areas was greatly increased by many consultations and field
conferences with fellow graduate students working in the region. These
include: Neil Bingert, Debra Cannon, Rich Conrey, Tom Dill, Glenn
Hayman, Britt Hill, Scott Hughes, Jere Jay, Angela McDannel, Dave
Thormahlen, and Gene Yogodzinski.
Special thanks to Donald Stensland
for sharing with me his invaluable geologic maps of the southern part
of the basin.
Most majorelement chemical analyses were conducted at Oregon
State University by Dr. Edward Taylor and Mrs. Ruth Lightfoot.
Additional analyses were performed at Washington State University under
Isotopic
the direction of Dr. Peter Hooper and supported by RHOBWIP.
age determinations were obtained by Dr. Lawrence Snee, Oregon State
University, in the laboratory of Dr. John Sutter, U. S. Geological
Survey
Reston.
The electron microprobe analyses presented in this dissertation
were obtained with the assistance of Brewster Strope (RHO) and Scott
Traceelement analyses of
Cornelius (Washington State University).
basalts were courteously arranged by Dr. Peter Hooper (Washington State
Support for
University) with able assistance from Diane Johnson.
obtaining these analytical data was provided by RHOBWIP.
All geologists working in central Oregon benefit greatly from
contact with Larry Chitwood of the Deschutes National Forest, Bend.
appreciate my numerous discussion with Larry and with his colleagues
Bob Jensen and Terry Brock.
I
My special thanks to Mel Ashwill of Madras for assistance in the
evaluation of fossil floras and faunas and for memorable field sorties
My appreciation to both
to discuss and ponder stratigraphic problems.
Mel and his wife, Betty, for their hospitality during visits in their
home.
Dr. Alan Niem and the OSU graduate classes in Sedimentary
Petrology (Winter 1983) and Sedimentation (Spring 1984) offered
valuable insights on Deschutes Formation sedimentology when I was most
in need of devil's advocates to improve my objectivity.
Discussions with Dr. George Priest, Oregon Department of Geology
and Mineral Industries, contributed greatly toward my understanding of
Cascade volcanic development.
I especially appreciate unpublished
observations of early High Cascade volcanic products in the Western
Cascades which George shared with me.
Consideration of the Prineville chemicaltype basalts, in Chapter
2, benefited greatly from discussions with Dr. Donald Swanson (U. S.
Vancouver), James Anderson (University of Southern
Geological Survey
California), and Dr. Gordon Goles (University of Oregon), each of whom
also shared unpublished data with me.
Observations of modern sedimentation near Mount St. Helens,
Washington, were instrumental in developing my understanding of volI extend my
caniclastic sedimentation in the Deschutes basin.
appreciation to fellow graduate student Rick Smith for introducing me
to the barely cold deposits related to the 1980 eruptive activity and
Corvallis) for providing
to Dr. Fred Swanson (U. S. Forest Service
logistical support for my first study of sediments in the restricted
area.
Thanks also for the discussions and/or field trips with Dr. Kevin
Scott, Dr. Tom Pierson, Dr. Richard Waitt, Mike Doukas, and Pat Pringle
of the U.
S. Geological Survey, Johnston Cascades Volcano Observatory.
Further insight into deposition of volcanogenic sediments was gained
from discussions with Dr. Richard Fisher (University of California,
Santa Barbara) and Dr. Paul Hammond (Portland State University).
Dr. J. Platt Bradbury, U. S. Geological Survey
Denver, analyzed
and interpreted the Deschutes basin diatom floras and introduced me to
the complex, but intriguing, problems of terrestrial biostratigraphy.
The contributions of my dissertation committee, Drs. Ed
Taylor, Larry Snee, Steve Reidel, Keith Oles, and Vern Kulm, toward
improving the text are greatly appreciated. The text was also reviewed
by Terry Tolan (RHOBWIP) and Dr. Gordon Goles (University of Oregon)
for Chapter 2, Karl Fecht (RHOBWIP) for Chapter 3, and Bruce Bjornstad
(RHOBWIP), Dr. Sam Johnson (U. S. Geological Survey
Denver), and Dr.
William Fritz (Georgia State University) for most of Chapter 8.
Drafting of most text figures was coordinated by Mrs. Connie Poe
William Crowley
(RHO) and ably done by Linda Lang (Kaiser Engineering).
(RHO) and Carol Johnston (RHO) assisted me in the preparation of the
plates.
Five very special people receive my heartfelt appreciation for
their encouragement and support during the course of this study. Ed
Taylor ably supervised the project, helped to maintain its focus, and
provided the type of education which one cannot receive in a classroom.
Chuck and Arlene Gilderoy, of Crooked River Ranch, provided me with an
extra home and family which was largely responsible for maintaining my
motivation during what would otherwise have been long, lonely field
seasons; they hold a special place in my memories of working in central
Oregon.
And to my parents, Howard and Marjorie Smith, a special thank
you for the encouragement to pursue my love of geology.
TABLE OF CONTENTS
Page
CHAPTER 1: INTRODUCTION AND OVERVIEW
Purpose
Terminology
1
1
7
PART I: MIDDLE MIOCENE STRATIGRAPHY AND PALEOGEOGRAPHY
11
CHAPTER 2: STRATIGRAPHY OF THE PRINEVILLE CHEMICAL-TYPE BASALT
IN THE DESCHUTES BASIN, OREGON, AND CORRELATION TO
THE COLUMBIA RIVER BASALT GROUP
Introduction
Middle Miocene Basaltic Volcanism in the Pacific Northwest
Petrology of the Prineville Chemical-Type Basalt
Stratigraphy of the Type Section
Stratigraphy in the Deschutes Basin
Occurrences of Prineville Chemical-Type Basalt in NorthCentral Oregon
Correlation of Prineville Chemical-Type Flows
Stratigraphic Nomenclature
Conclusions
11
11
12
17
21
22
27
31
35
40
CHAPTER 3: SIMTUSTUS FORMATION: PALEOGEOGRAPHIC AND STRATIGRAPHIC
SIGNIFICANCE OF A NEWLY DEFINED MIOCENE UNIT IN THE
DESCHUTES BASIN, CENTRAL OREGON
Introduction
Previous Work
Definition of Simtustus Formation
Sedimentology of the Simtustus Formation
Middle Miocene Deschutes Basin Paleogeography
Relationship to Cascade Volcanism
Regional Stratigraphic Correlation
Conclusions
42
PART II: GEOLOGY OF THE DESCHUTES FORMATION: THE RECORD OF
EARLY HIGH CASCADE VOLCANISM IN CENTRAL OREGON
69
CHAPTER 4: INTRODUCTION TO THE GEOLOGY OF THE DESCHUTES FORMATION
Location and Purpose
Previous Work
69
69
72
CHAPTER 5: GEOLOGIC SETTING
Geomorphology
Pre-Deschutes Formation Stratigraphy
General Features of The Deschutes Formation
Post-Deschutes Formation Stratigraphy
Structural Geology
Summary
76
76
76
83
86
97
42
44
46
52
57
60
62
66
109
CHAPTER 6: VOLCANIC STRATIGRAPHY OF THE DESCHUTES FORMATION
Introduction
Pelton Basalt Member
Chinook Ignimbrite Member
Seekseequa Basalt Member
Juniper Canyon Basalt Member
Opal Springs Basalt Member
Hollywood Ignimbrite Member
Jackson Buttes Ignimbrite Member
Big Canyon Basalt Member
Lower Bridge Ignimbrite Member
Cove Ignimbrite Member
McKenzie Canyon Ignimbrite Member
Balanced Rocks Ignimbrite Member
Fly Creek Ignimbrite Member
Tenino Ignimbrite Member
Coyote Butte Ignimbrite Member
Steelhead Falls Ignimbrite Member
Peninsula Ignimbrite Member
Deep Canyon Ignimbrite Member
Six Creek Ignimbrite Member
Tetherow Butte Member
Lower Desert Basalt Member
Steamboat Rock Member
Round Butte Member
Rattlesnake Ignimbrite Member
CHAPTER 7: VOLCANIC GEOLOGY OF THE DESCHUTES FORMATION
Introduction
Distribution of Volcanic Rocks
Basalts
Diktytaxitic Olivine Basalts
Nondiktytaxitic Basalts
Basaltic Andesites and Andesites
Dacites, Rhyodacites, and Rhyolites
Physical Features of Ignimbrites
Depositional Structure and Texture
Welding
GasEscape Structures
Cogenetic AirFall Deposits
Compositional Heterogeneity in Deschutes Formation Ignimbrites
Relationship of Deschutes Magmatism to the High Cascade Graben
CHAPTER 8: SEDIMENTARY GEOLOGY OF THE DESCHUTES FORMATION
Facies and Facies Associations
Fades of the Deschutes Formation
Facies Associations
Facies Association 1: Fluvial Channel Deposits
Fades Association 2: Floodplain Deposits
Fades Association 3: Sheetflood Deposits
Facies Association 4: DebrisFlow and Hyperconcen
trated Floodflow Deposits
111
111
121
123
127
129
130
133
134
136
138
141
143
147
148
151
152
155
155
158
159
161
167
169
174
175
178
178
179
184
184
198
203
213
216
216
222
224
225
226
233
240
240
240
250
250
250
253
255
Facies Association 5: PaleosolDominated Deposits
Paleodrainage and Depositional Settings
ArcAdjacent Alluvial Plain
Description
Discussion
Ancestral Deschutes River
Description
Discussion
Inactive Basin Margin
Description
Discussion
Causes of Aggradation
Distinctive Sedimentary Units
SubPelton Conglomerate
SubLower Bridge DebrisFlow Deposit
SupraMcKenzie Canyon DebrisFlow and Flood Deposits
Street Creek DebrisFlow Deposit
Dry Canyon Flood Deposit
Tetherow DebrisFlow Deposit
Petrology of Deschutes Formation Sedimentary Rocks
Introduction
Conglomerates
Sandstones
Framework Composition
Cements
Discussion
260
262
267
267
278
281
281
285
288
288
289
290
292
293
294
294
296
299
303
305
305
305
310
310
315
318
CHAPTER 9: LATE NEOGENE VOLCANOTECTONIC DEVELOPMENT OF THE CENTRAL 320
OREGON HIGH CASCADES
Key Features of the Deschutes Formation Critical to
320
Regional Tectonics
The Nature of Cascade EastFlank Structure North and
322
South of Green Ridge
Relationship of the High Cascade Graben to Basin and
333
Range Extension
Formation of Intraarc Graben
337
Conclusions
345
CHAPTER 10: THE DESCHUTES FORMATION AND THE EARLY HIGH CASCADES
CONCLUSIONS
348
CHAPTER 11: NEOGENE STRATIGRAPHY OF THE DESCHUTES BASIN
GENERAL CONCLUSIONS AND PERSPECTIVES
354
REFERENCES CITED
358
APPENDICES
381
APPENDIX I
MAJORELEMENT ANALYSES OF DESCHUTES BASIN VOLCANIC
382
ROCKS
APPENDIX II
TRACEELEMENT ANALYSES OF DESCHUTES BASIN BASALTS
415
APPENDIX III - ELECRON MICROPROBE DATA FOR SILICATE MINERALS IN
SELECTED DESCHUTES FORMATION IGNIMBRITES
417
APPENDIX IV - TYPE LOCALITIES OF DESCHUTES FORMATION MEMBERS
423
APPENDIX V - MEASURED SECTIONS OF SIMTUSTUS FORMATION
428
APPENDIX VI - MEASURED SECTIONS OF DESCHUTES FORMATION
434
APPENDIX VII - MEASURED SECTION OF "CAMP SHERMAN BEDS"
459
APPENDIX VIII - DESCHUTES BASIN DIATOM FLORAS
461
40
APPENDIX IX - PRELIMINARY
39
Ar/
Ar AGE DATES, DESCHUTES BASIN
466
LIST OF FIGURES
Page
Figure
1.1
Location map of the Deschutes basin
2
1.2
Place name location map for the Deschutes basin
3
1.3
Location of mapping included in Oregon State
University theses
6
2.1
Distribution of the Columbia River Basalt Group and
middle Miocene basalts of the Blue Mountains
13
2.2
Generalized-geologic map of the eastern Deschutes basin
and western Ochoco Mountains.
16
2.3
Outcrop photos in the type area of the Prineville
chemicaltype basalt
20
2.4
Outcrop photos of Prineville chemicaltype basalt in
the northern Deschutes basin
26
2.5
Map showing location of Prineville chemicaltype basalt
northcentral Oregon
in
28
2.6
Fence diagram illustrating proposed correlation of PCT
basalt in central Oregon
32
2.7
Variation diagrams for compositional units within the
Columbia River Basalt Group
38
3.1
Graphic measured sections of Simtustus Formation
49
3.2
Basal Deschutes Formation conglomerate resting
unconformably upon tuffaceous mudstone of the Simtustus
Formation
51
3.3
Finingupward fluvial cycles in Simtustus Formation
51
3.4
Outcrop photos of finegrained sandstone and mudstone
facies association
54
3.5
Photographs showing Celtis endocarps in Simtustus
Formation
54
4.1
Generalized geologic map of the Deschutes basin
70
5.1
Distribution of preDeschutes Formation rocks in and
near the Deschutes basin
77
5.2
Representative exposures of John Day Formation in the
Deschutes basin.
79
5.3
Prineville chemicaltype basalt and Simtustus Formation
at Pelton Dam
82
5.4
View of the west face of the north end. of Green Ridge
showing crosssection of the Castle Rocks volcano
82
5.5
Typical exposure of Deschutes Formation
84
5.6
Distribution of postDeschutes Formation lavas in, and
near, the Deschutes basin
86
5.7
Squawback Ridge, a Pliocene basaltic andesite shield
volcano, as seen from The Peninsula
90
5.8
Erosional remnants of Pleistocene Newberry (?)
intracanyon basalt flows
94
5.9
Distribution of Pleistocene pyroclastic deposits
adjacent to the central Oregon High Cascades
96
Structural features in and adjacent to the Deschutes
98
5.10
basin
5.11
Residual gravity anomaly map (contoured in miligals) of
central Oregon
101
5.12
Major fault zones adjacent to the Cascade Range in
central Oregon
104
5.13
Landsat (RBV) image of central Oregon
105
6.1
Stratigraphic positions of informally named members of
the Deschutes Formation
112
6.2
Distribution of Pelton basalt member
122
6.3
Distribution of Chinook ignimbrite member within the
Deschutes basin
125
6.4
Outcrop views of Deschutes Formation marker units
6.5
Distribution of Seekseequa basalt member
128
6.6
Distribution of Juniper Canyon basalt member within the
Deschutes basin
131
6.7
Outcrop views of Deschutes Formation marker units
132
6.8
Distribution of the Jackson Buttes ignimbrite member
I
II
126
135
within the Deschutes basin
Distribution of Big Canyon basalt member within the
Deschutes basin
137
6.10
Distribution of Lower Bridge ignimbrite member within
the Deschutes basin
139
6.11
Outcrop views of Deschutes Formation marker units
III
140
6.12
Outcrop views of Deschutes Formation marker units
IV
145
6.13
Distribution of McKenzie Canyon ignimbrite member
within the Deschutes basin
146
6.14
Distribution of Balanced Rocks ignimbrite member within
the Deschutes basin
149
6.15
Distribution of Fly Creek ignimbrite member within the
Deschutes basin
150
6.16
Distribution of Tenino ignimbrite member within the
Deschutes basin
153
6.17
Distribution of'Coyote Butte ignimbrite member within
the Deschutes basin
154
6.18
Distribution of Peninsula ignimbrite member within the
Deschutes basin
156
6.19
Outcrop views of Deschutes Formation marker units
157
6.20
Distribution of Deep Canyon ignimbrite member within
the Deschutes basin
160
6.21
Distribution of Six Creek ignimbrite member
160
6.22
Distribution of lava flows and pyroclastics of the
Tetherow Butte and Round Butte members
162
6.23
Photographs of Tetherow Butte member
164
6.24
Distribution of Lower Desert basalt member
168
6.25
Distribution of dikes, lava flows, and pyroclastics of
the Steamboat Rock member
171
6.26
Exposure of Steamboat Rock member pyroclastics 1.5 km
north of Steelhead Falls
172
6.27
Round Butte from CovePalisades State Park
172
6.9
V
6.28
Distribution of Rattlesnake ignimbrite in eastern
Oregon
176
7.1
SiO
182
7.2
Field and petrographic features of diktytaxitic basalts
185
7.3
Variation in Fe' with MgO for Deschutes basin
diktytaxitic basalts
190
7.4
Variation in the ratio CaO/Fe0 with increasing TiO as
an indicator of increasing fractionation for Deschutes
basin diktytaxitic basalts
190
7.5
Variation in the ration CaO/A1 0
for diktytaxitic basalts
191
7.6
Covariation of CaO/A1 0
diktytaxitic basalts
7.7
Comparison of CaO/A1 0
diktytaxitic basalts
7.8
Comparison of Fe' versus MgO for porphyritic and
diktytaxitic basalts
202
7.9
Fe' versus MgO for Cascadian basalts, basaltic
andesites, and andesites in the Deschutes basin
206
7.10
Harker diagrams for selected majorelement oxides and
ratios for Deschutes Formation basaltic andesites and
andesites
210
7.11
TiO histogram for Deschutes Formation basaltic
andesites and andesites compared to the compilation of
Gill (1982) for rocks in orogenic settings with SiO
between 53 and 58 wt %.
212
7.12
Harker diagrams for selected majorelement oxides and
ratios for Deschutes Formation dacites and rhyodacites,
in closed symbols, and rhyolites, in open symbols
215
7.13
Standard ignimbrite flow unit of Sparks and others
218
histogram of Deschutes Formation volcanics
2
with increasing TiO
and Sc for selected
versus TiO
for porphyritic and
191
202
(1973)
7.14
Groundsurge deposits in Deschutes Formation
ignimbrites
220
7.15
Grading in Deschutes Formation ignimbrites
221
7.16
Examples of Deschutes Formation airfall units
227
7.17
Photos of compositionally heterogeneous pyroclastic
229
units
7.18
Diagram illustrating compositional range of selected
Deschutes Formation ignimbrites
230
8.1
Comparative examples of clastsupport conglomerate
fades Gm(b), on left, and Gm(a), on right
243
8.2
Horizontal stratification in Deschutes Formation
sandstones
246
8.3
Massive paleosol sandstones (facies Sm(p)) in the
Deschutes Formation
248
8.4
Primary (a) and reworked (b) pumice lapillistones
248
8.5
and 2 exposed in roadcuts in
Facies associations
CovePalisades State Park
252
8.6
Typical outcrops of sheetflood fades association in
the Deschutes canyon opposite the mouth of Squaw Creek
256
8.7
Examples of facies association 4
257
8.8
Complex vertical sequences of debrisflow and
hyperconcentrated floodflow facies
259
8.9
Typical exposures of facies association 5, east of
Madras
261
8.10
Diagrams illustrating paleodrainage and depositional
settings in the Deschutes basin
264
8.11
Approximate position of ancestral Deschutes River in
the northern Deschutes basin
266
8.12
Graphic measured sections of typical vertical sequences
in the arcadjacent alluvial plain setting
269
8.13
Example of a paleochannel, about 15 m deep, in the
alluvialplain sequence
270
8.14
Facies association
conglomerates and sandstones in
the alluvialplain sequence
272
8.15
Section in Crooked River canyon illustrating transition
in depositional style at horizon of McKenzie Canyon
ignimbrite member (MC)
273
8.16
Graphic measured sections in the Crooked River canyon
(left) and CovePalisades State Park (right)
274
1
1
illustrating vertical transition from streamflow to
and debrisflow sedimentation
flood
8.17
Mean diameter of ten largest clasts from streamflow
conglomerates (facies (Gm(b)) plotted against distance
east of Green Ridge
275
8.18
Paleosol and tephradominated sequence capping typical
alluvialplain fades in Deschutes canyon near Geneva
277
canyon
8.19
Exposures representing the ancestral Deschutes River
depositional setting
283
8.20
Graphic measured sections from the Round Butte Dam type
section illustrating fades sequences representing
ancestral Deschutes River sedimentation
284
8.21
Photographs illustrating debrisflow and
hyperconcentrated floodflow deposits immediately
overlying and underlying the McKenzie Canyon ignimbrite
member
295
8.22
Drawings illustrating lateral variation in texture of
the Street Creek debrisflow deposit
298
8.23
Photographs of the Dry Canyon flood deposit
301
'8.24
John Day Formation clasts in the Deschutes Formation
307
8.25
Photomicrographs of Deschutes Formation sandstones
312
9,1
Generalized geologic map of the central Oregon Cascades
and northern Deschutes basin
323
9.2
Structural features of the Oregon Cascade Range
326
9.3
Physiographic map of the Deschutes basin and adjacent
Nigh Cascades
331
9.4
Crosssections through intraarc grabens in Central
America, Kamchatka, and Japan
339
Schematic crosssection of the central Oregon Cascade
Range and Deschutes basin
351
10.1
LIST OF TABLES
Page
Table
9
1.1
Terminology and classification of volcaniclastic rocks
2.1
Average composition of the Prineville chemicaltype
basalt
19
2.2
Composition of Prineville chemicaltype basalt
Deschutes basin exposures
24
-2.3
Composition of Prineville chemicaltype basalt
occurrences north of the Deschutes basin
30
6.1
Summary table: Deschutes Formation lava flow members
114
6.2
Summary of characteristics of Deschutes Formation
ignimbrite members
115
6.3 '
Average major and traceelement compositions for
Deschutes Formation basalt and basaltic andesite
116
members
6.4
Average majorelement compositions of Deschutes
Formation ignimbrite members
118
7.1
Representative analyses of Deschutes basin
diktytaxitic basalts
186
7.2
Comparisoh of Deschutes basin diktytaxitic basalts with
other Pacific Northwest basalts
196
7.3
Representative Deschutes Formation nondiktytaxitic
basalts
199
7.4
Representative Deschutes Formation basaltic andesites
and andesites
205
8.1
Facies nomenclature for the Deschutes Formation
241
8.2
Facies associations
251
8.3
Depositional settings
265
8.4
Clast counts, Deschutes Formation conglomerate, Round
Butte Dam section
309
8.5
Analyses of components of Deschutes Formation sediments
314
LIST OF PLATES
I.
Geologic map of the Madras West, Seekseequa (in back pocket)
Junction and east half of the Metolius
quadrangles, Jefferson County, northcentral
Oregon.
Geologic map of the northeastern Deschutes
basin, central Oregon, emphasizing distribution of the middle Miocene Simtustus
(in back pocket)
Formation.
Neogene stratigraphy of the Deschutes basin. (in back pocket)
"111-
STRATIGRAPHY, SEDIMENTOLOGY, AND PETROLOGY OF NEOGENE ROCKS IN THE
DESCHUTES BASIN, CENTRAL OREGON: A RECORD OF CONTINENTAL-MARGIN
VOLCANISM AND ITS INFLUENCE ON FLUVIAL SEDIMENTATION IN AN ARCADJACENT BASIN
CHAPTER 1: INTRODUCTION AND OVERVIEW
PURPOSE
This dissertation describes the middle Miocene to middle Pliocene
volcanic and volcanogenic sedimentary rocks of the Deschutes basin in
central Oregon and their significance to regional paleogeography and
volcanotectonic evolution.
The region considered is located in
Jefferson, Deschutes, and westernmost Crook counties and is bordered by
the High Cascade Range, to the west, Mutton Mountains, to the north,
Ochoco Mountains, to the east, and High Lava Plains, to the south (Fig.
1.1).
The dissertation is presented in manuscript format, to
facilitate subsequent publication; thus some introductory material is
repeated near the beginning of chapters.
When originally proposed, the emphasis of this project was compilation of a composite volcanic stratigraphy for the Deschutes Formation
and presentation of a basin analysis of associated sedimentary rocks.
However, previous usage of Deschutes Formation to include all rocks
overlying basalts assigned to the middle Miocene Columbia River Basalt
Group was found to be inappropriate.
Volcaniclastics lithologically
dissimiliar to the Deschutes Formation as originally described by
Russell (1905), and hosting older Barstovian vertebrate fossils, were
found to be conformable upon and interstratified with the middle
2
25
0
KILOMETERS
q0
-N
Gateway
_
LAKE
SIMTUSTUS
Madras
<cs
0
co
0
LAKE
CHINOOK
C.)
Tr
0
U)
ct
0
"
CD
Sisters
0
0
cc.
"ED
0
Redmond
44(.7
NI P.
%01
'tk
0 Bend
vvi*
PS8509-132
Fig. 1.1. Location map of the Deschutes basin.
3
® Simnasho
HEHE X
-s0
BUTTE
X0
<CC'4
0
14`'
'tP X
VS
4
Kah-nee-ta
II
e
CREEK
o South
Junction
OLLAUE X
BUTTE
ANYON 4:74 ,
C./
SHITIKE BUTTE
$(247.1/40 BENCH DRY H OZ,
MIDDLE
Ft VE.9
Junction
BALD
tW
(414.7
0
?-
Madras 3
0
J.
ROUND BUTTE DAM
X ROUND
S.A. 8ENCkf
poi< '''IUS RIVER
4,41,
0
71, 0
LAKE
SIMTUSTUS
4,4".004,
0
DAM
X ,...--JACKSON
X
BUTTES
1^'
tn.`
.51
X BUCK BUTTE
BUTTE
L,
TELLER
FLAT
N
EX
SB
7CREe
eOL
IT
P"
vAG
# SIX cv4.
THE COVE-
LAKE
BILLY
CHINOOK
4°
CANYON
Camp
<2.
Sherman LITTLE SQUAW/to
BACK
STATE
PARK
0
L1 ..1.-.
Culver
B
HAYSTACK
RESERVOIR
ill
Grizzly
X
a.
x HAYSTACK
JUNIPER
9
9
ecb
GRIZZLY
BUTTE
0 BUTTE
99 % c"P
X
cc,
:PALISADES
0
.zo, co
SQUAW BACK
2 Opal Springs
GENEVA ',$.
RIDGE
o
PEEK
JACK
PELTON
Seekseequa
PETER
THREE
X FINGERED
Paxton
4'4 CREe,
X BUTTE
40,1
00
Gateway
INDERS
TE NINO CREE
BUxTTEx S
Willowdale
FR
CHUTES
Warm
Springs
'S)
PEE
X NORTH BUTTE
MT. X
JEFFERSON
LI
REOL
MOUNTAIN
X
GRAY BUTTE
0
X
u+
x BLACK
S's
BUTTE
er.
Terrebonne X
Lowe
Bridge
MT. WASHINGTON
X
X
SMITH
ROCK
x
LINE
FALLS
st
O'Neil
TETHEROW X
BUTTE
Sisters
w
z
BIG FALLS
Prineville
Redmond
x BLACK
CRATER
NORTH SISTER
X
LAVA
Tumalo
X
SOUTH
BOWMAN
DAM
BADLANDS
MIDDLE SISTER
x TRIANGLE
XSISTER
x BROKEN
1,0
HILL
TOP
Bend
KILOMETERS
TUMALO
BEAR CREEK
x
BUTTE
PS8509-232
Fig. 1.2. Place name location map for the Deschutes basin.
4
Miocene basalts and separated from the "type" Deschutes Formation by an
unconformity (Smith and Hayman, 1983).
Combined with a new isotopic
age of 7.6 + 0.3 Ma for the lowest volcanic unit in the Deschutes
Formation, these field observations required a revision in the
stratigraphy of the basin (Smith and Snee, 1984).
Deschutes Formation
is retained for the late Miocene to early Pliocene lavas and
volcaniclastics and Simtustus Formation proposed for the older volcaniclastics.
Inclusion of the Simtustus Formation into this study necessitated
consideration of the stratigraphy of the intercalated middle Miocene
basalts.
Chemical analyses showed that all such basalts in the
Deschutes basin were Prineville chemical type, a compositional variant
originally assigned to the Columbia River Group by Uppuluri (1974) but
subsequently excluded from the redefined Columbia River Basalt Group
(Swanson and others, 1979) where not associated with the Grande Ronde
Basalt.
Exclusion of the type Prineville chemicaltype basalts, south
of Prineville, and basalts in the Deschutes basin from the larger group
reflected uncertainity in the stratigraphic equivalence of the type
Prineville basalts to flows within the Grande Ronde Basalt section
showing similar compositional traits (D. A. Swanson, U.
person. commun., 1983).
S. Geol. Surv.,
The Deschutes basin lies between the type
Prineville basalts and the occurrence of similar flows within the
Grande Ronde Basalt farther north and is a key area for assessing the
stratigraphic relationship between the type Prineville and Grande Ronde
basalts.
The text of the dissertation is divided into two parts.
Part I
5
considers the geology of the Prineville chemical type basalts and the
Simtustus Formation and emphasizes the stratigraphic and paleogeographic significance of these units.
Part II is a discussion of the
stratigraphy, petrology, and sedimentology of the Deschutes Formation.
This latter part emphasizes the nature of the early High Cascade
eruptive episode, which produced the Deschutes Formation, and the
culminating development of an intraarc graben that terminated
Deschutes deposition.
Stratigraphic relationships of Deschutes volcanic units and major
element chemical analyses are, in part, compiled from theses by Hewitt
(1970), Stensland (1970, and in progress), Hales (1975), Jay (1982),
Hayman (1983), Cannon (1984), Thormahlen (1984), Conrey (1985),
Yogodzinski (1986), Dill (in progress), Wendland (in progress), and
McDannel (in progress) conducted under the direction of Dr. Paul
Robinson, through 1970, and by Dr. Edward Taylor, following 1970, at
Oregon State University.
Geologic maps prepared as a part of these
theses were a primary resource in this investigation and cover most of
the Deschutes basin (Fig. 1.3).
Areas lacking detailed geologic
mapping were studied in a reconnaissance manner, with local detailed
mapping on the Warm Springs Indian Reservation included here as Plate
I,
independent of the text.
Plate I also includes a large portion of
the area mapped by Jay (1982) because of inaccuracies in the earlier
map and because new mapping to the west and south allowed for better
understanding of the stratigraphic position of units originally mapped
by Jay.
6
WENDLAND
JAY (1982)
HAYMAN (1983)
(IN PROGRESS)
YOGODZINSKI (1985)
SMITH THIS STUDY
NT.
JEFFERSON
,61/%1611.41
MADRAS
,1411111111101111
HALES (1975)
NA"
DONKEY (1985)1
DILL, (1985)
;II
HEWITT (1970)
A1
'THORMAHLEN:
N(1 84N\
STENSLAND
STENSLAND (1970)*:
SISTERS
(IN PROGRESS)
-
10 km
REDMOND
McDANNEL
(IN PROGRESS
Fig. 1.3. Location of geologic mapping included in Oregon State
University theses concerning the Deschutes basin.
7
TERMINOLOGY
A variety of classifications exist for volcanic and volcaniclastic
rocks leading to a bewildering profusion of terms.
Therefore, it is
important, at the outset, to establish the usage of terms in this
dissertation.
Although the International Union of Geological Sciences
(Streckeisen, 1979) has adopted a classification of volcanic rocks
based on primary mineralogy, such a classification is difficult to use
because of the fine grain size and varying degrees of crystallinity
exhibited by volcanic rocks.
Instead, the waterfree compositional
classification used by Taylor (1978), developed from study of Oregon
Cascade volcanics, is generally applied here:
< 53 wt. %
Basalt: SiO
2
> 53 wt. % and < 58 wt. %
Basaltic andesite: SiO
2
> 58 wt. % and < 63 wt. %
Andesite: SiO
2
> 63 wt. % and < 68 wt. %
Dacite: SiO
2
> 68 wt. %, and K 0 < 4.0 wt. %
Rhyodacite: SiO
2
2
> 68 wt. % and K 0 > 4.0 wt. %.
Rhyolite: SiO
2
2
The reader will note minor divergence from this scheme in Chapter 2,
concerning the Columbia River Basalt Group.
Many of these "basalts"
are actually basaltic andesites by the above classification.
However,
historical precedence dictates that these lava flows be collectively
called basalts, a practice which is followed here.
The term volcaniclastic is used here as introduced by Fisher (1961)
to "include all clastic volcanic materials formed by any process of
fragmentation, dispersed by any kind of transporting agent, deposited
8
in any environment or mixed in any significant portion with nonvolcanic
fragments" (Fisher and Schmincke, 1984,
p. 89).
Most volcaniclastic
rocks can be named on the basis of grain size and relative proportion
of pyroclastic and epiclastic material (Table 1.1).
However,
recognition of the pyroclastic versus epiclastic origin of some grains
is virtually impossible and requires the frequent use of nongenetic
terms (e.g. volcanic sandstone instead of coarse tuff or tuffaceous
sandstone).
Because volcaniclastic material includes primary
pyroclastic deposits (pyroclasticflow and airfall deposits) in
addition to water and wind reworked debris derived from them, the
term "volcanogenic" is adopted from Grechin and others (1981) to refer
to clastic sediments of volcanic composition as separate from primary
pyroclastics.
Two terms have received wide usage in geologic literature to
describe the deposits of pumiceous pyroclastic flows: ashflow tuff and
ignimbrite.
Both terms have disadvantages; ashflow tuff implies that
over 50% of the material is ashsize and Marshall's (1935) definition
of ignimbrite suggests an origin by "fiery showers", rather than
pyroclastic flows, and includes welding as a characteristic feature.
Nonetheless, the term ignimbrite has emerged in recent years as the
preferred term for pumiceous pyroclastic flow deposits, welded or
unwelded (MacDonald, 1972; Williams and McBirney, 1979; Wright and
others, 1980; Walker, 1983).
Ignimbrite is used here as defined by
Walker (1983, p. 66) "as a pyroclastic deposit or rock body, made
predominantly from pumiceous material, which shows evidence of having
been emplaced as a concentrated hot and dry particulate flow."
9
TABLE 1.1. TERMINOLOGY AND CLASSIFICATION OF VOLCANICLASTIC ROCKS
GRAIN SIZE
(mm)
PYROC LAST
PYROCLASTIC
ROCK
Pytrorocecicaisatic
Block, bomb
64 .
.
agglomerate
EPICLASTIC
ROCK*
Epiclastic
EQUIVALENT
NONGENETIC
TERMS*
Volcanic breccia,
volcanic breccia,
conglomerate
Lapillus
Lapillistone
Coarse ash
Coarse tuff
Fine
Fine
ash
tuff
conglomerate
- 2 -
Epiclastic
volcanic
Volcanic
Epiclastic
volcanic
siltstone
'Agit
Epiclas/ic
volcanic
daystone
Volcanic
claystone
sandstone
sandstone
-1/16 -
1/256.
* add adjective "tuffaceous 'to rocks containing pyroclastic material
10
Watermobilized volcanogenic sediment is transported by flows with
a wide range in sediment/water ratio.
The term "lahar" has frequently
been used to describe any poorlysorted volcanogenic deposit even
though this Indonesian term, in a strict sense, only refers to debris
flows.
Research by the writer, conducted as a companion study to this
dissertation, led to the recognition of criteria by which deposits
resulting from processes intermediate in character between more
familiar stream flow and debris flow could be distinguished from those
of the end members.
The ambiguous term "lahar" is dropped in favor of
"debris flow", for flow which deposits material en masse when shear
stress diminishes below yield strength, and hyperconcentrated flood
flow for partly turbulent, highconcentration flow in which sediment is
neither supported by turbulence alone, nor deposited en masse (Smith,
in press).
Unconsolidated sediment is typically distinguished from consolidated sedimentary rocks by use of the terms mud, sand and gravel versus
mudstone, sandstone, and conglomerate. respectively.
is difficult to make in the Deschutes Formation.
This distinction
Most Deschutes
sedimentary units are poorly consolidated and very friable, others
tightly cemented, and some completely unconsolidated.
Degree of
consolidation is often variable within a single depositional unit.
For
simplicity and brevity all Deschutes sedimentary units are treated as
rocks.
11
PART I: MIDDLE MIOCENE STRATIGRAPHY AND PALEOGEOGRAPHY
CHAPTER 2:
STRATIGRAPHY OF THE PRINEVILLE CHEMICALTYPE BASALT IN THE DESCHUTES
BASIN, OREGON, AND CORRELATION TO THE COLUMBIA RIVER BASALT GROUP
INTRODUCTION
As part of a geochemical study of basalts previously mapped as
middle Miocene Columbia River basalt in the western Blue Mountains of
central Oregon, Uppuluri (1973) recognized a section of lavas whose
composition is distinct from other Miocene basalts in the region.
These flows, best exposed at Bowman (formerly Prineville) Dam show
general petrographic and compositional affinities to Columbia River
Basalt lavas but contain anomalously large concentrations of
incompatible elements, most notably Ba (2000-2300 ppm) and P 0
(1.15
25
to 1.3 wt.%).
On the basis of this distinctive composition Uppuluri
(1974) designated the Prineville chemical type (PCT) as a newly
recognized variant within the Columbia River Group.
Subsequently, PCT basalts have been recognized elsewhere in north
central Oregon (Nathan and Fruchter, 1974; Smith and Priest, 1983) and
in the northern Oregon Cascade Range (Anderson, 1978; Beeson and Moran,
1979b) intercalated, in some places, with other Columbia River basalt
flows.
However, no effort was made to conclusively correlate these
occurrences to the type section of Uppuluri (1974), primarily because
of discrepencies in magnetic stratigraphy.
When stratigraphic revision led to definition of the Columbia
River Basalt Group (Swanson and others, 1979) the Prineville chemical
12
type was excluded from the group where it occurred separate from other
chemical types, including at the type locality.
This potentially
confusing assignment of some, but not all, occurrences of these
distinctive flows to the Columbia River Basalt Group resulted from
uncertainty about the stratigraphic equivalence of the type PCT flows,
south of the Columbia Plateau, with Ba and P 0 rich basalts occuring
25
with Columbia River Basalt Group flows farther north (D. A. Swanson,
person. commun., 1983).
The purpose of this report is to reevaluate the type section of
Ba and P 0 rich basalts at Bowman Dam and discuss the correlation of
25
these flows to compositionally similar basalts which occur within the
Columbia River Basalt Group.
The report emphasizes the distribution of
these flows in and near the Deschutes basin which, because of its
position between the Bowman Dam type area and the Columbia River Basalt
Group on the Columbia Plateau, is a key region for addressing the
stratigraphic uncertainities of these compositionally distinctive lavas
(Fig. 2.1).
MIDDLE MIOCENE BASALTIC VOLCANISM IN THE PACIFIC NORTHWEST
The stratigraphy and paleogeographic significance of the PCT
basalts must be evaluated within the framework of previous studies of
extensive Miocene basaltic volcanism in the Pacific Northwest.
The
most wellknown representative of this period of regional basaltic
volcanism is the Columbia River Basalt Group, with a volume of about
3
200,000 km
(Swanson and others, 1985) which inundated eastern
Washington and adjacent Oregon and Idaho between 17.0 and 6.0 Ma (McKee
and others, 1977, 1981; Swanson and others, 1979).
More than 98% of
13
E]
SADDLE MOUNTAINS
BASALT
WANAPUM BASALT
0
*Seattle
GRANDE RONDE
BASALT
PICTURE GORGE
BASALT
BASALT
(GENERALLY OVERLAIN
ElIMNAHA
BY GRANDE RONDE)
PRINEVILLE CHEMICAL
TYPE BASALT
E]
fl
STRAWBERRY VOLCANICS
BEAR CREEK
BASALT
\ DIKE SWARM
CJ- CHIEF JOSEPH DIKE SWARM
IH - ICE HARBOR DIKE SWARM
M - MONUMENT DIKE SWARM
TA
TYGH RIDGE
MM MUTTON MOUNTAINS
DB
DESCHUTES BASIN
BD - BOWMAN DAM
200
100
KILOMETERS
Fig.
2.1.
PS8509-129
Distribution of the Columbia River Basalt Group and middle
Miocene basalts of the Blue Mountains.
14
this volume was extruded between 17.0 and 14.0 Ma. South of the
Columbia Plateau, basalts of calcalkaline affinity were erupted in or
near the Blue Mountains (Robyn, 1979; Goles,
in press), and generally
highalumina diktytaxitic basalts were erupted in the northern Basin
and Range (Gunn and Watkins, 1970; Hart and others, 1983)
contemporaneous with, and subsequent to, Columbia River Basalt Group
volcanism.
The Columbia River Basalt Group is divided into 5 formations based
on chemical composition, paleomagnetic polarity, and stratigraphic
position (Fig. 2.1; Wright and others, 1973; Swanson and others, 1979).
The Picture Gorge Basalt occurs in the Blue Mountains of Oregon and was
erupted from the Monument dike swarm.
The Imnaha Basalt is restricted
to the Snake River canyon region of eastern Oregon, southeastern
Washington, and western Idaho.
The largely younger Yakima Basalt
Subgroup is composed of the Grande Ronde Basalt, Wanapum Basalt, and
Saddle Mountains Basalt and forms the largest volume of the group.
The
Chief Joseph dike swarm on the eastern margin of the Columbia Plateau
served as a source for many, if not all, Grande Ronde and Wanapum flows
and some Saddle Mountains Basalt.
Source dikes for other Saddle
Mountains flows have been recognized in western Idaho and in south
central Washington near Ice Harbor Dam.
Yakima Basalt Subgroup lavas
flowed westward down a gentle paleoslope and offlapped topographic
highs along the plateau margin.
The broad Blue Mountains anticlinorium
and Mutton Mountains uplift form the southern boundary of Yakima Basalt
distribution.
Several flows continued eastward through the Cascade
Range (Tolan and Beeson, 1984; Anderson, 1978) and reached the Pacific
1 5
Ocean (Beeson and others, 1979).
The type locality of Prineville chemicaltype basalt is located
near the southwest end of the Blue Mountains anticlinorium.
Source
dikes have not been found but because the number of flows is greatest
near Bowman Dam, the source is assumed to be nearby (Uppuluri, 1974).
Therefore, it is likely that PCT basalts were erupted from sources
located farther west than dikes which fed contemporaneous Columbia
River Basalt Group flows.
The PCT basalts are exposed northward in a
narrow belt around the west end of the Blue Mountains structure,
referred to by the local name Ochoco Mountains, through the Deschutes
basin, and across a broad area of northern Oregon (Figs. 2,2 and 2.5).
Two other middle Miocene basalt sequences have been recognized in
the Blue Mountains region.
.
Basalts mapped as Columbia River basalt in
the Bear Creek drainage (Walker and others, 1967), 15 km south of
Bowman Dam, were found by Osawa and Goles (1970) and Uppuluri (1973) to
have calcalkaline affinities distinguishing them from the tholeiites
of the Columbia River Basalt Group.
Gales (in press) has informally
named these the Bear Creek basalts.
Unequivocal sources for these
flows are not known but the two lowest flows of this sequence (27A and
27B of Osawa and Goles, 1970) are similar to two dikes of the Monument
swarm (MD-16 and MD-17 of Fruchter and Baldwin, 1975).
A dike exposed
along Bear Creek approximately 2 km north of the basalt outcrops has a
major element composition similar to the upper flows of the sequence
(Appendix Jo, analysis BN7).
Another sequence of calcalkaline basalts
comprises the Slide Creek member of the Strawberry volcanics (Thayer,
1957; Robyn, 1979).
Although partly coeval with the Picture Gorge
16
POST-COLUMBIA RIVER
BASALT GROUP
(VOLCANICS,
VOLCANICLASTICS,
AND ALLUVIUM
POST-PRINEVILLE
GRANDE RONDE
BASALT
PRINEVILLE
CHEMICAL
TYPE BASALT
CLARNO AND JOHN
DAY FORMATION
(MIDDLE EOCENE
TO EARLY MIOCENE
VOLCANICS AND
VOLCANICLASTICS
PELTON
;
DAM
Ci)
ROUND
BUM
44/0
Madras
DAM
-N-
0
5
10
KILOMETERS
.. X
GRAY BUTTE
LONE
PINE
FLAT
Redmond
0
GRASS
BUTTE X
p
POWELL
BUTTES
PRINEVILE
RESERVOIR
BOWMAN
DAM
Bend
ALKALI
BUTTE x
BEAR CREEK
X BUTTE " :::.rS8509-128
Fig.
2.2.
Generalized geologic map of the eastern Deschutes basin and
western Ochoco Mountains.
17
Basalt (Robyn and others, 1977) the basalts of the Strawberry Volcanics
are'excluded from the Columbia River Basalt Group because of their
local nature, eruption from central vents rather than fissures, calcalkaline composition, association with more evolved rocks, and earlier
onset of eruptive activity (Swanson and others, 1979).
PETROLOGY OF THE PRINEVILLE CHEMICALTYPE BASALT
The primary petrologic features of the Prineville chemicaltype
basalt at Bowman Dam are discussed at length by Uppuluri (1973, 1974)
and are briefly summarized here.
The basalts are typically hyalophitic
with microlites of plagioclase, pyroxene, and apatite.
The latter are
relatively abundant for a typical accessory mineral (up to 5 %), reach
2 mm in length, and reflect the large P 0
content.
Olivine is rarely
25
observed and the basalts are generally aphyric with very rare
phenocrysts of plagioclase.
Uppuluri's analyses (Table 2.1) from the
Bowman Dam locality are very uniform.
abundances of Ba and P 0
,
Besides the unusually large
Uppuluri (1974) noted that the PCT is also
25
enriched in Sr,
Sm, Yb, and Lu, and depleted in Ni, Co, and Cr relative
to most Columbia River Basalt Group lavas.
Although falling in the
alkali basalt field on the total alkalies
silica variation diagram of
MacDonald and Katsura (1964), the PCT basalts are quartz normative
suggesting a tholeiitic affinity (Uppuluri, 1973; Goles, in press).
Analyses of basalts at the type section obtained during this study
(Table 2.1), at a different laboratory, are also uniform but are
slightly different from those reported by Uppuluri (1974).
Two varieties of PCT basalt occur outside the type section, both
with the characteristic large Ba and P 0
2 5
contents.
One variety is
18
The other variety has
similar to the basalts at the type locality.
distinctly greater SiO ,
2
K 0 and lower Fe 0
,
MgO, CaO, and trace
23
2
element contents (Table 2.1).
Although not represented in the type
section, the large Ba and P 0
contents serve to define this higher
25
composition as belonging to the Prineville chemical type.
SiO
2
However, the two varieties cannot be related by crystal fractionation
alone because the twofold enrichment in K 0 with depletion in other
2
incompatible elements (Sr, Zr,
Y,
Ba) exhibited by the highSiO
2
variety relative to the flows at the type section is inexplicable by
extraction of observed silicate phases.
Goles (in press) suggested that the enrichment in K 0, P 0
2
,
and Ba
25
in PCT basalts may be a reflection of metasomatism in the mantle source
region or contamination of the magma by crustal rocks.
If the PCT
basalts were erupted in the PrinevilleBowman Dam region, it
is
interesting to note that large Oligocene rhyolite dome complexes at
Powell Buttes and Bear Creek Butte may be indicative of granitic bodies
at lower crustal levels in that region which could serve as contaminants.
Also, analyses of some Pliocene olivine basalts erupted in
this vicinity exhibit enrichment in selected incompatible elements (see
Chapter 7 and Appendices In and II).
Examples include the basalt
erupted at Grass Butte with 1.07 wt. % K 0 and 1144 ppm Sr, and basalt
2
erupted at Alkali Butte with 1.04 wt.% K 0, 965 ppm Sr, and 1123 ppm
2
Ba.
Other analyzed Pliocene basalts east of the Deschutes basin with
similar Mg numbers, typically contain <0.8 wt.% K 0, 250 to 400 ppm Sr,
2
and 250 to 500 ppm Ba.
19
TABLE 2.1: AVERAGE COMPOSITION OF PRINEVILLE CHEMICAL-TYPE BASALT
S102
TiO2
Al203
Fe203
MgO
CaO
Na20
K2O
P2°5
MnO
Rb
Sr
Zr
Y
Ba
Sc
Ni
V
50.54
2.67
13.59
13.38
4.35
7.96
3.29
1.98
1.36
0.24
+ 0.27
1 0.02
1 0.09
1 0.17
T 0.17
T 0.06
T 0.08
1 0.08
1 0.02
1
45 +
3
2
1
0.01
2
389 T 12
-
51.22
2.78
14.50
13.37
4.22
7.89
2.68
+
1
T
1
T
T
T
0.28
0.02
0.07
0.34
0.21
0.12
0.06
1.86 1 0.05
1.24 + 0.02
0.24 1 0.01
51 +
396 T
2
8
176 T 12
43+
8
1987 +100
2159 +106
361
371
1
151 4
-
1
53125 25
354+ 4
54.49
2.51
15.36
10.55
3.25
6.08
2.99
3.37
1.17
0.23
+
1
1
1
0.38
0.07
0.22
0.28
T 0.21
T 0.18
T 0.34
1 0.11
+ 0.04
T 0.01
52 + 5
287 T 10
149 T 4
42+
1
2108 T 42
331
1
36 1- 20
4
230 1-
NOTE: Major-element analyses normalized to 100% on a water-free basis;
ranges represent one standard deviation.
Average composition of low-Si02 Prineville chemical-type basalt
at Bowman Dam from Uppuluri (1974). Analyses performed at University
of Oregon (n = 15, except for Rb, Sr, Ni for which n = 4).
Average composition of low-Si02 Prineville chemical-type basalt at
Bowman Dam from this study. Analyses performed at Washington State
University; major elements under the direction of P. R. Hooper (n =
10), trace elements by G. A. Smith (n = 7).
Average composition of high-Si02 Prineville chemical-type basalt in
central Oregon. Analyses performed at Washington State University;
major elements under the direction of P. R. Hooper (n = 10), trace
elements by G. A. Smith (n = 4).
20
PB -
.
%..
...
-4'
:"".' S-14'
"
Tr.
'
917'"rIrr":74
'a
P
-
17i
_
nti;
.7eLf
.
a
Erlefo.."
Pliocerie basalt
mtitl
Bowman maar
7ilit4EVILLE
HeiArCA L-T V PE
Ax,S ALT%
if
-*"
111
Fig. 2.3. Outcrop photos in the type area of the Prinevillechemicaltype basalt. a) Portion of the type section, 1 km north of
Bowman Dam, showing flows 2 through 6 overlain by Pliocene
basalts (PB); p indicates pillowed zone at base of flow 3.
b) View to the north of the west end of Prineville Reservoir.
Steeply dipping Prinville chemicaltype basalt overlies poorly exposed, and slumped, John Day Formation and is unconformably overlain by Pliocene basalt erupted from the "Bowman
maar
.
21
STRATIGRAPHY OF THE TYPE SECTION
The largest continuous exposure of Prineville chemicaltype basalt
2
covers an area of about 180 km
between the town of Prineville and
Prineville Reservoir on the Crooked River (Fig. 2.2 and 2.3).
The type
section at Bowman Dam is 210 m thick and composed of 13 flows according
to Uppuluri (1973, 1974).
Magnetic polarities, determined by fluxgate
magnetometer, were reported as normal for the lowest flow and reverse
for the other 12 (Uppuluri, 1974).
Reevaluation of the type section (Smith and Cushing, 1985) indicated that only 6 flows are present.
Uppuluri (1973) duplicated his
section by not recognizing that flows on opposite sides of the Crooked
River are the same and do not represent separate, stratigraphically
unequivalent sections.
The basal flow is exposed in contact. with
white, massive tuff, mapped as Oligocene to early Miocene John Day
Formation by Swanson (1969), along State Route 27, 2 km south of Bowman
Dam.
Similar silicic tuffs, each 3 m thick, occur between flows 1 and
2, and 2 and 3, south of the dam, but no interbeds occur between flows
elsewhere in the type section.
The third flow contains a thick pillow-
palagonite zone at its base (Fig. 2.3), as much as 12 m thick, which
can be traced north and south of the dam and along both sides of the
Crooked River.
This pillowed flow is a useful marker and demonstrates
the equivalency of the exposures on both sides of the river.
The PCT
basalts are unconformably overlain by locally erupted Pliocene
diktytaxitic basalts (Fig. 2.3).
Magnetic polarity was determined by fluxgate magnetometer using 3
to 6 oriented samples of basalt from each flow.
A tentative
22
magnetostratigraphy of reverse/normal/normal/normal/reverse/normal,
from bottom to top of the section, was designated by Smith and Cushing
(1985).
STRATIGRAPHY
IN THE DESCHUTES BASIN
The Prineville chemicaltype basalts occur in two areas of exposure
in, and marginal to, the Deschutes basin (Fig. 2.2).
Scattered
exposures from the town of Prineville westward to Gray Butte consist of
1
to 3(?) flows overlying tuffs of the John Day Formation and overlain,
with angular unconformity, by Pliocene basalts.
In the northern
Deschutes basin two flows crop out in the Deschutes River canyon,
downstream from the vicinity of Madras, and along the northeast basin
margin.
These flows overlie tuffaceous sediments of the John Day
Formation and are intercalated with and conformably overlain by
volcanic sandstones and tuffaceous mudstones of the Simtustus Formation
or unconformably overlain by late Miocene to early Pliocene Cascade
derived volcanics and volcanogenic sediments of the Deschutes
Formation.
Distribution of the flows and paleogeographic relationship
with the contemporary Simtustus Formation (Chapter 3) suggest that the
western Blue Mountains (Ochoco Mountains) and Mutton Mountains stood as
topographic highs and directed basalts and sediments along a drainage
course similar to the present Crooked and Deschutes Rivers.
The basalts in the Prineville region are best exposed west of town
in the core of a southplunging syncline (Fig. 2.2; Table 2.2).
lowest flow is a normalpolarity, lowSiO
The
PCT basalt that is a
2
prominent ridgeformer east of Lone Pine Flat and thins appreciably to
the north and northeast.
On the south side of the Crooked River the
23
flow.
first flow is overlain by a normal-polarity, high-SiO
This is
2
the southernmost occurrence of the more evolved composition. Eight
kilometers southwest of Prineville, the top of a PCT flow projects
This flow is
above the plateau of younger olivine basalts (Fig 2.3).
of low-SiO
composition and has normal magnetic polarity.
Although the
2
major-element composition is similar to the flows of the type section,
this basalt contains much greater abundances of Rb, Sr,
(Table 2.2) precluding correlation to the
position
of
this
flow
relative to the two
Y, and Zr
flows at Bowman Dam. The
flows exposed in the syncline
to the north is unclear but it is probably higher in the section.
A single
flow
of PCT basalt, overlying John Day Formation and over-
lain by Pliocene basalt, crops out 2 km southeast of Prineville.
flow has normal polarity and is
of low-SiO
composition.
This
The exposure
2
is approximately 5 km north of the main PCT outcrop area and probably
represents onlap
of
flows from the type area onto a John Day high.
Early mapping in the Gray Butte area by Williams (1957) reported
several occurrences of Columbia River basalt bounded by northeasttrending faults.
Robinson and Stensland (1979) mapped these flows
within the Eocene Clarno Formation.
Several of the fine-grained,
aphyric flows were analyzed as a part of this study and found to have
major element compositions indistingushable from John Day Formation
trachyandesites (Appendix Ia, analyses GB2, GB3, GB5, 0C14).
Associa-
tion of rhyolitic ignimbrites and sedimentary rocks bearing typical
Oligocene floras (Ashwill, 1983 and person. commun., 1985) with these
lavas suggests that most of the rocks in the Gray Butte area belong to
the John Day Formation and not the Clarno Formation or Columbia River
24
TABLE 2.2: COMPOSITION OF PRINEVILLE CHEMICAL-TYPE BASALT - DESCHUTES
BASIN EXPOSURES
1
Si02
TiO2
Al203
Fe2O3
M90
CaO
Na2O
K20
P205
MnO
Rb
Sr
Zr
51.21
2.75
14.41
13.69
4.46
7.79
2.42
1.78
1.24
0.24
-
Y
Ba
Sc
Ni
V
2
3
4
5
54.33
2.56
15.14
10.69
3.22
51.17
2.79
51.41
54.27
2.60
15.52
10.08
3.49
6.39
2.80
6.11
3.14
3.40
1.21
0.22
56
282
150
43
2123
34
27
225
14.71
13.53
3.89
7.86
2.72
1.83
1.26
0.24
2.82
14.62
13.11
4.00
8.09
2.77
1.68
1.24
0.25
3.41
1.20
0.24
47
51
405
181
302
144
2304
45
2270
2154
37
93
350
38
100
358
34
64
232
135
586
371
87
41
NOTE: Major-element analyses normalized to 100% on a water-free basis.
First
Second
flow
flow
in sequence west of Prineville (sample GB1, Appendix lb).
in sequence west of Prineville (sample 0N2, Appendix Ib).
Third
flow
in sequence west of Prineville (sample P1, Appendix lb).
First
flow
in sequence at Pelton Dam (sample LS1, Appendix lb).
Second
flow
in sequence at Pelton Dam (sample LS2, Appendix lb).
25
Basalt Group (see also Chapter 5).
Queried exposures of Columbia River
Basalt on Robinson and Stensland's (1979) map near the west base of
Grizzly Mountain are altered, coarsegrained, porphyritic flows
resembling basalts of the Clarno Formation (Appendix 1a, analysis GB8).
Goles and others (unpub. mapping) located poorly exposed breccia outcrops on the northeast flank of Gray Butte which they tentatively
assigned to the PCT based on trace element composition, in particular
Using
Ba content of 2140 ppm (G. G. Goles, person. commun., 1985).
field notes and maps courteously supplied by Dr. Gordon Goles, the
author located this exposure.
A majorelement analysis of this unit
(Appendix Ia, analysis GB5) is generally similar to John Day lavas and
lacks the large P 0
content characteristic of PCT basalts.
25
Two normalpolarity PCT basalt flows are widespread in the northern
Deschutes basin and have been described by Jay (1982) and Hayman
(1983).
Analyses of these basalts near Pelton Dam show that the lower
composition and the upper flow to be the highSiO
flow is of lowSiO
40
2
variant (Table 2.2).
15.7
The lower flow has yielded a
0.1 Ma (Appendix IX).
39
Ar/
2
Ar age of
The flows are separated at many
localities by a sedimentary interbed, as much as 15 m thick, assigned
to the Simtustus Formation.
The upper flow exhibits an invasive
relationship with this interbed in several localities (Fig. 2.4b).
The
thickness of the flows varies from 10 m to 180 m and reflects the
paleorelief developed on the underlying John Day Formation.
The lower
basalt was restricted to the topographic lows but the combined
thickness of this flow and the overlying sediment was sufficient to
fill the low areas and allowed the upper flow to spread as a sheet over
26
-
ogr--
41'
.,-..
r....-1.4.11.,..7
...,:
.-. _.
...
;,
,..._, /1-1- -..
Ai-qr.::
' ...:, .4.. ':.,.1,,-,,
V.1.,
-
--
-
,
,..
i
ft
-
il
..,-.
1
-
-'-'
14,,,b^
.,
-.14
6 '
.
.
6
4w
!
'-itr
A
.;
,
4._dc
Fig. 2.4. Outcrop photos of Prineville chemical-type basalt in the
northern Deschutes basin. a). View to the west from Trout
Creek valley to Webster Flat on the west side of the Deschutes
River. Dark line highlights irregular contact between John Day
Note that the lowest PCT flow is
Formation and PCT basalts.
restricted to the paleotopographic lows and that the upper
flow forms a continuous sheet. Slope between flows in formed
in Simtustus Formation interbed.
Landslide debris of
John Day Formation and PCT basalt east of the Deschutes River
forms irregular topography in foreground.
b). Intermixed
basalt and light-colored sediment at invasive contact
between evolved PCT flow and Simtustus Formation.
Roadcut on
Highway 97, 12.5 km northeast of Madras.
27
much of the northern Deschutes basin (Fig. 2.4a).
Both flows onlap
John Day Formation and pinch out east and west of Madras and in the
northwest part of the basin on the south flank of the Mutton Mountains.
In Cow Canyon, 30 km northeast of Madras, these same two PCT flows
have a combined thickness of 150 m and are overlain by a highMg°
chemicaltype Grande Ronde Basalt flow (sample CC1, Appendix lo).
Waters (1961) and Watkins and Baksi (1974) reported an 11 flow sequence
in Cow Canyon and the latter authors identified three magnetic
reversals in the section.
These studies are difficult to resolve with
the present observation of only 3 flows, all with normal polarity.
K-
Ar ages for two samples in this section are 15.4 + 0.3 and 15.3 + 0.3
Ma (Watkins and Baksi, 1974).
Four meters of volcanogenic sediments
lithologically similar to the Simtustus Formation separate the
Prineville and highMg° Grande Ronde flows.
OCCURRENCES OF PRINEVILLE CHEMICAL TYPE BASALT IN NORTHCENTRAL OREGON
North of the Deschutes basin, high P 0
and Ba basalts have been
25
recognized in several localities, intercalated with lowMg° chemical;
type Grande Ronde Basalt (Fig. 2.5).
Swanson and others (1979)
assigned these PCT basalts to the Grande Ronde Basalt.
A single normalpolarity PCT basalt occurs along the John Day River
and was first analyzed by Brock and Grolier (1973) in Butte Creek (Fig.
2.5).
The flow is distinguished from intercalated lowMg° Grande Ronde
Basalt flows by its gray weathering color, in contrast to the reddish
brown weathering typical of the other flows (Nathan and Fruchter,
1974).
Because of its distinctive appearance and composition and its
usefulness as a marker, this basalt was named the Buckhorn flow for
28
Portland
c14c4.
41w
0 The
Dallas
*HOOD' X 4E°
U
9,4.
co
o
<is
co
V'
0 )(1
TYGH RIDGE
*
X
*
**
ct,
lb
0
i
-N-
i
049
1405110"14
M°°4
44/1.
A.
,4o
,
*
*
4
0
14
0
y
0-s.
-4
in
i?trl
o
04,7,
* '4-* 0-9
0I.
*
*ANALYZED OUTCROPS OF PRINEVILLE
CHEMICAL-TYPE BASALT
44
RIVER
50
KILOMETERS
Madras
PS8509-127
Fig. 2.5. Map showing location of Prineville chemicaltype basalt in
northcentral Oregon.
29
Buckhorn Canyon in the Butte Creek drainage (Cockerham and Bentley,
cited by Nathan and Fruchter, 1974).
of lowSiO
The composition of this flow is
type (Table 2.3).
2
On Tygh Ridge, 120 km-north of the Deschutes basin (Fig. 2.5),
Nathan and Fruchter (1974) recognized two P 0 rich flows separated by
25
five lowMg0 Grande Ronde Basalt flows and two sedimentary interbeds
(Table 2.3).
The lower flow (TY4) has major and trace element contents
similar to those of the basalts at Bowman Dam.
is the highSiO
The upper flow (TY10)
variant of the PCT with major and trace element
2
composition comparable to the similar flow in the Deschutes basin.
Both flows have normal magnetic polarity and were assigned to the N
2
chron by Nathan and Fruchter (1974).
flow
the lower
upper
flow
with the Buckhorn
flow
VandiverPowell (1977) correlated
at Butte Creek and named the
for Buck Hollow on the east end
of
Tygh Ridge.
Anderson
(person. commun., 1985) has traced these flows westward to Gunsight
Butte where they disappear beneath High Cascade volcanics.
Anderson (1978) recognized two flows of PCT basalt within the lowMg0 Grande Ronde Basalt section in the Clackamas River drainage in the
Oregon Western Cascades (Fig. 2.5).
The flows occur together at the R
2
magnetic break and were determined to have reverse polarity by
N
2
Kienle (1971).
lowSiO
From partial analyses it appears that these flows are
and resemble flows in the type section (Table 2.3).
2
Another lowSiO
PCT basalt was recognized in a deep geothermal
2
drill hole just west of Mount Hood in the Oregon High Cascades (Beeson
and Moran, 1979a,b).
The flow occurs near the top of the lowMg0
Grande Ronde Basalt section and is probably in the N
chron.
2
30
TABLE 2.3: COMPOSITION OF PRINEVILLE CHEMICAL-TYPE BASALT - OCCURRENCES
NORTH OF THE DESCHUTES BASIN
1
2
4
3
6
5
7
8
9
10
Si02 50.57 50.98 54.17 51.0 50.50 51.22 51.57 52.47 54.91 54.67
T102
2.69 2.79 2.52 2.64 2.73 2.70 2.69 2.73 2.44 2.44
Al203 13.64 14.60 15.29
14.81 14.57 14.90 15.05 15.55 15.50
13.37 13.53 13.70 13.15 10.66 10.76
Fe203 13.44 13.54 10.81
3.18
MgO
3.21
4.55 4.40 3.62 4.60 3.81
4.07 4.20 3.56
5.99
6.04
7.71
7.75
9.05
8.01
7.87 7.82 5.92 8.15
CaO
2.73
Na2O
3.27 2.59 2.61
2.82 2.26 2.20 2.16 2.57
3.25 3.35
1.71
K20
1.66
1.88 1.78 3.63
1.12 1.91
1.15
P205
1.16
1.40
1.17
1.19
1.22
1.24
1.20 1.44 1.21
0.23
0.22
0.20
0.23
MnO
0.24 0.25 0.23 0.24 0.26 0.23
Rb
Sr
Zr
Y
Ba
Sc
44
400
-
55
398
175
280
42
41
153
2135
2175
2055
37
19
38
20
32
37
234
Ni
V
55
-
351
-
2300
39
-
-
-
310
1730
36
-
-
46
-
-
384
180
-
-
-
41
2168
37
20
360
-
45
284
149
43
2099
34
18
231
NOTE: Major-element analyses normalized to 100% on a water-free basis.
"Buckhorn flow", Butte Creek section (Nathan and Fruchter, 1974;
anal. at Univ. of Oregon).
First PCT flow in Tygh Ridge section (TY4 of Appendix Ib).
Second PCT flow in Tygh Ridge section (TY10 of Appendix lb).
Partial analysis, average of 2 PCT flows in Clackamas River section
(Anderson, 1978; anal. at Portland State Univ.).
PCT flow in Old Maid Flat drill hole, west of Mount Hood (Priest and
Vogt, 1982; major elements analyzed at Washington State Univ.; trace
elements at Portland State Univ.).
through 10. First through fifth PCT flows, respectively, in Pacquet
Gulch section (analyses performed at Washington State University on
samples collected by J. L. Anderson as part of USGS-DOE cooperative
mapping of the Columbia River Basalt Group and were obtained from
Basalt Waste Isolation Project, Rockwell Hanford Operations,
Richland, Washington; original sample numbers for columns 6 through
10 are, respectively, JA80446, JA80445, 3A80443, JA80440, JA80439).
31
While mapping on the north flank of the Mutton Mountains, James
Anderson (person. commun., 1985) measured a section in Pacquet Gulch
and N
which includes 5 PCT basalt flows, intercalated with both R
2
2
Anderson interprets
lowMg0 Grande Ronde Basalt.
3 lowSiO
PCT flows
2
of
section (Table
in the N
2
2
Because
2.3).
flows
section and two highSiO
in the R
2
poor exposure it is possible that the
flow
pairs 1
and 2 and 4 and 5 represent only one flow each.
CORRELATION OF PRINEVILLE CHEMICALTYPE FLOWS
At least 8 P 0
and Barich
flows were probably erupted in
25
central Oregon, south of Prineville.
Six of these flows, with uniform
composition, comprise the type section at Bowman Dam.
Flows 3 and 6,
both normal polarity, are the thickest flows and are the most likely to
be correlated to other localities.
Another flow with similar major
element composition but notably enriched in incompatible trace
elements, crops out west of Prineville.
A
flow
and
rich in SiO
2
alkalies, but with smaller trace element abundances, also crops out
west
of
Prineville.
Through most of the Deschutes basin there are two normal polarity
PCT flows.
flow rests unconformably upon John Day Formation
A lowSiO
2
and is overlain by a highSiO
flow.
Because these flows are
2
conformably overlain by highMg0 Grande Ronde Basalt, Swanson (person.
commun., 1984) favors extension of the stratigraphy of Swanson and
others (1979) into the northern Deschutes basin to include these flows
in the Columbia River Basalt Group.
Relative position, polarity, and
composition suggest that these two flows are correlative to the two
lowest flows exposed in the syncline west of Prineville (Fig. 2.8).
32
TR
OMF
CR
N2
ENE
R2
SWIM R2
N2
Portland
o OMF
0
CR
2
TRo
PDo
N2
)
N1
)
)
0 BC
o CC
Pr)
c.)
GO
0405
00°
PW
60
NORMAL
POLARITY
REVERSE
POLARITY
BD
KILOMETERS
LOCALMES:
BD - BOWMAN DAM
PVV - PRINEVILLE-WEST
PO - PELTON DAM
CC - COW CANYON
BC - BUTTE CREEK
PG - PACQUET GULCH
TR - TYGH RIDGE
CR - CLACKAMAS RIVER
OMF - OLD MAID FLAT
HIGH-Mg0
GRANDE RONDE BASALT
VA
LOW-Si02
PRINEVILLE BASALT
LOW-Mg0
GRANDE RONDE BASALT
PICTURE GORGE
BASALT
HIGH-Si02
PRINEVILLE BASALT
VOLCANICLASTIC
INTERBEDS
INTRUSIONS
,PS8509-233
Fig. 2.6. Fence diagram illustrating proposed correlation of Prineville
chemicaltype basalt in central. Oregon.
33
flow
The lowSiO
may be equivalent to
flow
3 or 6 at Bowman Dam.
2
The occurrence of two normalpolarity PCT flows on Tygh Ridge,
with highSiO
above lowSiO , suggests that these flows are
2
2
correlative to those in the Deschutes basin.
flows in the Deschutes basin are in the N
If this is so, then the
chron.
Also, if the flows
2
at Bowman Dam are entirely time equivalent with Grande Ronde Basalt and
if
the fluxgate magnetostratigraphy is correct, then the R
,
N
1
R
,
2
1
chrons are represented at the type section and flow 6 is the
and N
2
flow most likely to correlate with the lowest flow in the Deschutes
basin and on Tygh Ridge.
If
VandiverPowell's (1977) correlations are
correct then the lowest flow in the Deschutes basin is the Buckhorn
flow of Nathan and Fruchter (1974).
These correlations are shown in
Figure 2.6.
To this point in the discussion it appears that sufficient evidence
exists to correlate PCT basalts in the Prineville area, though not
entirely in the type section, through the Deschutes basin and onto the
Columbia Plateau where they occur intercalated with
Basalt.
Grande Ronde
However, promise of a simple stratigraphic picture ends there.
If all Prineville chemicaltype basalts were erupted in the region
south of Prineville, as is commonly assumed, then:
Why are there apparently more
flows on the north side of
the
Mutton Mountains than in the Deschutes basin?
How could reversepolarity
flows
occur in the Mutton Mountains
and in the Clackamas valley without cropping out in the
Deschutes basin?
If the basalts were diverted westward around the nose of the
34
Blue Mountains anticlinorium, then how did the Buckhorn flow
become so widespread to the east?
A potential solution to all of these questions is that PCT flows
were not all erupted south of Prineville but from numerous sources or
long linear dikes distributed over a large area, including well to the
north of Prineville.
The greater number of flows in the Pacquet Gulch
section, compared to the Deschutes basin, may reflect a local source in
the poorly mapped Mutton Mountains region.
The reverse polarity flows
in the Clackamas drainage may correlate to this hypothetical source
rather than to the Prineville area.
Some Columbia River Basalt Group
flows were erupted from fissure systems as much as 100 km long and by
analogy, the widespread distribution of the Buckhorn flow and its
presumed correlatives, may reflect nearly contemporaneous extrusion
from dikes north and south of the Blue Mountains axis.
These solutions
remain untested, however, because of the lack of known exposures of
dikes which fed Prineville basalt flows.
Problems of correlation because of conflicting magnetic polarities
may reflect problems of magnetic overprinting which are not correctable
with a fluxgate magnetometer.
Further paleomagnetic studies are
required in order to develop a reliable magnetostratigraphy.
An alternative explanation for the distribution of the Buckhorn
flow would still be consistent with its derivation from dikes near
Prineville.
In this scenario the flow is envisioned as having been
restricted to deeply incised valleys south of the Mutton Mountains,
where it is now lying directly on John Day Formation, but would have
spread out to the north as a broad sheet where older lowMg0 Grande
35
Ronde flows previously buried the paleotopographic relief.
STRATIGRAPHIC NOMENCLATURE
When Swanson and others (1979) formally revised the stratigraphy of
the Columbia River Basalt Group they favored including Prineville
chemicaltype flows within the Grande Ronde Basalt where they were
intercalated with Grande Ronde lavas, as at Butte Creek, Tygh Ridge,
Pacquet Gulch, and in the Western Cascades.
They excluded from the
group the type section of the PCT and other occurrences where these
basalts were not interbedded with Grande Ronde Basalt.
These authors
also suggested that, with further fieldwork, it might become advisable
to assign the PCT basalts to member or formational status within the
Yakima Basalt Subgroup.
Goles (in press) has suggested renaming this chemical type for
Bowman Dam because the flows are not prominently exposed near the town
of Prineville.
Prineville chemical type is retained here because
although the name was never assigned formal stratigraphic status the
treatment of geographic names applied to rock units should follow the
same rules as developed for formal units in order to avoid confusion.
The name Prineville chemical type has historic priority and has been
widely used in the literature.
The name should not change simply
because the geographic name of the type locality has changed from
Prineville to Bowman Dam (North American Commission on Stratigraphic
Nomenclature, 1983).
Changing the name at this time will likely cause
confusion and is therefore deemed inappropriate.
Although problems remain in understanding the stratigraphy of the
Prineville basalts, work completed since 1979, including the
36
correlations proposed here, allows for more thorough discussion of
appropriate stratigraphic nomenclature.
Several options requiring
consideration include:
removal of all PCT basalts from the Columbia River Basalt Group,
as recently suggested by Goles (in press);
inclusion of all high P 0
and Ba basalts as a chemical
25
type or member within the Grande Ronde Basalt; or
designation of a separate formation within the Columbia River
Basalt Group to accomodate Prineville chemicaltype lavas.
Uppuluri (1973) included PCT within the Columbia River Basalt
because flows of similar composition were intercalated with Grande
Ronde Basalt.
Although Swanson and others (1979) expressed concern
over correlation of Grande Ronde Basalt and the type section of PCT,
their decision to include PCT flows as Grande Ronde Basalt where
intercalated with Grande Ronde Basalt sets the precedent of
interstratification as a primary criterion for including PCT within the
Columbia River Basalt Group.
Correlation of the type area of PCT
basalt to occurrences with Grande Ronde Basalt farther north now
warrents inclusion of all PCT basalts in the Columbia River Basalt
Group.
Goles (in press) agrees that all PCT flows should be treated
together stratigraphically rather than assigning some to the Columbia
River Basalt Group while excluding others.
He gives no specific
reason(s) for supporting his proposal to remove all PCT flows from the
Columbia River Basalt Group, although their unusual composition is the
implied motive.
Given the compositional variability of the Columbia
37
River Basalt Group lavas (e.g. Wright and others, 1973) this, alone,
does not seem to be a sufficient reason for dispensing with, rather
than extending, the stratigraphic assignments of Swanson and others
(1979).
Because the PCT basalts cannot be confidently distinguished
from texturally similar Grande Ronde Basalt flows without petrographic
or chemical analyses, the two basalt types cannot be accurately mapped
separately.
Therefore, it seems unwise to remove all PCT flows from
the Columbia River Basalt Group and judicious to extend the usage
proposed by Swanson and others (1979) to include all PCT basalts within
the Grande Ronde Basalt.
Assignment of the Prineville chemicaltype basalts to the Grande
Ronde Basalt is not free from objection, however.
The stratigraphic
scheme of Swanson and others (1979) is not only based on position and
mappability but also on compositional traits.
Wright and others (1973)
demonstrated that the basalts could be divided into chemical types
distinguishable on variation diagrams (Fig. 2.7), especially SiO
vs.
2
MgO, which are useful stratigraphically as well.
The Imnaha, Picture
Gorge, Grande Ronde, and Wanapum Basalts form compositionally distinct
fields on these diagrams, and the Saddle Mountains Basalt, the youngest
and least voluminous flows in the group, while occupying distinct
fields, are extremely variable in their composition (Fig. 2.7).
Thus
although the Columbia River Basalt Group as a whole is compositionally
diverse, the Grande Ronde Basalt is restricted in composition.
Compositionally the PCT basalt is unlike Grande Ronde Basalt (Fig.
2.7).
Trace element contents of PCT are unlike any other Columbia
River Basalt Group flows.
The characteristic large Ba and P 0
2 5
38
2.0
1.0
12.0
11.0
EDSaddle Mountains Basalt
IH Ice Harbor Member
EM Elephant Mountain Member
P Pomona Member
U Umatilla Member
10.0
9.0
Wanapum Basalt
PR Priest Rapids Member
R Roza Member
FS - Frenchman Springs Member
8.0
ev
7.0
Grande Ronde Basalt (GR)
Prineville Chemical-type Basalt (PCT)
6.0
EjPicture Gorge Basalt (PG)
50
WgAI lmnaha Basalt (I)
57
56
55
54
53
52
51
50
49
48
47
90
8.0
7.0
6.0
111g0
5.0
4.0
3.0
2.0
Fig. 2.7. Variation diagrams for compositional units within the
Columbia River Basalt Group.
39
contents of PCT flows are most comparable to the Umatilla Member (0.88
P 0
,
3000 ppm Ba) and Goose Island flow of the Ice Harbor Member (1.54
25
% P 0
,
750 ppm Bo) in the Saddle Mountains Basalt.
25
PCT basalt is also
206
more enriched in
Pb than is Grande Ronde Basalt (Church, 1985).
Therefore, although field relationships argue for including PCT
within the Grande Ronde Basalt, composition argues for exclusion from
that formation.
The problem becomes one of relative importance of
field criteria and compositional criteria in the develOpment of
stratigraphic nomenclature.
The author feels that it would be unwise
to establish formation status for the PCT basalts because 1) they
cannot always be reliably distinguished from Grande Ronde Basalt in the
field; and 2) the extent of PCT basalts is presently known only from
scattered sections and has not been mapped.
Although assignment of
these flows to the Grande Ronde Basalt would dissolve the compositional
coherency of the formation, this solution appears to be most reasonable
in a stratigraphic sense because the PCT flows are timeequivalent to
Grande Ronde, occur in sequences including other Grande Ronde chemical
types, and are presently included within the Grande Ronde over most of
their area of distribution.
The author, therefore, recommends that all of the high P 0
and Ba
25
flows correlative with Grande Ronde Basalt be included within that
formation and designated Prineville chemical type, regardless of their
location and occurrence with or without intercalated Grande Ronde
Basalt flows.
If the Grande Ronde Basalt is later divided into
members, as have the Wanapum and Saddle Mountains basalts, member
status would be appropriate for the PCT flows.
Designation of member
40
status is inappropriate at this time because the Grande Ronde Basalt is
Distinction should be made
not otherwise subdivided on a formal basis.
and highSiO
between the relatively lowSiO
2
type.
SiO
variants of this chemical
2
The term Buckhorn subtype is informally proposed for the lowcompositions after the Buckhorn flow.
The first geographic name
2
applied to the highSiO
variant was Buck Hollow (VandiverPowell,
2
1977), but because this name is easily
confused with Buckhorn, the
term Paquet Gulch subtype, proposed by J. L. Anderson (see Gales, in
press) is suggested instead.
CONCLUSIONS
Study of middle Miocene basalts in
Oregon shows that P 0
the Deschutes Basin in central
and Barich flows designated as Prineville
25
chemical type by Uppuluri (1973, 1974), south of the basin, can be
correlated through the basin to similar flows mapped as part of the
Grande Ronde Basalt of the Columbia River Basalt Group.
Prior concern
with the correlation of PCT basalts at Bowman Dam with similar flows
farther north is greatly diminished by recognizing errors in Uppuluri's
designation of lithostratigraphy and magnetostratigraphy at the Bowman
Dam type section.
Although compositionally distinct from other Grande Ronde Basalt
flows, the present understanding of PCT distribution favors a
stratigraphic scheme which extends the existing nomenclature of Swanson
and others (1979) to include all PCT basalt within the Grande Ronde
Basalt over proposals to exclude them from the Columbia River Basalt
Group altogether.
If further work allows for mapping the extent of PCT
basalts as a separate unit, then separate formational status within the
41
Columbia River Basalt Group should be considered. Following Vandiver-
Powell (1977) and Gobs (in press), flows whose major-element
composition is distinct from the uniform compositions reported by
Uppuluri (1973, 1974) but sharing the P 0 - and Ba-rich character are
25
included in the PCT because the P 0
and Ba contents are the
25
distinguishing features of the chemical type and flows of both
compositions occur together in close spatial and stratigraphic
proximity.
variant near Prineville
The occurrence of the higher SiO
2
suggests that at least one flow of this composition was erupted near
the type area.
Vents or dikes which fed these compositionally distinctive basalts
remain undiscovered.
However, flow distributions and magnetostrati-
graphy suggest that not all of the flows were erupted in the Prineville
- Bowman Dam vicinity as has been previously assumed.
42
CHAPTER 3
SIMTUSTUS FORMATION: PALEOGEOGRAPHIC AND STRATIGRAPHIC SIGNIFICANCE OF
A NEWLY DEFINED MIOCENE UNIT IN THE DESCHUTES BASIN, CENTRAL OREGON
INTRODUCTION
The purpose of this chapter is to describe a newly recognized
Miocene unit, herein named the Simtustus Formation, in the Deschutes
basin of central Oregon, and to discuss
its depositional environment,.
relationship to previously defined units, and significance to regional
stratigraphy.
Of particular importance, is the relationship between
the Simtustus Formation and the Columbia River Basalt Group with which
it is interstratified
.
As discussed by Swanson and others (1979), a
recognized stratigraphy for sedimentary rocks interbedded with the
flood basalts exists in most of central and eastern Washington but such
an understanding.of interbed stratigraphy is lacking in Oregon.
Although the Simtustus Formation is presently mapped over an area of
only about 250 square kilometers (Plate II) it is appropriate to
propose nomenclature at this time to establish precedent for
stratigraphic nomenclature of volcaniclastic rocks conformable upon,
and interbedded with, the Columbia River Basalt Group elsewhere in
northcentral Oregon.
As used here, the Deschutes basin refers to that area of central
Oregon south of the Mutton Mountains, north of the High Lava Plains,
east of the High Cascade Range, and west of the Ochoco Mountain foothills (Fig. 1.1).
This region, currently integrated into the Columbia
River drainage system via the northflowing Deschutes River, has been
43
the site of episodic emplacement of volcanic and volcaniclastic sedimentary rocks since at least middle Eocene time.
Surface exposure is
dominated by rocks of Neogene age derived both from the Cascade Range
and volcanic sources within and east of the basin.
Exposure of the middle to upper Eocene Clarno Formation and
Oligocene to lower Miocene John Day Formation is largely restricted
to structurally high areas north and east of the Deschutes basin.
Large
rhyolite domes and smaller knobs of dacite assigned to the John Day
Formation also occur as inliers within the basin, surrounded and
partially buried by younger rocks.
The John Day Formation is overlain by middle Miocene Prineville
chemicaltype basalt which is stratigraphically equivalent to the
Grande Ronde Basalt of the Columbia River Basalt Group (Uppuluri, 1974;
Smith, Chapter 2).
