AN ABSTRACT OF THE THESIS OF
ROBERT HOLMES NEEL for the degree MASTER OF SCIENCE
in
GEOLOGY
presented on
July 10, 1975
Title: GEOLOGY OF THE TILLAMOOK HEAD - NECANICUM
JUNCTION AREA, CLATSOP COUNTY, NORTHWESTERN
OREGON
Abstract approved:
Alan R. Niem
Five distinct lithologic units compose the Tertiary rocks in the
Tillamook Head - Necanicum Junction area of the northern Oregon
Coast Range. They are: the late Eocene to early Miocene Oswald
West mudstones, the middle Miocene Angora Peak sandstone and
Silver Point mudstone members of the Astoria Formation, and the
middle Miocene intrusive and extrusive Depoe Bay and Cape
Foulweather basalts. These units are locally overlain by Pleistocene
marine terraces and Holocene beach and dune sands, stream alluvium,
and landslide deposits. The Oswald West mudstones, Angora Peak
sandstones, and Silver Point mudstones are informal stratigraphic
names used in this study.
The Oswald West mudstones consist of over 2, 000 feet of well-
bedded, intensely burrowed, silty mudstones and siltstones interstratified with minor glauconitic sandstones, tuff beds, and thick
bedded tuffaceous siltstones Foraminifera, trace fossils, glauconite,
and the general fine-grained character of these rocks suggest that
deposition occurred in an open marine deep-water environment,
possibly as part of a prodelta or on the outer continental shelf.
The Angora Peak member is composed of several hundred feet
of moderately sorted, medium- to coarse-grained quartzose-feldspathic and lithic sandstones. The sandstones unconformably overlie
the Oswald West mudstones and are thick bedded, laminated, and.less
commonly cross-laminated. Sedimentary structures, sorting, and
stratigraphic relationships suggest that the sandstones were deposited
in a high energy, wave-dominated environment, possibly as delta
sheet sands.
The sands were, in part, redistributed by waves and
longshore drift to form offshore coastal barrier bars and.linear
clastic shoreline deposits.
The overlying 650-foot thick Silver Point member consists
dominantly of dark gray, micaceous, laminated mudstones and very
thin siltstones. Rhythmically interbedded mudstones and fine-grained
turbidite sandstones occur in the lower part of the unit. Deposition
occurred in cool, low energy, open marine conditions, probably of
sublittoral to upper bathyal depths. Oversteepening of Angora Peak
sheet sands on the delta front may have resulted in periodic slumping
of sands which were transported by turbidity currents into a shelf
basin of the deeper water Silver Point delta-slope environment. An
east to west paleocurrent pattern in the Salver Point turbidite
sandstonesis consistent with this model.
Sandstone petrography and heavy mineral suites of the Astoria
Formation suggests a dominantly volcanic and sedimentary prove-
nance for these strata, probably the pre-Miocene volcanic rocks of
the western Cascades and local Coast Range Eocene basalts and sandstones.
Mineralogy and rare rock fragments in the sandstones also
suggest that some detritus was derived from the granitic, metamorphic, and Paleozoic sedimentary terrains of eastern Oregon and
Washington, and from British Columbia and western Idaho, possibly
transported via an "ancestral Columbia River drainage system.
Numerous dikes, sills, and irregular-shaped plutons of Depoe
Bay Basalt and Cape Foulweather Basalt intruded the Oswald West
mudstones and Astoria Formation. The major intrusive body in the
area is the 900-foot thick sill that forms Tillamook Head. At Ecola
State Park, the forceful intrusion of this sill into the semi-consolidated Silver Point strata at very shallow depths produced local synsedimentary folds.
Over 1, 000 feet of extrusive Depoe Bay Basalt palagonitized
pillow lavas and breccias and 2.00 feet of very local Cape Foulweather
pillow lavas lie with angular unconformity over the Silver Point
member.
Deformation which accompanied the general Coast Range uplift
during the late Miocene to Pleistocene formed four northeast-trending
folds in the thesis area and two sets of high angle faults which strike
north-south and northwest- southeast through the area..
In addition,
active landslides are abundant throughout the area, particularly along
the coast.
Economic resources of the area include basalt quarry rock for
rip-rap and road base, basalt stream gravels for road aggregate,
and potential petroleum resources may occur in the adjacent offshore
area.
Geology of the Tillamook Head - Necanicum Junction Area,
Clatsop County,. Northwest Oregon
by
Robert Holmes Neel
A THESIS
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Master of Science
June 1976
APPROVED:
Assistant Professor of Geology
in charge of major
Herad of Department of Geology
Dean of G
duate School:
Date thesis is presented
July 10, 1975
Typed by Susie Kozlik for Robert Holmes Neel
ACKNOWLEDGEMENTS
My deepest appreciation goes to Dr.. Alan R. Niem who pro-
vided enthusiastic support, advice, and complete cooperation during
every aspect of this study. My gratitude is likewise extended to Dr.
Cyrus W. Field and Dr. J. G.. Johnson for their counsel, critical
reading of the manuscript, and patience.
In addition, I wish to acknowledge the following persons for
their prompt assistance and vital contributions to this thesis. Dr.
W. W. Rau of the Washington Department of Natural Resources,
Geology and Earth Resources Division and Dr. W. O. Addicott of the
United States Geological Survey identified Foraminifera and Mollusea,
respectively, and interpreted their depositional environments. Trace
fossils and their depositional environments were -identified and inter-
preted by Dr. K. C. Chamberlain of Ohio University. Dr. R. O.
Dennis, Oregon State University, examined plant fossils and postulated a paleoclimate.
.A very special thank you goes to my wife, Janet, for her
understanding., patience, and help in preparing the manuscript, and
to my parents for their financial assistance.
TABLE OF CONTENTS
Page
INTRODUCTION
Purposes of Investigation
Location and Accessibility
Principal Geographic Features
Climate
Methods of Investigation
Field Procedures
Laboratory Procedures
Previous Work
1
1
3
5
5
5
7
9
REGIONAL GEOLOGY
12
DESCRIPTIVE GEOLOGY OF THE THESIS AREA
20
Oswald West Mudstones
Nomenclature
Distribution
Lithologies and Structures
Petrology
Contact Relations
Age and Correlation
Depositional Environment
Astoria Formation
Angora Peak Sandstone Member
Nomenclature
Distribution
Lithology and Structures
Petrology
Contact Relations
Age and Correlation
Silver Point Mudstone Member
Nomenclature
Distribution
Lithology and Structures
Petrology
Contact Relations
.Age and
Correlation
Depositional Environments of the Angora Peak and
Silver Point Members of the Astoria Formation
Depoe Bay Basalt
20
20
21
21
30
33
36
38
41
42
42
43
43
45
52
55
57
57
58
58
67
72
73
75
80
Page
Nomenclature
Intrusive Rocks
80
Relations
Lithology and Petrology
Extrusive Rocks
81
81
Distribution, General Features, and Contact
Distribution and General Features
Lithology and Petrology
Contact Relations
Chemistry
Age and Correlation
Cape Foulweather Basalt
Nomenclature
Intrusive Rocks
Distribution, General Features, and Contact
Relations
Lithology and Petrology
Extrusive Rocks
Distribution and General Features
Lithology and Petrology
Contact Relations
Chemistry
Age and Correlation
Quaternary Deposits
Beach Deposits
Marine Terraces
River Alluvium
Landslide Deposits
86
90
90
91
93
95
97
98
98
99
99
101
105
105
105
107
108
110
110
110
ill
112
113
SIZE ANALYSES
117
STRUCTURAL GEOLOGY
Folds
Synsedimentary Folds
Faults
121
121
GEOLOGIC HISTORY
130
123
127
Transport Directions
130
Provenance
Summary and Conclusions
131
.138
HYPOTHESES FOR THE ORIGIN OF THE OREGON COAST
RANGE UPLIFT
145
Page
MINERAL RESOURCES
Crushed Rock Resources
Petroleum
Coal
148
148
149
155
REFERENCES CITED
156
APPENDICES
164
LIST OF FIGURES
Page
Figure
1
Index map showing the location of the thesis area
2
2
Scenic view looking south into the thesis area
4
3
Aerial photograph showing prominent relief of
Tillamook Head
4
4
Regional geologic map of the northwest Oregon
Coast Range
5
Rock correlation chart for the northern Oregon
Coast Range
6
26
Close-up of the glauconitic sandstone showing
characteristic spheroidal weathering pattern and
hematite rims
10
24