The Grande Ronde Basalt is the oldest of three
formations in the Yakima Basalt SubGroup that are widespread in
central and eastern Washington and northern Oregon and were erupted
from fissure vents in southeastern Washington and northeastern Oregon.
The Prineville chemicaltype flows in the Deschutes basin were probably
erupted from nowburied vents somewhere south of Powell Buttes, flowed
northward through the Deschutes basin and became intercalated with
Grande Ronde Basalt flows north of the Mutton Mountains.
Correlation
of Prineville chemicaltype flows in the Deschutes basin with flows of
similar composition within the Grande Ronde Basalt farther north
warrants inclusion of the Deschutes basin basalts within the Grande
Ronde Basalt (Chapter 2).
The flood basalts are overlain by and intercalated with volcanic
44
and volcaniclastic rocks of largely Cascade provenance, the Simtustus
Formation and Deschutes Formation, whose stratigraphy is considered
here. Pleistocene basalt flows and pyroclastic deposits locally overlie
the Neogene section and partly fill 50 to 250 m deep canyons incised
during late Pliocene and early Pleistocene time by the Deschutes River
and its tributaries.
The base of the exposed section becomes older
northward because of northward increase in the depth of incision and
southerly dips on the south flank of the Mutton Mountains.
PREVIOUS WORK
Volcanic and sedimentary rocks of Miocene to early Pliocene age
overlying the Columbia River Basalt Group in the Deschutes basin have
been referred to by three names. These rocks have been named Deschutes
sands (Russell, 1905) or Deschutes Formation (Stearns, 1930; Moore,
1937; Stensland, 1970 Taylor, 1973, 1980a; Peterson and others, 1976;
Jay, 1982; Hayman, 1983; Farooqui and others, 1981a,b; Smith and
Priest, 1983), Madras Formation (Hodge, 1928,1940; Williams, 1957;
Hewitt, 1970; Robinson and Price, 1963; Robinson and Stensland, 1979;
Robinson and others, 1984), and Dalles Formation (Hodge, 1942; Waters,
1968a; Robinson, 1975; Robison and Laenen, 1976).
Farooqui and others
(1981a,b) proposed retaining usage of Deschutes Formation, because the
name Deschutes has historic priority, and placed the formation, along
with other units in northcentral Oregon which had been previously
mapped as Dalles Formation, into a newly defined Dalles Group.
No Deschutes Formation type section was defined by previous
workers but most have referred to exposures in the canyons of the
45
Deschutes and Crooked Rivers upstream from Round Butte Dam as typical
of the formation.
In this region the Deschutes Formation consists of
dark gray to black, pebbly, coarsegrained sandstones, cobble to
boulder conglomerates, and minor tuffaceous mudstones and diatomites,
interbedded with pumice lapillistones and more than one hundred
ignimbrites and lava flows. As part of the present study of the basin,
a type section illustrating this lithologic character has been defined
at Round Butte Dam (Chapter 5, Appendix VI).
The age of the Deschutes Formation has been determined by both
paleontologic and radiometricage dating studies.
Fossil leaves
(Chaney, 1938; Ashwill, 1983) and fish bones (Cavender and Miller,
1972) indicate a late Miocene to early Pliocene age.
40
KAr and
Isotopic dates by
39
Ar/
Ar methods indicate a range in age from about 7..6 Ma
(Smith and Snee, 1984) for the Felton basalt member, the lowest basalt
flow in the Deschutes section, to about 4.0 Ma (Appendix IX) for
basalts near the top of the formation.
Vertebrate fossils indicative
of a HemingfordianBarstovian age (12.0 to 21.0 Ma; all land mammal
ages from Berggren and others, 1985) were described by Downs (1956)
from localities near Gateway, subsequently mapped as Dalles Formation
(Waters, 1968a; Robinson, 1975) or lower Deschutes Formation (Hayman,
1983), which are stratigraphically between the Columbia River Basalt
Group and Hemphillian age (5.0 to 9.0 Ma) fish fossils (Cavender and
Miller, 1972) below the Pelton basalt member.
Jay (1982) and Hayman (1983) were the first workers to make a
detailed evaluation of the stratigraphy of the Columbia River Basalt
Group and overlying rocks in the Round Butte Dam to Gateway area.
They
46
designated all rocks overlying the Columbia River Basalt Group as
'Deschutes Formation, including those hosting Downs' (1956) fossils,
thus extending the age of the base of the Deschutes Formation to middle
Miocene.
These two workers also recognized that the Columbia River
Basalt Group is represented by two flows separated by a sedimentary
interbed.
Jay (1982) assigned the interbed to the Deschutes Formation
but Hayman (1983) mapped the interbed as a separate, unnamed unit.
Smith and Hayman (1983) gave a preliminary report of evidence for an
unconformity separating Hemphillian fossil localities beneath the
Pelton basalt member and Downs' (1956) HemingfordianBarstovian fossil
localities.
They proposed retaining Deschutes Formation for the upper
unit and informally used Lake Simtustus formation for rocks below the
unconformity and interbedded with the Columbia River Basalt Group.
The
name was shortened to Simtustus formation by Smith and Priest (1983)
and Smith and See (1984) and has been reserved by the U.
Survey Geologic Names Committee (V.
S. Geological
Langenheim, person. commun., 1984).
DEFINITION OF SIMTUSTUS FORMATION
Simtustus Formation is proposed for the volcaniclastic rocks
conformable upon, and interbedded with, the Columbia River Basalt Group
in the Deschutes basin and is lithologically distinct from other rocks
in the Deschutes basin.
Probable extension of the unit outside of this
area is left for future workers.
The name is derived from Lake
Simtustus, the reservoir impounded behind Pelton Dam on the Deschutes
River, west of Madras.
The type section is defined from a composite of
three exposures on the eastern canyon wall near the reservoir and two
47
reference sections are designated near Gateway (Fig. 3.1).
These
sections illustrate most of the lithologic diversity of the formation
but do not include an areally restricted rhyodacitic ignimbrite
The
(Hayman, 1983) exposed on a hill 1.5 km southeast of Gateway.
distribution of the Simtustus Formation is shown in Plate II.
As thus defined, the Simtustus Formation is 1 to 65 m thick, and
composed, in decreasing order of abundance, of tan, massive and
laminated tuffaceous mudstone to finegrained sandstone, light gray to
tan, crossbedded medium to very coarsegrained tuffaceous sandstone,
smallpebble volcanic conglomerate, tuff, debrisflow breccia, and
rhyodacitic ignimbrite.
Deschutes Formation is retained for the
coarsegrained volcanogenic sediments and interbedded lava flows and
ignimbrites, of variable composition, that characterize the exposures
first described by Russell (1905) and Stearns (1930) and unconformably
overlie the Simtustus Formation.
These lithologies, distinct from the
Simtustus Formation, have been considered in further detail by
Stensland (1970), Hewitt (1970), Jay (1982), Hayman (1983), Conrey
(1985), Dill (1985) and Smith (Part II).
the usage
This represents a revision in
by Farooqui and others (1981b) which placed all Neogene
rocks overlying the Columbia River Basalt Group in the Deschutes
Formation, including those now assigned to the Simtustus Formation.
Because of lithologic similarity between that portion of the
Simtustus Formation interbedded with the basalt and the portion
overlying the basalt, a single stratigraphic name is proposed.
This
designation follows the precedent of Swanson and others (1979), farther
north on the Columbia Plateau, to
restrict the Columbia River Basalt
48
Group to basalt lithologies alone and assign sedimentary interbeds to
the immediatley overlying sedimentary formation of similar lithology.
This approach is preferable to schemes that define formational
boundaries in lithologically indistinct sedimentary units on the basis
of position in the basalt sequence, because at the plateau margin
definitive basalt flows may not occur as markers (Schmincke, 1964;
Swanson and others, 1979).
Nonetheless, formal and informal division
of the Ellensburg Formation, in Washington, into members is based on
position of sedimentary interbeds relative to distinctive basalts of
the Columbia River Basalt Group (Mackin, 1961; Schmincke, 1964, 1967;
Bentley, 1977) rather than on the lithologic characteristics of the
sediments themselves.
This results in ambiguous correlations of sedi-
mentary units between sections with dissimilar basalt stratigraphy.
To
avoid introducing such ambiguity in Oregon nomenclature, member status
in the Simtustus Formation is not designated for the prominent interbed
in the basalts in the Deschutes basin which, hereafter, is referred to
informally simply as the lower Simtustus Formation for convenience in
this paper.
Field relationships indicate that the Simtustus Formation is
conformable with the Columbia River Basalt Group.
In the Deschutes
basin no Simtustus Formation occurs below the Columbia River Basalt
which lies upon the John Day Formation with up to 200 m of erosional
relief (Fig. 2.4a) and up to 10
Simtustus Formation sedimentation
of angular discordance.
Lower
was probably initiated soon after
the emplacement of the first basalt flow, because no paleosol occurs on
the basalt. The second flow was emplaced during Simtustus deposition
A. TYPE SECTION
LAKE SIMTUSTUS
B. REFERENCE SECTION
CLARK DRIVE
C. REFERENCE SECTION
GATEWAY GRADE
PELTON BASALT
MEMBER
RIVER
T590 m
Gateway
.18
8
650 m
650 m
(COVERED)
DIATOMITE
5
4. A2
I
1525
)
KILOMETERS
U3 2
D
1-P
fn
Al
&30m
A3
2
2 cc
a2
LAKE
SIMTUSTLIS
m
610 m
Madras
500 m
\
625 m
600 m
ACCRETIONARY LAPILLI
COLUMBIA
RIVER
(COVERED)
566m
CLAST-SUPPORT
CONGLOMERATE
fa 2 BASALT
0
(4 r"
1- 2
2 cc
a2
GROUP
\\
ff.\ pc
vf
MUD SAND GRAVEL
MATRIX-SUPPORT
CONGLOMERATE
(COVERED)
566m
vc
COLUMBIA
MEDIUM-VERY
COARSE SANDSTONE
RIVER
BASALT
GROUP
485 m
&e TROUGH X-STRAT.
TABULAR X-STRAT.
HORIZONTAL STRAT.
AA A
1111111 11
CLASTIC DIKES
CfLTIS ENDOCARPS
ROOT HORIZONS
VERTEBRATE FOSSILS
LEAF FOSSILS
MEAN PALEOCURRENT
VECTOR (NORTH IS UP)
Fig. 3.1. Graphic measured sections of Simtustus Formation
VERY FINE-MEDIUM
SANDSTONE
MUDSTONE /TUFF
F-1
BASALT
PS8509-137
50
because there is no evidence of disconformity and locally the flow is
invasive into lower Simtustus
siltstone (Fig. 2.4b).
The invasive
relationship is recognized by the occurrence, along the top of the
flow, of crude pillows, chilled rinds, and intermixed baked siltsone.
The Deschutes Formation overlies the Simtustus Formation with
angular and erosional unconformity.
Formation dips 5
In the Gateway area, Simtustus
to the south, as does the Columbia River Basalt
(Hayman, 1983), while the Deschutes Formation dips less than 1
southward.
East of Gateway there is at least 30 m of relief on the
contact between Deschutes Formation cobble conglomerate and underlying,
finer grained Simtustus Formation lithologies (Figs. 3.1 and 3.2).
West of the Deschutes River and along the eastern margin of the basin,
Deschutes Formation rests directly on John Day Formation or Columbia
River Basalt Group indicating that any Simtustus Formation that may
have been deposited in those areas was removed by erosion before
Deschutes Formation deposition commenced.
Reverse faults near the east
abutment of Pelton Dam offset Columbia River Basalt and upper and lower
Simtustus Formation by the same amount but do not affect the Deschutes
Formation (Plate II).
The age of the Simtustus Formation can be determined only from
paleontologic data since no appropriate, fresh, primary volcanic
material has been found within the formation for isotopic dating.
The
Prineville chemicaltype basalt is interstratified with lowMg0
chemicaltype Grande Ronde Basalt north of the Deschutes basin (Nathan
and Fruchter, 1974; Chapter 2) indicating an age of about 15.5 million
years for the base of the Simtustus Formation (Swanson and others,
51
Fig. 3.2. Basal Deschutes Formation conglomerate resting unconformably
upon tuffaceous mudstone of the Simtustus Formation. Roadcut
Photo courtesy of G.
on Clark Drive, 1.5 km south of Gateway.
A. Hayman.
Fig. 3.3. Finingupward fluvial cycles in Simtustus Formation.
Each
cycle commences with trough crossbedded sandstone and grades
upward into massive, blockyjointed mudstone. a) Upper
Simtustus Formation, Clark Drive, 1.0 km south of Gateway. b)
Lower Simtustus Formation at Pelton Dam.
Arrow points to air
fall tuff within mudstone.
52
1979).
This is consistant with the occurrence of the middle Miocene
Pelton flora of Ashwill (1983) which is located in the lower Simtustus
Formation (not in the Deschutes Formation as reported by Ashwill,
1983).
The age of the top of the Simtustus Formation, as it is
preserved, is uncertain.
However, less than 30 m separates Downs'
(1956) pre-12 m.y. faunal localities in the upper Simtustus Formation
from the unconformity with the Deschutes Formation.
The 7.6 m.y.
Pelton basalt member occurs near the unconformity (Fig. 3.1). This
suggests that the preserved Simtustus Formation is entirely middle
Miocene in age (12 to 15.5 Ma) and that a 5 m.y. or more hiatus is
represented by the SimtustusDeschutes unconformity.
SEDIMENTOLOGY OF THE SIMTUSTUS FORMATION
Typical vertical sequences of lithofacies in the Simtustus Formation
are reflected in the measured sections (Fig. 3.1).
Two facies
associations are apparent in these sections: crossbedded sandstone with
minor mudstone, and massive finegrained sandstone and mudstone.
The crossbedded sandstone and minor mudstone facies are arranged
in finingupward cycles 1-6 m thick, averaging 2.5 m thick (Fig. 3.3).
These sequences commence with trough crossbedded coarsegrained,
pumicebearing sandstone or massive to horizontallystratified pebble
conglomerate.
Height of crossbed sets generally decrease, and
abundance of pumice lapilli increases, upward in a cycle, sometimes
passing into ripple crosslaminated, fine to mediumgrained sandstone.
Paleocurrent directions measured from crossbedding, in both lower and
upper Simtustus Formation, vary widely from N20 W to N75 E with mean
orientations at individual locations in northeastward directions.
The
53
upper portion of each cyc.le is represented by massive, light tan,
blocky jointed, finegrained sandstone and mudstone with dispersed,
rounded, pumice lapilli.
This finegrained interval frequently
contains bone fragments, partial leaf and stem impressions, and rare
permineralized root molds.
These massive sedimentary units are
interpreted as bioturbated overbank deposits.
The massive finegrained facies are wellexposed southeast and
southwest of Gateway.
These deposits share much in common with
inferred overbank deposits capping cycles in sandstonedominated
deposits.
They are dominantly light tan, massive, blocky jointed,
finegrained sandstone and mudstone with beds of pumice lapilli
interupted by burrows and abundant lapilli dispersed through the
sediment (Figs. 3.4 and 3.5a).
Because of poor sorting, massive
character, dominance of pyroclastic fragments, and randomly dispersed
pumice lapilli, these sediments closely resemble ignimbrites (Fig.
3.5a).
However, close examination shows rare discontinuous sedimentary
structures, epiclastic sandstone lenses lacking finer grained ash, and
gradational lower contacts with crossbedded sandstones.
These
features argue against these units being ignimbrites and suggests that
the massive, poorly sorted character reflects homogenization and mixing
of pyroclastic sediment by plant roots and animal burrows.
Exposures
on Gateway Grade, southwest of the village, and along U. S. 97 suggest
that these lithologies are organized into crude finingupward cycles,
0.5 to 2 m thick, in which the abundance of lapilli and grain size of
enclosing sediment decreases upward (Fig. 3.4a).
Bone fragments are
54
-
!..41,10.547,7$"
r
r
-AO
1104
°.. -
-,;-
.;,,,..0
..!..pk.
4.
1._,.
..
.
-,
i
:-. s.
.
3.4. Outcrop photos of finegrained sandstone and mOstone fades
association. a) Thin finingupward sequences of ripple cross
b) Clastic dike of
laminated sandstone to massive mudstone.
vertically laminated mudstone cutting massive mudstone.
Fig.
Poadcut on Gateway Grade,
',,fr
.,
II:
.H.
PM- -.
1
km southwest of Gateway.
4-
r
..9
c_
I
..o...
rod
Ot;q7;.1.7.
.
"
N'AP
^
A. .
A
.
.
us
.
.,
V
pa
...,.
' '411
.. ,
.!
4
%
...
._.1.
.I.k .4-:
-
.P.r.f."
''''..,....
Fig. 3.5. Photographs showing Celtis endocarps in Simtustus Formation,
a) Lapillibearing mudstone with scattered endocarps; most
prominent endocarp at tip of knife blade. b) Handsample of
tuffaceous mudstone with numerouw endocarps; sample is 8 cm
across.
55
very common and opalreplaced Celtis (hackberry) 'endocarps are
ubiquitous (Fig. 3.5).
In some localities remnant sedimentary
structures within the generally massive units are represented by thin
bedded, ripplecrosslaminated finegrained sandstones (Fig. 3.4a) and
planelaminated siltstones and claystones.
Clastic dikes 1
to 2 cm
wide, are filled with vertically laminated mudstone, and occur in
several exposures near Gateway (Fig. 3.4b).
Units interpreted as volcanic debrisflow deposits have also been
recognized in the Simtustus Formation.
They are massive,
1
to 3 m
thick, and dominated by pebble to cobblesize clasts supported in a
matrix of sand and mudsize material.
Pelton Dam,
One deposit occurs north of
in the type section. Another occurs south and east of
Gateway and contains flame structures and clastic dikes of underlying
tuffaceous mudstone at its base, resulting from rapid loading of
saturated sediments.
This latter unit thickens eastward from 1.5 m
thick near Gateway to 3 m thick in Old Maids Canyon.
The finingupward cycles of crossbedded sandstone to massive mud
stone, and highly variable paleocurrent directions, are suggestive of
sedimentation on point bars in a meandering river (Allen, 1964, 1970).
Crossbedded sandstone represents deposition by subaqueous dunes in the
river channel and, as the channel migrated, was succeeded by fine
grained overbank sedimentation.
Epsilon crossstratification, repre-
senting successive pointbar lateralaccretion surfaces, has not been
recognized in the Simtustus Formation but large exposures necessary for
observing this sedimentary structure (Jackson, 1978) are rare.
The massive finegrained sandstone and mudstone facies association
56
probably represents floodplain deposition adjacent to, but beyond the
extent of lateral migration of a river channel.
This relationship is
suggested by: 1) bioturbation indicated by the massive character of
these fades with occassional remnant structures; 2) the abundant
fossil remains; 3) similarity to fine-grained upper parts of preserved
point-bar facies; and 4) thickness in excess of 20 m without
intervening cross-bedded sandstone or conglomerate.
Petrographic examination indicates that most Simtustus sandstones
are feldspathic volcanic arenites with subordinate volcanic plagioclase
arkoses and volcanic arenites, by the classification of Folk (1968),
and contain about equal proportions of pyroclastic and epiclastic
volcanic fragments as defined by Fisher (1961).
Quartz and potassium
feldspar (sanidine) compose less than 1 volume percent of the
sandstones.
Heavy minerals, mostly pyroxene, hornblende, and iron-
titanium oxides, are present to as much as 5 volume percent and usually
display alteration rims of hematite and unidentified clay minerals.
The lithic fraction is all volcanic and, in most sandstones,
consists of 50-75%, slightly- to highly-altered, light brown to
colorless glass, mostly of coarse ash to lapilli size.
appear green, lavender, and pink in hand sample.
Altered lapilli
This material is
probably derived from reworking of pyroclastic air-fall deposits
originating from the Cascade Range and is largely of dacitic
composition (G. Hayman, unpub. data).
Rarely, as much as 50% or more
of the lithic fragments are epiclastic volcanic grains.
Mineralogy and
texture of the epiclastic grains suggests that most are basaltic
andesites and andesites derived from the Cascades, with a subordinate
57
contribution from the interbedded Columbia River Basalt (the Prineville
chemicaltype is characterized by an abundance of groundmass apatite
making it petrographically distinct from Cascade basaltic rocks).
As
much as 10% of some sandstones consists of devitrified rhyolite grains
that were probably eroded from John Day Formation rhyolite domes and
ignimbrites, which are also the likely source for the minor quartz and
sanidine.
True rhyolites, with phenocrystal quartz and sanidine, are
virtually unknown in the Oregon Cascades (Priest and others, 1983).
The sandstones are poorly to well cemented by opaline silica and
unidentified clay minerals.
Scanning electron microscope examination
of one lower Simtustus sandstone also disclosed the occurrence of an
unidentified, acicular zeolite.
-
Green cryptocrystalline silica, known
to local rock collectors as "wascoite", forms concretions up to 25 cm
across in the lower Simtustus and also occurs as amygdules within the
lower Columbia River Basalt flow.
MIDDLE MIOCENE DESCHUTES BASIN PALEOGEOGRAPHY
Only a general paleogeographic picture can be constructed for the
Deschutes basin during Simtustus Formation deposition because 1)
exposure is restricted to the area north of Madras; 2) the main outcrop
areas at Lake Simtustus and in the Gateway region are separated by an
intervening area of no exposure (Plate II); and 3) an unknown volume of
the unit was removed by preDeschutes erosion.
The Columbia River basalt flows largely buried a terrain with
erosional relief in excess of 100 m (Fig. 2.4).
The distribution of
the lower of the two flows was strongly controlled by this
paleotopography and lower Simtustus deposition was also largely
58
restricted to the location of pre-existing valleys.
The combined
thickness of the lower basalt flow and the lower Simtustus sediment was
sufficient to allow the upper flow to cover most remaining hills to
produce a gently sloping plain almost 20 km wide on which upper
Simtustus deposition occurred.
Because lower Simtustus deposition was
confined, the thick floodplain facies association is not as well
represented as in the upper Simtustus Formation where there was no
confinement.
Fluvial aggradation to produce the Simtustus Formation may largely
have been the result of drainage disruption by Columbia River Basalt
Group lava flows (Smith, 1984).
Notably, there is no record of middle
Miocene deposition prior to emplacement of the lowest basalt flow in
the Deschutes basin but, as discussed previously, deposition did
probably begin soon after the emplacement of the flow.
The two basalt
flows in the Deschutes basin have a combined thickness of 15 m to 150 m
and subdued and buried much of the paleotopography to produce a low
gradient surface on which Simtustus streams flowed.
This abrupt
modification of gradients could produce aggradation in a previously
non-depositional system.
Sediments deposited would be relatively fine-
grained, because of decreased competence, and include broad floodplain
deposits such as seen in the Simtustus Formation.
Aggradation would continue until a pause in basaltic volcanism of
sufficient duration occurred to allow uninhibited downcutting by the
rivers.
Over 200,000 cubic kilometers of basalt of the Grande Ronde
Basalt was erupted onto the Columbia Plateau between about 17.0 and
15.5 Ma (Swanson and others, 1979).
Basalt flows of the upper Grande
59
Ronde Basalt, Wanapum Basalt (16.5 to 14.5 Ma), and Saddle Mountains
Basalt (13.5 to 6 Ma) of the Columbia River Basalt Group are restricted
north of the Deschutes basin.
Although only the two flows of
Prineville chemicaltype occur within the basin, the contemporaneous
Grande Ronde Basalt inundated ancestral Columbia, "Clearwater" and
Deschutes Rivers north, and downstream, of the Deschutes basin,
severely disrupting drainage to produce lakes adjacent to the
thickening basalt plateau and raised local base level (Fecht and
others, in press).
Degradation in the Deschutes basin could commence only after
headward erosion by the ancestral Columbia River progressed far enough
eastward to integrate these lakes and the ancestral Deschutes
drainage..
River
It is not clear where the confluence of the ancestral
Deschutes and Columbia Rivers was at this time but basalt distribution
maps (Swanson and others, 1979) and study of the evolution of the
Columbia River drainage (Anderson and Vogt, in press) suggest that
headward incision of the ancestral streams significantly east of the
Cascade Range did not occur until sometime between 12 and 14 million
years ago.
Aggradation in the ancestral Deschutes River, imposed by
flood basalt volcanism, would then extend over the period from about 16
Ma to sometime before 12 Ma, consistent with available information for
the age of the Simtustus Formation.
The general northeasterly course for the Deschutes River during
Simtustus time, indicated by paleocurrent analysis and distribution of
the interstratified basalt flows, reflects the topographic influence of
the Mutton Mountains to produce an eastward deflection in the generally
60
northflowing river.
This influence is also indicated by the Deschutes
Formation paleodrainage (Chapter 8) and in the modern drainage.
The
Mutton Mountains are abroad anticlinal uplift of east to northeast
trend.
However, much of the topographic relief is constructional, not
structural, and defined by a northnortheast trending line of John Day
Formation rhyolite domes.
Uplift of the anticline commenced prior to
the emplacement of the Columbia River basalt flows as indicated by the
underlying angular unconformity with the John Day Formation.
The
Prineville chemicaltype basalts lapped onto the south flank of the
highland, progressed around its eastern end, and spread out again to
the north. Contemporary Grande Ronde Basalt flows erupted in
northeastern Oregon and southeastern Washington onlapped the north
flank of the Mutton Mountains, and some of the youngest flows extended
a short distance southward around the east end of the anticline.
Further uplift resulted in the angular unconformity between Deschutes
and Simtustus Formations.
A broad, northeastsouthwest trending
syncline, with opposing dips up to 12
in the basalt, is developed in
the preDeschutes Formation rocks in the northern part of the basin
(Plate II).
This syncline, south of the Mutton Mountains anticline,
has apparently controlled the location of the Deschutes River since at
least middle Miocene time.
RELATIONSHIP TO CASCADE VOLCANISM
Although aggradation to produce the Simtustus Formation was probably
a result of drainage disruption by the Columbia River Basalt Group
lavas, most of the sediment within the formation is of Cascade Range
provenance.
The late Western Cascade volcanic episode (18-9 m.y.b.p.)
61
of Priest and others (1983) is represented in the central Oregon
Western Cascades by basaltic andesite and andesite lavas with
subordinate dacitic pyroclastic units (Priest and others, 1983).
The
Simtustus Formation, composed of primary and reworked dacitic and
rhyodacitic pyroclastic material and epiclastic fragments of basaltic
andesite and hornblende or pyroxene andesite, is a distal reflection of
this volcanic episode.
The proportion of pyroclastic to basaltic andesite and andesite
epiclastic material
in the Simtustus Formation is about 2 to 1
although Priest and others (1983) indicate that pyroclastic material is
subordinate to other lithologies in the proximal Western Cascades, 75
km to the west.
This difference illustrates the hazards of
characterizing volcanism on the basis of distal sediment composition.
Pyroclastic debris is more widespread and more easily eroded than are
lava flows
and therefore dominates over epiclastic grains in
transported sediment.
These characteristics tend to produce relative
enrichment of pyroclastic sediment in distal depositional basins when
compared with the record of pyroclastic volcanism in the source. areas.
However, there are also problems in assuming that proximal volcanic
rocks accurately reflect the eruptive behavior of a volcanic episode.
Because pyroclastic material is usually unconsolidated it is easily
removed from steep slopes and highgradient stream valleys typical of
the proximal setting of mature volcanic arcs and, hence, not preserved
there. The enrichment of pyroclastic material in the Simtustus
Formation, when compared to the Western Cascade rocks of similar age,
probably reflects a combination of preferential removal of pyroclastic
62
material by erosion in the source area and preferential enrichment of
this material in distal sedimentary deposits.
It is also possible that
some of the silicic tuffaceous material in the Simtustus Formation was
derived from erosion of the widespread dacitic air-fall deposits of the
John Day Formation (Robinson and others, 1984).
REGIONAL STRATIGRAPHIC CORRELATION
The Simtustus Formation is the first well-studied sedimentary unit,
in Oregon, which is demonstrated to interfinger with the Columbia River
Basalt Group.
In eastern Oregon, sedimentary interbeds within the
Columbia River Basalt Group, and age-equivalent Strawberry Volcanics,
have been recognized for their fossil floras but have generally not
been named.
These include sedimentary material hosting the Blue
Mountains flora (Chaney and Axelrod, 1959) in Grant County, 220 km east
of the Deschutes basin, and the Sparta flora (Hoxie, 1965) northeast of
Baker, 300 km east of the Deschutes basin.
Although roughly
contemporaneous in age and sharing floral elements with the Simtustus
Formation, the distribution and sedimentological characteristics of
these units remain unstudied.
The Simtustus Formation is also correlative with the Mascall
Formation of Merriam (1901) and Merriam and others (1925).
The Mascall
Formation is defined (Merriam, 1901) as a dominantly lacustrine
sequence in the John Day valley, 125 km east of the Deschutes basin,
where it is conformable upon and locally interfingers with the Picture
Gorge Basalt of the Columbia River Basalt Group.
The Picture Gorge
Basalt is elsewhere intercalated with the Grande Ronde Basalt (Nathan
and Fruchter, 1974).
The Mascall Formation is overlain, with angular
63
unconformity, by the Rattlesnake Formation of Merriam (1901) and Enlows
(1976) (redefined as the Rattlesnake AshFlow Tuff and unnamed
conglomerate by Walker (1979)), which is dated at about 6.6 m.y.b.p.
(Enlows, 1976).
The Rattlesnake ignimbrite is also interbedded with
the Deschutes Formation in the eastern Deschutes basin (Smith and
others, 1984).
The Simtustus and Mascall Formations thus share a
similar structural and stratigraphic position relative to the Columbia
River Basalt Group and overlying upper Miocene rocks,
and' also share a
similar vertebrate fauna (Downs, 1956).
Unnamed sedimentary rocks with Mascall faunal components (Downs,
1956), or similar stratigraphic position, occur at several localities
east of the Deschutes basin (Walker, 1977) and are probably generally
correlative with the Simtustus Formation.
Until more sedimentologic
and stratigraphic work is done it seems prudent to restrict the name
Mascall Formation to the dominantly lacustrine, pyroclastic sediments
of the John Day basin, restrict Simtustus Formation to the dominantly
fluvial, mixed pyroclastic and epiclastic sediments of the Deschutes
basin, and leave other middle Miocene volcaniclastic rocks unassigned
at this time.
The absence of rocks of lacustrine origin within the Simtustus
Formation indicates that the Deschutes basin was not a closed basin at
that time.
Therefore, deposits of the aggrading fluvial system with
characteristics similar to the Simtustus Formation can be expected to
occur farther north.
Reconnaissance investigation confirms this.
In
Cow Canyon, 30 km northeast of Madras, fluvially deposited sediment,
lithologically identical to the Simtustus Formation, occurs between a
64
Prineville-chemical type basalt flow and a high-Mg0 type Grande Ronde
Basalt flow, probably from the upper half of the Grande Ronde section
(Swanson and others, 1979).
Similar interbeds occur with Prineville
chemical-type and low-Mg0 Grande Ronde flows on the north flank of the
Mutton Mountains.
In Butler Canyon on Tygh Ridge, 70 km north of
Madras, two Prineville flows, believed to be the same two as in the
Deschutes basin (Chapter 2), are separated by four low-Mg0 chemical
type Grande Ronde Basalt flows (Nathan and Fruchter, 1974).
Two
sedimentary interbeds occur within this interval that are lithologically similar to the Simtustus Formation, and are stratigraphically
equivalent to the lower Simtustus in the Deschutes basin.
Similar
tuffaceous sediments occur as interbeds higher in the Columbia River
Basalt Group section on Tygh Ridge and also conformably above the
basalt.
The sediments conformable upon the basalt (mapped as
Ellensburg Formation by Waters, 1968b) are lithologically distinct from
the overlying sediments of the Dalles Formation, of Waters (1968b), or
Tygh Valley Formation, of Farooqui and others (1981b), and are
separated from them by a 40
angular unconformity. It therefore seems
improper to include the sediments conformable upon the basalt in the
Tygh Valley Formation of Farooqui and others (1981b). Thin laminated
mudstones occur between Grande Ronde flows on the north flank of Tygh
Ridge and probably represent the lakes into which the ancestral
Deschutes River flowed.
Formal stratigraphic designation of these
sedimentary units should await more detailed study but the observations
described above indicate northward continuation of Simtustus Formation
lithologies and suggest that the portion of the Simtustus overlying the
65
Columbia River Basalt Group in the Deschutes basin is intercalated with
younger Columbia River Basalt flows to the north.
Beyond northcentral Oregon, the Simtustus Formation is
correlative to middle Miocene sedimentary units which are interbedded
with the Columbia River Basalt Group in Washington and to others in the
Basin and Range province in southeastern Oregon.
The Ellensburg
Formation and Latah Formation are interbedded with and locally overlie
the Columbia River Basalt Group in central Washington and northeastern
Washington and adjacent Idaho, respectively (Swanson and others, 1979).
The Ellensburg Formation, as redefined by Swanson and others (1979) to
include all sedimentary interbeds within the basalt and sediments above
the basalt in the western Columbia Plateau, is a lithologically diverse
unit of volcaniclastic and arkosic, fluvial and lacustrine sediments
(Schmincke, 1964; Mackin, 1961) which locally accumulated to great
thickness in basins of the Yakima foldbelt.
The Ellensburg Formation
is also interbedded with and overlies Wanapum and Saddle Mountains
Basalt and contains a Hemphillian fauna near Yakima making only the
lower part correlative with the Simtustus Formation.
The Latah
Formation is composed of finegrained arkosic sediments largely
deposited in lakes (Pardee and Bryan, 1926) that formed along the
eastern margin of the basalt plateau, presumably because of drainage
disruption by the lava flows.
Based on similaraged fauna and flora, the Simtustus Formation is
also correlative, in part or whole, with the Sucker Creek Formation,
Deer Butte Formation, Drip Spring Formation, and Butte Creek Volcanic
Sandstone, and interbedded basalts, rhyolites and ignimbrites in
66
southeastern Oregon (Kittleman and others, 1965).
These volcaniclastic
and arkosic sediments filled faultbounded basins in the Basin and
Range province.
CONCLUSIONS
The Tertiary nonmarine sedimentary rocks of central and eastern
Oregon require detailed study to obtain useful stratigraphic and
sedimentologic information needed to evaluate the stratigraphic
nomenclature, paleogeography, and tectonic development of the region.
Over eighty years of geologic endeavor in the Deschutes basin by almost
a dozen workers failed to recognize the occurrence of an unconformity
within the sedimentary rocks overlying the Columbia River Basalt Group.
This oversight reflects the reconnaissance nature of most stratigraphic
studies in the eastern twothirds of the state.
Detailed stratigraphic
study indicates that this unconformity, representing a depositional
hiatus of 5 million years or more, separates two lithologically
distinct volcaniclastic sequences of largely Cascade provenance, the
newly defined Simtustus Formation, and revised Deschutes Formation.
The Simtustus Formation represents channel and floodplain
deposition by lowgradient, mixedload, possibly highly sinuous
streams. Based upon compelling circumstantial evidence, aggradation was
mostly the result of drainage disruption and gradient diminishment by
basalt flows of the Columbia River Basalt Group with which the
Simtustus Formation is demonstrably contemporaneous.
Basin analysis of
the Simtustus Formation and distribution of intercalated Grande Ronde
Basalt (Prineville chemicaltype) suggests that the Mutton Mountains, a
volcanic and structural high north of the Deschutes basin, had already
67
begun uplift prior to the middle Miocene and has influenced the
regional drainage pattern since.
The Simtustus Formation is the first wellstudied sedimentary unit,
in Oregon, shown to interfinger with the Columbia River Basalt Group.
Other sedimentary units interbedded with the basalts in Oregon are
unnamed, with the exception of the Mascall Formation, and known only
for their faunal and floral contents.
The sedimentary rocks
interbedded with the Columbia River Basalt Group in most of the
Washington portion of the Columbia Plateau are assigned to the
Ellensburg or Latah Formations which are lithologically distinct from
the only partly ageequivalent rocks in the Deschutes basin, warranting
use of a separate name.
Following the practice of Schmincke (1964) and
Swanson and others (1979) in Washington, sediment interbedded with the
Columbia River basalt in the Deschutes basin is assigned to the
overlying Simtustus Formation because it cannot be lithologically
distinguished from the volcaniclastic rocks conformable above the
basalt.
The practice, in Washington stratigraphic usage, of subdividing
sedimentary units deposited contemporaneously with the Columbia River
Basalt Group on the basis of stratigraphic position relative to the
named basalt members is not perpetuated because of potential ambiguity
when definitive basalts do not occur in a sedimentary sequence. The
author hopes that future stratigraphic assignments of interbedded
sedimentary units in Oregon will be based upon the lithostratigraphic
character of the sedimentary rocks alone, irrespective of the basalts
with which they are associated.
Relative to this problem, it is
68
notable that while most of the type Simtustus Formation overlies the
Columbia River Basalt Group in the Deschutes basin, at the plateau
margin, it probably interfingers with younger flows northward toward
the plateau interior.
Reconnaissance observations suggest continuation
of the Simtustus Formation at least as far north as Tygh Ridge but
stratigraphic assignments of volcaniclastic rocks interbedded with the
Columbia River Basalt Group north of the Deschutes basin is left for
future detailed work.
69
PART II: GEOLOGY OF THE DESCHUTES FORMATION: THE RECORD OF EARLY HIGH
CASCADE VOLCANISM IN CENTRAL OREGON
CHAPTER 4: INTRODUCTION TO THE GEOLOGY OF THE DESCHUTES FORMATION
LOCATION AND PURPOSE
The Deschutes Formation is a diverse assemblage of volcanic and
volcanogenic sedimentary rocks, of late Miocene to early Pliocene age,
exposed on the east flank of the Cascade Range in central Oregon (Fig.
The Deschutes basin is conveniently defined by the exposed
4.1).
extent of the Deschutes Formation.
To the north and east the basin
extends to the slopes of the Mutton and Ochoco Mountains, respectively,
composed of older Tertiary volcanics and minor exposure of preTertiary
rocks.
To the south the Deschutes Formation disappears beneath a
carapace of Pliocene to Holocene basalts, largely erupted from sources
east of the Cascade Range.
The Cascades themselves form the western
boundary to the basin.
This report provides a comprehensive survey of the geology of the
Deschutes basin and defines a composite volcanic stratigraphy for the
Deschutes Formation.
This stratigraphy serves as a framework for
describing the petrology and sedimentology of the formation and
evaluating the volcanotectonic evolution of the Cascade Range and
paleogeography of the Deschutes basin.
Work by Hammond (1979) and Priest and others (1983) has
established four stages in the development of the central Oregon
Cascade Range.
The highly dissected Western Cascades are dominantly
composed of pre late Miocene rocks emplaced during two eruptive
70
KEY:
LATE PLIOCENEHOLOCENE VOLCANICS
AND SEDIMENTS
DESCHUTES FORMATION
(LATE MIOCENEEARLY PLIOCENE)
PRE-DESCHUTES
FORMATION
X MT. JEFFERSON
1
1
4-eo
RIVER
Prineville
Sisters ®
0 Redmond
X
X THREE
SISTERS
X
CA;A
0
5
KILOMETERS
10
Nic\
H'GH L41,4
PL4ovs
Bend
PS8505-211
Fig. 4.1. Generalized geologic map of the Deschutes basin.
71
episodes.
Late Eocene to early Miocene volcanism of the early Western
Cascade episode produced a severalkilometerthick sequence of
rhyodacitic tuffs, tuffaceous sediments, and tholeiitic basalts which
was followed by the dominantly basaltic andesite and andesite lavas of
the early to middle Miocene, late Western Cascades episode (Priest and
others, 1983).
Volcanism subsequent to 8 to 10 Ma has primarily been
localized farther east in the High Cascade Range (Fig. 4.1) and
represents either a narrowing or shifting of the volcanic axis in late
Miocene time.
With the spatial reorganization came a petrologic shift
to generally more mafic magmatism with extrusion of an unprecedented
volume of basalt and basaltic andesite lavas.
During the late Miocene
to early Pliocene early High Cascade eruptive episode, extrusion of the
mafic lavas was accompanied by eruption of voluminous pyroclastic flows
of mostly andesitic to rhyodacitic composition.
This episode
culminated in the subsidence of the early central Oregon High Cascades
into an intraarc graben where the volcanic centers were subsequently
buried by late Pliocene to Recent, late High Cascade eruptive products
(Taylor, 1981; Priest and others, 1983).
This latest eruptive episode
has produced a broad platform of basalt and basaltic andesite upon
which the modern glaciated stratovolcanoes have been constructed.
Andesitic and more siliceous lavas and pyroclastics are prominent near
Mt. Jefferson and at the Three Sisters but are subordinate to mafic
lithologies in the intervening region.
The Deschutes Formation is temporally equivalent to the early High
Cascade eruptive episode.
The shift to more mafic volcanism and the
extension which culminated in the development of the central Oregon
72
High Cascade graben reflect an important stage in the volcanotectonic
evolution of the Cascade Range.
However, the proximal volcanic record
of this eruptive episode is buried from view within the graben leaving
stratigraphic study of the more distal Deschutes Formation as an
integral part of our understanding of early High Cascade volcanism.
In part, this report represents a synthesis of thesis research
conducted at Oregon State University over the past 15 years (Hewitt,
1970; Stensland, 1970; Hales, 1975; Jay, 1982; Hayman, 1983; Cannon,
1984; Conrey, 1985; Dill, in prep.; Yogodzinski, 1986).
The reader
interested in the detailed geology of specific areas of the Deschutes
basin is referred to these theses and maps contained therein (Fig.
1.2).
PREVIOUS WORK
Geologic study in the Deschutes basin has progressed in three
stages.
Initial observations were published by Russell in 1905 and
were followed by a hiatus of two decades.
.
From 1925 to 1968 numerous
reconnaissance studies were published, most notably by Stearns (1930),
Hodge (1928, 1940, 1942), Williams (1957), and Waters (1968a).
From
1970 to the present, detailed evaluation of most of the basin has been
recorded in a number of theses (referred to above) at Oregon State
University.
Stratigraphic nomenclature of the rocks herein referred to as
Deschutes Formation has a complicated history.
These rocks have been
named Deschutes sands (Russell, 1905) or Deschutes Formation (Stearns,
1930; Moore, 1937; Taylor, 1973, 1980; Peterson and others, 1976;
Farooqui and others 1981a,b; Smith and Priest, 1983), Madras Formation
73
(Hodge, 1928, 1940; Williams, 1957; Robinson and Price, 1963; Robinson
and Stensland, 1979; Robinson and others, 1984) and Dalles Formation
(Hodge, 1942; Waters, 1968; Robinson, 1975;. Robison and Laenen, 1976).
Farooqui and others (1981b) proposed retaining usage of Deschutes
Formation, because the name Deschutes has historic priority, and placed
the formation, along with other units in northcentral Oregon which had
been previously mapped as Dalles Formation, into a newly defined Dalles
Group.
Wheeler and Coombs (1967) assigned basalts near the top of the
Deschutes Formation to the Mesa Basalt which they regarded as a
regional stratigraphic unit of late Pliocene or early Pleistocene age
in Oregon, northeastern California, northwestern Nevada, and
southwestern Idaho resulting from a single volcanic event.
The term
Mesa Basalt was also used in the Deschutes basin by McBirney and others
(1974) even though Walker and Swanson (1968) demonstrated that the
basalts discussed by Wheeler and Coombs (1967) exhibit a wide range in
age and composition, were erupted from vents near their respective
outcrop areas, and were not the product of a single floodbasalt event.
The type Mesa Basalt in Nevada is early Pleistocene in age (Walker and
Swanson, 1968) and is not correlative to the Deschutes Formation.
Until recently Deschutes Formation, and equivalent formational
names, have been applied to all rocks in the Deschutes basin that
overlie the middle Miocene Columbia River Basalt Group and are
unconformably overlain by late Pliocene and Pleistocene basalts
(Farooqui and others, 1981a,b).
Smith and Hayman (1983) reported
preliminary evidence of an unconformity and distinct lithologic break
74
in this sequence, warranting the use of two stratigraphic names.
Simtustus Formation (Chapter 3) has been proposed for the light
colored, relatively finegrained volcaniclastics conformable upon, and
interstratified with, the Columbia River Basalt Group.
Deschutes
Formation is retained for the unconformably overlying coarsegrained
volcanogenic sediments and intercalated lava flows and ignimbrites
typical of exposures to which Russell (1905) first applied the name
Deschutes.
Paleontologic study of the Deschutes Formation has included
reports on fossil floras (Chaney, 1938; Ashwill, 1983) and a vertebrate
fauna (Cavender and Miller, 1973).
Age diagnostic taxa are indicative
of a late Miocene to earliest Pliocene age.
An older, middle Miocene
fauna collected by Downs (1956) from rocks overlying the Columbia River
Basalt Group near Gateway is now recognized as belonging to the
Simtustus Formation (Chapter 3).
Isotopic dating of Deschutes Formation basalts by KAr and
40
39
Ar/
Ar radiometric techniques have yielded disparate results.
Evernden and James (1964) report KAr ages of 4.3 Ma and 5.3 Ma for
plagioclase separated from a Deschutes tuff.
Armstrong and others
(1975) reported KAr ages generally between 6.0 Ma and 4.7 Ma
(recalculated by Fiebelkorn and others, 1983) with a 16.3 + 3.0 Ma date
for the Pelton basalt, near the base of the formation.
Farooqui and
others (1981) and Bunker and others (1982) obtained KAr dates on
Deschutes basalts ranging from 10.7 + 1.2 Ma to 22.0 + 8.0 Ma,
including factor of two differences with rocks also analyzed by
Armstrong and others (1975).
Smith and Snee (1984) reported an
75
40
39
Ar/
Ar age of 7.6 + 0.3 Ma for the Felton basalt which is consistent
with the occurrence of Hemphillian (5.0 to 9.0 Ma) fossils (Cavender
40
and Miller, 1973) beneath the basalt.
Preliminary
39
Ar/
Ar data
presented in Appendix IX indicate an age of about 4.0 Ma for the top of
the Deschutes Formation with the bulk of the unit being older than 5.3
Ma.
76
CHAPTER 5: GEOLOGIC SETTING
GEOMORPHOLOGY
The Deschutes basin is a plateau capped by Pliocene basalts and
incised to a depth of 250 m by the Deschutes, Crooked, and Metolius
Rivers and their tributaries (Fig. 4.1).
The plateau merges with the
dissected highlands of the Ochoco and Mutton Mountains, to the east and
north.
Incision decreases southward toward the High Lava Plains.
The
plateau slopes northward and ranges in elevation from 1000 m, near
Bend, to 700 m above sea level, near Gateway.
Isolated volcanic
centers, such as Round Butte and Cline Buttes, stand up to 300 m above
the rimrock basalts.
Elevation increases westward to the crest of
Green Ridge at about 1570 m.
Elevations in the adjacent Mutton and
Ochoco Mountains are between 1200 and 1850
m.
PREDESCHUTES FORMATION STRATIGRAPHY
Exposure of rocks older than the Deschutes Formation is largely
restricted to the eastern and northern basin margins.
These rocks are
older Tertiary volcanics with minor exposure of preTertiary units
(Fig. 5.1).
The oldest rocks in the vicinity of the Deschutes basin are
poorlyexposed, lowgrade metasedimentary rocks which crop out in a 25
2
km
area, about 20 km eastsoutheast of Madras (Fig. 5.1).
The age of
these rocks is not known but Swanson (1969) and Kleinhans and others
(1984) suggested that they are correlative with Cretaceous rocks
exposed farther east near Mitchell, Oregon.
The Eocene Clarno Formation is a widespread unit of calcalkaline
volcanics and locally abundant sedimentary rocks representing a vast
77
SIDWALTER
aurrEs
SHITIKE
BUTTE
0 Madras
CASTLE
ROCKS
RIVER
DESCHUTES
FORMATION AND
YOUNGER VOLCANICS
AND SEDIMENTS
SMITH
ROCK
PRE-DESCHUTES
CASCADE VOLCANICS
0IrED
SIMTUSTUS FORMATION
COLUMBIA RIVER
BASALT GROUP
atIP
4116
RivER
Prineville
CLINE
surrEs
JOHN DAY
FORMATION
CLARNO
FORMATION
101
UNDIFFERENTIATED
KILOMETERS
PS8505-210
Fig.
5.1.
Distribution of preDeschutes Formation rocks in, and near,
the Deschutes basin.
78
volcanic field which may originally have covered much of central and
eastern Oregon.
Clarno eruptive centers are marked by shallowlevel
intrusions and plugs that establish some of the higher topographic
features of the Oehoco Mountains.
Volcaniclastic rocks are generally
well cemented and component grains are altered to clay minerals and
zeolites.
Volcanic rocks exhibit slight to intense alteration.
The Oligocene to early Miocene John Day Formation unconformably
overlies the Clarno Formation and is represented in the vicinity of the
Deschutes basin by the western and southern fades of Robinson and
others (1984).
Four lithologies dominate the John Day Formation in
this region: 1) light colored, generally massive, dacitic tuffs and
lapillistones representing airfall pyroclastic material erupted in the
Western Cascades (Fig. 5.2a); 2) single and composite rhyolite domes
(Fig. 5.2b); 3) widespread rhyolitic ignimbrites whose sources may be
concealed beneath the large dome complexes; and 4) a minor volume of
trachyandesite and trachybasalt lava flows, to the east, and calcalkaline dacite to the west.
The widespread rhyolite domes and
ignimbrites suggest that these components of the John Day Formation are
representative of the "great ignimbrite flareup" which occurred
contemporaneously across Nevada and Arizona (Coney, 1976).
Dacite
= 63.8 wt.% ) in the western
eruptive centers (e.g. Eagle Butte, SiO
2
exposures of the formation are representative of Cascade calcalkaline
volcanism.
Some of the highstanding eruptive centers occur within the
Deschutes basin where they protrude through the younger cover (e.g
Juniper Butte, Sidwalter Buttes, Hehe Butte, Cline Buttes, Powell
Buttes).
Dacitic inliers at Forked Horn Butte and along the Deschutes
79
-"
-
-
.
_,,
4
fritilar.441V
ts0,414k '3417
.iretrou
-
-
.
.
.
:Ver
#44`
Ii
.1
'
,K-142ttNrs,
mar -
igm:P
.1g4
Fig. 5.2. Representative exposures of John Day Formation in the
Deschutes basin. a) Roadcut exposures of massive tuffs,
lapillistones, and sandstones along U. S. 26 near Warm
Springs.
b) Juniper Butte silicic dome complex viewed from
the west; Deschutes Formation exposed in Crooked River canyon
in foreground. c) Moderatelydipping variecolored siltstones
and sandstones overlain by trachyandesite (T) along Main Unit
Canal southwest of Gray Butte. d) View westward of Smith
Rock tuff; Terrebonne and Tetherow Butte in middle ground,
Cline Buttes in background, High Cascades on skyline.
80
River north of Lower Bridge may also be of John Day age.
As a part of
this study reconnaissance investigation of the Gray Butte - Smith Rock
area along the eastern margin of the Deschutes basin was undertaken to
resolve stratigraphic controversy there.
Williams (1957) mapped this
area as John Day Formation and Columbia River Basalt.
Robinson and
Stensland (1979) mapped the same area as Clarno Formation and Tertiary
volcanics of uncertain age between what they considered unequivocal
Clarno and John Day.
The rocks consist of a southeast-dipping
homoclinal sequence of dark, aphyric lava flows, fine- to coarsegrained volcanic sandstones, tan to dark red mudstones, a rhyolitic
ignimbrite, silicic lavas, and a massive tan tuff forming Smith Rock
(Fig. 5.2c,d).
The rhyolitic ignimbrite contains large sanidine
phenocrysts and resembles the basal ignimbrite in
John Day member G
(Robinson and Brem, 1981). The aphyric lavas, mapped as Columbia River
Basalt by Williams (1957), are similar in major element chemistry to
John Day trachyandesites of member F (Appendix Ia; Robinson and Brem,
1981).
The mudstones contain a varied flora (Ashwill, 1983) rich in
Metasequoia, a taxon common in the John Day but rare in the Clarno
Formation (Orr and Orr, 1981).
Therefore most, if not all, of the
rocks composing the highland at Gray Butte are interpreted as John Day
Formation.
"Pre-Tertiary limestone" described from the northwest flank
of Gray Butte by Ashwill (1979) was found, on closer examination, to be
thick calcite veins within the Tertiary volcaniclastics.
The John Day Formation is unconformably overlain by one to three
middle Miocene basalt flows of "Prineville chemical-type" that were
probably erupted from vents south of the town of that name (Uppuluri,
81
1973).
The basalts are sparsely phyric, hyalophitic tholeiites whose
distinctive compositional traits are unusually large abundances of P 0
25
The high P 0
(§1.2 wt%) and Ba (§2000ppm).
content is reflected
25
petrographically by abundant apatite microlites.
North of the
Deschutes basin, on Tygh Ridge, Prineville chemical-type basalts are
interstratified with low-Mg0 chemical-type Grande Ronde Basalt of the
Columbia River Basalt Group (Nathan and Fruchter, 1973).
Correlation
of the Deschutes basin basalts with those on Tygh Ridge (Chapter 2)
warrants southward extension of the stratigraphic nomenclature of
Swanson and others (1979).
Thus the Prineville chemical-type basalts
in the Deschutes basin are assigned to the Grande Ronde Basalt of the
Columbia River Basalt Group (Chapter 2)
The basalts range in thickness
from 10 to 150 m; the variation reflects the paleotopography on the
underlying John Day Formation.
southeast of Lone Pine Flat,
The Columbia River Basalt is exposed
in scattered exposures east of Madras, on
the southeast flank of the Mutton Mountains, and in the Deschutes River
canyon downstream from Willow Creek (Fig. 5.1, 5.3).
The Columbia River Basalt Group,
in the Deschutes basin, is
conformably overlain by, and interbedded with (Fig. 5.3), the middle
Miocene Simtustus Formation (Chapter 3).
The Simtustus Formation is a
thin (<100 m) sequence of tuffaceous mudstones and sandstones, with
minor primary pyroclastic lithologies, deposited by the ancestral
Deschutes River in response to drainage disruption by the Columbia
River Basalt flows.
Eroded volcanic centers of late Miocene age occur along the
western margin of the Deschutes basin where they are partially buried
82
s
1.132C 1
1 ,.
ri
o.
--1rot--....
. s ....,......; ,
.----e-
7r
..w
3.1.:
/
-vir;aimmr_,'
%IS'', s.
.1.40
'IL
4
...
p
r.,49,
....
,....,,
e
lyffil; -14
,,-
.,A,4
elia?..7411r7
'
-
1.
' ,..:. s
ft
',
.
:`
,1
71 1
rtr
6, 1'
,('
,.."
,s, ,
Fig. 5.3. Prineville chemicaltype basalt and Simtustus Formation at
Pelton Dam.
Two basalt flows are separated by Simtustus
Pelton basalt on right skyline is near the base of
interbed.
the Deschutes Formation.
CASTLE ROCKS VOLCANO
Fig. 5.4. View of the west face of the north end of Green Ridge showing
exposure of
crosssection of the Castle Rocks volcano. CR
agglomerates and breccias comprising the Castle Rocks; P
conduit plug.
Deschutes basin in background; forested
Pliocene Bald Peter basaltic andesites in foreground.
83
beneath the Deschutes Formation (Fig. 5.1).
The most prominent of
these older centers is the "Castle Rocks Volcano", a composite cone of
andesite and basaltic andesite flows which forms the north end of Green
Ridge (Fig. 5.4; Hales, 1975; Wendland, in prep.).
The Castle Rocks
center was built upon a foundation of older dacites (Hales, 1975) and
was active prior to an adjacent basaltic andesite center to the south
(Conrey, 1985).
KAr ages reported by Armstrong and others (1975) for
these volcanic centers range from 7.5 + 0.1 Ma to 9.4 + 0.6 Ma
(recalculated by Fiebelkorn and others, 1983).
Shitike Butte, Twin
Buttes, and other topographic highs north of Green Ridge may represent
other volcanic centers which were active just prior to Deschutes
Formation time (Yogodzinski, 1985).
GENERAL FEATURES OF THE DESCHUTES FORMATION
The Deschutes Formation is composed of gray to black volcanic
sandstone, conglomerate, minor mudstone, and diatomite interbedded with
basalt to rhyolite volcanics including lava flows, pumice
lapillistones, and ignimbrites (Fig. 5.5).
In its westernmost
exposures, along Green Ridge, the formation is over 700 m thick and
composed almost entirely of lava flows.
The Deschutes Formation thins
eastward to about 250 m along the Deschutes River where it is dominated
by volcanogenic sedimentary rocks.
The formation also thins and
pinches out northward against older rocks on the south flank of the
Mutton Mountains.
East and northeast of Madras the formation is
generally less than 75 m thick and is dominantly epiclastic material
eroded from John Day Formation domes and ignimbrites.
Pumice
lapillistones of probable Deschutes age occur in isolated exposures on
84
'.'
-
Outcrop on west
Fig. 5.5. Typical exposure of Deschutes Formation.
wall of Deschutes canyon, 0.5 km south of CovePalisades
Coarsegrained volcanogenic sedimentary rocks
State Park.
are intercalated with ignimbrites (I) and basalt flows (8).
For scale, the lowest ignimbrite is about 4 m thick at right
edge of photo.
85
the slopes of Grizzly Mountain (Thormahlen, 1984) and may occur
elsewhere in the Ochoco Mountains.
No type section was designated for the Deschutes Formation by
previous workers.
As a part of this study a type section is defined
just north of Round Butte Dam.
Appendix VI.
The lithologic description is given in
This section illustrates nearly all of the physical
characteristics of the Deschutes Formation but no single section can
represent the compositional variation of Deschutes volcanic units.
Other measured sections in Appendix VI, and also in theses by Stensland
(1970), Hewitt (1970), and Dill (1985), serve as reference sections.
Several features suggest that the bulk of the Deschutes Formation
was derived from the site of the presentday High Cascade Range.
Grainsize trends and paleocurrent data in the sedimentary units
indicate sediment dispersal to the east and northeast through most of
the basin (Chapter 8).
Volcanic units become more abundant, exhibit
steeper initial dips, and ignimbrites are thicker, coarsergrained, and
exhibit greater degrees of welding as one goes westward across the
basin.
However, Deschutes Formation rocks cannot, generally, be traced to
potential source volcanoes in the modern Cascade Range.
Along Green
Ridge the Deschutes Formation is truncated by faults bounding the
central Oregon High Cascade graben (Taylor, 1981).
To the north and
south of Green Ridge the Deschutes Formation disappears beneath a
carapace of younger High Cascade volcanics which also obscure any
continuation of the grabenbounding faults at Green Ridge.
A minor volume of Deschutes volcanics was derived from sources
86
These include basalts erupted from sources
outside the Cascade Range.
within the basin (at Tethrow Butte, Round Butte and in the Lower Bridge
to Steelhead Falls vicinity) and from cinder cones and small shield
volcanoes between Bend and Sisters.
Also, the rhyolitic Rattlesnake
ignimbrite, erupted from a source in, or near, the Harney basin (250 km
east of the Deschutes basin), occurs along the eastern Deschutes basin
margin.
40
Highresolution
39
Ar/
Ar dates were obtained by L. W. Snee
(Oregon State University and U. S. Geological Survey) to complement
this study.
These dates, summarized in Appendix IX, indicate that the
base of the Deschutes Formation in the basin is about 7.6 Ma and that
the oldest exposed Deschutes volcanics on Green Ridge are about 7.3 Ma.
The youngest date, about 4.0 Ma, represents the age of Round Butte, an
intrabasinal volcanic center whose lavas locally cap the Deschutes
Formation.
Basalts underlying the Round Butte lavas are about 5.5 Ma
and indicate that the bulk of the Deschutes Formation was emplaced in
little more than 2 million years.
Basaltic andesites on the crest of
Green Ridge are about 5.3 million years old and probably represent the
approximate time of faulting at Green Ridge.
Close correspondance in
age for the youngest Green Ridge rocks and the top of the bulk of the
formation in the basin suggests that aggradation ended when development
of the intraarc graben isolated the Deschutes basin from the High
Cascade source area.
POSTDESCHUTES FORMATION STRATIGRAPHY
A variety of volcanic rocks and unconsolidated sediments overly
the Deschutes Formation.
The younger rocks are recognized by
87
disconformable contact with the Deschutes, resulting from incision of
most drainages following Deschutes aggradation, or paraconformities
indicated by lithologic changes or isotopicage determinations.
Late Pliocene High Cascade basalts flowed eastward into the
northwestern Deschutes basin onto an erosion surface developed on the
Deschutes Formation (Fig. 5.6).
These diktytaxitic olivine basalts
form the rimrocks on the Warm Springs Indian Reservation and thicken
from 10 m, west of Warm Springs, to 140 m,
drainage.
in the lower Mill Creek
Northeast of Warm Springs the basalts rest on John Day
Formation along an ancestral Deschutes River channel.
Yogodzinski
(1986) traced some of these rimrock basalts to an area near the
confluence of the Whitewater and Metolius Rivers where they fill a
paleocanyon incised over 150 m into the Deschutes Formation.
A KAr
age of 9.1 + 1.0 Ma, obtained by Bunker and others (1982; recalculated
by Fiebelkorn and others, 1983),
is inconsistent with the position of
these basalts over the Deschutes Formation.
In the lower Whitewater
canyon the rimrock basalts overlie Deschutes Formation basaltic
andesites dated at 4.27 + 0.75 Ma (Yogodzinski, 1986).
An age of 3.7 +
0.1 has been determined for the lowest, exposed Pliocene diktytaxitic
basalt in Mill Creek canyon (L. W. Snee, person. commun., 1985;
Appendix IX).
On the western half of the Warm Springs Indian
Reservation the basalts are overlain by up to 25 m of poorly
consolidated sand and gravel of unknown, but possibly Pleistocene, age.
Eruptive centers southeast and south of the central Deschutes
basin, such as Horse Butte and Grass Butte near Prineville, erupted
diktytaxitic olivine basalts during the Pliocene. These flows form the
88
KEY:
PLEISTOCENE
NEWBERRY BASALTS
PLIOCENE-HOLOCENE
HIGH CASCADE
LAVAS
PLIOCENE BASALTS
ERUPTED SOUTHEAST OF
DESCHUTES BASIN
*
VENTS
RIV
Gateway
WHITEWA1681"FI'3'.
MT. JEFFERSON :;:.:7
()Madras
ROUND BUTTE
DAM
0-
7
10
KILOMETERS
C:z2
Fig. 5.6. Distribution of postDeschutes Formation lavas in, and near,
the Deschutes basin.
89
plateau between Prineville and Redmond and extend northward to underlie
the village of Terrbonne (Fig. 5.6).
Basalts are over 60 m thick along
the north side of the Crooked River east of O'Neil and, therefore, were
erupted after initial incision of the Crooked River.
Sutter (unpub.
data) obtained a KAr age of 3.36 + 0.08 Ma for the basalt at Coombs
The basalt at Terrebonne is 3.4 + 0.5
Flat, just east of Prineville.
40
Ma according to a recent
39
Ar/
Ar age determination (L. W. Snee,
person. commun., 1985; Appendix IX).
Pliocene volcanism also occurred in the Deschutes basin and
constructed the basaltic andesite shield volcanoes of Squaw Back Ridge
(Fig. 5.7) and Little Squaw Back and numerous small shields and cinder
cones along the southwestern basin margin.
Armstrong and others (1975)
reported a KAr age of 2.9 + 0.2 Ma for a sample collected at the summit
of Squaw Back Ridge.
Pliocene and Pleistocene basalts, basaltic andesites and sediments
are present along the west base of Green Ridge. Most of this material
is Pleistocene outwash and lavas of the late High Cascade eruptive
episode described by Hales (1975), Scott (1977), and Conrey (1985).
Scattered exposures of Pliocene fluvial and lacustrine sediments
on.the west face of Green Ridge and in the Metolius valley represent
the upper portion of a lowdensity volcaniclastic grabenfill proposed
by Couch and others (1982) on the basis of gravity data.
face of Green Ridge,
On the west
20 m of lacustrine and fluvial sediments are
exposed in roadcuts on Forest Road
1490,at an elevation of 940 m.
sediments rest on a fault block of Deschutes Formation lavas and dip
15
to the south.
A measured section of these sediments, informally
The
90
Fig. 5.7. Squawback Ridge, a Pliocene basaltic andesite shield volcano,
as seen from The Peninsula.
91
named the Camp Sherman beds, is provided in Appendix VI.
Most of the
section is laminated mudstone interbedded with tephras of basaltic and
more silicic composition.
Massive diatomite near the top of the
section is interstratified with 2 m of rhythmically bedded basaltic ash
composed of equant sideromelane shards, plagioclase, and olivine.
Normal grading, occassional ripple marks, convolute bedding, and flame
structures suggest that the ash was deposited as turbidites and
interrupted diatomite sedimentation.
Because sideromelane glass is
most often the product of phreatomagmatic eruptions (Fisher and
Schmincke, 1984), the basaltic rhythmites may represent subaqueous
deposition
from base surges erupted from a tuff cone within the lake
in which the diatomite was being deposited.
The diatom assemblage
shows an affinity with 2 to 4 million year old lacustrine sequences
elsewhere in the western United States (J.
P. Bradbury, person.
The top of the section is composed of 2.5 m of
commun., 1983, 1984).
crossbedded, coarsegrained sandstone and conglomerate deposited in a
fluvial setting.
The sediment on lower Green Ridge may be correlative to fluvial
sand, silt, and pebble gravel exposed near the mouth of Jack Creek, 8
km to the west within the Metolius valley.
of sediment, dipping 20
outwash gravel.
to the east,
Here, at least five meters
is unconformably overlain by
Crossbedding in the older sediment yields northeast
paleocurrent directions.
Sediment and pyroclastic units of the Camp
Sherman beds are probably widespread in the Metolius Valley beneath a
thin veneer of Pleistocene basalts and glacial outwash.
Northeast of Madras a veneer of unconsolidated gravel,
1
to 2 m
92
thick, caps flattopped, northwesttrending ridges of Simtustus
Formation.
The gravel is composed of pebbles and cobbles weathered
from Columbia River Basalt Group lavas and John Day Formation rhyolite
flows and ignimbrites.
This gravel does not resemble Deschutes
Formation sediments typically found along the eastern basin margin
(Chapter 8) and was probably deposited as a sheet on late Pliocene or
early Pleistocene pediment surfaces developed on the Simtustus
Formation.
The distribution of this gravel is shown on Plate II.
Late Pliocene and early Pleistocene basalts erupted from sources
in the High Cascades entered the Deschutes basin from the west and were
confined to canyons of modern dimensions previously incised into the
Deschutes Formation (Fig. 5.6).
Normal polarity diktytaxitic olivine
basalts erupted through dikes in the Green Ridge fault zone flowed
north and then east into the Deschutes basin via the Metolius River
and extended nearly to the confluence of the Metolius with the
Deschutes River (Conrey, 1985).
Younger, reverse polarity intracanyon
lavas, dated at 1.6 + 0.3 Ma (Armstrong and others, 1975), occur in
the upper Metolius canyon (Hales, 1975).. Another prominent intracanyon basalt was erupted in the High Cascades south of Sisters and
flowed down Deep Canyon (Fig. 5.6).
Basalts entered the basin from the south in two episodes during
the Pleistocene and were probably erupted near Newberry volcano (Fig.
5.6).
The older flows, with reverse magnetic polarity and dated at 1.2
+ 0.1 Ma (L. W. Snee, person. commun., 1985; Appendix IX), entered the
Crooked River at O'Neil and continued as intracanyon flows to a point 2
km north of Round Butte Dam.
At least 15 flows are present in the
93
intracanyon complex with local pillows and intercalated hyaloclastite.
The flows cooled together in many places to produce a single cooling
unit composed of upper and lower colonnade and intervening entablature.
These basalts are up to 125 m thick, form a prominent bench in the
Crooked River canyon from the U. S. 97 bridge to the CovePalisades
State Park and comprise The Island, a 3 kmlong ridge separating the
Crooked and Deschutes Rivers (Fig. 5.8a).
The basalt also flowed 4.5
km up the Deschutes River from the contemporary confluence with the
Crooked River at the south end of The Island.
Younger, normalpolarity
basalt flows are widespread in the southern Deschutes basin and form
the "Lava Badlands" between the Deschutes River and Powell Buttes (Fig.
5.6).
From south of Bend to Lower Bridge the Deschutes was forced to
its present position along the western margin of these flows.
Most of
the basalt became confined to the narrow Deschutes canyon just north of
Lower Bridge and flowed another 20 km as intracanyon flows (Fig. 5.8b).
Remnants of these intracanyon flows occur at a lower elevation in the
Deschutes canyon than the reversepolarity lavas (Fig. 5.8) indicating
that a time interval sufficient for eroding most of the older basalts
elapsed between the two eruptions.
Normal polarity basalts also overly
the reverse polarity lavas in the Crooked River valley and extend
northward to Crooked River Ranch.
At Lower Bridge, the normalpolarity basalts from Newberry overlie
up to 20 m of diatomite (Moore, 1937).
Most previous workers have
assigned the diatomite to the Deschutes Formation (Stearns, 1925;
Moore, 1937; Stensland, 1970; Peterson and others, 1976), but Williams
(1957) suggested that it is Pleistocene in age.
Williams' assignment
94
N
1
Fig. 5.8. Erosional remnants of Pleistocene Newberry (?) intracanyon
a) View to the south of the confluence of
basalt flows.
Deschutes (right) and Crooked (left) rivers at CovePalisades
Intracanyon basalt forms The Island, between the
State Park.
two rivers, and other remnants can be seen along the walls of
b) View to the north in the Deschutes canyon
both canyons.
R denotes reverse polarity intracanyon
south of The Cove.
basalts which backed up the Deschutes River from its
N denotes normal polarity
confluence with the Crooked.
basalts which flowed directly down the Deschutes River.
95
is supported by the restriction of the diatomite to the present valley
at the confluence of Buckhorn and Deep Canyons with the Deschutes River
P.
and an abundance of extant species in the diatom assemblage (J.
Bradbury, person. commun., 1983).
Interbedded rhyodacitic ashes near
the top of the diatomite are compositionally similar to Pleistocene
tephras erupted in the High Cascades (A. SarnaWojiciki, person.
commun., 1985).
The diatomite is overlain by 3 to 4 m of fluvially-
deposited sand and gravel which includes diatomite clasts.
Five Pleistocene ignimbrites and a thick pumicelapilli unit, of
2
andesitic to rhyodacitic composition, are exposed over a 225 km
area
south, west, and northwest of Bend, including the southwestern margin
of the Deschutes basin (Fig. 5.9; Taylor, 1980b).
Distribution and
lithologic character suggest that these units were erupted from the
early Pleistocene "silicic highland" of Taylor (1978) which extends
eastward from the Three Sisters to Bend and is now largely obscured by
a mantle of younger mafic lava flows (Fig. 5.9).
Detailed discussion
of some of these pyroclastic units is provided by Peterson and others
(1976), Taylor (1980a, 1981), Mimura (1983) and Hill (1984).
Late Pleistocene eruptive activity in the vicinity of Mount
Jefferson is represented by at least two airfall lapilli units and
cogenetic pyroclasticflow deposits in the northern Deschutes basin
(Yogodzinski and others, 1983).
The youngest eruption emplaced a
pumiceous pyroclastic flow in the Whitewater River (Fig. 5.9;
Yogodzinski, 1986) and covered most of the northern Deschutes basin
with fresh, dacitic pumice lapilli locally preserved in deposits up to
2 m thick (Beget, 1981,1982).
Pumice isopleths (Fig. 5.9) point to
96
CZ=
smNaEW4
*
10
" RIVER
PELTON
DAM
MT
JEFFERSON
Madras
<T\
PYROCLASTIC
FLOW DEPOSITS
MT. JEFFERSON
PUMICE
ISOPLETHS
0
(cm)
Sisters
* THREE
SISTERS
SILICIC
HIGHLAND
0
5
10
Bend
KILOMETERS
PS8505-193
Fig. 5.9. Distribution of Pleistocene pyroclastic deposits adjacent to
the central Oregon High Cascades.
97
Mount Jefferson as the source for this eruption, probably from a
glaciated dome complex on the northeast side of the mountain.
Pyroclastic deposits have been removed by glacial erosion close to the
volcano and do not occur on moraines or outwash of the Jack Creek
formation which is probably 40,000-140,000 years old (Scott, 1977).
Beget (1982) has suggested that blockandash flow deposits exposed
along the North Santiam River, west of Mount Jefferson may be
correlative with this eruptive episode.
An older eruption is recorded
by a rhyodacitic ignimbrite exposed in gravel terraces within the
Deschutes canyon near Pelton Dam and above U.
S. 26 (Fig. 5.9).
A
compositionally similar airfall pumicelapilli deposit is exposed in
roadcuts along U.
S.
26, east of the Deschutes River.
The source of
the rhyodacitic pyroclastics is not clearly defined but mean lapilli
size of the airfall deposit is similar to that in adjacent outcrops of
the younger Mount Jefferson tephra, suggesting that the source was in
the Mount Jefferson vicinity. The rhyodacitic airfall and pyroClasticflow deposits in the Deschutes basin may be related to rhyodacitic air
fall pumice with olivine xenocrysts and basaltic andesite xenoliths
found on the west face of Green Ridge (Conrey, 1985).
STRUCTURAL GEOLOGY
The Deschutes basin lies near the intersection of four, diverse
structural provinces (Fig. 1.1, 5.10).
To the east is the Blue
Mountains anticlinorium (locally named the Ochoco Mountains), a broad,
complexly folded and faulted region that is transitional in crustal
properties and structures between the Columbia Plateau, to the north,
and the Basin and Range, to the south.
The Deschutes basin is also
98
Simnasho®
04 s
1,A0
-22e,
0 Madras
c.cs%
RI Ve/fi,
* ROUND
/1*.
BUTTE
0,
JUNIPER
BUTTE
X
0 A x GRAY BUTTE
BLACK*
BUTTE
4-C.c
\:\
4-*
TETHER OW
BUTTE
Sisters 0
Tumalo
V
ANTICLINE
SYNCLINE
FAULT (BALL
AND BAR ON
DOWNTHROWN SIDE)
*
.>\
Bend
0
I
5
I
10
1
KILOMETERS
VOLCANIC VENTS
PS8505-194
Fig. 5.10. Structural features in and adjacent to the Deschutes basin.
99
adjacent to the Columbia Plateau,
the western portion of which belongs
to the Yakima foldbelt (Meyers and others, 1979) characterized by
generally eastwest trending anticlinal ridges and synclinal valleys
developed in the Columbia River Basalt Group.
The basin merges to the
south and southeast with the High Lava Plains, an elevated plateau of
faulted, late Cenozoic volcanics along the northern margin of the Basin
and Range province.
To the west is the Cascade Range with prominent
faults and lineaments along primarily northsouth and northwest
With the
southeast trends (Venkatakrishnan and others, 1980).
exception of largescale faulting at Green Ridge, the Deschutes
Formation exhibits only minor, local deformation and obscures
underlying structures.
Based onthe distribution of ignimbrites in the John Day
Formation, Swanson and Robinson (1968) suggested that uplift of the
Blue Mountains began at about 36 Ma, and produced a substantial
'topographic high before extrusion of Columbia River Basalts began at
about 16 Ma (Nathan and Fruchter, 1974).
Fisher (1967) argued that
uplift along some structures in the anticlinorium preceded John Day
time. Along the eastern margin of the Deschutes basin the Columbia
River Basalt is gently folded, with dips of 5 to 15
by flatlying Pliocene basalts.
,
and is overlain
The lack of deformation in Pliocene
units indicates that uplift along the western end of the Blue Mountains
structure had largely ceased by Deschutes Formation time.
northeast trending faults east and north of Madras
North
(Fig. 5.10) offset
the John Day Formation and Columbia River Basalt Group but not the
Deschutes Formation basalts erupted at Teller Flat and near Grizzly
100
(Swanson, 1969).
However, these shield volcanoes are located in the
fault system suggesting structural control on the volcanism.
Perhaps the most curious structural problem in central Oregon is
the nature of termination of Blue Mountains structures along the
eastern margin of the Deschutes basin.
A high residual Bouguer gravity
anomaly, attributed to shoaling of preTertiary basement along the axis
of the anticlinorium, is abruptly truncated in the eastern Deschutes
The steep gradient in residual gravity
basin (Couch and others, 1982).
values (Fig. 5.11) suggests faulting along a NS or NNWSSE trend and
-the low residual anomaly under the center of the basin may represent as
much as 3 km of lowdensity volcaniclastic material (R. Couch, person.
commun., 1984).
No faulting is apparent at the surface indicating that
truncation of the western Blue Mountains trend occurred prior to
Deschutes Formation time.
Subsidence of 3 km,
if it occurred, was
likely complete before the Oligocene and certainly before the Miocene
because the Deschutes Formation in the basin is no more than 0.3 km
thick and presumed John Day Formation volcanic highs occur west of the
zone of steep gravity gradients.
The structural significance of the Mutton Mountains, a broad
anticline about 15 km across,
geologic maps.
is unclear because of a lack of detailed
Clarno Formation is exposed in the structural center of
the anticline and flanked by John Day Formation which forms the heights
at the east end (Waters, 1968a,b).
On the basis of gravity data, Couch
and others (1982) suggest that preTertiary rocks occur at a shallow
level under the Clarno Formation.
The Mutton Mountains were
established prior to extrusion of the Columbia River basalt and served
101
MT. JEFFERSON x
Fig. 5.11. Residual gravity anomaly map (contoured in milligals) of
central Oregon. Major surface structures superimposed.
Modified from Couch and others (1982)
102
as a barrier to southward onlap of the Yakima Basalt SubGroup lavas
flowing westward through. northcentral Oregon (Swanson and others,
1979).
Prineville chemicaltype basalts that flowed northward through
the Deschutes valley were diverted eastward around the anticline
(Chapter 2).
Further uplift deformed the Columbia River Basalt Group,
including the development of thrust faults on the north flank of the
Mutton Mountains (Swanson and others, 1981).
Several volcanic units in
the lower Deschutes Formation dip southward at less than
1
,
calculated
from their distribution over a large area, even though interbedded
sediments yield paleocurrent data indicating a northward inclined
paleoslope (see Chapter 8).
The rimrock basalt in the northern
Deschutes basin dips gently northward at an angle similar to the modern
Deschutes River gradient.
Thus minor uplift of the Mutton Mountains
probably continued into Deschutes time but had ceased by latest
Miocene.
The Mutton Mountains anticline is located at the edge of the
Yakima foldbelt but differs from these structures in that it already
stood as a topographic high prior to extrusion of the Columbia River
Basalt flows.
Anticlines in the Yakima foldbelt rose at a slow,
continuous rate during extrusion of the basalts (Reidel, 1984) and
although previous growth on these structures cannot be ruled out it
must have been minor because no preMiocene rocks are exposed within
the foldbelt.
Thus, the uplift history of the Mutton Mountains is more
similar to the Blue Mountains than the Yakima foldbelt but the
anticline is not located on the Blue Mountains trend.
Few structural features can be delineated within the Deschutes
103
Along the southeast margin of the Warm Springs Indian
basin.
Reservation the Columbia River Basalt Group and Simtustus Formation dip
up to 20
to the southeast and overlie folded John Day Formation at
Seekseequa Junction.
These observations suggest that a structural high
A
occurs farther west but no other evidence for this structure exists.
northverging thrust fault (trend N 75 E) and two highangle reverse
faults (trend N 30 W) are exposed in road cuts in the northern
Only one
Deschutes basin (Plate II) and exhibit offsets of 1 to 2 m.
of these faults obviously disrupts the Deschutes Formation; the other
two appear to deform only older rocks.
Other small faults may be
common in the basin but have no obvious surface expression.
The most prominent structures affecting the Deschutes Formation
are the faults at Green Ridge associated with the central Oregon High
Cascade graben (Figs. 5.10, 5.12, and 5.13).
Several, roughly
parallel, northsouth trending faults have been mapped by Conrey (1985)
and Wendland (person. commun., 1984).
Along the central portion of
Green Ridge displacements of several hundred meters can be documented
and other faults probably exist in the Metolius valley, buried beneath
late High Cascade volcanics.
Faults along the north end of Green Ridge
have documented displacements of only 10 to 50 m and no major fault is
apparent.
Continuation of the major faults may be concealed beneath
Fault
the east flank of the midPliocene, Bald Peter shield volcano.
bounded blocks along the base of Green Ridge are tilted up to 15
north or south.
to
Pliocene (?) and Pleistocene basaltic volcanism is
locally coincident with these faults (Fig. 5.10) including Black Butte
and a line of cinder cones forming Wizard Ridge (Figs. 5.13).
The
104
NEWBERRY LAVAS
FAULT (BAR AND
BALL ON
DOWNTHROWN
SIDE)
BROTHERS FA (it
0
10
r
zoivE
20
KILOMETERS
PS8505-197
Fig. 5.12. Major fault zones adjacent to the Cascade Range in central
Modified from Peterson and others (1976), MacLeod
Oregon.
and others (1982), and Chitwood (1984).
105
r14.21
VIM
...
00k.
IM.k. - .
BP
, :0
.- -,,,,,-,
-r
I
CR,.
.
agt.;
-.
-:.:
;-
f %
-».-1....
'..
.
..... ,
fl. a')
1,-
..;''.*1
7-.7.151;
cy)
gi.. : ir, or4
i
-
.-.11
1:3 A3
7-t.......
..
',ill
-
A.
--,-
,
c
.
.4.
.
..TFJ
.
..4*
-.." ..._
,
m.
4?
.,
...-
.:!
.'
,
,
,
-..,,"z
C.-.0
..m
4
P
Fig. 5.13. Landsat (RBV) image of central Oregon. Green Ridge fault
scarp separates the Deschutes basin from the High Cascades.
Key: MJ = Mount Jefferson; TFJ = Three Fingered Jack; BP =
Bald Peter; CR = Castle Rocks volcano; WR = Wizard Ridge;
crosses are spaced approximately 10 km apart.
106
northnorthwest orientation of the Wizard Ridge cones suggests that
some faulting associated with the graben may diverge from the
topographic trend of Green Ridge and extend along the southwest flank
of Bald Peter in an area where detailed mapping is not available and
faults may be buried.
The inclination of volcanic units within the Deschutes Formation
decreases uniformly eastward from 4
less than 1
,
,
near the crest of Green Ridge, to
near the Deschutes River.
These dips are similar to
modern stream gradients and are probably initial, not structural,
attitudes.
The paucity of Sediments in the Deschutes Formation near
Green Ridge suggests a westward increase in paleogradient as indicated
by these presumed initial dips.
Thus, there is no indication of
absolute uplift and tilting along the Green Ridge faults.
Topographic and structural relief decreases southward along Green
Ridge. and the major faults take on a northwestsoutheast trend and
merge with a broad zone of faults that extend another 55 km
southeastward to the flank of Newberry volcano (Fig. 5.12).
Some
writers (Peterson and others, 1976; Smith and Taylor, 1983) have
attributed this latter system of faults to the Brothers fault zone
which is inferred to represent a major structural boundary between the
Basin and Range and the Blue Mountains (Lawrence, 1976; Robyn and
Hoover, 1982),
However, several features suggest that these fault
systems are different structures.
The Brothers fault zone trends approximately N70 W from the Harney
basin to the northeast flank of Newberry volcano, however, component
enechelon faults are oriented between N30 W and N50 W (Walker, 1977).
107
Lawrence (1976) suggested that the disparity between the orientation of
the zone and its component faults reflects burial of an active right
lateral, strikeslip fault by lava flows so that subsequent movement
has produced Riedel shears in the surface basalts whose orientation
differs from that of the master strikeslip fault at depth.
The faults between Newberry and Green Ridge, on the other hand,
o
o
are oriented at N25 30 W and parallel the trend of the zone suggesting
a different structural style from the Brothers fault zone (Fig. 5.12).
Mapping by MacLeod and others (1982) shows that the Brothers fault zone
does not displace Pleistocene and Holocene lavas of Newberry volcano
but that faults extending southeast from Green Ridge do, suggesting
different ages of last movement for these structures as well (Fig.
5.12).
Furthermore, the latter fault zone appears to be one arm of an
arcuate zone of faults and aligned cinder cones extending northwest and
southwest from Newberry caldera, the other arm extending to the major
downtothewest fault escarpment at Walker Rim (Fig. 5.12; MacLeod and
others, 1982).
The name Tumalo fault zone is proposed for the faults between
Green Ridge and Newberry volcano, and is named for one of the largest
faults in the zone located on the northeast side of the Tumalo
Reservoir.
The zone of faults is about 20 km wide and the sense of
relative motion varies on individual faults, with either northeast or
southwest side downthrown, to produce small horst and graben.
net displacement across the zone is down to the west.
However,
West of Redmond
individual faults offset the Deschutes Formation by 5 m to perhaps as
much as 20 m with no indication that the faults were active during
108
Deschutes time.
Farther south late Pliocene (?) and Pleistocene
basalts and i-gnimbrites are faulted in some places and banked up
against fault escarpments in others.
Dozens of small shield volcanoes
and cinder cones, probably Pliocene in age, are distributed throughout
the Tumalo fault zone between Black Butte and Bend (Fig. 5.10).
Only
in rare cases are these eruptive centers aligned (e.g. Garrison Buttes)
but the general concurrence of the faults and cones suggests a
structural control on the volcanism (Fig. 5.10).
South of Bend, dozens
of Pleistocene and Holocene cinder cones and fissure vents are
associated with this fault zone on the northwest flank of Newberry
The possible relationship between the Tumalo fault zone and
volcano.
the central Oregon High Cascade graben will be discussed in Chapter 9.
North of Green Ridge, a zone of small' normal faults with downto-
thewest displacements extends from Seekseequa Creek to Simnasho (Fig.
5.10).
The zone is about 5 km wide and is east of the northward
projection
of
faults at Green Ridge.
Displacements are on the order of
5 to 10 m and are equivalent in both the Deschutes Formation and the
overlying Pliocene basalts.
Near Simnasho, the faults bend to the
northeast but no effort has been made to map them into the Mutton
Mountains.
Several regional geologic maps (Wells and Peck, 1961; Waters,
1968a; Venkatakrishnan and others, 1982) illustrate a northwest
trending fault along the lower Metolius River which truncates the Green
Ridge faults.
The fault was inferred by Waters (1968a) on the basis of
the linear course of the river and the interpretation of Columbia River
basalt on the south side of the river, necessitating uplift.
Mapping
109
by Hales (1975) shows that the basalt in question is not Columbia River
basalt and work by Yogodzinski (1986) and Dill (1985) demonstrates
correlation of Deschutes Formation volcanic units across the river and
excludes the possibility of vertical or lateral displacements along
hypothetical faults in the Metolius canyon.
SUMMARY
Eocene and Oligocene volcanism was widespread across northcentral
Oregon and became more localized to the Cascade Range during the
Miocene.
Gravity data (Couch and others, 1982) suggest that several
kilometers of subsidence has occurred in the Deschutes basin but
surface geology constrains this deformation to preMiocene, and
probably preOligocene, time.
Uplift of the Blue and Mutton Mountains commenced in the early
Oligocene and controlled the distribution of middle Miocene basalts of
the Columbia River Basalt Group some of which flowed northward through
the Deschutes basin from inferred sources south of Prineville.
These
basalts buried a mature, dissected topography developed on the
tuffaceous John Day Formation and disrupted drainage to induce modest
fluvial and floodplain aggradation represented by the Simtustus
Formation.
Volcanism along the western margin of the Deschutes basin
constructed several volcanic centers of basaltic andesite to dacite
composition between about 10 and 7.5 Ma.
Beginning near 7.5 Ma the Deschutes basin rapidly aggraded with
coarsegrained volcanogenic sediments and interbedded ignimbrites and
lava flows.
Aggradation virtually ended at about 5.3 Ma when
development of the central Oregon High Cascade graben isolated the
110
depositional basin from the Cascade source area.
Faults bounding the
east side of the graben form Green Ridge and the upper portion of a
thick sequence of grabenfill volcaniclastics is exposed in scattered
localities near the base of the escarpment.
Obvious faults cannot be traced north or south of Green Ridge,
but, to the north, may diverge westward from the trend of Green Ridge
and lie concealed beneath late High Cascade volcanics.
Southward, the
Green Ridge faults merge into the Tumalo fault zone which is part of an
arcuate fault system centered on Newberry volcano that has been active
into the Pleistocene and possibly the Holocene.
A zone of faults with
small offsets occurs north of Green Ridge and probably developed in
late Pliocene or Pleistocene time.
During dissection of the Deschutes basin, folloWing initial
development of the graben, volcanism occurred within and adjacent to
the basin.
Pliocene basalts entered the northern part of the basin
from the High Cascades on the west, and other flows entered the basin
from source areas to the south and southeast. Two large shield
volcanoes, Squawback Ridge and Little Squaw Back. were constructed
within the basin east of Green Ridge.
Pleistocene basalts erupted in
the High Cascades and near Newberry Volcano entered the basin as
intracanyon flows in the Deschutes, Crooked, and Metolius Rivers and in
Deep Canyon.
Airfall tephras and ignimbrites erupted in the highland
west of Bend and near Mount Jefferson are also locally present in the
Deschutes basin.
111
CHAPTER 6
VOLCANIC STRATIGRAPHY OF THE DESCHUTES FORMATION
INTRODUCTION
A stratigraphic framework is necessary before considering the
petrology and sedimentology of the Deschutes Formation and the
relationship of these rocks to Cascade evolution.
The internal
stratigraphy of the Deschutes Formation is best defined on the basis of
distinctive widespread volcanic units because sedimentary units, with a
few exceptions, are not laterally continuous for more than a few
hundred meters.
There are countless lava flows and ignimbrites within
the Deschutes Formation which are candidates as markers. However, lava
flows were generally confined to channels and form restricted
shoestring lenses of limited stratigraphic utility and many of the
ignimbrites_, though originally extending as sheets over large areas of
the basin, were removed by subsequent erosion to leave isolated,
irregularly distributed outcrops.
Fortunately, there are, out of the
several hundred volcanic units in the Deschutes Formation, about a
score whose distributions are wide enough to serve as valuable
stratigraphic markers (Fig. 6.1).
In the following pages, twentyfour volcanic units are described
and assigned informal member status within the Deschutes Formation
(Tables 6.2 and 6.3).
These named units provide a stratigraphic
framework to facilitate discussion of the Deschutes Formation in
succeding chapters, and to guide subsequent studies in the Deschutes
basin.
Most of these units are distinctive in their outcrop appearance
and have been selected not only because of their widespread
112
(NORTH)
(SOUTH)
ROUND BUTTE MEMBER
LOWER DESERT BASALT MEMBER
STEAMBOAT ROCK MEMBER
TETHEROW
BUTTE
SIX CREEK IGNIMBRITE MEMBER
DEEP CANYON IGNIMBRITE MEMBER
PENINSULA IGNIMBRITE MEMBER
STEELHEAD FALLS IGNIMBRITE MEMBER
MEMBER
COYOTE BUTTE IGNIMBRITE MBR.
TENINO IGNIMBRITE MEMBER
FLY CREEK IGNIMBRITE MEMBER
BALANCED ROCKS IGNIMBRITE MEMBER
MCKENZIE CANYON IGNIMBRITE MEMBER
LOWER BRIDGE IGNIMBRITE MBA.
COVE
IGNIMBRITE
MEMBER
BIG CANYON BASALT MEMBER
JACKSON BUTTES IGNIMBRITE MEMBER
HOLLYWOOD IGNIMBRITE MEMBER
OPAL SPRINGS BASALT MEMBER
JUNIPER CANYON BASALT MEMBER
I? RATTLESNAKE
IGNIMBRITE
MEMBER?)
SEEKSEEOUA BASALT MEMBER
CHINOOK IGNIMBRITE MEMBER
PELTON BASALT MEMBER
Fig. 6.1. Stratigraphic position of informally named members of the
Deschutes Formation. Approximate stratigraphic order is
indicated by vertical position of name (younger to the top).
Unambiguous relative stratigraphic position is defined only
for those units whose names occur above one another in the
Position of the Rattlesnake ignimbrite member
diagram.
relative to other units is not certain.
113
distribution but so that several markers are defined in virtually all
localities of good exposure within the basin.
A few of the members are
laterally restricted in outcrop but are treated with the more extensive
units because they occupy key stratigraphic positions allowing
correlation between areas of the basin (e.g. Cove ignimbrite member) or
A
are regionally important (e.g. Rattlesnake ignimbrite member).
correlation diagram (Plate III) of Deschutes Formation sections, east
of Green Ridge, has been constructed by using the members for
correlation between sections.
Formal nomenclature for these members seems inappropriate because
of the large number of new formal names that would necessarily become
established.
The members serve their primary purpose as markers
regardless of formal or informal designation and
names provide a
convenient means of referring to the marker units. To avoid future
ambiguity, easily accessible type localities are defined for each
member in Appendix IV, and the members are named for prominent features
at, or near, their type locality which, with few exceptions, is labeled
on 7.5' topographic maps.
Future workers requiring stratigraphic
information should refer to these type localities in order to observe
the intent of the author in establishing each unit.
Physical and compositional characteristics of ignimbrites at type
localities may not be representative over their entire outcrop area.
The degree of welding and the size and abundance of pumice lapilli
increase toward the ignimbrite source and results in a laterally
variable appearance to each unit.
Many of the ignimbrites were
emplaced as multiple flow units whose contacts are recognized by
114
TABLE M. SUMMARY TABLE: DESCHUTES FORMATION LAVA FLOW MEMBERS
Member
Pelton basalt
Seekseequa basalt
Juniper Canyon basalt
Opal Springs basalt
Big Canyon basalt
Tetherow Butte
Lower Desert basalt
Steamboat Rock
Round Butte
Magnetic
Polarity
N
N
N
N
R
N
N
R
R
Essential Mineralogy
Groundmass
Phenocrysts
plag cpx ol
plag
cpx 01
An65-70
An53-65
An60-65
An65-70
An75-85
An55-65
An75-80
An50-55
An65-70
-
X
X
X
X
-
-
X
-
X
X
X
X
An60-70
An53-57
An55-60
An60-70
An65-70
An65-70
An65-70
An50
An60-65
X
X
X
X
X
X
X
v
X
X
-
X
X
X
-
A
X
115
TABLE 6.2. SUMMARY OF CHARACTERISTICS OF DESCHUTES FORMATION
IGNIMBRITE MEMBERS
Ignimbrite
Member
Chinook
Hollywood
Jackson. Buttes
R
Lower Bridge
R
Cove
McKenzie Canyon
R
Balanced Rock
R
Fly Creek
Tenino
Coyote Butte
Steelhead Falls
Peninsula
Deep Canyon
Six Creek
Rattlesnake
Essential Mineralogy
hb other
cpx
plag opx
Mag. Matrix Pumice Si02
Pol. Color Color wt.%
R
R
pink- white
gray
gray
orange white
black
pink- gray
gray
pink white
gray
white gray
white, white
red,or black
gray
gray
black
gray,
gray
orange black
gray
black
white
white gray
gray,
white
pink
brown, white
gray
gray
black
yellow, graygray
black
brown black
gray
white, white
70-72
67-71
72
63
70-71
X
X
An21 En57
X
X
X
X
X
X
X
X
An22 En55 Wo4lEn39
An35 En42 Wo40En40
An38 En42 Wo40En44
70
An20 En57 Wo42En39
70-72 An30 En55 Wo40En36
59-61
An62 En60 Wo37En41
70
An21 En40 Wo40En40
65
An24 En52 Wo45En42
X
70-72 An20 En50
52-55 An58 En75 Wo45En44
(?)
64-66 An26 En69
(no analytical data)
X
X
64-66
X
70
An21 En50
70
65-69
72
ol
X
X
X
X
X
X
X
X
X
An20
X
X
Wo48En10
qtz.
Or33
gray,
orange black
Bio
X
64-68
61-63
64-66
60
69
76
X
X
(no analytical data)
Data compiled from Cannon (1984), Conrey (1985), Dill (1985), and
Appendix III.
Data for Rattlesnake ignimbrite courtesy of E. M.
Taylor.
116
TABLE 6.3. AVERAGE MAJOR AND TRACE ELEMENT COMPOSITIONS FOR DESCHUTES
FORMATION BASALT AND BASALTIC ANDESITE MEMBERS
PEL
SEEK
JC
OS
BC
TBtb
TBap
TBcr
(n)
(8)
(2)
(2)
(2)
(4)
(2)
(16)
(8)
Si02
TiO2
Al203
FeO
49.4
1.85
15.8
51.6
1.74
16.9
10.05
6.7
8.25
3.6
1.19
49.5
0.93
16.4
9.29
9.5
11.92
2.4
0.05
50.3
1.46
16.8
9.86
9.0
9.67
51.4
1.05
16.6
9.32
9.3
11.26
2.4
0.15
51.3
99.78 100.03
99.99
99.53 101.48
MgO
12.01
7.0
9.73
3.4
CaO
Na20
K20
Total
0.61
(n)
Rb
Sr
Zr
Y
Ba
Sc
Ni
V
(1)
-
-
22
320
113
29
402
38
112
229
-
-
-
14.7
13.35
4.7
8.74
3.0
0.61
51.4
2.56
14.0
13.60
4.8
8.78
3.6
0.65
51.9
2.49
14.3
13.43
5.2
8.14
3.6
0.62
99.21
99.65
99.69
-
(1)
(2)
(1)
-
-
19
19
-
-
374
160
35
446
44
50
418
382
153
37
457
43
20
411
22
387
155
38
2.1
0.34
-
-
2.81
484
41
38
444
Major elements from Jay (1982).
PEL - Average Pelton basalt member.
Trace elements from Smith (Appendix II).
SEEK - Average Seekseequa basalt member from Jay (1982) and Hayman
(1983).
JC - Average Juniper Canyon basalt member from Dill (1985).
OS - Average Opal Springs basalt member from Smith (Appendix Ic).
BC - Average Big Canyon basalt member from Dill (1985).
TBtb - Average Tetherow Butte spatter, Tetherow Butte member, from Smith
(Appendices le and II).
Major
TBap - Average Agency Plains basalt flow, Tetherow Butte member.
elements from Jay (1982), Haymam (1983), and Smith (Appendix le);
trace elements from Smith (Appendix II).
TBcr - Average Crooked River basalt flow, Tetherow Butte member, from
Smith (Appendices le and II).
117
TABLE 6.3. (continued)
SRf
RB
LDcb
LDfl
SRd
(6)
(4)
(2)
(5)
(5)
(8)
50.7
0.85
17.9
8.8
8.9
11.30
2.4
0.27
51.0
0.84
17.5
8.55
8.3
11.15
2.5
0.35
55.3
2.14
15.6
10.19
4.3
7.86
3.3
1.20
54.4
2.07
16.2
9.93
4.2
7.88
3.3
1.09
55.0
2.09
15.6
10.07
4.4
7.75
3.3
1.13
51.4
1.78
16.5
9.48
6.3
8.56
3.3
1.15
Total 101.12 100.19
99.89
99.07
99.34. 98.47
(1)
(2)
(n)
Si02
TiO2
Al203
FeO
MgO
CO
Na20
K20
(n)
Rb
Sr
Zr
(2)
(1)
91
24
362
153
34
556
44
36
27
111
42
205
118
255
7
9
337
89
22
Ni
292
82
26
118
43
169
V
201
217
Y
Ba
Sc
SRp
21
658
214
30
448
Major
LDcb - Average Canadian Bench flow, Lower Desert .basalt member.
Trace elements from
elements from Conrey (1985) and Dill (1985).
Smith (Appendix II).
LDfl - Average Fly Lake flow, Lower Desert basalt member. Major
Trace elements from
elements from Conrey (1985) and Dill (1985).
Smith (Appendix II).
SRd - Average Steamboat Rock member dikes from Smith (Appendix If).
SRp - Average Steamboat Rock member pyroclastics (bombs, spatter,
cinder) from Smith (Appendices If and II).
SRf - Average Steamboat Rock member lava flows from Smith (Appendix If).
RB - Average Round Butte member. Major elements from Jay (1982) and
Smith (Appendix Ig); trace elements from Smith (Appendix II).
118
TABLE 6.4. AVERAGE MAJOR ELEMENT COMPOSITIONS OF DESCHUTES FORMATION
IGNIMBRITE MEMBERS
(n)
CV
CHw
CHb
HWw
HWb
JB
LBw
(6)
(4)
(2)
(2)
(7)
(11)
(4)
(1)
67.0
0.72
16.3
73.2
0.27
63.0
1.16
14.1
17.1
71.4
0.30
14.4
4.31
1.5
2.18
0.3
1.09
2.7
5.56
66.7
0.73
16.2
3.74
2.94
4.4
3.39
70.4
0.55
15.7
3.02
0.8
1.63
4.3
3.95
S102 71.1
TiO2
0.46
Al203 15.6
FeO
3.09
MgO
1.3
CaO
2.15
Na20
3.4
K2O
3.20
3.38
4.3
2.76
Total 100.30 99.27
5.79
2.6
2.31
1.8
70.7
0.54
15.5
3.00
0.9
4.25
1.61
1.61
4.7
1.73
3.2
4.08
3.9
4.16
99.40 100.33
LBg
1.9
99.10 100.31 100.00 100.35
CHw - Average Chinook ignimbrite member white rhyodacitic pumice from
Dill (1985) and Smith (Appendix Ih).
CHb - Average Chinook ignimbrite member banded brown and white dacitic
pumice from Dill (1985).
HWw - Average Hollywood. ignimbrite member white rhyolitic pumice from
Canon (1984) and Smith (Appendix Ih).
HWb - Average Hollywood ignimbrite member black andesitic-dacitic pumice
from Smith (Appendix Ih).
JB - Average .Jackson Butte ignimbrite member gray rhyolitic pumice from
Jay (1982), Dill (1985), and Smith (Appendix Ih).
LBw - Average Lower Bridge ignimbrite member white rhyolitic pumice from
Canon (1984).
LBg - Average Lower Bridge ignimbrite member gray dacitic-rhyodacitic
pumice from Canon (1984).
CV - Cove ignimbrite member white rhyodacitic-rhyolitic pumice from
Smith (Appendix Ih).
119
TABLE 6.4. (cantinued)
(n)
S102
TiO2
MCw
MCb
BRg
BRb
FCw
FCb
TN
CB
SF
(21)
(14)
(7)
(1)
(11)
(5)
(2)
(2)
(1)
69.6
0.62
15.2
3.66
0.7
70.6
0.40
15.7
2.84
0.5
1.67
4.0
3.77
52.9
1.58
17.2
9.30
5.4
66.4
0.85
16.2
4.42
1.6
2.62
5.0
0.74
2.51
65.0
0.85
17.2
4.60
1.9
3.63
4.4
1.80
69.8
0.52
16.3
3.23
2.52
2.62
2.67
2.14
99.50
98.72
99.60
99.38
99.70
71.6
0.27
5.54
4.12
1.52
2.11
4.91
2.90
65.0
0.93
15.6
4.84
1.3
3.08
5.3
2.24
Total 100.41 99.98
99.70
98.29
Al203 15.5
FeO
2.54
MgO
0.9
Ca0
1.53
Na20
3.63
K20
4.44
60.6
1.51
16.2
7.49
3.1
8.40
3.2
MCw - Average McKenzie Canyon ignimbrite member white rhyolitic pumice
from Canon (1984).
MCI, - Average McKenzie Canyon ignimbrite member black andesitic pumice
from Canon (1984).
BRg - Average Balanced Rocks ignimbrite member gray rhyodacitic pumice
from Conrey (1985) and Dill (1985).
BRb - Balanced Rocks ignimbrite member black dacitic pumice from Dill
(1985).
FCw - Average Fly Creek ignimbrite member white rhyodacitic-rhyolitic
pumice from Conrey (1985) and Dill (1985).
FCb - Average Fly Creek ignimbrite member black basaltic andesite pumice
from Conrey (1985) and Dill (1985).
TN - Average Tenino ignimbrite member dacitic pumice from Smith
(Appendix Ih).
CB - Average Coyote Butte ignimbrite member dacitic pumice from Smith
(Appendix Ih).
SF - Steelhead Falls ignimbrite member rhyodacitic pumice from Smith
(Appendix Ih).
120
TABLE 6.4. (continued)
PNw
PN1g
PNdg
PNb
DC
SCw
SCb
RAT
(1)
(2)
(10)
(2)
(3)
(7)
(6)
(2)
67.8
0.74
16.9
4.16
1.9
2.60
65.1
64.3
0.90
2.05
68.9
0.56
15.7
4.14
0.7
2.24
5.3
2.29
59.3
1.57
16.8
7.87
2.3
5.88
4.8
1.19
76.3
0.11
11.4
1.07
0.4
0.53
2.7
1.93
0.85
16.6
4.53
2.3
2.86
5.0
1.95
61.8
1.35
16.9
3.23
2.3
4.64
4.9
1.47
Total 99.71 100.13
99.19
99.59
99.34
99.86
99.71
98.32
(n)
Si02 72.2
TiO2
0.26
Al203 15.7
FeO
2.08
MgO
0.4
CaO
0.96
Na20
4.0
K20
4.11
4.1
16.1
5.28
1.5
3.31
5.9
5.81
PNw - Peninsula ignimbrite member white rhyolitic pumice from Smith
(Appendix Ih).
PN1g - Average Peninsula ignimbrite member light gray daciticrhyodacitic pumice from Smith (Appendix Ih).
PNdg - Average Peninsula ignimbrite member, dark gray and porphyritic
black dacitic pumice from Smith (Appendix Ih).
PNb - Average Peninsula ignimbrite member aphyric black andesitic pumice
from Smith (Appendix Ih).
DC - Average Deep Canyon ignimbrite member dacitic pumice from Smith
(Appendix Ih).
SCw - Average Six Creek ignimbrite member, white rhyodacitic pumice from
Hales (1975), Conrey .(l985), and Dill (1985).
SCb - Average Six Creek ignimbrite member black andesitic pumice from
Hales (1975), Conrey (1985), and Dill (1985).
RAT - Average Rattlesnake ignimbrite member white rhyolitic pumice from
analyses by E. M. Taylor (person. commun., 1983).
121
intervening airfall and/or faintlybedded pyroclastic surge deposits,
or lithicrich zones representing the base of a flow unit.
The number
of preserved flow units generally increases toward the source area.
Also, many Deschutes Formation ignimbrites record the comingling of
magmas or eruption from heterogenous magma chambers (Chapter 7).
As a
result, the composition of pumice lapilli, the most representative
samples of the magma(s) which produced the ignimbrite, are not uniform
throughout the unit.
Because of this variability, the type locality
should be regarded as the best exposure of an ignimbrite member and the
following descriptions of each unit attempts to cover all significant
lateral variability.
PELTON BASALT MEMBER
The oldest exposed volcanic unit in the Deschutes Formation is a
sequence of diktytaxitic olivine basalts which forms a prominent bench
along the Deschutes River canyon from Round Butte Dam to Pelton Dam
(Figs. 6.2, 6.4a).
The name Pelton basalt was first proposed for this
sequence by Ira Williams (1924).
Jay (1982) reported 4 to 8 flows
ranging in thickness from 20 to 45 meters with an upward increase in
the abundance of olivine phenocrysts.
Prior to construction of Round
Butte Dam Stearns (1930) traced the Pelton basalt 1.5 km up the
Metolius River and 3 km up the Crooked River.
of Hayman (1983), near Gateway,
The "Clark Drive basalt"
is petrographically and compositionally
similar to the Pelton basalt and occupies the same stratigraphic
position.
Study of waterwell logs suggest that the basalt is
continuous in the subsurface from the Deschutes River to the Gateway
area warranting inclusion of the "Clark Drive basalt" within the Pelton
122
Madras
PELTON BASALT MEMBER
tRobNb
.
1111 OUTCROP
BUTTE
DAM
PROBABLE EXTENT
LAKE
BILLY
CH/NOOK
o
I
5
i
10
1
KILOMETERS
PS8505-189
The basalt extends an
Fig. 6.2. Distribution of Pelton basalt member.
unknown distance to the south and was probably erupted
southeast of the Deschutes basin. Based on mapping by Jay
(1982), Hayman (1983), and the author.
123
West of the Deschutes River the Pelton basalt thins and
basalt member.
pinches out against an erosional high of Columbia River Basalt Group
and John Day Formation (e.g. Plate I).
Its eastward extent beneath
Deschutes Formation sediments is unknown.
The Pelton basalt member has normal magnetic polarity and has been
40
dated by
39
Ar/
Ar method at 7.6+0.3 Ma (Smith and Snee, 1984).
Analyses by Jay (1982) indicate a small range in composition for all
flows of the Pelton basalt.
However the occurrence of weathered zones
on flow tops and local, thin (<1 m) sedimentary interbeds between flows
precludes contemporaneity of eruption for the entire member.
Nonethe-
less, uniform polarity and composition and widespread distribution
makes these basalts a useful marker. The elevation of the base of the
Pelton basalt member increases northward from
Butte Dam, to
2160 ft. south of Gateway.
1700 ft. near Round
Because imbrication of
underlying conglomerate cobbles indicate northward sediment dispersal,
the low southerly dip of the member must represent subsequent southward
tilting.
Because of burial by younger rocks of the Deschutes Formation it
is impossible to trace the Pelton basalts to their source.
However,
their relatively low alumina content (avg. 15.8 wt%) suggests a source
southeast of the Deschutes basin where Miocene and Pliocene basalts
have similar low alumina (avg. 15.7 wt %, n=13), rather than in the
High Cascades where highalumina basalts (Al 0
> 16.7 wt.%)
23
predominate.
CHINOOK IGNIMBRITE MEMBER
A distinctive pinkishgray, unwelded ignimbrite is exposed at
124
water level along Lake Billy Chinook on the west side of the Deschutes
River arm and along both sides of the Metolius River arm (Figs. 6.3,
6.4b).
Light olivegray to white pumice lapilli are rhyodacitic in
composition (Table 6.4) but Dill (1985) noted the occurrence of banded
pumice with dacitic bulk composition suggesting comingling of this
silicic magma with a more mafic melt.
A prominent feature of this
ignimbrite, throughout the area of its occurrence, is a lithicrich
zone, 0.5 to 1.5 m thick, at the base of the unit representing rounded
cobbles entrained by the pyroclastic flow from underlying gravel.
Hewitt (1970) referred to this ignimbrite as "unit 1".
Stensland (unpub. map) used the name Chinook tuff.
Subsequently,
The name Chinook
ignimbrite member is proposed here.
Dill (1985) recognized multiple flow units within this ignimbrite,
composing a single cooling unit, and provided detailed descriptions of
its features along the south side of the Metolius River.
In the
vicinity of Fly Creek and Spring Creek the Chinook ignimbrite is up to
30 m thick (Dill, 1985).
Farther east the base of the unit dips below
the surface of the lake and its thickness is unknown (Fig. 6.4d).
Based on its distribution, Dill (1985) suggested that the ignimbrite
was emplaced on a terrain with up to 30 m of relief.
Near the confluence of the presentday Metolius and Deschutes
Rivers the pyroclastic flow turned northward and followed an ancestral
Deschutes River channel, approximately 5 m deep, for at least an
additional 25 km (Fig. 6.3).
The most distal, known exposure is 4 km
southwest of Gateway.
In lower Fly Creek canyon a 3 mthick stratified interval occurs
125
Gateway 0
CHINOOK
IGNIMBRITE MEMBER
OUTCROP
PROBABLE EXTENT
SEEKS
4 CREEK
am (I)
AlltE
rSINITUSTUS
?o_
£1/(
"kce,f,
(.F.,77AvvER
0
Madras
LAKE BILLY
CHINOOK
0
5
KILOMETERS
PS8505-202
Fig. 6.3. Distribution of Chinook ignimbrite member within the
Pyroclastic flow was probably erupted in
Deschutes basin.
Based on
the High Cascades, 20 km west of the map margin.
mapping by Dill (1985) and the author.
126
_J
-
I
'0
It) 41
-r.
.i.-011Vgr
at 4 r..#
a)
I.
Fig. 6.4. Outcrop view of Deschutes Formation marker units
View to the south of the Pelton basalt member along Lake
Higher basaltic
Simtustus, 3 km north of Round Butte Dam.
cliffs in background are Pleistocene intracanyon lavas. b)
Outcrop of Chinook ignimbrite member at the mouth of Fly
Dashed line separates massive pyroclasticflow
Creek.
deposit (above) from bedded pyroclasticsurge deposit
Photo
Note contact between flow units at arrow.
(below).
c) Seekseequa basalt member at
courtesy of T. E. Dill.
Seekseequa Junction; thickest portion of unit is confined to
d) Westward view up
ancestral Deschutes River channel.
Chinook
from the mouth of Juniper
Metolius arm of Lake Billy
Canyon showing Chinook ignimbrite (C), Juniper Canyon basalt
(JC), and Big Canyon basalt (BC) members.
127
within the member (Fig. 6.3).
In this section the unit exhibits plane
beds, about 1 cm thick, grading upward into largescale crossbeds with
wavelengths of about 4 m and amplitudes near 40 cm (Dill, 1985).
These
structures indicate turbulent deposition characteristic of pyroclasticsurge deposits.
The Chinook ignimbrite dips eastward at 6.5 m/km along the
Metolius River (Dill, 1985).
This dip probably represents the
depositional slope on which the unit was emplaced.
From Round Butte
Dam to near Gateway, the base of the unit dips southward at
3.4 m/km
in opposition to northdirected paleocurrent indicators in the
underlying conglomerate and sandstone.
Thus, like the Pelton basalt
member, the attitude of the ignimbrite in the northern Deschutes basin
reflects minor southward tilting.
SEEKSEEQUA BASALT MEMBER
The prominent physiographic feature at Seekseequa Junction, on the
Warm Springs Indian Reservation, is a spectacular, cliffforming
columnarjointed basalt up to 25 m thick (Fig. 6.4c).
This flow,
herein named the Seekseequa basalt member, can be traced for 40 km,
from CovePalisades State Park to Trout Creek, where it filled, and
overflowed, an ancestral Deschutes River channel (Fig. 6.5).
Exposure
at Seekseequa Junction provides a crosssectional view of this channel,
which was about 50 m wide and 20 m deep (Fig. 6.4c).
The basalt filled
the channel and extended as a §3 to 4 mthick sheet up to 150 m on
either side.
A prominent northeasttrending ridge southeast of Warm
Springs is capped by this basalt and represents exhumed topographic
inversion of the ancestral Deschutes River channel.
That portion of
128
4
0
0 err
/5"EE
SEEKSEEQUA BASALT MEMBER
OUTCROP
PROBABLE EXTENT
5
0
,.,Seekseequa
Junction
KILOMETERS
Madras
COVE-PALISADES
STATE PARK
PS8505-201
The basalt flow
Fig. 6.5. Distribution of Seekseequa basalt member.
filled and overflowed an ancestral Deschutes River channel,
delineated by the outcrop distribution, and originated an
Based on mapping by Jay
unknown distance to the south.
(1982), Hayman (1983), and the author.
129
the Seekseequa basalt that occupies the paleochannel has a convexup
upper surface so that it stands 3 to 5 m higher than the adjacent out-
ofchannel portion of the flow (Fig. 6.4c).
Like the older Pelton
basalt and Chinook ignimbrite members, the Seekseequa basalt is
inclined gently southward in opposition to the flow direction of the
ancestral Deschutes River indicated by paleocurrent indicators in
underlying conglomerates and sandstones.
The Seekseequa basalt is coarsely porphyritic with plagioclase
glomerophenocrysts up to 1
mm.
cm across and olivine phenocrysts up to 4
Hayman (1983) noted that three distinct plagioclase phenocryst
populations are present in this basalt.
zoned crystals ranging from An
,
Most common are euhedral,
in the core, to An
,
at the rim.
53-57
65
Less common euhedral crystals are unzoned with a composition of An
75
.
As many as 5% of these phenocrysts are anhedral to subhedral,
80
zoned, with embayed margins, and compositions near An
.
The andesine
40
and bytownite crystals may be xenocrysts.
JUNIPER CANYON BASALT MEMBER
The lowest diktytaxitic olivine basalt presently exposed in the
lower Metolius canyon was named the Juniper Canyon basalt by Dill
(1985; Figs. 6.4d, 6.6).
The unit is composed of at least four flows
and varies in thickness from 5 to 13 m.
The bottom flows often contain
pipe vesicles and, on the east side of The Cove north of the marina,
include a basal pillowed zone.
The basalt is compositionally and
petrographically like other diktytaxitic basalts in the vicinity of the
CovePalisades State Park and is recognized on the basis of
stratigraphic position.
It overlies the Chinook ignimbrite member
130
along the Deschutes and lower Metolius rivers.
In its northernmost
exposures, on the southeast corner of the Warm Springs Indian
Reservation, the Juniper Canyon basalt member occurs at the same
elevation as the Seekseequa basalt member.
The relative ages of the
two units, both with normal magnetic polarity, are not known because
they never occur in the same vertical section.
OPAL SPRINGS BASALT MEMBER
Two to four flows of diktytaxitic olivine basalt, up to 40 m
thick, are exposed at the bottom of the Crooked River canyon from 3 km
south of Osborn Canyon to Opal Springs (Fig. 6.7a).
These flows are
compositionally similar and are all of normal magnetic polarity but
were not erupted at the same time because thin paleosols intervene
between the flows.
The Opal Springs basalt member resembles the Pelton
basalt member but occurs at a higher stratigraphic level and has higher
alumina and lower titania contents (Table 6.3).
L.
W. Snee (person.
commun., 1985; Appendix IX) has dated the Opal Springs basalt member at
40
6.3 + 0.1 Ma by
39
Ar/
Ar method.
The Opal Springs basalts were
probably erupted within the High Cascades, based on their high alumina
character, but exposure does not allow the flows to be traced westward.
Large springs enter the Crooked River from many different levels
within this flow sequence.
Most of the springs are on the west side of
the river but some, such as at Opal Springs, are on the east side.
Gaging information available in 1925 indicated that 620 million gallons
of water was introduced to the Crooked River each day by these springs
(Stearns, 1930).
131
JUNIPER CANYON BASALT MEMBER
IIOUTCROP
Fig. 6.6. Distribution of Juniper Canyon basalt member within the
Lava flow was probably erupted in the High
Deschutes basin.
Cascades, 15 to 20 km west of the map margin.
132
ts
-A4
4,
4'4°
-.
-
n
s"="
°
44"
-
74,
11_.
C-ior
:Sri
,
s
I,
ow
Nik
.
7%F'
,-S
II.
Fig. 6.7. Outcrop views of Deschutes Formation marker units
a) Hollywood ignimbrite member overlying Opal Springs basalt
member along the Crooked River near Crooked River Ranch.
Pleistocene intracanyon basalts form cliff in background. b)
Jackson Buttes ignimbrite member near mouth of Willow Creek.
Columnar jointing in center of photo is developed in welded
zone which overlies a lightercolored, basal unwelded zone
and is overlain by a slopeforming upper unwelded zone.
Total thickness of unit is 23 m.
133
HOLLYWOOD IGNIMBRITE MEMBER
An orange ignimbrite, as much as 60 m thick, crops out immediately
above the Opal Springs basalt member in the Crooked River gorge (Fig.
6.7a).
The unit is named for prominent exposures on Hollywood Road on
Crooked River Ranch.
This ignimbrite was misidentified by Stensland
(1970) as his "ashflow tuff 1", or Lower Bridge ignimbrite member of
this study, which occurs higher in the section. Cannon (1985) briefly
discussed this ignimbrite as "unit 0".
In exposures on the east side of the Crooked River, two flow units
are represented, each exhibiting welldeveloped reverse grading of
pumice.
The Hollywood ignimbrite is unwelded and the orange color is a
result of fumarolic alteration which caused devitrification of some
pumice lapilli and bombs and oxidized the glass. Orange lapilli and
bombs up to 20 cm across are most common, and less abundant black
lapilli are up to 5 cm across.
Banded black and orange lapilli and
bombs are prominent in the upper 5 to 10 m of the ignimbrite.
Black
transitional andesitedacite lapilli are less vesiculated and less
altered than the orange rhyolite pumice lapilli which, in rare cases,
contain fresh, lightgray cores.
Because this thick ignimbrite is not exposed in the Deschutes
canyon it probably was erupted in the Cascades at a more southerly
latitude than its outcrop area and then flowed northward along the
ancestral Deschutes valley which was located just east of the modern
Crooked River in late Miocene time (see Chapter 8).
Analyses of
lapilli from a 15 mthick orange ignimbrite at an elevation of 2800
feet in a geothermal gradient well 6 km northeast of Powell Butte
134
closely resemble the Hollywood ignimbrite member and further suggest a
source southsouthwest of its type locality.
JACKSON BUTTES IGNIMBRITE MEMBER
A lightgray rhyodacitic ignimbrite crops out discontinuously
throughout most of the central and northern Deschutes basin and is
named for exposures at Jackson Buttes on the Warm Springs Indian
Reservation (Fig. 6.8).
In its type area the ignimbrite is up to 23 m
thick and thins eastward and westward defining a broad ancestral
Deschutes River channel.
At its thickest exposure, near the mouth of
Willow Creek, the central portion of the ignimbrite is welded and
displays crude columnar jointing (Fig. 6.7b).
This member is
equivalent to "Willow Creek ashflow tuff 1" and "access road ashflow
tuff" of Jay (1982) and to "tuff ten" of Dill (1985)
The pyroclastic flow entered the Deschutes basin from the west and
turned northward near the confluence of the present Metolius and
Deschutes Rivers.
The ignimbrite occurs on the east side of the
Crooked River in the northern part of the CovePalisades State Park and
on the west side of the Deschutes River in the southern part of the
park.
Erosion removed the unit from near the Deschutes Arm bridge at
The Cove northward to the Metolius River.
A remnant of the ignimbrite
can be seen within conglomerate above the first switchback on the road
leading westward from the Deschutes Arm bridge.
Pumice lapilli in the Jackson Butte ignimbrite range from 1 to 6
cm across and increase in diameter upward.
The lapilli are generally
white to light gray in color with orange oxidation.
The ignimbrite is light gray where fresh but more typically is
135
COVE-PALISADES
S !ATE PARK
Fig. 6.8. Distribution of the Jackson Buttes ignimbrite member within
the Deschutes basin. Pyroclastic flow was probably erupted
in the High Cascades, 30 km west of map margin.
136
pinkishgray or light orange in color as a result of fumarolic
oxidation.
Three exposures of a pink ignimbrite with extensive vapor
phase alteration are tentatively assigned to this member and are
queried on Figure 6.8.
BIG CANYON BASALT MEMBER
A thick (18 to 24 m), widespread sequence of reverse polarity
diktytaxitic olivine basalt flows was named for exposures at the mouth
of Big Canyon by Dill (1985; Fig. 6.4d).
The basalts were apparently
erupted in the High Cascades at the latitude of Green Ridge and flowed
eastward into the central Deschutes basin (Fig. 6.9).
In the vicinity
of the The Cove the flows advanced northeastward, apparently in two
lobes, along the ancestral Deschutes River valley.
The Big Canyon
basalt includes the upper Deschutes canyon intraformational basalt
(note that sample analysed by Jay, 1982, was collected from the
Newberry intracanyon basalt and not from the intraformational lava as
reported), Dry Canyon basalt flow, and lower Willow Creek basalt flow
of Jay (1982).
From the latter outcrop the flow can be traced up the
Willow Creek canyon into Madras where the uppermost flow top crops out
extensively in the western part of town (Plate I).
The Big Canyon basalt member is compositionally and
petrographically similar to other diktytaxitic basalts in the Deschutes
basin and is recognized on the basis of stratigraphic position.
It is
the only widespread sequence of reverse polarity basalt in the central
portion of the Deschutes Formation section.
North of Round Butte Dam
the Big Canyon basalt member overlies the Jackson Buttes ignimbrite
member.
The Big Canyon basalt member overlies the Juniper Canyon
137
BIG CANYON BASALT MEMBER
IIOUTCROP
PROBABLE EXTENT
Madras
KILOMETERS
Psnos-13s
Fig. 6.9. Distribution of Big Canyon basalt member within the Deschutes
Lava flow was probably erupted in the High Cascades
basin.
Based on mapping by Dill
15 to 20 km west of the map margin.
(1985) and the author.
138
basalt member in the lower Metolius canyon and along the Deschutes
River south of its confluence with the Metolius.
The elevation of the base of the Big Canyon basalt member is
variable but, in general, slopes eastward from 2350 feet, near the
mouth of Fly Creek, to 2190 feet along the Deschutes River.
This
presumed paleogradient is also reflected in the northward decrease in
elevation to about 2100 feet in the westernmost exposures in Willow
Creek canyon.
However, the basalt dips gently southwestward
in the
Willow Creek exposures and reaches an elevation of about 2200 feet near
Madras.
The southwestward inclination represents tilting as is also
recognized in several lower members.
LOWER BRIDGE IGNIMBRITE MEMBER
One of the most widespread ignimbrites in the southern Deschutes
basin is a pink, unwelded unit overlying 1.0 to 1.5 m of bedded air
fall tuff with accretionary lapilli (Figs. 6.10, 6.11).
The unit is
best exposed on both sides of the Deschutes River at Lower Bridge,
where it is 16 m thick.
Stensland (1970) referred to this unit as
"ashflow tuff one" and Cannon (1984) named it the Lower Bridge tuff.
Detailed discussion of the petrology and distribution of the Lower Bridge
ignimbrite is given by Cannon (1984).
Pumice lapilli and bombs within the Lower Bridge ignimbrite are
mostly white in color and of rhyolite composition.
Gray dacitic
lapilli occur near the top of the unit but this upper zone was stripped
by erosion over most of the area of distribution of the member.
Plagioclase, augite, hypersthene, and rare hornblende and biotite
comprise the essential mineralogy of the unit.
139
Fig. 6.10. Distribution of Lower Bridge ignimbrite member within
the Deschutes basin. Pyroclastic flow was probably
erupted in the High Cascades, at least 25 km southwest of
the lower map margin. Based on mapping by Cannon (1984).
140
ER:
-
,
"
my-
4sr
o.
t.
e
-
.4 ea-
1:':
lot_4014
-
"
-
4Sic
r'e? 4
,
,Th?
jO
,
r,
41-Y
'
-
**
d::"AlFt*;
Z...?;':-
^
'`'
^
_
"re-
Fig. 6.11. Outcrop view of Deschutes Formation marker units
III.
McKenzie Canyon (MC) and Lower Bridge (LB) ignimbrite
members north of Deep Canyon along the Deschutes River.
Bedded, airfall lapillituff beneath Lower Bridge
ignimbrite near Big Falls.
Top of hammer is at the base of
the ignimbrite. c) Close view of McKenzie Canyon ignimbrite
at Lower Bridge showing rhyolite (white), andesite (black),
and banded lapilli.
d) Section exposed on east side of
Deschutes canyon, 3 km northwest of Steelhead Falls (LB =
Lower Bridge ignimbrite member; MC = McKenzie Canyon
ignimbrite member; SF = Steelhead Falls ignimbrite member;
P = Peninsula ignimbrite member).
Note prominent, white,
airfall units beneath SF and LB, channel incised into
McKenzie Canyon ignimbrite, and dark, ledgeforming debris
flow breccia beneath Lower Bridge ignimbrite.
141
2
The ignimbrite is exposed over an area of 100 km
and constituent
pumice lapilli coarsen and thickness increases toward a presumed
Cascade source area southwest of the Deschutes basin (Cannon, 1984).
Stensland (1970) incorrectly stated that this unit thickened
northeastward by erroneously correlating the Lower Bridge ignimbrite to
the more prominently exposed Hollywood ignimbrite in the Crooked River
canyon.
From Lower Bridge to Squaw Creek, the unit is exposed as a
nearly continuous sheet in the Deschutes canyon except in the area
around Steelhead Falls.
The absence of the ignimbrite at Steelhead
Falls probably reflects diversion of the pyroclastic flow by the John
Day (?) dacitic highland near McKenzie Canyon resulting in a "shadow"
region of nondeposition on its northeast side (Fig. 6.10).
North of
the mouth of Squaw Creek, the Lower Bridge ignimbrite occurs as
isolated erosional remnants.
The lower portion of the ignimbrite also
crops out in the Crooked River canyon in discontinuous exposures up to
3 m thick.
From Lower Bridge to Squaw Creek the base of the cogenetic air
fall deposit is virtually planar and nearly always overlies a tan,
sandy paleosol which, in turn, overlies
deposits (Fig. 6.11d).
1
to 3 m of debrisflow
Thus, in the southern portion of the basin the
ignimbrite appears to have been emplaced on a nearly flat plain above
an extraordinarily widespread sheetlike sequence of debris flow
deposits which had previously smoothed over erosional irregularities.
COVE IGNIMBRITE MEMBER
Many ignimbrites are exposed within the CovePalisades State Park,
the most conspicuous of which is a white rhyodacitic unit that forms
142
the prow of The Ship, a prominent landmark in the park, and is also
exposed in roadcuts on the east and westside entrance roads
6.12a).
(rig.
Though not widespread, the Cove ignimbrite member is important
stratigraphically because its position is nearly equivalent to, but
slightly below, the McKenzie Canyon ignimbrite, a widespread marker
farther south.
The two units are never seen in the same vertical
section but can be traced within 100 m of each other where both are at
the same elevation.
Clasts of the McKenzie Canyon ignimbrite are
common in sediments immediately overlying the Cove ignimbrite but are
never found below it.
The Cove ignimbrite member thus provides a
stratigraphic tie between the central and southern Deschutes basin.
The Cove ignimbrite is unwelded, white in color, and contains
scattered white to lightgray pumice lapilli up to 2 cm across.
Both
the lapilli and the ignimbrite matrix contain abundant plagioclase
crystals.
A discontinuous, finesdepleted layer, 2-6 cm thick,
composed primarily of rounded pumice lapilli 4-8 mm across occurs at
the base of the unit and may represent deposition from the fluidized
head of the pyroclastic flow.
Below this layer is a continuous layer
of plane bedded ash and accretionary lapilli 0.5 to 1.0 m thick which
represents cogenetic airfall pyroclastic material.
The Cove ignimbrite has a limited distribution and only extends
over an 8 km length of the Deschutes Canyon. In exposures on The Ship
and in the Deschutes and Crooked River canyon walls the ignimbrite
overlies conglomerate in a paleochannel which trends N70 E suggesting
derivation from a source almost due west of the park.
143
MCKENZIE CANYON IGNIMBRITE MEMBER
The most widespread ignimbrite exposed in the Deschutes Formation
is a conspicuous red to orange unit that crops out from Lower Bridge to
This unit
the vicinity of the CovePalisades State Park (Fig. 6.13).
is "ashflow tuff two" of Stensland (1970) and the McKenzie Canyon tuff
of Cannon (1984).
north and east.
The ignimbrite is up to 15 m thick and thins to the
In its type area, along McKenzie and Deep Canyons, the
ignimbrite is composed of at least 5 flow units, ranges in color from
white at the base to brickred at the top, and forms a single, firmly
Degree
welded cooling unit with crude columnar jointing (Fig. 6.11a).
of welding and number of flow units decrease to the north and east and,
in conjunction with thickness and lapillisize variation, indicate a
source to the southwest (Cannon, 1980. The most diagnostic features of
this unit are its orange and red colors and prominence of white, black,
and banded (black and white) pumice lapilli.
Petrologic study by Cannon (1984) showed that the McKenzie Canyon
ignimbrite is the product of an eruption involving comingling of two
compositionally distinct magmas (Fig. 6.11c, Table 6.5).
White pumice
lapilli are rhyolitic in composition and contain phenocrysts of augite,
hypersthene, and plagioclase.
Black pumice lapilli are andesitic with
phenocrysts of augite, hypersthene, olivine, and plagioclase.
Oxidation of the andesitic component probably accounts for the red and
orange colors in the ignimbrite.
dominated by white pumice.
The lower three flow units are
Black pumice and banded pumice,
representing incomplete mixing of the rhyolitic and andesitic magmas,
become more abundant upward.
The vertical compositional variation in
144
the ignimbrite also accounts for the transition from a white color at
.
the base to orange or red at the top.
In its northernmost exposures
the McKenzie Canyon is represented by a single unwelded, dominantly
andesitic, flow unit.
Like the Lower Bridge ignimbrite, the McKenzie Canyon ignimbrite
is almost continuously exposed in the Deschutes canyon from Lower
Bridge to Squaw Creek but is limited to scattered exposures farther
north and east.
The McKenzie Canyon ignimbrite is also missing at the
Steelhead Falls section as a result of diversion around the John Day
(?) dacitic highland (Fig. 6.13).
Exposures on the southwest side of
the high show that the pyroclastic flow ramped up the stoss side of the
highland to an elevation
35 m above its adjacent depositional surface
but did not completely surmount the barrier.
From Lower Bridge to
McKenzie Canyon the ignimbrite has a nearly planar lower surface and
either lies directly on the Lower Bridge ignimbrite or is separated
from the lower unit by 1 to 2 m of tephra and a sandy paleosol (Fig.
6.11a).
Farther north and east the two ignimbrites are separated by as
much as 10 m of coarsegrained sediment and the upper part of the Lower
Bridge ignimbrite is missing (Fig. 6.11d).
Deep channels were eroded into the McKenzie Canyon and Lower
Bridge ignimbrites and are well exposed at several localities.
A
channel 10 m deep, incised through the McKenzie Canyon and into the
Lower Bridge ignimbrite, can be traced 2 km northeastward from the
Lower Bridge diatomite mine and is partly filled with an unnamed,
white, dacitic ignimbrite with ubiquitous molds of logs, plant stems,
and rare leaves.
In exposures on the east side of the Deschutes River
145
_,.... r_
4k
..PS,
'
1
-5.:::
'
.
-..----,..e.
. 44.3 .
`Y.;
4.... :
-tr. Er
:,,....
":10WFT, L-
,::
.,
-
' qs- :'7,1::11'''
...' -V.'
.
4*--'''
...;
Ai
.
:
74 ;,-,
: ...."'
u,
4.*
4. .
"flig.""-'
:14N0141;"'
L
-
PAN--
'7
JB
-
r.
r
1
\V
,
j
V
'_;1)
-;1(':'
/
J
t
Nk
/
IV.
Fig. 6.12. Outcrop views of Deschutes Formation marker units
a) Cove (C) and Jackson Buttes (JB) ignimbrite members forming The Ship, CovePalisades State Park. Black Butte (left)
and Squawback Ridge (right) on the skyline.
b) Pinnacles of
Balanced Rock ignimbrite member capped by resistant slabs of
Fly Creek ignimbrite member at The Balanced Rocks. View to
the west with the north end of Green Ridge and 011alie Butte
on the skyline.
c) Tenino ignimbrite member and overlying,
unnamed, white ignimbrite near the mouth of South Fork
Seekseequa Creek.
Arrow points to ashcloudsurge deposit
between pyroclastic flow units.
d) Welded Steelhead Falls
ignimbrite member in the Deschutes canyon opposite the mouth
of Squaw Creek.
146
McKENZIE CANYON
IGNIMBRITE MEMBER
Fig. 6.13. Distribution of McKenzie Canyon ignimbrite member within the
Pyroclastic flows probably erupted in High
Deschutes basin.
Cascades, at least 20 km southwest of lower map margin.
Based on mapping by Cannon (1984).
147
at the mouth of Squaw Creek, both McKenzie Canyon and Lower Bridge
ignimbrites are truncated by a channel at least 30 m deep and partly
filled with welded Steelhead Falls ignimbrite member.
A 60 mdeep
channel was eroded through the Lower Bridge and McKenzie Canyon
The
ignimbrites in the vicinity of Alder Springs on Squaw Creek.
channel was filled with sediment and a normal polarity, dacitic
ignimbrite prior to eruption of the Peninsula ignimbrite member.
BALANCED ROCKS IGNIMBRITE MEMBER
A distinctive light to darkgray, unwelded ignimbrite is a
prominent unit in the Street CreekFly Creek area of the central
Deschutes basin (Fig. 6.14).
This ignimbrite was included in Hewitt's
In
(1970) "unit 5" and was named the Hoodoos tuff by Dill (1985).
several localities,-most notably above the Metolius River east of the
mouth of Spring Creek, this ignimbrite has weathered into pinnacles
several meters high that are capped by resistant slabs of the
overlying, welded, Fly Creek ignimbrite member (Fig. 6.12b).
A
particularly picturesque locality exhibiting this differential
weathering was named the "Balanced Rocks" by Brogan (1973) and herein
serves as the type locality for the member.
Detailed descriptions are
provided by Conrey (1985) and Dill (1985).
The Balanced Rocks ignimbrite is composed of 2 or 3 flow units in
most exposures and becomes increasingly darker in color with height
above the base.
Lightgray rhyodacitic pumice lapilli are prominent
throughout the unit but become subordinate to black andesitic lapilli
and bombs, to 30 cm across, in the upper third.
Banded, lightgray and
black, lapilli and bombs are common and represent incomplete mixing of
148
these two components.
The thickness of the Balanced Rocks ignimbrite
increases westward from 10 m, near the mouth of Big Canyon, to 45 m in
its westernmost exposures (Dill, 1985).
Xenoliths of a variety of rock
types, including granulitegrade metamorphics, occur near the base of
the ignimbrite, especially near Fly Creek Ranch (Conrey, 1985).
FLY CREEK IGNIMBRITE MEMBER
A widespread, often welded, ignimbrite in the central Deschutes
basin is named for exposures along Fly Creek (Fig. 6.15).
This unit is
part of Hewitt's (1970) "unit 5" and the Fly Creek tuff of Dill (1985).
West of Fly Creek, the ignimbrite lies 1
Rocks ignimbrite member.
to 4 m above the Balanced
Detailed descriptions of this unit can be
found in Dill (1985) and Conrey (1985).
2
The Fly Creek lgnimbrite is exposed over a 175 km
area from the
Deschutes River, westward to the confluence of Six Creek and Fly Creek.
The ignimbrite is 10 m thick and unwelded along the Deschutes River and
thickens to as much as 50 m thick in the Fly Creek area where the basal
portion is welded.
(Dill, 1985).
Welding increases westward to produce a vitrophyre
In most exposures the Fly Creek ignimbrite is lightgray
to lightorange with lightgray or orange pumice lapilli and bombs.
Intensity of orange coloration increases upward in the unit and
probably represents fumarolic oxidation.
Where preserved, the upper 5
to 10 m of the ignimbrite in the western portion of its outcrop area is
mediumgray in color with white, black, and banded (white and black)
pumice lapilli and bombs to 30 cm across.
White lapilli and bombs are
rhyodacitic in composition and contain plagioclase, augite, and
hypersthene.
Black pumice lapilli are basalt and basaltic andesite and
149
Fig. 6.14. Distribution of Balanced Rocks ignimbrite member within the
Pyroclastic flow probably erupted in High
Deschutes basin.
Based on mapping by
Cascades, 20 km west of the map margin.
Conrey (1985) and Dill (1985).
150
Fig. 6.15. Distribution of Fly Creek ignimbrite member within the
Pyroclastic flow probably erupted in High
Deschutes basin.
Based on mapping by
Cascades, 20 km west of map margin.
Conrey (1985) and Dill (1985).
151
contain phenocrysts of plagioclase, augite, hypersthene, and
hornblende.
TENINO IGNIMBRITE MEMBER
Two thick darkgray, dacitic ignimbrites are exposed in roadcuts
along Tenino Creek and at least one of these units is widespread over
the southern part of the Warm Springs Indian Reservation (Fig. 6.15).
The ignimbrites are indistinguishable with regard to mineralogy or
major element composition (Table 6.4) and are, thus, grouped together
in this member.
Along Tenino Creek the lower unit is 15 to 60 m thick
and contains a lower, platyjointed, welded zone where it is thickest.
The upper unit is 20 to 25 m thick and exhibits no welding.
Both
ignimbrites contain multiple flow units and are bright pink or orange
at the top as a result of fumarolic oxidation.
Along Tenino Creek at
the type locality the two ignimbrites are separated by 15 m of coarse
sandstone, lapillistone, and a paleosol developed on top of the lower
unit.
The lower ignimbrite pinches out rapidly to the east suggesting
that it was largely confined to a channel with an orientation different
from the present canyons to which exposure at this stratigraphic level
is limited.
Southward, in the Seekseequa Creek drainage, only one
cooling unit, up to 30 m thick, occurs.
The elevation of this
ignimbrite suggests that this is the lower unit exposed in Tenino Creek
but evidence of paleorelief on the order of 40 to 60 m
A prominent exposure along the
correlation on this basis very tenuous.
South Fork of Seekseequa Creek (S.
22,
(Plate I) makes
T.
10 S.,
R.
11
E.) includes an
ashcloud surge deposit (see Chapter 7) preserved between two flow
units near the top of the cooling unit (Fig. 6.12c).
152
Black dacitic pumice lapilli and bombs up to 25 cm in diameter are
prominent throughout these ignimbrites.
White lapilli up to 2 cm across
are present in most exposures but are rare and have not been analyzed.
No banded pumice lapilli have been seen.
COYOTE BUTTE IGNIMBRITE MEMBER
A relatively thin (2 to 10 m) ignimbrite forms a conspicuous
marker over most of the southeastern corner of the Warm Springs Indian
Reservation (Fig. 6.17).
It is named for prominent exposures along the
south flank of Coyote Butte and is frequently exposed above the Tenino
ignimbrite member.
This ignimbrite is white to light gray in color and contains about
20% small white dacite pumice lapilli up to 3 cm across in a matrix of
glass shards, crystals, and ubiquitous angular fragments of black,
dacitic vitrophyre.
The vitrophyre fragments are also prominent
constituents in debrisflow and flood deposits in the northern
Deschutes basin and may be derived from the same source (see Chapter
8).
In most exposures the Coyote Butte ignimbrite overlies 0.5 to 1.0
m of airfall tuff with scattered small pumice lapilli, accretionary
lapilli, and angular, gray andesitic (?) accidental lithic fragments.
About 0.5 km south of Coyote Butte the airfall deposit is overlain by
2 m of rounded pumice lapilli supported in an ash matrix which may
represent a pumicerich levee fades at the margin of the ignimbrite.
The base of the ignimbrite is commonly composed of a 0.5 mthick zone
of faintly stratified, crystalrich ash that probably represents a
groundsurge deposit.
In several outcrops along Tenino Creek the
Coyote Butte ignimbrite is composed of two flow units separated by 1 m
153
TENINO IGNIMBRITE MEMBER
OUTCROP
Warm
Springs
PROBABLE
EXTENT
TEN/NO ORE E/(
PS8509-138
Fig. 6.16. Distribution of Tenino ignimbrite member within the Deschutes
Pyroclastic flows probably erupted in the High
basin.
Based on
Cascades, 15 km westsouthwest of map margin.
mapping by the author (Plate I).
154
COYOTE BUTTE IGNIMBRITE MEMBER
III OUTCROP
PROBABLE EXTENT
Warm
Springs 0
-N-
5
KILOMETERS
PS8505-188
Fig. 6.17. Distribution of Coyote Butte ignimbrite member within the
Pyroclastic flow probably erupted in High
Deschutes basin.
Based on mapping by the
Cascades, 15 km west of map margin.
author (Plate I).
155
of accretionary lapilli.
Along Tenino Creek the base of the Coyote Butte ignimbrite member
is at 2710' but just 5 km to the south the base is at 2600'.
Because
paleocurrent indicators in associated sediments indicate an eastward
slope, and no faults are apparent, this elevation difference must
represent the paleorelief in the area.
STEELHEAD FALLS IGNIMBRITE MEMBER
A pink, unwelded, rhyodacitic ignimbrite is prominent on the east
wall of the Deschutes canyon in the vicinity of Steelhead Falls where
it is about 6 m thick.
This ignimbrite was discussed by Stensland
(1970) as "ashflow tuff 3".
The unit is nearly continuous for 2.5 km
north of Steelhead Falls where it is truncated by a channel that is,
in
The ignimbrite also occurs
turn, filled with a basaltic andesite flow.
in an isolated exposure near the confluence of Squaw Creek and the
Deschutes River where it partly fills a channel incised through the
McKenzie Canyon and Lower Bridge ignimbrites.
thick and is welded in the center (Fig. 6.12d).
Here it is 20 to 25 m
South of Steelhead
Falls the ignimbrite extends only to the north flank of the dacite
inlier north of McKenzie Canyon.
The Steelhead Falls ignimbrite is
also exposed in the Crooked River Canyon.
In all outcrops the
ignimbrite overlies a cogenetic, white, pumice lapillistone up to 1.5 m
thick.
PENINSULA IGNIMBRITE MEMBER
A widespread, though discontinuously exposed, ignimbrite crops out
in the canyons of Squaw Creek, Deschutes River, and Crooked River from
the latitude of Steelhead Falls to the CovePalisades State Park (Fig.
156
PENINSULA
IGNIMBRITE MEMBER
OUTCROP
PROBABLE EXTENT
KILOMETERS
COVE-PALISADES
STATE PARK
JUNIPER
BUTTE X
PS8505-204
Fig. 6.18. Distribution of the Peninsula ignimbrite member within the
Deschutes basin.
Pyroclastic flow was probably erupted in
Based on
the High Cascades, 20 km west of map margin.
mapping by Stensland (1970, and unpub. map), Dill (1985),
and the author.
157
V.
a)
Fig. 6.19. Outcrop views of Deschutes Formation marker units
Peninsula ignimbrite member above Squaw Creek near Alder
Note inverse grading of black pumice lapilli and
Springs.
b) Large, black, andesitic bombs typical of
bombs.
proximal exposures of the Six Creek ignimbrite member.
158
6.18).
The ignimbrite is named for prominent exposures along The
Peninsula, the mesa separating the Deschutes and Crooked Rivers. The
unit is light brown to brownish gray in color and contains black pumice
lapilli and bombs 2 to 15 cm across (Fig. 6.19a), gray pumice lapilli 2
to 5 cm across, and altered, white pumice lapilli 0.5 to 2 cm in
diameter.
Banded, black and light gray, lapilli are uncommon but
ubiquitous components of this ignimbrite.
White lapilli are rhyolite
and gray and black lapilli are mostly dacite (Table 6.5).
Some of the
large black bombs are notably aphyric and are andesitic in composition.
Thickness varies from 2 to 12 m with erosional contacts at base and
top.
The pyroclastic flow traveled through a series of parallel,
northeasttrending channels and is confined north of the dacite inlier
near McKenzie Canyon.
Because of its channelfilling nature, the base
of the member varies in elevation by as much as 30 m in less than 2 km.
A discontinuous finesdepleted layer, a few centimeters to 1.5 m
thick, occurs locally at the base of the ignimbrite.
This layer
contains rounded juvenile lapilli, lithic fragments, and sediment
ripups up to 8 cm across and features rare flame structures extending
into the overlying matrixsupport ignimbrite (Fig. 7.14b).
Some
lapilli exhibit breadcrusted surfaces indicative of in situ cooling
from high temperature.
This layer was produced by a turbulent
pyroclastic surge which preceded the pyroclastic flow.
DEEP CANYON IGNIMBRITE MEMBER
Three Deschutes Formation ignimbrites are exposed above the
McKenzie Canyon ignimbrite in Deep Canyon (Stensland, 1970).
The
159
second of these, here named the Deep Canyon ignimbrite member, is.
nearly 20 m 'thick and is prominently exposed along Oregon State Route
126 with its base about 2 m above the bridge at the bottom of the
grade.
The ignimbrite is dacitic in composition and varies in
thickness from 5 m to 30 m.
Exposure is limited to a narrow belt
Welding is
between Buckhorn Canyon and Fremont Canyon (Fig. 6.20).
characteristic of the basal portion of the unit in its southwesternmost exposures suggesting that the pyroclastic flow entered the basin
from that direction.
Pumice lapilli, up to 8 cm across, are light
brown to black in color with lightercolored lapilli exhibiting a
greater degree of vesiculation.
In welded zones, the lapilli are col-
lapsed into black vitrophyre fragments.
abundant upward in the unit.
Pumice lapilli become more
Overall color of the ignimbrite is light
brown, where unwelded, and brownishgray, weathered to yellow, where
welded.
SIX CREEK IGNIMBRITE MEMBER
Only one ignimbrite has been successfully correlated from the west
face of Green Ridge into the Deschutes basin (Fig. 6.21).
named for prominent exposures along Six Creek,
This unit,
is the youngest
Deschutes ignimbrite recognized on the west face of Green Ridge and
only one, thin ignimbrite with restricted distribution is known to
overly the Six Creek ignimbrite farther east (Dill, 1985).
This member
was described in detail by Conrey (1985) as the Six Creek tuff.
The Six Creek ignimbrite member is brown in color and contains
gray, crystalpoor rhyodacitic pumice
pumice, and banded pumice.
,
black, aphyric andesitic
Gray pumice occurs as lapilli and bombs up
160
Fig. 6.20. Distribution of Deep Canyon ignimbrite member within the
Pyroclastic flow probably erupted in
Deschutes basin.
High Cascades, at least 20 km southwest of lower map
Based on mapping by Stensland (1970, and unpub.
margin.
map).
'J METQLII,c
SIX CREEK
IGNIMBRITE MEMBER
OUTCROP
0
KILOMETERS
PROBABLE EXTENT
PS8505-207
Pyroclastic
Fig. 6.21. Distribution of Six Creek ignimbrite member.
flow erupted in High Cascades within 10 km of western map
margin. Based on mapping by Conrey (1985) and Dill (1985).
161
qo cm across and black pumice occurs as lapilli and bombs as large
as 1.5 m across (Fig. 6.19b).
Ths ignimbrite contains 2 to 3 flow
units in most localities and exhibits welding only in the basal portion
on the west slope of Green Ridge.
The ignimbrite is approximately 80 m
thick where it occupies an easttrending paleovalley on Green Ridge and
thins to 35 m near Fly Lake.
TETHEROW BUTTE MEMBER
The Tetherow Butte member represents the first eruptive event to
occur within the Deschutes basin in the late Miocene.
The member
consists of the Agency Plains and Crooked River basalt flows, and the
cinder cones of Tetherow Butte, near Terrebonne (Fig. 6.22).
The cinder cones are composed of red and black basaltic cinder and
spatter and were orginally at least 120 m high (Fig. 6.23a).
highest cones form a 5 km long, N35 W trend.
The
Smaller accumulations of
cinder to the north may represent portions of the cones which were
rafted on top of flows extruded from the base of the cones.
The basalt
is fine grained with scattered glomerophenocrysts of plagioclase and
conspicuously zoned green augite in a groundmass of glass and opague
minerals (Fig. 6.23c).
Olivine is rarely observed.
Plagioclase
phenocrysts often exhibit a resorbed core which is more sodic than the
rim.
The majorelement composition is notable for its high TiO
and
2
FeO contents and low Al 0
and MgO which makes it distinct from other
23
Deschutes Formation basalts (Table 6.4).
The cinders are overlain by
younger tephras and paleosols with a combined maximum thickness of 20
m.
Most of this overlying volcaniclastic material has been removed by
erosion, along with some of the cinder, except where subsequently
162
Fig. 6.22. Distribution of lava flows and pyroclastics of the Tetherow
Butte and Round Butte members. Contacts are locally obscured
Based on mapping by Jay (1982),
by sedimentary lithologies.
Hayman (1983), and the author.
163
South ç1'
Junction
iI
5
0 Gateway
Madras
.;
COVE-PALISADES
STATE PARK
ROUND BUTTE
MEMBER:
I CINDER AND SPATTER
LAVA FLOWS
0
SOURCE VENTS
TETHEROW BUTTE
MEMBER
III CINDER AND SPATTER
LAVA FLOWS
0
SOURCE VENTS
A
ROOTLESS VENTS
FLOW FRONT OF
CROOKED RIVER
FLOW
0
5
KILOMETERS
en
Terrebonne
-
Fig. 6.22
P58505-196
164
-r
.......,
41).
_44,4
.743.
'47:%V
...N10..17
11;
il,. 774.7.,:e
41'
5
16-
ck-
°,17.41.
N
...46.7.1*
.4
I*.
cd;.,.1,
t-1,13
VI'
!I I
!ft.'
°
,
s' V;;; 4
°
"
;7141,.
t
.J.*"
t
'1
ni
; -
,
41'
.
.'er4
-
7
..14,t-
iktpc..V 4N
,
.
-
-eitti'
INA
2:t.41.'r;"
.
t-.
.-145L
Fig. 6.23. Photographs of Tetherow Butte member.
a) Tetherow Butte
cindercone complex from the north.
Cultivated fields and
pastures in foreground are developed on a veneer of sediment
which obscures the lava flows erupted from these vents.
b)
Agency Plains and Crooked River basalt flows along east side
of Crooked River canyon near Crooked River Ranch.
Arrow
points to discontinuous break in coolingjoint pattern
marking contact between the two flows. c) Photomicrograph
'(plane light) of Crooked River flow showing plagioclase and
augite glomerophenocrysts and abundant opague irontitanium
oxides.
165
overlain by the Pliocene basalt on which Terrebonne is constructed.
Robinson and Stensland (1979) mapped a lava flow associated with
Tetherow Butte extending northward from the cinder cones.
However,
this basalt is only inferred since at least 1 to 2 m of younger
sediments and modern soil obscure it from exposure (Fig. 6.23a).
The Agency Plains basalt flow is a widespread unit which forms the
rimrock over most of the region from just south of the CovePalisades
State Park northward to South Junction (Fig. 6.22).
North of The Cove,
this unit has been previously mapped as the "rimforming basalt lava"
by Jay (1982) and the Agency Plains basalt by Hayman (1983).
The flow
varies in thickness from 2 m to more than 50 m where it filled, and
overflowed, an ancestral Deschutes River channel.
North of the Cove
the Deschutes River was relocated to its present position along the
-west edge of the flow.
The composition and petrographic features of
the Agency Plains basalt are indistinguishable from the spatter and
cinders at Tetherow Butte except for a higher degree of crystallinity
in the groundmass.
Jay (1982) proposed that a small accumulation of
cinder northwest of Madras, about 250 m in diameter and 15 m high, was
a source for the Agency Plains basalt.
However, this is unlikely
because most of the basalt lies at higher elevations than the proposed
vent.
This small cinder accumulation is situated above the previous
course of the Deschutes River, as indicated by the thickness variation
in the basalt, and probably is a rootless vent produced by escaping
steam blasting through the basalt.
The Agency Plains basalt is
overlain by up to 50 m of sediment, mainly sandy paleosols, which has
been largely stripped off by erosion except where preserved beneath
166
basalts from Round Butte.
South of the Cove, on both sides of the Crooked River, Agency
Plains basalt is overlain by another basalt flow with a 5 to 10 m high
flow front near Opal Springs (Fig. 6.22).
This flow is also composi-
tionally and petrographically identical to Tetherow Butte ejecta (Fig.
6.23c).
The two flows can be traced southward as separate units for
several kilometers until the contact becomes obscure.
On the east side
of the Crooked River, across from Crooked River Ranch, the two flows
have a combined thickness of 70 m and in most places cooled together to
form a single thick entablature with thin upper and lower colonnades.
The contact between the two flows can be detected by an occassional
cooling break in the middle of the thick basalt sequence (Fig. 6.23b)
or by a prominent vesicular zone
with flow breccia that,
in places,
has led to the development of an erosional bench in the cliffforming
basalt.
The upper flow is herein named the Crooked River basalt flow.
Robinson and Stensland (1979) informally used this name to refer to the
entire cooling unit which is recognized here as including, in its lower
half, the Agency Plains basalt.
More than a dozen low mounds of spatter occur on top of the
Crooked River flow from Ogden State Park to Juniper Butte.
Similarity
in dimension, shape, and structure to the rootless vent near Madras
suggests that these accumulations of spatter are of the same origin
(Fig. 6.22).
In further support of this conclusion, the spatter mounds
are roughly aligned on northwest and northeast trends which parallel
the modern Crooked River and western tributary orientations,
respectively.
Paleogeographic considerations (see Chapter 8) show
167
that, at this time, a single northflowing river occurred in the basin,
just east of the present Crooked River, and was fed by northeast
flowing tributaries.
Thus, the orientation of the spatter mounds may
represent the position of drainage buried by the lavas.
The cinder cones at Tetherow Butte are believed to be the source
for both Agency Plains and Crooked River basalts with the first flow
being more extensive, but the second being erupted soon enough after
the first to allow both to cool as a single cooling unit close to the
source.
An
40
39
Ar/ Ar age of 5.5 + 0.2 Ma has been determined by L. W.
Snee (person*. commun., 1985; Appendix IX) for the Agency Plains flow.
LOWER DESERT BASALT MEMBER
At least two flows of normal polarity diktytaxitic olivine basalt
form the rimrock above the Metolius and Deschutes River canyons from
Fly Creek eastward to Canadian Bench and southward to the Geneva
townsite (Fig. 6.24).
These compositionally similar flows, celled the
Canadian Bench and Fly Lake basalts by Dill (1985) and upper Canadian
Bench and Fly Lake basalts by Conrey (1985), are the youngest volcanic
units which can be correlated from the central Deschutes basin to Green
Ridge.
Both flows have normal magnetic polarity and form the basalt
rimrock between Fly Creek and the Deschutes canyon.
is younger and slightly less extensive in area.
The Fly Lake flow
Armstrong and others
(1975) reported a date of 5.0+/-0.5 Ma (recalculated by Fiebelkorn and
others, 1983) for the Canadian Bench flow at The Cove.
On the Peninsula and on the east side of the Crooked River above
the marina at CovePalisades State Park the Canadian bench flow overlies
the Agency Plains basalt flow of the Tetherow Butte member.
Near
168
COVEPALISADES
STATE PARK
SQUAWBACK
" RIDGE
SQUAWBACK
RIDGE
LAVAS
UTILE
X sQUAWBACK
LOWER DESERT BASALT MEMBER:
5
CANADIAN BENCH FLOW
I.
:1
FLY LAKE FLOW
KILOMETERS
PS8505-208
Fig. 6.24. Distribution of Lower Desert basalt member.
Both the Fly
Lake and Canadian Bench flows were probably erupted just
west of the Green Ridge fault zone.
Based on mapping by
Conrey (1985), Dill (1985), and the author.
169
Geneva the Lower Desert flows are overlain by younger Deschutes
Formation lavas.
Conrey (1985) correlated these flows to diktytaxitic basalts on
the east flank of Green Ridge.
Discontinuity in outcrop between these
exposures and those on the Lower desert is a result of burial of the
Lower Desert member by Pliocene basaltic andesite from Squaw Back Ridge
and erosion of the proximal exposures west of Fly Creek.
On the east
flank of Green Ridge the Lower Desert member is overlain by younger
Deschutes Formation basaltic andesites.
The basalt flows were probably
erupted from vents in the High Cascades near Green Ridge.
The composition and textural features of these basalts are similar
to other basalts in the Deschutes section in the vicinity of the Cove
Palisades State Park.
Stratigraphic position is the only means of
distinguishing these flows.
However, because of the widespread
distribution of the Canadian Bench and Fly Lake basalts and the
importance of their correlation to Green Ridge, assignment of member
status on an informal basis is appropriate.
STEAMBOAT ROCK MEMBER
The Steamboat Rock member is a lithologically diverse unit of lava
flows, lapillituff, breccia, and dikes of basaltic andesite composition, produced during a fissure eruption within the Deschutes basin
(Fig. 6.25).
The name is derived from Steamboat Rock, the site of a
basaltic andesite dike, located about 2 km east of Lower Bridge.
The
products of the fissure eruption extend for 15 km on two enechelon
N15 W trends along the east side of the Deschutes River, centered
around Steelhead Falls.
170
South of Steelhead Falls the member is composed of a thin sheet of
lava with several exposures of feeder dikes.
Steamboat Rock, on the south end
The dikes are exposed at
of the mesa 1 km north of Steamboat
Rock, and along the Deschutes River 2 km southeast of Steamboat Rock.
The eroded remains of a north-northwest trending spatter rampart and a
low shield cone, each about 10 m high, were the sites of extrusion of
basaltic andesites that cap the mesa north of Steamboat Rock, and a
40
sample collected here has yielded an
39
Ar/
Ar age of 5.1 + 0.2 Ma (L.
W. Snee person. commun., 1985; Appendix IX).
Another eroded spatter
rampart is present south of Steamboat Rock.
Lava flowed northward
into the channel of the ancestral Deschutes River which was located
along the western margin of the Crooked River flow of the Tetherow
Butte member.
North of Steelhead Falls, the Steamboat Rock member is comprised
largely of sideromelane lapilli-tuff.
Phreatomagmatic eruptions
produced two tuff cones which were originally at least 60 m high.
Slightly palagonitized, cinder-rich sideromelane lapilli-tuff, with
surge bedding features, is well exposed in the east wall of the
Deschutes canyon, downstream from Steelhead Falls (Fig. 6.26).
The
tuff is underlain by 1 to 3 m of pyroclastic breccia composed of
accidental blocks up to 3 m across and juvenile cauliflower bombs up to
50 cm across.
The lapill-tuff is overlain by spatter and fusiform and
ribbon bombs and lastly by 1 to 2 m thick scoriaceous, flow-banded
basaltic andesite which dips radially inward toward the center of the
tuff cones.
The texture and attitude of this lava suggests that it was
erupted by fire fountains in the center of the tuff cones and
171
STEAMBOAT ROCK
MEMBER
-
LAVA FLOW
FT:7 TUFF, TUFF BRECCIA
DIKE
d
VENT, LOW SHIELD
EXHUMED CONDUIT
FILLED WITH TUFF
SPATTER RAMPART
T TOPOGRAPHIC RIM
<9
T 4,9
Op
STEELHEAD
FALLS
so
o\
\ "it_
<1"
A
\
STEAMBOAT
ROCK
LOWER
BRIDGE
0
1
2
KILOMETERS
PS8505-195
Fig. 6.25. Distribution of dikes, lava flows, and pyroclastics of the
Steamboat Rock member. Based on mapping by the author.
172
r.
a
rt."'
1"
,
4.
ote '
1
ult.
f6t".--74"
P
'Mawr
AAV4,-
t
19Erzaw.zkall
'
4
I
P".."
,
4,1Z4
Fig. 6.26. Exposure of Steamboat Rock member pyroclastics 1.5 km north
of Steelhead Falls. Bedded, surgedeposited lapillituff
overlies coarse breccia (b) which, in turn, lies upon a
massive paleosol (p). Massive tuff to right of figure is
infilling of a small, cylindrical conduit.
Note downward
drag of units along left side of conduit.
(0
L
-
-
'
_
4'''
""=---',--7..'"
*!1-*
.
f1,
-...
%N.
,
1
m-,,lk
1
u
_
Fig. 6.27. Round Butte from CovePalisades State Park. Note cinder
cones at summit and on northwest (left) flank.
173
accumulated on the crater walls.
The eruption sequence was completed
with the construction of three, low, lava shields, two within the tuff
cones and one just to the north.
The eruption stratigraphy suggests that initial, strong,
phreatomagmatic explosions created the basal tuffbreccia, followed by
less violent eruption of base surges and tephra plumes.
The influence
of water on the eruptive character diminished with time and Strombolian
activity produced bombs.
As explosiveness further diminished, fire
fountains developed within the craters and were succeeded by quiet
effusion of lava to produce the shields within the craters.
Contemporaneity of activity at vents north and south of Steelhead Falls
is demonstrated by the occurrence of bombs from the northern vents
lying upon the basalt flow from the southern vents which,
in turn,
overlies the lapillituff.
Although the tuff cones were constructed along what may have been
the course of the Deschutes River (Fig. 6.25), surface water was not
responsible for the phreatomagmatic explosions.
The basal breccia is
dominated by clasts of basaltic lithologies not exposed in the
Deschutes canyon walls, suggesting that explosions originated at least
150 m below the paleosurface.
Furthermore, exhumed conduits (Fig.
6.26) filled with lapillituff form a field of pinnacles, about 1 km
north of Steelhead Falls, and can be traced downward tens of meters
into the Deschutes Formation stratigraphy. These conduits are southwest
of the trend of the tuff cones but are along the extension of the trend
defined by dikes and spatter ramparts south of Steelhead Falls.
174
ROUND BUTTE MEMBER
40
The youngest Deschutes Formation basalts (
39
Ar/
0.1 Ma; Appendix IX) were erupted from Round Butte, west
(Figs. 6.22, 6.27).
of 4.0 +
Ar age
of
Madras
Round Butte is a cinder-spatter cone surmounting
broad shield volcano over 6 km in diameter.
a
A smaller cinder cone
occurs low on the northwest slope suggesting that eruptions occurred on
a N 30 W-trending fissure (Jay, 1982).
The basalt has reverse magnetic
polarity and is dark gray in color with phenocrysts
olivine up to 1.5 mm across.
of
plagioclase and
Angular intercrystalline vesicles are
locally prominent but the basalt, in general, lacks a diktytaxitic
texture.
Along Belmont Lane, on the west side
of
Dry Canyon, a single
flow
of basalt rests on a 50 m-thick volcaniciastic section overlying the
Agency Plains basalt flow.
of
The base
the Round Butte basalt here is
at 2530 ft.
West of Round Butte, along the access road to Round Butte Dam, the
base of the basalt is at 2360 ft. and four flows are present.
flows are separated by thin interbeds
of
pinch out in less than 100 m to the east.
The
crossbedded sandstone which
The lowest flow is invasive
into fluvial sandstones and features chilled margins, pillow-like
structures along the flow top, and associated peperite.
elevation of the base of the basalt, greater number
The lower
of flows,
intercalated sediments, and invasive contacts, suggest that the Round
Butte basalts flowed into the Deschutes River channel at this locality.
The basalts can be traced northward where they overly the Agency
Plains basalt flow.
The southward extent
of
the Round Butte basalt
175
member is obscured by windblown sediments.
RATTLESNAKE IGNIMBRITE MEMBER
A distinctive rhyolitic ignimbrite is exposed in two areas along
the eastern margin of the Deschutes basin (Fig. 6.28). The major
element composition of pumice lapilli and the composition of alkali
feldspar (anorthoclase), and Ferich clinopyroxene (ferrohedenbergite)
are unlike known ignimbrites of Cascade provenance or those of the
Clarno and John Day formations.