Green glauconitic sandstone unit in the Oswald
West mudstones
9
22
Bedded medium gray Oswald West mudstones with
concretionary and lighter colored tuff horizons
8
15
Map showing outcrop distribution of the Oswald West
mudstones
7
13
26
Very thinly bedded tuffaceous and calcareous
siltstones and darker silty mudstones of the upper
Oswald West mudstones
11
28
Terebellina and Scalarituba burrows in the siltstones
of the Oswald West mudstones
28
12
Classification of point counted sandstone samples
31
13
Photomicrograph of glauconitic sandstone of the
Oswald West mudstones
14
34
Map showing outcrop distribution of the Angora
Peak sandstones
44
Page
Figure
15
16
Outcrop of typical feldspathic sandstone. of the
Angora Peak member
46
Typical poorly sorted Angora Peak sandstone
consisting of angular and subangular clasts of
monocrystalline quartz, poly crystalline quartz,
microcline, orthoclase, plagioclase, and
diagenetically altered rock fragments
48
17
Photomicrograph of Angora Peak sandstone
48
18
Framework grains of Angora Peak sandstone
including altered volcanic rock fragments and
devitrified glass in a mixed-layered vermiculitechlorite clay matrix
51
Map showing outcrop distribution of the Silver Point
member
59
Well-bedded turbidite sandstones and mudstones of
the Silver Point member
62
Close-up of thin turbidite sandstones and mudstones
of the Silver Point member
62
Graded turbidite sandstones in Silver Point mudstone
member. of the Astoria Formation
66
Clastic dike of dark mudstone truncating blocky,
light gray, siltstones of the upper Silver Point
member
66
Large, dark mudstone rip-up and Foraminifera test
in laminated, calcite cemented feldspathic wacke
of the Silver Point member
68
Fine-grained, poorly sorted clay-rich Silver Point
turbidite sandstones
68
Dike-like apophses of the underlying Tillamook
Head sill intruding Silver Point mudstones
83
Irregular contact between Silver Point mudstones
and an overlying brecciated Depoe Bay Basalt
intrusive in Ecola State Park
83
19
20
21
22
23
24
25
26
27
Figure
Page
Depoe Bay Basalt intrusive exposed in a quarry
northwest of Sugarloaf Mountain
85
Typical intergranular texture in weathered Depoe
Bay Basalt from Tillamook Head
89
Fracture filling of actinolite or hornblende with
hexagonal crystals of apatite in Depoe Bay Basalt
from Tillamook Head near West Point
89
31
Outcrop of Depoe Bay Basalt breccia
92
32
Exfoliation boulders of Depoe Bay Basalt breccia
92
33
Photomicrograph of cavity fillings of apophyllite and
heulandite between angular basalt glass fragments
in Depoe Bay Basalt breccia
94
Silica variation diagram of selected middle Miocene
basalt samples from the thesis area
96
28
29
30
34
35
Hyalopilitic texture in Cape Foulweather Basalt
102
36
Photomicrograph showing intergranular texture in
Cape Foulweather Basalt
102
Structural map of the Tillamook Head - Necanicum
Junction area
122
Synsedimentary chevron folds of well-bedded
turbidite sandstones and mudstones of the lower
Silver Point member
124
Isoclinal syncline in Silver Point strata at Ecola
Point in Ecola State Park
125
Rose diagrams of paleocurrent measurements
from the Silver Point sandstones and siltstones
132
37
38
39
40
LIST OF TABLES
Pa e
Table
Modal analyses of selected samples of Depoe Bay
Basalt
2
3
Modal analyses of selected samples of Cape
Foulweather Basalt
104
Summary of size parameters
118
LIST OF PLATES
Plate
I
II
87
Geologic map of the Tillamook Head Necanicum
Junction area, Clatsop County, northwestern Oregon
(in pocket)
Diagrammatic geologic cross-sections ( in pocket)
GEOLOGY OF THE TILLAMOOK HEAD - NECANICUM
JUNCTION AREA, CLATSOP COUNTY,
NORTHWEST OREGON
INTRODUCTION
Purposes of Investigation
Little is known about the detailed geology and economic potential
of the Tertiary rocks in the northwest Coast Range of Oregon. Previous geologic mapping has been done on a reconnaissance scale.