These compositions are, however,
identical to those of the Rattlesnake ignimbrite in the John Day valley
(Enlows, 1976).
North of Grizzly Moutain, Thormahlen (1984) mapped
three outcrops of this-ignimbrite where it overlies rocks of the John
Day and Clarno Formation.
South of Prineville,
in Swartz Canyon, the
ignimbrite is interstratified with sedimentary rocks characteristic of
the Deschutes Formation.
A 6.5 Ma age for the Rattlesnake ignimbrite,
where dated farther east (Enlows, 1976),
is consistent with its
occurrence within the Deschutes Formation.
The unit is recognized by its light orange color, with or without
a light gray zone at the base, the presence of both light brown and
black glass shards and pumice lapilli, and low phenocryst content
including diagnostic bipyramidal quartz, anorthoclase, oligoclase and
ferrohedenbergite.
At both the Grizzly and Swartz Canyon localities
the ignimbrite is about 7 m thick, but the base is not exposed.
Pumice
lapilli are up to 6 cm across at the Swartz Canyon locality but rarely
exceed 2 cm near Grizzly.
Pumice lapilli exhibit a slight degree of
flattening and lower parts of the unit are slightly welded at Swartz
Canyon but no evidence of welding occurs in Grizzly outcrops.
These
176
John
Daydp
Prineville
ER
Burns
DISTRIBUTION OF
RATTLESNAKE IGNIMBRITE
0
50
HARNEY
BASIN
100
KILOMETERS
PS8505-198
Fig. 6.28. Distribution of Rattlesnake ignimbrite in eastern Oregon.
Note that newly recognized occurrences near Grizzly and in
Swartz Canyon extend the limit of this ignimbrite by almost
100 km beyond previous mapping (modified from H. E. Enlows,
unpub. map).
177
observations indicate that the Grizzly exposures are more distal than
those at Swartz Canyon, and combined with the occurrence of firmly
welded Rattlesnake ignimbrite 20 km south of Swartz Canyon (Lowry,
1944; Smith and others, 1984), suggests that the pyroclastic flow
entered the Deschutes basin from the southsoutheast.
The Rattlesnake ignimbrite was erupted within the Harney basin
(Enlows, 1976; Walker, 1979), 250 km southeast of the Deschutes basin,
2
and crops out over an area in excess of 30,000 km
.
In its original
type locality, in the John Day valley, the ignimbrite is intercalated
with locally derived fanglomerate and collectively called the
Rattlesnake Formation by Merriam (1901) and Enlows (1976).
In the
Harney basin, the ignimbrite is interstratified with volcaniclastic
sediments and other ignimbrites collectively named the Danforth
Formation by Piper and others (1939).
Walker (1979) proposed moving
the type locality of the Rattlesnake ignimbrite to Poison Creek, north
of Burns, so that it would be closer to the source area and thus,
include more of the lithologic variability of the unit.
Walker (1979)
also renamed the unit Rattlesnake AshFlow Tuff and raised it, and
other ignimbrites within the Danforth Formation, to formation rank
while abandoning future use of Danforth and Rattlesnake Formations.
Formation status is inappropriate for the Rattlesnake ignimbrite
outcrops near Grizzly Mountain and in Swartz Canyon because of its
thinness in distal exposures. Therefore, at these exposures, the
Rattlesnake ignimbrite is assigned member status within the Deschutes
Formation.
This assignment does not preclude formation status else-
where (North American Commission on Stratigraphic Nomenclature, 1983)
178
CHAPTER 7: VOLCANIC GEOLOGY OF THE DESCHUTES FORMATION
INTRODUCTION
The lava flows, ignimbrites, and air-fall pyroclastics of the
Deschutes Formation record the nature of magmas and character of
eruptions during the early High Cascade eruptive episode.
Where
previously studied in the central Western Cascades, rocks of this
episode consist mostly of basalt and basaltic andesite lavas (Flaherty,
1981; Priest and others, 1983).
Similar lavas dominate the most
proximal Deschutes Formation exposures on Green Ridge (Conrey, 1985)
but occurrence of widespread ignimbrites in the Deschutes basin, of
andesite to rhyolite composition, records more diverse magmatism during
this episode than is reflected by the Western Cascade studies.
This chapter discusses the general features of early High Cascade
magmatism with emphasis on Deschutes Formation volcanics.
Detailed
consideration of petrogenesis is beyond the scope of this report and
requires trace element and isotopic data not collected during this
study.
Quantitative discussion of petrologic relationships between
units is also inappropriate because Deschutes volcanic rocks represent
almost .4 million years of volcanism along a volcanic front 100 km long
and reflects innumerable volcanic centers and magma batches.
Nonetheless, field relationships, petrographic features, and majorelement analytical data do allow for generalized discussion of early
High Cascade magmatism and its relationship to the extensional
tectonism which culminated in development of the central Oregon High
Cascade graben.
179
DISTRIBUTION OF VOLCANIC ROCKS
The Deschutes Formation section at Green Ridge is composed almost
entirely of volcanic rocks and is dominated by basaltic andesite and
basalt lavas.
Outcrop distribution indicates that most of these lavas
and subordinate ignimbrites fill east and northeasttrending
paleocanyons up to 80 m deep (Hales, 1975; Conrey, 1985; Yogodzinski,
1986).
The proportion of volcanic rocks, relative to sedimentary
units, decreases eastward but ignimbrites increase in abundance.
The
dominance of lavas on Green Ridge reflects the steep gradients of
proximal stream channels which were inappropriate for significant net
sediment deposition and resulted in erosive removal of unconsolidated
pyroclastic debris.
The locus of sedimentation was farther east, near
the center of the basin, where relief was lower.
Pyroclastic flows
spread out as sheets in the center of the basin, enhancing their
preservation potential when compared to their restriction to narrow,
steep canyons farther west.
This tendency for preferential
preservation in more distal exposures may,
in part, account.for the
paucity of ignimbrites among contemporary rocks exposed in the Western
Cascades.
Some basalt and basaltic andesite lavas extend eastward up
to 65 km from the flanks of the modern High Cascades and are prominent
within the Deschutes basin.
Other mafic lavas of the Deschutes
Formation were erupted from sources within and east of the basin.
Volcanic rocks are most prominent in the Deschutes Formation at,
and south of, the latitude of Green Ridge.
All intraformational lavas
on the Warm Springs Indian Reservation, 6 km or more east of the
longitude of Green Ridge, can be traced southwestward across the
180
Metolius River and were not erupted at the latitude of the reservation.
Only about a dozen ignimbrites have been recognized in this northern
area as compared to at least fifty separate units farther south.
These
ignimbrites occur mostly in the southern part of the reservation,
between Metolius and Tenino benches, and many were probably erupted at
the latitude of Green Ridge and entered the basin through a northeast
trending paleocanyon near the modern confluence of the Metolius and
Whitewater rivers (Yogodzinski, 1986).
The distribution and lateral variation in physical characteristics
of Deschutes Formation lava flows and ignimbrites, south of the
reservation, indicates dispersal eastward and northeastward from the
latitude of Green Ridge, and northeastward into the southern Deschutes
basin from near the present site of the Three Sisters and Broken Top.
The soutwesttonortheast dispersal of volcanic units in the southern
Deschutes basin is most notable in the inverted topographic ridges of
basalt and basaltic andesite lavas between Sisters and Redmond and the
grain size and welding variation in several ignimbrite units including
Lower Bridge, McKenzie Canyon, and Peninsula ignimbrite members:
Paleocurrent data reveal a SWNE inclined paleoslope (Chap. 8) and
together with the distribution of volcanic units suggests the presence
of a volcanic highland extending eastward from the High Cascades toward
the site of Bend during Deschutes Formation time (Smith and Taylor,
1983).
Basalt and basaltic andesite lavas are more abundantly exposed in
the Crooked River canyon than in the Deschutes River canyon to the west
(Plate III).
Most of these lava flows were probably erupted south of
181
the Deschutes basin and flowed northward along the ancestral Deschutes
River valley.
In some cases these lavas backed up northeasttrending
tributary valleys for short distances so that exposures of these units
in the Crooked River canyon are channelform with northeastsouthwest
orientation but cannot be correlated southwestward to the Deschutes
River canyon.
It is not possible to make accurate estimates of volumes
represented by various rock types erupted during the early High Cascade
episode because proximal exposures are lost from view within the graben
and only rocks on the east flank of the arc are considered in this
study.
-
It is likely that the Deschutes Formation record gives a biased
view toward bimodal magmatism involving contemporaneous extrusion of
basalt and basaltic andesite lavas with dacitic to rhyolitic
ignimbrites (Fig. 7.1).
Andesitic magma is generally too volatile poor
to produce large pyroclastic flows which might reach the Deschutes
basin and too viscous to generate extensive lava flows.
Andesite domes
and short lava flows may have been important constituents of the early
High Cascade volcanic pile but, because of their general restriction to
proximal positions, have subsided into the High Cascade graben and been
subsequently buried.
Distribution patterns of Deschutes ignimbrites (e.g. figures in
.
Chapter 6) and extent from inferred sources near the modern High
Cascade axis suggest, by comparison to studies of completely exposed
ignimbrites elsewhere (Smith, 1979; Bacon, 1983), that magma volumes on
3
the order of 50-200 km
were erupted.
The thickest and most
extensive, and presumably the most voluminous, ignimbrites occur in the
182
171 IGNIMBRITES
15-
1111 LAVA FLOWS
10
0
5-
J
50
55
60
65
SiO2
Fig. 7.1
S102 histogram of Deschutes Formation volcanics.
Data
compiled from Conrey (1985), Dill (1985), Yogodzinski (1985)
and this study (Appendix I).
183
lower half of the Deschutes Formation section (e.g. Chinook, Jackson
Buttes, Lower Bridge, McKenzie Canyon, and Fly Creek ignimbrite
members).
Ignimbrites are notably missing from the upper 165 m of the Green
Ridge section which is dominated by basalt and basaltic andesite lavas
(Conrey, 1985).
The Six Creek ignimbrite member was the last
widespread ignimbrite emplaced in the Deschutes basin.
However, thick
silicic airfall lapillistones occur at higher stratigraphic positions
in the basin and indicate that the paucity of ignimbrites is not
representative of diminished pyroclastic volcanism.
Some Deschutes Formation basalts were erupted within and east of
the Deschutes basin.
Lava flows with intrabasinal sources were
discussed in the previous chapter.
Although lavas and shield volcanoes
presently exposed east and southeast of Redmond are younger than the
Deschutes Formation, older volcanism in this area is thought to account
for the Pelton basalt member and the occurrence of similar
lowalumina diktytaxitic olivine basalts encountered at depths in
excess of 100 m in geothermal gradient wells near Powell Buttes.
Shield volcanoes near Grizzly and at Teller Flat were the source of
basalts which flowed as intracanyon flows down ancestral Willow and Hay
creeks and spread out as sheets near their distal ends where relief in
the actively aggrading Deschutes basin was low.
Although presently
forming rimrocks, an isotopic age of 6.4 + 0.1 Ma (L.
W. Snee, person.
commun., 1985; Appendix IX) for the flow erupted near Grizzly indicates
that these basalts were not erupted near the end of,
Deschutes Formation deposition.
or following,
184
BASALTS
Most Deschutes Formation basalts were erupted in the High Cascades
with lesser contributions from intrabasinal vents and volcanism east
and southeast of the basin.
Younger Pliocene basalts from the same
provinces overlie the Deschutes Formation and are included in this
discussion.
Most of the Deschutes Formation and younger basalts
exhibit distinctive diktytaxitic textures and these units are discussed
separately from other basalts.
Diktytaxitic Olivine Basalts
Diktytaxitic olivine basalts in the Deschutes basin are typically
coarse grained and medium gray in color.
Single flows rarely exceed 4
m in thickness and as many as eight flows compose each mappable basalt
unit (Figs. 6.3a, 6.6a).
Vesicle sheets and/or cylinders (Fig. 7.2a)
are ubiquitous in diktytaxitic basalts but are notably more
characteristic of some units (e.g. Fly Lake flow of the Lower Desert
basalt member) than others.
Most diktytaxitic basalts contain plagioclase and olivine
crystals, 0.25 to 2 mm across, with intergranular to ophitic augite
(Fig. 7.2b).
Because of the coarse grain size it is difficult to
distinguish phenocryst and groundmass assemblages.
However, most thin
sections contain 1.5 to 2 mm long subhedral plagioclase grains which
are typically more calcic (An
)
than the slightly smaller, euhedral
70-80
Olivine is usually subhedral and exhibits varying
).
60-70
degrees of alteration to iddingsite although the outermost margin of
grains (An
each crystal is invariably fresh (Fig. 7.2b).
TiO
Olivine in basalts with
less than about 1.1 wt.% frequently contains euhedral
2
185
Fig. 7.2. Field and petrographic features of diktytaxitic basalts. a)
- Crosssectional and plan views of vesicle cylinders in the
Fly Lake flow of the Lower Desert basalt member on Fly Creek
grade.
b) Photomicrograph (crossed polarizers) of typical
Deschutes basin diktytaxitic basalt (Pliocene flow capping
section at Mill Creek canyon). Black areas are irregular
void spaces.
Note coarse grain size, subophitic augite (a)
and iddingsite (i) alteration near, but not at, margins of
olivine (o) crystals.
186
TABLE 7.1. REPRESENTATIVE ANALYSES OF DESCHUTES BASIN DIKYTATAXITIC
BASALTS
6
51.9
1.99
15.6
9.36
7.20
9.06
0.21
1.03
17.5
8.53
7.33
11.54
2.07
0.28
0.16
0.16
50.3
1.49
16.7
10.9
7.09
9.75
2.41
0.16
0.12
0.17
99.50 100.00
99.70
99.09
99.25
2
3
S102
TiO2
Al203
FeO
MgO
CaO
Na20
K20
P205
MnO
50.5
0.98
17.0
8.53
8.62
10.93
2.33
0.13
0.15
0.17
50.5
1.56
16.3
9.86
8.18
9.74
2.58
0.30
0.31
0.17
50.3
1.79
Total
99.34
Rb
Sr
Zr
9
304
84
Y
21
Ba
Sc
98
42
Ni
V
151
190
4
5
1
-
-
16.1
11.46
9.47
7.49
2.40
0.41
0.37
22
320
113
29
402
38
112
229
51.1
2.54
1.07
0.35
0.15
10
10
20
332
96
22
255
242
104
1144
249
39
127
256
27
27
112
32
508
25
125
224
111
203
Canadian Bench flow of the Lower Desert basalt member on Canadian
Bench.
Opal Springs basalt member at base of Hollywood Road, Crooked River
Ranch.
Pelton basalt member, fourth flow unit from base of unit, at mouth
of Willow Creek.
Pliocene basalt rimrock northwest of Seekseequa Junction, Warm
Springs Indian Reservation.
Pliocene basalt rimrock at U. S. 26 bridge over Mill Creek canyon,
Warm Springs Indian Reservation.
Pliocene basalt from summit of Grass Butte shield volcano, west of
Prineville.
All analyses performed at Washington State University; major elements
under the direction of P. R. Hooper, trace elements by G. A. Smith.
187
Pyroxene
inclusions of brown spinel which are thought to be picotite.
is generally ophitic.or subophitic near the center of flows, and is
intergranular near base and top.
Glassy flow margins are intersertal
with plagioclase and olivine set in black glass.
These observations
indicate that augite crystallized after olivine and plagioclase and
that ophitic texture did not result from simultaneous growth of phases
possessing different nucleation and growth rates as is commonly assumed
(Cox and others, 1979).
In those basalts where titania exceeds
approximately 1.5 wt.%, the clinopyroxene is usually pink or light
brown in plane light, a feature charateristic of titanaugite.
Most diktytaxitic basalts in the Deschutes basin are highalumina
tholeiites (Al 0
> 16.5 wt.%, and typically > 17 wt.%) erupted within
23
the High Cascades.
This highalumina character is shared by other
Oregon High Cascade basalts (S. Hughs, 1983; Priest and others, 1983;
Tolan and Beeson, 1984).
Highalumina basalts, in general, probably
result from partial melting of plagioclase peridotite in the upper
mantle, thus accounting for the high Al 0
content and the frequently
23
observed enrichment in Eu relative to other basalts (Wyllie, 1971).
In
most basalts studied experimentally by Yoder and Tilley (1962)
C of plagioclase.
clinopyroxene crystallizes within 20
The absence of
clinopyroxene phenocrysts and the abundance of plagioclase in Deschutes
basin diktytaxitic basalts suggests a high crystallization temperature
for plagioclase.
This relationship is best explained by derivation of
the basaltic magma from an H 0rich mantle source where the first2
produced melts would be enriched in plagioclase component relative to
an anhydrous melt (Kushiro, 1979).
A volatilerich source is
188
independently suggested by the diktytaxitic texture and
vesicle
cylinders (Goff, 1977).
Basalts along the eastern and southeastern basin margins, erupted
in the Ochoco Mountains and High Lava Plains, generally contain about
15 to 16.5 wt.% Al 0
.
This difference in alumina is the primary
23
compositional difference between High Cascade and other diktytaxitic
basalts in the Deschutes basin and may provide a means of determining
the provenance of those basalts whose exposure precludes unequivocal
designation of a source direction.
On this basis, all analyzed
diktytaxitic basalts within the Deschutes basin west of the longitude
of Redmond are believed to represent High Cascade magmas, with the
exception of the Pelton basalt member.
All analyzed Deschutes
Formation and younger basalts east of the longitude of Redmond, and the
Pelton basalt member, were probably erupted from vents east and
southeast of the Deschutes basin.
content of Deschutes basin basalts shows only a
Although SiO
2
small variation (49 to 52 wt.%) other major oxides vary greatly.
The
molar ratio Fe0/Fe0+Mg0, referred to as Fe', ranges from 0.33, for the
most primitive diktytaxitic basalts, to 0.5 for more evolved flows.
(0.85 to
The enrichment in FeO is concomittant with enrichment in TiO
2
3.2 wt.%) and depletion of MgO ( 10.5 to 6.7 wt.%) and CaO ( 12.0 to
8.3 wt.%).
Most Deschutes Formation diktytaxitic basalts of Cascade
provenance (e.g. Lower Desert basalt member) are relatively primitive
(Fe' < 0.40).
Pliocene basalts on the Warm Springs Indian Reservation,
with the exception of the basalts of Tenino Bench (Plate I), are more
189
evolved (Fe' = 0.42 to 0.50).
Titania abundances in the evolved
basalts range from 1.3 wt% to 2.4 wt.% and are higher than typical
convergent margin basalts (Green, 1980).
It is also notable that the
primitive Deschutes Formation basalts are most prominent in the
Deschutes basin at the latitude of Green Ridge (e.g. Juniper Canyon,
Cascade
Big Canyon, and Lower Desert basalt members; see Chapter 6).
diktytaxitic basalts are less common in the southern Deschutes basin
and are generally more evolved (Fe' > 0.40).
Deschutes Formation and younger Pliocene diktytaxitic basalts
erupted from vents east and southeast of the Deschutes basin are relatively evolved (Fe'= 0.42 to 0.58).
basalts distinctly lower in Al 0
,
Not only are these nonCascade
as mentioned previously, but also
23
(1.82 + 0.30 wt.% vs. 1.59 +
higher in TiO
0.31 wt. %) and P 0
25
2
(0.41 + 0.07 vs. 0.21 + 0.11 wt.%) than contemporary Cascade basalts
with similar Fe' values.
Binary variation diagrams offer constraints on potential
Systematic
fractionation schemes which may relate basaltic magmas.
decrease in MgO with increasing Fe', for both Cascade and nonCascade
diktytaxitic basalts is consistent with, but not unambiguous evidence
for, fractionation of ferromagnesian silicates (Fig. 7.3).
2
2
is
wt.%), TiO
is large (+ 1
is slight and analytical error for SiO
variation in SiO
Because the
a better indicator of degree of differentiation.
2
(Fig. 7.4) is
Variation of CaO/Fe0 ratio with increasing TiO
2
consistent with extensive plagioclase fractionation.
The role of clinopyroxene in basalt evolution is best evaluated by
variation of the ratio CaO/A1 0
23
with increasing differentiation
190
DIKTYTAXMC BASAL'fS
Deschutes Formation Cascade basalts
0.6
Pelson bimalt member
Pliocene basalts, northern Deschutes basin
Neogene basalts, eastern basin margin
. ..
0.5
o
o
o
0
0
0
0.4
0
0.3
7.0
6.0
5.0
8.0
10.0
9.0
MgO
Fig. 7.3. Variation in Fe' with MgO for Deschutes basin diktytaxitic
Increase in Fe' with decreasing MgO reflects
basalts.
differentiation involving ferromagesian silicates, primarily
olivine.
DIKTYTAXITIC BASALTS
Deschutes Formation Cascade basalts
Felton built member
.0
Pliocene basalts, northern Deschutes basin
Neogene basalts, eastern basin margin
00
BO
0.8
o
0.7
0.6
0.8
1.0
I
1.2
I
1.4
I
1.6
I
1.8
2!0
TiO2
I
2.2
24
1
2.6
I
2.8
I
3.0
3.2
Fig. 7.4. Variation in the ratio CaO/Fe0 with increasing Ti02, as an
indicator of increasing differentiation for diktytaxitic
basalts.
The welldefined inverse relationship of these
variables suggests extensive plagioclase fractionation
(calcic plagioclase has CaO/Fe0 greater than 10 and lacks
titania).
0.8
0.7
**elf. m .0,
et
0
0.6-
3..
0
anild3
An
85
0
Ce°°°°
fl
0
1.8
2.0
22
6.
0.5
soAn
0.4
00
0.2
0.4
0.6
1.0
0.8
1.2
1.6
1.4
2.6
2.4
2.8
3.0
32
1102
Fig. 7.5. Variation in the ratio CaO/Al203 with increasing TiO2 for
Note the slight decrease in this ratio
diktytaxitic basalts.
with increasing TiO2 for Cascadian basalts (solid symbols)
Symbols
suggesting clinopyroxene influence on petrogenesis.
as in Fig. 7.4.
0.7
D1KTYTAXITIC BASALTS
Deschutes Formation Cascade basalts
Felton basalt member
Pliocene basalts, northern Deschutes basin
Neogene basidtS, eutern basin margin
0
8
0.5
1
30
35
40
45
Sc
Fig. 7.6. Covariation of CaO/Al203 and Sc for selected diktytaxitic
Increase in CaO/Al203 with increase in Sc for
basalts.
Cascade basalts suggesting clinopyroxene influence.
192
because clinopyroxene is the only basaltic phase that significantly
fractionates these elements from each other.
Because petrographic
observations indicate that augite is not a phenocryst phase, pyroxene
Non
fractionation is not expected to play a role in basalt evolution.
Cascade basalts, including the Pelton basalt member, show no variation
(Fig. 7.5), consistent with petrographic
with TiO
in CaO/A1 0
23
observations.
2
However, the slight decrease in CaO/A1 0
for
with TiO
23
2
Cascade basalts suggests clinopyroxene involvement in opposition to
Olivine fractionation is not
petrographic observations (Fig. 7.5).
likely to produce this variation because, although forsterite has a
high CaO/A1 0
ratio, the abundance of both elements is so low that a
23
large olivine volume would have to be removed to account for the
observed variation.
Available Ni data precludes such extensive olivine
fractionation (144 + 28ppm Ni for 7 basalts with Fe'<0.40, vs. 129 + 22
ppm Ni for 6 basalts with Fe'>0.40). Plagioclase in diktytaxitic
basalts ranges from An
and, as is apparent from Figure 7.5,
to An
80
65
fractionation of this phase cannot account for the decrease in
CaO/A1 0
with increasing TiO
.
23
The role of clinopyroxene is further
2
indicated by Fig. 7.6 in which CaO/A1 0
is plotted against Sc.
The
23
correlation of CaO/A1 0
with Sc for Cascadederived basalts can be
23
interpreted to indicate clinopyroxene fractionation or varying degrees
of partial melting of clinopyroxene in the source material. The only
other basaltic or mantle mineral which has a scandium distribution
coefficient >1 is garnet.
CaO/A1 0
Mantle garnets have an extremely low
ratio (C. Hughes, 1983) and would result in an inverse
23
relationship between CaO/A1 0
23
and Sc if it played a role in magma
193
genesis.
An alternative interpretation for the decrease in CaO/A1 0
with
23
fractionation (Figs. 7.5 and 7.6) is that two data populations
exhibiting stable CaO/A1 0
exist and are roughly separated by a
23
CaO/A1 0
Rareearth element data should be collected to
value of 0.6.
23
test the suggestion that different degrees of partial melting produced
two primary basalt types (higher CaO/A1 0
for
and Sc and lower TiO
23
2
relatively large degrees of melting and the reverse for smaller degrees
of melting) followed by fractionation of olivine and plagioclase which
and FeO contents, diminished CaO and Mg0, and only
increased the TiO
2
slightly affected SiO
,
CaO/A1 0
,
and Sc.
23
2
Based on available data it seems unlikely that Cascade
diktytaxitic basalts are related by fractionation from a common parent.
Enrichment in FeO and TiO
with only slight increase in SiO
suggests
2
2
olivine and plagioclase fractionation and is consistent with
petrographic observations that these phases come onto the liquidus at
an early stage of crystallization.
The CaO/A1 0
versus Sc
23
relationship indicates that clinopyroxene plays a role in the variable
basalt chemistry but petrographic observation that augite is a late
crystallizing phase precludes clinopyroxene fractionation.
Therefore,
Cascade basalts appear to result from varying degrees of partial
melting from a mantle source region where both pyroxene and plagioclase
are stable (e.g. plagioclase peridotite) combined with olivine and
plagioclase fractionation.
Conrey (1985) pointed out that the variation in K 0 between
2
Deschutes Formation diktytaxitic basalts with the same Fe' cannot be
194
explained by fractionation.
For example, the Fly Lake flow and the
Canadian Bench flow of the Lower Desert basalt member both have Fe' of
0.36 but the Fly Lake flow contains 0.34 + 0.03 wt.% K 0 and the
2
Canadian Bench flow contains 0.23 + 0.09 wt. %.
Conrey (1985)
suggested that potassium was added to the basaltic magma by crustal
assimilation or a small amount of mixing with more silicic magma.
Although assimilation or mixing are possible, the variation in K 0
2
alone is not sufficient for justifying these processes.
Study of
diktytaxitic basalts by Goff (1977) demonstrated the inhomogeneous
distribution of incompatible elements within these flows.
Potassium is
enriched by a factor of two or more in segregation veins, vesicle
linings, and vesicle cylinders or sheets relative to the bulk rock
(Goff, 1977).
The difference in bulk rock K 0 between two flows with
2
similar Fe' may reflect the difference in the extent of volatile
transfer of incompatible elements, rather than contamination.
It is
also noteworthy that trace element data for the Fly Lake and Canadian
Bench flows are very similar and provide no indication of mixing with
material enriched in incompatible elements.
Taylor (1980), Flaherty (1981), Priest and others (1983), and
Conrey (1985) have previously noted the similarity of High Cascade
diktytaxitic basalts to contemporary basalts erupted in the Basin and
Range.
Although late Cenozoic Basin and Range basalts are largely more
alkalic than High Cascade basalts (Leeman and Rogers, 1970), Deschutes
basalts are similar to the widespread diktytaxitic highalumina olivine
tholeiites (HAOT) of the northwestern Great Basin (Table 7.2).
HAOT is
restricted in occurrence to northeastern California, northcentral
195
Nevada, southwestern Idaho, and southeastern Oregon and was erupted
over the period 16 to 0 Ma, concommitant with widespread rhyolitic
volcanism and highangle normal faulting (Hart and others, 1984).
Table 7.2 presents a comparison of basalts of the High Cascades
with those of the northwestern Great Basin and also with average
Primitive (i.e. Fe'<0.40)
basalts from various tectonic settings.
Deschutes basin basalts of Cascade provenance are most comparable with
LowK, lowTi transitional tholeiites (LKLT) were defined by
HAOT.
< 2.0%
Hart and others (1984) as basalts with K 0 < 0.5 wt.% and TiO
2
2
<2.0
and are compared here to evolved Deschutes basin basalts with TiO
2
wt.% and normal basalts of the High Cascade mafic platform (S. Hughes,
1983).
High Cascade basalts of the Deschutes basin are remarkably
similar, on average, to the northwestern Great Basin basalts in both
major and trace elements.
The most notable difference is the slightly
higher silica and rubidium contents of the Deschutes basin flows.
Early High Cascade basalts in the Western Cascades and the younger
mafic platform basalts are notably more alkaline and enriched in Sr
than Deschutes basin and Great Basin basalts.
Late Miocene to
Pleistocene basalts along the eastern margin of the Western Cascades
are also enriched in Ba.
An unusual feature of the northwestern Great Basin basalts is
their enrichment in alkaline earth (Sr, Ba) relative to alkali (K,Rb)
elements, resulting in K/Ba ratios among the lowest known for
terrestrial basalt (McKee and others, 1983); ten times lower than in
midocean ridge basalt (MORB) and three to five times lower than in
196
TABLE 7.2. COMPARISON OF DESCHUTES BASIN DIKTYTAXITIC BASALTS WITH
OTHER PACIFIC NORTHWEST BASALTS
HAOT
DBPB
Si02
TiO2
47.66 50.1
1.00 0.99
Al203 16.91 17.2
Fell
9.88 8.81
MgO
9.06 8.75
CaO
11.20 11.29
Na2O
2.53 2.06
K2O
0.23 0.17
P205
0.13 0.16
MnO
0.17 0.17
Fe'
Rb
Sr
0.38
2.1
Ni
255
154
Ba
141
Zr
95
V
191
Y
20
13.5
.008
K/Ba
Rb/Sr
0.36
7.0
293
144
210
81
249
22
6.3
.02
LKLT
DBEB
DBEM
HCNB
WCDB
MORB
BABB
IAT
51.6
50.5 49.8 51.4 49.34 50.7
1.49 1.23 0.80
1.48 1.28
1.68 1.47 2.01
17.04 16.6 15.9
16.02 16.7 15.9 17.3 17.1
7.84 9.51
11.20 10.91 11.45 9.70 9.32 8.61
6.73
7.19
7.40
6.57
8.20 7.26 7.18 8.3
8.85
11.72
10.89
11.74
10.90 9.65 9.43 8.80
2.41
3.06
2.73
2.44 2.39 2.61
3.41
3.3
0.44
0.39
0.76 0.16
0.36 0.23 0.49 0.71
0.11
0.28 0.17
0.38
0.36 0.16 0.17
0.17
0.16 0.17 0.16
0.18 0.17 0.19
47.29 50.6
0.43
7.3
246
130
253
140
230
0.46
15.0
260
24
11.8
.03
111
0.47
18.0
622
134
487
214
198
301
26
37
12.3
.03
8.4
.03
119
162
0.39
5
486
150
198
124
-
29.8
.01
0.44
7.2
672
113
381
-
0.44
0.40
0.37
1.2
140
97
8
95
-
5
5
208
200
66
65
106
-
30
75
70
48.1
.03
16.6 161.5
.009
.01
49.8
.02
HAOT - Average high-alumina olivine tholeiite (K2O < 0.39% andTiO2 < 1.35 %) of the northern Great Basin (Hart and others,
1984;n=50).
DBPB - Average Deschutes basin primitive (Fe' < 0.40) diktytaxitic
basalt (from 6 analyses of units for which trace elements are
available).
LKLT - Average low-K, low-Ti transitional tholeiite (K20 < 0.5% and
TiO2 < 2.0%) of the northern Great Basin (Hart and others, 1984;
n=32):
DBEB - Average Deschutes basin evolved (Fe' > 0.40, TiO2 < 2.0%)
diktytaxitic basalt of Cascade provenance (from 9 analyses of
units for which trace elements are available).
DBEM - Average eastern Deschutes basin margin diktytaxitic basalt (n=8)
HCNB - Average central Oregon High Cascade "normal" basalts of the
mafic platform (S. Hughes, 1982; n = 13).
WCDB - Average late Miocene to Pleistocene High Cascade diktytaxitic
basalt in the Oregon Western Cascades (Preist and Vogt, 1983,
n=15)
MORB - Average mid-ocean ridge basalt, compiled by McKee and others,
1983.
BABB - Average back-arc basin basalt, compiled by McKee and others,
1983.
IAT
- Average island-arc tholeiite, compiled by McKee and others,
1983.
197
islandarc tholeiite (IAT) or backarc basin basalt (BABB).
Deschutes
basin basalts share this low K/Ba ratio with the Great Basin basalts.
Contemporary and younger High Cascade lavas elsewhere have higher K/Ba
ratio but the ratio is still less than average IAT.
The composition of Cascade basalts, and Deschutes basin
diktytaxitic basalts in particular, provides no clear indication of a
specific tectonic affinity.
In a general sense, the majorelement
compositions of Deschutes basin basalts are not significantly different
from MORB, IAT, or BABB.
Potassium depletion in Deschutes basin
basalts is similar to MORB but Rb is not as depleted; Ni abundance is
higher than MORB, BABB, or IAT.
Alkalineearth elements are greatly
enriched in Deschutes basin basalts relative to MORB; Sr abundances
Similar failure in discriminating
more closely resemble IAT and BABB.
tectonic affinity based on composition has been observed for HAOT by
McKee and others (1983).
The gross similarities between the Deschutes
and northern Great Basin basalts suggest that they do share a similar
Also, all Cascade basalts, Deschutes basinmargin
tectonic regime.
basalts, and northern Great Basin basalts share unusually low K/Ba
ratios implying that they were derived from a similar mantle source
severly depleted in alkalis but not so depleted in alkalineearth
metals.
Perhaps the mantle over this large region has experienced a
similar history of previous melting episodes.
Diktytaxitic basalts erupted east and southeast of the Deschutes
basin are distinct from neighboring northwestern Basin and Range
basalts.
The basinmargin basalts are lower in Al 0
(avg. 15.7 wt%)
23
(2.05 wt%) and K 0 (0.49 wt.%) than HAOT or LKLT.
and higher in TiO
2
2
198
Some of these flows show obvious evidence of contamination by material
Examples of contaminated basalt
enriched in incompatible elements.
include Alkali Flat (1.04 wt. % K 0; 965 ppm Sr; 1123 ppm Bo) and Grass
2
Butte (1.07 wt.% K 0, 1144 ppm Sr).
However, these flows still have
2
unusually low K/Ba ratios (8-12).
Nondiktytaxitic Basalts
Deschutes Formation basalts lacking diktytaxitic texture can be
generally divided into three groups.
The first group includes basalts
near the top of the Green Ridge section which have conspicuous olivine
phenocrysts, lowalumina content (15.5 to 16 wt.%), and minor normative
nepheline (Conrey, 1985).
The second group is porphyritic highalumina
basalts with olivine, plagioclase, and occasional augite phenocrysts or
glomerocrysts up to 1 cm across.
Petrographically similar flows
contents as high as 55 %, and are thus basaltic andesites,
contain SiO
2
but are discussed with these basalts.
The third category represents
-intrabasinal basalts of the Tetherow Butte member and Round Butte
member and the related basaltic andesite of the Steamboat Rock member.
The olivine basalts near the crest of Green Ridge are described by
Conrey (1985) and are stratigraphically between the Six Creek ignimbrite
member and Lower Desert basalt member.
These are the only Deschutes
Formation basalts with alkaline character (i.e. normative nepheline).
Rare hypersthene phenocrysts are incompatible with the composition of
these basalts and may represent crustal contamination (Conrey, 1985).
These basalts are volumetrically inferior to intercalated olivinebearing basaltic andesites and collectively represent the east flank of a
shield volcano transected by the Green Ridge fault scarp (Conrey, 1985).
199
TABLE 7.3. REPRESENTATIVE DESCHUTES FORMATION NONDIKTYTAXITIC BASALTS
1
2
3
K20
49.5
1.73
15.8
10.4
7.6
10.0
3.0
1.0
0.46
52.8
1.35
19.4
8.23
4.7
8.10
4.0
0.97
TOTAL
99.03 100.12
99.55
Si02
TiO2
Al203
Fe0
MgO
CaO
Na20
53.1
1.18
20.3
7.00
4.7
10.28
.3.1
4
51.1
99.68
1.63
16.50
9.90
8.3
7.80
3.11
1.34
5
51.1
0.82
20.1
6.8
6.7
9.4
2.8
0.62
98.34
Nepheline-normative porphyritic olivine basalt, Green Ridge (Hales,
1975).
Porphyritic basalt - basaltic andesite, Big Falls.
Porphyritic basalt - basaltic andesite rimrock southeast of Lower
Bridge.
Seekseequa basalt member, east side of Deschutes River, north of
Round Butte Dam (Jay, 1982).
Porphyritic olivine basalt, lower Whitewater River canyon
(Yogodzinski, 1986).
200
The relatively steep gradient,of these flows (400 ft./mi.) and thick
flow breccias led Conrey (1985) to suggest that vents for this shield
were within 2 to 3 km of the Green Ridge crest.
Although the basalts
and basaltic andesites are intimately interfingered, it seems unlikely
that they are related by fractionation because the higher alumina and
lower titania and lime contents of the basaltic andesites cannot be
explained by fractionation of the basalt phenocryst assemblage of olivine, plagioclase, and hypersthene.
The porphyritic high-alumina basalts and related basaltic
andesites were largely erupted south of the latitude of Green Ridge and
flowed along northeast-trending drainages into the Deschutes basin.
The Seekseequa basalt member is probably the most extensive of these
flows and other prominent examples occur in the lower half of the
section, including the basalts and basaltic andesites forming Steelhead
and Big Falls, the rapids in the Deschutes River at the mouth of Squaw
Creek, and a flow beneath the Opal Springs basalt member in the Crooked
River canyon.
Similar lavas were erupted from a vent just north of
Green Ridge and are now exposed in the Whitewater canyon (Yogodzinski,
Al 0
1986).
as high as 21.5 wt% and CaO to 10.0 wt% suggests that
23
plagioclase accumulated in the magmas represented by some these flows.
Several observations suggest that the porphyritic basalts are
unrelated to diktytaxitic basalts.
CaO/A1 0
The porphyritic flows have
ratios consisently less than diktytaxitic flows with similar
23
and Fe' (Fig. 7.7).
TiO
2
The stability of CaO/A1 0
with increasing
23
indicates little or no influence of augite fractionation, in
TiO
2
contrast to the diktytaxitic basalts, despite the presence of augite
201
phenocrysts in some of the porphyritic basalts.
The presence
of
augite
phenocrysts and groundmass hypersthene also distinguishes the
of
Covariance
porphyritic basalts from the diktytaxitic varieties.
Fe'
and MgO (Fig. 7.8) suggests that porphyritic basalts may be related to
each other by fractionation of ferromagnesian silicates but cannot be
so derived from the diktytaxitic basalts.
spatially separated as
well;
The two basalt types are
diktytaxitic basalts are dominant in the
central and northern Deschutes basin whereas porphyritic
flows
are
dominant to the south.
Lavas erupted at Tetherow Butte are compositionally and petrographically distinct from basalts erupted in the High Cascades or
along the eastern basin margin.
flows of
In terms of Fe', the
the
Tetherow Butte member are the most evolved basalts in the Deschutes
Formation (Fe'=0.60).
The near absence of olivine is also distinctive
and the Tetherow Butte
flows
are the only Deschutes Formation basalts
The Tetherow Butte
with more abundant augite than olivine phenocrysts.
lavas are enriched in the incompatible elements (K,
Rb, Sr, Zr, Y, and
Ba) relative to Cascadederived basalts in the Deschutes basin and, in
this respect, are more similar to basalts erupted along the eastern
basin margin.
Very low Ni abundance (17 to 50 ppm) suggests that
olivine fractionation may have played a role in the extreme FeO, TiO
2
and V enrichment exhibited by Tetherow Butte basalts.
Flows, bombs, and cinders of the Steamboat Rock member, although
basaltic andesite, are petrographically similar to Tetherow Butte
basalt and exhibit some compositional similarities.
With the exception
of the much higher K 0 content (1.13 % vs. 0.64 %) the
2
202
0.8
Deschutes bum Cascade
Porphyritic basalts
diktytaxitic basalts
0.7
0.(5
.(5
(3
0.5
e, An
A
An60
0.4
00
I
0.2
0.4
0.8
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
no2
Fig. 7.7. Comparison of CaO/Al203 versus TiO2 for porphyritic basalts
Note the stability of CaO/Al203
and diktytaxitic basalts.
with increasing TiO2 for porphyritic basalts suggesting
fractionation of calcic plagioclase and little or no
clinopyroxene influence.
0.6
Deschutes basin
0.5
/ Cascade diktytaxitic basalts
Fe'
0.4
Porphyritic basalts
03
A
5.0
6.0
7.0
MgO
8.0
9.0
10.0
Fig. 7.8. Comparison of Fe' versus MgO for porphyritic and diktytaxitic
Note that although the inverse relationship
basalts.
illustrated suggests that porphyritic basalts are related by
fractionation of ferromagnesion silicates, principally
olivine, they cannot be related by fractionation to the
diktytaxitic basalts.
203
conspicuous differences in Steamboat Rock member chemistry,
to the Tetherow Butte member, such as higher Al 0
relative
and lower TiO
23
,
V,
2
and FeO, are consistent with fractionation of augite and
titanomagnetite which are both prominent phenocrysts in the Tetherow
Butte flows (Fig. 6.22c).
Although the difference in KO precludes a
2
simple fractionation model to relate these two magmas, the generally
similar composition, close proximity of eruptive centers, and eruption
of the basaltic andesite following the basalt, suggest a relationship
between Tetherow Butte and Steamboat Rock lavas.
Basalts of the Round Butte member are distinctly olivinephyric
flows with incompatible element contents close to eastern basin margin
basalts.
The similarities end there, however, because Round Butte
basalt is notably higher in Al 0
and lower in FeO, CaO, and MgO when
23
compared to basin margin basalts with similar Fe' (0.45).
The great
differences in composition and mineralogy between Round Butte basalts
and flows from the other intrabasinal vents precludes any relationship
between them by fractionation.
BASALTIC ANDESITES AND ANDESITES
Because of similar petrographic character and behavior on
variation diagrams, basaltic andesites and andesites are treated
together.
Basaltic andesite is volumetrically more important than
andesite in the Deschutes Formation and the latter compositional range
is better represented by ignimbrite pumice than by lava flows.
The petrographic and compositional characteristics of basaltic
andesites and andesites are quite variable but they can be generally
divided into porphyritic varieties and sparsely phyric to aphyric
204
(Table 7.4;
varieties; the latter being notably richer in FeO and TiO
2
Fig. 7.10).
The most important phenocryst is plagioclase, as calcic as
in andesites.
in basaltic andesites to An
An
Olivine occurs in
65
80
most porphyritic basaltic andesites, singularly or in glomerocrysts
with plagioclase, and is usually subhedral and partly altered to
iddingsite.
Olivine is less common in andesites and is typically
rimmed by pyroxene.
Augite is a conspicuous phenocryst in most
> 56 wt. % and occurs with
porphyritic flows and pumice with SiO
2
Microphenocrysts
hypersthene phenocrysts in most andesites.
tentatively identified as pigeonite occur in some of the aphyric
basaltic andesites.
Hornblende, a common phenocryst in andesites from
continental margin arcs (Gill, 1982), is extremely rare in Deschutes
Formation andesites.
Several lines of evidence suggest that basaltic andesites and
andesites were not derived by fractional crystallization of
diktytaxitic basalts.
The mean CaO/A1 0
ratio for basaltic andesites
23
is 0.45 and for Cascadederived diktytaxitic basalts is 0.60.
Augite
is the only phase that can affect this ratio and the late
crystallization of pyroxene in the diktytaxitic basalts eliminates it
from consideration as a fractionating phase.
Variation of MgO vs Fe'
(Fig. 7.9) shows that basaltic andesites lie along a trend different
from diktytaxitic basalts but colinear with the porphyritic basalts.
Andesite compositions, especially for the aphyric varieties, generally
lie on the extrapolation of the basaltic andesite trend suggesting that
there may be a relationship by fractionation of ferromagnesian phases.
Study of mafic platform lavas by S. Hughs (1983) convincingly showed
205
TABLE 7.4. REPRESENTATIVE DESCHUTES FORMATION BASALTIC ANDESITES AND
ANDESITES
Si02
TiO2
Al203
FeO
MgO
CO
Na20
K20
1
2
3
4
55.0
1.24
17.3
8.34
7.0
7.40
3.2
1.07
55.0
1.22
18.0
8.05
5.3
8.10
3.4
0.94
53.5
1.)6
17.8
8.61
57.2
2.01
17.0
9.26
3.7
6.81
4.2
0.99
7.1
8.49
3.6
0.53
6
7
8
9
58.8
0.95
18.4
6.30
3.6
7.35
3.7
1.07
59.3
1.05
16.7
6.80
3.2
6.60
3.8
1.55
58.2
1.67
15.7
9.10
6.80
4.3
0.98
60.6
1.47
17.0
7.80
1.5
5.40
4.4
1.47
98.93 100.17
99.00
99.82
99.60
5
TOTAL 100.55 100.03 100.79 101.17
54.7
1.91
16.0
10.00
3.6
7.60
4.4
0.78
3.1
Columnar-jointed, porphyritic basaltic andesite below rimrock near
top of Crooked River grade, Cove-Palisades State Park.
Olivine-bearing basaltic andesite near crest of Green Ridge (Conrey,
1985).
Sparsely-phyric basaltic andesite near Monty Campground, lower
Metolius River (Dill, 1985).
Aphyric basaltic andesite at Pipp Spring, Warm Springs Indian
Reservation.
Aphyric basaltic andesite,
1
km west of Fly Lake (Conrey, 1985).
Porphyritic andesite, ridge-forming lava between S.
and N. Fk. Spring Creek (Conrey, 1985).
Fk. Street Creek
Porphyritic two-pyroxene andesite, west flank of Green Ridge east of
Canyon Creek (Hales, 1975).
8, Aphyric andesite, ridge-former above Fly Creek Ranch (Conrey, 1985).
9. Aphyric andesite, overlying Six Creek ignimbrite member,
of Prairie Farm Spring (Conrey, 1985).
1
km south
206
0
°
o
A Porphyritic basalts
Porphyritic basaltic andesites
0 Aphyric basaltic andesites
Porphyritic andesites
U Aphyric andesites
vi,in
0.6
11140,0
Fe'
0.5
0.4
0.3
2.0
4.0
6.0
8.0
10.0
MgO
Fig. 7.9. Fe' versus MgO for Cascadian basalts, basaltic andesites,
Porphyritic basalts,
and andesites in the Deschutes basin.
basaltic andesites, and andesites form a single trend
Note
suggesting that they may be related by fractionation.
that the field of diktytaxitic basalts lies parallel to, but
not coincident with, this trend.
207
that PlioPleistocene High Cascade basaltic andesites are primary
magmas, unrelated to the diktytaxitic basalts.
The data presented here
support separate parentage for Deschutes basin basaltic andesites and
diktytaxitic basalts as well.
Further work is needed to determine
whether the basaltic andesites are primary magmas or the product of
fractionation of porphyritic basalts.
Alternatively, Conrey (1985) suggested that basaltic andesites and
andesites were derived from parental basaltic magmas which were mixed
with more silicic magmas to produce the intermediate compositions.
Conrey (1985) favored the mixing hypothesis over separate magmas or
fractionation because of abundant petrographic evidence for the former.
This evidence consists largely of observations of resorbed phenocrysts
and complicated zoning patterns in plagioclase indidative of
disequilibria.
These petrographic features are not unambiguous
evidence of magma mixing and the role of mixing basalt with silicic
magmas to produce basaltic andesites and andesites is controversial
(Gill, 1982).
In the absence of trace element data the magma mixing
hypothesis cannot be adequately assessed.
The important point, whether
magma mixing or separate primary magmas are invoked, is that Deschutes
Formation diktytaxitic basalts and basaltic andesites are not related
by crystal fractionation.
More convincing evidence of magma mixing or contamination is
illustrated by basaltic andesites which contain multiple phenocryst
assemblages and/or streaks and bands of different composition.
Conrey
(1985) describes several such flows on both the east and west flanks of
Green Ridge, the most extensive of which is a 140 mthick sequence of
208
lavas overlying the Six Creek ignimbrite member and erupted, in part,
Called the "mixed
from dikes now exposed along the Green Ridge crest.
lavas" by Conrey, these basaltic andesites contain phenocrysts of
plagioclase, olivine, augite, hypersthene, opagues, and rare hornblende
tridymite, and
in a groundmass of plagioclase, two pyroxenes, opagues,
Plagioclase phenocryts occur as normally
rare biotite and hornblende.
cores and reversezoned crystals with An
zoned crystals with An
60-65
80
cores.
of
The multiple phenocryst populations suggest hybridization
andesite and basaltic andesite magmas (Conrey, 1985).
Variation diagrams in Figure 7.10 illustrate the compositional
TiO
traits of Deschutes Formation basaltic andesites and andesites.
2
2
2
%, beyond which TiO
is not an accurate
Therefore, TiO
declines.
reaches about 57 wt.
until SiO
increases slightly with increasing SiO
2
2
indicator of magma evolution for compositions more evolved than
basalts.
as a measure of the degree of differentiation
The use of SiO
2
imparts scatter to the diagram, because of analytical error, but,
nonetheless, important trends are apparent.
versus SiO ,
significant slope in CaO/A1 0
23
The lack
of
any
between 53 and 56
for SiO
2
2
wt. %, suggests little or no fractionation of clinopyroxene.
increases beyond 56 wt. %
as SiO
Systematic decrease in CaO/A1 0
23
2
possibly signals the onset of clinopyroxene fractionation and is
consistent with the increasing abundance of augite phenocrysts in these
more silicic rocks.
2
2
diminishes.
57 % where it levels off as TiO
reaches
until SiO
Fe' rises with increasing SiO
The end
of
iron
2
content probably indicates
enrichment and reduction of TiO
2
fractionation of FeTi oxides.
The only other mineral that could
209
produce such a fractionation effect is hornblende which is virtually
non-existant in Deschutes Formation andesites.
.
The high FeO, and especially high TiO
,
contents of Deschutes
2
basaltic andesites and andesites are atypical of convergent margin
magmatism (Fig. 7.11).
Magmas generated above subduction zones are
content less than 1.2 wt % because
thought to be characterized by TiO
2
- bearing phase (e.g. sphene)
of hypothesized stabilization of a TiO
2
under hydrous mantle conditions and fractionation of TiO -rich minerals
2
(e.g. titanomagnetite, hornblende) to produce andesitic and more
evolved magmas (Green, 1980).
content and
The unusually high TiO
2
extensive iron enrichment, apparent in basalts as well as intermediate
rocks, is an important petrologic characteristic of Deschutes Formation
rocks and is critical to the evaluation of the tectonic setting of
early High Cascade volcanism, as discussed at the end of this chapter.
The relationship of the aphyr'ic basaltic andesites
to the porphyritic varieties is unclear.
and andesites
The aphyric or sparsely
phyric character suggests that the fine-grained flows are either a high
temperature primary magma type or a residual liquid from which crystals
have been extracted.
The large Fe' values (mostly between 0.55 and
0.70) argue against a primary magma generated in equilibrium with the
same mantle that produced the primitive High Cascade basalts (Fe'
0.35).
The mechanisms by which such complete separation of liquid and
crystals occurred is not clear but a residual magma origin seems more
likely.
Because of their high FeO and TiO
contents, the aphyric magmas
2
were probably denser, yet less viscous, than the phenocryst-bearing
210
Fig. 7.10. Harker diagrams for selected majorelement oxides and ratios
for Deschutes Formation basaltic andesites and andesites.
Closed symbols represent porphyritic samples; open symbols
represent aphyric samples.
LLZ
V
TO
V
WV
WV
V
V
Vv v
00
V
v
3
voqrv
40
T
,c7
V
0
W
V
V
V
7
V
V
£'0
V
8
V
V
V
V
V
V
Z'O
V
V
80
V
CO
V
V
v
v"
7
,v
vg v
v
0.0
V
v
V VV
V
V
v
V
w
v v
v
7
0,,,
v
V
v
..,
TO
n
011.
OU
o oo
001
a
0
20
00
0
1). 0. sr
0
1
io 0 0 so
'
at
0
02
0
FF
0
, 4,
0
,
.
00
0
B
O
0
11
0...
0
48° 0 0
iii
§
cc
8c°
se°,
0
O'Z
0
0
i
0
° 00 g
o
0
°
0
°0
0
0
0.Z
00
0
0
0
..
0
0
0
08
0
00
00
0
0
60
0
.
'
e0
00
00
oto
o
o
0
0
cf)
0
0
,0
00
CI.
'0
SS
95
85
zOtS
'6!-J
'occ
85
09
l9
Z9
£9
212
0
5
GLOBAL COMPILATION
400
2
0
_
300
DESCHUTES FORMATION
(c421
WI-
-
2
-8
ci
u.
0
2
D
100
0
0.0
0.5
1.0
1.5
2.0
2.5
TiO2
Fig. 7.11. TiO2 histogram for Deschutes Formation basaltic andesites
and andesites compared to the compilation of Gill (1982)
for rocks in orogenic settings with Si02 between 53 and
58 wt. %.
213
These characteristics are
basaltic andesite and andesite magmas.
important for two reasons.
First, such dense magmas would probably be
unable to reach the surface and be extruded unless tectonic pathways,
Second, probably as a
i.e. dilational faults, were present.
consequence of lower viscosity, the aphyric lavas in the Deschutes
Formation are far more extensive than the porphyritic basaltic
andesites and andeistes, with one such flow occurring 30 km northeast
of Green Ridge (Jay, 1982; Plate I).
DACITES, RHYODACITES, AND RHYOLITES
Dacite, rhyodacite, and minor rhyolite are represented by a few
lava flows on the west face of Green Ridge and widespread ignimbrites
in the Deschutes basin.
Plagioclase (An
)
is the dominant
45-60
phenocryst in Deschutes Formation dacite lavas with subordinate
These
hypersthene, augite, and occassional olivine (Conrey, 1985).
flows are glassy, plat' jointed, and 25 to 35 m thick.
A single
rhyodacite flow, nearly 95 m thick, crops out on Green Ridge and
contains sparse phenocrysts and microphenocrysts of plagioclase,
augite, and hypersthene.
Pumice in dacitic to rhyolitic ignimbrites
contains prominent plagioclase phenocrysts (up to 35 wt. % of the
lapilli) and subordinate augite and hypersthene.
Representative
electron microprobe data (Appendix III) indicate that plagioclase
to An
ranges in composition from An
Wo
En
45
,
,
clinopyroxene from Wo
to En
and orthopyroxene from En
45
45
recognized in only seven ignimbrite units.
and K 0,
although rich in SiO
2
.
to
En
42
40
15
36
Hornblende has been
70
Rhyolite ignimbrites,
lack quartz or K-feldspar phenocrysts.
2
Silicic compositions are also represented by conspicuous
214
vitrophyre clasts in Deschutes Formation debris-flow units.
These
vitrophyres are generally black in color with varying proportions of
plagioclase phenocr'ysts and occassional augite and hypersthene
phenocrysts.
The clasts are generally angular and in some cases
exhibit radial prismatic fractures indicative of in situ cooling from
high temperature.
This observation suggests that the host debris flow
was generated by moblization of lithic-rich pyroclastic debris such as
commonly occurs near silicic domes.
Analyzed vitrophyre clasts range
from andesite to rhyodacite with dacitic clasts being most common
(Table 8.5; Appendix 1k).
Some of the dacitic and rhyodacitic clasts
have unusually high Na 0 contents (6.0 to 6.5 wt% compared to 3.5 to
2
5.5 wt % for most other Deschutes silicic units) of uncertain
petrologic significance.
Another notable clast lithology is restricted
to a single debris-flow deposit exposed in lower Street Creek canyon,
near Seekseequa Junction, at Jackson Buttes, and in lower Willow Creek
canyon.
This debris-flow deposit is characterized by light gray, soda6.2 wt. % Na 0) to 30 cm across with
rich dacite clasts (64.9 wt% SiO
2
2
small plagioclase, hypersthene, and hornblende phenocrysts and cognate
xenoliths of hypersthene-hornblende diorite up to 15 cm across.
Air-fall pumice lapillistones occur throughout the Deschutes
Formation and are particularly prominent near the top of the section in
the central and southern Deschutes basin where unreworked units are as
thick as 1.5 m.
Most air-fall lapilli are sparsely phyric with
plagioclase, augite, hypersthene, magnetite, and occassional hornblende
crystals.
Because of their high porosity, most lapillistones are
hydrated and difficult to analyze accurately.
Air-fall deposits near
215
_
0.3
A
A
0
.1
AAA
0.2
......
0
A
AA
AA
A
A
A
AAAA.A
A
0.1
.
A
A
LA A
A
t
AA
A
0.8
-
A
A
.:1
AA A
0.7
.2
0.6
-'
A
A
A
AA
A
A
A
A
A
A
AA
A
0.5
0.4
s a 6.0
.,
...
0
IN 5.0
3.0
.
2.0
,
nil
0
I2
0 , s 9,
: 1 **or? lir
.
4.0
0
5r
....p.
o
OA
7.0
6.0
5.0
L6
am di
4.0
IF a a
a
1 11.
a"
p a
a
o 3.0
II
0
2 2.0
:go O.
4.. :
1.0
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s
0.
, i
.1.
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eieg
loci
6:s3
.
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00
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1.7
0.4 -
a,
o
-
0
0.0
63
64
65
66
68
67
69
70
71
72
SiO2
Fig. 7.12. Harker diagrams for selected major-element oxides and ratios
for Deschutes Formation dacites and rhyodacites, in closed
symbols, and rhyolites, in open symbols.
216
the top of the section are generally the freshest and analyses of
several of these units indicate rhyodacite and, less commonly, dacite
compositions similar to Deschutes ignimbrites (Appendix Ij).
Variation diagrams (Fig. 7.12) illustrate systematic decline in
TiO
,
FeO, MgO, and CaO/A1 0
,
slight increase in Fe', significant
23
2
increase in K 0 and little variation in Na 0 with increasing SiO
relatively steep slope of the CaO/A1 0
23
The
.
2
2
2
vs. SiO
plot suggests
2
may
continued fractionation of clinopyroxene and decrease in TiO
2
reflect fractionation of titaniferrous oxides and, to a lesser extent,
hornblende.
PHYSICAL FEATURES OF IGNIMBRITES
Depositional Structure and Texture
In the past decade there has been a considerable number of papers
written concerning the structure and texture of deposits resulting from
pyroclastic flows and related processes, directed at understanding
transport and depositional mechanisms (Sparks and others, 1973; Sparks,
1976; Fisher, 1979; Wohletz and Sheridan, 1979; Walker and others,
1980a, 1981a,b; Wright and others, 1980; Wilson and Walker, 1982;
Walker, 1983).
2
(i.e 10
These papers are primarily based on study of very large
33
to 10 km ) ignimbrites and several (Walker and others, 1980a;
1981a,b; Wilson and Walker, 1982) draw strongly on work on the Taupo
3
ignimbrite in New Zealand which, although modest in volume (30 km ),
was the product of an unusually powerful eruption (Walker, 1980).
Excellent exposures of a large number of ignimbrites in the Deschutes
Formation allow for evaluation of the applicability of this recent
literature to the modestsized pyroclastic flows most common in
217
continental-margin arcs.
Sparks and others (1973) introduced the concept of a standard
ignimbrite flow unit (Fig. 7.13) composed of a turbulently deposited
layer 1 (resulting from fluidization by air ingested at the head of a
flow), a poorly-sorted, mass-emplaced layer 2 (representing the body of
the pyroclast"ic flow) and a turbulently deposited layer 3 of crystal-
depleted ash (representing deposition of fines elutriated by
fluidization from the flow).
Considerable debate has recently arisen
over the use of the terms pyroclastic flow versus pyroclastic surge
(more specifically, ground surge) to describe layer
1
deposits (Fisher
and others, 1980: Walker and others, 1980b; Wilson and Walker, 1982;
Walker, 1983; Walker and McBroome, 1983).
The controversy largely
centers on whether the distinction between pyroclastic flow and surge
is to be made on the basis of concentration of particulate matter
relative to a continuous gas phase (Walker, 1983) or on the basis of
flow behavior - laminar versus turbulent (Fisher, 1982).
In this
discussion, the latter approach is adopted and layer 1 deposits are
considered to be the product of turbulent surges of uncertain
particulate concentration but, undoubtedly, less than that of the
pyroclastic flow which produced overlying layer 2.
Turbulent
deposition is inferred from the presence of sedimentary structures
produced by migrating bedforms and/or clast support of lapill-size and
larger fragments reflecting winnowing of finer-grained particles.
Two types of layer 1 deposits occur in Deschutes Formation ignimbrites.
The first, and most common, is a generally thin (<30 cm),
plane-laminated and/or cross-laminated layer of ash and/or small
218
IDEALIZED SEQUENCE
TYPE OF DEPOSIT
LAYER
ash-cloud surge
3
111,
2b
pyroclastic flow
2a
=MI
ground surge
precursor air fall
Fig. 7.13. Standard ignimbrite flow unit of Sparks and others (1973).
219
rounded pumice lapilli (Fig. 7.14a).
The second type is a 15 cm to 1 m
layer of fines-depleted juvenile lapilli and bombs with or without
admixed angular and rounded lithic fragments and sediment ripups (Fig.
7.14b).
Only in one case, at the base of the Cove-Palisades ignimbrite
member, have both types been observed in the same exposure and in this
outcrop the first type overlies the second.
An important feature of
Deschutes Formation layer 1 deposits is their lack of continuity.
In
most ignimbrites it would be more proper to refer to these surge
deposits as lenses rather than layers.
Therefore, precursor, cogenetic
surges apparently were local, transient phenomena (possibly influenced
by topography) during the emplacement of most Deschutes pyroclastic
flows and may indicate low transport velocities in the distal Deschutes
basin.
An exception to this generalization is the extraordinarily
thick, bedded deposit at the base of relatively proximal exposures of
the Chinook ignimbrite member which suggests a very powerful surge
(Fig. 6.,4b).
In the general scheme of Sparks and others (1973) the pyroclastic
flow (sensu stricto) is represented by massive, matrix-support layer 2
whose texture and lack of structure suggests deposition by laminar flow
(Sparks, 1976).
Dispersive pressure is thought to account for a
relatively fine-grained layer 2a which is gradational to layer 2b,
characterized by coarse-tail inverse grading of pumice lapilli and
bombs and coarse-tail normal-grading of denser lithics (Sparks and
others, 1973).
The grading is thought to result from settling of dense
lithic clasts and lifting of buoyant pumiceous clasts in a fluidized
matrix (Sparks, 1976).
Deschutes Formation ignimbrites exhibit the
220
-WS
0,I
ofhai.ti
1
-
4.".
.
'
4
Sir'
I4
.4*
Fig. 7.14. Ground-surge deposits in Deschutes Formation ignimbrites.
a) Plane-laminated and cross-laminated ash at base of Steelhead Falls ignimbrite member east of the mouth of Squaw
Cogenetic air-fall lapillistone underlies the ignimCreek.
b) Loading of massive, matrix-support ignimbrite
brite.
into underlying fines-depleted ground-surge deposit composed
primarily of juvenile, prismatically fractured lapilli.
Peninsula ignimbrite member southeast of the mouth of Squaw
Creek.
221
a) Finegrained
Fig. 7.15. Grading in Deschutes Formation ignimbrites.
layer 2a is overlain by layer 26 with distinct normal
gradingof darkcolored lithic fragments and reverse grading
of lightcolored pumice lapilli. Note upward increase in
flattening of lapilli and development of platy jointing
indicative of welding in the central portion of the
Fly Creek ignimbrite member at The Balanced
ignimbrite,
Rocks.
b) Two cogenetic, reversegraded flow units in
ignimbrite beneath Lower Bridge ignimbrite member, 3 km
north of Steelhead Falls.
Note thin, cogenetic airfall tuff
at base (af) and laminated surge zone at base of first flow
unit(s).
For scale, bottom flow unit is 0.9 m thick.
222
massive, matrixsupport characteristics of pyroclastic flow deposits.
In many units there is no consistent development of grading or of a
layer 2a, suggesting relatively low flow velocities and limited
fluidization.
However, other units show both characteristics and
closely resemble the Sparks and others (1973) model (Fig. 7.15).
The third layer of the standard ignimbrite flow unit is composed of
a thin stratified zone, often extending laterally beyond the lower
layers.
This ashcloud surge deposit is the result of 1) elutriation
of ash from a fluidized pyroclastic flow to produce an overriding
turbulent cloud (Sparks and others, 1973), or 2) gravity segregation of
an originally turbulent flow into a basal highconcentration
pyroclastic flow and resultant, upper lowconcentration, still
turbulent surge (Fisher and Heiken, 1982).
Because ashcloud surge
deposits are typically only a few centimeters thick they are rarely
preserved in the geologic record.
Only one unequivocal ashcloud surge
dep-osit has been recognized in the Deschutes Formation and occurs
between flow units of the Tenino ignimbrite member (Fig 6.12c).
The
crossbedded ash deposit is inferred to represent ashcloud surge
related to the lower flow unit rather than ground surge associated with
the upper one because the ash is crystaldepleted relative to the
matrix of the massive ignimbrite, is gradational to the lower
ignimbrite flow unit, and bedforms are locally truncated by the base of
the upper flow unit.
Welding
Fewer than a dozen Deschutes Formation ignimbrites exhibit
welding; in most units pumice lapilli are undeformed and there is
223
Therefore, many
little or no plastic deformation of matrix shards.
Deschutes ignimbrites are friable slope-formers and induration is
primarily a result of iron oxides produced by fumarolic alteration
shortly after emplacement or by secondary clay and opaline silica
produced by subsequent weathering.
Extensive vapor-phase
devitrification has not been recognized.
Welding may have occurred in
many ignimbrites emplaced west of Green Ridge but these proximal
deposits are buried in the intra-arc graben.
Welding, when observed, is of two types.
Some ignimbrites show a
lateral decrease in welding with increasing distance from presumed
source areas.
These ignimbrites (e.g. Fly Creek ignimbrite member,
McKenzie Canyon ignimbrite member) are unwelded in their most distal
exposures but exhibit well-developed welded zones in more proximal
outcrops in the western part of the basin.
Two such units, the Fly
Creek ignimbrite member and the red ignimbrite (unit 5 of Stensland,
1970) exposed beneath the Deep Canyon ignimbrite member in Deep Canyon,
contain densely-welded vitrophyre zones.
This lateral variation sug-
gests predominant control on welding by temperature.
As pyroclastic
flows ingest air and cool, more distal deposits are less likely to
become welded.
The second type of welding is represented by local
development of welded zones where ignimbrites form unusually thick,
channel-filling units.
In these cases a central welded zone, bounded
by lower and upper unwelded zones, was probably the combined results of
greater heat retention and lithostatic load imposed by the thicker
nature of the unit.
Examples of the second type of welding include the
Tenino ignimbrite member along Tenino Creek, the Jackson Buttes ignim-
224
brite member in lower Willow Creek canyon (Fig. 6.7b), and the
Steelhead Falls ignimbrite member at the confluence of Squaw Creek and
the Deschutes River (Fig. 6.11d).
Most welded ignimbrites contain an unwelded upper zone where heat
lost to the atmosphere and lack of lithostatic load prevented welding
(Smith, 1960).
This upper unwelded zone may be removed by erosion
leaving the more resistant welded portion as the only record of the
ignimbrite.
For example, exposures of the Fly Creek ignimbrite member
between Fly Creek Ranch and the Metolius River consist only of the
welded lower part of the ignimbrite.
The McKenzie Canyon ignimbrite member lacks an upper unwelded zone
over the entire area of distribution of the welded zone.
It seems
unlikely that an unwelded zone, if it existed, could be so thoroughly
removed by erosion over such a large area, especially since the more
distal entirely unwelded ignimbrite is preserved.
Rather, welding of
the ignimbrite may have proceeded tothe top of the unit, even in the
absence of lithostatic load, because of the high emplacement
temperature of the upper flow units dominated by highiron andesite.
GasEscape Structures
Gases escaping from hot pyroclasticflow deposits produce narrow
pipes (generally less than 5 cm in diameter) where ash is winnowed away
and coarse lapilli and lithic fragments are concentrated.
Such
structures are present in many Deschutes Formation ignimbrites but they
are generally not abundant.
Degassing pipes are typically regarded as
representing exsolution of volatiles from vesiculating pumice or
release of gases trapped within the pyroclastic flow deposit (Fisher
225
and Schmincke, 1984).
However, virtually all pipes observed in
Deschutes Formation ignimbrites appear to be related to consumption of
organic material within the deposit or to steam liberated from
underlying watersaturated sediment.
The first relationship is
demonstrated by the common origin of pipes, or sheets, of fines
depleted material along zones of permineralized branches or hollow
branch molds.
The second relationship is represented by pipes
originating at the base of an ignimbrite and containing sand and small
pebbles entrained from the underlying sedimentary unit.
The apparent
paucity of pipes related to degassing of the pyroclastic flows is
consistent with the lack of significant vaporphase alteration and
indicates that, by and large, Deschutes pyroclastic flows were not
highly fluidized by the time they reached the central Deschutes -basin.
Dill (1985) described clastic dikes within three Deschutes
Formation ignimbrites, and best developed in the Balanced Rocks
ignimbrite member near Fly Creek Ranch, which may be related to
degassing.
The dikes are filled largely by material derived from the
host ignimbrite; size segregation and vertical lamination suggesting
flowage indicate that the dikes are not the result of passive in
filling of fractures from above.
However, the lack of finesdepletion
and dike, rather than pipe, morphology makes these features distinct
from previouslydescribed gasescape structures.
Cogenetic AirFall Deposits
Many Deschutes Formation ignimbrites overly lapillistones or tuffs
which represent cogenetic Plinian airfall deposits.
In only a few
cases have critical studies of chemistry or mineralogy been undertaken
226
to demonstrate the consanguinity of air-fall and pyroclastic-flow
deposits but field relationships argue strongly for such a
relationship.
The air falls and ignimbrites occur in contact with each
other without intervening deposits and the air-fall deposits lack
evidence of reworking, which is typical of lapillistones not overlain
by ignimbrites (see Chapter 8), and suggest a short time interval
between fall and flow events (Figs. 6.11, 6.12, 7.14a, 7.15b).
Air-
fall tuffs are rare in the Deschutes Formation except where overlain by
presumed cogenetic ignimbrites which protected the fine-grained
pyroclastics from erosion.
Lapillistones, often reworked at least in part by wind or water,
.
also occur without overlying ignimbrites.
These beds, 10 cm to 1.5 m
thick, are composed of angular to rounded (if reworked) pumice lapilli
or,
in some cases, accretionary lapilli (Fig. 7.16).
Though not
directly overlain by ignimbrites the grain size and thickness of these
units implies powerful Plinian eruptions capable of producing
pyroclastic flows which may be preserved as ignimbrites elsewhere in
the basin.
Future geochemical and mineralogical work aimed at
correlating air-fall units with ignimbrites would provide a
stratigraphic framework for the east side of the basin which lacks an
ignimbrite record but contains innumerable air-fall deposits.
COMPOSITIONAL HETEROGENITY IN DESCHUTES FORMATION IGNIMBRITES
Many Deschutes ignimbrites are compositionally heterogeneous, a
common feature of ignimbrites everywhere (Smith, 1979; Hildreth, 1981;
Spera, 1984).
Heterogenity is indicated by pumice populations with
distinctly different composition and mineralogy and occurrence of
227
a) Massive,
Fig. 7.16. Examples of Deschutes Formation air-fall units.
unreworked pumice lapillistone which mantled underlying
Note thin zone of fluvial reworking near top of
topography.
unit indicated by admixed, dark-colored, rounded lithic
Thin white bed above lapillistone is an air-fall
grains.
tuff containing accretionary lapilli. Outcrop along Deschute
b) Accretionary lapilli
River, 7.5 km northeast of Tumalo.
in interval between Balanced Rocks and Fly Creek ignimbrite
members at The Balanced Rocks.
228
banded pumice consisting of streaks of glass of different color and
composition (the adjective "banded" is preferred to the commonly used
"mixed" because the discreet bands indicate that although different
The
magmas have comingled they have failed to mix and form a hybrid).
compositionally heterogenous ignimbrites are readily recognized in the
field by the presence of pumice lapilli and bombs of different color
within the same unit (Fig. 7.17a).
Care must be taken to recognize
mineralogical differences (phenocryst type or abundance) between
different colored lapilli because coloration is also a function of the
degree of vesiculation; dark pumice may be the less vesiculated
representatives of the same magma that produced more vesiculated light
pumice.
Compositional heterogeneity is either the reflection of eruption
from compositionally zoned magma chambers or contemporaneous eruption
of genetically unrelated magmas whose comingling may have caused the
eruption because of rapid vesiculation
of a silicic melt upon
introduction of a hotter, more mafic one (Sparks and others, 1977).
Although it is unusual for any intermediate to silicic magma body with
3
a volume in excess of 10 km
to be homogeneous (Hildreth, 1981), the
compositional variability exhibited by most heterogeneous Deschutes
ignimbrites is best attributed to the comingling of magmas.
Analyses of pumice from 17 heterogenous ignimbrites (Conrey, 1985;
Dill, 1985; Appendix I) reveals that most are composed of two end
member compositions separated by a compositional gap (Fig. 7.18).
some cases the gap is extremely large, such as the combination of
aphyric andesite and rhyolite in the McKenzie Canyon and Hollywood
In
229
rt
ze.. .7
411A
I
.
1.11
?
)0
P
0c
Astqlk,
42
t
4
,
^.
,a
,
"
4,
.11
-
1..44
.s
ler
'
7exi.
.
-
77
-
olt-
-
.cru
1%0 .4.
"4b
!
,
Fig. 7.17. Photos of compositionally 'heterogeneous pyroclastic units.
a) Dacitic (black), rhyodacitic (gray), and banded pumice
lapilli and bombs in the Balanced Rocks ignimbrite member at
b) Zoned airfall lapillistone in upper
The Balanced Rocks.
White lapilli are
Deschutes Formation at Cline Falls.
rhyodacite and black lapilli are basaltic andesite.
230
71111?
CLINE FALLS AIRFALL
PINK IGN1M. IN DEEP CANYON
Awn?
STENSLANDT (1970) UNIT 6 AT STEELHEAD FALLS
MN
:51E:
?maw
MOM?
GRAY IGNIAL AT BASE OF RIVER PLACE SECTION
mit
"TUFF 28" WILL, 19851
71.?
711?
"TUFF 11" (DILL. 1965)
mats?
namiam?
"TUFF 6" (DILL. 19610
mgm
"RC 402 TUFF" (CONREY, 1968)
ins
SIX CREEK IGNIM. MBR.
7610BININNN/AMMVP?
PENINSULA IGNIM. MBR.
111111.11M
FLY CK. IGNIM. MBA.
MUM IMSAMM7a1MMIN
BALANCED ROCKS IGNIM. MBA.
=IMMO
IIRAMINEN
MCKENZIE CANYON IGNIM. MBR.
Ellifil=111=11=1.11=
LOWER BRIDGE IGNIM. MBR.
no?
HOLLYWOOD IGNIM. MBA.
I
7111P
?MAMMON
ROMOUM?
CHINOOK IGNIM. MBA.
52
?MON?
417
JACKSON BUTTES IGNIM. MBA.
50
7
-
I
54
56
58
62
60
64
66
68
70
72
SW2
Fig. 7.18. Diagram illustrating compositional range of selected
Deschutes Formation ignimbrites.
Lighter patterns and
question marks reflect uncertainity of compositional range
because of the limited number of analyses.
231
ignimbrite members and basaltic andesite and rhyodacite in the Fly
Creek ignimbrite member.
It is difficult to conceive how these magmas
could have evolved through crystal fractionation in a single chamber
without producing magmas of intermediate composition.
The sparsely
phyric nature of the more mafic melts likewise suggests that the two
magmas are unrelated by fractionation.
It is also unlikely that the
silicic melts were the result of crustal fusion in the presence of the
hotter, more mafic magmas because such partial melts should be more
silicic and potassic than those generally observed in the Deschutes
Formation.
A few ignimbrites contain multiple pumice populations without
significant compositional gaps and may represent eruption of related
magmas from zoned chambers.
Good examples include the Peninsula
ignimbrite member, which contains mostly homogeneous dacite pumice with
lesser volumes of andesitic and rhyolitic pumice, and the Balanced Rock
ignimbrite member, which contains homogenous pumice over most of the
range from 65 to 70 wt. %.
SiO
Conrey (1985) suggested comingling of
2
separate dacite and rhyodacite magmas to produce the Balanced Rock
ignimbrite member because of prominent banded pumice lapilli and
crystalpoor nature.
However, because of the difficulty in recognizing
a significant compositional gap among analyses of homogenous lapilli,
eruption from a zoned chamber can not be dismissed without study of
trace element distributions.
The Lower Bridge ignimbrite member
exhibits a wide range in composition.
Although rhyolite dominates,
rhyodacite and dacite lapilli also occur, especially in upper flow
units (Canon, 1984).
No compositional gaps exist (Fig. 7.18)
232
suggesting eruption from a zoned magma chamber.
Ignimbrite cooling units involving large compositional gaps (e.g.
McKenzie Canyon, Hollywood, and Fly Creek ignimbrite members) typically
contain only the silicic component in lower flow units and exhibit an
upward increase in the abundance of the mafic component in succeeding
flow units
.
This suggests that eruption began with extrusion of the
silicic magma and was joined by the mafic one whose injection into the
base of the magma chamber may have initiated the eruption.
Density
differences between the disparate magmas would have prohibited
introduction of the relatively dense, more mafic magma into the
eruptive column at the onset of the eruption (Blake, 1981).
Ignimbrites with small compositional gaps and probably, or
possibly, representing partial evacuation of zoned magma chambers,
exhibit pumice of all compositions at all levels with an upward
increase of more mafic components apparent in some units (e.g. Balanced
Rocks ignimbrite member) but lacking in others (e.g. Peninsula
ignimbrite member).
Literature concerning ignimbrites abounds with
descriptions of pyroclastic flows erupted from zoned chambers and whose
deposits are conspicuously zoned themselves and record the zonation of
the chamber in inverted form with the top of the chamber erupted first
and thus forming the bottom of the ignimbrite (see Hildreth, 1981, for
a review).
Such welldeveloped zonation of ignimbrite cooling units is
probably restricted to largevolume eruptions along ring fractures
which allow for simultaneous tapping of a large crosssectional area of
the magma chamber.
In centralvent eruptions the conduit diameter may
be much smaller than the magma chamber diameter and evacuation of the
233
chamber can be envisioned as sequential eruption of concentricshell
volumes of magma over a large depth range within the chamber (Blake,
1981; Spera, 1984).
Therefore, resulting ignimbrites, although
compositionally heterogenous, exhibit little or no compositional
zoning.
Although compositionally heterogenous ignimbrites are common in
the Deschutes Formation, only one heterogenous airfall lapillistone
known.
is
The rarity of heterogeneous air falls is probably a reflection
of the relatively volatilepoor nature of intermediate magmas, as
evidenced by the near absence of hydrous minerals, prohibiting
development of highstanding Plinian eruption plumes capable of
The
producing a preservable airfall record in the Deschutes basin.
one known heterogenous lapillistone is prominently exposed along state
route 126 and an adjacent side road just west of the Deschutes River at
Cline Falls (Fig. 7.17b).
Fifty centimeters of white rhyodacite
pumice (69% SiO ) is overlain by 20 cm of aphyric, Fe and Tirich
2
basaltic andesite lapilli (57% SiO
,
2
2.04% TiO
,
Fe' = 0.60).
Banded
2
black and white lapilli occur in an 8 cmthick zone along the sharp
interface between homogenous black and white lapilli.
RELATIONSHIP OF DESCHUTES MAGMATISM TO THE HIGH CASCADE GRABEN
Synthesis of the preceeding discussion of Deschutes Formation
volcanic rocks permits interpretation of the nature of volcanism in the
early High Cascades and its relationship to the formation of the
central Oregon High Cascade graben.
The volcanic record in the Deschutes Formation provides a
different view of early High Cascade volcanism than that expressed by
234
investigators working on contemporary rocks in the Western Cascades
(Priest and others, 1983).
Although the volume of erupted basaltic
andesite, and particularly basalt, was undoubtedly larger than in
previous volcanic episodes in the central Oregon Cascades,
volume of more silicic magma was also erupted.
a large
Dacitic to rhyolitic
ignimbrites are widely distributed in the Deschutes Formation but are
uncommon among contemporary rocks exposed along Western Cascade ridge
crests. -The ignimbrites require a lowrelief setting for preservation.
Such a setting occurred in the central Deschutes basin but apparently
was lacking in the Western Cascades or else the ignimbrite record there
has been lost because of subsequent uplift and incision.
It is noteworthy to speculate on how the early High Cascade
episode would be interpreted if only the exposures on Green Ridge or
those along the Deschutes River were available for study.
The Green
Ridge exposures, like their counterparts in the Western Cascades, would
suggest a period of basaltic andesite and basalt volcanism with minor
pyroclastic eruptions of more silicic magmas.
The Deschutes canyon
exposures would bias the observer to interpret a period of bimodal
volcanism involving eruption of modestvolume dacitic to rhyolitic
ignimbrites and smaller basalt flows.
reflection.
Neither is an accurate
Combined observations of Green Ridge and the central
Deschutes basin provide a more accurate picture but is still not
complete because of the lack of exposure of the most proximal rocks
which may include a large volume of intermediate and silicic lavas.
The spatial and temporal distribution of volcanics within the
Deschutes Formation offers important insight into the location of
235
active eruptive centers and subsidence history of the graben.
Lavas
and -pyroclastic flows extended eastward and northeastward into the
Deschutes basin from the latitude of Green Ridge and also followed a
northeastinclined
paleoslope into the Deschutes basin from a.
postulated highland west of Bend.
Volcanism in the Cascades north of
Green Ridge was apparently less intense because no lava flows and few
ignimbrites entered the Deschutes basin from this region.
The top of
the Deschutes Formation section is dominated by mafic lavas and the
most voluminous ignimbrites, though still modest in size compared with
Basin and Range calderarelated units (Smith, 1979), are located in the
lower half of the section.
Therefore, the fault scarps at Green Ridge
are not part of a large caldera structure.
The lack of ignimbrites at the top of the Deschutes section, but
abundance of thick airfall lapillistones is best explained by early
occurrence of graben subsidence west of Green Ridge which prohibited
subsequent pyroclastic flows from entering the Deschutes basin. Because
the High Cascades south of Green Ridge were also a primary source of
pyroclastic flows to the Deschutes basin, structural isolation of the
High Cascade axis must also have occurred there.
Following initial
subsidence, basalt and basaltic andesite lavas were erupted between
Green Ridge and the original fault scarps and continued to flow into
the basin while more silicic magmas were erupted within the early
graben, along the volcanic axis.
The silicic magmas may have produced
a "shadow zone" that inhibited the passage of denser, mafic magmas to
the surface near the center of the High Cascades resulting in a
peripheral field of mafic magmatism (Hildreth, 1981).
Faulting
236
subsequently stepped eastward to Green Ridge and truncated these mafic
sequences.
Intrabasin volcanism developed late in Deschutes Formation time
and postdated the initial graben faulting west of Green Ridge.
The
intrabasin basalts and basaltic andesite may also have used structural
pathways opened during continued extension.
Fissures which fed the
eruptive products of the Steamboat Rock member coincide with a zone of
steep residual gravity gradients possibly reflecting a major basement
fault (compare Figs. 5.11 and 6.25).
The compositional character of Deschutes Formation volcanic rocks
is atypical of convergentmargin volcanism.
Cascadederived
diktytaxitic basalts are notably enriched in TiO , Ni, and Ba, and
2
depleted in K relative to islandarc tholeiites but closely resemble
contemporary high alumina olivine tholeiite and lowK, lowTi
transitional tholeiite of southeastern Oregon.
Deschutes Formation and
younger Cascade diktytaxitic basalts in the northern Deschutes basin,
although remarkably similar to the northwestern Basin and Range lavas,
differ from contemporaneous rocks analyzed in the Western Cascades and
those comprising the High Cascade mafic platform which are more
enriched in K, Sr, and Ba.
The similarity of the Deschutes basin and
Basin and Range basalts suggest derivation from a similar mantle
source, severely depleted in K and Rb, and evolution by similar
petrogenetic processes.
It is not clear why similar low K,
Sr, and Ba
basalts have not yet been recognized in contemporaneous basalts on the
west side of the High Cascades or in the mafic platform.
The
contents of all High Cascade diktytaxitic basalts
relatively large TiO
2
237
suggests a mantle source where a titaniferous phase is not stabilized
in the mantle as is thought to be typical of convergentmargin arcs
(Green, 1980) even though texture and highalumina character suggest a
hydrous source region.
Interestingly, Deschutes basin basalts erupted along the eastern
and southeastern basin margins bear no resemblance to the high alumina
The higher alkali element,
tholeiites of the adjacent Great Basin.
lower alumina, and variable incompatibleelement contents of the basin
margin basalts may reflect derivation from deeper depths beneath
thicker crust of the Blue Mountains province and contamination by
crustal material.
Intrabasinal lavas portrj compositional traits
intermediate between High Cascade basalts and the basinmargin lavas.
Basaltic andesites and andesites of the Deschutes Formation are
and FeO compared to other convergentmargin
greatly enriched in TiO
2
intermediate volcanics.
Preliminary interpretations, based on bulk
rock, majorelement analyse
and petrographic observations suggest that
approached or surpassed 56
augite was not on the liquidus until SiO
2
wt. 74 titaniferous oxides were not a prominent fractionating phase
reached 57 wt. %; and hornblende is rare in
until SiO
andesites and
2
not common in more silicic rocks.
This petrologic character is
consistent with, although not unambiguous evidence for, primarily low
pressure fractionation of plagioclase + olivine followed by plagioclase
+ olivine + augite (Grove and Baker, 1984).
The implied dominance of
lowpressure fractionation in the development of Deschutes Formation
volcanics suggest an extensional environment that allowed magmas to
rise to high crustal levels.
In more typical convergentmargin arcs
238
magmas fractionate deep in the crust or at the crustmantle interface
where augite is an early liquidus phase (Grove and Baker, 1984) and
allow magnetite and hornblende fractionation (Kay
higher PH 0 and fo
2
2
and others, 1981) leading to more typical calcalkaline rocks.
The
tholeiitic character of Deschutes volcanism is a reflection of
extension which culminated in the development of the High Cascade
graben.
Basaltic andesites and andesites were not derived by fractionation
from diktytaxitic basalts.
The intermediate magmas may be related by
fractionation to porphyritic basalts which also appear unrelated to the
diktytaxitic basalts.
It is interesting to note that a volcanic suite grossly similar to
early High Cascade volcanics occurs in middle Miocene rocks of the
southern California borderland.
These rocks (variously known as the
Conejo Volcanics, Glendora Volcanics, Santa Cruz Island Volcanics, and
Catalina Island Volcanics), ranging in composition from basalt to rhyolite, contain abundant basaltic andesite and andesite flows which
exhibit iron and titanium enrichment outside the realm of typical con-
vergentmargin volcanics (Crowe and others, 1976; Higgins,
and others, 1982).
contents of basalts, as well
The low K 0 and TiO
2
1976; Hurst
2
as trace element abundances, show some affinities to MORB but the
voluminous intermediate and more evolved compositions are incompatible
with an oceanic setting.
Hurst and others (1982) suggest that the
Miocene borderland volcanics represent modified midocean ridge
magmatism associated with interaction of the Farallon Ridge with the
subduction system into which it was ultimately consumed.
Primary melts
239
possibly were generated by shallow, partial melting of oceanic mantle
but were then trapped in the crust because of ridge
interactions (Hurst and others, 1982).
trench
transform
High Cascade magmatism may
represent the same general style of evolution
basalts tapped in an
extensional regime in a selectively depleted mantle and evolving within
continental, or thickened oceanic, crust.
Faulting and extension within the central Oregon High Cascades had
other influences on magmatism other than their probable role in
determining fractionation assemblages.
and TiO
Primitive basalts and high FeO
andesite magmas were likely too dense to pass through the
2
Cascade crust.
magmas.
However, faults probably provided pathways for these
Faulting also enhanced the potential for magmas evolving at
different levels in the crust or upper mantle to come in contact with
each other.
Comingling of unrelated magmas is widely recorded by
Deschutes Formation ignimbrites and probably triggered many of the
ignimbriteforming eruptions.
Whether such chance collisions of
different magmas resulted in the production of hybrid magmas requires
further testing by microprobe and traceelement analyses but may have
been a major influence on the development of early High Cascade magmas
(Conrey, 1985).
240
CHAPTER 8: SEDIMENTARY GEOLOGY OF THE DESCHUTES FORMATION
FACIES AND FACIES ASSOCIATIONS
Fades of the Deschutes Formation
Miall (1977, 1978) and Rust (1978b) introduced a set of facies
codes for evaluating braidedriver deposits, based on descriptive
classification of lithologies and sedimentary structures,
that is
applicable to most fluvial deposits, and has received wide usage in
sedimentology literature.
Table 8.1 lists Miall's (1978) facies codes
appropriate for the Deschutes Formation and those of Mathisen and
Vondra (1983) for describing pyroclastic rocks.
The descriptions for
some fades have been modified and several additional facies codes are
introduced to describe facies in the Deschutes Formation which have not
previously been identified with codes.
Three of the additional fades codes described below (Gm(a),
Sm(g), Sh(b)) are introduced to describe deposits resulting from hyper
concentrated flood flow.
Hyperconcentrated flood flows are highdis-
charge events intermediate in sediment/water ratio and flow character
between debris flows and normal, usually dilute, stream flows and
produce deposits which are distinct from those resulting from the end
member processes (Smith, in press).
The term is modified from Beverage
and Culbertson (1964) who suggested, on the basis of empirical data,
that flows with 40 to 80 weight percent suspended sediment are intermediate in their sediment transport mechanics between debris flow and
normal stream flow.
Turbulence serves as the dominant sediment support
mechanism with important contributions from grain interactions and
buoyancy because of high sediment concentration (Smith, in press).
241
TABLE 8.1. FACIES NOMENCLATURE FOR THE DESCHUTES FORMATION
Facies Identifier
Gm
(1)
Interpretation
Sedimentary Structures
lithofacies
(General identifier for missive, clast-support gravel; divided into two facies in the
Deschutes Formation.)
Gm/b)
gravel, massive or crudely
bedded, minor sand lenses;
claSt-support, relatively
well-sorted; rounded clasts
gravel imbricated dominantly traction deposition; longon b-axis (i.e. a-axis trans- itudinal bars, channel lag.
verse to flow direction.
Gm(*)
gravel, massive or crudely
bedded; abundant sand matrix,
largely clast-support; poorly sorted, subangular to
round chats.
gravel clasts oriented with
both a and b axes transverse to flow direction;
poor imbrication; may be
normally graded.
coarse-grained hyperconcenCrated flood flow deposits;
rapid deposition both froM
suspension and by traction.
gravel, stratified
trough cross beds
channel fill
G.
gravel, stratified
planar tabular cross beding. solitary or gripped
straight crested transverse
GI
gravel, stratified
low angle (5-200) inclined
stratification; sets to
4m thick
lateral accretion surfaces
gravel, massive, matrix
support, very poorly
sorted; chants may be
angular to rounded.
possible reverse grading
throughout or only at base;
Possible coarse-tail normal
grading, especially in upper
Portion of deposit
debris
sand, medium to very
coarse, may be pebbly
trough cross beds, single
or grouped
sinuous crested dunes
sand, medium to very
coarse, nay be pebbly
planar tabular cross beds,
single or grouped
straight crested bars (sand
waves, transverse bars) and
linguoid bars
sand, fine to coarse
low angle ( .10°) cross
beds
scour fills and antidunes (?)
Gt
(1)
(1)
1'27
St
(1)
SO
(1)
(1.2)
Sr
01
St
(1)
se
(1)
SAW
Sm(S
.SmiP)
(I)
Fm
sand, very fine to
medium grained
sand, fine to coarse.
may be pebbly
Te.t.e
(3)
flow
ripples (bar-toP. Secondary
channel, flood plain, lacustrine)
-
broad shallow scours,
including eta cross
stratification
scour fills
(This identifier is used for horizontal lamination and bedding. This encompasses a
variety of structures and is divided into two facies in the Deschutes Formation)
sand, fine to medium
grained
thin, parallel strata (up
to 0.5cm thick); possible
parting lineation
sand, fine to very coarse
grained, may be pebbly
parallel strata (0.5 to
hyperconcentrated flood flow;
5cm thick), laterally discon- possibly produced by low-amplitude.
tinuous over 1-5m; gradation- long wavelength dunes
al contacts between coarse
and fine strata
pebbly sand, medium to
very coarse grained
massive, normally graded.
poorly sorted; usually 0.5
to 2m thick
hyperconcentrated flood flow;
rapid deposition from suspension,
analogous to Gm(a)
sand, fine to very coarse,
may be pebbly; several
Percent Silt and clay;
usually oxidized
massive or patches of stratification; evidence of pedogenesis: burrows, rootlets,
clay cutans on sand grains
paleosols
sand (very fine to fine),
silt, clay, interbedded
ripple marks, plane lamination, convolute bedding,
burrows, plant rootlets,
leaf impressions
deposits of waning floods, overbank
deposits, restricted ephemeral lakes
formed in shallow, upper flow
regime, possibly by processes in
the turbulent boundary layer
silt. clay
generally massive, rootlets
abundant, dessication cracks
mud drapes
diatomite
massive or thin horizontal
bedding
lacustrine, low clastic
ash or lapilli; well sorted
angular grains; rare lithic
grains (accidental ejecta)
massive or horizontal bedding airfall pyroclastics
may show evidence of burrowing or root disturbance; may
be inverse, normal graded, or
(1)
0
ripple marks and ripple
cross-lamination of all
types
bars
flout
both
Tr,Lr,
ash or lapilli, poor to
(3)
well sorted, rounded grains;
abundant lithic grains
References:
(I): adapted from Mall (1977,
adapted from Rust (1978)
adapted from Mathison and
massive Or stratified
cross strittified
1978)
Vondra (1983)
y be reworked airfall pyroclastics
242
Hyperconcentrated floodflow deposits are distinguished from debris
flow deposits by absence of features indicative of mass deposition,
such as matrix support and lack of stratification, and, instead, exhibit clast support, normal grading, and horizontal stratification.
Hyperconcentrated floodflow deposits are distinct from normal stream
flow deposits because of their lack of crossstratification or recognizable bar morphologies. Hyperconcentrated floodflow deposits are
abundant in the Deschutes Formation and their widespread
occurrence in
other modern and ancient nonmarine volcaniclastic deposits in the
Pacific Northwest suggests that hyperconcentrated flood flow is an
important sedimentation process in volcanic regions (Smith, in press).
Miall's (1977,1978) massive, clastsupport gravel facies is
divided into two facies on the basis of sorting, grading,
framework/matrix relationship, relationship to sand facies, and fabric.
Fabric provides a'convenient means of defining facies codes for the two
types of massive clastsupport gravel: Gm(b) gravels exhibit a dominant
baxis paralleltoflow fabric; Gm(a) gravels exhibit a prominent a
axis paralleltoflow fabric, especially in small clasts, in addition
to atransvers orientations.
Traction deposition of gravel produces a fabric in which clasts
show a strong preference for orientation of the
flow direction (Rust, 1972a; Walker, 1975b).
b axis parallel to
Clasts transported in a
dispersion above the bed, and rapidly deposited without significant
traction reworking, tend to be oriented with the a axis parallel to
flow direction (Walker, 1975b).
Therefore, channellag and
longitudinalbar gravels show a strongly developed bparalleltoflow
243
Fig. 8.1. Comparative examples of clastsupport fades Gm(b), on left,
Note close packing of cobbles, sand
and Gm(a), on right.
matrix, and high degree of rounding in streamflow
conglomerate (Gm(b)) and very poor sorting and more angular
clasts in hyperconcentrated floodflow conglomerate (Gm(a)).
244
fabric (Rust, 1972a), as represented in fades Gm(b).
Hyperconcen-
trated floodflow gravels, facies Gm(a), usually exhibit both b-
paralleltoflow, for large cobbles and boulders, and aparalleltoflow, for pebbles and small cobbles, representing deposition from flow
that was competent to transport pebbles and small cobbles above the bed
while rolling large clasts along the bed (Smith, in press).
Although fabric provides a convenient means of defining facies
.
codes, other features serve to distinguish these two gravel fades as
well.
Gm(b) gravel is generally wellimbricated, and exhibits a clast-
support framework in which intervening space is void, or filled by
finergrained
sediment that infiltrated the gravel following its depo-
sition to produce a bimodal grain size distribution (Fig. 8.1a).
Gm(a)
gravel is very poorly sorted, poorly imbricated or nonimbricated, and
frequently exhibits distribution normal grading.
wide range of grain sizes results in
Rapid deposition of a
clast support gravel in which the
space between framework cobbles and boulders is occupied by a poorly
sorted, very coarse sand to pebble matrix, that is too coarse to represent infiltration between the larger clasts (Fig. 8.1b).
Lenses of
stratified sand are common in Gm(b) gravels but are absent in Gm(a)
gravels, though the latter frequently grade upward into horizontally
stratified sand.
Facies code Gi is introduced to describe gravel characterized by
low angle (5 to 20
)
inclined stratification in sets up to 4 m thick.
Sets have erosional bases and are tabular or lenticular in shape with
concaveup bases.
This fades has also been described in Holocene
gravels where it is interpreted to represent lateral accretion of
245
gravel point bars (On, 1982; Arche, 1983).
The fades code Sh is a general identifier for horizontal lamination and bedding.
The origin of horizontal lamination and bedding in
sand is not well understood and a variety of structures, of probable
different origins,
have been referred to by the terms horizontal,
flat, parallel, plane, or planar lamination and bedding (Allen, 1984).
Two types of horizontal stratification
are recognized in the Deschutes
Formation.
The code Sh(1) is used here to describe thin (<0.5cm), parallel
laminae of fine to mediumgrained sandstone frequently associated with
parting lineation (Fig. 8.2a).
formed
This type of stratification is probably
in the turbulent boundary layer of the upperflow regime
(Allen, 1984).
More common in the Deschutes Formation are parallel beds of
medium to very coarsegrained sandstone up to 5 cm thick that are
laterally continuous for only 1 to 5 m and have gradational interstratal contacts (Fig. 8.2 b,c).
This structure is assigned the facies code
Sh(b), the "b" emphasizing beds as opposed to lamination.
These strata
are composed of sediment that generally is too coarse grained (Bridge,
1978) and individual strata too thick (Allen, 1984) to represent turbulent boundary layer processes.
Smith (in press) presents several
arguments that suggest that this structure results from rapid deposition by hyperconcentrated flood flow.
These include, 1) common
gradational contacts of Sh(b) with other hyperconcentrated floodflow
and debrisflow facies (Gm(a), Sm(g), Gms,) 2) common occurrence of
isolated, outsized clasts up to 1 m or more across (Fig. 8.2c),
246
r
7:=1
-.B.
"
701
..
izr-rt:
t
-=
;
.''
-
-
-
, or
6,-;-1^4"
As
AIKC"
..,
1
%.*
-
MI
'1.1.
'b
°
.16
.vr-44::.vi.,.`
-
Fig. 8.2. Horizontal stratification in Deschutes Formation sandstones..
Horizontal lamination (Sh(1)) in mediumgrained sandstone.
and c) Horizontal bedding (Sh(b)) in coarsegrained,
pebbly sandstone.
Note discontinuity of strata, indistinct
interstratal contacts, and outsized clasts, in b) and c),
characteristic of hyperconcentrated floodflow deposits.
247
3) similarity in grain size, strata] thickness, and gradational strata]
contacts to horizontallystratified sand associated with subaqueous,
resedimented conglomerate (Walker, 1975a), 4) formation of similar
strata during known (Pierson and Scott,
in press) or independently
inferred (Smith and Smith, 1983) hyperconcentrated flood flows near
Mount St. Helens, Washington, and 5) flume (Simons and others, 1963)
and field (Bradley and Graham, in press) observations indicating that
high suspended sediment load alone can prohibit formation of bedforms
that produce crossstratification.
Rare observation of lowangle
stratification within these horizontal beds and similarity in size and
geometry to strata produced in flume experiments by Einstein and Chien
(1953) led Smith (in press) to suggest that such stratification is
produced by the migration of lowamplitude, longwavelength, dunelike
bedforms which result from transport of a wide range of sediment sizes
combined with a continuous and high rate of deposition (Einstein and
Chien, 1953).
Structureless sandstones are either the result of rapid deposition
or
postdepositional modification by pedogenic processes.
Facies
Sm(g) represents normally graded, pebbly, massive, coarsegrained sandstone in units 0.5 to 2 m that frequently grades upward into Sh(b).
Poor sorting, normal grading, and common association with Sh(b) suggest
that Sm(g) results from hyperconcentrated flood flow and is the finer
grained equivalent of facies GM(a).
Facies Sm(p) (Fig. 8.3) is recog-
nized by pedogenic features, including 1) oxidation and formation of
hematite and clay rims on lithic and mafic mineral grains (Fig. 8.25b)
which is rarely observed in other Deschutes sandstones, 2) burrow and
248
Fig. 8.3. Massive paleosol sandstones (fades Sm(p)) in the Deschutes
Formation,
a) fine to mediumgrained sandstone with silcapermineralized root traces. b) Massive pebbly sandstone
with remnant patches of stratification.
-
Fig. 8.4. Primary (a) and reworked (b) pumice lapillistones.
Note
angularity of lapilli, crude horizontal stratification, and
accidental lithic grains in primary lapillistone (facies La).
Reworked deposit (Lr) is crossbedded, includes lenses of
volcanic sandstone, and occurs between units of crossbedded,
pebbly sandstone.
249
root traces (pedotubules), 3) isolated patches of remnant stratifi-
cation, and 4) downward gradation into wellstructured, unoxidized
sandstone.
In the Deschutes Formation these massive sandstones,
resulting from pedogenesis of other sandy facies, usually lack distinct
soil horizons and are claypoor suggesting that they represent immature
soils developed in welldrained areas in a dry environment.
Airfall pyroclastic debris occurs either as primary, unreworked
deposits or in units which have been reworked but not extensively mixed
with other sediment.
The fades codes Ta and La are used for primary
airfall tuffs and lapillistones, respectively.
The codes Tr andLr
refer to reworked airfall tuffs and lapillistones.
These codes
assignments are modified, and expanded to include coarser fragments,
from Mathisen and Vondra (1983) who proposed Ts for stratified airfall
tuff and Tr for reworked tuff.
Primary airfall deposits are not
always stratified and so the "s" is dropped in favor of "a", for air
fall textures.
Primary airfall facies contain angular lapilli and
shards, may be crudely stratified but not crossstratified, and often
contain several percent angular dense, lithic fragments representing
accessory or accidental ejecta (Fig. 8.4a).
Air falls reworked by
fluvial or eolian processes are often composed almost entirely of
pumice lapilli and/or ash shards but these fragments are rounded,
reflecting traction transport.
Reworked pyroclastic facies are often
crossbedded and usually contain several percent rounded, dense, lithic
fragments which are too large to be aerodynamicallty equivalent to the
pyroclasts (Fig. 8.4b).
250
Facies Associations
Facies are indicative of processes which transport and deposit
sediment.
Specific depositional settings tend to be the site of
specific sedimentary processes and, therefore, are represented by
characteristic assemblages of facies. The key to defining depositional
settings is to recognize these facies associations.
Based on relative
abundance of facies in vertical sequences, five fades associations can
be defined in the Deschutes Formation (Table 8.2).
Facies Association 1: Fluvial Channel Deposits
Massive or crudely stratified, clastsupported conglomerate
(Gm(b)) with lenses and/or thin beds of stratified, medium to coarse
grained sandstone (St, S1) is a prominent facies association in the
Deschutes Formation (Fig. 8.5a).
Other facies occassionally found in
this association are Sp, Sh(1), Gi, and Gt.
The conglomerates consist
of wellrounded and imbricated pebbles, cobbles, and boulders.
The prominence of imbricated conglomerates and crossstratified
sandstones indicates traction deposition in fluvial channels.
Massive
and crudely stratified conglomerate are probably longitudinalbar
deposits and associated sandstone lenses represent waning flow deposition on bar tops and margins, or in channels abandoned following
periods of high discharge (Doeglas, 1962; Rust, 1972b; Miall, 1977).
Fades Association 2: Floodplain Deposits
The fluvialchannel facies association is sometimes interbedded
with sequences of massive or ripplelaminated, very finegrained sandstone and siltstone (Fm, Fl), ripplelaminated and trough crossbedded
fine to coarsegrained sandstone (Sr, St), and planelaminated
251
TABLE 8.2. FACIES ASSOCIATIONS
Fades
Dominant
Association
Fades
Fades
Gm(b), Sl,
Sh(1), Sp,
Gi, Gt
Basaltscommon Fluvialchannel
deposits
Ignims.rare
Sr, St,
Sh(1),
Sm(p).
D, La, Lr
Basaltscommon
Ignims.common
Floodplain
deposits
Basaltscommon
Fl, Gms Ignims.common
Sheetflood
deposits
FA1
St
FA2
Fm,
FA3
Fl
Si, Ss,
Sh(b), Gm(b)
Minor
Sp, Sm(p)
Fm,
Sm(p), St
Gm(a), Gms
Sm(g), Sh(b) Sp, Ss,
Gm(b)
FA4
Intercalated
Volcanic Units
Basaltscommon
Ignims.common
Interpretation
Debrisflow &
Hyperconcen-
trated flood
flow deposits
FA5
a
Sm(p), La
(laterally
continuous)
Sm(P), La
(laterally
discontinuous)
Ta, Sh(1),
Sh(b), Fm
Basaltsrare
Ignims.common
Ta, Gm(b),
St, Sh(1),
Sh(b)
Basaltsrare
to common
Ignims.rare
Interfluve
area
Broad region
w/low sedimentation rate;
isolated from
pyroclastic
flows
-
252
fp.;
4-
-ia'
v7;-(47.:
"-p44-
,e4-
"-:pr '7
-"
%..-fi
--Of.
-
4r;
tr-
,A5.-
,- .,,4,- 1,ritse,:g..
,.",..apt'-:
r
3,
,
tcf . v * * c ',"
)
I..-- if, -Y
,r
;',APr 'i. ,...-V"
! -ifEr ."
-
r' .,-.
-,-.
,a':
..'..: ,,,,e'7...- .0e,
4,''
.
,,,
.
r.i i-i'...J.,,,,,...."..i...4.-
7.1,,,..--
.4i
1 E.
-ZE-iiri..,-, -
.10,.
.FA!es'-'":
,4,-
-
.
'
,-,
-
..-
-
-Y
- - w- a.
-
%.,,,,*:-.
-,7.- Y
b,70171*L4
.., -
vc-
air a
t'
, ..----..---4'
'',.33*.t.
..f:-.-1, - , ..- 4
-
--4 r;
+f_11,t1W-11,;',.,!,4-'-j-40..04
r.,:. e
4.4-f-.
,
_er"--
r-
-..
7:41;r
,
7-*
iflAi
,
P
..
...,..
,;(1-..4rxe
3.
/-
'
3.;
'
.G-7
:tify4rm
k
Awm-
9
If]
Fig. 8.5. Facies associations 1 and 2 exposed in roadcuts in Cove
Palisades State Park. a) Facies association 1: Clastsupport
cobble conglomerate with lenses of lowangle, crossbedded
b) Facies association 2: Massive, finegrained
sandstone and siltstone with lenses of crossstratified
sandstone.
mediumgrained sandstone.
association
1
Overlying conglomerate of facies
visible at upper right corner.
253
fine to mediumgrained sandstone (Fig. 8.5b).
Siltstones and most
sandstones contain abundant root traces and burrow molds and grade
upward into oxidized paleosols.
Both reworked and primary airfall
lapillistones also occur in this facies association.
Impressions of leaf and stem fragments are common in the fine
grained facies along with rodent bone fragments and
some of which were buried in growth position.
molds of logs,
Fossil floras are
generally suggestive of streamside vegetation in semiarid environments
and include the genuses Platanus, Quercus, Populus, Salix, Acer, and
Typha (Chaney, 1938; Ashwill, 1983, and personal communication).
Massive diatomite and laminated diatomaceous mudstone occur in
this facies association and are generally less than
1
m thick. The
diatom assemblages suggest shallow, fresh to slightly saline, slightly
alkaline to slightly acidic, lakes and ponds (J.
P. Bradbury, U.
Geological Survey, person. commun., 1983; Appendix VIII).
S.
Occurrence
of leaf fragments, chryophycean cysts and phytoliths in some diatomites
indicates marsh settings.
Facies association 2 occurs in sequences 4 to 10 m thick that may
be laterally continuous for a kilometer or more and interfinger with
conglomerates of facies association 1.
The occurrence of finegrained
lithologies, abundant fossil remains, extensive bioturbation, and shal-
low lacustrine fades all suggest that this facies association represents deposition in floodplain environments.
Facies Association 3: Sheetflood Deposits
Thinly interbedded sandstone and conglomerate composed of parallel, lenticular strata,
5 to 25 cm thick, separated by lowangle
254
erosion surfaces exhibiting less than 10 cm of relief,
facies association (Fig. 8.6a).
is a common
Sequences of such strata may be 10 m
or more thick and extend laterally, transverse to dispersal direction
indicated by paleocurrent data, for more than 100 m.
The thin bedding
and lateral extent of bed sets exaggerates the sheetlike nature of the
beds, especially when viewed from a distance (Fig. 8.6b).
However,
close examination indicates that individual strata rarely extend more
than 1-2 m (Figs. 8.6 a,c) and are lenticular in form, albeit with
length/thickness ratios often exceeding 10 to 1.
Strata are poorly
sorted and sandstone is more voluminous than pebble conglomerate
(Gm(b)) in most sequences. Sandstone is pebbly, medium to very coarse
grained, and represented by horizontal lamination (Sh(1)), lowangle
stratification (Si) laterally transitional to Sh(1), scourfill cross
bedding (Ss), and rare planartabular crossbedding
10 cm high (Fig. 8.6c).
(Sp) in sets up to
Massive or laminated mudstone (Fm, Fl) in
layers up to 5 cm thick, or pedogenically altered sandstone (Sm(p)),
often cap these sequences.
The thinbedded nature, lateral extent, dominance of upperflow
regime structures, low relief of scours, and low height of cross
stratification suggest that these deposits were produced by shallow
flow in broad, braided channels.
Migration of channels and bars pro-
duced the complex, lenticular interbedding of sandstone and conglomerate.
This facies association resembles proglacial sandur deposits
resulting from shallow, supercritical flow during periods of considerable discharge over broad areas (Church, 1972; Ruegg, 1977; HoumarkNielson, 1983), and
alluvialfan sheetflood deposits pictured and
255
described by Bull (1972), on modern fans, and by Gloppen and Steel
(1981), for Devonian fans.
Although the Deschutes Formation examples
probably were broadly confined, the term sheetflood seems most
appropriate for describing the process which produced these facies.
Fades Association 4: DebrisFlow and Hyperconcentrated FloodFlow
Deposits
The thickness and texture of debrisflow and hyperconcentrated
floodflow facies in the Deschutes Formation is quite variable.
Most
debrisflow deposits are less than 5 m thick with maximum clast size
between 10 cm and 3 m, and
and sand (Fig. 8.7a).
clasts are supported in a matrix of silt
Clay is present in only trace quantities as is
typical of unaltered volcanic debrisflow deposits (Fisher and
Schmincke, 1984; Smith, in press).
Most hyperconcentrated floodflow
deposits are 1 to 10 m thick and represented by facies Sh(b), Gm(a), or
Sm(g) alone, or more commonly, in vertical sequences of Gm(a) or Sm(g)
grading upward into Sh(b) (Fig. 8.7b).
which can be traced for up to 100 m.
These facies often form sheets
In other cases the deposits are
lenticular and separated from similar or other facies by erosion
surfaces.
Thin, crossbedded sandstones (St. Sp) or clastsupport boulder
lags (Gm(b)) often occur within sequences of debrisflow and
hyperconcentrated floodflow fades and represent reworking of these
deposits by normal stream flow.
Some gradedstratified hyperconcen-
trated floodflow deposits grade upward into 0.5 to 1.0 m thick cross
bedded sandstone (St. Ss) enriched in pumice lapilli relative to the
lower part of the unit.
This thin, crossbedded zone with low density
256
irgi
a*.
.
Cat
r
Fp,,t
*
,smma
7
;
1,?fr::-
,-...,....
_
,-A-Iosf7.*,*;..-Lzrkv,;AmNtt:.<-[
_--
-
-
.......... 7
I.
7
Am
-,r3-174-1
-
.
_
--As
N:e
--
^.1
rOP
..7-11r4M71
r
4t
kV.
°3P....../ri"
3
-
-..0
'
. -..e -
8.
.
'r
1614
7
-
F.
Fig. 8.6. Typical outcrops of sheetflood facies association in theDeschutes canyon opposite the mouth of Squaw Creek. a) Thin,
interstratified lenses of sandstone and sandy conglomerate
with prominent scourfill crossbedding.
b) Three sheetflood
sequences and associated hyperconcentrated floodflow
deposits (Sh(b)), debrisflow deposits (Gms), basalts (B) and
an ignimbrite (I). Note the typical lenticular nature of
bedding in outcrop in foreground (hammer for scale) and great
lateral extent of sheetflood sequences (outcrop is approximately perpendicular to paleoslope).
c) Thin, eroded debris
flow deposit (Gms) intercalated with sheetflood facies.
257
_. - '
-
'41 '
' 4.71947.- ..'4 ' .
4 4:
'
,..
.,
-
-
.
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.7,
.
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.
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"Ow
-
1187,
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.
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e.
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,.,
.,
4.
511e:
74
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,
. 11141.'4 Itt 74. :? '
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.4.4..4.14. .
4- ,
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,,...,47111..
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- , rs-:
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,C.,
........-
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40.1%-'; -
-arn
4411.1.
411-7'.0.,
:41E
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r: -;"4,4,A
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4,
... ,..,
I,' 7, "c- -.- - i -.
0 ..-
.16.'111r
:4 i
[
4
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I
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AS,
,
r
.1041:;;LA,
A. XV, ;*
'
"d!
Sh OA
-7--
!'/:
%-4 47!?
Fig. 8.7.
Examples of fades association 4. a) Massive, ungraded debrisflow deposit in lower Street Creek canyon.
b) Graded-stratihyperconcentrated flood-flow deposit (Gm(a) to Sh(b)) overlain by sheetflood facies association (SF).
Outcrop in Squaw
c) Debris-flow deposit
Creek canyon near Alder Springs.
(Gms) transitional at the base to stratified hyperconcentrated flood-flow deposit (Sh(b)) produced by dilution of the
debris flow.
Exposure in Deschutes canyon opposite the mouth
of Squaw Creek.
258
grains may reflect deposition by dilute, waning flow at the end of the
flood event.
Paleosols (Sm(p)) are common on top of both debris-flow
and hyperconcentrated flood-flow fades.
Debris-flow and hyperconcentrated flood-flow facies occur throughout the Deschutes Formation as single depositional units or in multistory sequences up to 25 m thick.
Many debris-flow deposits are tran-
sitional at the base to a finer-grained, clast-support, faintly-stratified hypercOncentrated flood-flow deposit up to 20 cm thick (Fig.
8.7c).
This vertical sequence records the dilution of the front of a
debris flow by stream water to produce hyperconcentrated flood flow,
the resulting deposit of which was immediately over-run by the yet
undiluted portion of the debris flow.
This dilution-transformation
process may be the primary mechanism of producing hyperconcentrated
flood flows (Smith, in press).
The genetic relationship between these
two processes is also suggested by proximal-to-distal changes in which
debris-flow deposits are at least twice as abundant as hyperconcentrated flood-flow deposits in sections within 20 km of Green Ridge but
are subordinate to hyperconcentrated flood-flow deposits farther east.
Most debris-flow deposits that do occur beyond 20 km from Green Ridge
are associated with an underlying hyperconcentrated flood-flow deposit.
Several debris flows and hyperconcentrated flood flows may be
generated within a short period of time to produce complicated fades
sequences.
The sequence pictured in Figure 8.8a suggests that two
debris flows, or pulses within a single debris-flow event, moved
through the same channel.
The first flow veneered the channel wall
with a thin, matrix-support bed and related hyperconcentrated flood-
259
Fig. 8.8. Complex vertical sequences of debrisflow and hyperconcentrated floodflow fades, a) Channelized debrisflow deposit.
emplaced against earlier debrisflow and associated hyper
concentrated floodflow deposits which formed a veneer on the
b) Sequence
Outcrop below The Balanced Rocks.
channel wall.
of hyperconcentrated floodflow and debrisflow facies exposed beneath the Lower Bridge ignimbrite member, about 3 km
All units pictured share distinctnorth of Steelhead Falls.
ive clast lithologies not present in adjacent units and were
presumably deposited in a short time period.
Thin debris
flow deposit above hammer may be a veneer of sediment deposNote
ited by a debris flow that continued downstream.
bedded, pebbly sandstone below the uppermost Gms probably
produced by dilution of the debris flow.
260
flow deposit which was in turn overlain by a thick debrisflow unit.
Figure 8.86 pictures a complicated sequence of thin debrisflow veneers
(?) and thicker debrisflow deposits and thin hyperconcentrated flood
flow fades separated by scour surfaces.
Lack of intervening facies of
other types and the common occurrence of distinctive clasts in these
units not present in adjacent units suggests that these deposits are
genetically related and were emplaced over a short period of time.
Fades Association 5: PaleosolDominated Deposits
Generally massive, light brown sandstones, representing paleosols
(Sm(p)), not only occur as thin units, 0.2 to 3.0 m thick, in facies
associations 2,
thick.
3, and 4, but also dominate other sequences up to 50 m
The designation of paleosoldominated sequences as a separate
facies association is important for recognizing regions or strati
graphic intervals characterized by low sedimentation rates.
In sequences dominated by paleosols, facies Sm(p) occurs in units
up to 10 m thick (Fig. 8.9a).
These thick, massive sandstones repre-
sent superimposed paleosols derived from occassional periods of
deposition of thin sandy units, followed by longer periods of bioturbation which homogenized the new deposits and obscured their depositional surface.
Slow sedimentation rates are thus implied because
vertical accretion of sediment occurred at a slower rate than downward
homogenization and oxidation by pedogenic processes.
Remnant sediment-
ary structures suggest that original sand was deposited as facies
Sh(1), Sh(b), and St with lenses of Gm(b) (Fig. 8.9b).
The most common associated facies are La and Ta.
Lapillistone
beds 1 to 2 m thick are common and show little evidence of traction
261
-
Fig. 8.9. Typical exposures of facies association 5,
east of Madras.
a) Part of a 10 mthick, massive, fine to coarsegrained
Small
sandstone representing superimposed paleosols.
b) Interstratiwhite spots are dispersed pumice lapilli.
fied paleosols (Sm(p)), airfall pyroclastics (La, Ta), and
minor conglomerate (Gm(b)) and sandstone of probable sheet
Note
Prominent burrows marked as "b".
flood origin.
blocky jointing in lower two paleosols.
262
reworking (Fig. 8.9b). However, disruption of airfall facies by burrow
and root traces (Fig. 8.9h) and dispersed pumice lapilli within the
paleosols (Fig. 8.9a) indicates that pedogenesis has also affected
these fades.
Ignimbrites are present in some occurrences of this facies
Where ignimbrites are
association but are notably absent in others.
present, facies association 5 is gradational laterally to other facies
associations.
Where a paleosoldominated section is continuous for
several kilometers, or more, ignimbrites are rare or absent.
These
observations suggest two different environments, with similar
depositional processes, for this facies association.
Association 5a,
laterally discontinuous and including ignimbrites, suggests an interfluve environment standing above frequent depositional tracts but still
mantled by pyroclastic airfalls and flows.
Association 5b, laterally
continuous and generally lacking ignimbrites, indicates broad regions
characterized by slow sedimentation rates and also located outside the
distribution area of Cascade pyroclastic flows.
PALEODRAINAGE AND DEPOSITIONAL SETTINGS
Paleocurrent measurements from channel orientations, sandstone
crossbedding, and conglomerate imbrication, show that sediment dispersal was away from high areas east and west of the basin and into a
longitudinal, northflowing river and closely approximated the present
day drainage pattern (Fig. 8.10a).
These data are in opposition to the
suggestion by Hodge (1940) that the basin was drained to the south
during Deschutes time.
The rarity of lacustrine units also argues
against Hodge's later contention (Hodge, 1960) that the Deschutes basin
263
was closed at this time and that an outlet near South Junction gave
birth to the Deschutes River when the basin became filled with sediment.
The occurrence of anadromous fish fossils in the Deschutes
Formation (Cavender and Miller, 1973) indicates that the basin was
integrated into a larger fluvial system connected to the ocean, presumably the ancestral Columbia River.
Three depositional settings can be recognized on the basis of
paleocurrent data and distribution of fades associations (Fig. 8.10b,
Table 8.3):. 1) a northflowing ancestral Deschutes River; 2) tributaries to the major river that flowed eastward and northeastward from
the Cascades; and 3) regions east of the major river and along the
northern basin margin adjacent to, and onlapping, the older Tertiary
highlands. Boundaries between the settings were gradational and migrated with time causing interfingering of diagnostic facies associations and accounting for the overlap in fields on Figure 8.10b.
The only prominent difference in the modern and Neogene drainage
patterns is the present occurrence of not one, but two, northflowing
rivers, Deschutes and Crooked, through most of the basin.
Deschutes
Formation exposures in the Crooked River canyon, south of Cove
Palisades State Park, provide crosssections through northeasttrending
paleochannels (Fig. 8.10a) filled with ignimbrites, lava flows and
sediment facies typical of the arcadjacent alluvial plain.
In the
central and southern Deschutes basin the northflowing river must have
been confined to the 5 km wide belt between the present Crooked River
canyon and the highlands between Juniper Butte and Smith Rock (Fig.
8.10b).
Rapid progradation of the arcadjacent alluvial plain probably
264
DEPOSITIONAL SETTINGS
ARCADJACENT
ALLUVIAL PLAIN
ANCESTRAL
DESCHUTES
I
RIVER
INACTIVE
BASIN MARGIN
OGateway
;'//;
Madras ,
/
MEAN
PALEOCURRENT
DIRECTION
7 TREND OF
INVERTED
TO
RIDGE
I1 jPALEO.COURSES
OF DESCHUTES
I
I IRIVER
DEFINED BY
DISTRIBUTION OF
II
VOLCANIC UNITS
'Redmond
ct
5
mww.
P58509- 26
10
*Redmond
PS8505-186
Fig. 8.10. Diagrams illustrating paleodrainage and depositional settings in the Deschutes basin. a) Deschutes Formation
sedimentdispersal pattern as indicated by paleocurrent
data, topographically inverted valleyfilling lava flows,
and distribution of volcanic units that filled the ancestral
b) Approximate positions of
Deschutes River channel.
depositional settings based on paleocurrent data and
distribution of fades associations. Overlap in fields
reflects migration of setting boundaries during Deschutes
Formation time.
265
TABLE 8.3. DEPOSITIONAL SETTINGS
Depositional Paleocurrent
Directions
Setting
Arcadjacent
E to NE
Facies Association
FA1
Common
FA2
Rare
FA3
FA4
FA5a
FA5b
Abun Abun Common Dominates
dant
top of
section
rare to otherwise
absent
east
to
west;
alluvial
plain
dant
Major
River
-NNE to NNW Abun Abun Absent Common Absent Dominates
dant
top of
section
where
preserved
dant
East side:W Absent Absent
Inactive
Basin Margin North side:?
Rare
Absent Absent Abundant
266
APPROXIMATE POSITION OF
DESCHUTES RIVER AT TIME OF:
TETHEROW BUTTE MEMBER
77.7:7:773K7
SEEKSEEQUA BASALT MEMBER
CHINOOK IGNIMBRITE MEMBER
PELTON BASALT MEMBER
Madras
OP
lo
1
KILOMETERS
PS8505-192
Fig. 8.11. Approximate position of ancestral Deschutes River in the
Positions based on distribution
northern Deschutes basin.
and thickness variation of volcanic units which filled, and
overflowed, the river channel.
267
forced the river against the east side of the basin.
Farther north,
where Cascade volcanism and related sedimentation were less voluminous,
ancestral Deschutes River occupied a more central position in the
basin.
Division of the northflowing drainage into two streams was
initiated late in Deschutes time by intrabasin volcanism that produced
the Tetherow Butte and Steamboat Rock members, and by Pleistocene
basalts from the north flank of Newberry volcano that diverted the
Deschutes and Crooked Rivers to their present positions.
The course of the ancestral Deschutes River in the northern part
of the basin is not only defined by the occurrence of diagnostic facies
and paleocurrent indicators but also by the outcrop pattern of basalt
flows and ignimbrites which filled and overflowed, the channel.
The
position of the channel in the northern part of the basin at four
different times is defined by the distribution and thickness variation
of the Pelton basalt, Chinook ignimbrite, Seekseequa basalt, and
Tetherow Butte members (Fig. 8.11).
The present canyon was incised
along the western flowmargin of the Agency Plains basalt flow.
The
paleocourses reflect the same deviation from north to eastnortheast
as exhibited in the modern Deschutes River.
structural control of the Deschutes River to
This deviation represents
flow parallel to the
south flank of the Mutton Mountains and provides confirming evidence of
the presence of the Mutton Mountains as a major topographic feature
prior to Deschutes Formation time.
Arcadjacent Alluvial Plain
Description
An eastward sloping and thinning wedge of Deschutes Formation
268
volcanic and sedimentary rocks extended about 45 km from the site of
the High Cascades to the longitude of the present Crooked River. North
of the latitude of Green Ridge the Deschutes Formation contains fewer
volcanic units (Smith and Taylor, 1983) and is only about half as thick
as it is to the south.
Sediments and 'volcanic units emplaced on this
eastwardtapering apron represent the bulk of the Deschutes Formation
and show the greatest variety of facies associations of the three
depositional settings.
Dominant facies associations are those attributed to sheetflood
and debrisflow/hyperconcentrated floodflow processes (Figs. 8.6b and
8.12).
These two facies associations comprise 40 to 70% of the sedi-
mentary sections exposed in the western twothirds of the Deschutes
basin, and are interbedded with ignimbrites and lava flows to form
sequences 10 to 70 m thick bounded by erosion surfaces.
Paleochannel depth ranges from 2 m to 70 m (Fig 8.13) and generally increases westward, although channels over 30 m deep occur more than
20 km east of Green Ridge.
Most channels are less than 50 m wide and
are filled by lava flows,. ignimbrites, and debrisflow or hyperconcen-
trated floodflow deposits.
Broad, shallow, valleys filled with sheet
flood deposits are probably over 100 m wide but are difficult to define.
General westward increase in the erosional paleorelief probably
reflects increasing paleogradients closer to the Cascades.
dips of Deschutes Formation lavas decrease from 4
Green Ridge, to 2
Deschutes River.
,
,
Eastward
near the crest of
8 km east of the fault scarp, to 1
,
at the
These attitudes are comparable to modern stream
269
Ss,
SNIL
GeNW
{.1
Ss, SI,
Sh(b ),
Sp.
Gr11(0)
'III
II
C
cs,),tmcjc pc
I
I
MUD SAND GRAVEL
MUDSTONE
Ss, SI,
SNIL
Sp.
Gm(b)
III
Svi
p
I
c
CONGLOMERATE
SANDY
CONGLOMERATE
SANDSTONE
TUFF
MATRIX-SUPPORT
C
IGNIMBRITE
BRECCIA/
CONGLOMERATE
MUD SAND GRAVEL
) \ )\
ROOT
TRACES
PS8509-131
PEBBLY
[:;:11 SANDSTONE
LAPILLISTONE,
svf mcvc p
MUD SAND GRAVEL
,-----..._
--..._....
SCOUR-FILL
CROSSBEDDING
TROUGH
CROSS-
BEDDING
PLANAR
CROSSBEDDING
---
HORIZONTAL
STRATIFICATION
---s -.4
RIPPLE
CROSSSTRATIFICATION
Fig. 8.12. Graphic measured sections of typical vertical sequences in
Fades and facies
the arcadjacent alluvial plain setting.
associations abbreviated as in Tables 8.1 and 8.2.
270
?",04
_
-`
.ss
4.* .
$4124'
,,':.2turiv
Fig. 8.13. Example of a paleochannel, about 15 m deep, in
Channel is incised through and
plain sequence.
flood facies. Massive unit capping section is
Exposure in Deschutes canyon opposite
deposit.
Squaw Creek.
the alluvial
filled by
a debrisflow
mouth of
271
gradients suggesting that the dips are primary and not structural.
This observation supports westwardincrease in paleogradients and is
Because
consistent with westwardthinning of the sedimentary section.
of greater incision westward, volcanic and sedimentary units are more
lenticular to the west and highstanding interfluves represented by
intercalated paleosols, airfall tephras, and ignimbrites of facies
association 5a separate channels filled with volcanics or debrisflow
and flood deposits.
In the southwestern part of the basin channels are conspicuous in
the upper half of the section but are less evident in the lower portion
where they exhibit no more than 5 m of relief.
The Lower Bridge and
McKenzie Canyon ignimbrite members occur as sheets with nearly flat
bases indicative of a lowrelief depositional surface. However, these
units are truncated by channels 10 to 60 m deep and subsequent ignimbrites (e.g. Steelhead Falls and Peninsula ignimbrite members) and lava
flows generally lack the sheetlike characteristics of the older units
and are confined to channels or exhibit undulatory basal contacts
representing burial of topography with up to tens of meters of erosional relief.
Fluvialchannel conglomerates and sandstones of facies association
1 are locally present in the arcadjacent alluvial plain sequence but
are rarely over 10 m thick.
Thicker sequences, up to 50 m,
are
resticted to exposures along the Metolius River and in the lower half
of the section in an 8 kmwide belt south of the confluence of the
Deschutes and Crooked rivers (Fig. 8.14).
In the latter area,
deposition of these facies came to an abrupt halt at the horizon of the
'
272
'0V41-gP,61;
"lc,'
0-491P-.;
-
4
-
413/4
a
1
.
.
..:
,in.k
A
...,111
"
N
w
'
'
...
--
t:t...",_
ea.
A
...
.
2-1
"-:- 4.,,citu ..
I'
.
Irn0
1.,
-
-
s
7
.111tk4...f.:
11Ar ''-
_..a
117r.*.::-,,
.
" 414
'
° it/14.7
-
.. -
'
..*..% ;e;
5,-..3
ca.
t
=
I
.
"A.
_3
'
iP'1
, '1
'' -
,,...,
--:.:=14.1
-
"
N .
r,
4
lee
:
"
.1
-
Fig. &14. Fades association 1 conglomerates and sandstones in the
alluvial-plain sequence. a) cobble to boulder conglomerate
near the mouth of Street Creek; flow was from right to left
and toward the viewer. A light-colored ignimbrite is
visible at the top of the photo. b) Pebble to cobble
conglomerate with sandstone lenses in Deschutes canyon
opposite Geneva Canyon; stratigraphically beneath McKenzie
Canyon ignimbrite member. Flow direction was away from the
viewer.
273
Fig. 8.15. Section in Crooked River canyon illustrating transition in
depositional style at horizon of McKenzie Canyon ignimbrite
Facies association 1 channel conglomerates and
member (MC).
sandstones dominate the lower part of the section and are
intercalated with overbank facies association 2 and minor
hyperconcentrated floodflow deposits. Only sheetflood,
debrisflow, and hyperconcentrated floodflow fades occur
above the ignimbrite.
See also Figure 8.16.
274
Fig. 8.16. Graphic measured sections in the Crooked River canyon (left)
and CovePalisades State Park (right) illustrating vertical
transition from streamflow to flood and debrisflow
Lines drawn
Sections are 2 km apart.
sedimentation.
between sections show correlation of McKenzie Canyon ignimbrite member in the Crooked River section to a position just
above the Cove ignimbrite member in the Cove section.
and 2 on the right diagram are also
Facies associations
illustrated in Figure 8.5 and a photo of facies association
Photo in Figure 8.15 was
4 in this section is Figure 8.21a.
taken less than 1 km south of the Crooked River section
illustrated here.
-
1
275
500
250
DESCHUTES
FORMATION
100
d
50
2
2 25
57(.
2
10
-
BRAIDED RIVERS
AND ALLUVIAL
PLAINS
ALLUVIAL
FANS
5
10
15
30
20
25
DISTANCE FROM SOURCE.(km )
35
40
45
50
Fig. 8.17. Mean diameter of ten largest clasts from streamflow
conglomerates (facies Gm(b)) plotted against distance east
Data collected on an eastwest transect in
of Green Ridge.
Fields for lateral grainsize trends
the Metolius canyon.
on alluvial fans and alluvial plains adapted from Rust and
Koster (1984).
276
McKenzie Canyon ignimbrite member and was followed by debrisflow and
flood deposition more representative of sedimentation on the arcadjacent alluvial plain (Figs. 8.15 and 8.16);
Sequences of floodplain
fades up to 8 m thick occur with the fluvialchannel facies, especially in eastern exposures, but the ratio of channel to overbank deposits
is 3 to 1 or greater.
Dominance of gravel and resemblance to published
fades models (Miall, 1977; Rust, 1978) suggests that these sequences
represent braided river deposition.
50 cm,
Maximum clast size decreases from
10 km east of Green Ridge, to 20 cm, along the Deschutes River
(Fig. 8.17).
Distinctly outsized clasts, probably derived from
winnowing of matrix from debrisflow deposits or from bank erosion of
basalt flows, were not measured.
The upper 10 to 50 m of the Deschutes Formation, stratigraphically
above the Six Creek ignimbrite member, exhibits an abrupt shift from
the sheetflood, debrisflow
hyperconcentrated floodflow, and ignim-
brite fades to a paleosoldominated fades association lacking intercalated ignimbrites (Fig. 8.18).
Paleosols occur with lapillistones up
to 1.5 m thick, basalts erupted east of the High Cascade crest, and
rare, thin units of pebble conglomerate or crossbedded sandstone up to
3 m thick.
This abrupt lithologic break can be traced throughout the
central and southern part of the basin and is correlated westward to
the upper 150 m of section on Green Ridge which is composed of basalt
and basaltic andesite lavas and is also lacking ignimbrites (Conrey,
1985).
However, the thick lapillistones in the central basin indicate
that largevolume, pyroclastic volcanism was still occurring to the
west.
277
I
Fig. 8.18. Paleosol and tephradominated sequence capping typical
alluvialplain facies in Deschutes canyon near Geneva
Canyon.
Approximately 100 m of section pictured.
Arrows
mark prominent exposures of white lapillistones and light
colored paleosols.
Tetherow Butte member basalt forms the
rimrock.
278
Discussion
The wedge of volcaniclastic material deposited on the east flank
of the early High Cascades was a gently sloping alluvial plain on which
most aggradation occurred during episodes when streams were choked with
debris.
The dominance of debrisflow and flood deposits would general-
ly be interpreted to indicate deposition on arid alluvial fans (Bull,
1972; Collinson, 1978; Nilsen, 1982).
However, several observations
indicate that deposition was on an alluvial plain, not an alluvial fan.
An alluvial fan represents a special type of fluvial environment
where a distinctive fan geometry results from deposition along a
mountain front as flow leaves the confines of erosional channels
(Blissenbach, 1954).
In the case of the Deschutes Formation, dispersal
patterns were parallel over large areas and not divergent as on a fan
(Fig. 8.10a).
This is particularly apparent in the southwestern part of
the basin where modern, parallel, northeastflowing streams are
separated by Deschutes Formation basalt ridges formed by topographic
inversion of a similar, parallel drainage pattern (Fig. 8.10a).
Also,
the gradual fades change from volcanicdominated to sedimentdominated
and the gradual decrease in depositional slopes do not indicate an
abrupt mountain front.
The extent of the flood and debrisflow facies assemblage to more
than 40 km east of Green Ridge is greater than in arid alluvial fans,
which have an average radius of about 10 km (Nilsen, 1982), but is
typical of modern occurrences of volcanisminduced sedimentation (Smith,
in press).
Because of abrupt loss of competence by transporting flows,
arid alluvialfan deposits are typified by rapid downstream facies and
279
grainsize changes from debrisflow and flood dominated sequences to
finegrained fluvial and lacustrine sediments in distances as little as
2 km (Allen, 1981; Gloppen and Steel, 1981; Nilsen, 1982; On,
Hayward, 1983; Kerr, 1984; Rust and Koster, 1984).
1982;
Although lateral
changes do occur in the Deschutes Formation, these changes are gradual
and all fades are represented over a large area.
Conglomerate grain
size trends are also more like those expected on an alluvial plain than
on an alluvial fan (Fig. 8.17; Rust and Koster, 1984).
Thus, although
Deschutes Formation facies are consistent with alluvial fan facies
models, the geometry and lateral extent of these facies are not.
Initial aggradation developed a lowrelief plain which is
especially evident in the southern part of the basin.
Debris flow and
flood deposits and volcanic units formed broad sheets separated by
channels generally less than 5 m deep.
Between volcanic and high-
sedimentload deposition events, sedimentation was focused in grave)
bedload, braided streams.
The lack of braidedstream deposits and the ubiquity of deep
channels
in the upper half of the section suggest that later aggra-
dation occurred in punctuated episodes of highsedimentload deposition
separated by periods of incision to depths of 10 to 70 m.
Alternating
aggradation and degradation on this scale is uncommon in descriptions
of nonvolcanic alluvium which, typically, is pictured as representing
continuous deposition in subsiding basins (e.g. Miall, 1981; Nilsen,
1984).
Deschutes deposition probably resulted from Cascade volcanic
events that provided large sediment loads in excess of geomorphic
thresholds allowing aggradation.
Shortterm aggradation ended when
-
280
eruption sequences ceased and vegetation stabilized fragmental debris.
Streams then attempted to establish their former graded elevations and
incised channels through the volcaniclastic debris.
If not filled by
lava or pyroclastic flows these channels were filled with sediment
during the next aggradational episode.
When narrow channels were
filled, continued aggradation constructed broad sheetflooddominated
sand and gravel sheets in broad valleys.
Between aggradational epi-
sodes most streams on the alluvial plain were dominantly erosive and
left no depositional record.
Several factors may have contributed to the transition from rela-
tively continuous aggradation, that produced a broad, lowrelief plain,
to development of aggradation/degradation cycles characterized by deep
incision and subsequent infilling of channels.
Volcanisminduced
aggradation episodes may have occurred so frequently during early
Deschutes time that widespread
incision was never initiated.
The most
voluminous ignimbrites occur in the lower half of the section and
correlation of tentative volcanic magnetostratigraphy from the central
part of the Deschutes basin to Green Ridge suggests a hiatus of one
magnetopolarity zone in the basin above the McKenzie Canyon ignimbrite
member.
Rapid aggradation induced by the period of large volume pyro-
clastic volcanism may have temporarily ceased during the period of
relative quiescence resulting in widespread incision.
Alternatively,
or in conjunction with this mechanism, widespread aggradation may have
been terminated by external causes such as regional uplift, climatic
change, or changes in base level, but existing data are insufficient to
evaluate these effects.
281
The abrupt decrease in sedimentation rate suggested by the abundance of paleosols near the top of the section is a reflection of
initial graben development in the Cascades west of Green Ridge isolating the Deschutes basin from its primary sediment source.
causal relation is suggested by the lack of
This
ignimbrites in the upper
Deschutes Formation even though thick airfall deposits are abundant.
Pyroclastic volcanism continued but lack of an ignimbrite record indicates that
the
pyroclastic flows that one would expect to have been
generated contemporaneously with the thick airfall deposits were
unable to
enter the basin; presumably, they were ponded in the graben
along with eruptioninduced sedimentary deposits (Smith, 1985).
These
grabenfill volcaniclastics are thought to account for the negative
residual gravity anomaly west of Green Ridge (Couch and others, 1982;
Fig. 5.11) and are locally exposed at the surface as the Camp Sherman
beds in the Metolius valley (see Chapter 5).
Ancestral Deschutes River
Description
Ancestral Deschutes River sedimentation is represented by coarse
units of conglomerate and minor sandstone alternating with fine units
of
fine to mediumgrained sandstone and mudstone, representing facies
associations 1 and 2, respectively (Figs. 8.19, 8.20a).
In most
sections coarse and fine units are of approximately equal thickness in
alternating sequences 4 to 10 m thick.
In vertical sequences conglo-
merates show an erosive contact with underlying siltstones and fine
grained sandstones (Figs. 8.19a, 8.20a) and an abrupt transition to
these same facies above.
Exposures in road and railroad cuts illus-
282
trate lateral interfingering of the coarse and fine memiQers (Fig.
8.19b).
Hyperconcentrated floodflow deposits are far more abundant than
debrisflow deposits but even then compose a smaller volume of the
Deschutes river facies than those of the alluvial plain.
The hypercon-
centrated floodflow deposits are restricted to occurrences within
floodplain sequences
or cap channelfill sequences (Fig. 8.19b, 8.20).
Basalt flows are well preserved but modestsize ignimbrites were
largely, or completely, removed by erosion in the river channels with
preservation limited to the floodplain sequences.
volcanic
units fill broad, shallow troughs up to 1
In some cases the
km wide, overlying
conglomerate, and extend for another 0.5 km or more beyond this trough
overlying finegrained facies.
In other instances the volcanic units
occupy channels 50 to 150 m wide and up to 25 m deep incised through a
variety of facies (Fig. 8.19c).
Sedimentary and volcanic rocks emplaced in the ancestral Deschutes
river valley thin from 150 m to 0 m from the latitude of Madras to the
northern extent of Deschutes Formation outcrop (about 20 km).
As the
formation thins northward, basalt, debrisflow, and hyperconcentrated
floodflow deposits dominate over the facies representing normal
streamflow aggradation.
The most distal Deschutes Formation exposures
are composed of flows of the Seekseequa basalt and Tetherow Butte
members in a paleovalley incised into Columbia River Basalt Group on
the southeast flank of the Mutton Mountains near South Junction (Plate
U.
Three Deschutes Formation basalt flows and an ignimbrite can be
283
r
Fig. 8.19.
Exposures representing the ancestral Deschutes River
a) Channel conglomerates and
depositional setting.
sandstones (FA1) overlying overbank mudstones and sandHeight of exposure about 12 m. Exposure is
stones (FA2).
Abandoned
capped by an aphyric basaltic andesite flow.
railroad cut in Willow Creek canyon, 3 km west of Madras
b) Conglomerate and coarsegrained sandstone of FA1 inter
fingered with, and partly overlain by, lightcolored silt
Both channel
stones and finegrained sandstones of FA2.
and overbank deposits are overlain by a gradedstratified
hyperconcentrated floodflow deposit (HFF) which extends
upward, under colluvium, to the base of the overlying
c) Exposure of the north wall of
Seekseequa basalt member.
Jackson Buttes ignimbrite member (1)
Willow Creek canyon.
overlies slopeforming conglomerate and is overlain by a
dark, ledgeforming FA4 sequence and lightcolored, slope
forming mudstones of FA2. These units can be traced as
Ignimbrite 2 filled
sheets for another 3 km to the west.
and overflowed a Deschutes River channel incised at least
25 m into the older units.
284
Fig. 8.20. Graphic measured sections from the Round Butte Dam type
section illustrating fades sequences representing ancestral
Deschutes River sedimentation. Section on left illustrates
FA1 conglomerates interbedded with FA2 sandstones, mud
Section on right illustrates
stones, and lapillistones.
thick gradedstratified hyperconcentrated floodflow
Symbols
deposits interbedded with overbank deposits of FA2.
as in Fig. 8.12.
285
traced through the northern Deschutes basin where they filled, and
overflowed, the ancestral Deschutes River channel (Fig. 8.11b).
Paleocurrent indicators in underlying sedimentary lithologies indicate
that the river flowed northward,
but the bases of the volcanic channel
fill units are inclined southward at low angles (<1
) that can be
calculated from the outcrop distribution over a large area (see Chapter
6).
The depositional record of the ancestral Deschutes River is dis-
tinct from that of the arcadjacent alluvial plain in the dominance of
normal streamflow fades and northwardoriented paleocurrent indicators.
Although the fluvialchannel facies association occurs in both
settings, it is subordinate to other fades and associated with a
smaller proportion of floodplain facies in the arcadjacent alluvial
plain sequence. No sheetflood deposits, a prominent facies association
of the arcadjacent alluvial plain, occur in the major river deposits.
Discussion
The depositional record of the ancestral Deschutes river reflects
the difficulty of recognizing criteria for distinguishing braided and
meandering alluvium in the rock record (c.f. Jackson, 1978; Rust,
1978a; Galloway, 1981; Friend, 1983;).
The fluvialchannel facies
association exhibits features suggestive of braidedriver deposition,
but the occurrence of thick finegrained facies association 2 sequences
(up to 50% of exposures of ancestral Deschutes River deposits) are
atypical of braided systems (Miall, 1977).
that braidedriver fades models
Rust (1978b) points out
emphasize the lack of overbank
deposits, but are strongly influenced by study of modern proglacial
286
rivers confined to glaciated troughs and that ancient, unconfined
braided systems should have had space for extensive inactive areas to
develop and be represented as floodplain facies.
Nonetheless, Rust
(1978b) contends that the record of gravel-bedload, braided streams
should still be dominated by framework conglomerate.
However, in the
Deschutes Formation the channel and floodplain facies associat are
about equal in volume.
The occurrence of low-angle inclined strata
(Gi) suggests that some of the conglomerate was deposited on point-bars
in a sinuous system. Likewise, vertical sequences in the Deschutes
Formation are similar to those shown by Gustayson (1978) to result from
lateral accretion of gravel meander lobes and vertical floodplain
accretion along the modern Nueces River in Texas.
Morphologic
classification of the ancestral Deschutes River is thus deemed
inappropriate.
The rarity of debris-flow deposits in the tract of the ancestral
Deschutes River may reflect the ability of this larger river to dilute
debris flows to hyperconcentrated flood flows over a short distance.
The restriction of hyperconcentrated flood-flow facies to floodplain
sequences or capping channel-fill deposits suggests that the aggradation during floods was capable of diverting channels but that flood
deposits were reworked by stream flow in the channels and not preserved
when such diversion did not occur.
As in the case of the arc-adjacent alluvial plain, the major-river
setting was not the site of continuous aggradation but was charac-
IL
terized by alternating periods of deposition and incision.
The occur-
rence of the fluvial-channel fades association grading laterally into
287
the floodplain fades association implies a very lowrelief valley
(Fig. 8.19b,c) but is inconsistent with the complimentary occurrence of
narrow, deeplyincised channels whose morphology is preserved by channelfilling lava flows and ignimbrites (Fig. 8.19c).
Aggradation pro-
bably occurred during times of high sediment load input with intervening periods of incision to maintain grade when sediment contribution
was lower.
Although the majorriver deposits are dominated by normal
streamflow fades aggradation was still likely related to Cascade
volcanism with the lesser abundance of debrisflow and flood deposits,
compared to the alluvial plain, reflecting the ability of the larger
river to transform these flows by dilution and to rework their
deposits.
Aggradational episodes on the alluvial plain were likely
contemporaneous with aggradation in the ancestral Deschutes valley,
and incision of tributary streams following rapid aggradation by sheet
flood, debris flow and hyperconcentrated flood flow would contribute
large volumes of sediment to the lower gradient, northflowing river
where aggradation by streamflow processes would continue until the
sediment supply diminished.
Vessel and Davies (1981) discuss eruption
initiated aggradation that continues for 20-30 years following eruptions in Guatemala where the tropical climate should allow sediment
stabilization by rapid revegetation.
In the semiarid Deschutes basin
such episodes may have had longer duration.
Northward increase of basalt flows and debrisflow and hyperconcentrated floodflow deposits indicates that the whole fluvial system
did not undergo net aggradation during Deschutes Formation time.
Incision of the ancestral Deschutes River across the Mutton Mountains
288
produced a valley which became more confined northward so that sediment
deposited during aggradation episodes was removed during intervening
periods.
Lava flows and some thick, rapidly deposited debrisflow and
hyperconcentrated floodflow deposits diverted the river and were preserved.
The lowangle dips of volcanic channelfill units in opposition to
expected depositionaldip direction indicates uplift of the Mutton
Mountains during Deschutes Formation time.
However, this deformation
is not likely to have produced the northward decrease in aggradation.
Studies by
Schumm and others (1982), Burnett and Schumm (1983) and
Ouchi (1985) indicate that upliftinduced incision should occur downstream of the axis of uplift and greatest aggradation should be
immediately upstream from the axis, where Deschutes deposition is
observed to have been minimal. Northward decrease in aggradation probably reflects the greater distance from the majdr sediment source. The
Cascade source area
north of the latitude of Green Ridge was not as
energetic a sediment source as farther south, as evidenced by the thin-
ner nature of the arcadjacent alluvial plain sequence and paucity of
intercalated volcanics (Smith and Taylor, 1983; Chapter 7).
Inactive Basin Margin
Description
On the east side of the basin the Deschutes Formation is composed
of laterally extensive paleosoldominated sections, not restricted to
specific stratigraphic intervals (Fig. 8.9b).
Paleosols also dominate
scattered outcrops along the south flank of the Mutton Mountains, north
of characteristic arcadjacent alluvialplain deposits (Fig. 8.10a).
289
Primary structures are rare; the most common being stratified
sandstone (Sh(1), Si, Sp) and lenticular conglomerate (Gm(b)) diagnostic of the sheetflood facies association. However, these sheetflood
units are only 1 to 2 m thick as compared to the common occurrence of
10 m thick sequences on the west side.
were extensively burrowed and rooted and
well-developed paleosols.
The thin sheetflood deposits
frequently grade upward into
Conglomerate imbrication indicates sediment
dispersal to the west, consistent with the dominance of angular rhyolite clasts from Oligocene volcanic highs located 5 to 10 km to the
east.
The paleosol-dominated facies association includes massive, sandy
paleosols up to 10 m thick separated by air-fall tephras or rare sheetflood facies.
Because of location east of the basin axis, Cascade
volcanic products.are largely restricted to air-fall components.
The
inactive basin margin sequence is similar to the upper portion of the
arc-adjacent alluvial plain section but is distinguished on the basis
of clast composition, more abundant interbedded sheetflood fades, and
better development of zonation in paleosols.
Discussion
The dominance of paleosols indicates very low sedimentation rates
from the highlands of older Tertiary volcanics.
As the western and
central areas of the basin aggraded with Cascade detritus, occassional
flash floods deposited thin, poorly sorted units along the eastern
margin.
Periods between depositional events were long enough to allow
obliteration of most primary structures by pedogenic processes.
290
CAUSES OF AGGRADATION
Of fundamental importance in stratigraphic studies of fluvial
sedimentary rocks is consideration of the cause, or causes, of aggradation on an appropriate scale to leave a preserved depositional record.
Aggradation reflects disequilibrium between sediment supply and a
river's capability to transport sediment. Most sedimentology studies
point to tectonism as the ultimate cause of aggradation
by choking
fluvial systems with detritus from adjacent areas of rapid uplift
and/or by diminishing stream gradient in subsiding basins or upstream
from active uplifts which cross drainages.
Aggradation of the Deschutes Formation, on the other hand, appears
more closely related to volcanic, than tectonic, controls.
Several
observations suggest that aggradation was the response of a semiarid
fluvial system to the pyroclastic volcanism of the early High Cascade
eruptive episode.
The Deschutes Formation is temporally equivalent to the early
High Cascade eruptive episode of Priest and others (1983).
The occur-
rence of at least 75 ignimbrites occur within the Deschutes Formation
(Chapter 7) chronicles a period of pyroclastic volcanism which is not
matched by the record of the immediately preceding or following eruptive episodes (Priest and others, 1983) for which there is little
depositional record in the Deschutes basin.
The Deschutes Formation represents a constructional wedge
volcanic and sedimentary rocks which thins eastward and
from the major locus of contemporary volcanism,
position of Mount Jefferson (Smith and
of
northward away
south of the present
Taylor, 1983).
291
Deposition on an alluvial plain adjacent to the arc was
by sheetflood, debrisflow, and
hyperconcentrated floodflow events
which produced thick deposits extending over 40 km from
areas.
mostly
inferred source
thickness and extent of such deposits is far greater than
The
similar facies in nonvolcanic alluvialfan settings but is
consistent
with modern observations of synvolcanic sedimentation in which sediment
is mobilized on a scale unmatched in nonvolcanic environments.
The extensive development of channels, in excess of 10 m to
indicates that aggradation was not a continuous process.
60 m deep,
sug-
Coupled with the observed facies, this characteristic strongly
gests that tens of meters of aggradation occurred in short punctuated
episodes, when large sediment loads were introduced, and separated by
periods of incision when streams attempted to achieve their previous,
graded
elevations.
This aggradationdegradation cycle is similar
that observed for Pleistocene to Recent sedimentation in
Range (Smith, in prep.), in Central America
to
the Cascade
(Kuenzi and others,
1973; Vessel and Davies, 1981) and adjacent to the Andes (Van Houten,
1971) but is not a typical feature of
syntectonic clastic wedges or
basin fills.
Deposits adjacent to uplifts east and north of the basin are
dominated by
paleosols indicating slow sedimentation rates. Aggrada-
tion could not have resulted from high detrital input from these basin
margins, which were
inactive as sediment sources.
Although very lowangle, southward dips of Deschutes
Formation
volcanic units south of the Mutton Mountains indicate contemporary
uplift north of the basin, tectonism is not likely
to have caused
292
aggradation.
increasing stratigraphic
Structural dip increases with
age throughout the Tertiary section and the Deschutes Formation lies on
the next oldest
uplift north
unit with a distinct angular unconformity.
Thus,
and east of the basin was probably at a slow, continuous
rate through the mid and late Tertiary but is not equated
significant sedimentary record prior to Deschutes
with any
Formation time.
have a profound effect on
While slow uplift across river courses can
the geomorphology of a river (Burnett and Schumm, 1983), that effect
was, at most, a minor contribution to Deschutes Formation deposition
because the lack of aggradation immediately upstream of the Mutton
Mountains is contrary to predicted tectonic influences. Northward thinning of the
Deschutes Formation favors an upstream control on
aggra-
dation, by the Cascade sediment source, over downstream control, by
gradient
diminishment across the
Mutton
Mountains.
7. Abrupt decrease in sedimentation rates across the arcadjacent
alluvial plain, near the end of Deschutes
in the thick accumulation of
listones which
Formation time, is reflected
successive paleosols and airfall lapil-
overly the record of punctuated highsedimentload
aggradation events.
The sudden transition of the High Cascades from a
major sediment source to an inactive status,
basin margins, resulted from
similar to the other
subsidence of the central Oregon High
Cascade graben that isolated the basin from its sediment source.
This
relationship documents the dependence of Deschutes Formation aggradation on Cascade volcanism.
DISTINCTIVE SEDIMENTARY UNITS
The choice of volcanic units as stratigr'aphic markers in the
293
Deschutes Formation (Chapter 6) was made because of the general lack of
continuity and/or distinctive features in sedimentary units.
Nonethe-
less, there are a few widespread sedimentary units, or genetically
related sequences of units, which are important in a stratigraphic
sense.
Other sedimentary units, though not widespread, exhibit dis-
tinctive features deserving separate discussion.
Sub-Pelton Conglomerate
As much as 25 m of sedimentary section separates the Pelton basalt
member from the uncomformity with the Simtustus Formation and is exposed on the hills south of Gateway and along the Deschutes River
canyon near Willow Creek.
This interval is composed almost entirely of
conglomerate making it distinct from other exposures of ancestral
Deschutes River deposits which typically consist of approximately equal
thicknesses of alternating conglomerate and floodplain facies.
Promi-
nent fine-grained overbank deposits occur immediately above the Pelton
basalt member, including a 10 m-thick section of lacustrine mudstone
north of Round Butte Dam, but are restricted to discontinuous beds,
1
to 2 m thick, below the basalt.
The rarity of preserved overbank deposits at the base of the
Deschutes Formation section may indicate restriction of the ancestral
Deschutes River to narrow valleys incised into the Simtustus Formation
that prohibited development of broad floodplains.
The combined thick-
ness of the conglomerate and the Pelton basalt member may have filled
most of these lows on the unconformity and led to the development of
inactive tracts adjacent to the subsequently unconfined stream.
294
Sub-Lower Bridge Debris-Flow Deposits
From Lower Bridge to beyond the mouth of Squaw Creek, the Lower
Bridge ignimbrite member typically lies upon a paleosol which overlies
a 1
to 4 m thick sequence of debris-flow deposits (Fig. 6.11d).
As
many as 4 depositional units can be recognized in some exposures of
this sequence which has a flat top and an undulating base, with up to 2
2
m of relief, and covers an area of more than 100 km
.
Some debris-flow
deposits are gradational at the base to hyperconcentrated flood-flow
facies.
The internal stratigraphy of this interval is often a compli-
cated mixture of relatively thick debris-flow deposits, thin debrisflow veneers, and hyperconcentrated flood-flow deposits sometimes
separated by scour surfaces (Fig. 8.8b).
The source(s) of these debris flows is not clear but because their
distribution is similar to that of the Lower Bridge ignimbrite, it is
likely to have been to the southwest.
The debris flows filled in most
of the minor erosional topography in the southern portion of the basin
to provide a low-relief surface on which the Lower Bridge ignimbrite
was emplaced.
Supra-McKenzie Canyon, Debris-Flow and Hyperconcentrated-Flood Flow
Deposits
The thickest sequence of multiple debris-flow and hyperconcentrated flood-flow deposits in the Deschutes Formation (up to 30 m)
directly overlies the McKenzie Canyon ignimbrite member (Fig. 8.21).
This sequence is well exposed from Lower Bridge to The Cove-Palisades
State Park.
In its northern occurrences it is beyond the extent of the
McKenzie Canyon ignimbrite and overlies the Cove ignimbrite member
295
k
Fig. 8.21. Photographs illustrating debrisflow and hyperconcentrated
floodflow deposits immediately overlying and underlying the
McKenzie Canyon ignimbrite member. a) and b) illustrate
supraMcKenzie Canyon sediments at CovePalisades State Park
Large, lightcolored
and near Big Falls, respectively.
boulders
in a) are clasts of McKenzie Canyon ignimbrite.
Section in a) also is illustrated in Figure 8.16 and overc) McKenzie Canyon ignimbrite
lies Cove ignimbrite member.
member, at top of photo, overlying flood deposit which
contains lightcolored boulders (like that indicated by the
geologist) of the ignimbrite.
296
(Fig. 8.21a).
Incision to a depth of 10 to 70 m occurred over much of
the basin after the emplacement of these debris-flow and hyperconcentrated flood-flow deposits and limits the continuity of outcrop of the
unit.
Clasts of the McKenzie Canyon ignimbrite are common within this
sequence but are rare at higher stratigraphic levels.
The association of this thick interval of high-sediment load
facies with the most extensive ignimbrite in the formation suggests a
genetic relationship between them.
Thd McKenzie Canyon pyroclastic
flow probably devastated several thousand square kilometers; the
resulting devegetation and the availability of easily eroded pyroclastic debris undoubtedly led to widespread flooding and debris flows.
Some of the hyperconcentrated flood flows and debris flows occurred
simultaneously with the eruption as evidenced by several exposures near
the distal extent of the ignimbrite where a single pyroclastic flow
unit overlies hyperconcentrated flood-flow deposits containing clasts
of the ignimbrite (Fig. 8.21c).
The most likely explanation for such
an occurrence is that erosion of early flow units led to flood events
whose deposits were subsequently covered by a later flow unit.
Because
the McKenzie Canyon ignimbrite occurs as only one cooling unit, the
various pyroclastic flow units and contemporary sediments must have
been emplaced within a matter of hours to days.
The sedimentary units
intimately associated with the McKenzie Canyon ignimbrite offer impressive testimony to the influence of pyroclastic eruptive events on
fluvial sedimentation.
Street Creek Debris-Flow Deposit
Of the many debris-flow deposits in the Deschutes Formation, only
297
one can be traced a significant distance on the basis of its clast
composition.
This unit is-characterized by ubiquitous cobbles of
distinctive, gray, glassy dacite with needleshaped microphenocrysts of
hornblende
and hypersthene.
The dacite clasts contain cognate xeno-
liths of hornblendehypersthene diorite. The clasts frequently exhibit
radial, prismatic joints indicating that they cooled in place from high
temperature.
This distinctive lithology has not been recognized in any
other Deschutes Formation unit.
The westernmost exposure of this debrisflow deposit is at
2500
feet on the north side of Street Creek, near its confluence with the
Metolius River, and it can be traced northeast to Willow Creek canyon,
a distance of 23.5 km.
The recognition of this deposit at four locali-
ties (Street Creek, Seekseequa Junction, Jackson Buttes, Willow Creek)
over such a large area is important for two reasons.
First, because of
the paucity of volcanic units in the northern Deschutes basin, this
unit provides critical stratigraphic control.
Second, this unit pro-
vides a rare opportunity to observe lateral variation in debrisflow
depositional texture over a long distance.
Figure 8.22 summarizes the
textural and structural features of the deposit at its three best
exposures.
These observed features suggest that grading becomes better
developed with distance; the unit is ungraded at its most proximal
exposure, becomes inversetonormal graded, and is coarsetail normal
graded at its most distal exposure.
Maximum grain size diminshes with
distance and probably reflects settling and depositon of the largest
clasts.
Dilution to produce hyperconcentrated flood flow is recorded
in the Seekseequa Junction and Willow Creek exposures.
The presence
298
SEEKSEEOUA
JUNCTION
STREET CREEK
16 km
WILLOW CREEK
7.5 km
1m
Fig. 8.22. Drawings illustrating lateral variation in texture of the
Street Creek debrisflow deposit. The debrisflow unit (DF)
changes from ungraded to reversetonormal graded, to
coarsetail normal graded with increasing distance from
Hyperconcentrated flood flow (HFF) unit
(left to right).
occurs at the base of the deposit in more distal exposures.
Note representation of radialprismatic fractures in clasts
in Street Creek and Seekseequa Junction exposures.
299
of prismatically jointed blocks indicates that the debris flow originated on the flanks of a volcano from hot material derived from a
pyroclastic flow, lava flow, or dome.
Such a source would have been
located west of Green Ridge making the total distance of flow at least
35 km.
Dry Canyon Flood Deposit
Sandpits in Dry Canyon, east of Round Butte, reveal a sequence of
horizontally stratified sand, in excess of 35 m thick, that generally
lacks internal scour surfaces and appears to have been emplaced during
a single depositional event (Fig. 8.23a).
The same sequence is exposed
in the Round Butte Dam measured section, where it is 35 m thick, and in
roadcuts southwest of Gateway, where it is 15 m thick.
In Dry Canyon
and at Gateway this deposit is directly overlain by the Agency Plains
basalt flow of the Tetherow Butte member.
In the Round Butte Dam
section the deposit is overlain by the Round Butte member.
In all observed localities the dominant sedimentary structure in
this deposit is horizontal bedding.
The beds are 0.5 to 5 cm thick
with gradational contacts and are similar to those attributed to rapid
deposition from sedimentladen dispersions by Smith (in press; Fig.
8.23 b,c).
The sediment is poorly sorted and ranges from finegrained
sand to pebbles with occassional cobbles and boulders.
Pumice lapilli
up to 2 cm across are prominent constituents of coarsegrained beds.
Pumicedominated beds occur in closely spaced groups, separated by
thin, finergrained and darker strata that are randomly distributed
throughout the deposit.
The alternation of lightcolored, pumice
dominated intervals and dark colored intervals with less pumice pro-
300
duces a firstorder stratification, on the order of 50 cm to 1
m, which
is easily recognized when viewing this deposit from a distance (Fig.
8.23a).
In all three localities there is crudely defined normalgrading of
nonpumiceous grains throughout the deposit.
In the Round Butte Dam
section, the basal 15 m is massive, pebble gravel, generally lacking
pumice, and grades upward into horizontally bedded coarsegrained
lithic sand with pumice lapilli.
In Dry Canyon the base of the deposit
is not exposed but the mean diameter of nonpumiceous grains decreases
from coarse sand, at the base of the exposure, to fine sand at the top.
Cobbles, and boulders to 1.5 m across, are dispersed throughout the
lowest 5 m (Fig. 8.23b) but are absent above.
Grain size at the
Gateway exposure is similar to the top of the Dry Canyon section and
also includes thick beds of rounded pumice lapilli, up to 1 m thick,
which are not observed at the other localities.
In Dry Canyon, the
upper 5 m of the unit contains broad scour surfaces up to 1.5 m deep
and 4 m across that are filled with strata of the same texture and
composition deposited conformably on the scour.
Similar erosion sur-
faces occur in the exposure near Gateway where microfaults and convolute bedding adjacent to the scours suggest that erosion occurred while
the sediment was watersaturated (Fig. 8.23c).
The normal grading, uniform composition, and lack of scour surfaces, except near the top of the unit, suggest that this thick
sequence was deposited during a single event.
The massive nature of
the lower part of the unit, prominence of horizontal bedding of poorly
sorted sand with gradational stratal contacts, and presence of out-
301
MODv.
.
Pao_
At.
I
1.-ik
ov,
,
t j
-
-
_
-
.
7.4
*kjAki
-
fir
,G0
16741'
4
,707
.
-wold
Yr) °
.".:ok
e
,
LVc..-
Fig. 8.23. Photographs of the Dry Canyon flood deposit. a) Sand pit in
Dry Canyon exposing-30 m of horizontally bedded sandstone
Base of deposit is not exposed; top occurs
(facies Sm(p)).
b) Large
beneath Tetherow Butte member basalt at arrow.
boulders near base of flood deposit in Dry Canyon sand pit.
c) Closeup of a part of the flood deposit exposed on
Arrows point to margin of broad scour
Gateway Grade.
surface delineated by bedding truncation and inclined
Note slumped bedding along channel margin between
bedding.
d) Telephoto view of channelfilling
hammer and lower arrow.
boulder breccia (highlighted) on west side of Crooked River
canyon south of the CovePalisades State Park. Boulders near
Tetherow Butte
left margin of photo are up to 8 m across.
member basalt forms the rimrock.