The purposes of this study were: 1) to map the rocks and structure
of the Tillamook Head - Necanicum Junction area in detail; 2) to
describe the sedimentary and volcanic units and to reconstruct the
geological history of the area; 3) to determine the paleoenvironments
and the provenances of the sedimentary rocks; and 4) to evaluate the
economic potential of these rock units.
Location and Accessibility
The study area is located along the northern Oregon coast in
southwestern Clatsop County, approximately 60 miles west-northwest
of Portland. The 52 square mile area extends approximately eight
miles along the Pacific Ocean from the city of Seaside to one mile
south of the town of Cannon Beach and extends inland six to ten miles
(Figure 1).
2
"Columbia River
Astoria /J
Clatskanie
Mist
z
Ca
U
O
U
Ca
F'7
FF- . -
rMn
Nehalem
Jewell
I
Elsie
- - .r1n+-Qr%-n
_. . rn*-,
Miles
Columbia(Count
.
- - --\. - .
M Thesis Area
Map
A rea
OREGON
Figure 1. Index map showing the location of the-thesis area.
3
Access to and within the area is excellent. Two highways, U. S.
Highway 101 in the western part and State Highway 53 in the eastern
portion, traverse the area from north to south. U. S. 26 from
Portland cuts across the north central part of the thesis area from
east to west. A number of city roads and several county roads provide further access within parts of the area., and many gravel logging
roads maintained by Crown Zellerbach Corporation provide easy
access to forested mountain areas.
Principal Geographic Features
The northern Oregon coastline is characterized by rugged headlands, sea stacks, and beaches. The interior of the thesis area is
best described as a region of small stream valleys, hummocky low-
lands, and isolated steep hills, prominent ridges, and rugged
mountains composed of resistant basalt intrusives and submarine lava
flows .(Figure 2).
Prominent topographic features in the area are
Twin Peaks and the unnamed mountains northeast of Kidder's Butte,
which are 1, 621 and 2, 350 feet high respectively, and Tillamook Head,
a 1400-foot high basalt sea cliff (Figure -3 and Plate I).
Other geo-
graphic features include the Necanicum River, Ecola State Park, the
towns of Seaside, Cannon Beach, and Tolovana Park, and settlements
at Cannon Beach Junction and Necanicum Junction..
4
Figure 2. Scenic view looking south into the thesis area. Protrusive
peak on the right is Sugarloaf Mountain; rugged mountains
on the left are Kidder's Butte.
— — - S.
Figure 3. Aerial photograph showing prominent relief of Tillamook
Head (picture taken due west of Indian Beach; view is
northeast).
Climate
The climate of the northwest coast of Oregon is temperate and
moist due to the moderating effect of the ocean. Winters are mild
and wet, and summers are dry, cool, and sunny. Eighty percent of
the rainfall occurs during October through March (U. S. Department
of Commerce, 1972).
Annual rainfall varies from 80 inches in the
lowlands near Seaside to over 130 inches in the nearby mountains.
The average temperature for Seaside in August, the warmest month,
is 62. 4o F, and 42. 4o F in December, the coldest month. Summer
mornings are often accompanied by coastal fog that extends inland
several miles. Winter storms are characterized by heavy rains with
accompanying gale-force winds along -the coast. High runoff from
severe storms in the mountain areas occasionally results in flooding
along the Necanicum River and other small coastal streams (Schlicker
and others, 1972).
Methods of Investigation
Field Procedures
Field work was conducted for three months during the summer of
1973 and on numerous weekends and holidays late that year and in
early 1974. Field work consisted of geologic mapping, measurement
6
and description of stratigraphic sections, collection of rock and fossil
samples., and measurement of paleocurrent indicators..
Geologic mapping and field observations were initially compiled
on Oregon State Forest Service Aerial photographs taken in 1971 at a
scale of 1:12., 000 and were later transferred to an enlarged (1;12., 000)
portion of the 15' Cannon Beach quadrangle map (1955) (Plate I).
Stratification and cross stratification terminology in the described partial measured sections (Appendices I, II, and III) is in
accordance with the terminology of McKee and Weir (1953) and Bouma
and Brouwer (1964).
Paleocurrent measurements were collected with a Brunton
compass and were corrected for tectonic tilt by the stereonet method
discussed by Potter and Pettijohn (1963).
More than 100 samples of representative sedimentary and igneous rock lithologies were collected for laboratory study. In addi-
tion, 80 mudstone samples were collected for micro-fossil extraction
and identification. Several suites of molluscan and Ichno-fossils and
samples of sedimentary structures were selected for further study in
the laboratory. Geographic locations and stratigraphic positions of
samples collected in the area are presented in Appendices N-X and
on Plate I.
7
Laboratory Procedures
Laboratory studies of field samples consisted of the following:
1) grain size and heavy mineral analyses of sandstones; 2) thin section
study and modal analyses of sandstones and basalts; 3) chemical
analyses of basalts; 4) x-ray diffractions and powder photographs of
mudstones, sandstones matrices, and zeolites; and 5) determination
of organic content of sandstones and mudstones.
Grain size analyses of seven disaggregated sandstone and siltstone samples (Appendix VII) were performed by using the pretreatment, sieving and pipette settling techniques outlined by Royse (1970).
Folk and Ward's (1957) statistical grain size parameters (mean,
mode, standard deviation, skewness and kurtosis) were calculated
from the cumulative weight percent curves of the sieved and pipette
grain size fractions. These statistical parameters were plotted on
Friedman's (1962) and Passega's (1957) binary graphs to aid in the
interpretation of depositional environments.
Tetrabromoethane (S. G. = 2. 953) and the Franz magnetic separ-
ators were used to separate the 40 size fraction into light and heavy
mineral groups. The heavy fractions were mounted on slides for
petrographic identification (Appendix VIII).
Nineteen sandstone billets were stained for a quantitative determination of plagioclase and potash feldspar following the method
8
devised by Laniz and others (1964).
Twenty-nine thin sections of sandstones and basalts were ex-
amined with a petrographic microscope. Modal analyses of 13 thin
sections were performed by counting approximately 600 points per
slide. The classification of sandstones and basalts followed in this
report is that of Williams, Turner, and Gilbert (1954). Mudstones
and siltstones are classified in accordance with Folk's (1968) size
classification of clastic rocks.
Clay and zeolite minerals in mudstones and matrixes (< 40) of
disaggregated sandstones and siltstones were identified using a
Norelco X-ray diffractometer. 'Diffraction patterns were compared
with known clay diffraction patterns obtained by Grim (1968), Carroll
(1970), and Brown (1969). A detailed description of X-ray analysis
and pretreatment techniques appears in Appendix IX.
Six basalt samples from sills and submarine breccias were
chemically analyzed by a combination of X-ray fluorescence, atomic
absorption spectrophotometry, and visible light spectrophotometry.
The results were compared to chemical analyses of basalts of the
Oregon Coast Range by Snavely and others (1973). Weight percents
of SiO2, A12O3, FeO (total iron), CaO, K2O, and TiO 2 were deter-
mined by X-ray fluorescence. Atomic absorption spectrophotometry
was used to determine percents of Na2O and MgO. A more precise
weight percent value for SiO 2 was obtained from the visible light
9
spectrophotometer. Buttons and chemical solutions for the analyses
were prepared in accordance with Taylor's Cookbook for Standard
Chemical Analysis (1974).
Dr.. Weldon Rau of the Geology and Earth Resources Division
of the Washington Department of Natural Resources examined and
identified the Foraminiferra recovered from disaggregated mudstone
samples. Mollusca collected in the field were sent to Dr. Warren
Addicott of the U. S. Geological Survey, Menlo Park, for identification and paleoecological interpretation. Ichno-fossils (trace fossils)
were sent to Dr. C. K. Chamberlain of Ohio University for examination and interpretation. Dr. Robert Dennis of the Department of
Botany, Oregon State University, identified plant fossils.
Previous Work
Regional geologic investigations of the northern Oregon Coast
Range have been conducted by a number of scientists during the past
78 years. The Tillamook Head - Necanicum Junction area was first
scientifically investigated by J. S. Diller (1896) in a pioneer study of
northwest Oregon. Washburne (1914) conducted a reconnaissance
study of the geology and oil potential of northwest Oregon in which he
said of the Tillamook Head area, "Owing to the absence of roads and
the almost impenetrable brush,. little work was attempted in the
southwest corner of the (Clatsop) county, except along the seacoast. "
10
However, he did note several isolated basalt and sandstone exposures
along the coast.