302
sized clasts, indicates that this large volume of sediment was
As sediment concen-
deposited rapidly by hyperconcentrated flood flow.
tration diminished, local erosion produced scour surfaces which were
mantled by deposition during later flood pulses.
The similarity in
sediment composition, texture, and structure across the scours, and
associated softsediment deformation features, suggest that the scours
developed by shortlived erosion during a single depositional event.
Therefore, this deposit represents a flood event of cataclysmic proportions and is, perhaps, one of the largest such events yet recognized
in the geologic record.
Assuming that the deposit was uniformily
distributed over the area bounded by the three exposures, a minimum of
3
3.5 km
of sediment was deposited.
The finer average grain size of the deposit at Gateway, as compared to farther south, suggests that the flood which deposited this
sediment was flowing northward.
Scour surfaces in the Gateway exposure
trend N10 E and those in Dry Canyon trend N20-35 E.
Because of the
unconsolidated nature of the deposit exposures are limited to roadcuts
and sand pits inhibiting efforts to trace it to the south or southwest.
However, this deposit may be correlative to another unusual unit ex-
posed on the west wall of the Crooked River canyon, 15 km south
southwest of the sand pits in Dry Canyon.
Here, the Agency Plains
basalt overlies a megabreccia which filled a northnortheast trending
channel about 65 m deep and at least 100 m across (Fig. 8.23d).
The
breccia is composed of angular and subangular basalt clasts 10 cm to 8
m across.
These blocks are in clast support with an interstitial
matrix of horizontally bedded, coarsegrained, pebbly sand.
All clasts
303
over 20 cm across are diktytaxitic olivine basalt and an analysis of
one of these clasts (Appendix Ik, sample RB50) shows compositional
similarity to a basalt flow which occupies the same stratigraphic
position a few kilometers to the south (Appendix Ic, sample SF134).
The thick sandy flood deposit, and possibly correlative breccia,
is similar in texture and sedimentary structures to other flood deposits in the Deschutes Formation but is at least an order of magnitude
larger in scale.
The volume of sediment and water involved in this
flood event far exceeds even those floods typically produced during
explosive volcanic events.
The most likely explanation for this flood
deposit is that it resulted from tne emptying of a lake, perhaps by
failure of a lava-dam.
Tetherow Debris-Flow Deposit
Stensland (1970) described a spectacular debris-flow breccia, over
60 m thick, which is well-exposed in the Deschutes River canyon southwest of Tetherow Butte, near Tetherow Bridge.
The unit is massive,
ungraded, and contains clasts up to 15 m across.
Most of the largest
clasts resemble poorly-exposed lithologies on Forked Horn Butte while
others resemble Deschutes Formation basaltic andesites.
Clasts of
sediment and unwelded ignimbrites, similar to Deschutes Formation
lithologies, are common, as are fragments of light gray perlite up to 5
cm across.
The base of the debris flow is not exposed but the unit is
at least 60 m thick along the Deschutes River; water-well logs (Sceva,
1968) suggest that it may be over 100 m thick south of Forked Horn
Butte. The breccia is overlain by Deschutes Formation basalts southwest
of Forked Horn Butte, and by younger Pliocene and Pleistocene basalts
304
farther north (Robinson and Stensland, 1970).
Burial by younger units and lack of deep dissection precludes
observation of thickness variation and distribution which are necessary
to establish a source for the breccia.
The northernmost exposures,
near Tetherow Butte, form lobate, northsouth trending ridges and may
represent the distal end of the debris flow and, thus, indicate a
source to the south.
Large clasts were incorporated from Forked Horn
Butte, a poorly exposed dacitic volcanic high of presumed John Day
Formation age, but this is not a likely source for the bulk of the
debris flow.
Clasts of Deschutes Formation lithologies indicate that
it is Deschutesage and
if the debris flow was related to volcanic
activity it is too young to be related to Forked Horn Butte.
If the
debris flow was the result of a late Miocene or early Pliocene mass
failure at Forked Horn Butte it is difficult to envision how it incorporated so many Deschutes Formation clasts and became so thoroughly
homogenized in only a few kilometers of flow.
The debris flow probably
originated closer to, or in, the High Cascades to the southwest and
incorporated large blocks of Forked Horn Butte lithologies as it impinged upon, and flowed around the John Day high.
Debrisflow deposits as thick as the Tetherow breccia are rarely
observed in the geologic record.
The scale of this unit is further
magnified if it originated over 30 km away in the High Cascades.
Per-
haps the closest analog for the Tetherow debris flow is the early
Holocene "Osceola mudflow" that resulted from failure of the summit of
Mount Rainier, Washington.
This debris flow traveled over 75 km and
includes clasts up to 15 m across in distal exposures (Crandell and
305
Waldron, 1956).
PETROLOGY OF DESCHUTES FORMATION SEDIMENTARY ROCKS
Introduction
Deschutes Formation sedimentary rocks are texturally and compositionally diverse.
In general, the sediments are very poorly sorted and
although this reflects, in part, the rapid deposition of many units
during highdischarge and often highsedimentload events, it is also a
reflection of the complicated hydraulic equivalence of vesicular volcanic grains with a wide range in specific gravity (Smith and Smith,
1985).
Compositional variability is probably a reflection of the ever
changing nature of unconsolidated pyroclastic material which was provided to the basin as sediment.
.
It is likely that detailed geochemical
studies of Deschutes sandstones and conglomerates would allow correSome
lation of flood and debris flow events with specific eruptions.
such correlations are obvious from field observations.
Sheetflood
deposits overlying, and adjacent to, the Peninsula ignimbrite member
near the mouth of Squaw Creek are identical in color to the ignimbrite
matrix and rich in black and gray pumice lapilli similar to those found
in the ignimbrite.
In the Deschutes River canyon opposite Geneva
Canyon the McKenzie Canyon ignimbrite is locally overlain by sheet
flood deposits comprised almost entirely of sand to smallpebblesize
grains of the distinctive orangecolored ignimbrite.
Conglomerates
The composition of conglomerate clasts is one of the most useful
sedimentological tools for determining sediment provenance.
Clast
counts in wholly volcanic conglomerates are not very useful, however,
306
because accurate determination of composition of many clasts requires
chemical analyses.
However, distinctive clasts may be recognized
in
the field and provide useful stratigraphic or provenance information.
For example, clasts of distinctive welded ignimbrites (e.g. McKenzie
Canyon, Fly Creek, Deep Canyon ignimbrite members) are commonly en-
countered in hyperconcentrated floodflow conglomerates (e.g. Fig.
8.21a) and,
in a stratigraphic sense, usually appear abruptly and de-
crease in abundance upward.
These conglomerates probably represent
floods initiated during or relatively soon after eruption of the ignimbrites and provide approximate stratigraphic markers where the ignimbrites are not present.
John Day Formation rhyolitic ignimbrites were the source for a
distinctive suite of red, gray, and white clasts with prominent fiamme
and lithophysae and rare to common quartz, sanidine, and biotite phenocrysts (Fig. 8.24).
These ignimbrite lithologies provided almost all
of the pebble to bouldersize clasts found in sedimentary units along
the eastern basin margin.
Other clasts were derived from the Columbia
River Basalt Group flows and John Day rhyolite domes, particularly Buck
Butte.
Gray perlite clasts (Fig. 8.24), ubiquitous in the eastern
sedimentary units, are undoubtedly derived from John Day rocks as well,
although the author is not familiar with similar .perlite outcrops east
of Madras.
Occassional agate pebbles are probably from lithophysal
John Day ignimbrites.
Cobbles of petrified wood also occur.
Andesite
and porphyritic basalt and basaltic andesite clasts which might represent a Clarno Formation source have been recognized in the eastern
Deschutes basin conglomerates only in the vicinity of Prineville.
307
1
..
-.."..-
Fig. 8.24. John Day Formation clasts in the Deschutes Formation.
Light colored clasts were derived from rhyolitic ignimbrites
at the base of member A of the John Day Formation (Robinson
Dark clast near right edge of photo is
and Brem, 1981).
perlite, possibly eroded from the vitrophyric welded zone of
a John Day ignimbrite.
308
Clast counts in fluvialchannel conglomerates (facies Gm(b)) within the lower half of the Round Butte Dam type section illustrate the
compositional variability typical of Deschutes Formation conglomerates
(Table 8.4).
All clast count data is for conglomerates representing
ancestral Deschutes River sedimentation and illustrate mixing of the
eastern, predominantly John Day Formation source terrane, and the
western High Cascade sediment source.
The abundance of John Day clasts
is highly variable and some conglomerates completely lack the
distinctive rhyolites.
The dominance of Cascade lithologies in the
conglomerates supports the observations set forth earlier in this chapter that the active volcanic chain was the primary source of Deschutes
Formation sediment.
John Day Formation and Columbia River Basalt Group
clasts may have been locally important constituents in Deschutes River
gravel bars near the mouths of streams draining westward from the
Ochoco Mountains but probabl
became quickly diluted with Cascadian
clasts farther downstream.
Gray andesite clasts with plagioclase and hornblende or augite
phenocrysts and glomeropheoncrysts are a rare but ubiquitous component
of conglomerates below the Pelton basalt member and some conglomerates
a short distance above these basalts, but are not found higher in the
section (Table 8.4).
These clasts are very similar to andesites of the
Castle Rocks volcanic center on the north end of Green Ridge.
Erosion
of this older volcanic center provided clasts to early Deschutes
Formation conglomerates but subsequent partial burial by Deschutes
lavas (Hales, 1975; Conrey, 1985; Wendland, personal communication, 1983)
apparently eliminated this area as a sediment source early in Deschutes
309
TABLE 8.4. CLAST COUNTS, DESCHUTES FORMATION CONGLOMERATE, ROUND BUTTE
DAM SECTION.
Stratigraphic
Position
(meters above
Pelton basalt
member)
4.6
13.9
36.9
49.2
58.5
70.2
93.5
John
Day
Fm.
0%
11%
48%
28%
5%
0%
1%
Columb.
River
Basalt
Group
0%
4%
12%
10%
1%
0%
0%
"Aphric"
(<5%
phenos.)
bas. and.
and and.
"Phyric"
(>5%
phenos.)
bas. and.
and and.
Dikty
taxit
58%
42%*
42%
18%
35%
42%
50%
55%
0%
0%
5%
9%
2%
4%
3%
43%
15%
15%
35%
43%
39%
* includes 3% gray, porphyritic hornblende andesite
(Based on counts of 200 cobbles per sample)
ic
basalt
Vitrophyres and
Cascadian
ignims.
0%
0%
2%
3%
15%
3%
2%
310
time.
Dark gray to black vitrophyre clasts of andesite to rhyodacite
composition (Table 8.5, column 6; Appendix Ik) comprise up to 25 % of
the clasts in many Deschutes Formation debrisflow and hyperconcentrated floodflow deposits (Chapter 7) but are rare in fluvial conglomerates.
The paucity of the distinctive vitrophyres in streamflow
conglomerates suggests that these lithologies composed a very minor
volume of early High Cascade eruptive products.
The enrichment of
vitrophyre fragments in debrisflow and hyperconcentrated floodflow
conglomerates suggests that the extrusion of these lavas was related to
the highsedimentload discharge events.
Occassional occurrence of
radial prismatic fractures in the vitrophyre clasts, indicative of
insitu cooling from high temperature, also relates the eruptive and
depositional events.
The vitrophyres may have been extruded as domes
whose instability lead to frequent avalanches of hot debris which mixed
with water and flowed into the Deschutes basin.
Sandstones
Framework Composition
Deschutes Formation sandstones are generally poorly to very poorly
sorted and contain 50% or more volcanic lithic fragments.
The sand-
stones are typically friable, gray to black in color, and contain
variable proportions of rounded, lightcolored pumice lapilli.
Free crytals and crystals mantled by glass are primarily plagioclase (75 to 80%) followed in abundance by augite, hypersthene,
opagues, and olivine.
Hornblende and biotite occur rarely.
Plagio-
clase composition, based on MichelLevy measurement technique, ranges
311
probably being most abundant. Russell
40-50
(1905, pg. 91) and Marlatte (1931) described Deschutes sandstones as
from An
with An
to An
30
80
quartzose but apparently confused quartz with plagioclase.
Examination
of 70 thins section failed to reveal the presence of quartz or potassium feldspar in Deschutes Formation sandstones.
These minerals are
also lacking as phenocryst phases in Deschutes volcanic rocks and are
generally rare as phenocrysts in other Oregon Cascade volcanics (Priest
and others, 1983).
Mechanical weathering and disintegration of John
Day Formation rhyolites should have liberated quartz and sanidine into
Deschutes sediment but apparently in such small relative volume that
they have yet to be recognized in thin sections.
The mineral fraction
of Deschutes Formation sandstones along the eastern basin margin is
mostly plagioclase and pyroxene derived either from John Day Formation
dacitic tuffs or Deschutes Formation airfall deposits.
The dominant lithic fraction of Deschutes Formation sandstones is
comprised of three components: 1) lightcolored pumice lapilli and
glass shards; 2) dark brown to black glass, often vesicular or with
vesiclewall margins, containing plagioclase microlites and occassional
plagioclase, pyroxene, or olivine phenocrysts; and 3) holocrystalline
grains dominated by plagioclase and intergranular pyroxene, often with
pilotaxitic texture.
The lightcolored glass is pyroclastic in origin
and derived from reworking of airfall deposits and unwelded ignimbrites.
The holocrystalline grains are mineralogically and texturally
like basaltic andesite lava flows.
The dark, glassy grains comprise
50% or more of the lithic fraction in most of the sandstones and accounts for their dark color.
The texture and mineralogy of the dark
312
3571L2511-
/ TM
.t,
_
-e
,..1.
'21'.
j
.
r
.-
'A.
A.,:-,,,,..,
,,S l'"Ir's
4:
Er ?
s- " t
''.3*f.`"....: *
'
. '
.hill( .4.:: .4%.
4.,,74,-,1
-.,
.
_
'' :. -IC
.ft, ,,,,up jig,.
' AF.11-.it'lgt
t.:(
47
-.,...,
.15k
'rait'i''
100 um
'44
ri
Fig. 8.25. Photomicrographs of Deschutes Formation sandstones.
a)
Typical ,sandstone showing abundance of black, glassy grains.
A lightcolored pumice lapillus can be seen in the lower
right corner.
An epiclastic, intersertal basaltic
andesite(?) grain at the left margin is marked "e". High
relief, angular mineral grain at center of view is
hypersthene (plane light).
b) Paleosol sandstone showing
dark rims of hematite and clay surrounding framework grains
(plane light). c) Lightcolored opaline cement partly
filling the interstices of a sandstone (plane light). Note
fractures caused by dessication of the opal. d) SEM image
of matrix between two dark sand grains.
Matrix is primarily
composed of very finegrained detrital dust, probably ash.
Cluster of platy grains marked "z" may be zeolites.
Thin rim of opal on lefthand grain marked "o".
313
hyalophitic and hyalopilitic grains is variable.
Some contain olivine
suggesting derivation from basalts or basaltic andesites while others
From petro-
contain hypersthene and are probably dacite or rhyodacite.
graphic observations it is unclear whether these grains were derived
from the weathering of glassy lava flows or flow tops, and thus epiclastic, or from reworking of pyroclastic units.
Because the dark, glassy grains are the dominant constituent of
Deschutes sandstones, chemical analyses were used to aid in evaluation
of their origin.
Four samples of coarsegrained, poorly consolidated
sandstones rich in dark, vitric grains were disaggregated and the
glassy grains handpicked with forceps and analyzed for major element
oxides by xray fluorescence methods used to analyze the volcanic
rocks.
The resulting analyses (Table 8.5, columns 1-4) show yariable
compositions but suggest that, despite the dark color, most of these
Because
grains are probably andesitic or more felsic in composition.
these are bulk analyses they provide an average composition for the
sandstones and provide little information on the relative abundance of
different rock types.
It is also possible that the analyses are in-
fluenced by minor amounts of cement adhering to the sandstone grains.
Because the cements are largely opal (see below) this could inflate the
values but, because the volume of grains to cement rinds is large,
SiO
2
this influence is probably minor.
Electron microprobe analyses of grains from three sandstones provides a better indication of the composition of glassy grains.
general types of grains are indicated in these analyses.
Three
The first
type is probably pyroclastic grains representing basaltic andesite
314
TABLE 8.5. ANALYSES OF COMPONENTS OF DESCHUTES FORMATION SEDIMENTS
1
Si02
TiO2
Al203
FeO
MgO
CaO
Na2O
K2O
2
3
4
5
6
7
8
9
10
68.8
61.3 61.8 57.3 65.8 56.31 58.86 63.07 72.13 68.1
0.82 0.75
1.10
1.55 1.46 1.02 1.73 2.88 2.09 0.42
15.17 13.82 12.11 14.26 14.78 15.6
16.8 16.1
18.0 15.1
9.28 12.62 11.79 3.73 4.60 3.89
6.12 8.22 8.27 5.31
0.75 0.5
1.70 0.21
4.0
1.84 3.25 2.01
2.7
2.7
8.45 4.12 2.09 1.67 2.53 1.93
4.93 5.54 6.59 3.91
4.1
4.38 3.95 3.93 4.56 6.52 6.3
4.4
4.1
3.9
2.27
1.96
1.96
1.39 1.65 3.17 2.94
1.73 1.20 1.25
(Analyses normalized to 100% on a water-free basis; all iron expressed
as FeO.)
Bulk lithic sandstone.
Bulk lithic sandstone.
Bulk lithic sandstone.
Bulk lithic sandstone.
Probable high Fe-Ti basaltic andesite tephra sand grain.
Probable basaltic andesite groundmass sand grain.
Probable basaltic andesite groundmass sand grain.
Rhyodacite sand grain.
High Na2O rhyodacite sand grain.
High Na20 rhyodacite conglomerate clast.
315
These grains, although largely glass, have major element
tephras.
Deschutes lavas
compositions similar to the aphyric high FeO and TiO
2
(e.g. Table 8.5, column 5).
The second type probably represents inter-
stitial glass from basaltic andesite, and perhaps andesite, lava flows.
These grains (e.g. Table 8.5, columns 6 and 7) are unusually enriched
in TiO ,
FeO, and K 0 and depleted in Al 0 , MgO, and Ca0 relative to
2
2
23
analyses of Deschutes intermediate rocks.
This suggests that these
glasses were the uncrystallized portions of magmas which had extensively crystallized plagioclase, olivine and perhaps clinopyroxene, the
common phenocrysts of Deschutes Formation basaltic andesites and
andesites.
These grains are probably epiclastic in origin and eroded
from vesicular flowtop breccias.
The third type of glassy grain has a
dacite or rhyodacite (Table 8.5, columns 8 and 9) composition and it is
not clear whether these are epiclastic obsidian and vitrophyre frag-
ments or pyroclastic grains derived from erosion of Deschutes ignimbrites which commonly contain dark dacitic, and sometimes dark rhyodacitic, pumice lapilli and bombs.
The texture and composition of
these dark silicic glasses is variable. The analysis in Table 8.5,
column 9 is notably very similar to the highNa 0 dacite vitrophyre
2
clasts found in some debrisflow deposits (column 10).
The relative
abundance of different grain types could not be determined from the
small number of analyses made.
Cements
Consideration of the diagenesis of the Deschutes Formation was
considered outside the purpose of this study.
Nonetheless, several
observations were made which may be of interest to the reader.
316
Volcanogenic sediments are renowned for their diagenetic complexities
primarily resulting from the abundance of metastable glass and ferromagnesian minerals which are unstable in the weathering environment and
in presence of typical pore fluids.
Deschutes Formation sandstones
show very little diagenetic alteration.
Their relatively pristine
condition is undoubtedly the result of a combination of factors,
including: 1) deposition in a semi-arid environment lacking the capacity for extensive chemical weathering; 2) the thinness of the unit and
lack of a significant thickness of overlying rocks (i.e. no "burial
diagenesis"); and 3) occurrence above the regional groundwater table.
Most Deschutes Formation sandstones are very poorly consolidated.
Cements are rarely resolvable in thin section or hand sample but, when
they are visible, appear to be dominated by opaline silica (Fig.
8.25c).
The silica is milky-white to pink in color and is frequently
dessicated.
Amorphous silica also occurs as permineralized replace-
ments of stems and roots, frequently fills fractures, and produces
conspicuous white seams or layers along surfaces where sediment grain
size changes or previous groundwater levels were established.
These
ubiquitous white coatings have been misinterpreted as caliche by some
previous workers (e.g. Farooqui and others, 1981a).
Pedogenically modified sandstones (facies Sm(p)) are often wellcemented.
Stringers of opal are common and, in some thin sections,
silica replacement of framework grains is apparent.
The most charac-
teristic petrographic feature of paleosols is the development of red or
brown rims of hematite and clay around the sand grains (Fig. 8.25b).
This feature is not exhibited by other Deschutes sandstones and is
317
taken to represent weathering in the solum and is further evidence of a
paleosol origin for this facies.
Debrisflow and poorly sorted hyperconcentrated floodflow
deposits tend to form the most indurated outcrops in the Deschutes
Formation.
These units often produce dark ledgeforming exposures,
particularly in the northern part of the basin (e.g. Fig. 8.8 and
center of Fig. 8.19c).
Induration may be so complete that outcrops
break indiscriminantly across clasts and matrix when struck by a hammer.
Because of their induration and poorly sorted character, these
units were informally dubbed "concretes" in the field (e.g. Jay, 1982;
Hayman, 1983; Dill, 1985).
These previous investigators were perplexed
by the origin of such extreme cementation and Jay (1982) and Hayman
(1983) suggested that the debris was emplaced hot and "baked" itself
dry to produce concrete consistency.
While evidence exists that hot
clasts were transported by some of these flows, this explanation is not
very compelling.
This author's study of debrisflow deposits in other
Neogene volcaniclastic units in Washington, Oregon, and northern
California indicates that these facies typically form wellindurated
outcrops like those in the Deschutes basin.
An instructive exposure on
the access road to Round Butte Dam shows that the concretelike induration is only a surficial phenomenon.
A natural outcrop of a ledge
forming, wellcemented hyperconcentrated floodflow deposit can be
traced into the roadcut where it is very friable.
A similar observa-
tion was made by Anderson (1933) in the Tuscan Formation in northern
California where poorly sorted volcanogenic sediments which formed
hard, cliffforming outcrops, were found to be poorly indurated a few
318
meters beneath the surface when efforts were made to excavate railroad
The origin of the cementation is not clear.
tunnels.
Scanningelec-
tron microscope examination of one Deschutes Formation sample (Fig.
8.25d) showed the presence of opal rinds, as in other sandstones, and a
matrix of finegrained ash.
Clusters of platy, euhedrallooking
mineral grains, possibly representing zeolites, also occur in the
matrix.
Perhaps this cementation is a casehardening phenomenon which
preferentially affects low permeability, matrixrich units.
Discussion
The relatively sodic composition of plagioclase, dominance of
pyroxene over olivine, and andesitic bulk composition of the dominant
dark glass fraction suggest that Deschutes Formation sandstones were
derived primarily from andesitic and more silicic materials.
This
observation is in contrast to the relative abundance of Deschutes
Formation volcanic units which although representing all compositions
from basalt to rhyolite, are dominated by basaltic andesites.
Two
explanations are offered to explain this discrepency and are difficult
to resolve because of the problems in assessing epiclastic versus pyroclastic origin of Deschutes sands.
First, as noted in Chapter 7,
intermediate compositions may be underrepresented among Deschutes
volcanic units because eruptive style and magma viscosity largely
restrict distribution of these compositions to the proximal volcanic
setting, which in this case is not exposed for study.
The sandstones,
it might be argued, represent a lessbiased view of sourcerock lithologies and thus indicate a great abundance of andesitic and dacitic
lavas in the early High Cascades.
Alternatively, sandstone composition
319
might be biased toward silicic compositions because of preferential
incorporation of easily eroded pyroclastic material.
Compared with
observations of conglomerate composition, the latter explanation
appears more probable.
The composition of conglomerate clasts is based
on mesoscopic examination in the field and is not as reliable as the
petrographic and analytical data obtained for sandstones.
Nonetheless,
the conglomerates appear dominated by basaltic andesites, with perhaps
some andesites and basalts, and the low abundance of more silicic
clasts suggests that basaltic andesite was the dominant flowrock type
in the early High Cascades.
320
CHAPTER 9.
LATE NEOGENE VOLCANOTECTONIC DEVELOPMENT OF THE CENTRAL OREGON HIGH
CASCADES.
KEY FEATURES OF THE DESCHUTES FORMATION CRITICAL TO REGIONAL TECTONICS
The preceding chapters set the stage for evaluating the tectonic
significance of the Deschutes Formation relative to the origin of the
High Cascade graben.
Detailed study of the Deschutes Formation reveals
several features of early High Cascade volcanism and tectonism which
are critical to such a discussion.
These critical features are
summarized below.
The Deschutes Formation provides the best exposed and most
lithologically diverse record of early High Cascade volcanism yet
studied.
This record clearly indicates that although basaltic andesite
and basalt magmatism occurred On a large scale at this time, early High
Cascade volcanism was more compositionally diverse than recognized in
Western Cascade studies.
It is conceivable that the explosive vol-
canism represented in Deschutes Formation ignimbrites and air falls was
restricted to a small segment of the Cascades.
Alternatively, the more
diverse volcanic record in the Deschutes basin, compared to other
accumulations of early High Cascade volcanics, may reflect the requirement of a low-relief basin to preserve silicic pyroclastic debris.
The lack of evidence for significant intrabasinal influence on
Deschutes sedimentation and the temporal correspondance of aggradation
to the early High Cascade eruptive episode strongly suggest that sedimentation was volcanism induced.
The absence of a similar depositional
record correlative with previous and subsequent Cascade eruptive epi-
321
sodes suggests that pyroclastic volcanism occurred on a larger scale in
the central Oregon Cascades during Deschutes Formation time.
3) The petrology of Deschutes Formation volcanics with Cascade
provenance is atypical for convergentmargin arcs.
The abundance of
primitive basalts and petrologic features consistent with highlevel
fractionation to produce unusually iron and titaniumrich magmas are
probable indicators of an extensional tectonic environment in the
adjacent High Cascades.
4)Faults bounding the east side of the central Oregon High Cascade
graben occur along Green Ridge.
The Deschutes Formation volcanic
record is most voluminous at and south of the latitude of Green Ridge.
The lack of a comparable volcanic record in the northern Deschutes
basin, where there is no evidence for a graben, strongly suggests that
subsidence was localized to those portions of the High Cascades where
the largest volumes of magma had been extruded prior to formation of
the graben.
5) The distribution of volcanic units and variation in depositional style of Deschutes Formation sedimentary rocks indicates that ini-
tial subsidence along westfacing faults occurred west of Green Ridge
and also isolated the Three Sisters region from the Deschutes basin.-
Explosive eruption of silicic magmas continued following initial subsidence and the products of these eruptions may have accumulated to great
thicknesses within the graben before being buried by mafic platform
lavas.
Following initial subsidence basalts and basaltic andesites
were erupted east of the first escarpments, in part from vents along or
near the site of subsequent faults at Green Ridge.
Later eruption of
322
mafic lavas along older fault trends within the Deschutes basin suggests that the effects of extension migrated outward from the Cascades.
THE NATURE OF CASCADE EASTFLANK STRUCTURE NORTH AND SOUTH OF GREEN
RIDGE
Green Ridge provides a prominent structural discontinuity along
the east flank of the central Oregon High Cascades.
However, the
abrupt termination of the feature, at both ends, raises critical
questions about the continuity of the High Cascade graben and its
structural geometry.
Do major faults continue to the north and south
which have been buried by younger volcanics?
Does offset along the
Green Ridge fault zone gradually diminish to an insignificant amount
along strike?
Are the faults along Green Ridge truncated by.cross-
cutting structures?
Because of the the large volume of postearly
Pliocene volcanics which now obscure structures associated with the
early High Cascade graben, it is impossible to provide specific, conclusive answers to these questions.
However, study of the Deschutes
basin in the context of regional geologic and geophysical investigations offers insight to the problems.
There is no compelling evidence for northward continuation of the
Green Ridge faults beyond the latitude of Mount Jefferson.
Relief
along the northern end of Green Ridge is constructional, not structural, and faults which are present exhibit less than 100 m of offset
(Wendland, person. commun., 1984).
The Pliocene basalts capping the
Deschutes Formation on the Warm Springs Indian Reservation probably
represent early, mafic platform basalts which were not impeded by
significant fault scarps from flowing into the Deschutes basin.
323
PRE-4.0 M.Y.
VOLCANIC
CENTER?
EOCENE TO MIDDLE
MIOCENE VOLCANICS
20
0
KILOMETERS
x OLLALIE
BUTTE
PRE-4.0 M.Y.
VOLCANIC
CENTERS
MT.
JEFFERSON
ANTIA
7.5-10 M.Y.
VOLCANIC
CENTERS \
X
HIGH
CASCADE
MAFIC
PLATFORM
DESCHUTES
FORMATION
CASCADE
LAVAS
PS8509-134
Fig. 9.1. Generalized geologic map of the central Oregon Cascades and
northern Deschutes basin.
324
Shitike Butte and other volcanic centers standing above the platform
lavas probably represent Miocene volcanics whose exposure precludes
significant subsidence along the High Cascade axis north of Mount
Jefferson.
High
No faults have been mapped along the Western Cascade
Cascade boundary west of Green Ridge but the linear nature of this
boundary and its influence on the course of the North Fork of the
Santiam River strongly suggests structural control (Fig. 9.1).
East
facing fault scarps have probably been eroded back and the faults
themselves partially buried by mafic platform lavas.
Southwest of
Mount Jefferson the contact between mafic platform lavas and older
volcanics changes from a probable structural relationship to a
depositional contact (Fig. 9.1).
Mapping by Rollins (1976), Hammond
and others (1982), and G. Priest and M. Ferns (Dept. Geol. Min. Ind.,
in progress) shows that lavas and volcaniclastics probably correlative
to the Deschutes Formation occur eastward to within a few kilometers of
the base of Mount Jefferson.
These rocks are overlain by mafic
platform basalts and basaltic andesites mapped as the Minto Lavas by
Thayer (1939) which have yielded KAr ages between 3.06 + 0.05 Ma and
0.68 + 0.03 Ma (G. Priest and R. Duncan, unpub. data, 1984).
Lacustrine sediments occur locally beneath the Minto Lavas over an area
2
of at least 80 km
(M. Ferns, person. commun., 1985) and contain a
diatom flora similar to the Camp Sherman beds in the Metolius valley
(J. P. Bradbury, person. commun., 1985).
The Minto Lavas impinge upon
a highland to the west which is capped by middle to upper Miocene lavas
and minor pyroclastic debris some of which was erupted from nearby
325
vents represented by dikes and intercalated tuff-cone deposits (Priest
and others, 1984).
The occurrence of lacustrine sediments below the
oldest known mafic platform lavas and the probable correlation of the
volcanics capping the highland to those beneath the Minto Lavas 100 to
200 m lbwer in elevation suggests that early Pliocene faulting did
affect this area.
However, the offset on these faults must be much
less than that suggested by the 500 to 700 m-high linear escarpment to
the south and, unless a larger fault occurs beneath Mount Jefferson,
suggests that displacement on graben-bounding faults on the west side
decreases northward as is also implied on Green Ridge.
Geophysical data also fail to support continuation of the High
Cascade graben north of Mount Jefferson.
Gravity anomaly lineations
show a strong N-S or NNE-SSW orientation south of Mount Jefferson which
may represent faults produced by extension normal to the Cascades
(Couch and others, 1982).
These lineations are not prominent farther
north where northeast and northwest trends are dominant (Fig. 5.11;
Couch and others, 1982).
Interpretation of magnetotelluric profiles
are consistent with north-south-trending normal faults in the High
Cascades at the latitude of Santiam Pass but extensional structures are
not apparent on a transect located 30 km north of Mount Jefferson
(Stanley, 1983, and person. commun., 1983).
Extensional structures reappear in the Mount Hood region (Priest,
1982; Williams and others, 1982) but are largely absent between Mount
Hood and Mount Jefferson (Fig. 9.2).
Mapping by Hammond and others
(1982) indicates the presence of a zone of northwest to north-northwest
trending faults, at least 40 km wide, as the dominant structural pat-
326
100
KILOMETERS
OREGON
HIGH CASCADES
<>
MAJOR VOLCANO
C3) CALDERA
GRABEN-RELATED
FAULTS
HOOD RIVER
GREEN RIDGE
TUMALO
McKENZIE
BRIDGE - HORSE
CREEK
COUGAR
RESERVOIR
WALDO LAKE
GROUNDHOG
CREEK
WALKER RIM
UJ
FLU
0
PNVel
KLAMATH
GRABEN
PS8509-136
Fig. 9.2. Structural features of the Oregon Cascade Range.
327
tern in this intervening region.
Where north-south faults occur they
generally truncate northwest-trending faults and are believed to be
generally younger than 5 Ma (Hammond and others, 1980).
White (1980)
illustrated that Miocene to early Pliocene volcanics can be mapped at
the surface nearly to the High Cascade axis in this region and are
overlain by a relatively thin veneer of Pleistocene basalt and basaltic
andesite.
The faults transecting the Cascades north of Mount Jefferson are
part of a broad belt of northwest-trending faults which continue westward across the Willamette valley and into the Coast Range.
The most
prominent of these structural zones is coincident with the Clackamas
River (Fig. 9.2).
Study of the Clackamas River fault zone by Anderson
(1978) showed that displacements are oblique slip with 100 to 500 m of
normal displacement demonstrated on individual faults and an uncertain
magnitude of right-lateral strike-slip movement.
horizontal slickensides.
Fault planes contain
Beeson and others (1985) argue that the
Clackamas River fault zone continues westward, where it is expressed by
the Portland Hills anticline and related faulting, and that other
northwest-trending zones can be traced from the Cascade foothills
across the Willamette Valley and can be connected to prominent Coast
Range structures. Movement along these structures during the middle
Miocene is indicated by thickness variations and distribution of flows
of the Frenchman Springs Member of the Wanapum Basalt of the Columbia
River Basalt Group (Beeson and others, 1985).
Hammond and others
(1982) show many of these northwest-trending faults offsetting Pliocene
volcanics near the High Cascade axis and White (1980) inferred dis-
328
placement of Quaternary units.
The geophysical character of the region north of Mount Jefferson
(Couch and others, 1982; Stanley, 1983) suggests that the zone of
northwesttrending faults represents a fundamental crustal structure.
Based on heatflow data, the Clackamas River zone also separates an
area of regionally high heat flow to the south from an area of generally lower heat flow and local "hot spots" to the north (Black and
others, 1983).
Combined with geologic evidence these geophysical
observations suggest that the transition from northsouth normal
faults, south of Mount Jefferson, to northwesttrending obliqueslip
faults to the north may reflect a structural truncation of the graben.
Structural truncation of the graben may have been dynamic, by
crossfaulting on northwest trends, or passive, reflected by the lack
of extension in the relatively cold crust north of the presumed intracrustal boundary.
In order to resolve this question it is important to
determine if the northwesttrending structures continue across the
Cascades and connect with fault zones on the east side.
Assuming no
change in strike, the Clackamas River fault zone does not trend toward
the Brothers or Tumalo fault zones as proposed by Anderson (1978) and
Hammond and others (1980).
The Clackamas River faults do, however,
-
line up with the Metolius River lineament along the north end of Green
Ridge.
As discussed in Chapter 5, there are no faults within the Metolius
canyon which can be related to the prominent lineament.
However, that
does not exclude the lineament from having tectonic significance.
Structurally controlled drainages in volcanic areas may not directly
329
coincide with the influential structure because lava flows may fill a
faultcontrolled valley and force the stream to cut a hew channel which
will be parallel to, but not coincident with, the fault.
With this
problem in mind it is worthy to note that the lower,Metolius River
The
flows adjacent to the south margin of the Metolius Bench basalts.
morphology of Metolius Bench suggests that these basalts filled an
ancestral NWSE trending Metolius River Valley.
This orientation is
unusual because paleocurrent data, channel orientations, and distribu-
tion of lava flows indicates that this area had an east to northeast
trending paleoslope during Deschutes time (Figs. 8.10a, 9.1, 9.3).
This abrupt change in drainage direction, the linear nature of the
valley, and the coincidence with the trend
of the Clackamas River
fault zone suggest that faulting, though perhaps involving only minor
displacements, occurred along northwest trends across the north end of
Green Ridge during the same time interval that graben subsidence was
occuring.
Northwesttrending late Miocene dikes in the lower
Whitewater River canyon (Yogodzinski, 1986) are further evidence of the
continuation of this structural zone across the Cascades.
Other evidence for deformation along a northwest trend can be
found along the Metolius River west of Castle Rocks.
Bedded basaltic
andesite tuff, associated with the Castle Rocks volcano or an older
vent, has been sheared and resulting granulation has reduced porosity
to create features analogous to deformation bands observed in sandstones (Aydin, 1977; Smith, 1983).
deformation bands is N 35 W.
The dominant orientation of the
The offset across this zone and the age
of deformation relative to northsouth trending faults which strike
330
toward this area from less than 1
km to the north is not known.
Although the north end of Green Ridge appears to represent the
north end of the High Cascade graben, the depression undoubtedly
extended a considerable distance south of Green Ridge.
Downtothe-
east faults along the McKenzie River and Horse Creek, west of the Three
Sisters (Fig. 9.2), are interpreted to represent southward continuation
of grabenbounding faults on the west side of the depression (Flaherty,
1981; Priest and others, 1983).
Faulting on the east side at the lati-
tude of the Three Sisters is impled by the stratigraphy of the
Deschutes Formation.
Voluminous pyroclastic flows entered the southern
Deschutes basin from the present Three Sisters
Broken Top vicinity
until near the end of Deschutes Formation time when coarsegrained
sedimentation with numerous pyroclastic flows abruptly changed to widespread paleosol development occurred.
As argued in Chapters 7 and 8,
this abrupt change is best explained by intraarc subsidence isolating
the High Cascades from the Deschutes basin.
If this subsidence was
restricted to the latitude of Green Ridge there would be no marked
change in the stratigraphic sequence in the southern Deschutes basin.
As discussed in Chapter 5, northsouth trending faults on the
south end of Green Ridge bend to a northwest trend and merge with the
Tumalo fault zone (Fig. 5.10, 5.12).
Taylor (1978) suggested that the
northwesttrending faults, being of much smaller displacement and cutting Quaternary rocks, were not related to Pliocene graben development
and that a southward extension of the Green Ridge fault likely exists
beneath the highland of Pleistocene silicic rocks capped by mafic lavas
and surmounted by Broken Top (Fig. 9.3).
There are problems with
Madras
LAKE
CHINOOK
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BLACK
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MtKENZIE
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IGNIMBRITE
MEMBER-
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SPRINGS 1KILOMETERS
INLIER
1
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i
rl
THREE
SISTERS
Send
PS8509-135
Fig.
9.3.
Physiographic map of the Deschutes basin and adjacent High
Present difference in drainage direction from west
to east, along Green Ridge, to southwest to northeast, east
of the Three Sisters, reflects the influence of the eastern
protuberance of the Cascades referred to as the "silicic
Paleocurrents in the Deschutes Formation (arrows)
highland".
and characteristics of ignimbrites (e.g. McKenzie Canyon
ignimbrite member) suggesting southwestern sources, imply a
similar paleogeography during the late Neogene. The remnant
of an earlier highland crops out near Bull Springs.
Cascades.
-
332
extending the Green Ridge faults southward, however.
1) the Green
Ridge fault zone bends to a northwest trend, it does not appear to be
truncated or cut by the Tumalo fault zone (Figs. 5.10 and 5.12).
2) If
the Green Ridge fault zone extended south from Black Butte it is
difficult to explain the low relief of the area west of Sisters which
should be a horst block (Fig. 9.3).
3) Distribution of ignimbrites and
paleocurrent data indicate that a volcanic highland occupied the
position of the Pleistocene silicic highland during Deschutes time.
A
lone exposure of this older highland, northwest of Bend, suggests that
it may have been similar in size and elevation to the present highland
(Fig. 9.3).
Faulting on a northsouth trend on strike with Green Ridge
should have left .a highstanding horst similar to Green Ridge, where
the highland was transected.
The absence of such a feature suggests
that displacement across a broad zone of northwesttrending faults led
to subsidence of this entire highland and nearly complete burial beneath volcanics of the present highland.
he above evidence favors southeastward continuation of eastside
grabenbounding faults along the Tumalo fault zone and/or parallel
faults southwest of the present scarps.
Subsidence may have occurred
across a broad zone with most fault scarps buried by younger volcanics.
If the Pleistocene silicic volcanism was a continuation of Deschutes
Formation volcanism then the approximately 3 to 4 million year hiatus
between the last Deschutes ignimbrites and coarsegrained sediments,
and the Pleistocene ignimbrites and similar intercalated sediments may
reflect the time period required
o sufficiently bury the fault scarps
and allow Cascade eruptive produc s to enter the basin once again.
The
333
young faults along the Tumalo zone would then represent reactivation of
some of the grabenrelated
faults.
RELATIONSHIP OF THE HIGH CASCADE GRABEN TO BASIN AND RANGE EXTENSION
Taylor (1980a), Magill and others (1982) and Priest and others
(1983) suggested that the preponderance of mafic magmatism during the
early High Cascade episode and formation of the intraarc graben reflects a modification of Cascade magmatism and stress regime by Basin
This influence of Basin and Range processes in
and Range extension.
the central Oregon Cascades is supported by; 1) proximity of the two
provinces; 2) similarity in timing of development of the present extensional topography; and
and others, 1983).
3) eruption of similar basaltic magmas (Priest
Distinguishing between Basin and Range influence
and arc processes in the formation of the High Cascade graben and
contemporary magmas is critical to evaluating late Cenozoic tectonics
of the Pacific Northwest and the geothermal potential of the Cascades.
Petrologic arguments for Basin and Range influence on High Cascade
magmatism are ambiguous.
compositional data
Priest and others (1983), using majorelement
and petrographic observations, illustrated presumed
similarites between High Cascade mafic lavas and Basin and Range basalts.
Specifically, these similarities were illustrated by overlap of
fields on variation diagrams, alkaline nature of some High Cascade
basalts, and common occurrence of diktytaxitic basalts in both provinces over the last 10 million years.
It is important to note that the
bulk of High Cascade basalts are distinctly enriched in K and Sr relative to the voluminous high alumina olivine tholeiites of the northwestern Basin and Range.
The low potassium content of these Basin and
334
Range basalts is their distinctive petrologic characteristic (Hart and
others, 1983).
To date, the only analyszed High Cascade basalts with
similar chemistry are the Deschutes Formation diktytaxitic basalts.
The alkaline nature of High Cascade basalts (Priest and others,
1983) has probably been overemphasized.
Although many late Miocene to
Recent Cascade basalts plot in the alkali basalt field of the total
alkalies versus silica variation diagram of MacDonald and Katsura
(1963) and frequently contain clinopyroxene with petrographic features
of titanaugite, nepheline normative rocks are very unusual.
It is
important to remember that titanaugite is a reflection of titanium
content, not alkaline chemistry, and although considered characteristic
of Hawaiian alkali basalts, is commonly seen in tholeiites (C. Hughes,
1983).
Also, caution should be exercised when applying the MacDonald
and Katsura (1963) diagram, developed from study of Hawaiian suites, to
highalumina basalts.
The occurrence of diktytaxitic texture in High
Cascade basalts is also not compelling evidence for a relation to Basin
and Range magmatism because this texture occurs in basalts of a variety
of compositions in many tectonic environments (Goff, 1977).
The large volume of basalt erupted during the development of the
High Cascades and the evidence that highlevel fractionation produceed
iron and titaniumrich basaltic andesites and andesites is supportive
of intraarc extension. However, influence of Basin and Range magmatism
is not clearly defined.
Another critical observation is that the unequivocal Pliocene
intraarc depressions occurred north of the Brothers fault zone which
forms the northern boundary of Basin and Range structures in central
335
Oregon (Lawrence, 1976).
Other than the central Oregon High Cascade
graben, another depression is centered on Mount Hood (Williams and
others, 1982) and mafic magmatism, associated with north-south trending
faults and fissure systems continue into southern Washington (Hammond
and others, 1976).
Clearly defined intra-arc grabens are lacking in
southern Oregon where Basin and Range grabens impinge upon the High
Cascades.
Available geologic mapping shows that High Cascade rocks in
southern Oregon are largely basalts and basaltic andesites (Woller and
Black, 1983), like farther north, but potential graben-bounding faults
(Waldo Lake and Groundhog Creek faults) are only inferred on the basis
of tenuous field relationships (Woller and Black,
1983) and gravity
anomalies (Couch and others, 1982; Blake and others, 1985) and have no
topographic expression (Fig. 9.2).
Sherrod (1985) argues that these
faults are limited in both displacement and continuity and are not
likely to represent graben-bounding structures.
The Basin and Range
Klamath graben trends toward the High Cascades (Fig. 9.2) but is not a
well-defined structure within the Cascades (Smith and others, 1982).
If formation of intra-arc graben was a reflection of invasion of Basin
and Range extension into the Cascades it is difficult to explain why
graben structures are poorly-defined, or lacking altogether, in the
southern Oregon High Cascades but are clearly evident to the north
beyond the latitudes of adjacent Basin and Range extension.
Equally
important is the interpretation of eastward stepping of faulting at
Green Ridge followed by structurally controlled volcanism within the
Deschutes basin, suggesting that the Cascade chain itself was the locus
of extension.
336
Also important to evaluating the influence of Basin and Range
processes is consideration of structural and petrologic evidence for
earlier episodes of extension within the Cascade Range.
Basalts and
basaltic andesites illustrating iron and titanium enrichment similar to
Deschutes Formation rocks occur in the Eocene and Oligocene section in
the Washington Cascades (Wise, 1970; Ort and others, 1983) and are
widespread in the early Western Cascade sequence in Oregon (White,
1980; Lux, 1981; Priest and others, 1983).
Many of the early and
middle Miocene basalts in the Oregon Western Cascades are olivine
phyric and diktytaxitic resembling younger High Cascade lavas (Woller
and Black, 1983; G. Walker, person. commun., 1985).
Thus petrologic
processes similar to those operative during the Oregon High Cascade
eruptive episodes are' represented by mafic lavas over a large area of
the Cascade Range throughout much of the Tertiary.
Evidence for earlier periods of intraarc graben development is'
found in gravity data compiled and interpreted by Couch and others
(1982) suggesting that the Pliocene central Oregon High Cascade graben
is nested within an older, more continuous Cascade graben.
Priest and
others (1983) interpret subsidence on these older faults west of the
Three Sisters (Cougar Reservoir, Fig. 9.2) to have occurred between 8.5
and 13 Ma.
Late Eocene to early Miocene volcanics and volcanogenic sediments
accumulated to thicknesses of 3 to 6 km in the Washington Cascades
while topographic relief remained subdued (Fiske and other, 1963; Wise,
1970).
This observation suggests repeated or continuous subsidence of
the early Cascades but unequivocal evidence of graben structures is not
337
apparent from currently available maps.
Burial by younger volcanics makes interpretation of the structures
associated with early Cascade development difficult to evaluate.
None-
theless, the observations summarized above indicate that extension has
episodically influenced Cascade magmatism and structural development
and is not entirely correlative, temporally or spatially, with Basin
and Range processes.
FORMATION OF INTRA-ARC GRABENS
The intra-arc grabens of the Cascade Range are not unique to the
northwestern United States but have counterparts throughout the circumPacific region.
Studies of convergent-margin arcs generally emphasize
volcanological and petrological aspects and detailed structural
evaluations are typically lacking.
Nonetheless, considerable insight
into the origin of the central Oregon High Cascade graben may come from
comparing it to similar intra-arc depressions.
Perhaps the best documented occurrences of intra-arc extensional
structures are the grabens associated with the Central American arc
(Fig. 9.4).
The Nicaraguan Depression contains the present arc within
most of Nicaragua and Costa Rica and is bounded on the southwest by a
1000 m high, 70 km long fault escarpment and on the northeast by a
faulted monoclinal flexure (McBirney and Williams, 1965).
Subsidence
occurred during late Pliocene or early Pleistocene following eruption
of andesitic and dacitic ignimbrites with subordinate olivine basalt
flows from the site of the graben (McBirney and others, 1965; Dengo and
others, 1970).
The depression is partly filled with more than 1 km of
alluvium, lake sediments, and ash that buries all but a few isolated
338
hills of Tertiary rocks.
Quaternary volcanism within the graben has
been dominated by basalt and basaltic andesite lavas with subordinate
eruption of dacitic pyroclastics (McBirney and Williams,
1980).
1965; Weyl,
Smaller north-south trending grabens cut across the larger
northwest-southeast trending depression.
graben and half-grabens
Discontinuous development of
can be traced northwestward into El Salvador.
In Guatemala the arc strikes nearly east-west but most faulting within
the arc bounds graben with north-south trends and is associated with
bimodal volcanism (Williams, 1960; Williams and others, 1964).
There has been no consensus on the origin of the Central America
graben structures.
Carr (1976) interprets the intra-arc graben to be
analogous to pull-apart basins between graben-bounding, northwest
trending strike-slip faults combined with subsidence because of longterm magmatic withdrawl.
interpretation.
Earthquake focal-mechanism data support this
Weyl (1980) calls for magmatic withdrawl and crustal
loading by volcanic edifices to produce subsidence.
Wadge and Burke
(1983) point to extension in northern and compression in southern
Central America as confirmation of counterclockwise rotation of the
Caribbean plate predicted by plate geometry considerations.
This rota-
tion model is most suitable for explaining the north-south trending
grabens.
Extensional structures also occur within and parallel to the
Andean volcanic chain in South America.
Segments of the volcanic chain
which have experienced the largest volume of late Cenozoic magmatism
are associated with grabens and half-grabens, exhibiting 2 to 10 km of
subsidence, both within (Altiplano of Peru and Bolivia) and immediately
339
NICARAGUA (McBirney and Williams, 1965):
PACIFIC
OCEAN
QUATERNARY
VOLCANICS
TERTIARY
SEDIMENTS
TERTIARY LAVAS
AND TUFFS CAPPED
BY IGNIMBRITES
QUATERNARY
SEDIMENTS
IGNIMBRITE
PLATEAU
NICARAGUAN DEPRESSION
COASTAL
PLAIN
10
0
20
KILOMETERS
HOLOCENE
AiVOLCANICS
KAMCHATKA (Erlich, 1968):
TERTIARY
ROCKS
PLEISTOCENE
VOLCANICS
20
0
I
I
40
VE = 8:1
I
KILOMETERS
KYUSHU (Yamasaki and Hayashi, 1976):
PLEISTOCENE
VOLCANICS
PRE-TERTIARY
ROCKS
MIOCENE VOLCANICS
10
KILOMETERS
VE = 2:1
PS8509-133
Fig. 9.4. Crosssections through intraarc grabens in Central America,
Kamchatka, and Japan.
340
Zeil
trenchward (Valle Longitudinal of Chile) of the arc (Zeil, 1979).
(1979) believes the coincidence of volcanism and graben structures is
evidence for magmatic processes producing extension.
Alternatively,
Suarez and others (1983) suggest that extension of the Altiplano is a
result of gravitational body forces acting upon the highstanding Andes
causing extension at high elevations and eastdirected thrust faulting
a few tens of kilometers away at lower elevations.
The andesitic to rhyolitic calcalkaline volcanics of North
Island, New Zealand are largely confined within the Taupo Volcanic
Zone, a graben up to 40 km wide and at least 180 km long, which has
subsided 2 to 4 km in the last 1 million years (Healey, 1962; Cole,
1979; Cole and Lewis, 1981).
Volcanism within this zone has been
dominantly rhyolitic, including the formation of numerous calderas and
extensive ignimbrites, with less voluminous andesite and dacite strato-
volcanoes. The depression is separated from the forearc-basin by a
highland of preTertiary marine sedimentary rocks which ii also a zone
of strikeslip faulting that reflects the highly oblique nature of
subduction of the Pacific plate beneath New Zealand (Cole and Lewis,
1981).
The Taupo zone is considered by Cole (1979) and Cole and Lewis
(1981) to be an onshore extension of the LauHavre back arc basin
which borders the Kermadec and Tonga island arcs to the north.
This
interpretation is difficult to justify with Cole and Lewis' (1981) maps
(their Figure 10) showing that the calcalkaline rocks of the Taupo
zone are on strike with the island arcs to the north and that the
Lau
Havre rift intersects New Zealand 75 km farther west in an area of
alkalic and pantelleritic volcanism more typical of rift environments.
341
Thus the graben of the Taupo zone appears to be an intraarc graben
whose development may be related to transtension along an oblique
subduction boundary.
Intraarc grabens have been shown to develop intermittently on
Kyushu, Japan by Yamasaki and .Hayashi (1976).
The present graben
structure is 12 to 40 km wide, at least 150 km long, and is associated
with several major calderas (Fig. 9.4).
A larger depression of uncer-
tain structural style stretches for over 800 km across southwest Japan,
is up to 150 km wide, and contains most of the region's volcanic
centers.
This larger depression was the site of extensive lacustrine
deposition during the late Miocene and was invaded by marine waters
during the PlioPleistocene.
Volcanic centers of the Kamchatka Peninsula are almost entirely
confined to Pleistocene graben 10 to 75 km wide, up to 300 km long, and
1.5 to 2.0 km deep ( Fig. 9.4; Erlich, 1968).
The graben occur along
two parallel trends and are nested within a larger Neogene graben up to
300 km wide which contains at least 1 km of Neogene sedimentary fill.
Although a wide compositional spectrum is exhibited by Kamchatka volcanics, basalts and basaltic andesites predominate.
Spence (1977) and Kay and others (1982) have noted that local
regions of extension occur along the Aleutian Ridge where the
subducting Pacific plate is segmented, in part coincident with fracture
zones.
Large, primarily basaltic volcanoes, exhibiting highlevel
tholeiitic fractionation trends similar to Deschutes Formation
volcanics are located in these areas of extension and are separated by
smaller, generally more silicic calcalkaline eruptive centers (Kay and
342
others, 1982).
Large submarine volcanotectonic depresssions on the
order of 50 to 100 km long, 20 to 35 km wide, and 0.5 to 2.0 km deep
occur on the axis of the Aleutian Ridge at these segment boundaries
(Perry and Nichols, 1966; Marlow and others, 1970).
Spence's (1970)
suggestion that these depressions represent areas lacking arc development because of lack of magmatic head is inconsistent with their
location atop a constructional ridge standing over 2 km above adjacent
sea floor and bathymetric evidence of highangle bounding faults
(Marlow and others, 1970).
From this review it is clear that intraarc graben occur with a
variety of dimensions, tectonic settings and associated magmatism.
Strikeslip faulting probably plays an important role in the
development
of the New Zealand and Central American graben but is not
obviously apparent in the other localities.
Notably, the Oregon
Cascades are located along an oblique subduction margin (Wells and
others, 1984) and strikeslip displacement on grabenbounding faults
would be consistent with regional compressive stress orientation.
The
Washington Cascades, north of Mount Rainier, are oriented at a higher
angle to the Juan de Fuca convergence vector and are not associated
with extensional structures or mafic volcanism (Duffield, 1983; Rogers,
1985).
Fitch (1972) illustrated that oblique subduction is decomposed
into a thrust component on the convergent boundary and a strikeslip
component in or near the volcanic arc.
Dewey (1980) further suggests
that extension within arcs along oblique subduction zones would result
because of this strikeslip faulting.
Transtension within thermally
343
weakened arc crust may account for intra-arc extension within the
Oregon Cascades (as well as New Zealand and Central America) and account for the differences in structural and magmatic character of the
Oregon and Washington High Cascades.
Reorganization of relative
rotation poles for the Juan de Fuca ridge at 8.5 Ma and 5.0Ma (Wilson
and others, 1984) may have increased the obliquity of convergence; the
earlier date possibly related to the onset of extension in the Oregon
High Cascades and the later date to graben formation.
Mafic volcanism is predominant within intra-arc depressions of the
Cascades, Nicaragua, Kamchatka, and the Aleutians.
Conversely, large-
scale rhyolitic magmatism is localized in the Japan and New Zealand
graben and volcanism in Guatemala is bimodal.
If the rhyolitic volcan-
ism is a reflection of anatexis of thick crust by rising mafic magmas
(Hildreth, 1981) it is possible to relate all of these varieties of
magmatism to extensional tapping of mafic magmas with the disparities
representing differences in crustal thickness and flux of mafic magmas
from the mantle.
It is also interesting to note that silicic pyro-
clastic volcanism was volumetrically important during volcanic episodes
culminating in graben development in the central Oregon Cascades and
Nicaragua but is subordinate to mafic volcanism now.
The volcano-tectonic history of Japan and Kamchatka suggests
several episodes of graben development and formation of graben within
graben.
Likewise-the Cenozoic volcanic record of the Cascades indi-
cates multiple periods of graben formation, and the geophysical interpretations of Couch and others (1982), in conjunction with field
studies, suggests the occurrence of nested graben.
344
Thus the central Oregon High Cascade graben shares many features
with other intraarc depressions surrounding the Pacific basin.
However, development of a hypothesis for graben formation is difficult
because there does not appear to be a set of common denominators link-
ing circumPacific depressions. It is notable, however, that the occur-
rence of backarc extension is not requisite for the formation of
intraarc graben.
Fyfe and McBirney (1975) and Hildebrand and Bowring (1984) relate
the formation of intraarc depressions to subsidence resulting from
The
withdrawl of material from below and thus not requiring extension.
model of Hildebrand and Bowring (1984) is based on semiquantitative
calculations which show that there is a mass balance between the volume
of mafic magmas arriving at the base of the crust and ash removed by
highlevel atmospheric transport during Plinian eruptions.
This model
requires that intraarc depressions be longactive synclinal downwarps
and that they be associated with voluminous pyroclastic eruptions.
Because nearly all such depressions described in the literature are
fault bounded, develop episodically, and are not all related to periods
of voluminous pyroclastic extrusion, the model of Hildebrand and
Bowring is not very tenable as a general explanation for the origin of
intraarc graben.
The importance of pyroclastic volcanism in the formation of the
central Oregon High Cascade graben is difficult to address.
The record
of pyroclastic volcanism in the Deschutes Formation is much larger than
that exhibited by early High Cascade volcanics in southern Oregon where
graben structures are poorly defined or nonexistant (Woller and Black,
345
1983; D. Sherrod, person. commun., 1984).
However, the Deschutes pyro-
clastic record is also much larger than that reported from contemporaneous Western Cascade rocks at the latitude of the Deschutes basin suggesting that the absence of pyroclastics in southern Oregon could be a
matter of nonpreservation or buried rather than nondeposition.
However, a relationship between magmatism and extension is suggested by the larger volume of Deschutes Formation volcanics at the
latitude of known graben faults relative to the northern Deschutes
basin where subsidence of the adjacent Cascades cannot be documented.
Does this mean that significant subsidence occurs only where large
volumes of magma have been withdrawn from the lower crust and large
volcanic edifices place additional load on thermally thinned and weakened crust?
Or, does this observation reflect passage of large volumes
of magma to the surface over restricted regions which are experiencing
extensional strain?
Stratigraphic evidence of relatively rapid subsi-
dence near the end of the early High Cascade eruptive episode, rather
than slow subsidence throughout this period, and the inferred highlevel fractionation of early High Cascade magmas suggests that the
latter explanation, requiring extension, is more likely.
It is tempt-
ing to speculate that the subsequent development of normal faults in
the northern Deschutes basin (Chapter 5) and adjacent High Cascades
(Hammond and others, 1982) signals an approaching period of intra-arc
extension between the central Oregon High Cascade graben and the depression at Mount Hood.
CONCLUSIONS
Formation of the central Oregon High Cascade graben cannot be
346
unambiguously related to Basin and Range tectonomagmatism or to processes indigenous to the arc itself.
However, prevailing inter-
pretations of a strong Basin and Range influence on the High Cascades
should be tempered by: 1) the occurrence of intraarc graben in other
circumPacific arcs lacking backarc rifts; 2) episodic development of
intraarc depressions within the Cascades throughout the Cenozoic which
are not temporally or spatially related to Basin and Range processes;
and 3) the best development of Neogene intraarc extensional features
at more northerly latitudes than Basin and Range extension.
The
development of the central Oregon High Cascade graben may be a
reflection of the oblique orientation of the arc relative to the Juan
de Fuca convergence vector resulting in transtension of thermally
weakened arc crust.
Other contributing causes of intraarc extension
include decrease in convergence rate (Wells and others, 1984) and/or
coupling of the Juan de Fuca and North American plates at the latitude
of Oregon (Weaver and Michaelson, 1985).
The High Cascade graben does not appear to be a continuous feature
as first proposed by Allen (1966).
The depression between the lati-
tudes of Mount Jefferson and the Three Sisters ends northward in a
region of cooler, thicker (?) crust transected by northwesttrendingfaults with long deformation histories.
There is a suggestion that
some of these northwesttrending faults were active during intraarc
extension and truncated the resulting graben.
South of Green Ridge the
eastern boundary of the Pliocene High Cascade graben is probably coincident with the Tumalo, and possibly Walker Rim, fault zones which have
continued to be active into the Quaternary.
Northsouth vent align-
347
ments and dominance of mafic volcanism suggests southward continuation
of intraarc extension but graben structures, if they exist, lack
surface expression.
348
CHAPTER 10
THE DESCHUTES FORMATION AND THE EARLY HIGH CASCADES
CONCLUSIONS
Although the ancestral central Oregon High Cascade volcanic
centers are not presently exposed, the stratigraphy, petrology, and
sedimentology of the Deschutes Formation provide considerable insight
into late Miocene to early Pliocene volcanism.
Early High Cascade
magmatism was atypical of continental-margin arcs and high-level fractionation, favored by crustal extension, best explains its compositional traits.
This period of extension culminated in the development of
an intra-arc graben spatially related to that portion of the High
Cascades which had experienced eruption of the largest volume of volcanic products.
Transtension within the arc, because of the oblique
orientation of the Juan de Fuca convergent vector relative to the
Oregon High Cascades, is at least as tenable as more popular suggestions that extension reflects an invasion of the Cascade Range by Basin
and Range tectonomagmatic processes.
Within a stratigraphic framework defined by widespread volcanic
units, basin analysis of the Deschutes Formation illustrates several
important features of early High Cascade volcanism.
First, ,although
basalts and basaltic andesites were volumetrically important components
of this volcanic episode, early High Cascade volcanism emplaced a
larger proportion of silicic pyroclastic units than during immediately
previous or subsequent episodes.
This aspect of early High Cascade
volcanism has not previously received much attention, apparently because pyroclastic deposits were not extensively preserved on the west
side of the arc.
Second, initial subsidence of the graben occurred
349
along faults located west of Green Ridge at about 5.6 Ma and isolated
the Deschutes basin from pyroclastic flows and eruptioninduced sedimentation which dominate the volcaniclastic portion of the Deschutes
Formation.
Subsequent mafic lavas were erupted from vents near and
coincident with the now prominent Green Ridge fault escarpment, which
formed at about 5.3 Ma, and also from structurally controlled sites
within the Deschutes basin.
Third, the late Miocene graben was trun-
cated to the north along a crustal transition marked, in part, by a
major zone of northwesttrending faults.
Intraarc subsidence also
occurred south of Green Ridge where bounding structures on the east
side of the graben are probably, in part, coincident with Quaternary
faults of the Tumalo zone.
The sedimentology of the Deschutes Formation indicates a strong
influence of pyroclastic volcanism on fluvial sedimentation.
Deposition occurred spasmodically, during periods when large, eruption
related sediment loads were introduced into the basin, and was
separated by periods of degradation when streams became incised to
regain previous graded profiles.
Deposition on a broad alluvial plain
adjacent to the arc was largely by debris flow, sheetflood and hyper
concentrated floodflow events.
Resulting facies resemble those
typically restricted to much smaller alluvial fans in nonvolcanic
regions.
Existing fluvial facies models are not adequate for inter-
preting the sedimentology of volcanisminduced deposition sequences.
The absence of significant Deschutes basin aggradation during
other eruptive episodes probably is related to the large volume of
pyroclastic material erupted during early High Cascade volcanism.
350
Other factors, such as climate, relief of the Cascades, tectonism in
and around the Deschutes basin, and location of the Cascade volcanic
axis may also have influenced the long-term sedimentation history of
the basin.
Nonetheless, the geomorphic thresholds considered by
Vessell and Davies (1981) to produce short-term aggradation, related to
single eruptions, can be extended to the larger scale of evaluating the
potential for long-term net aggradation related to eruptive episodes,
millions of years in duration.
The temporal correspondance of
Deschutes Formation sedimentation to the early High Cascade eruptive
episode strengthens the arguments for designating this as a distinctive
period in central Oregon Cascade evolution.
The cross-section in Figure 10.1 updates and expands upon the
earlier schematic section illustrated by Taylor (1981).
The stuctural
origin of the Deschutes basin is not well-understood but probably
involved subsidence and truncation of the Blue Mountains structural
trend.
This subsidence occurred long before Deschutes Formation depo-
sition and probably predates at least the uppermost John Day Formation.
The Deschutes Formation is temporally equivalent to the early High
Cascade eruptive episode and is composed of an eastward-thinning wedge
of volcanics and sediments which onlap the older rocks of the Ochoco
Mountains to the east.
The wedge is dominated by volcanic rocks to the
west and by volcanogenic sedimentary lithologies to the east.. Subsidence of the ancestral High Cascades brought an end to in-filling of
the Deschutes basin.
However, explosive volcanism continued and con-
351
Fig. 10.1. Schematic crosssection of the central Oregon Cascade Range
and Deschutes basin.
PLEISTOCENE-HOLOCENE CASCADE VOLCANO
LATE MIOCENE
VOLCANICS
ANCESTRAL HIGH
CASCADES
PLIO-PLEISTOCENE MAFIC PLATFORM
EARLY PLIOCENE
VOLCANICLASTICS
EOCENE MIOCENE
DESCHUTES FORMATION
VOLCANIC
DOMINATED
SEDIMENT
DOMINATED
5
10
KILOMETERS
PRE-TERTIARY
ROCKS
EOCENE -
VOLCANICS
M. MIOCENE
ROCKS
'k
WESTERN
CASCADES
HIGH CASCADE GRABEN
GREEN
RIDGE
Figure 10.1
JO)
ao
y
DESCHUTES BASIN
PS8509-231
353
tributed to a thick sequence of fluvial, lacustrine and pyroclastic
facies, the Camp Sherman beds, which are interpreted to underlie
younger lavas in the Metolius valley.
The late Pliocene and Pleisto-
cene mafic platform lava flows were erupted from cinder cones and
coalesced shield volcanoes which are the foundation for the modern
crestline cones.
Pliocene basalts and basaltic andesites were ponded
within the graben and also flowed around the north end of Green Ridge
and covered a broad area of the northern Deschutes basin.
Latest
Pliocene (?) and Pleistocene Cascade lavas and pyroclastic flows which
entered the Deschutes basin were largely confined to canyons resulting
from postDeschutes Formation incision except in the southern part of
the basin beyond the erosional knickpoint.
354
CHAPTER 11
NEOGENE STRATIGRAPHY OF THE DESCHUTES BASIN
GENERAL CONCLUSIONS AND
PERSPECTIVES
The Tertiary stratigraphy of central and .eastern Oregon is charac-
terized by sequences of volcanic and nonmarine, largely volcanogenic,
sedimentary rocks (Walker, 1977).
Although many of the volcanic rocks
have been the subject of petrologic and straigraphic study, little
effort has been made to evaluate the stratigraphy and sedimentology o
the sedimentary units, or their paleogeographic and tectonic significance.
Many of the sedimentary units host fossil floras and faunas
which have been the subject of paleontological scrutiny for ever a
century, but rarely were these studies coupled with stratigraphic
invesitgations.
As a result, the contact relationships of paleon-
tologically dated units with adjacent rocks are generally unknown, the
lithologies often undescribed, and stratigraphic nomenclature either
lacking altogether or often ambiguously defined on the basis of reconnaissance.mapping.
This study of the Deschutes basin suggests that the existing
reconnaissance studies are insufficient for defining regional stratigraphy.
Also, more detailed analysis not only contributes to a better
understanding of basin stratigraphy but also provides important insight
into paleogeography, paleovolcanism, and paleodrainage, which are
critical to evaluating the tectonic development of central and eastern
Oregon.
Clearly, a firm stratigraphic foundation, involving combined study
of volcanic and sedimentary lithologies, is required to make important
355
conclusion regarding paleogeography and tectonics.
Because of the
complexity of the stratigraphic relationships the author questions the
validity of recent revisions to the stratigraphy of northcentral
Oregon (Farooqui and others, 1981 a,b), including the Deschutes basin,
based on one to two mandays effort per quadrangle for both fieldwork
and office compilation (Farooqui and others, 1981a).
As discussed in
Chapter 3, the authors of this newlyerected Dalles Group stratigraphy
failed to recognize the DeschutesSimtustus unconformity in the
Deschutes basin and chose to ignore the previous designation (Waters,
1968b) of a similar formationbounding unconformity in the Tygh Valley
basin.
The Tygh Valley and Chenoweth formations of Faroolui and others
(1981a,b), formerly Dalles Formation, have been determined by the
author to be in facies relationship to each other and are not separately mappable units.
The inability to map these units separately is also
evident by disparate interpretations of the nature of contact between
the two "formations" by the same compiler in Farooqui and others
(1981a) and Bela (1982).
Also, lumping together lithologically
distinct units in the Deschutes, Tygh Valley, Dalles, Arlington, and
Umatilla basins into the Dalles Group, concommitant with abandonment
of some wellestablished stratigraphic names, failed to consider the
differing ages of these units.
Available data (Farooqui and others,
1981a; Martin, 1979; Smith and Snee, 1984; Keith and others, 1985).
suggest that the original Dalles Formation (Tygh Valley and Chenoweth
formations of Farooqui and others, 1981b) is largely, if not entirely,
older than the Deschutes Formation; the Alkali Canyon Formation is
entirely younger than Tygh Valley and Chenoweth formations and possibly
356
similar in age to the Deschutes Formation; and the McKay Formation is
youn er than all other units in the group.
Although the stratigraphy
of t e "Dalles Group" basins requires more study and probable revisions
from original designations, rapid reconnaissance work clearly does not
just.fy the sweeping revisions proposed by Farooqui and others (1981b).
Neogene rocks of the Deschutes basin illustrate diverse contin-
enta margin volcanic and tectonic processes and their influence on
sedi entation in nonmarine, arcadjacent basins.
the
Backarc volcanism of
olumbia River Basalt Group is represented in the Deschutes basin
by t o thick basalt flows which are intimately associated with the
midd e Miocene Simtustus Formation.
the
This association, combined with
edimentological features of the Simtustus Formation, strongly
sugg st that aggradation was a response of the ancestral Deschutes
Rive
to drainage disruption and local baselevel elevation resulting
from emplacement of the lava flows.
It is unlikely that a significant
sedi entary record of contemporary Cascade volcanism would have been
prod ced if it had not been for this influence of flood basalts to
prod ce fluvial aggradation.
This portion of the stratigraphy is in
cont ast to the overlying Deschutes Formation which records an episode
of 1
rgevolume volcanism within the Cascades which, in itself, caused
depo ition by periodically introducing large pyroclastic sediment loads
in e cess of geomorphic thresholds allowing aggradation.
The Camp
Sher an beds illustrate a third influence of continentalmargin arc
procss on sedimentation
the formation of intraarc depressions which
acco odate great thicknesses of fluvial and lacustrine sediments in
addi ion to volcanics.
357
The diversity of volcanic and sedimentary processes displayed
among Neogene rocks of the Deschutes basin is likely to be recorded in
other nonmarine basins in the Pacific Northwest.
Reconnaissance study
by the author suggests that concepts of the influence of volcanic and
tectonic processes on nonmarine sedimentation developed in the
Deschutes basin are widely applicable in Oregon and Washington (Smith,
1984, 1985b, and in prep.).
358
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Union, v. 64, p. 899.
,
Zeil, W., 1979, The Andes, a geological review: Berlin, Gebruder
Borntraeger, 260 p.
381
APPENDICES
382
APPENDIX I: MAJOR ELEMENT ANALYSES OF DESCHUTES BASIN VOLCANIC ROCKS
Most major element analyses reported in this appendix were
obtained by Xray fluorescence (for oxides of Si, Al, Ti, Fe, Ca,
and K) and atomic absorption spectrometry (for oxides of Na and Mg) at
Oregon State University under the direction of Dr. Edward M. Taylor.
Selected basalts were analyzed by Xray fluorescence using a set of
basalt standards at Washington State University under the direction of
Dr. Peter R. Hooper. Analyses from the two different laboratories can
be distinguished in the following tables by the presence of values for
MnO and P205 in the WSU analyses which were not analyzed for at OSU.
The analytical procedure at OSU includes dehydration of samples whereas
WSU analyses are performed without elimination of volatiles before
preparation of lithium tetraborate beads; as a result some WSU analyses
Iron is reported as FeO at
sum to values significantly less than 100%.
OSU and as Fe203 at WSU.
In most of the analytical tables the WSU data
has been recalculated with iron as FeO to facilitate comparison with
OSU analyses.
Analytical precision at the two laboratories is summarized in the table below.
Precision (95% confidence limit)
OSU
WSU
Element
0.550
0.310
0.050
0.350
Si 02
Al203
TiO2
Fe203
FeO
MnO
CaO
MgO
K20
Na20
P205
1.000
0.500
0.050
0.200
0.010
0.220
0.150
0.030
0.160
0.014
0.100
0.200
0.050
0.200
Geographic Location System
R. 12 E. R. 13 E.
7S/12E/36Abc
Township Range '4, 14, 1/4 Section
1.12 S.
1.13 S.
383
Appendix 1a: Clarno and John Day Formations
EB8
FHB
GB2
GB3
GB5
0C14
SID
SF14
Si02 51.83
TiO2
1.71
Al203 15.74
FeO
9.41
MgO
5.40
Ca0
8.31
Na20
1.94
K20
0.95
P205
0.25
MnO
0.18
63.8
0.77
18.0
5.03
2.4
5.42
3.6
1.14
67.1
51.8
3.23
14.2
13.64
3.50
7.34
2.03
1.19
0.40
0.22
53.8
2.01
13.9
13.58
1.83
5.80
2.00
1.89
0.59
0.23
52.77
2.99
14.72
12.92
3.62
7.29
2.33
0.38
0.44
0.23
53.6
71.9
0.36
66.9
0.68
15.1
16.1
Total 95.72
100.16
97.55
95.63
97.69
94.82
GB8
0.96
14.4
5.94
1.0
5.94
4.2
3.15
99.95
2.00
13.8
13.44
1.78
5.97
2.26
1.14
0.60
0.23
0.79
4.6
3.47
3.58
0.4
2.37
5.4
2.55
99.12
97.98
2.90
<0.1
GB8
Coarse-grained, porphyritic basalt, Clarno Formation (?),
northwest of Grizzly Mountain, 3420', 13S/14E/11Dda.
EB8
Eagle Butte dacite, John Day Formation, 2310', 8S/13E/31Ddb.
FHB
.
Forked Horn Butte dacite, John Day Formation (?), 25' depth in
Century West Engineering core, 15S/13E/19Dda.
GB2
John Day Formation trachyandesite, north-northeast of Gray Butte,
3910', 13S/14E/19Daa.
GB3
John Day Formation trachyandesite, Kings Gap, northwest of Gray
Butte, 3915', 13S/13E/24Abd.
GB5
John Day Formation trachyandesite (?), northwest of Red Top
Spring, northeast of Gray Butte, 3850', 13S/14E/20Ccb.
0C14
John Day Formation trachyandesite, quarry north of Crooked River
and west of Gray Butte, 2850', 13S/13E/28Dda.
SID
Sidwalter Buttes rhyodacite, John Day Formation, roadcut on
Sidwalter Buttes road, 3020', 8S/10E/2Cbc.
SF14
Steelhead Falls inlier dacite, John Day Formation (?), 2650',
145/12E/4Aac.
384
Appendix Ib: Prineville
Si02
TiO2
Al203
Fe203
MgO
CaO
Na20
K20
P205
MnO
PD2
PD3
PD4
PD5
PD6
PD7
Ni
51.43
2.79
51.22
2.77
14.41
14.51
13.77
4.22
7.85
2.60
1.92
1.25
0.24
51.48
2.79
14.65
13.47
4.06
7.94
2.68
51.30
2.78
14.56
12.88
4.16
8.04
2.67
1.87
1.24
0.24
51.39
2.74
14.45
13.02
4.26
7.89
2.67
1.89
1.25
0.24
51.64
2.79
14.53
13.22
4.35
7.92
2.69
1.97
1.23
0.25
51.58
2.78
14.65
13.23
4.23
7.92
2.68
1.83
1.26
0.24
51.57
2.80
14.59
13.03
99.99 100.45
99.80
99.72 100.57 101.46 100.26
PD10
PD11
PD12
51.19
2.75
51.09
2.76
13.73
51.58
2.75
13.66
4.27
7.85
2.77
1.84
1.24
0.24
PB2
PD9
51.07
2.78
13.83
51.62
2.76
13.37
3.90
7.99
2.52
1.92
1.23
0.24
3.81
8.05
2.79
1.82
1.28
0.24
1.91
1.21
0.24
P1
P3
7.80
2.71
1.84
1.23
0.24
0N2
52.62
2.48
10.35
3.12
5.92
3.04
3.29
1.17
96.86
8.21
2.61
1.73
1.58
0.24
Total 100.49 100.14 100.47 100.08 100.68 100.39
99.57
99.85
7.79
2.69
1.83
1.24
0.25
P4
P5
14.01
4.26
7.78
2.77
1.84
1.24
0.24
G1
4.11
7.91
2.62
1.94
1.25
0.24
LS1
LS2
ME5
EB5
51.27
52.71
2.81
2.53
15.07
9.79
3.39
50.79
2.74
14.54
13.50
1.17
0.23
52.90
2.59
15.00
10.73
3.42
6.30
2.74
3.18
1.23
0.23
97.13
98.32
50.39
3.12
14.20
13.74
4.82
8.39
2.57
1.46
1.57
0.26
51.68
0.21
0.22
14.58
13.07
3.99
8.07
2.76
1.68
1.24
0.25
Total 100.52
96.77
96.61
99.73
2.61
14.54
10.43
3.62
6.58
2.90
2.72
1.26
51.70
2.56
14.45
11.17
3.09
6.60
2.79
2.81
1.22
3.91
6.21
2.72
3.31
51.07
2.81
14.61
13.68
4.13
7.91
2.73
1.82
1.26
0.23
GB1
7.89
2.73
1.84
1.26
0.24
4.51
51.37
2.80
13.58
50.70
3.12
13.24
3.95
14.01
4.61
51.13
2.75
15.15
4.45
7.78
2.42
1.78
1.24
0.24
Si02
TiO2
Al203
Fe203
MgO
CaO
Na20
K20
P205
MnO
N2
PD1
Total 100.01 100.35
Si02
TiO2
Fe203
MgO
CaO
Na20
K20
P205
MnO
Chemical-type Basalt
S318
0.21
CC3
7.70
2.77
2.03
1.24
0.23
52.49
2.45
15.14
10.29
2.98
5.78
2.90
3.32
1.14
0.23
52.83
2.62
15.05
10.95
2.97
6.30
2.73
3.16
1.24
0.22
99.83
96.72
98.28
4.31
385
Si02
TiO2
Al203
Fe203
MgO
CaO
Na20
K20
P205
MnO
Total
PD1
FP5
TY4
50.68
2.76
14.34
13.93
4.46
51.31
2.81
TY10
1.79
1.25
0.25
53.04
2.47
14.97
10.59
3.54
5.80
2.56
3.56
1.15
0.23
99.73 100.64
97.91
7.61
2.79
1.68
1.24
0.23
14.69
13.63
4.43
7.88
2.61
Flow one at type section, roadcut south of Bowman Dam, 3490',
175/16E/14Cba.
PD2
Flow two at type section, roadcut south of Bowman Dam, 3500',
17S/16E/14Cba.
PD3
Flow three at type sect-kin, pillowed zone, roadcut south of
Bowman Dam, 3530', 175/16E/14Cba.
PD4
Flow three at type section, north end of Bowman Dam, 3290',
175/16E/11Bcb.
PD5
Flow four at type section, north of Bowman Dam, 3500',
175/16E/11Bbb.
PD6
Flow five at type section, north of Bowman Dam, 3520',
175/16E/2Ccc.
PD7
Flow six
N1
Flow three (?), east of Chimney Rock, 3090', 165/16E/33Dbd.
N2
Flow three (?), east of Chimney Rock, 3200', 16S/16E/33Bdd.
PB2
Flow two, below Bowman Dam, 3160', 17S/16E/10Daa.
PD9
Roadcut on Juniper Canyon Road, 3900', 16S/16E/12Bab.
PD10
Roadcut on Juniper Canyon Road, 3975', 15S/16E/34Bad.
at type section, north of Bowman Dam, 3560',
175/16E/3Ddd.
386
PD11
Pillowed flow in roadcut on Juniper Canyon Road, 3880',
155/16E/16Cac.
PD12
Quarry at mouth of Juniper Canyon, southeast of Prineville,
3160', 15S/16E/16Cac.
P1
Quarry along highway 126, northeast of Powell Buttes,
15S/15E/21Ddb.
P3
Roadcut west of U.
GB1
Quarry near headwaters of Japanese Creek, east of Lone Pine Flat,
13S/14E/35Daa.
0N2
Roadcut on Highway 126, north of Houston Lake, 2840',
14S/15E/19Bbd.
P4
Roadcut on Highway 126 north of Houston Lake, 2840',
14S/14E/24Dad.
P5
Roadcut on Highway 126 north of Houston Lake, 2840',
14S/15E/19Dbd.
G1
Quarry on South Junction Road, 2240', 8S/14E/27Acb.
LS1
First flow at Pelton Dam, 1540', 10S/12E/18Cac.
LS2
Second flow at Pelton Dam, 1600', 10S/12E/240db.
ME5
Quarry east of Mud Springs Creek, east of Madras, 2750'
11S/13E/2Abc.
EB8
North of Kahneeta Lodge, 2220', 8S/13E/16Ddd.
SJ18
Roadcut on Jackson Trail Road east of Seekseequa Junction,
1910', 10S/12E/27Acc.
CC3
Roadcut on U. S. 97 in Cow Canyon, 2890', 8S/15E/15Bbd.
FP5
Roadcut west of Foreman Point, 3250', 5S/11E/34Dcb.
TY4
Roadcut on U. S. 97 in Butler Canyon, Tygh Ridge (also sample TY4
of Nathan and Fruchter, 1974).
TY10
Roadcut on U. S. 97 in Butler Canyon, Tygh Ridge (also sample
TY10 of Nathan and Fruchter, 1974).
S.
26, north of Round Butte, 14S/15E/8Bda.