The northern Coast Range of Oregon was first mapped on a reconnaissance scale by Warren and others (1945) and published as a
U. S. Geological Survey oil and gas investigations map (1:143,.000).
Because of the similar lithologies of Tertiary strata in the area, only
basalts were delineated from undifferentiated Tertiary sedimentary
rocks on the map.
Wells and Peck (1961) compiled a geologic map of western
Oregon (west of the 121st meridian) in which the general. outcrop dis-
tribution of the Astoria Formation was distinguished from undiffer-
entiated Eocene-Oligocene strata in the thesis area. Schlicker and
others (1961) investigated the destructive landslides at Ecola State
Park that occurred during the winter of 1960-1961 and presented a
general geologic engineering sketch and map of the southern park area.
Schlicker and others (1972) remapped the northwest Coast Range on a
regional scale for an environmental and geological engineering study.
On the maps of Clatsop and Tillamook counties, Miocene basaltic
submarine breccias were differentiated from basaltic intrusives, the
Astoria Formation was restricted to a middle Miocene sandstone unit,
and the remainder of the Tertiary strata, including the type Astoria
Formation at Astoria, Oregon, were mapped as undifferentiated
Oligocene-Miocene sedimentary rocks.
11
Snavely and others (1963, 1964, 1969a) and Baldwin (1964) have
discussed the general region in various sketches and overviews of the
geology and geologic history of northwestern Oregon.
The chemical
composition and petrology of middle Miocene basaltic rocks in the
Oregon Coast Range including samples from the Tillamook Head-
Necanicum Junction area, have been reported by Snavely and others
(1968, ,1969b, 1973).
Turner (1970) and Niem and Cressy (1973)
have reported K/Ar middle Miocene radiometric dates for two basalt
intrusives in the thesis area.
Approximately three miles south of the town of Cannon Beach a
1000-foot thick sedimentary sequence of shallow marine and fluvial
sandstones and conglomerates has been informally erected as Angora
Peak sandstone member of the Astoria Formation by Niem and Van
Atta (1973) and Cressy (1974).
Their studies indicate that these strata
are equivalent in age to the type Astoria Formation at Astoria., Oregon.
Niem and Van Atta (1973) also recognized that a deep-water, turbidite
sandstone facies of the Astoria Formation is exposed in Ecola State
Park near Cannon Beach.
12
REGIONAL GEOLOGY
Rock units exposed in the northern Coast Range of Oregon range
from early Eocene to Quaternary in age. The Tertiary sedimentary
and volcanic rocks attain a composite thickness of over 25, 000 feet
(Suavely and Wagner, 1964).
Quaternary deposits are composed of
river alluvium, marine terrace deposits, and beach and dune sands.
Snavely and Wagner (1964) interpreted the structure of the
northern Oregon Coast Range as a northward plunging anticlinorium
with the oldest rocks, the Eocene Siletz River and Tillamook Volcanics,
exposed in the center (Figure 4). A sequence of late Eocene to middle
Miocene sedimentary rocks and middle Miocene volcanic rocks dip
from 100 to 400 to the north, east, and west away from the uplifted
volcanic core.
The anticlinorium contains a number of smaller anti-
clines and synclines which trend northwest in the northern part of the
northern Oregon Coast Range and northeast in the southern part (Niem
and Van Atta, 1973). Faults in the northern Oregon Coast Range are
high-angle normal and reverse faults which commonly strike northwest-southeast. Displacements on the faults range from a few feet
to over 3,000 feet.
The Tillamook Volcanic Series (Warren and others, 1945) and
the Siletz River Volcanic Series (Snavely and Baldwin, 1948) are
thick sequences of lower to middle Eocene tholeiitic and alkalic basalt
13
Astoria
Map Area
Oregon
T it lamook
Head
Silver
Point
Hug Point
Oswald West
xx
xx,
xXxx
xx
Tas
Cane
xXxx x
xxxx
,
NNNNNN
x
'
NNNN\
-
Tgv
xxx
I--
Tpb
EXPLANATION
Quaternary/Pliocene
sediments
Columbia River
Basalts
Astoria Formation
Middle Miocene
Cape
Meares
Eocene to Oligocene -sedi-
mentary rocks (undifferentiated)
Pittsburg Bluff Formation
Keasey Formation
Cape
Lookout
0
5
Miles
OVA
Cowlitz Formation
Eocene volcanics (Tillamoo
and Goble series)
Figure 4. Regional geologic map of the northwest Oregon Coast
Range. (Map after Wells and Peck, . 1961).