387
Appendix Ic: Deschutes
Formation Diktytaxitic huh§
0299
H-150
J203
0C2a
0C2b
0C2c
P-460
51.06
0.96
16.98
8.01
8.04
10.79
2.34
0.24
0.14
0.17
51.4
1.44
15.5
50.32
1.79
16.09
11.46
7.49
9.47
2.4
0.41
0.37
49.3
1.36
17.0
51.2
1.56
16.6
10.31
9.0
9.86
50.45
1.56
16.27
9.86
8.18
11.10
2.58
0.30
50.6
0.98
17.02
8.53
8.62
10.93
2.33
0.13
0.15
0.17
99.37
98.73
99.78 100.01
98.99
99.90 100.78 100.82
RB25
RB31
RB47
RB51a
RB51b
RB51c
RB52
SJ47
SR2
5102
TiO2
Al203
Fe0
MgO
CaO
Na20
K20
P205
MnO
50.18
0.88
49.3
0.85
17.4
9.02
50.43
0.95
16.96
8.25
8.99
11.10
2.26
0.17
0.17
0.17
48.0
0.92
17.6
9.56
10.6
11.25
2.3
0.03
49.2
0.85
18.0
8.25
9.2
11.13
2.4
0.14
50.08
1.06
17.10
50.8
0.88
17.7
8.72
8.4
10.90
2.6
0.16
0.15
49.31
52.85
0.88
16.88
9.77
6.16
9.12
1.88
0.87
Total
98.98
99.90
99.45 100.32
99.17 100.29 100.16
SR4
SF130
SF134
SF139
SF142
S102
TiO2
Al203
FeO
MgO
CaO
Na20
K20
P205
MnO
50.10
1.64
16.88
10.47
7.08
9.36
49.77
1.12
16.73
9.86
9.37
10.99
2.36
0.05
50.14
0.94
16.82
8.40
9.15
10.92
2.33
0.22
0.16
0.17
50.27
1.04
17.07
8.82
9.2
50.64
0.97
11.41
10.91
Total
98.94 100.65
S102
TiO2
Al203
Fe0
MgO
CaO
Na20
K20
P205
MnO
B3
097
50.1
50.51
1.39
17.2
8.90
9.8
10.07
3.0
0.47
Total 100.93
17.21
8.24
8.90
10.91
2.12
0.22
0.15
0.17
2.31
0.87
0.16
0.14
10.1
10.66
2.4
0.17
0.21
0.19
11.01
8.1
8.59
3.2
0.54
9.41
9.0
9.47
3.1
1.1
0.35
0.31
2.30
0.00
0.16
0.17
8.38
9.27
2.28
0.18
0.17
0.17
99.25 100.44 100.08
11.94
8.7
6.81
3.1
0.34
0.31
0.17
0.21
17.11
1.71
15.1
8.75
9.25
11.36
2.31
0.05
0.16
0.17
0.98
17.15
9.08
9.63
11.62
1.63
0.04
0.16
0.18
99.77
0.14
98.71
388
83
D97
Diktytaxitic olivine basalt, west side of Deschutes River north
of Sawyer State Park, 3460', 17S/12E/7Ccc, normal polarity.
Diktytaxitic olivine basalt, Canadian Bench flow of the Lower
Desert Basalt member, Canadian Bench, 11S/12E/34Cdb (also
analyzed by Dill, 1985; sample TED 97)
D299
Diktytaxitic olivine basalt, Fly Lake flow of the Lower Desert
basalt member, south rimrock at Big Canyon, 11S/12E/31Dcd (also
analyzed by Dill, 1985; sample TED 299).
H-150
Diktytaxitic olivine basalt, base of 30' thick flow at 150'
depth in State Highway #1 geothermal gradient test well
northwest of Powell Buttes, 16/14/17Ddd.
J203
Diktytaxitic olivine basalt, Pelton basalt member, Willow Creek
canyon, 1780', 10S/13E/29Ca (also analyzed by Jay, 1982; sample
203).
0C2
Diktytaxitic olivine basalt, Opal Springs basalt member, at
bottom of Hollywood Rd. grade, 2200', 13S/12E/24Bab (3
analyses).
P-460
Diktytaxitic olivine basalt, 20' thick
Powell Buttes #1 geothermal test well.
flow
at 460' depth in
RB25
Diktytaxitic olivine basalt, Canadian Bench flow of the Lower
Desert basalt member, on The Peninsula, 2675', 12S/12E/28Add.
RB31
Diktytaxitic olivine basalt, Canadian Bench flow of Lower
Desert basalt member, The Peninsula, 2600', 12S/12E/21Dbd.
RB47
RB51
Diktytaxitic olivine basalt below Tetherow Butte member on The
Peninsula, 2540', 12S/12E/28Daa.
Diktytaxitic olivine basalt, Canadian Bench flow of Lower
Desert basalt member, east side of The Cove, 2480',
11S/12E/26Cdd (2 analyses).
RB52
Diktytaxitic olivine basalt below Tetherow Butte member, east
side of The Cove (correlative to Tdb7 of Dill, 1985, and Lower
Canadian Bench basalt of Conrey, 1985), 2360', 11S/12E/26Cdc,
reverse polarity.
SJ47
Diktytaxitic olivine basalt, Juniper Canyon basalt member,
northwest of Round Butte Dam, 1990', 11S/12E/9Daa.
SR2
SR4
Diktytaxitic olivine basalt, Canadian Bench flow of the Lower
Desert basalt member, north of Squaw Flat Ranch, 2770',
13S/11E/11Bda.
Diktytaxitic olivine basalt, west side of Squaw Flat, 2780',
389'
13S/11E/22Ccc, reverse polarity.
SF130
Diktytaxitic olivine basalt overlying Hollywood ignimbrite
member in Crooked River canyon, 2170', 13S/12E/10Dab, reverse
polarity.
SF134
Diktytaxitic olivine basalt near top of section on east side of
Crooked River canyon south of Opal Springs, 2620',
13S/12E/3Cdb, normal polarity.
SF139
Diktytaxitic olivine basalt, Opal Springs basalt member at
Opal Springs, 2200', 12S/12E/33Acd.
SF142
Diktytaxitic olivine basalt on Opal Springs access road
(probably correlative to SF 134), 2650', 12S/12E/33Ddb,
normal polarity.
390
Appendix Id: Deschutes Formation Porphyritic Basalts, Basaltic
Andesites, and Andesites
Si02
TiO2
Al203
FeO
MgO
CaO
Na20
K20
CF23
CF43
0C1
RB62
SR9
53.1
52.8
1.35
19.4
8.23
4.7
8.10
4.0
0.97
53.7
1.58
18.9
52.7
1.35
18.3
9.21
8.55
6.9
7.85
52.8
1.12
18.6
8.17
1.18
20.3
7.00
4.7
10.28
3.1
0.46
Total 100.12
4.5
9.01
3.4
0.97
3.7
0.69
99.55 101.27 100.04
6.1
8.91
3.5
0.73
SF43
SF69
SF124
52.7
2.12
16.9
10.72
4.0
7.62
4.3
0.74
49.5
52.6
1.02
21.5
1.11
16.4
9.37
9.2
10.62
2.8
0.11
7.21
4.6
10.00
3.2
0.42
99.93
99.10
99.11 100.55
SF141
CF40.
CF42
HB15
RB48
RB61
SR1
SR3
SR5
Si02
TiO2
52.6
55.9
1.61
Al203
16.4
9.34
3.9
6.95
4.2
55.0
1.24
17.3
8.34
7.0
7.40
3.2
1.07
53.8
1.60
17.2
9.29
54.9
0.89
16.6
57.3
1.13
17.5
7.84
5.5
6.70
3.5
0.98
53.6
1.90
17.4
K20
17.7
9.66
4.7
8.89
3.4
0.80
53.6
1.05
53.6
7.23
53.1
1.71
Total
99.06
99.31
99.93 100.91 100.55
99.26
99.16 100.45
99.12
SR6
SR8
SF25
SF47
SF59
SF64
SF70
SF85
SF135
S102
TiO2
Al203
FeO
MgO
CaO
Na20
K20
54.3
53.9
1.37
53.9
1.37
53.6
1.69
18.1
17.1
7.97
3.7
8.18
3.6
1.24
54.0
2.21
16.2
10.36
4.7
7.80
3.4
1.02
53.1
10.52
3.8
7.12
4.3
0.78
55.5
1.58
18.0
8.63
3.6
7.44
4.4
0.92
53.8
1.15
18.2
8.17
53.7
1.63
18.7
8.62
4.5
Total
99.03
99.87
99.00
99.69
FeO
MgO
Ca0
Na20
2.11
16.1
1.01
1.75
16.7
10.98
5.1
5.1
8.75
3.8
0.90
8.89
3.7
0.69
19.1
1.55
18.4
5.1
7.71
7.1
7.73
3.8
0.74
8.39
2.8
0.77
8.73
4.9
8.45
8.74
4.0
7.94
4.1
4.1
0.69
0.99
98.38 100.24
98.16
8.40
4.2
7.86
3.7
1.17
6.1
7.25
3.7
0.71
9.97
4.0
7.77
3.7
0.78
8.49
4.0
0.93
99.08 100.57
391
SF140
SF146
SF147
SJ26
HB17
Si02
TiO2
Al203
Fe0
MgO
CaO
Na20
K20
54.1
54.0
1.39
16.9
9.47
4.9
9.03
3.6
0.78
53.3
1.40
17.7
9.02
5.8
8.45
57.2
Total
99.78 100.07
CF23
Porphyritic olivine basalt of Big Falls, 2525',14S/12E/9Dbd.
CF43
Porphyritic olivine basalt (Buckhorn basalt of Stensland,
unpub. map) on Lower Bridge Estates, 2890', 14S/12E/22Bab,
reverse polarity.
OC1
Porphyritic olivine basalt below Opal Springs basalt member in
Crooked River canyon, 2160', 13S/12E/24Bab, normal polarity.
RB62
Sparsely glomeroporphyritic olivine basalt (w/ augite
phenocrysts), middle of three basalts exposed in Crooked River
canyon south of The Ship, 2420', 12S/12E/15Aaa), reverse
1.30
18.5
7.99
4.6
8.50
4.0
0.79
3.1
6.81
4.2
0.92
0.99
59.3
1.75
15.7
9.25
1.5
3.83
5.0
1.73
99.69 101.17
98.06
2.01
17.0
9.26
3.7
polarity.
SR9
Sparsely porphyritic, hyaloophitic basalt rimrock east of
Squaw Creek ford, 2790', 13S/11E/26Abc, reverse polarity.
SF43
Platy-jointed, glomeroporphyritic olivine basalt-basaltic andesite, south side of Chandler Ridge, 2540', 13S/12E/17Daa),
reverse polarity.
SF69
Glomeroporphyritic olivine basalt rimrock, east side of
Deschutes canyon opposite mouth of Squaw Creek, 2710',
13S/12E/8Bcb, reverse polarity.
SF124
Porphyritic olivine basalt at river level, Deschutes canyon at
mouth of Squaw Creek, 2100', 13S/12E/8Cba.
SF141
Glomeroporphyritic olivine basalt along Opal Springs access
road, 2600', 12S/12E/33Acd, reverse polarity.
CF40
Porphyritic olivine-bearing basaltic andesite at river in
Deschutes canyon, upstream from Lower Bridge, 2540',
14S/12E/15Bdb, reverse(?) polarity.
CF42
Porphyritic olivine basaltic andesite, roadcut on Teator Road,
392
Lower Bridge Estates, 2840', 14S/12E/22Bab, reverse polarity.
HB15
Porphyritic pilotaxitic olivine basaltic andesite, rimrock at
top of Deep Canyon grade, 3050', 15S/11E/10Bbc.
RB48
Porphyritic olivine basaltic andesite, columnarjointed flow
near top of grade, Crooked River access road, The Cove, 2420',
12S/12E/11Dbb, reverse polarity.
RB61
Glomeroporphyritic olivine basaltic andesite, lowest of three
flows in Crooked River canyon south of The Ship, 2300',
12S/12E/15Aaa, reverse polarity.
SR1
Porphyritic olivinebearing basaltic andesite (w/ few augite
and hypersthene phenocrysts) overlying Lower Desert basalt
member north of Squaw Flat, 2780', 13S/11E/11Bda, normal
polarity.
SR3
Sparsely porphyritic olivinebearing basaltic andesite (w. few
augite and hypersthene phenocrysts), Squaw Flat, 2780',
13S/11E/21Dad, reverse polarity.
SR5
Porphyritic olivine basaltic andesite northwest of Holmes
Ranch, 3000', 14S/11E/sDbc.
SR6
Platyjointed, pilotaxitic, microporphyritic olivinebearing
basaltic andesite (w/ augite phenocrysts), east side of Squaw
Creek north of Holmes Ranch, 2750', 13S/11E/26Daa.
SR8
Porphyritic olivinebearing basaltic andesite (w/augite
phenocrysts) above Peninsula ignimbrite member at Squaw Creek
ford, 2660', 13S/11E/26Bad, reverse(?) polarity.
SF25
Glomeroporphyritic olivine basaltic andesite at Steelhead
Falls, 2310', 13S/12E/28Daa, reverse polarity.
SF47
Microporphyritic hypersthenebearing basaltic andesite,
Deschutes canyon south of Squaw Mouth, 2540', 13S/12E/8Cad,
reverse polarity.
SF59
Porphyritic olivine basaltic andesite (w/ few augite
phenocrysts), Deschutes canyon, Squaw Mouth section, 2300',
13S/12E/7Ada, reverse polarity.
SF64
Glomeroporphyritic olivine basaltic andesite, Deschutes canyon,
Squaw Mouth section, 2370', 13S/12E/7Ada, reverse polarity.
SF70
Porphyritic olivine basaltic andesite (w/ augite phenocrysts),
Deschutes canyon, rimrock north of River Place section,
2500', 13S/12E/20Adc, reverse polarity.
SF85
Porphyritic olivine basaltic andesite, columnar jointed flow in
393
Deschutes canyon north of Geneva canyon, 2500', 12S/12E/29Daa,
reverse polarity.
SF135
Glomeroporphyritic olivine basaltic andesite (w/ augite
phenocrysts), thick flow in Crooked canyon at Crooked River
Ranch, 2520', 13S/12E/10Daa, reverse polarity.
SF140
Porphyritic olivinebearing basaltic andesite (w/ few augite
phenocrysts) along access road to Opal Springs, 2360',
12S/12E/33Dbd, normal polarity.
SF146
Microporphyritic basaltic andesite (w/ augite phenocrysts) at
Alder Springs, 2410', 13S/12E/18Cca, normal(?) polarity.
SF147
Microporphyritic basaltic andesite (w/ few augite phenocrysts)
in Squaw Creek valley north of Squaw Creek ford (correlative to
SF146), 2400', 13@/12E/24Bcc, normal(?) polarity.
SJ26
Pilotaxitic aphyric basaltic andesite of Pipp Spring
(incorrectly mapped as Columbia River Basalt by Waters, 1968a),
2280', 11S/12E/6Aaa, normal (?) polarity.
HB17
Pilotaxitic, platyjointed, aphyric andesite, rimrock at
McKenzie Canyon reservoir, 30451, 145/11E/33Acd.
394
Appendix le: Tetherow Butte Member
R1
M13
Rbb6
RB44
RB45
RB46
RB49
RB66
SF92
50.90
2.56
52.2
2.42
13.7
13.04
4.6
8.56
3.8
0.59
51.08
2.66
14.8
13.56
4.74
8.65
3.03
0.69
0.53
0.25
50.82
2.55
14.64
13.70
4.89
9.29
2.92
0.52
0.51
0.23
51.30
2.67
51.21
51.5
2.49
14.3
13.18
4.8
8.65
3.4
0.60
98.91
99.99 100.05 100.24 100.01
R10
R832
52.9
2.04
14.7
13.12
5.0
8.37
3.6
0.70
Si02
TiO2
Al203
FeO
MgO
CaO
Na20
K20
P205
MnO
50.97
2.52
14.83
13.30
4.74
8.67
2.93
0.62
0.49
0.23
51.6
2.49
14.6
13.4
4.7
8.80
0.60
8.77
2.94
0.45
0.49
0.23
Total
99.30
99.29
98.99
0C10
0C13
Si02
TiO2
Al203
FeO
MgO
CaO
Na20
K20
P205
MnO
52.1
52.1
2.42
14.2
13.34
5.2
8.24
3.6
0.61
51.8
2.52
13.0
13.06
5.2
8.64
3.6
0.60
2.44
14.3
13.35
5.0
8.45
3.7
0.59
51.5
2.41
14.2
13.37
5.3
8.53
3.8
0.56
Total
99.71
98.42
99.93
99.67 100.43
R1
Bomb, Tetherow Butte cinder (rafted cone fragment?), cinder pit
at north end of Terrebonne, 2780', 14S/13E/16Bcb.
M13
Spatter, quarry on southwest side of Tetherow Butte (E. M.
Taylor, person. commun., 1983).
Rbb6
Agency Plains flow, top of Hurber's Canyon grade on Elk Drive.
RB44
Agency Plains flow, rimrock on The Peninsula, 2550',
12S/12E/22Dad.
RB45
Agency Plains flow, east side of Crooked River, south of
entrance to The Cove, 2560', 12S/12E/23Aba.
RB46
Agency Plains flow, The Peninsula, 2620', 12S/12E/28Add.
3.1
14.71
13.13
4.81
R8
14.61
13.98
4.57
8.55
3.12
0.64
0.55
0.25
2.67
14.78
13.67
4.63
8.58
3.11
0.57
0.54
0.25
98.92
SF143a SF143b
SF144
SF129A
51.60
2.65
14.66
13.08
4.60
8.56
3.06
0.54
52.2
2.68
13.7
13.96
51.3
2.92
13.5
52.6
2.45
14.44
6.0
13.09
5.9
8.29
3.7
0.45
99.52
100.26-100.46 100.83
5.1
7.53
3.7
0.62
0.53
0.24
8.11
3.6
0.59
14.1
395
RB49
Agency Plains flow, The Peninsula, 2520', 12S/12E/22Ddb.
RB66
Agency Plains, flow, quarry at east entrance to The Cove, 2550',
125/12E/11Dab.
SF92
Agency Plains flow, The Peninsula, 2650', 12S/12E/29Dcb.
°CIO
Crooked River flow, west of Opal City, 2800', 13S/12E/24Baa.
0C13
Crooked River flow, top of Badger grade, Crooked River Ranch,
2760', 13S/12E/36Ddb.
R8
Crooked River flow (?), south flank of Tetherow Butte along NW
Market Rd., 2840', 14S/13E/29Bca.
R10
Crooked River flow (?), platyjointed flow on southwest flank
of Tetherow Butte along Pershall Rd., 2840', 14S/13E/29Ddc.
RB32
Crooked River flow, The Peninsula, 2770', 12S/12E/28Dbb.
SF143
Crooked River flow, east of Crooked River, south of Opal
Springs, 2760', 12S/12E/33Ddc (2 analyses).
SF144
Crooked River flow, The Peninsula southwest of Opal Springs,
2760', 125/12E/33Cdc.
SF129A Glassy basaltic spatter from rootless (?) vent material, quarry
at top of grade on road to Opal Springs, 2790', 13S/12E/21Cbd.
396
Appendix If: Steamboat Rock Member
Si02
TiO2
Al203
FeO
MgO
Ca0
Na20
K20
P205
MnO
CF39
SF117
BLK
SF39
SF105
SF113
WTHa
WTHb
CF41
55.7
2.18
15.3
10.21
4.3
7.89
3.3
54.8
2.09
15.9
10.17
4.3
7.82
3.3
1.19
54.7
2.12
16.4
10.26
54.2
2.10
15.7
9.91
4.0
8.03
3.3
1.24
54.3
2.01
16.8
9.80
4.3
8.06
3.3
0.91
55.0
2.08
15.5
9.96
4.3
7.74
3.3
1.23
55.13
2.18
15.39
10.04
4.27
7.69
54.4
2.12
15.9
3.5
1.03
53.9
2.03
16.6
9.72
4.2
7.74
3.3
1.05
1.16
0.68
0.19
1.23
99.55 100.15
98.58
98.51
99.52
99.12
99.54
99.12
SF38
SF112
SF116
SF104 SF148B SF109
55.1
55.1
2.14
15.4
10.03
4.4
7.96
3.3
1.09
2.10
15.8
50.0
0.71
17.2
8.53
0.02
50.03
0.86
17.07
8.45
8.87
11.99
2.23
0.00
99.42 100.00 100.08
99.50
1.21
Total 100.02
R4
4.3
7.84
2.81
10.01
4.3
7.82
3.3
53.3
1.53
Si02
TiO2
Al203
FeO
MgO
CaO
Na20
K20
55.5
1.99
15.0
9.66
4.4
1.02
54.9
2.08
15.8
9.96
4.4
7.66
3.2
1.21
Total
98.28
99.16
CF39
Dike, Steamboat Rock, 2700', 14S/12E/11Ccc.
SF117
Dike, north of Steamboat Rock, 2700', 14S/12E/10Aab.
BLK
Cauliflower bomb, Panorama Canyon, contains §2% xenoliths,
2680', 13S/12E/16Bba.
SF39
Cinder within lapilli-tuff north of Steelhead Falls, 2730',
13S/12E/22Ccc.
SF105
Juvenile blocks in basal explosion breccia, north of Steelhead
Falls, 2650', 13S/12E/22Ccd.
SF113
Cinder within lapilli-tuff north of Steelhead Falls, 2670',
13S/12E/21Cbc.
WTH
Agglutinate from summit of shield underlying water tower,
2850', 13S/12E/16Dab.
CF41
Lava flow, lower rimrock on Lower Bridge Estates, 2720',
14S/12E/14Bcd.
7.41
3.3
10.11
4.4
7.90
3.5
1.09
9.1
12.41
2.1
20.1
9.13
4.4
9.26
3.5
0.92
102.22
397
R4
Lava flow, mesa surrounded by Pleistocene basalt west of
Tetherow Butte, 2780', 14S/12E/24Bdc.
SF38
Agglutinate from inward-dipping lava sheet at southern edge of
tuff ring complex, 2785', 13S/12E/22Cca.
SF112
Lava flow capping lapilli-tuff, 2680', 13S/12E/21Cbc.
SF116
Lava flow capping mesa north of Steamboat Rock, 27301,
14S/12E/10Ddc.
SF104
Accidental block of diktytaxitic basalt from agglutinate
capping lapilli-tuff, 2650', 13S/12E/22Ccd.
SF148A Accidental block of diktytaxitic basalt from basal explosion
breccia, 2670', 13S/12E/16Dcb.
SF109
Accidental block of porphyritic olivine basalt from basal
explosion breccia, 2650', 13S/12E/21Cbc.
Appendix Ig: Round Butte Member
RB39
J109
Rbbl
Rbb2
Rbb3
Rbb4
Rbb5
52.40
1.78
16.83
9.23
5.90
8.76
2.14
51.64
1.84
16.62
9.58
5.42
8.16
2.93
51.13
1.79
16.32
9.20
6.60
8.72
51.06
1.79
51.22
1.84
16.40
9.48
51.07
6.31
6.55
8.59
8.91
2.51
2.49
K20
P205
MnO
1.01
1.01
0.54
0.17
0.84
0.39
0.16
0.91
0.38
0.15
0.83
0.38
0.15
6.64
8.69
2.47
0.86
0.37
0.15
51.22
1.82
16.37
9.25
6.47
8.65
2.47
0.75
0.38
0.16
Total
98.57
97.91
97.73
97.65
97.54
97.74
97.46
Si02
TiO2
Al203
FeO
MgO
CaO
Na20
2.61
16.31
9.31
1
77
16.12
9.10
0.39
0.15
RB39: Lower flow on Round Butte Dam access road, invasive into
sandstone; 2340', 11S/12E/10Dda.
Rbbl: Lower flow on Round Butte Dam access road, at invasion point,
2350', 11S/12E/11Cbd.
Second flow on Round Butte Dam access road, 2370',
115/12E/11Cca.
Third flow on Round Butte Dam access road, 2385', 11S/12E/11Cca.
Fourth flow on Round Butte Dam access road, 2420',
11S/12E/11Ccb.
Rbb5: Thin flow overlying Tetherow Butte member, 1.0 km north of Round
398
Butte Dam gate, 2370', 11S/12E/11Bdd.
J109: Rimrock flow on west side of Dry Canyon, 2560', 11S/13E/17Ab
(Also analyzed by Jay, 1982, sample 109).
399
Appendix Ih: Deschutes Ignimbrite Members
S102
TiO2
Al203
FeO
MgO
CaO
Na20
K20
MElw
SJ19w SF132b SF132w
H400b RB141
72.2
0.48
15.5
3.33
1.38
71.6
0.48
15.7
3.37
2.00
2.35
2.06
2.47
72.1
63.1
0.27
14.8
2.12
0.5
1.09
3.3
5.29
1.18
17.6
5.64
2.2
3.99
5.0
1.58
2.01
3.38
2.49
62.9
1.12
16.6
5.94
2.8
4.51
4.4
1.88
Total 100.77 100.07 100.15
99.47 100.29
71.2
0.27
15.0
2.23
0.6
1.05
3.2
4.70
RB531
68.9
0.52
16.1
3.39
0.9
2.13
4.0
4.09
RB551
RBlOw
70.4
0.55
15.7
3.02
0.8
1.63
4.3
3.95
70.6
0.50
15.2
2.70
0.9
1.98
4.1
4.17
98.28 100.03 100.15.100.35
MB3b
MB6b
MB171
SJ24b
0C6w
SR7b
RB27b
RB271
SF26d
S102
TiO2
Al203
FeO
MgO
Ca0
Na20
K20
65.2
0.93
67.5
0.77
16.3
64.8
0.90
16.4
65.1
62.4
1.28
17.0
4.21
1.5
4.51
1.5
2.34
4.8
2.72
3.47
5.3
1.96
69.8
0.52
16.3
3.23
2.5
2.62
2.7
2.14
61.2
0.79
18.0
4.69
2.2
3.79
3.4
1.64
4.28
4.7
1.54
64.7
0.97
16.2
4.73
2.7
3.33
4.8
1.73
65.3
0.85
17.8
4.53
1.7
2.97
5.4
1.90
Total
98.84 100.14
98.84
99.61
99.70
99.85
99.11
99.16
SF26d
SF261
SF261
SF26w
SF37d
SF37d
SF78d
SF78d
SF97d
68.1
67.5
0.78
16.8
4.28
1.5
2.70
64.5
0.83
16.3
4.52
1.4
5.1
1.84
2.02
2.71
5.8
1.98
65.2
0.83
16.0
4.46
1.4
2.72
6.2
1.97
65.0
0.88
16.9
3.1
72.2
0.26
15.7
2.08
0.4
0.96
4.0
4.11
65.1
0.69
16.9
4.03
2.3
2.50
65.9
0.92
MgO
Ca0
Na20
K20
65.9
0.82
16.9
4.53
1.7
2.94
4.5
1.99
Total
99.06
99.46 100.66
98.74
98.06
99.23
S102
TiO2
Al203
FeO
16.1
4.62
1.6
2.89
5.2
2.30
1.41
16.8
6.65
2.4
4.99
5.0
1.40
1.87
0.90
17.5
4.73
1.5
2.96
5.2
1.77
99.71 100.10
99.66
17.1
4.69
1.6
2.92
5.1
5.81
2.1
-
99.06
4.51
1.4
2.70
5.9
2.60
400
Si02
TiO2
Al203
FeO
MgO
CaO
Na20
K20
SF99d
BUCKb
CF1d
HB14b
65.7
0.74
16.4
63.4
0.99
16.0
5.08
1.6
3.25
5.7
2.15
65.5
63.9
0.80
16.9
5.20
1.3
3.50
6.0
1.90
4.51
1.9
4.15
3.8
2.60
0.91
15.3
5.05
1.5
3.17
5.8
2.11
b = black pumice, d = dark-gray pumice,
w = white pumice.
s
1
= light-gray pumice,
ME1
Chinook ignimbrite member, Mud Springs valley, southwest of
Gateway, 2180', 9S/13E/36Aca.
SJ19
Chinook ignimbrite member, Dry Hollow, 1980', 10S/12E/26Bbd.
SF132 Hollywood ignimbrite member, Crooked River canyon,
13S/12E/3Cdd.
2260',
H400
Hollywood ignimbrite member (?), pumice cuttings from orange
ignimbrite at 400' depth in the State Highway #1 geothermal
gradient north of Powell Buttes, 16/14/17Ddd.
RB14
Jackson Buttes ignimbrite member, The Ship, 2150',
12S/12E/10Dda.
RB53
Jackson Buttes ignimbrite member, east side of The Cove, north
of the marina, 2080', 11S/12E/26Cbd.
RB55
Jackson Buttes ignimbrite member, east side of The Cove, north
of the marina, 2100, 11S/12E/35Bda.
RB10
Cove ignimbrite member, roadcut on east side of the Crooked
River, Cove Palisades State Park, 2380', 12S/12E/11Acc.
MB3
Tenino ignimbrite member, lower cooling unit, roadcut on Tenino
Road, 2530', 10S/11E/3Bac.
MB6
Tenino ignimbrite member, upper cooling unit, roadcut on Tenino
Road, 2650', 10S/11E/4Aad.
MB17
Coyote Butte ignimbrite member,
10S/11E/22Bdd.
SJ24
Coyote Butte ignimbrite member, south flank of Coyote Butte,
2380', 10S/12E/31Bba.
S.
Fk. Seekseequa Creek, 2600',
401
006
Steelhead Falls ignimbrite member in Crooked River canyon, 2400',
13S/12E/24Baa.
SR7
Peninsula ignimbrite member, southeast of Squaw Creek ford,
2650', 13S/11E/26Bad.
RB27
Peninsula ignimbrite member, Deschutes River canyon, south of
The Cove, 2450', 12S/12E/21Dbc.
SF26
Peninsula ignimbrite member, campground south of Steelhead
Falls, 2420', 13S/12E/28Dad.
SF37
Peninsula ignimbrite member, River Place section, 2440',
13S/12E/21Cba.
SF78
Peninsula ignimbrite member, Chandler Ridge Road, 2540'
13S/12E/17Dad.
SF97
Peninsula ignimbrite member, Squaw Mouth section, 2580',
13S/12E/7Ada.
SF99
Peninsula ignimbrite member (bomb from groundsurge deposit),
Squaw Mouth section, 2580', 13S/12E/Ada.
BUCK
Deep Canyon ignimbrite member, Deep Canyon grade.
CF1
Deep Canyon ignimbrite member, mouth of Buckhorn Canyon, 2725',
14S/12E/29Aac.
HB14
Deep Canyon ignimbrite member, Deep Canyon grade, (E. M. Taylor,
person. commun., 1982).
402
Appendix Ii: Unnamed Deschutes Formation Ignimbrites
MW11b
HB8w
CF38g CF38w CF44g CF45g HB8g
67.1
0.72
17.0
4.12
1.0
2.47
5.4
2.12
69.0
0.59
16.2
3.42
0.4
1.24
4.6
3.93
69.3
0.62
15.8
3.32
0.5
K20
68.0
0.74
15.9
4.14
1.0
2.52
5.4
2.12
Total
99.82
99.93
99.29
99.36
MB15d
M8169
MB16w
RB12d
Si02
TiO2
Al203
FeO
MgO
Ca0
Na20
K20
63.6
1.04
16.3
5.28
1.9
3.93
5.0
1.75
67.5
0.81
68.2
0.77
70.7
0.39
16.1
15.4
3.99 2.67
0.4
1.0
1.54
2.67
4.6
4.7
Total
98.80
Si02
TiO2
Al203
FeO
MgO
CaO
Na20
16.1
4.08
1.3
2.87
4.8
2.25
1.71
4.3
3.81
64.6
0.73
17.2
4.30
1.5
3.05
4.6
2.79
69.4
0.61
17.0
3.43
0.5
63.4
0.94
17.0
-4.85
1.9
MB141
MB151
64.6
1.04
16.4
5.19
64.4
1.7
2.01
4.02
3.88
4.5
3.13
5.1
5.1
1.85
1.75
4.1
1.71
98.81 100.58
99.06
99.66
99.09
RB16d
RB65b
RB191
RB301
RB42w
70.3
0.41
15.4
2.66
70.6
0.41
15.5
2.74
0.6
70.3
0.44
69.7
0.44
15.8
2.85
68.8
0.53
15.7
3.60
1.6
1.66
3.4
1.2
1.59
4.7
3.48
3.03
98.93
99.15
SF32b SF32b
SF76b
0.8
16.1
1.51
4.6
3.44
2.88
2.0
1.99
3.3
2.92
99.70
99.93
2.41
3.81
1.59
4.5
3.73
99.71
99.75
99.59
99.40
SJ53w
SJ62d
S6w-
SF4w
SF8w
Si02
TiO2
Al203
FeO
MgO
CaO
Na20
K20
63.7
1.14
15.9
5.29
2.7
71.0
0.38
15.0
2.67
0.3
1.45
4.5
3.48
68.0
0.90
66.7
1.00
17.2
5.17
1.23
69.5
0.43
16.9
3.03
1.82
2.49
2.7
3.24
63.5
0.99
16.5
5.32
2.0
3.40
5.4
1.85
64.2
1.52
71.4
0.28
15.3
2.19
0.8
1.24
3.5
4.40
Total
99.36
99.11
98.78 100.35 100.49 100.10
99.00
4.01
5.1
SF54d SF128b SF128w
Si02
TiO2
Al203
FeO
MgO
CaO
Na20
K20
61.3
1.28
Total
99.22
16.1
6.56
3.0
4.65
4.5
1.81
60.3
1.27
16.2
6.65
2.8
5.08
5.5
1.78
70.0
0.77
15.9
18.1
4.63
1.54
2.57
2.4
2.24
SF145b TD2d
70.1
70.3
0.32
65.4
0.79
16.5
3.80
2.5
16.1
3.41
1.99
99.58 101.83
99.02
2.92
3.03
2.8
2.48
SF91w
2.79
1.5
1.48
3.0
3.65
3.81
3.91
SF12w
SF84g
0.40
1.01
17.0
5.09
2.0
3.78
15.1
2.41
0.6
1.22
3.0
5.17
98.14
4.74
2.0
2.92
60.7
1.26
17.0
7.10
2.2
4.73
5.0
2.01
5.3
1.44
97..78
99.73
0.91
16.0
WS111
5.4
2.15
7.09
0.8
3.40
5.7
1.59
70.6
0.40
15.7
2.77
0.6
1.48
3.9
3.73
99.75
99.14
99.18
3.21
61.9
1.58
17.1
403
pumice colors: w = white, b = black, g = gray (1 = light, d = dark).
CF38
Pink ignimbrite in roadcuts north of 126 and west of Cline
Falls, 2835', 15S/12E/15Bcd.
CF44
Gray ignimbrite underlying Deep Canyon ignimbrite member at
mouth of Buckhorn Canyon (Stensland, unpub. map, correlates this
unit to "unit 5" of Stensland, 1970), 2715', 14S/12E/20Cdb,
reverse magnetic polarity.
CF45
Gray ignimbrite overlying McKenzie Canyon ignimbrite member near
base of Teator Road grade (probably the same unit as CF44),
2630', 14S/12E/16Dda, reverse (?) polarity.
HB8
Pink ignimbrite near top of Deep Canyon grade, 3000',
15S/11E/10Bbd, reverse (?) magnetic polarity.
MW11
Lightgray to orange ignimbrite filling paleocanyon near mouth
of Willow Creek (unit Tdaw4 of Jay, 1982), 2200', 10S/12E/19Ccb.
MB14
Gray to pink ignimbrite underlying Tenino ignimbrite member in
upper Seekseequa Creek, 2500', 10S/11E/16Bdb.
MB15
Gray to pink ignimbrite underlying Tenino ignimbrite member
along Tenino Creek (same .unit as MB14), 2300', 10S/11E/2Bab,
reverse (?) polarity.
MB16
White ignimbrite between Tenino and Coyote Butte ignimbrite
members, S. Fk. Seekseequa Creek, 2580', 10S/11E/22Acc,
reverse (?) polarity.
RB12
Orange ignimbrite overlying Cove ignimbrite member, roadcut on
east side of Crooked River, CovePalisades State Park, 2420',
12S/12E/11Dbb, reverse polarity.
.
RB16
Orange ignimbrite overlying Cove ignimbrite member, The Ship
(correlative to RB12), 2400', 12S/12E/10Ddd, reverse
polarity.
RB65
Orange ignimbrite overlying Cove ignimbrite member, roadcut on
east side of Crooked River, CovePalisades State Park (same
outcrop as RB12), 2420', 12S/12E/11Dbb, reverse polarity.
RB19
Lightgray ignimbrite at top of old Deschutes Arm grade, 2400',
12S/12E/16Aba, reverse polarity.
RB30
Lightgray ignimbrite on east wall of Deschutes canyon south of
The Cove (same unit as RB19), 2400', 12S/12E/21Dbc.
RB42
Lightgray, lithic rich ignimbrite (debrisflow deposit?) near
water level in Crooked River canyon south of The Cove, 2010',
404
12S/12E/27B, normal (?) polarity.
SJ53
Gray ignimbrite prominently exposed at top of tributary canyon
to Deschutes River, 2 km west of Indian Park campground, 2200',
11S/12E/9Abd, reverse (?) polarity.
SJ62
Gray ignimbrite overlying Seekseequa basalt member, east wall of
Deschutes canyon opposite Indian Park campground, 2040',
11S/12E/11Bac, normal polarity.
S6
White, poorly consolidated ignimbrite, roadcut east of Simnasho
near cemetary, 2650', 7S/12E/20Abd.
SF4
Lightgray ignimbrite with ubiquitous stem impressions overlying
McKenzie Canyon ignimbrite member north of Big Falls, 2620',
14S/12E/9Aad.
SF8
Light gray ignimbrite with ubiquitous stem impressions overlying
McKenzie Canyon ignimbrite member (same unit as SF4)
SF12
Lightgray ignimbrite with ubiquitous stem impressions (probably
same unit as SF4 and SF8), overlying McKenzie Canyon ignimbrite
member south of Steelhead Falls, 2600', 14S/12E/4Adc.
SF32
Brown igimbrite beneath Steamboat Rock member north of Steelhead
Falls (unit 6 of Stensland, 1970), 2540', 13S/12E/22Ddc, reverse
polarity.
SF76
Brown ignimbrite beneath Steamboat Rock member near Steelhead
Falls (unit 6 of Stensland, 1970), 2540', 13S/12E/27Cbb, reverse
polarity.
SF54
Lightgray to pink ignimbrite below Lower Bridge ignimbrite
member, Deschutes canyon north of Squaw Creek, 2220',
13S/12E/3Ddd, reverse polarity.
SF128 Lightgray ignimbrite below Lower Bridge ignimbrite member,
River Place section (probably correlates to SF54), 2280',
13S/12E/21Cbd, reverse (?) polarity.
SF84
Orange ignimbrite above McKenzie Canyon ignimbrite, Deschutes
canyon near Geneva Canyon (may correlate to RB12, RB65), 2290',
12S/12E/290cb, reverse polarity.
SF91
White to pinkish gray ignimbrite below thick debrisflow deposit
in Deschutes canyon opposite Geneva Canyon, 2180',
12S/12E/29Dcb, reverse polarity.
SF145 Gray ignimbrite in paleochannel incised through McKenzie Canyon
and Lower Bridge ignimbrite members at Alder Springs, 2330',
13S/12E/18Cac, normal polarity.
405
TD2
Red ignimbrite at east abutment of Tumalo Dam, 3520',
16S/11E/29Abb, reverse polarity.
WS11
Lowest of two pink ignimbrites in roadcuts on U. S. 26, Warm
Springs grade, 2300', 9S/12E/17Dad, normal polarity.
Appendix Ij: Deschutes Formation Air-Fall Deposits
Si02
TiO2
Al203
FeO
MgO
CaO
Na20
K20
Total
CF36
CF37
HB10
HB12
57.0
2.04
16.4
9.64
3.6
6.04
69.0
0.65
16.6
68.8
0.89
17.4
65.9
0.79
3.51
4.31
4.09
0.92
1.69
4.5
3.04
0.8
1.3
2.61
3.7
2.44
2.66
4.5
2.53
99.81 100.95
98.87
4.1
0.94
99.76
17.1
SF148A
SF149
SF150
SF151
SF152
57.2
1.38
16.7
7.60
3.8
73.3
70.0
0.54
16.7
3.30
1.8
69.9
0.52
15.5
2.72
2.0
7.30
3.8
0.96
56.3
1.46
17.4
8.67
3.8
6.95
3.6
1.08
2.51
3.2
2.51
2.46
3.2
2.78
98.74
99.26
99.49 100.51
99.13
0.21
15.0
1.84
1.2
1.18
2.8
3.96
CF36
Black pumice lapilli from heterogeneous air-fall deposit above
Cline Falls, 2860', 155/12E/11Dcc.
CF37
White pumice lapilli from heterogeneous air-fall deposit above
Cline Falls, 2860', 155/12E/11Dcc.
HB10
White pumice lapilli from air-fall deposit above pink ignimbrite
near top of Deep Canyon grade, 3010', 155/11E/10Bbc.
HB12
White pumice lapilli from air-fall deposit near top of section
on Deep Canyon grade, 3035', 155/11E/10Bbd.
SF148A Cinder, near base of paleosol-dominated section just north of
Panorama Canyon above the Deschutes River, 2590', 13S/12E/8Dcd.
SF149 Cinder, approx. 2 m above SF148, 2595', 13S/12E/8Dcd.
SF150 White pumice lapilli from air-fall deposit approx.
SF149, 2600', 13S/12E/8Dcd.
1
m above
1
SF151 White pumice lapilli from air-fall deposit approx. 1.5 in above
SF150, 2605', 13S/12E/8Dcd.
SF152 White pumice lapilli from air-fall deposit approx.
SF151, 2610', 135/12E/8Dcd.
1 m above
406
Appendix Ik: Clasts in Deschutes Formation Sedimentary Units
MW9
MW29
MW30
MW36
MW39
MB10
RB67
RB68
SJ36
68.8
0.75
15.6
3.89
0.5
1.93
6.3
2.27
64.3
1.24
67.8
0.74
15.4
3.87
68.1
67.2
0.99
4.03
4.2
2.49
62.0
1.46
16.0
6.84
2.2
4.37
4.6
2.22
66.6
0.80
16.7
4.37
0.80
2.30
6.4
2.05
66.0
0.92
16.5
4.17
0.90
2.30
6.3
2.16
58.9
1.52
16.2
7.66
3.0
5.86
4.5
1.45
Total 100.04 100.18
99.69
98.89 100.59 100.26 100.02
99.25
98.99
Si02
TiO2
Al203
Fe0
MgO
Ca0
Na20
K20
16.1
5.72
2.1
1.1
2.14
5.3
2.54
0.87
16.0
3.93
0.9
1.92
6.3
2.29
16.1
5.75
1.1
2.99
4.7
2.70
SF129B
SJ11d
SJ11x
CF6
MW23
RB50
SJ60
Si02
TiO2
Al203
FeO
MgO
CaO
Na20
K20
70.0
0.59
14.2
3.20
0.5
1.66
5.6
3.03
64.9
0.90
17.0
4.59
1.4
2.99
6.2
1.83
57.5
1.12
60.1
1.45
61.8
18.1
16.7
7.90
3.0
5.59
4.7
1.35
50.6
0.94
17.4
8.79
9.7
10.83
2.5
0.18
48.9
1.79
16.5
9.33
9.0
9.97
2.9
1.34
Total
99.28
99.81
99.50 100.79 100.50 100.94
99.43
6.76
4.2
7.12
3.4
1.34
1.05
16.9
5.75
3.7
5.55
4.3
1.45
MW9
Vitrophyre fragments from lithic-rich base of ignimbrite (Tdaw3
of Jay, 1982) near mouth of Willow Creek, 2220', 10S/12E/19Ccb.
MW29
Vitrophyre fragments from debris-flow deposit, Vanora cliff,
2160', 10S/12E/6C.
MW30
Vitrophyre fragments from debris-flow deposit, Vanora cliff,
2180', 10S/12E/6C.
MW36
Vitrophyre fragments from debris-flow deposit, Vanora cliff,
2230', 10S/12E/6C.
MW39
Vitrophyre fragments from debris-flow deposit, Willow Creek
canyon, 2100', 10S/13E/33Bcb.
MB10
Vitrophyre fragments from Coyote Butte ignimbrite member above
Tenino Road, 2710', 10S/11E/3Bba.
RB67
Vitrophyre clast from debris-flow deposit above Cove ignimbrite
member on Deschutes Arm grade, Cove-Palisades State Park, 2400',
12S/12E/21Bac.
407
RB68
Vitrophyre clast from debrisflow deposit above McKenzie Canyon
ignimbrite member in Crooked River canyon south of The Cove
(approx. same stratigraphic position as unit containing RB67),
2360', 12S/12E/27Bad.
S336
Prismatically jointed vitrophyre clast from debrisflow deposit
overlying Jackson Buttes ignimbrite member above Jackson Trail
Road, north of Dry Hollow, 2300', 10S/12E/11Cad.
SF129B Vitrophyre clast from debrisflow deposit overlying McKenzie
Canyon ignimbrite member, Deschutes canyon, north of Sundown
Canyon area of Crooked River Ranch, 2320', 13S/12E/21Cbd.
SJ11d Porphyritic dacite clast from "Street Creek debrisflow
deposit", west of Seekseequa Junction, 2160', 10S/12E/28Aba.
SJ11x Diorite xenolith from dacite clast (sample SJ11d), 2160',
10S/12E/28Aba.
CF6
Black pumice bomb (3 m dia.) in debrisflow deposit at base of
Lower Bridge section, 2540', 14S/12E/16Adb.
MW23
Black pumice bomb from debrisflow deposit, roadcut on U. S. 26
on Vanora grade (fossil locality described by Chaney, 1938),
10S/13E/8B.
RB50
Diktytaxitic olivine basalt clast from thick flood breccia on
west wall of Crooked River canyon, south of The Cove, 2400',
12S/12E/22Ddc.
SJ60
Basalt block from hyaloclastite exposed in Deschutes canyon
below Round Butte member, 2200', 11S/12E/11Bdb.
408
Appendix Ii: Deschutes Formation, Bulk Sandstones
CF3
DC5a
DC5b
RB43
SF15
Si02
TiO2
Al203
FeO
MgO
CaO
Na20
K20
60.9
1.09
17.9
6.08
2.7
4.90
61.7
1.76
16.8
8.92
3.4
6.68
3.15
1.17
61.9
1.56
15.2
8.23
2.7
5.56
3.9
1.20
56.5
1.44
16.6
8.16
3.9
6.52
4.3
1.23
65.0
Total
99.39 103.58 100.25
98.65
98.78
4.1
1.72
1.01
15.9
5.25
1.8
3.86
.
4.00
1.94
Note: Analyses represent 2 gms (each sample) of black vitric grains
separated by hand from disaggregated sandstones.
CF3
Coarse-grained, black lithic sandstone, Lower Bridge section,
2540', 14S/12E/16Adb.
DC5
Coarse-grained, dark lithic sandstone, Dry Canyon flood deposit,
sand pits north of Belmont Lane, 2300', 11S/13E/9Dcb (2
analyses).
RB43 Very coarse-grained black, lithic sandstone, Crooked River
canyon, south of The Cove, 2320', 12S/12E/27Aab.
SF15 Coarse-grained black lithic sandstone, sand pit on River Road,
Crooked River Ranch, 2565', 13S/12E/27Ccc.
409
Appendix Im: Pliocene Diktytaxitic Basalts - Warm Springs Indian
Reservation
EB7
MB1
MB7
MB8
PP1
-PP2
PP3
PP4
PP5
50.56
1.38
16.90
11.32
8.04
9.73
2.06
0.08
0.14
0.18
50.03
2.13
16.66
10.85
8.47
8.49
2.79
0.35
0.40
50.48
0.97
17.60
8.74
8.10
49.6
1.56
18.4
10.73
6.6
51.11
1.45
50.76
11.71
1.84
8.88
17.00
11.29
6.66
9.66
2.45
50.75
1.39
16.72
10.73
7.62
9.74
50.28
1.49
16.73
10.94
7.09
9.75
2.41
0.26
0.21
0.13
0.18
2.19
0.24
0.15
0.17
2.21
0.23
0.14
0.16
16.42
11.72
6.90
8.96
2.73
0.29
0.14
0.17
51.21
1.47
16.46
11.46
6.88
8.82
0.26
0.12
0.17
0.16
0.12
0.17
Total 100.39 100.38
99.97
99.13 100.04
99.60
00.05
99.71
99.14
SJ16
SJ31
S342
SJ56
WS15
50.14
0.95
17.64
8.36
8.34
11.44
1.70
0.17
0.17
0.17
50.58
51.41
1.41
16.71
2.06
16.14
10.22
7.80
8.35
2.33
50.31
1.55
99.08
99.04
S102
TiO2
Al203
Fe0
MgO
CaO
Na20
K20
P205
MnO
SB7
S3
Si02
TiO2
Al203
FeO
MgO
Ca0
Na20
K20
P205
MnO
50.07
1.42
16.55
9.44
7.77
10.36
2.15
0.32
0.23
0.16
0.16
0.18
51.10
1.03
17.47
8.53
7.33
11.54
2.07
0.28
0.16
0.16
Total
98.47 100.14
99.67
50.29
1.40
17.49
11.55
6.78
9.88
2.30
0.11
3.1
0.11
10.43
7.67
9.35
2.41
0.27
0.15
0.16
1.51
0.71
0.46
0.15
16.84
11.22
7.50
9.96
2.47
0.28
0.17
0.17
99.63 100.47
EB7
Diktytaxitic olivine basalt, rimrock above fish hatchery, 2360',
8S/12E/13Dba, reverse polarity.
MB1
Diktytaxitic olivine basalt, Metolius-Bench rimrock, 3220',
10S/10E/36Bda.
MB7
Diktytaxitic olivine basalt, Tenino Bench rimrock, 2800',
10S/11E/4Aad, normal polarity.
MB8
Diktytaxitic olivine basalt, north of Tenino Creek,2960',
10S/11E/4Aab, normal polarity.
PP1
Diktytaxitic olivine basalt, lowest flow exposed in Mill Creek
canyon below U. S. 26 bridge, 2420', 8S/11E/21Aad, normal
polarity.
PP2
Diktytaxitic olivine basalt, second flow exposed in Mill Creek
410
canyon below U.
polarity:
PP3
S.
26 bridge, 2450', 8S/11E/21Aad, normal
Diktytaxitic olivine basalt, third flow exposed in Mill Creek
canyon below U. S. 26 bridge, 2480', 8S/11E/21Aad, normal
polarity.
PP4
Diktytaxitic olivine basalt, fourth flow exposed in Mill Creek
canyon below U. S. 26 bridge, 2520', 8S/11E/22Bbc, normal
polarity.
PP5
Diktytaxitic olivine basalt, Miller Flat rimrock exposed along U.
S. 26 at Mill Creek canyon, 2580', 8S/11E/22Bbc, reverse
polarity.
SB7
Diktytaxitic olivine basalt, Metolius Bench rimrock southeast of
Shitike Butte, 3150', 10S/10E/27Bba. reverse polarity.
S3
Diktytaxitic glomeroporphyritic olivine basalt, rimrock at top of
Beaver Creek grade, southwest of Simnasho, 2650', 7S/11E/15Dbd,
normal polarity.
SJ16 Diktytaxitic olivine basalt, Tenino Bench rimrock northwest of
Seekseequa Junction, 2440', 10S/12E/28Aab, normal polarity.
SJ31 Diktytaxitic olivine basalt, rimrock north of Dry Hollow,
2400',10S/12E/3Dcb.
SJ42 Diktytaxitic olivine basalt, rimrock on southern butte of the
Jackson Buttes, 2240', 10S/12E/25Cbd, reverse polarity.
SJ56 Diktytaxitic olivine basalt, rimrock west of Indian Park
campground, 2350', 11S/12E/10Baa, reverse polarity.
WS15 Diktytaxitic olivine basalt, Miller Flat rimrock at top of Warm
Springs grade on U. S. 26, 2380', 9S/12E/17Acd, normal polarity.
411
Appendix In: Neogene Diktytaxitic Basalts Erupted East
Deschutes Basin
of the
BNW4
DT1
0T2
DT3
DT4
DT5
016
017
ERI
S102
TiO2
Al203
FeO
MgO
CaO
Na20
K20
P205
MnO
49.97
50.96
1.38
15.70
9.25
7.83
9.73
3.37
0.73
0.43
0.16
50.56
1.56
15.49
9.93
7.74
9.57
3.30
0.73
0.46
0.17
50.94
1.60
15.57
9.68
7.22
9.64
3.52
0.73
0.45
0.17
51.14
1.62
15.58
9.94
7.03
9.20
3.53
0.83
0.45
0.18
51.32
1.62
15.86
9.82
6.74
9.24
3.50
0.81
0.45
0.16
50.96
1.64
15.97
10.25
6.98
9.27
3.00
0.82
0.45
0.17
50.51
2.75
0.43
0.38
0.18
50.15
1.45
15.19
9.53
8.47
10.10
3.32
0.71
0.44
0.17
Total
99.22
99.53
99.54
99.51
99.52
99.50
99.52
99.51 100.38
0C12
PB1
PB3
P2
H176
49.67
50.22
1.57
16.61
10.96
49.65
2.25
15.76
12.92
7.25
9.20
2.64
0.42
0.50
0.22
51.88
1.99
15.63
9.36
7.20
9.06
2.54
1.07
0.35
0.15
51.54
3.22
15.27
11.92
5.96
8.50
2.62
0.23
0.38
0.16
Total 101.22 100.09 100.81
99.23
99.71
Si02
TiO2
Al203
FeO
MgO
Ca0
Na20
K20
P205
MnO
1.47
17.15
10.58
6.86
9.45
1.86
16.12
12.09
8.21
9.91
2.65
0.18
0.33
0.20
7.41
10,03
2.63
0.20
0.27
0.19
2.12
15.77
10.88
6.75
9.90
2.77
1.04
0.45
0.19
(Note: only the flow sampled as H176 is assigned to the Deschutes
Formation; all other samples are from younger basalts.)
BNW4
Diktytaxitic olivine basalt, rimrock south of Sage Hollow, north
of Millican,
DT1/DT7
Diktytaxitic olivine basalt, sequence of flows on north
side of Crooked River and east of Japanese Creek, 30003200', 14S/14E/24Bd (Analyses courtesy of D. Thormahlen;
performed at WSU with international standards).
ER1
Diktytaxitic olivine basalt, talus below Alkali Flat, 3400',
17S/17E/29Cc.
0C12
Diktytaxitic olivine basalt rimrock northwest of Terrebonne
(Redmond basalt of Robinson and Stensland, 1979), 2750',
14S/13E/6A, normal polarity.
412
PB1
Diktytaxitic olivine basalt, intracanyon flow north of Stearns
Dam on the Crooked River, south of Prineville, 3000',
16S/15E/1Db.
PB3
Diktytaxitic olivine basalt overlying sediments on State Route
27, south of Bowman Dam, 3840', 17S/16E/238d, reverse polarity.
P2
Diktytaxitic olivine basalt, summit of Grass Butte, 3600',
15S/15E/90c, normal polarity.
H-146 Diktytaxitic olivine basalt, Teller Flat flow west of Heisler
Station, 2000', 10S/15E/68d (also analyzed by Hayman, 1983,
sample 176).
413
Appendix lo: Miscellaneous, Non-Deschutes
Si02
TiO2
Al203
FeO
MgO
CaO
Na20
K20
P205
MnO
Basin Volcanics
CC1
FP1
FP3
GPG1
GPG2
TV1
1V3
TV4
54.23
52.78
2.20
14.16
12.33
53.41
2.23
52.99
2.19
14.23
12.00
3.39
6.68
2.10
1.78
0.33
0.19
50.44
1.42
16.99
10.90
7.33
9.55
2.47
0.31
0.20
0.17
73.2
50.85
1.57
17.19
10.36
6.68
9.75
2.82
0.39
95.88
99.78
99.34
2.01
15.89
12.37
5.12
9.26
2.47
0.74
0.29
0.18
Total 102.56
3.41
3.31
6.53
2.46
1.58
0.32
0.18
6.68
2.17
1.77
0.33
0.19
53.44
2.22
14.34
11.05
3.25
6.70
2.07
1.73
0.34
0.18
95.95
95.82
95.32
14.43
11.30
HR1
PEYRL
W1
BN7
Si02
TiO2
Al203
FeO
MgO
CaO
Na20
K20
P205
MnO
54.8
1.17
18.3
7.08
3.8
7.07
4.2
1.40
71.3
0.34
15.4
2.60
67.8
52.74
1.09
15.72
8.34
7.76
8.35
1.80
0.28
Total
97.82
0.1
0.92
4.7
4.43
0.61
17.3
4.14
0.4
2.94
4.6
2.67
0.31
14.3
2.23
0.4
1.33
4.0
3.57
0.21
0.17
99.99
0.21
0.15
99.79 100.46
96.44
CC1
High-Mg0 chemical-type Grande Ronde Basalt, top of Cow Canyon
grade on U. S. 97, 3040', 8S/15E/10Cd.
FP1
Low-Mg0 chemical-type Grande Ronde Basalt, quarry east of Foreman
Point, 2640', 5S/11E/35Ddd.
FP3
Low-Mg0 chemical-type Grande Ronde Basalt underlying Prineville
chemical-type basalt west of Foreman Point, 3240', 5S/11E/34Dcb,
reverse polarity.
GPG1 Low-Mg0 chemical type Grande Ronde Basalt, mouth of Pacquet
Gulch, 2360', 6S/12E/10Abb.
GPG2 Low-Mg0 chemical type Grande Ronde Basalt underlying Prineville
chemical-type basalt in quarry south of Pacquet Gulch, 2640',
6S/12E/10Cbd, reverse polarity.
414
TV1
-Diktytaxitic olivine basalt, Juniper Flat along Rock Creek, west
of Maupin, 1900', 4S/12E/28Db.
1V3
White ignimbrite near top of Tygh Valley Formation in White River
canyon, 1850', 5S/12E/48d.
TV4
DiktYtaxitic olivine basalt, Maupin, 1150', 4S/14E/32Cd.
HR1
Olivine basalt, Horse Ridge, Dry River viewpoint on U. S. 20 east
of Bend, 4200', 19S/14E/14Ddd.
PEYRL
Peyerl tuff (ignimbrite), roadcut on Highway 31 east of McCarty
Butte, 26S/13E.
W1
Microporphyritic dacite flow, north end of Long Ridge south of
headwaters of Butte Creek, Warm Springs Indian Reservation,
3640'.
BN7
Slightly altered, porphyritic basalt dike (feeder for Bear Creek
basalt(?) of Goles, in press), Bear Creek Road northeast of Sage
Hollow, 3640', 18S/17E/28Dbb.
415
APPENDIX II: TRACE ELEMENT ANALYSES OF DESCHUTES BASIN BASALTS
Trace element analyses were performed by the author, under the
direction of Dr. Peter Hooper, at Washington State University by X-ray
fluorescence. Sample locations can be found under the same headings
and sample numbers in Appendix I.
Rb
Precision 5
Sr
6
Zr
10
V
Ba
Sc
Ni
V
2
20
2
13
5
(PPm)
SAMPLE
Prineville Chemical-Type Basalts
P01
P02
PD3
PO4
P05
P06
P07
PD9
P010
PD11
P012
P1
0N2
LS1
LS2
PG
PG
TY4
TY10
48.30
51.30
52.40
51.80
50.10
50.00
55.50
51.30
53.00
56.00
50.40
134.60
55.90
46.90
51.40
45.00
45.90
55.00
55.10
387.40
401.50
397.80
401.60
385.10
405.30
390.10
386.40
395.80
391.20
397.70
585.70
281.40
405.10
301.60
283.80
384.00
398.20
280.40
166.00
188.50
165.10
187.00
161.20
187.10
177.10
164.00
185.30
178.80
162.20
370.90
150.00
181.30
144.30
149.10
180.20
174.90
153.20
42.70
44.10
43.50
42.90
42.10
44.10
42.80
41.90
46.80
47.00
42.00
86.60
43.20
45.40
41.40
42.60
41.30
42.40
40.50
2164.30
2236.00
2324.80
2201.90
2014.00
2061.80
2116.50
1986.70
2048.20
2048.20
2143.80
2304.30
2123.30
2270.20
2154.00
2099.40
2167.70
2174.50
2055.00
36.50
38.40
35.40
37.00
37.00
35.10
38.10
36.80
37.80
35.70
38.10
37.00
34.10
38.40
34.10
33.80
36.80
38.10
31.90
26.70
80.70
56.70
38.90
40.30
33.30
93.00
30.60
82.60
40.80
34.20
93.00
26.50
99.50
64.10
18.20
19.60
19.80
36.50
359.60
351.40
356.90
345.30
353.10
350.30
349.40
355.30
349.30
365.10
363.80
350.30
225.20
357.80
231.50
230.50
359.10
350.90
233.70
Deschutes Formation Diktytaxitic Basalts
D97
0299
J203
RB25
RB52
8.90
9.20
21.60
5.50
8.40
303.80
337.10
319.90
280.50
320.30
83.60
88.50
113.20
81.10
84.80
21.40
21.90
29.40
29.80
21.60
97.90
91.10
401.90
138.90
210.60
42.40
44.30
38.10
42.70
40.00
151.10
110.60
111.80
187.50
114.30
189.80
216.70
229.20
211.70
217.80
159.60
156.70
148.60
155.00
35.20
35.50
37.50
37.80
446.30
466.80
446.30
483.90
44.30
42.70
42.90
40.80
50.00
22.10
17.30
38.10
417.90
420.90
400.20
444.30
Tetherow Butte Member
R1
RB46
RB66
SF143
19.00
19.00
19.30
21.90
373.60
391.00
372.70
386.60
416
Rb
Sr
Zr
Y
Ba
Sc
Ni
V
Steamboat Rock Member
WTHb
23.90
362.30
152.80
34.00
555.60
35.70
42.40
205.10
215.10
213.10
30.10
28.80
439.50
456.50
26.50
26.80
96.00
139.80
303.70
207.10
Round Butte Member
J109
RB39
22.40
18.60
510.90
805.40
Pliocene Diktytaxitic Basalts - Warm Springs Indian Reservation
PP1
PP2
PP3
PP4
PP5
5J16
SJ31
SJ56
14.00
16.70
11.70
15.80
10.40
10.10
3.90
14.80
261.50
255.30
243.80
235.40
241.50
332.30
322.20
500.30
111.90
114.00
113.60
105.90
104.40
96.10
83.50
219.20
26.00
26.20
23.80
23.70
27.00
21.90
20.40
28.70
108.20
121.80
118.40
111.60
111.60
255.00
470.20
460.00
34.30
29.00
31.10
29.80
32.20
38.60
40.50
26.30
133.30
118.00
117.40
122.20
110.50
157.00
153.00
170.40
191.40
191.50
186.50
187.40
202.60
255.60
225.00
193.60
Pliocene Diktytaxitic Basalts Erupted East of the Deschutes Basin
ER!
H176
0C12
PB1
PB3
P2
21.90
36.10
2.30
7.00
17.90
19.50
964.80
589.50
256.60
334.00
367.60
1143.80
218.10
323.20
131.10
108.90
138.30
249.30
29.20 1122.60
45.00 231.10
30.20 251.60
25.20 285.80
32.10 818.60
26.60 507.80
37.00
24.90
38.60
33.80
38.60
24.90
83.00
76.90
145.30
120.80
121.90
124.60
284.00
221.90
299.70
265.30
354.80
224.10
417
APPENDIX III: ELECTRON MICROPROBE DATA FOR SILICATE MINERALS IN
SELECTED DESCHUTES FORMATION IGNIMBRITES
unit
Fly Creek ignimbrite member
/
pumice /
D145
Samp.
opx
min.
Si02
Al203
TiO2
FeO
CaO
MgO
MnO
K20
Na20
Total
Wo
En
Fs
D338
opx
D338
opx
51.12
0.42
0.24
28.87
1.36
16.53
1.40
0.00
0.00
51.96
0.43
0.20
27.70
1.33
17.88
1.38
0.00
0.00
51.80
3
3
3
0.41
0.24
29.01
1.31
17.36
1.39
0.00
0.00
49
48
Si02
Al203
TiO2
FeO
CaO
hb
41.91
10.34
3.91
MgO
MnO
10.16
11.27
13.93
0.16
K20
Na20
Total
2.56
94.43
Wo
En
Fs
An
0.01
D145
plag
D338
plag
61.88
24.02
0.03
0.30
62.22
23.72
0.03
0.28
5.64
0.02
0.16
0.43
7.92
61.85
24.13
0.00
0.24
18
-
5.72
0.05
0.07
5.81
0.04
0.00
0.43
8.14
D665
opx
53.89
1.58
0.44
15.40
1.38
27.01
0.42
0.00
0.16
50.94
2.88
52
45
50
47
-
-
20
19
-
3
45
44
19
74
23
/--Fly Creek ignim. mbr.--/---Chinook ignim. mbr.
pumice /
D665
Samp.
min.
61.47
24.13
0.00
0.33
5.93
0.05
black --D665
/
D338
plag
0.48
0.42
7.23
7.92
99.95 100.88 101.51 100.08 100.47 100.42 100.63 100.13
An
unit
gray
D145
plag
.021
-
black
D665
plag
hb
D665
41.61
11.38
3.91
11.00
11.16
55.15
28.16
0.08
0.42
10.42
0.05
0.00
0.00
white
D665
plag
D401
opx
D401
opx
D401
D401
plag
plag
53.11
52.21
60.85
24.53
0.02
60.89
29.26
0.10
52.78
0.48
0.20
25.78
1.24
09.38
1.23
0.63
0.22
0.41
25.15
6.67
1.13
0.02
19.08
0.00
1.22
0.39
0.00
0.01
7.76
0.00
0.00
99.64 101.10 100.64
0.51
11.59
99.79
0.04
0.00
0.05
4.64
99.30
-
-
-
-
52
58
14.41
0.13
0.33
2.47
96.40
-
5.51
2
2
56
56
42
-
42
-
21
.24.31
0.04
0.28
6.27
0.03
0.00
0.42
7.61
99.88
21
cpx
D.89
7.09
21.27
15.03
0.21
0.03
2.56
98.49
11
418
unit
Balanced Rocks ignimbrite member
/
pumice /
Samp.
0650
min.
opx
D654
cpx
D654
opx
Si02
52.87 52.47 52.15
Al203
0.55
0.50
1.06
T102
0.29
0.25 0.42
FeO
24.44 25.73 12.26
CaO
1.58
1.65 19.46
MgO
20.23 19.20 13.95
0.67
MnO
1.28
1.21
K20
0.00
0.00
0.00
Na20
0.14
0.00
0.08
Total 101.32 101.08 100.01
Wo
3
3
En
58
39
55
42
Fs
0.01
0.28
6.69
0.06
0.01
0.36
7.28
99.75
/
pumice -- black
0330
Samp.
0330
min.
plag
plag
/
_
_
En
Fs
20
-
27
61.11
60.37 59.66 52.55 59.01
24.75 25.20
1.10 25.56
0.05
0.02
0.32
0.00
24.79
0.50
0.41
0.32
7.44
1.56
7.98
6.84
0.02
20.94
0.04
0.02
0.07
1.00
0.00
0.06
0.00
0.33
0.33
0.36
7.02
0.00
7.18
7.26
99.94 100.31 102.26 100.54
24.09
0.07
0.33
6.20
0.06
0.06
0.43
7.45
99.79
2
21
23
27
25.
Steelhead Falls ignimbrite member --/
white
006
opx
006
opx
61.26 58.99 51.88 51.11
0.39
1.32
23.93 25.59
0.07
0.21
0.32
0.00
0.42 28.77 27.50
0.35
1.44
1.22
6.07
8.07
0.05
0.04 17.16 17.74
1.46
0.06
0.06
1.37
0.40
0.27
0.00
0.00
0.14
7.79
6.83
0.00
99.92 100.29 101.22 100.81
Wo
black --D330
D330
opx
plag
0654
plag
59
39
22
unit
An
60.47
24.60
/
D654
plag
0650
plag
40
40
20
An
S102
Al203
TiO2
FeO
Ca0
MgO
MnO
K20
Na20
Total
gray
0650
plag
3
50
47
006
plag
006
plag
42.59
10.33
60.10
24.78
61.71
3.51
0.01
13.25
10.87
13.69
0.34
0.33
2.57
0.23
7.14
0.05
0.04
006
hb
006
hb
42.29
11.18
3.78
11.63
11.17
14.36
0.16
0.08
2.68
97.44
0.31
7.52
97.48 100.18
23.43
0.02
0.32
5.62
0.02
0.10
0.45
8.02
99.68
3
52
45
23
18
419
Tenino ignimbrite member
upper cooling unit
/-- lower cooling unit
/
black
pumice /
Samp.
M83
MB3
MB3
M36
MB6
MB6
MB3
M86
opx
plag
min.
opx
opx
plag
plag
opx
opx
MB6
plag
Si02
Al203
TiO2
FeO
Ca0
MgO
MnO
K20
Na20
Total
57.96
25.67
0.08
0.42
8.07
0.02
0.00
0.26
6.86
99.35
unit
/
52.87
0.39
0.24
22.73
1.60
20.27
1.18
0.00
0.15
99.43 101.03
Wo
En
Fs
An
unit
An
0.01
52.66
0.78
0.39
22.14
1.68
21.42
1.08
0.00
0.00
52.67
0.72
0.34
22.53
1.67
21.57
1.13
0.00
0.13
52.25
0.70
0.34
23.41
1.89
20.40
1.19
0.00
0.02
0.30
6.90
0.00
99.86 100.75 100.15 100.21
3
3
3
4
61
38
41
*61
36
58
38
22
36
58.75
25.67
0.09
0.35
7.94
0.06
0.00
0.27
6.97
100.08
25
27
Cove ignimbrite member
/
51.90
0.31
0.24
25.77
1.35
19.09
1.60
0.00
0.00
Total 100.27
Fs
58.80
25.44
0.02
0.45
7.94
56
FeO
Ca0
MgO
MnO
K20
Na20
Wo
En
60.66
24.48
0.00
0.34
6.44
0.06
0.07
0.35
7.42
99.82
3
59
pumice /
Samp.
RB10
opx
min.
Si02
Al203
TiO2
52.32
0.73
0.30
25.39
1.49
19.59
1.17
0.05
0.00
RB10
opx
52.18
1.06
0.35
22.89
1.75
20.17
1.13
0.05
0.00
99.57
RB10
opx
white
RB10
RB10
opx
cpx
RB10
plag
RB10
plag
RB10
plag
52.44 64.19 60.49 63.14
0.80 22.69 24.73 23.57
0.02
0.02
0.32
0.09
0.35
11.68
0.38
0.29
5.16
20.05
4.13
6.55
0.03
13.53
0.00
0.04
0.02
0.86
0.00
0.00
0.51
0.37
0.00
0.70
8.43
0.33
9.20
7.82
100.92 100.41 100.02 101.38 100.41 101.24
53.10
0.32
0.22
23.94
1.44
20.32
1.43
0.00
0.15
52.79
0.24
0.18
24.39
1.37
19.80
1.49
0.00
0.14
3
4
3
3
55
42
59
37
58
39
57
40
42
39
19
-
-
-
12
21
16
27
420
/-- Jackson Buttes ignimbrite member ---/--Gray ignim. on old-Deschutes grade, Cove
gray
gray
pumice /
/
unit
Samp.
min.
RB15
opx
RB15
opx
RB15
cpx
RB15
plag
Si02
52.45
52.63 52.47 60.51
Al203
0.33
0.25
0.87 24.86
0.05
TiO2
0.19
0.20
0.38
0.22
FeO
25.98 25.60 12.11
6.69
Ca0
19.91
1.32
1.43
0.04
MgO
19.41
19.48 13.50
MnO
1.58
1.56
0.77
0.03
0.38
K20
0.00
0.00
0.00
Na20
7.55
0.00
0.14
0.38
Total 101.25 101.27 100.40 100.33 1
Wo
En
Fs
3
3
41
55
42
56
39
20
41
An
RB19
opx
RB15
plag
55.87
27.80
0.08
0.47
10.00
0.07
0.06
0.20
5.82
00.37
51.43 51.12
61.96
0.31
0.28
23.47
0.17
0.19
0.00
0.30 30.70 29.60
1.33
5.23
1.39
0.03 15.93 16.07
1.46
1.67
0.02
0.00
0.00
0.46
0.00
7.47
0.14
98.93 101.53 100.27
_
_
22
RB19
opx
RB15
plag
35
18
RB19
opx
53.98
0.66
0.26
17.34
1.40
25.61
0.67
0.00
0.00
99.95
3
3
3
46
48
49
70
27
51
Gray ignim. at top of old/--Orange ignimb. above Cove/
/mg. mbr., The Ship
/
Deschutes grade, Cove
gray
gray
/
pumice
/
RB16
RB16
RB16
RB19
RB16
RB19
RB19
RB19
Samp.
RB19
opx
plag
plag
opx
min.
hb
hb
plag
plag
plag
unit
Si02
Al203
TiO2
FeO
Ca0
MgO
MnO
K20
Na20
Total
Wo
En
Fs
An
62.54
23.97
42.83
10.44
3.64
12.50
10.90
13.93
0.26
0.32
2.69
42.60
10.13
3.45
13.09
10.86
13.48
0.27
0.40
97.51
96.78 101.27
-
2.51
-
0.07
0.30
5.64
0.05
0.05
0.49
8.16
51.80
58.04 57.41
0.42
26.14 26.58
0.09
0.19
0.03
0.24
0.32 28.02
1.36
9.01
8.22
17.64
0.07
0.01
1.44
0.02
0.00
0.26
0.28
0.00
6.39
0.04
6.63
99.64 100.10 100.99
-
-
18
-
-
28
31
-
51.83
0.47
0.07
25.60
1.25
19.29
1.22
0.02
0.00
99.89
3
3
51
57
46
46
62.13
23.45
0.03
0.24
5.41
62.34
23.76
0.05
0.27
5.53
0.01
0.02
0.03
0.00
0.54
0.49
7.87
8.02
99.79 100.39
18
18
421
Orange ignim. on Crooked River grade/
/at The Cove
gray
pumice /
RB12
RB12
Samp.
RB12
RB12
RB12
RB12
RB12
plag
plag
opx
min.
opx
hb
hb
opx
unit
Si02
Al203
TiO2
FeO
CaO
MgO
MnO
K20
Na20
Total
Wo
En
Fs
An
/
52.47
0.87
0.22
22.42
1.27
21.47
1.20
0.00
0.00
99.93
51.46
0.39
0.26
27.90
1.42
17.12
1.34
0.04
0.00
99.93
42.67
10.04
3.37
3
3
-
47
50
51
51.12
0.37
0.22
29.58
1.44
15.77
1.40
0.00
0.00
99.91
3
61
36
-
-
unit
46
-
12.61
10.87
13.64
0.31
0.39
2.21
96.10
-
-
-
gray
CF38
cpx
Samp.
min.
CF38
opx
CF38
opx
Si02
Al203
TiO2
51.20
0.63
0.23
26.60
51.41
1.61
0.76
0.31
26.58
2.17
19.44
17.97
1.64
1.59
0.00
0.00
0.00
0.00
99.87 100.27
Wo
En
53
Fs
44
An
59.86
24.85
0.05
0.29
7.23
0.05
0.00
0.35
7.08
99.77
-
-
19
24
-
---Pink ignim. at Cline Falls----/
pumice
FeO
CaO
MgO
MnO
K20
Na20
Total
43.45 61.59
9.96 23.82
3.27
0.08
0.21
13.57
10.82
5.91
0.00
13.55
0.28
0.04
0.47
0.20
8.09
2.70
97.80 100.21
3
4
52
44
CF38
plag
CF38
plag
51.00
2.23
0.82
10.28
60.18
24.93
0.03
0.23
59.93
24.70
0.09
19.81
14.53
0.61
6.69
0.03
0.00
0.32
7.44
99.85
6.69
0.00
0.00
0.29
7.55
99.57
0.00
0.29
99.58
0.31
41
42
17
22
22
CF38
opx
gray
CF38
53.28
1.01
0.38
18.18
1.46
24.65
0.96
0.00
0.00
99.92
opx
52.94
0.88
0.35
20.65
1.56
22.92
1.14
0.00
0.00
100.44
3
3
69
64
28
-
33
422
unit
/--Pink ignimbrite on Deep Canyon Grade---
pumice /
Samp.
min.
HB8
opx
HB8
opx
S102
Al203
TiO2
FeO
CaO
52.41
51.90
0.47
0.28
26.25
1.60
18.82
MgO
MnO
0.38
0.28
25.76
1.44
19.11
1.18
1.21
0.00
0.00
0.14
0.00
Total 100.69 100.54
K20
Na20
Wo
3
3
En
Fs
55
42
55
42
gray
HB8
HB8
plag
cpx
51.87 62.56 62.28
1.19 23.20 23.66
0.03
0.08
0.43
12.79
0.28
0.33
19.43
0.28
0.33
13.37
4.85
5.51
0.03
0.04
0.55
0.00
0.59
0.56
7.89
0.19
8.39
99.83 100.02 100.31
0.01
0.01
0.32
5.63
0.06
0.58
7.90
99.58
18
--Pink ignim. on Deep Canyon Grade-/
HB8
opx
5102
52.35
0.46
Al203
TiO2
0.22
FeO
25.53
CaO
1.49
MgO
19.72
MnO
1.17
K20
0.00
Na20
0.00
Total 100.93
Wo
En
Fs
An
61.48
23.54
21
15
pumice /
Samp.
min.
HB8
plag
40
39
An
unit
HB8
plag
HB8
opx
51.87
0.55
0.28
24.45
1.44
20.10
1.09
0.00
0.00
52.38
1.28
0.42
11.57
20.38
13.67
0.56
0.00
0.51
HB8
plag
HB8
plag
63.05
23.25
0.03
0.27
5.14
62.29
23.73
0.04
0.22
5.40
0.03
0.00
0.54
7.88
0.01
0.01
0.62
8.31
99.79 100.71 100.69 100.13
3
3
56
58
39
41
white
HB8
cpx
42
39
19
16
18
18
423
APPENDIX IV: TYPE LOCALITIES OF DESCHUTES FORMATION MEMBERS
PELTON BASALT MEMBER: Near confluence of Willow Creek and Deschutes
River (Lake Simtustus), SE 1/4 S. 25, T. 10 S., R. 12 E., Madras West
7.5'; 8 flow units of diktytaxitic olivine basalt, 30m thick, base at
1700'.
Other prominent exposures: continuous along both sides of
Deschutes River from Pelton Dam to Round Butte Dam; south of Gateway
along Clark Drive (2100', SW 1/4 S. 29, T. 9 S., R. 14 E., Gateway
7.5').
CHINOOK IGNIMBRITE MEMBER: Near confluence of Fly Creek and Metolius
River (Lake Billy Chinook), SW 1/4 S. 26, T. 11 S., R. 11 E., Fly Creek
7.5'; unwelded, pinkgray, rhyodacitic ignimbrite, 30 m thick, base at
Other prominent exposures: nearly continuous along Metolius
2000'.
River from Monty Campground to CovePalisades State Park; along Jackson
Trail Road east of Seekseequa Junction (1980', NE1/4 S. 27, T. 10 S.,
R. 12 E., Seekseequa Junction 7.5'); in Dry Hollow (2000', NW1/4 S. 14,
T. 10 S., R. 12 E., Seekseequa Junction 7.5'); in cliff face northeast
of Vanora townsite (2050', SW1/4 S. 6, T. 10 S., R. 13 E., Madras West
7.5'); in Mud Springs Valley southwest of Gateway (2120', NE1/4 S. 36,
T. 9 S., R. 13 E., Madras East 7.5').
SEEKSEEQUA BASALT MEMBER: Cliffs on either side of Seekseequa Creek
just east of Seekseequa Junction, E1/2 S.27, T. 10 S., R. 12 E., Seekseequa Junction 7.5'; columnar jointed, porphyritic, olivinebearing
basalt, 3-25 m thick, base at 2000'. Other prominent exposures: 1.5 km
north of Round Butte Dam along access road (2000', SE1/4 S. 15, T. 11
S., R. 12 E., Round Butte Dam 7.5'); both sides of Deschutes River about
1
km south of U. S. 26 bridge (2040', S1/2 S. 30, T. 9 S., R. 13 E.,
Eagle Butte 7.5').
JUNIPER CANYON BASALT MEMBER:. Lower cliffs at mouth of Juniper Canyon,
SW1/4 S. 28, T. 11S., R. 12 E., Round Butte Dam 7.5',12 m thick, base
at 2090'; multiple flow units of diktytaxitic olivine basalt. Other
Discontinuous along both sides of Metolius River
prominent exposures:
from Juniper Canyon to the mouth of Fly Creek; on west side of
Deschutes River below Canadian Bench (2070', SE1/4 S. 34, T. 11 S., R.
12 E.,Round Butte Dam 7.5')); in tributary canyon to Deschutes River
2.5 northwest of Round Butte Dam (2000', SE1/4 S. 9, T. 11 S., R. 12 E.
Seekseequa Junction 7.5')
OPAL SPRINGS BASALT MEMBER: Base of the west wall of Crooked River
canyon at north end of Hollywood Road, Crooked River Ranch, NW1/4 S. 24
T. 13 S., R. 12 E., Opal City 7.5'; four flow units of diktytaxitic
olivine basalt, 40 m thick, base at 2160'; Other prominent exposures:
along the Crooked River from Osborn Canyon to Opal Springs.
HOLLYWOOD IGNIMBRITE MEMBER: Roadcuts along Hollywood Road, Crooked
River Ranch, west side of Crooked River; SE1/4 S. 24, T. 13 S., R. 12
E., Opal City 7.5'; unwelded, orange ignimbrite with orange rhyolite
and black andesitedacite pumice, 30 m thick, base at 2240';
424
JACKSON BUTTES IGNIMBRITE MEMBER: On west flank of northern butte
of Jackson Buttes, NW1/4 S.
25,
T. 10 S., R. 12 E., Seekseequa
Junction 7.5'; unwelded, lightgray to pink rhyodactic ignimbrite, 15 m
thick, base at 2140 ft.,
Other prominent exposures: southwest of
Indian Park campground (2140', NW1/4, S. 10 S, T. 11 S., R. 12 E.,
Seekseequa Jct. 7.5'); near base of exposed section on north side of
lower Willow Creek canyon (2100', SW1/4 S. 19., T. 10 S., R. 13 E.,
Madras West 7.5').
BIG CANYON BASALT MEMBER: Cliffs at mouth of Big Canyon, NW1/4 S. 32,
T. 11 S., R. 12 E. Round Butte Dam 7.5', 20 m thick, base at 2220';
multiple flow units of diktytaxitic olivine basalt. Other prominent
exposures: Nearly continuous on both sides of Metolius River from
Juniper Canyon to the mouth of Fly Creek; on west side of Deschutes
River below Canadian Bench (2190', SE114 S. 34, T. 11 S., R. 12 E.,
Round Butte Dam 7.5'); in Willow Creek canyon immediately downstream
2180', SW1/4 S. 2, S. 3, T. 11 S., R. 13 E., Madras
from Madras (2100
West 7.5').
LOWER BRIDGE IGNIMBRITE MEMBER: Cliffs on either side of Lower Bridge
Market Road at Lower Bridge on the Deschutes River, NE1/4 S. 16, T. 14
S., R. 12 E., Cline Falls 7.5'; light gray to pink, unwelded, rhyolite
ignimbrite, up to 20 m thick, base at 2555'. Other prominent exposures:
nearly continuous along Deschutes River from Lower Bridge to the mouth
of McKenzie Canyon; both sides of Deschutes River 3km northwest of
Steelhead Falls (2380', NE1/4 S. 20, SW1/4 S. 21, T. 13 S., R. 12.E.,
Steelhead Falls 7.5').
COVE IGNIMBRITE MEMBER: Roadcuts along paved road on west side of the
Deschutes River, CovePalisades State Park, SW1/4 S. 16, T. 12 S., R.
12 E., Round Butte Dam 7.5'; white, unwelded rhyodacitic ignimbrite,
Other prominent occurrences: on The Ship
3 m thick, base at 2260'.
(2260', SE1/4 S. 10, T. 12 S., R. 12 E., Round Butte Dam 7.5'), road
cuts on east side of the CovePalisades State Park (2260', NW1/4 S.11,
11,
T. 12 S., R. 12 E., Round Butte Dam 7.5').
MCKENZIE CANYON IGNIMBRITE MEMBER: Exposure along lower McKenzie Canyon
at its confluence with the Deschutes River, SE1/4 S. 4, T. 14 S., R.-12
E., Steelhead Falls 7.5', 7 m thick, base at 2540'; three flow units of
welded ignimbrite, white at base to redorange at top, with white
rhyolitic pumice and black andesitic pumice. Other prominent exposures:
At top of exposed section on south side of Deschutes River and west of
road at Lower Bridge (2585', NE 1/4 S. 16, T. 14 S., R. 12 E., Cline
Falls 7.5'); both sides of Deschutes canyon- 3 km northwest of Steelhead
Falls (2420', NE 1/4 S. 20, SW 1/4 S. 21, T. 13 S., R. 12 E., Steelhead
Falls 7.5').
BALANCED ROCK IGNIMBRITE MEMBER: At the Balanced Rocks of Brogan (1973),
33, T. 11 S., R. 11 E., Fly Creek 7.5'; unwelded, gray ignimbrite with black andesitic, gray rhyodacitic, and mixed pumice lapilli
Other prominent exposures:
and bombs, 24 m thick, base at 2440 feet.
in a gully about 1 km west of the Balanced Rocks (2440', NE1/4, S. 32,
NW1/4 S.
425
11 S., R. 11 E., Fly Creek 7.5'), along Forest Road 1130, 0.75km
south of Fly Creek Ranch site (2520', SW1/4 S. 4, T. 12 S., R. 11 E.,
Fly Creek 7.5'), on bare hillside just west of Fly Creek Ranch site
(2520', NW1/4 S. 4, T. 12 S., R. 11 E., Fly Creek 7.5').
T.
FLY CREEK IGNIMBRITE MEMBER: North side of Fly Creek 1 km west of Fly
Lake, SW1/4 S. 8, T. 12 S., R. 11 E., Fly Creek 7.5'; welded light gray
to light orange ignimbrite with gray rhyolite pumice grading upward to
unwelded light-gray ignimbrite with rhyolite and black basaltic
andesite pumice, 30 m thick, base at 2650'.
Other prominent exposures:
along Forest Road 1130, 1 km south of Fly Creek Ranch site (2600',
SW1/4 S. 4, T. 12 S., R. 11 E., Fly Creek 7.5') at the Balanced Rocks
2500', NW1/4 S. 33, T. 11 S., R. 11 E., Fly Creek 7.5'), vitrophyre
south of Spring Creek (NE1/4 S. 31, T. 11 S., R. 11 E.,Fly Creek 7.5'),
along trail descending into Deschutes River canyon from Canadian Bench
2330' NE1/4 S. 34, T. 11 S., R. 12 E., Round Butte Dam 7.5').
TENINO IGNIMBRITE MEMBER:
Roadcuts on Tenino Road along Tenino Creek,
N1/2 S. 3, T. 10 S., R. 11 E., Metolius Bench 7.5'; two dark gray
ignimbrites with black dacitic pumice lapilli and bombs, locally welded,
pink to orange near the top of each unit, separated by 15 m of sediment
(not included in the member), 35-90 m thick, base at 2280'.
Other
prominent exposures:
along north side of South Fork Seekseequa Creek
(2360', SW1/4 S. 22, T. 10 S., R. 11 E., Metolius Bench 7.5'), north
side of Seekseequa Creek (2460', N1/2 S. 16, T. 10 S., R. 11 E.,
Metolius Bench 7.5').
COYOTE BUTTE IGNIMBRITE MEMBER: On north side of gully on south flank
of Coyote Butte, SW1/4 S. 30, T. 10 S., R. 12 E., Seekseequa Junction
7.5'; white to light gray, unwelded dacitic ignimbrite, 2.5 m thick,
base at 2390'. Other prominent exposures: on south- and west-facing
slopes 1.2 km northeast of Seekseequa Junction (2300', SE1/4 S. 22, T.
10 S., R.12 E., Seekseequa Junction 7.5'), above Tenino Road on north
side of Tenino Creek (2710', NW1/4 S. 3, T. 10 S., R. 11 E., Metolius
Bench 7.5').
STEELHEAD FALLS IGNIMBRITE MEMBER: Above Steelhead Falls, NW1/4 S. 27,
T. 13 S., R. 12 E., Steelhead Falls 7.5'; pink to light gray, unwelded,
rhyodacitic ignimbrite, 6.5 m thick, base at 2405'.
Other prominent
exposures: both sides of Deschutes River 3 km northwest of Steelhead
Falls (2440', SE1/4 S. 20, SW 1/4 S. 21, T. 13 S., R. 12 E., Steelhead
Falls 7.5'), east side of Deschutes River opposite mouth of Squaw Creek
(2360', NE1/4, S. 7, T. 13 S., R. 12 E., Steelhead Falls 7.5'), east
side of Crooked River canyon 2.5 km west-southwest of Opal City (2375',
NW1/4 S. 24, T. 13 S., R. 12 E., Opal City 7.5').
PENINSULA IGNIMBRITE MEMBER:
At unimproved Bureau of Land Management
campground on east side of Deschutes River 0.5km south of Steelhead
Falls, SE1/4 S. 28, T. 13 S., R. 12 E., Steelhead Falls 7.5'; light
gray to light brown, unwelded ignimbrite with white rhyolitic, gray
dacitic, and black andesitic to dacitic lapilli and bombs, 3 m thick,
base at 2445'. Other prominent exposures: on both sides of Deschutes
426
River 3 km northwest of Steelhead Falls (2470', S1/2, S. 21, T. 13 S.,
R. 12 E., Steelhead Falls 7.5'); east side of Deschutes canyon opposite
mouth of Squaw Creek (2590', NE1/4 S. 7, T. 13 S., R. 12 E., Steelhead
Falls 7.5'); on east side of Squaw Creek northeast of Alder Springs
(2600', NW1/4 S. 18, T. 13 S., R. 12 E., Steelhead Falls 7.5').
DEEP CANYON IGNIMBRITE MEMBER: Roadcuts along S. R. 126 on northwest
side of Deep Canyon, SW1/4 S.3, T. 15 S., R. 11 E., Henkle Butte 7.5';
gray-brown to yellow dacitic ignimbrite, welded at the base, 20m thick,
Other prominent exposures: near the northeast end of
base at 2870'.
Buckhorn Canyon (2720', NE1/4 S. 29, T. 14 S., R. 12 E., Cline Falls
7.5').
SIX CREEK IGNIMBRITE MEMBER: Along northwest-trending ridge south of
Prairie Farm Road and north of Forest Road 1100/800 on north side of
Six Creek, SW 1/4 S. 10, T. 12 S., R. 10 E Whitewater River 15', 50
m(?) thick, base near 3500'; brown, unwelded ignimbrite with black
andesitic pumice bombs to 1.5 m across and gray dacitic lapilli to 20
cm across.
Other prominent exposures: west face of Green Ridge, northeast of Allen Springs campground (4200', SW1/4 S. 12, T. 12 S., R. 9
E., Whitewater 15'); along Forest Road 1130 0.5 km north of Fly Lake
(2780', SE 1/4 S. 8, T. 12 S., R. 11 E., Fly Creek 7.5').
TETHEROW BUTTE MEMBER: Tetherow Butte cinder cones - south of
Terrebonne, S1/2 S. 20, N1/2 &E1/2 S. 29, WI/2 S. 28, N1/2 S. 33, T. 14
S., R. 13 E., .Redmond 7.5'.
Agency Plains basalt flow - quarry on
north side of U. S. 26 in Campbell Creek canyon, NE1/4 S. 8, T. 10 S.,
R.
13 E., Madras West 7.5'; sparsely phyric, fine-grained, columnar
jointed basalt, §60m thick, base near 2100'.
Crooked River basalt flow
- on Badger Drive, Crooked River Ranch, SW1/4 S. 36, T. 13 S., R. 12
E., Opal City 7.5'; sparsely phyric, fine-grained, crudely platy and
columnar jointed basalt, 15m thick, base at 2720'. Other prominent
exposures: both flows exposed along east side of Crooked River in 1 or
2 cooling units from Osborn Canyon to Opal Springs; Agency Plains
basalt flow also well exposed in quarries at east entrance to CovePalisades State Park (2520', SW1/4 S12, T. 12 S, R. 12 E., Culver 7.5')
and south of Straun Road in Willow Creek canyon (2300', SE1/4. S.33, T.
-
.
10 S.,
R.
13 E.).
LOWER DESERT BASALT MEMBER:
Canadian Bench basalt flow - rimrock at
north end of Canadian Bench, Nw1/4 S. 27, T. 11 S.; R. 12 E., Round
Butte Dam 7.5'; light to medium gray, porphyritic, diktytaxitic olivine
basalt, 18 m thick, base at 2360'. Other prominent exposures: head of
Juniper Canyon (2440', Sw1/4 S. 33, T. 11 S., R. 12 E., Round Butte Dam
7.5'), viewpoint on west rim of Deschutes Canyon above confluence with
Crooked River (2420', NW1/4 S. 35, T. 11 S, R. 12 E., Round Butte Dam
7.5'); top of highway grade west of Deschutes Arm of Lake Billy Chinook
(2620', NW1/4, S. 21, T. 12 S., R. 12 E., Round Butte Dam 7.5').
Fly
Lake basalt flow - at top of highway grade west of Fly Lake on Forest
Road 1130, SE1/4 S. 8, T. 12 S., R. 11 E., Fly Creek 7.5; gray diktytaxitic olivine basalt with prominent vesicle cylinders, 6 m thick,
base at 2980'. Other prominent exposures: rimrock north and south of
427
Big Canyon.
STEAMBOAT ROCK MEMBER: Steamboat Rock, SE1/4 S. 10, SW1/4 S. 11, T. 14
S., R. 12 E Cline Falls 7.5'; dike of aphyric basalt overlain by 2m
of flow of same composition. Other prominent exposures: rimrockforming basalt above Steelhead Falls (2680', S. 27 & S. 34 T. 13 S., R.
12 E., E1/2 S. 3, W1/2 S. 2, T. 14 S.; R. 12 E., Steelhead Falls 7.5'),
best exposures of basaltic tuff breccia and spatter are on west side of
Deschutes River north of Steelhead Falls (SE1/4 S. 21, T. 13 S., R. 12
E., Steelhead Falls 7.5') and along Canyon Drive, Crooked River Ranch
(SE1/4 S. 16, T. 13 S., R. 12 E., Steelhead Falls 7.5'), shieldforming
hill underneath Crooked River Ranch water tower (NE1/4 S. 16, T. 13 S.,
R. 12 E., Steelhead Falls 7.5').
ROUND BUTTE BASALT MEMBER: Roadcuts on access road to Round Butte Dam,
SW1/4 S. 11 T. 11 S., R. 12 E., Seekseequa Junction 7.5'; four flow
units of dark to light gray porphyritic olivine basalt; lower flow unit
is invasive and all flow units are intercalated with discontinuous
sandstone; 12 m thick, base at 2360'. Other prominent exposures:
Cinder pits near summit of Round Butte (3100', SE 1/4 S. 13, T. 11 S.,
R. 12 E., Culver 7.5'); roadcuts on Belmont Lane on west rim of Dry
Canyon (2350', NE1/4 S. 17, T. 11 S., R. 13 E., Culver 7.5'.
RATTLESNAKE IGNIMBRITE MEMBER: Type locality is outside of the
Prominent exposures marginal to
Deschutes basin; see Walker (1979).
north
of
Grizzly
Mountain
(3400', SE1/4 S. 36 and
Deschutes basin:
3680', SW1/4 S. 33, T. 12 S., R. 14 E., Prineville 15'), Swartz Canyon
(3080', NW1/4 S. 24, T. 16 S., R. 15 E., Powell Buttes 15').
428
APPENDIX V: MEASURED SECTIONS OF SIMTUSTUS FORMATION
APPENDIX V-1: Pelton Dam Measured Section
Location: Roadcuts at Pelton Dam and 1.5 km north of the dam,
railroad cuts 0.75 km south of the dam.
Measured by G. A. Smith, July 13, 1.983
UNIT
THICKNESS
UNIT TOTAL
DESCRIPTION
(Section exposed in cuts on abandoned
km south of Pelton Dam.)
grade,
railroad
23
Basalt,
Pelton Basalt
18.8
144.1
22
Conglomerate, pebble to cobble, with lenses of cross
stratified sandstone; erosive at the base.
7.1
125.3
2.1
118.2
40.0
116.1
diktytaxitic olivine basalt;
member, Deschutes Formation.
Top of the Simtustus Formation
21
tuffaceous, with dispersed, rounded pumice
Mudstone,
lapilli; light tan; base not exposed.
(Section offset 2.25 km to the north to roadcuts
Pelton Dam Road.)
COVERED
on
20
Sandstone, grading upward to siltstone, finegrained,
massive,
tan;
with dispersed pumice lapilli,
gradational at the base to:
0.9
76.1
19
Sandstone, grading upward to siltstone; lithic sand
at the base and fines
is
coarsegrained,
stone
upward into the silstone; sandstone is plane bedded
siltstone is massive;
and scourfill crossbedded,
tan; sharp base.
0.8
75.2
18
Sandstone,
finegrained, tuffaceous,
gradational at the base to:
massive, tan;
0.3
74.4
17
Sandstone,
pebbly,
plane bedded;
coarsegrained,
are
pebbles of andesite and basalt up to 2 cm dia.
concentrated near base and diminish in abundance
upward; light gray, sharp base.
0.9
74.1
16
Breccia,
angular to subrounded volcanic
lithic
fragments 5 mm to 3 cm across in a silt matrix;
cm is
plane bedded
coarsegrained
lowest
6
sandstone; tan to light brown; erosional base.
1.2
73.2
429
15
with dispersed pumice
coarsegrained
Sandstone,
lapilli,
trough crossbedded and plane bedded,
tuffaceous, sharp base.
0.9
72.0
14
pebbly,
coarsegrained at base fining
Sandstone,
tan; gradational at
upward to siltstone, massive,
base to:
1.1
71.1
13
Sandstone,
lag at
pebbly,
pebble
coarsegrained,
planebedded and scourfill crossbedded,
0.9
70.0
base,
fines upward and becomes more pumice rich;
tan; erosive base.
gray to
12
granules (3 to 4 mm) of reddishgray
Conglomerate,
andesite and pumice lapilli with scattered blocks
of basalt to 20 cm across,
planebedded; fines
sandstone;
upward
to coarsegrained tuffaceous
sharp base.
2.5
69.1
11
sandy with dispersed pumice lapilli, mas
Siltstone,
sive, tan, gradational at the base to:
1.5
66.6
10
of
pebbles
pebbly;
Sandstone,
coarsegrained,
rounded
basalt,
cobbles
of
andesite,
subangular
pumice lapilli in a tuffaceous, lithic sand matrix,
1.8
65.1
planebedded, sharp base.
(Section. offset 1.5 km to the south to road
adjacent to the east abutment of Pelton Dam.)
cuts
finegrained,
weathered
brown,
Ronde
Prineville chemicaltype Grande
Basalt, Columbia River Basalt Group.
-
15.1
63.3
9
Basalt,
glassy;
8
Conglomerate,
poorly sorted, granules of
planebedded
reddishgray andesite to 1 cm across,
and planar tabular crossbedded, gray; sharp base.
1.8
48.2
7
Sandstone,
medium to coarse grained.with dispersed
trough
and
bedded
pumice
lapilli,
plane
crossbedded., fines upward into laminated siltstone,
tan to gray; sharp base.
1.4
46.4
6
Tuff,
massive, brown;
0.2
45.0
5
Siltstone,
with dispersed white,
pumice lapilli to 1 cm
altered
tan; gradational at the base to:
green
pink,
and
massive,
across;
1.5
44.8
4
Sandstone, interbedded crossbedded, lightgray, well
sorted,
mediumgrained sandstone and lenses of
4.6
43.3
gray,
sandy,
ash and accretionary lapilli,
sharp base.
430
massive, tan, poorlysorted, pumicelapillibearing
sandstone; sharp base.
fine to coarse grained lithic sand with
pumice lapilli, trough and scourfill crossbedding;
some scourfill
sandstones capped by 1 to 4 cm
thick mudstones with dessication cracks and rootlet
impressions; erosive base.
4.6
38.7
pumice lapilli to 1 cm across dispersed in a
white ash matrix;
faint plane laminations and
crosslaminations near base; numerous leaf and stem
impresssions; sharp base.
3.1
34.1
dark gray,
fine grained, glassy; Prineville
chemicaltype Grande Ronde Basalt, Columbia River
Basalt Group, base not exposed.
31.0
31.0
3
Sandstone,
2
Tuff,
1
Basalt,
Base of Section at 1490'.
431
APPENDIX V-2: Gateway Grade Measured Section
Location: SW Gateway Grade Drive, southwest of Gateway.
Measured by G. A. Smith, July 14, 1985.
DESCRIPTION
UNIT
THICKNESS
UNIT TOTAL
12
Conglomerate,
cobbles to 20 cm with lenses of
crossbedded coarsegrained sandstone; sharp base.
15.4
53.9
11
angular volcanic lithic fragments to 15 cm,
Breccia,
in a wellindurated mud
rounded pumice lapilli
rare
matrix,
abundant leaf and stem impressions,
4.6
38.5
silty, root traces, massive, white; sharp
1.7
33.9
Sandstone, coarse grained, pebbly lithic sand, planar
fines upward into
tabular crossbedding (NlOW);
erosive
thinbedded tuffaceous siltstone beds;
4.6
32.2
1.2
27.6
Sandstone, fine grained with disperse pumice lapilli,
numerous mudstonefilled clastic dikes up to 5 cm
across, Celtis endocarps are common, massive, white
to tan; base not exposed.
2.2
26.4
COVERED
1.5
24.2
tuffaceous, with dispersed
Sandstone,
fine grained,
pumice lapilli, fines upward, Celtis endocarps are
fragments,
common,
scattered vertebrate fossil
3.7
22.7
finegrained
interbedded,
Sandstone and claystone,
sandstone with claystone intraclasts and claystone
beds to 10 cm thick, dispersed pumice lapilli, mud
base not
filled clastic dikes up to 2 cm thick;
exposed.
1.2
19.0
COVERED
3.1
17.8
tuffaceous with dispersed pumice
Siltstone,
sandy,
tan,
decreasing in abundance upward,
lapilli
massive; gradational at the base to:
5.2
14.7
petrified wood, massive, brown; sharp base.
10
9
Diatomite,
base.
base.
8
pebbles to 8 cm, with lenses of cross
stratified coarsegrained sandstone; erosive base.
Conglomerate,
Top of Simtustus Formation
6
massive, tan; gradational at the base to:
5
4
432
3
Sandstone,
mediumgrained sandstone fining upward
into
sandy siltstone,
dispersed pumice lapilli,
massive, light tan; gradational at the base to:
2
Siltstone,
numerous
sandy with dispersed
clastic dikes to 2
pumice lapilli,
cm across with
2.8
9.5
3.7
6.7
3.0
3.0
verticalbedded claystone margins, Celtis endocarps
massive, tan; gradational at the base
are common,
to:
1
fines upward,
medium to coarse grained,
dispersed pumice lapilli in lithic sand, trough and
scourfill crossbedding (N35E) gray; base not
exposed.
Sandstone,
Base of Section at 2010'.
433
APPENDIX V-3: Clark Drive Measured Section
on Clark Drive southeast
Roadcuts
Gateway.
Measured by G. A. Smith, July 14, 1983.
Location:
DESCRIPTION
UNIT
9
of
THICKNESS
UNIT TOTAL
3.1
58.6
horizontally
fine to medium grained,
Sandstone,
abundant
root
massive
at
top,
laminated at base,
baked to red color by overlying basalt;
traces,
discontinuous; sharp base.
1.8
55.5
cobbles and boulders to 25 cm across,
Conglomerate,
planar tabular and trough crossbedded
of
lenses
sandstone; erosive base.
22.0
53.7
3.7
31.7
16.0
28.0
Basalt, diktytaxitic, Pelton Basalt Member, Deschutes
Formation.
8
Top of Simtustus Formation
6
silty fine to mediumgrained sandstone
with dispersed pumice lapilli, Celtis endocarps are
common; massive, tan; base not exposed.
Sandstone,
COVERED
5
medium grained with lenses of rounded
Sandstone,
tan to light
trough crossbedded,
pumice lapilli,
gray; erosive base.
1.8
12.0
4
Sandstone, poorly sorted, fine to coarsegrained sand
dispersed
fining upward into sandy siltstone,
gradational at the
massive;
tan,
pumice lapilli,
2.5
10.2
base to:
3
Sandstone,
medium to coarse grained sand with lenses
of pumice lapilli, trough crossbedded (N45W), gray
to tan; erosive base.
3.7
7.7
2
Sandstone, mediumgrained, plane laminated and trough
crossbedded, tan, erosive base.
0.9
4.0
ash,
finegrained lithic sand and silt,
Siltstone,
base not
tan;
massive,
dispersed pumice lapilli,
exposed.
3.1
3.1
1
Base of Section at 1950'.
434
APPENDIX VI: MEASURED SECTIONS OF DESCHUTES FORMATION
APPENDIX
VI-1: Round Butte Dam Measured Section (Type section for the
Deschutes Formation)
Location: Roadcuts on the Round Butte Dam access road
Measured by G. A. Smith and R. A. McKenney, Sept. 12, 1982
and March 24, 1984.
THICKNESS (m)
UNIT TOTAL
DESCRIPTION
UNIT
and
6.3
283.6
coarse grained,
pebbly,
trough cross
bedded, gray, poorly consolidated, sharp base
4.3
277.3
dark gray,
phenocrysts of olivine and
plagioclase, fine grained, crude columnar jointing,
flow breccia; Round Butte member
4.6
273.0
trough
poorly
3.0
268.4
glassy,
phenocrysts of- olivine and
invasive relationship
discontinuous,
3.1
265.4
54
Basalt,
53
Sandstone,
52
Basalt,
51
coarse
Sandstone,
crossbedded,
gray,
defined base.
50
Basalt,
very
plagioclase,
dark gray,
phenocrysts
plagioclase; Round Butte member
of
olivine
grained,
pebbly,
poorly consolidated,
with surrounding sediment; Round Butte member.
49
Sandstone (fills paleochannel incised into under
lying unit),
medium to coarsegrained trough and
finegrained plane
planar crossbedded sandstone,
laminated and
ripple crosslaminated
sandstone;
rounded pumice
pebble to boulder lag at base;
lapilli throughout; light gray, erosive base.
15.4
262.3
48
Sandstone, coarse grained, pebbly, very poorly sorted
and poorly sorted, abundant pumice lapilli increase
upward,
plane bedded throughout (Dry Canyon flood
deposit), light gray; gradational at base to:
23.0
246.9
47
poorly sorted lithic gravel and coarse
Conglomerate,
sand,
subangular to subrounded,
5 mm to 1 cm in
diameter with cobbles up to 6 cm across at the
base;
clast support (Dry Canyon flood
massive,
deposit); erosive base.
13.8
223.9
46
Sandstone,
pebbly, plane bedded and
coarse grained,
massive at top;
crossbedded at base,
mostly gray but grades upward to tan; sharp base.
1.2
210.1
scourfill
435
45
tuffaceous, clasts of welded ignimbrites up
Breccia,
1
m across in a matrix of vitric and lithic
to
ubiquitous perlite and
pumice lapilli,
sand
and
boulders
obsidian fragments to 0.5 cm across,
concentrated near bottom of unit, massive except
for crude horizontal stratification in the
lower
1-.8
217.9
3.7
216.1
0.6 m, white; erosive base.
44
angular to rounded blocks of basalt and
Breccia,
ignimbrite up to 1.2 m across in a matrix of fine
to coarsegrained sand, massive, tan; erosive base.
43
pink,
crystalrich
eroded,
Ignimbrite,
highly
ignimbrite with white pumice lapilli to 2 cm
poorly
contains plant fragments at base;
across,
exposed; erosive base.
0.9
212.4
42
coarsegrained
Sandstone and Conglomerate,
lenses of basaltic pebbles and
sand with
2.5
211.5
pebbly
pumice
lapilli, horizontal lamination, scourfill, planar
relief of
tabular and trough crossbedding (N25E),
scours
and
sharp base.
crossbedding
is
less
than
10
cm,
41
Sandstone,
poorly sorted fine to coarsegrained sand
with dispersed, rounde pumice lapilli most abundant
near base, massive, tan; gradational at base to:
1.4
209.0
40
Lapillistone, rounded white pumice and black scoria up
to 2.5 cm across, poorly sorted; sharp base.
0.9
207.6
39
Sandstone, wellsorted fine to medium grained sand at
base coarseing upward to poorly sorted, pebbly sand
and
scourfill
plane
lamination
top,
at
numerous
lenses of pumice lapilli,
crossbedding,
subhorizontal limb casts up to 5 cm in diameter,
light gray, erosive base.
3.7
206.7
38
Breccia,
dense lithic
scoria,
pumice lapilli,
well
a
in
fragments to 3 cm across supported
massive, tan to
indurated matrix of sand and silt,
gray, sharp base.
3.1
203.1
37
Sandstone, fine to mediumgrained lithic sand, well
1.7
199.7
9.8
198.0
3.6
188.2
sorted, trough crossbedded, gray, base not exposed.
COVERED
36
poorly
coarse grained,
pebbly,
Sandstone,
horizontal bedding, gray, base not exposed.
sorted,
436
COVERED
24.6
184.6
35
Basalt,
diktytaxitic with coarse plagioclase and
olivine (Big Canyon basalt member), irregular base.
9.2
160.0
34
pink,
unwelded with light gray pumice
Ignimbrite,
concentrated near top
Lip to 12 cm in dia.
lapilli
sharp
of unit (Jackson Buttes ignimbrite member),
base.
8.4
150.8
33
Sandstone,
pebbly,
medium to very coarse grained,
poorly sorted, lenses of basalt pebbles, abundant
permineralized stem and root impressions,
massive,
tan, sharp base.
1.8
32
Conglomerate and sandstone, massive pebble to cobble
wellrounded clasts; wedgeshaped
conglomerate,
(N25W)
lenses of planar tabular crossbedded sand
with limb molds to 6 cm in diameter, erosive base.
3.7
31
Sandstone,
siltstone, and conglomerate, interbedded;
lithic, ripple crosslaminated
finegrained, gray,
tan,
tuffaceous siltstone and claystone,
sand;
and
root
impressions
with
vertical
massive
horizontal limb molds; basal 1.5 m is mostly lenses
conglomerate and planar
lithic sand, sharp base.
of
tabular
142.4
140.6
4.6 .136.9
crossbedded
30
Basalt,
coarsegrained,
porphyritic,
spiracles (Seekseequa basalt
jointing,
sharp base.
columnar
member),
13.5
132.3
29
pebbly with cobbles
very coarse grained,
Sandstone,
a
to
10 cm across scattered in the lower 1/3 and
6.8
118.8
basal lag of cobbles to 20 cm across in the basal 1
into
m,
basal
1/3 is massive and grades upward
horizontal stratification, gray, erosive base.
28
very fine sand and silt,
Sandstone and siltstone,
some
plane lamination and ripple crosslamination,
numerous impressions of stems,
beds are massive;
grades
light gray,
tan to
roots,and leaves,
downward and laterally into:
3.4
112.0
27
massive pebble to cobble
Conglomerate and sandstone,
gravel
with lenses of crossbedded sandstone and
thin
large lateral accretion sets; discontinuous,
in
places base is cut down completely
mudstone,
through;
6.5
108.6
26
Sandstone,
sorted,
pebbly,
poorly
very coarse grained,
scattered boulders up to 0.7 m across near
4.9
102.1
437
bottom half is massive,
base,
horizontally bedded, erosive base.
upper
half
is
25
Sandstone,
pebbles and cobbles
very coarse grained,
across near base,horizontally bedded
to
18 cm
except for trough crossbedding in upper 0.5 m,
erosive base.
2.5
97.2
24
rounded,
plane bedded,
Lapillistone and sandstone,
white pumice lapilli capped by discontinuous 5 cm
0.3
94.7
thick layer of orangebrown, finegrained sand with
root impressions, sharp base.
23
Sandstone, coarse grained with pumice lapilli to 2 cm
across, lapillibearing claystone intraclasts up to
2 m across near base, horizontally bedded, massive
near top,
capped by 5 cm of laminated veryfine
grained sandstone and claystone, sharp base.
3.7
94.4
22
Sandstone,
upward
fining
0.6
90.7
fine to medium grained,
massive,
sequences,
in two
softsediment
deformation, irregular, sharp base.
21
horizontal
grained,
pebbly,
Sandstone,
coarse
lowangle and scourfill crossbedding,
bedding,
sharp,
grades upward into massive brown sand,
irregular base.
1.8
90.1
20
Sandstone
conglomerate, massive conlgomerate,
pebbles
crossbeds in sand (N2OW),
to 5 cm across, erosive base.
1.8
88.3
19
Mudstone,
tan,
tuffaceous,
with dispersed pumice
lapilli, minor fine sandstone, massive, sharp base.
1.5
86.5
18
Basalt,
diktytaxitic, coarsegrained plagioclase and
olivine, locally thick breccia, irregular flow top,
sharp base.
3.1
85.0
17
medium to coarse grained,
Sandstone,
massive,
dispersed pumice lapilli,
poorly sorted,
2.2
81.9
and
planartabular
reddishbrown,
baked to red at top, gradational at base to:
16
Sandstone,
medium to coarse grained with pumice
two sequences of horizontally bedded sand
lapilli,
root
with
upward
into massive sand
grading
impressions, gray, sharp base.
2.2
79.7
15
interbedded siltstones,
Mudstone and lapillistone,
claystones and ripple crosslaminated finegrained
with 2 beds of rounded white pumice
sandstones,
lapilli, gradational at the base to:
1.2
77.5
438
14
into pebbly
fines
upward
Conglomerate,
sandy,
cobbles to
sandstone,
sand is trough crossbedded,
tan,
erosive
7.5 cm are wellimbricated (NlOW),
base
1.5
76.3
13
Sandstone,
fine to coarse grained, fines and becomes
upper 0.3 m is slightly fissile,
silty upward,
tan,
abundant
root
massive,
impressions,
1.4
74.8
gradational at the base to:
12
Sandstone,
very coarse grained,
pebbly, massive and
numerous scour surfaces,
horizontally stratified,
uppermost 0.5 m is extensively cemented by opal,
erosive base.
2.2
73.4
11
Sandstone,
medium to coarse grained with pumice
cm
gray
is
lapilli,
poorly sorted,
lowest
15
remainder
is
tan,
massive except for patches of
sharp
remnant stratification in basal
portion,
base.
0.8
71.2
10
Sandstone,
very coarse grained,
pebbly,
poorly
sorted, massive, brown, gradational at base to:
0.3
70.4
9
Lapillistone,
rounded pumice lapilli to 3 cm across,
massive, sharp base.
0.3
70.1
Sandstone,
coarse grained,
abundant black pumice
massive at base grading
lapilli,
poorly sorted,
upward into horizontal bedding with broad scour
in
surfaces,
white pumice lapilli
intraclasts,
upper 0.7 m of horizontally bedded and trough
capped by 25 cm of light
crossbedded sand (N36E),
gray massive sandstone and siltstone with root
impressions, erosive base.
7.1
69.8
7
Sandstone,
very coarse grained,
abundant
pebbly,
black lapilli, .massive at base grading upward
into
horizontal
bedding,
uppermost 30 cm is
oxidized brown and overlain by 0 to 4 cm of white
siltstone with root impressions, erosive base.
5.1
62.7
6
Siltstone and sandstone,
interbedded, pink and white
tuffaceous siltstone,
very finegrained white vitric sandstone,
gray and brown medium grained sandstone,
massive,
intraclasts,
abundant
root
impressions, sharp base.
3.7
57.6
Conglomerate and sandstone,
wellrounded cobbles to
sand,
gray,
12
cm with lenses of crossbedded
erosive base.
4.6
53.9
439
4
in fining upward sequences
Sandstone and mudstone,
0.6 to 1.5 m thick, trough and ripple crossstratiroot
abundant
plane
lamination,
fication,
impressions, hematiteopal concretions in mudstone,
grades downward and laterally into:
4.5
49.3
3
Conglomerate and sandstone, wellrounded cobbles to
10 cm across,
clast support, lenses of crossbedded
medium to coarsegrained sand, erosive base.
3.1
44.8
2
tan silt and finegrained
Siltstone and sandstone,
generally
sand with
dispersed pumice lapilli,
massive,
some horizontal lamination, abundant root
and stem impressions, sharp base.
3.7
41.7
diktytaxitic, coarsegrained plagioclase and
olivine (Pelton basalt member), base not exposed.
38.0
38.0
Basalt,
Base of section at 1580'.
440
APPENDIX VI-2: Measured Section at Lower Bridge
Location:
North side of Lower Bridge Road on west side
Deschutes River (NE1/4 S16, T. 14S., R. 12 E.).
Measured by Gary Smith, June 19, 1982
UNIT
15
THICKNESS (m)
UNIT TOTAL
DESCRIPTION
Diatomite,
of
sharp
3.5
30.2
Ignimbrite,
white at base grading up to orange at
lapilli to 8cm are all white at the
top;
pumice
5.2
26.7
0.1
21.5
0.1
21.4
white,
massive,
top not exposed,
base.
(Top of Deschutes Formation)
14
base; white, black, and banded (white and black) in
the central and upper portion; lapilli are slightly
flattened in upper portion; McKenzie Canyon ignimbrite member.
13
Sandstone,
12
Tuff, fine-grained,
11
Sandstone, fine- to coarse-grained, dark gray, lithic
sand, horizontal bedding, sharp base.
0.3
21.3
10
Sandstone, fine- to coarse-grained with pumice lapilli
to 6mm in lenses and dispersed throughout the
gray at the base to light brown at
unit;
massive,
the top; sharp base.
1.4
21.0
9
Ignimbrite, light gray to pink matrix with light gray
pumice lapilli up to 10cm in diameter;
to white
sharp base;
Lower Bridge ignimbrite member.
10.2
19.6
8
Tuff,
coarse ash and accretionary lapilli (up to 5mm
in diameter) in beds 5-15cm thick; sharp base.
1.0
9.4
Sandstone,
fine- to coarse-grained with dispersed
pumice lapilli
up to lcm across and discontinous
massive,dark gray
8cm thick;
ash,
bed of coarse
light brown at the top;
at the base grading to
gradational at the base to:
0.6
8.4
6
Sandstone, medium-grained, well-sorted volcanic lithic sand with 107 dispersed pumice lapilli up to 2cm
dark gray, massive; sharp base.
across;
0.4
7.8
5
Sandstone,
dispersed
5.0
7.4
medium-grained,
massive, sharp base.
fine- to
white,
dark gray,
massive,
coarse-grained
lithic sand;
sharp
with
base.
441
pumice lapilli up to 1.5cm across and scattered
pebbles of
basalt; massive; light brown, mottled;
gradational at the base to
4
Breccia,
angular to subrounded blocks of black,
dacitic pumice and basalt to 75cm in a matrix of
sand and ash;
matrix support, massive; gray; sharp
base.
3
Sandstone,
medium to coarsegrained,
dark gray, horizontally bedded;
lithic
sand;
0.6
2.4
0.3
1.8
0.9
1.5
0.6
0.6
gradational at the
base to:
2
1
Breccia, angular to rounded dacitic pumice and basalt
matrix
to 25cm across in a matrix of sand and ash;
support, massive; gray; sharp base.
Sandstone,
medium to coarsegrained,
lithic
sand;
poorly sorted, dark gray; horizontally bedded; base
not exposed.
Base of section at 2535'.
442
APPENDIX VI-3: STEELHEAD FALLS MEASURED SECTION
Location: East side of Deschutes River above Steelhead Falls
Measured by G. A. Smith, R. S. Sans, and M. Darrach, June 25 and
August 6, 1982,
UNIT
44
THICKNESS (m)
UNIT TOTAL
DESCRIPTION
'Basalt,
black,
aphyric,
vesicular.
Steamboat Rock
3.7
124.0
member
43
Sandstone,
fine-grained,
lithic-feldspathic
plane laminated, dark gray; sharp base.
sand,
1.9
120.3
42
Sandstone,
medium- to very coarse-grained, pebbly;
pumice lapilli to 3 cm comprise 40% of unit and
increase in abundance upward; massive, light gray;
sharp base.
2.5
118.4
41
Sandstone,
medium- to coarse-grained; in three, normally-graded beds, each 1-2 m thick; massive, wellcemented; light gray; sharp base.
3.7
115.9
40
Sandstone, medium- to coarse-grained, pebbly at base;
massive, normally graded; light gray; sharp base.
2.2
112.2
39
Ignimbrite,
andesitic
with black
light brown to gray matrix
puilice lapilli and bombs to 15cm across;
sharp
lapilli and bombs comprise 60% of the unit;
2.5
110.0
base.
38
Sandstone,
medium- to coarse-grained with dispersed
light
pumice lapilli up to lcm across; massive,
gray at base to light brown at top, gradational at
the base to:
1.4
107.5
37
Sandstone, medium- to very coarse-grained, cobble and
lamination,
pebble lenses,
poorly sorted; plane
planar tabular cross-stratification, and scour-fill
cross-stratification; gray; sharp base.
7.4
106.1
36
Sandstone,
coarse-grained,
boulders to 35 cm across
to 0.5 cm increase in
pumice lapilli
abundance
upward;
massive,
normal graded; light
brown; disconformable base.
gray to light
0.9
98.7
Sandstone,
medium- to coarse-grained with lenses of
basalt cobbles (to 15 cm across) and pumice lapilli;
scour-fill
crossbedding,
plane lamination;
gray; sharp base.
11.2
97.8
at
35
base,
443
34
1.2
86.6
4.0
85.4
2.0
81.4
plane
1.9
79.4
Sandstone,
medium- to coarse-grained, with rounded
basalt pebbles to 5cm across and dispersed pumice
lapilli to 3 cm across; massive, light brown; sharp
base.
33
medium-grained with lenses of pebbles and
Sandstone,
plane lamination and scour-fill
pumice lapilli;
cross-stratification; gray, sharp base.
COVERED
32
Sandstone,
coarse- to very coarse-grained,
bedded, well cemented, gray; sharp base.
31
Sandstone,
fine- to coarse-grained with lenses of
basalt pebbles and pumice lapilli; plane lamination
and scour-fill crossbedding; gray, sharp base.
3.1
77.5
30
subrounded, aphyric, white pumice to 3
Lapillistone,
cm across with §20% subrounded basalt pebbles to
sharp
horizontal stratification;
0.5 cm
across;
1.2
74.4
1.2
73.2
base.
29
Sandstone,
fine- to medium-grained with pebbles to 1
cm across near base; discontinuous 25 cm thick bed
subangular, pumice lapilli near middle of unit
of
by burrows; massive; light gray at base
disrupted
to light brown
at top; gradational at base to:
28
basalt and andesite (?) cobbles up to
Conglomerate,
10 cm across;
normal graded; upper 25 cm is poorly
sorted sand with pumice lapilli; gray, sharp base.
1.5
72.0
27
ignimbrite, light gray with white pumice lapilli to 2
subangular basalt pebbles to 2 cm concm across;
centrated at base; massive; sharp base.
2.2
70.5
26
Sandstone,
medium- to coarse-grained, with dispersed
pumice lapilli to 2 cm across; massive,
light
brown, sharp base.
1.2
68.3
25
Conglomerate, subrounded to rounded basalt and welded
with coarse
ignimbrites cobbles to 25 cm across,
sand
lenses up to 20 cm thick dominated by scourfill crossbedding; sharp base.
2.8
67.1
24
Conglomerate,
poorly-sorted, sandy; pebbles and cob=
normally graded, grades into
bles to 20 cm across;
coarse-grained,
horizontally bedded
sandstone:
sharp base.
3.1
64.3
444
23
Sandstone,
very coarse-grained, pebbly; cobbles to 8
cm across; horizontally bedded; gray, sharp base.
1.2
61.2
22
Ignimbrite,
medium gray to light brown with black,
gray,
and white pumice lapilli and bombs 1-18 cm
4.6
60.0
across; black and gray lapilli and bombs are larger
and more abundant than white lapilli;
disconformable base; Peninsula ignimbrite member.
21
Sandstone,
medium- to coarse-grained with pumice
lapilli to 1 cm across;
horizontal bedding, gray;
sharp base.
0.3
55.4
20
Sandstone,
medium- to coarse-grained with dispersed
light
pumice lapilli to 2 cm across; massive,
brown, gradationalat the base to:
4.6
55.1
19
Sandstone, coarse-grained, pebbly, with pumice lapilli to 1 cm across;
horizontally bedded,
fines upward; gray; sharp base.
0.6
50.5
18
Sandstone,
fine- to coarse-grained with dispersed
pumice lapilli to 3 cm across;
root traces, burrow
1.4
49.9
4.8
48.5
molds; massive; light brown; sharp base.
17
Ignimbrite,
pink with hydrated white lapilli up to 6
cm across;
sharp base;
Steelhead Falls ignimbrite
member
16
angular white pumice lapilli with
Lapillistone,
hypersthene,
and
hornblende phenoplagioclase,
crysts; massive; sharp base.
1.5
43.7
15
Sandstone,
pebbles
medium- to coarse-grained with basalt
5 cm across near the base and pumice
lapilli to 2 cm across in the upper 25 cm; massive,
normally graded; gray; base not exposed.
1.9
42.2
COVERED
5.1
40.3
14
fine sand and
interbedded,
Sandstone and siltstone,
silt in beds 2-20 cm thick; massive, plane-laminatroot traces
ripple crosslamination;
and rare
ed
and abundant impressions of leaf and stem fragments;
light gray; sharp base.
4.3
35.2
13
Sandstone, medium- to coarse-grained with pebbles to
1 cm and dispersed pumice lapilli; massive, mottled,
light brown; gradational at the base to:
2.2
30.9
12
two normalfine- to coarse-grained;
Sandstone,
graded, massive units; well-cemented; sharp base.
2.5
28.7
to
445
11
Sandstone, fine- to coarse-grained with pumice lapilli to 1 cm across;
in four normal-graded, massive
upward to horizontally bedded units each about 1 m
thick and
separated by scours up to 12 cm deep;
light gray; sharp base.
4.6
26.2
10
Sandstone and siltstone,
interbedded,
fine-grained
sand and silt with dispersed, rounded pumice lapilli
up to 5'.;r1 across;
leaf and stem immassive;
pressions; gray, sharp base.
1.2
21.6
9
Conglomerate,
matrix-supported subangular to subrounded pebbles
and cobbles to 10 cm across in a
matrix of and and silt;
about 15% of clasts are
John Day dacite remainder are basaltic andesite and
vesicular vitrophyre; well-cemented; dark brown;
sharp base.
2.2
20.4
8
Sandstone,
medium- to coarse-grained with scattered
basalt pebbles and pumice lapilli to 3 cm across;
massive; light gray; sharp base.
1.7
18.2
7
medium- to coarse-grained with pumice
Sandstone,
lapilli
to
2 cm across;
horizontal beds 3-5 cm
thick; light gray; sharp base.
0.5
16.5
6
Sandstone,
medium- to coarse-grained with pumice
lapilli to 2 cm across decrease upward;
horizontal
beds 0.5-5 cm
thick;
upper 12 cm is massive and
light brown sand; light gray; sharp base.
0.9
16.0
5
Sandstone,
lapilli
medium- to coarse-grained with pumice
2 cm across;
horizontal beds 1-4 cm
thick; gray; sharp
base.
15.1
to
4
Sandstone,
fine- to coarse-grained with dispersed
pumice lapilli to 3 cm across decreasing upward;
massive; light brown; sharp base.
1.2
11.4
3
Conglomerate,
matrix-support,
rounded cobbles and
boulders to 25 cm across in matirx of sand and
silt;
reverse-to- normal graded;
discontinuous
10 cm-thick layer of
horizontal bedded
coarsegrained sand at top; disconformable base.
2.5
10.2
2
Sandstone,
lapilli
medium- to coarse-grained with pumice
1
cm across;
horizontal beds 1-3 cm
thick, fines upward; sharp base.
4.6
7.7
Basalt,
coarse-grained, porphyritic; 20% plagioclase
phenocrysts
up to 8 mm long,
15% olivine pheno-
3.1
3.1
1
to
446
crysts
up to 4 mm across;
top; base not exposed.
black;
Base of Section: 2180 ft. (671m)
vesicular
flow
447
APPENDIX VI-4: Seekseequa Junction Measured Section
Location:
North side of gravel road, west of Seekseequa Junction
(NE1/4 S28, T. 10 S., R. 12 E.).
Measured by G. A. Smith on duly 15, 1982
UNIT
21
DESCRIPTION
THICKNESS (m)
UNIT TOTAL
2.5
187.7
59.2
185.2
1.6
126.0
COVERED
9.5
124.4
poorly-sorted, clast-support, subanguConglomerate,
lar to subrounded small pebbles in a sand matrix,
80% dense volcanic lithics, 20% rounded pumice
max. clast size: 3 cm; horicinder,
lapilli and
zontal bedding, gray,
well-cemented.
1.4
114.9
17.0
113.5
Basalt,
diktytaxitic high-alumina olivine tholeiite.
Top of Deschutes Formation
COVERED
20
Ignimbrite,
light-gray with abundant white pumice
lapilli
to 4 cm across and rare black lapilli to
ubiquitous angular fragments of
2.5 cm across;
black vitrophyre. Coyote Butte ignimbrite member.
19
COVERED
18
Sandstone,
pebbly,
medium- to very coarse-grained;
30%
25% dense lithic pebbles to 3 cm,
50% sand,
pumice
lapilli to 1 cm; normal graded, massive at
bedding at top with disconbase to horizontal
scour-fill crossbedding in upper
tinuous zone of
20 cm; gray, well-cemented, sharp base.
5.5
96.5
17
Sandstone,
medium- to coarse-grained with dispersed
light brown, maspebbly at base;
pumice lapilli,
sive; gradational at base to:
3.1
91.0
16
Conglomerate,
poorly-sorted, clast support, subanguclast
lar to subrounded pebbles and cobbles; max.
size: 16 cm; massive; sharp base.
0.7
87.9
15
Sandstone,
fine- to medium-grained with dispersed
pumice lapilli up to 1.5 cm in diameter; mottled
light brown with permineralized
light gray and
root traces; massive; base not exposed.
8.5
87.2
448
COVERED
13.5
78.7
1.5
65.2
(Note: Seekseequa basalt member occupies this stratigraphic position, 250 m east of the line of section)
14
Conglomerate, poorly sorted, matrix support; subangular to subrounded pebbles and cobbles in a
matrix
of sand and silt;
max.
21
cm;
60% of
clast size:
clasts are gray andesite with microphenocrysts of
hornblende and hypersthene and cognate xenoliths of
diorite;
some clast exhibit radial,
prismatic
joints;
reverse-to-normal
graded, massive; light
brown, sharp base.
13
Sandstone,
fine- to medium-grained with abundant
dispersed pumice lapilli up to 6 mm across; poorlyexposed slope
former; light brown, massive, gradational at the base to:
14.5
63.7
12
Sandstone,
medium- to very coarse-grained,
pebbly,
pumice lapilli to 1 cm across
increase upward;
normal graded,
massive,
grades from gray at the
base to light brown at
well-cemented,
the top;
disconformable base.
1.7
49.2
11
Sandstone,
medium- to coarse-grained,
pebbly,
dispoorly
persed pumice lapilli up to 1 cm across;
exposed slope
former;
massive (?),
light gray;
sharp base.
15.1
47.5
10
Breccia, angular lithic fragments to 8 cm in a matrix
sand and ash; matrix contains abundant
of fine
smaller voids
up to 2 mm across,
some of which
massive,
gray;
are filled by opaline
silica;
sharp base.
0.6
39.4
COVERED
6.8
31.8
9
Sandstone, fine- to coarse-grained with pumice lapilli up to 8 mm in diameter; horizontal beds 1-3 cm
gradational contacts are alternately
thick. with
pumice-rich and
lithic-rich,
light gray at the
toward the top, sharp base.
base to light brown
3.4
25.0
8
Sandstone, medium- to coarse-grained with 30% rounded
pumice lapilli to 8 mm in diameter largely altered
to orange clay,
basal 5 cm is a silcrete; poorlymassive; light gray; sharp
slope former,
exposed
4.8
21.6
2.0
16.8
base.
7
Breccia,
angular to subangular basalt boulders to 40
cm and pumice lapilli supported in a matrix of sand
449
massive, ungraded; well-indurated; light
and silt;
to dark gray; gradational at the base to:
6
4
Breccia, angular to subangular dense lithic fragments
clastto 6 cm across with matrix of coarse sand,
bedding;
light gray;
crude horizontal
support;
disconformable base.
1.8
14.8
Sandstone,
medium to very coarse-grained with 25%
angular, lithic pebbles to 4 across; crudely develgrading with horizontal bedding in
oped normal
to light brown; gradational
upper 1 m; light gray
at base to:
4.3
13.0
Conglomerate, sandy, subrounded to angular pebbles to
sharp
light gray;
well-cemented;
2 cm; massive,
1.7
8.7
Sandstone,
medium- to coarse-grained, poorly-sorted,
hori10% dispersed pumice lapilli to 2 cm across;
zontal beds 1-8 cm thick; light gray; sharp base.
4.0
7.0
well
0.8
3.0
2.2
2.2
base.
3
coarse-grained with ash matrix,
Sandstone,
cemented, massive; light gray; sharp base.
1
medium- to coarse-grained, massive, light
Sandstone,
gray; base not exposed.
Base of section at 1900'.
450
APPENDIX VI-6: Warm Springs Grade Measured Section
Location:
Roadcuts on north side of U.S. 26, west of
Warm Springs
Measured by G. A. Smith, July 12, 1982
UNIT
23
THICKNESS (m)
UNIT TOTAL
DESCRIPTION
coarse-grained,
olivine tholeiite.
6.5
103.0
COVERED
5.0
96.5
22
cemented,
medium-grained,
well-sorted,
Sandstone,
light gray with dispersed pumice
lapilli,
plane
with local
ripple cross-lamination.
laminations
(Note:
laterally
overlying a
adjacent to and
pumice lapilli).
pink ignimbrite with white
6.1
91.5
21
Lapillistone,
subangular pumice lapilli 4 mm to 1 cm
in diameter in horizontal beds 1-15 cm thick, sharp
0.5
85,4
0.6
84.9
in
0.8
84.3
Basalt,
diktytaxitic
high-alumina
Top of Deschutes Formation
base.
20
Sandstone,
medium- to coarse-grained with dispersed
light brown,
pumice lapilli in lower 1/3 of unit,
massive, gradational at the base to:
19
subangular pumice lapilli 1-3
Lapillistone,
diameter, massive, sharp base.
18
Sandstone,
lapilli
pumice
with dispersed
medium-grained
light brown,
in lowest 15 cm,
massive,
gradational at the base to:
0.9
83.5
17
subangular pumice lapilli 6 mm-3 cm in
Lapillistone,
diameter in beds 4-8 cm thick, sharp base.
0.5
82.6
16
Sandstone,
medium- to coarse-grained with rounded
pumice
lapilli to 10 cm across at the base,
pink,
light brown, massive, gradational at the base to:
1.1
82.1
15
angular to subangular pumice lapilli 4
Lapillistone,
mm-3 cm in diameter in horizontal beds 3-15 cm
thick, sharp base.
1.4
81.0
14
Sandstone, medium-grained with dispersed pumice
massive, gradational at the
light brown,
lapilli,
1.1
79.6
base to:
cm
451
13
angular pumice lapilli 3-8 cm
Lapillistone,
locally cemented with opaline
massive,
sharp base.
across,
silica;
0.9
78.5
12
rounded
poorly-sorted,
Sandstone.
coarse-grained,
pumice lapilli up to 3 cm across resemble those in
scour-fill
light gray,
underlying ignimbrite,
crossbedding, sharp base.
0.6
77.6
11
white pumice
pink to gray,
unwelded,
Ignimbrite,
in
10 cm
rims are 4 mm to
lapilli with pink
and
grading
crude
reverse
and
exhibit
diameter
center of the
also occur as lenses near the
across
2 cm
to
fragments
lithic
ignimbrite,
8.5
77.0
Lapillistone, subrounded to rounded pumice lapilli, 4
two, fining-upward,
and ash i
mm - 2 cm diameter,
thick,
cm
plane-bedded
units each ab ut 35
disconformable base.
0.6
68.5
e-grained, dispersed
wer 20 cm of unit
basalt
nce
upward,
the base, grades in
base to light brown
at the base to:
1
3.7
67.9
cm
0.2
64.2
medium- to coarse-g ained with dispersed
cm across decrease
rounded pumice lapilli 5 mm light
in
size
and abundance upwar in the unit,
brown, massive, gradational at the base to:
3.7
64.0
concentrated near the base, pumice are dacitic
composition (68wt% SiO ), sharp base.
in
2
10
9
medium- to very coar
Sandstone,
to 2 cm in 1
lapilli
pumice
size and abund
decrease in
occur nea
pebbles up to 5 cm
the
color from dark gray at
at the top, massive, gradation
8
Lapillistone,
subangular pumic
across, massive, sharp base.
7
Sandstone,
6
Lapillistone, subrounded to angul
diameter with abund
cm
in
an
phenocrysts,
hornblende
a
8 mm
lithic fragments to
massive except in
the unit;
plane laminated and contains
in an ash matrix; sharp base.
ar pumice lapill 1-3
nt plagioclase and
light-colored
ular
ross comprise 10% of
upper 30 cm which is
lapilli
subrounded
1.9
60.3
5
medium- to coarse-g
Sandstone,
pumice lapilli up to 2 cm in
varying abundance throughout
mot
90% of the lowest 40 cm;
b
light gray with ubiquitous
permineralized rootlets; massi
ained with dispersed
diameter occur in
he unit and comprise
led light brown and
rrow molds and rare,
e; sharp base.
14.8
58.4
lapilli up to 2
452
4
Sandstone,
medium- to coarse-grained with dispersed
weathered pumice lapilli 5 mm - 2 cm across which
comprise 65% of the basal 20 cm of the unit; light
5.1
43.6
Sandstone,
1.9
38.5
COVERED
4.3
36.6
Sandstone,
fine- to coarse-grained,
poorly sorted,
lithic-feldspathic wacke, cemented, crude horizontal stratification; base not exposed.
1.5
32.3
COVERED
3.4
30.8
Sandstone, medium- to coarse-grained, with dispersed,
weathered pumice lapilli 5 mm to 5 cm in diameter,
scattered pebbles of basalt, porphyritic andesite,
and
perlite; light gray at base to light brown at
top; base
not exposed.
15.2
27.4
COVERED
12.2
12.2
brown, mottled; massive, gradational at the base to:
medium- to coarse-grained with dispersed
pumice lapilli 3 mm - 3 cm in diameter;
scattered
basaltic cobbles up to 7 cm across; grades in color
from gray at the base
to light brown at the top;
massive; base not exposed.
2
1
Top of John Day Formation
Base of Section at 21801.
453
APPENDIX VI
7: Deschutes Arm Grade Measure Section
Location: Roadcuts on Deschutes Arm grade, west side of Cove
Palisades State Park.
Measured by G. A. Smith J. Givens and A. Church on Sept. 15, 1983
DESCRIPTION
UNIT
THICKNESS (m)
UNIT TOTAL
55
Basalt, diktytaxitic, coarse grained; Canadian Bench
flow of the Lower Desert basalt member.
4.6
177.6
54
Sandstone, medium grained with pumice lapilli;
massive, light brown; includes a discontinuous lens
of airfall pumice lapilli; well preserved burrow
molds and root traces; sharp base.
6.6
173.0
53
Basalt, diktytaxitic, coarse grained; sharp base.
4.0
167.0
52
Sandstone, medium to coarse grained with dispersed
pumice lapilli; includes several discontinuous beds
of airfall pumice lapilli; massive, light brown;
gradational at base to:
3.1
163.0
51
Conglomerate, pebbles in a sandy matrix; poorly
sorted and not well exposed; structure indistinct;
opaline cement; base not exposed.
1.5
159.9
COVERED
3.1
158.4
Sandstone, coarse grained, plane bedding, scourfill
crossbedding, lowangle crossbedding; gray with
dispersed white pumice lapilli and occassional
blocks of white tuff as much as 1 m across; sharp
3.7
155.3
3.1
151.6
COVERED
4.6
148.5
Sandstone, coarse grained, pebbly; pebbles are
subangular and as much as 5 cm across; gray,
weathered tan; plane bedded; base not exposed.
2.2
143.9
10.8
141.7
1.7
130.9
50
base.
49
Conglomerate, pebbles up to 3 cm in a coarsegrained
sand matrix; massive and plane bedded; base not
exposed.
48
COVERED
47
Conglomerate, clastsupported, wellrounded cobbles
as much as 10 cm in diameter; poorly exposed; sharp
base.
454
46
Sandstone, medium to coarse grained with about 40%
dispersed pumice lapilli; massive, light brown;
base not exposed.
0.9
129.2
45
Ignimbrite, white to light gray with lightgray,
plagioclaserich pumice lapilli 1-5 cm across;
unwelded; unit is largely concealed by colluvium in
roadcuts but is well exposed above the road; sharp
2:9
128.3
base.
44
Sandstone, medium to coarse grained; massive, light
brown; permineralized root traces; gradational at
the base to:
0.8
125.4
43
Sandstone, coarse grained, pebbly, pumiceous; gray to
light brown; in two units each composed of 25 cm of
plane bedded, coarsegrained sandstone grading up
into 60 to 70 cm of massive, matrixsupport pebbly
sandstone; sharp base.
1.8
124.6
42
Siltstone, massive to faintly laminated, stem and
root(?) impressions; light tan; sharp base.
0.1
122.8
41
Sandstone, medium grained, well sorted, plane
laminated; scourfill crossbedding at base.
0.5
122.7
40
Lapillistone, reworked, gray, accretionary lapilli,
0.5 to 1.0 cm in diameter; sharp base.
0.5
122.2
39
Sandstone, medium to coarse grained; gray, weathered
brown; plane bedded; sharp base.
0.7
121.7
38
Sandstone, medium to coarse grained with dispersed
pumice lapilli increasing in abundance downward;
massive, light brown; gradational at base to:
0.2
121.0
37
Lapillistone, subrounded white, hydrated pumice
lapilli; sharp base.
0.1
120.8
36
Sandstone, medium to coarse grained with dispersed
pumice lapilli; massive, light brown; sharp base.
0.8
120.7
35
Conglomerate, tightly packed, clast supported, poorly
sorted; subangular-to rounded cobbles 10 to 12 cm
across with abundant very coarsegrained sand;
boulders up to 75 cm in diameter are concentrated
in a train near the middle of the unit; basal 1.5 m
is crudely plane bedded, remainder is massive;
erosive base.
4.7
119.9
34
Conglomerate, pebbles from 5 mm to 1.5 cm comprise
most of the unit with interstitial fine to coarse-
1.5
115.2
455
grained sand and scatterd cobbles and boulders up
to 25 cm across; central third of unit is massive,
base and top display crude plane bedding; erosive
base.
33
Conglomerate, cobbles to 15 cm, rounded to subangular; clast-supported with interstitial coarsegrained sand; crude horizontal and low-angle
bedding; sharp base.
1.2
113.7
32
Conglomerate, angular to subrounded pebbles, 6 mm to
1.5 cm across with about 10% fine- to coarsegrained sand and 20% cobbles and boulders as much
as 75 cm in diameter; boulders are mostly from
McKenzie Canyon ignimbrite member; lowest 1 m is
plane bedded, central 1 m is massive, and upper 1 m
is plane bedded and scour-fill crossbedded; locally
capped by 0.3 m of medium- to coarse-grained
sandstone interbedded with pumice-bearing
siltstone; erosive base.
3.1
112.5
31
Conglomerate, angular to subrounded cobbles and
boulders up to 1.5 m across in a matrix of pebbles,
sand, and silt; lower 3 m is massive and largely
matrix support with largest clasts 2 to 3 m above
base; upper 1.5 m is plane bedded; includes clasts
of McKenzie Canyon ignimbrite member and black
vitrophyre; erosive base.
4.7
109.4
30
Conglomerate, poorly sorted, massive, clast support;
subangular to subrounded pebbles 5 to 8 cm across,
coarse-grained sand, and occassional boulders up to
75 cm across; erosive base.
4.0
104.7
29
Ignimbrite, unwelded, white with light gray and white
plagioclase-rich pumice lapilli up to 8 cm across;
0.8 m-thick plane bedded layer at base includes
accretionary lapilli in an ash matrix; rounded
cobbles up to 18 cm across are concentrated just
above plane-bedded zone; sharp base Cove ignimbrite
3.2
100.7
17.4
97.5
4.9
80.1
member.
28
Conglomerate, well-rounded cobbles up to 20 cm in
diameter; includes lenses of trough- and tabularcrossbedded coarse-grained sandstone; locally
capped by massive, brown, medium- to coarse-grained
sandstone containing root impressions; erosive
base.
27
Sandstone and mudstone, interbedded; lenses of pebble
conglomerate; fine- to medium-grained sandstone,
massive, ripple crosslaminated, in beds about 20 cm
456
thick; gray to tan laminated mudstone in beds 1 to
5 cm thick; lenses of pebbly, trough crossbedded
sandstone in upper half; uppermost 1 m is brown,
massive, mediumgrained sandstone with root
impressions; sharp base.
26
Sandstone, medium to very coarse grained with
dispersed, rounded pumice lapilli; plane bedding
and lowangle crossbedding; sharp base.
2.4
75.2
25
Conglomerate, angular to subrounded small pebbles to
scattered boulders up to 1 m across; clast support,
poorly sorted, generally massive with faint horizontal bedding in upper 20 cm; channelform with
erosive base.
0.9
72.8
24
Conglomerate, subangular to subrounded pebbles, cob
bles, and rounded boulders up to 0.75 cm across
supported in a matrix of sand and silt; massive,
inversetonormal grading; sharp base.
1.5
71.9
23
Sandstone, coarse grained, pebbly; trough crossbed
ding, discontinuous; sharp base.
0.6
70.4
22
Conglomerate, subangular to subrounded pebbles and
cobbles up to 15 cm across supported in a poorly
sorted sand and silt matrix; upper 0.8 m is
massive; lower 1 m is better sorted, finer grained
and plane bedded.
1.8
69.8
21
Sandstone, coarse grained, pebbly; plane bedding,
0.8
68.0
1.2
67.2
3.1
66.0
Conglomerate, poorly sorted, clast support; cobbles
and boulders up to 1 m across concentrated in lower
half of unit; massive at base with crude horizontal
bedding in upper half; abundant leaf, stem, and
branch remains at base; sharp base.
2.5
62.9
COVERED
1.7
60.4
scourfill crossbedding, lowangle crossbedding;
lag of rounded cobbles up to 25 cm in diameter at
base; sharp base.
20
Sandstone, very coarse grained, pebbly, rare cobbles
up to 20 cm in diameter; horizontally bedded; sharp.
base.
19
Conglomerate, pebbles, cobbles, and occassional boul
ders up to 0.75 m across, with lenses and matrix of
coarse and very coarsegrained sandstone; erosive
base.
18
457
17
Sandstone, well sorted, medium- to coarse-grained
sand, plane bedded, with two 1 m-thick beds of
massive coarse-grained sand and small pebbles;
locally contains pumice lapilli-bearing siltstone
near top; grades downward into:
6.2
58.7
16
Conglomerate, rounded and well-rounded cobbles and
boulders to 25 cm, clast support, massive to
crudely plane bedded; with lenses of tabular and
low-angle crossbedded coarse-grained sandstone;
some sandstones contain angular ripup clast of
finely laminated punk claystone; includes thin
(1 m) erosional remnant of an unwelded light gray
ignimbrite - Jackson Buttes ignimbrite member;
erosive base.
5.7
52.5
15
Sandstone and siltstone, interbedded; ripple
laminated siltstone with opalized leaf and stem
impressions with lenses of fine- to medium-grained
sandstone; abundant rounded, hydrated pumice
lapilli upper 80 cm is a clay- and silt-rich
paleosol with a dense network of fine root traces;
3.4
50.8
sharp base.
14
Sandstone, coarse- to very coarse-grained, plane
bedding, scour-fill crossbedding, and low-angle
crossbedding; gradational at the base to:
4.6
47.4
13
Sandstone, very coarse grained, pebbly, plane bedded;
sharp base.
2.6
42.8
12
Sandstone, medium to coarse grained, pebbly, with
cobbles up to 15 cm in diameter and scattered
pumice lapilli; lower half is coarse grained, more
poorly sorted, and plane bedded; upper half is
trough crossbedded.
6.2
40.2
11
Basalt, three thin flow units of vesicular, olivine
basalt; prominent pipe vesicles; includes large
blocks of sediment up to 3 m long; sharp base.
3.1
34.0
10
Sandstone, poorly sorted, fine to coarse grained with
scattered, rounded pumice lapilli; massive, gray to
tan with numerous root traces; occurs in beds 1 to
2 m thick with intervening siltstone beds up to
5 cm thick; gradational at the base to:
5.4
30.7
9
Sandstone, poorly sorted medium to very coarse
grained with pebble lenses; sharp base.
0.6
25.3
8
Sandstone, medium to coarse grained, pebbly, coarsens
upward; plane bedded, low-angle and scour-fill
0.8
24.7
458
crossbedded; capped by 3 cm of gray siltstone with
leaf and stem impressions; sharp base.
7
Conglomerate, sandy, poorly sorted, pebbles and
boulders up to 1 m across; massive to crude
horizontal bedding grading upward into plane bedded
pebbly sandstone; capped by 1 cm of tuffaceous
mudstone; erosive base.
8.7
23.9
6
Sandstone and mudstone, interbedded; massive and
plane bedded, poorly sorted, medium to very
coarsegrained sandstone in beds 0.3 to 0.8 m
thick; white, tuffaceous sandy siltstone and
mudstone in beds 1 cm to 50 cm thick; abundant root
and burrow traces; sharp base.
5.2
15.2
5
Sandstone with beds and lenses of sandy conglomerate;
greengray; rounded to subrounded pebbles and
cobbles are generally 1 to 5 cm across with
occassional boulders up to 80 cm; sandstone is
plane bedded and trought crossbedded; erosive base.
1.3
10.0
4
Sandstone, pebbly, medium to very coarse grained with
scattered cobbles and boulders up to 1 m in
diameter; plane bedded, lowangle and scourfill
crossbedded; sharp base.
2.8
8.7
3
Conglomerate, rounded cobbles of basaltic andesite
and andesite in a matrix of pink and gray ash
(Note: this unit is up to 6 m thick on the east
side of the CovePalisades State Park and fines
upward into a tuff); sharp base.
0.8
5.9
2
Sandstone, medium to very coarse grained, poorly
sorted; plane bedding and tabular crossbedding; in
beds averaging 0.75 m thick, each capped by several
centimeters of siltstone; sharp base.
1.8
5.1
Sandstone, medium to coarse graine; generally massive
with a thick (as much as 2.5 m) lens of pumiceous
sandstone exhibiting softsediment deformation;
capped by by 12 cm of rippled, pink to gray
siltstone with stem and leaf impressions; base not
3.3
3.3
1
exposed.
Base of section at 2000 feet.
459
APPENDIX VII: MEASURED SECTION OF "CAMP SHERMAN BEDS"
Roadcuts on west base of Green
Forest Road 1490
Measured by G. A. Smith, June 17, 1984.
Location:
Ridge,
THICKNESS (m)
UNIT TOTAL
DESCRIPTION
UNIT
18
Tuff,
plane
0.6
19.5
17
sand
to 2.5 cm in a coarse
Conglomerate,
pebbles
discontinuous
lense
of
poorly imbricated,
matrix,
white ash at base; gradational at base to:
0.6
18.9
16
Sandstone,
and
2.2
18.3
15
Tuff,
wellindurated,
0.8
16.1
medium to
coarsegrained
mafic ash,
bedded, black to yellowish brown, sharp base.
trough
medium to coarse grained,
scourfill crossbedded, dark gray; sharp base.
coarsegrained
massive to
mafic ash,
faintly plane bedded,
dark gray; sharp
base.
14
Diatomite, massive, white; sharp base.
1.5
15.3
13
Sandstone, coarsegrained, pebbly, massive to faintly
bedded, tan; sharp base.
0.8
13.8
12
Tuff,
fine to coarse mafic sideromelane ash with
is
1/3
lower
abundant plagioclase phenocrysts,
rhythmically bedded in beds 0.5 to 1.5 cm thick
ripple marks, and flame
exhibiting normal grading,
largely massive with
structures,
upper 2/3 is
remnant convolute bedding, dark gray; sharp base.
2.2
13.0
11
Diatomite, ashy, white, massive; sharp base.
0.6
10.8
10
Tuff,
massive,
dark
sharp
cement;
0.6
10.2
includes dispersed white pumice lapilli
brown;
massive,
which are highly altered to clay,
sharp base.
gray
coarse mafic ash and lapilli,
with orangered ferruginous
base.
9
Claystone,
0.6
9.6
8
coarse mafic ash and angular
Tuff and Lapillistone,
lapilli in beds 1.5 to 8 cm thick separated by gray
beds 1 to 2 cm thick and one contorted,
ash in
discontinuous lapillistone bed up to 3 cm thick;
and pillow structures at sharp basal contact
ball
1.8
9.0
with:
460
0.6
7.2
Tuff,
0.1
6.6
5
Tuff,
coarse,
light gray ash with dispersed pumice
burrow traces;
lapilli,
faintly crosslaminated,
sharp base.
0.1
6.5
4
Siltstone,
with
laminated
massive
to faintly
dispersed pumice lapilli and basalt pebbles to 3 cm
and thin beds of rounded lapilli up to 2 cm thick,
white to light brown; gradational at base to:
3.7
6.4
3
hydrated
rounded,
sparsely phyric,
Lapillistone,
of
lapilli
up
to
1.5
cm
across
in
a
matrix
pumice
gradational
at
the
fine feldspathic sand and silt;
base to:
0.3
2.7
2
silty with pumice lapilli
Claystone,
beds and dispersed throughout the
bedded, light brown; sharp base.
indiscontinous
faintly
unit,
1.8
2.4
0.6
0.6
7
rounded white pumice lapilli in beds 2
Lapillistone,
light
to 5 cm thick interbedded with thin beds.of
gray and white ash; sharp base.
6
1
coarse mafic ash with ferruginous opal cement,
dark gray to orange, massive; sharp base.
Lapillistone, white hornblendebearing pumice lapilli
1
to 4 cm across in a brown clay matrix.
461
APPENDIX VIII: DESCHUTES BASIN DIATOM FLORAS
Identification and interpretation of floras by J. Platt Bradbury,
S. Geological Survey, Denver.
U.
SAMPLE # 17 II 83-5
LITHOLOGY: Diatomite
SAMPLE LOCALITY: Gateway 7.5'; 2080', 9S-14E-19Dda
STRATIGRAPHIC POSITION: Deschutes Formation; 7m above contact
with Simtustus Formation; laterally correlative to section
below Pelton basalt member.
DIATOM ASSEMBLAGE:
dominant
Fragilaria virescens var. producta
F. construens var. venter
F. leptostrauron
Gomphonema tropicale
Achnanthes marginulata (?)
A. lanceolata
A. exigua
Rhopalodia gibba
Epithemia sorex
Opephora sp.
Navicula pupula
Cocconeis placentula
C. disculus
ENVIRONMENT: shallow water, low salinity
AGE: Miocene (?)
SAMPLE # 17 II 83-3
LITHOLOGY: Diatomite
SAMPLE LOCALITY: Round Butte Dam 7.5', 2000', 11S-12E-34Dbb
STRATIGRAPHIC POSITION: Deschutes Formation; 5m above Chinook
ignimbrite member.
DIATOM ASSEMBALGE:
Melosira sp.
Navicula semen
Tetracyclus lacustris
ENVIRONMENT: shallow water, acidic
AGE: Not age diagnostic
SAMPLE # 17 II 83-10a
LITHOLOGY: Diatomite
SAMPLE LOCALITY: Seekseequa Junction 7.5', 2100', 10S-12E-11Cbd
STRATIGRAPHIC POSITION: Deschutes Formation; approximately
equivalent to Seekseequa basalt member.
DIATOM ASSEMBLAGE:
Melosira italica
Fragilaria virescens
F. brevistriata
Meridian circulare
Eunotia pectinalis
E. curvata
462
Cymbella ehrenbergii
C. minuta
Pinnularia sp.
Navicula semen
N. fragilarioides
N. radiosa
Nitzchia
Gomphonema angustatum
Synedra rumpens
Caloneis bacillum
Achnanthes lanceolata
A. exigua
ENVIRONMENT: shallow water, low salinity, slightly acidic
AGE: Not age diagnostic
SAMPLE #17 II 83-10b
LITHOLOGY: Diatomite
SAMPLE LOCALITY: Seekseequa Junction 7.5', 2100', 11S-12E-11Bcd
STRATIGRAPHIC POSITION: Deschutes Formation; 10m above Seekseequa
basalt member.
DIATOM ASSEMBLAGE:
Melosira italica
Fragilaria virescens
Pinnularia viridis
Navicula semen
N. fragilarioides
N. amphibola
Stauroneis sp.
Hantzschia amphioxys
ENVIRONMENT: shallow water, low salinity
AGE: not age diagnostic
SAMPLE # 17 II 83-7
LITHOLOGY: Diatomite
SAMPLE LOCALITY: Madras West 7.5', 2140', 10S-13E-33Ddd
STRATIGRAPHIC POSITION: Deschutes Formation; between Pelton
basalt member and Agency Plains basalt flow of Tetherow
Butte member; precise position uncertain.
DIATOM ASSEMBLAGE:
Anomoeoneis costata
ENVIRONMENT: shallow water, alkaline, slightly saline
AGE: Not age diagnositc.
SAMPLE # 17 II 83-10d
LITHOLOGY: Mudstone
SAMPLE LOCALITY: Steelhead Falls 7.5', 2340', 13S-12E-8Ddc
STRATIGRAPHIC POSITION: Deschutes Formation, between McKenzie
Canyon ignimbrite member and Steelhead Falls ignimbrite member.
DIATOM ASSEMBLAGE:
Cymbella minuta
Nitzschia romana
N. inconspicua
463
SAMPLE # 17 II 83-10c
LITHOLOGY: Mudstone
SAMPLE LOCALITY: Steelhead Falls 7.5', 2530', 13S-12E-27Ccc
Deschutes Formation, 30m above Peninsula
STRATIGRAPHIC POSITION:
ignimbrite member.
DIATOM ASSEMBLAGE:
Pinnularia borealis (terrestrial diatom)
Hantzchia amphioxys (terrestrial diatom)
SAMPLE # 17 II 83-6
LITHOLOGY: Diatomite
SAMPLE LOCALITY: Culver 7.5', 2400', 11S-13E-17Acd
same elevation as
Deschutes Formation,
STRATIGRAPHIC POSITION:
Tetherow Butte member; 30m below
Agency Plains basalt flow,
basalt of Round Butte.
DIATOM ASSEMBLAGE:
Fragilaria virescens var. producta (dominant)
F. construens var. trigona
F. construens var. venter
F. pinnata
F. breviastriata
Melosira italica
Cymbella hauckii
C. muelleri
C. mexicana
C. cistula
Cocconeis placentula
Gompnonema turns
Rhopalodia gibba
Epithemia sorex
E. turgida
ENVIRONMENT: shallow water, possibly alkaline
AGE: Not age diagnostic
SAMPLE # 17 II 83-8
LITHOLOGY: Diatomite
SAMPLE LOCALITY: Whitewater River 15', 3200', 12S-9E-1Bcd
(Pliocene)
"Camp Sherman beds"
POSITION:
STRATIGRAPHIC
DIATOM ASSEMBLAGE:
Cyclotella elgeri (codominant)
Stephanodiscus carconensis (codominant)
S. astraea var. minutula
Cyclotella pygmaea (hannaites)
Melosira granulata var. angustissima
Opephora martyi
Pinnularia ruttneri
AGE: Pliocene
464
SAMPLE # 6 VIII 84-1
LITHOLOGY: Diatomite
SAMPLE LOCALITY: Whitewater River 15', 2780', 12S-9E-14Cbc
(Pliocene
"Camp
Sherman beds"
STRATIGRAPHIC
POSITION:
Pleistocene ?)
DIATOM ASSEMBLAGE:
Fragilaria construens var. venter
Stephandiscus astraea var. intermedia
S. astraea var. minutula
S. subtransylvanicus
S. transylvanicus
S. carconensis (?)
S. asteroides
Melosira solida (paucistriata)
Navicula aurora
Cocconeis placentula
Gomphoneis sp.
Neidium sp.
Pinnularia sp.
AGE: Pliocene (?)
SAMPLE # 17 II 83-4
LITHOLOGY: Diatomite
SAMPLE LOCALITY: Cline Falls 7.5', 2750', 14S-12E-16Acc
postDeschutes
Lower Bridge diatomite,
STRATIGRAPHIC POSITION:
Formation
DIATOM ASSEMBLAGE: (common forms only)
Cymbella mexicana
C. cistula
C. muelleri
C. minuta
Cocconeis placentula
Navicula radiosa
N. pupula var. rectangularis
Epithemia turgida
Rhocosphenia curvata
Rhopalodia gibba var. ventricosa
Nitzschia romana
Melosira italica
M. distans
Stephandiscus excentricus
S. hantzschii
Fragilaria pinnata
F. construens var. venter
F. construens var. subsalina
F.
brevistriata
Amphora ovalis
Asterionella formosa
Navicula cuspidata
ENVIRONMENT: shallow, nutrientrich lake
AGE: Pleistocene
465
SAMPLE # 17 II 83-9
LITHOLOGY: Diatomite
SAMPLE LOCALITY: Cline Falls 7.5', 2750', 14S-12E-24Bdc
Uncertain, appears to overly Pleistocene
STRATIGRAPHIC POSITION:
but may be underlying material related to 17 II 83-4
basalt
that was incorporated into the basalt flow.
DIATOM ASSEMBLAGE:
Nitzschia romana (codominant)
Fragilaria construens var. venter (codominant)
Rhoicosphenia curvata
Cymbella muelleri
Navicula huefleri
Amphora ovalis
Epithemia turgida
Cymbella cistula
C. mexicana
Rhopalodia gibba
Navicula ludloviana
Gomphoneis herculeana
Cocconeis placentula
C. disculus
ENVIRONMENT: shallow, warm, eutrophic environment of low salinity
and moderate alkalinity.
AGE: Pleistocene
466
40
APPENDIX IX: PRELIMINARY
39
Ar/
Ar AGE DATES, DESCHUTES BASIN
These data were obtained by Dr. L. W. Snee, Oregon State University, and are subject to slight revision.
Dates represent totalfusion
(t) andagespectrum (a) analyses. All analyses were performed on
whoierock samples.
Dia: Basaltic andesite, Steamboat Rock member, Deschutes Formation
(2720', 14S/12E/14Bcd, Cline Falls 7.5').
5.1 + 0.2 Ma (a)
4.9 + 0.1 Ma
(0
D3a: Diktytaxitic olivine basalt, unconformably overlies Deschutes
Formation [Redmond flow of Robinson and Stensland, 1979] near
Terrebonne (2760', 14S/13E/6Abc, Opal City 7.5').
3.4 + 0.5 Ma (a)
Diktytaxitic olivine basalt; reversepolarity Newberry(?) intracanyon flow, Crooked River Canyon (2300', 13S/12E/24Bad, Opal City
7.5').
1.2 + 0.1 Ma (a)
1.3 + 0.1 Ma
(0
Diktytaxitic olivine basalt, Opal Springs basalt member, Deschutes
Formation (2200', 13S/12E/24Baa, Opal City 7.5').
6.3 + 0.1 Ma (t)
Agency Plains basalt flow, Tetherow Butte member, Deschutes
Formation (2460', 12S/12E/11Daa, Round Butte Dam 7.5').
5.5 + 0.2 Ma (a)
-
Diktytaxitic olivine basalt below Lower Desert basalt member,
Deschutes Formation, on Deschutes Arm grade, CovePalisades State
Park (2580', 12S/12E/21Bcc, Round Butte Dam 7.5').
5.6 + 0.1 Ma (t)
Canadian Bench flow, Lower Desert basalt member, Deschutes
Formation (2610', 12S/12E/10Add, Round Butte Dam 7.5').
5.4 + 0.1 Ma
(0
Olivine basalt, Round Butte member, Deschutes Formation (2570',
11S/13E/17Aba, Culver 7.5').
4.0 + 0.1 Ma (a)
Diktytaxitic olivine basalt, unconformably overlies Deschutes
Formation (2440', 8S/11E/21Aad, Potters Ponds 7.5').
3.7 + 0.1 Ma (a)
3.8 T 0.1 Ma (t)
Lowest flow, Prineville chemicaltype basalt, Pelton Dam (1590',
10S/13E/18Cbb, Madras West 7.5').
15.7 + 0.1 Ma
(0
467
013: Diktytaxitic olivine basalt, Deschutes Formation, Willoe Creek
east of Madras (2395', 11S/14E/18Ccb, Buck Butte 7.5').
6.4 + 0.1 Ma (t)
RC89: Basaltic andesite near top of Deschutes Formation section on the
crest of Green Ridge (sample collected by R. M. Conrey).
5.3 + 0.1 Ma (a)
5.3 T 0.1 Ma (t)
RC808: Basaltic andesite near base of Deschutes Formation, Green Ridge
(sample collected by R. M. Conrey).
7.3 + 0.1 Ma (a)
7.1 T 0.1 Ma (t)
LH1: Dacite, Lionshead, High Cascades (sample collected by Gene
Yogodzinski).
2.4 + 0.1 Ma (a)
DKT: Diktytaxitic olivine basalt, unconformably overlies Deschutes
Formation in lower Whitewater River canyon (sample collected by
Gene Yogodzinski).
4.3± 0.1 Ma
(0