14
pillow lavas and breccias (Snavely and others, 1968). The base of
the volcanic series is not exposed in the Coast Range, but a minimum
thickness of 5, 000 feet has been measured and a maximum thickness
of 20, 000 feet near volcanic centers has been estimated by Snavely
and Wagner (1964) from geophysical studies.
Six formations are recognized in the eastern part of the northern
Coast Range. They are (from oldest to youngest) the Cowlitz Forma-
tion, Goble Volcanics, Keasey Formation, Pittsburg Bluff Formation,
Scappoose Formation, and the Columbia River basalts. Their corre-
latives in the central Oregon Coast Range are illustrated on the
correlation chart (Figure 5).
The 1, 000-foot thick.late Eocene C owlitz Formation has been
subdivided into four members: 1) a 200 -foot thick 'basal, basalt
boulder conglomerate; 2) a lower, dark shale member (locally glauconitic); 3) a massive micaceous sandstone member; and 4) a thick
fossiliferous, dark gray upper shale member (Warren and Norbisrath,
1946).
Pillow basalts and breccias of the Goble Volcanics have been
reported by Wilkinson and others (1946) to interfinger with the upper
shale member.
The overlying late Eocene to early Oligocene 1800 -foot thick
Keasey Formation consists of three members: a basal dark laminated siltstone overlain by a massive, tuffaceous and arkosic clayey
Pacific Coast
Megafossil
Foraminiferal
Terrace Mat.
Alluv ium & Terrace
Troutd a l e F ormati on
Jacalitos
Early
,
Late
>.
04
-Briones
---Temblor
a uero
Blakeleyy
Relizian
Early
Keasey
O
edim ent ary R ocks Upper Miocene
Sandstone
(at Clifton)
Col
o umbia River Basa lt Miocene Volcanic Rocks
Astoria Formation
Z emorri.an
Pittsburg Bluff Fm.
(includes Gries Ranch
Refu an
Narizian
Transition beds
E"
Middle
Domengine
Cape
oulweather Basalt
hale Cove Sandstone
salt
Astoria Formation
Cowlitz
Formation
---
Sediment
Rocks
Eocene
Volcanic
R oc ks
Ba
Ba sa
lt
Astoria Formation
Silver Pt. member
(-Angora Peak
member
Mudstones of
Alsea
Goble Eocene
Volcs
F orm at i on
Yaquina Formation
Siltstone of
C RI
D e Doe
Nye M udstone
Sedimentary Rocks
Keasey Formation
Y am hill
Ulatisian
- Miocene
Scappoose Formation
Late
Tejon
alluvium, landslides
beach & dune sand
SSaucesian
Lincoln
0o
on
Neroly
Cierbo
Troutdale Formation
This Thesis
Terrace gravel,
T errac e gr avels and
Portland Hills Silt
Borin Lava
Etchogoin
Middle
Central Oregon
Coast Area
beach & dune sand
San Joaquin Cl.
Late
Northwest Oregon
Coast Area
alluvium
deposits
Tulare
o
Northwest Oregon
Coast Range
olcanic
ocks
Nestucca
Formation
Yamhill Formation
Tyee
Formation
O swald West
- - - - - - not exposed
in
thesis area
ff siltstone
Uembe
Early
Capay
Siletz River
Penutian
Volcanics
Siletz River Volcani
Figure 5. Rock correlation chart for the northern Oregon Coast Range (modified from Niem and
Van Atta, 1973).
Un
16
siltstone and an upper member of tuffaceous sandy siltstones (Van
Atta, 1971).
Disconformably overlying the Keasey Formation is .the 850-
foot thick middle Oligocene Pittsburg Bluff Formation (Warren and
others,, 1945). This unit is composed of massive and laminated,
arkosic, and glauconitic sandstones interbedded with minor siltstones.,
mudstones, and conglomerates (Niem and Van Atta,. 1973). Coal beds
are reported in the upper part of the unit. The Keasey Formation
has been interpreted as deltaic in origin. The formation is exposed
only on the northeastern side of the.Oregon Coast Range (Beaulieu,
1971).
The overlying 1500-foot thick late Oligocene Scappoose Forma-
tion consists of three members.: a thin basal basaltic conglomerate,
a thick fossiliferous arkosic sandstone middle member, and a gray,
massive, tuf£aceous, sandy mudstone upper member (Niem and
Van Atta, 1973).
Columbia River Basalts unconformably rest upon the Scappoose
Formation and older formations in the -northeast part of the Oregon
Coast Range (Warren and Norbisrath, 1946). The Columbia River
Basalts consist of dense, aphanitic to very finely crystalline, dark,
basaltic subaerial flows containing well-developed columnar jointing..
The plateau derived middle Miocene basalts have been recognized
along the Columbia River as far west as Big Creek, 20 miles east of
17
Astoria (Snavely and others, 1968). Local non-marine, late Miocene
to early Pliocene strata, informally known as the "Upper Astoria
Beds" (Lowry and Baldwin, 1952), conformably overlie and interfinger with the Columbia River Basalts near Clifton and Brownsmead,
on the Columbia River east of Astoria. These strata.consist of
approximately 500 feet of massive buff and iron-stained., friable,
coarse grained arkosic sandstones interstratified with laminated,
dark gray, carbonaceous sandstones and silty mudstones (Niem and
Van Atta, 1973).
The Cowlitz, Keasey, Pittsburg Bluff, and Scappoose Formations and the Columbia River Basalts have not been recognized on the
west side of the northern Oregon Coast Range. Correlative units have
been mapped only, as undifferentiated Tertiary sedimentary rocks and
middle Miocene volcanics (Warren and others, .1945;; Wells and Peck,
1961; Schlicker and others, 1972) (Figures 4 and 5). Mapped units
recognized in the western part of the northern Oregon Coast Range
are the Astoria Formation, the Oswald West mudstones, and the
Depoe Bay and Cape Foulweather Basalts (Figure 5).
The middle Miocene Astoria Formation crops out in a series of
marine embayments from Astoria to Newport, Oregon. Howe (1926)
divided the Astoria Formation at Astoria, Oregon, the type area,
into three members: a lower 150 -foot member of thin-bedded,
laminated, fine-grained sandstones alternating with dark mudstones,
18
which is gradationally, overlain by a 1,000-foot thick middle mudstone
member that contains a few thin glauconitic sandstones, and an upper
member composed of massive to cross-bedded, coarse--grained,
arkosic and micaceous sandstones. Dodds (1969) presented evidence
for the existence of an unconformity between the upper sandstone and
middle shale member of Howe's Astoria Formation near Astoria,
Oregon. He assigned Howe's upper sandstone member to a new
formation called the "post-Astoria Sandstone. "
The contact relationships of the Astoria Formation with the
overlying and underlying Tertiary rock units are not exposed in the
type area. Niem and Van Atta (1973) and Cressy (1974) have des.
cribed an unconformable contact between the Astoria Formation and
underlying late Oligocene mudstones in Oswald West State Park,
approximately 30 miles south of the c-ity of Astoria along the Oregon
coast. These late Oligocene to early Miocene mudstones (informally
referred to as the "Oswald West mudstones") consist of a 1,500-foot
sequence of interbedded, dark gray, fossiliferous, burrowed, mudstones and light gray, tuffaceous siltstones with very minor graywacke sandstones. The deep marine mudstones correlate with the
sandstone-rich Scappoose Formation of the northeastern Oregon
Coast Range on the basis of age (Figure 5).
Middle Miocene submarine basalt pillow lavas and breccias and
associated intrusive rocks along the northern Oregon coast
19
unconformably overlie and intrude and Astoria Formation and older
Tertiary sedimentary rocks:. _Snavely and.others_(1973) divided these
locally derived basalts into three major petrologic types basea on
petrography, chemical composition, and stratigraphic position. From
oldest to youngest, they are Depoe Bay Basalt, Cape Foulweather
Basalt, and Pack Sack Basalt. The Depoe Bay and Cape Foulweather
Basalts are chemically and age equivalent to the Yakima and late
Yakima basalts, respectively, of the plateau derived Columbia River
Basalts of eastern Oregon and Washington (Snavely and others, 1973).
Local volcanic centers of Depoe Bay and Cape Foulweather submarine
basaltic breccias and intrus;ives form the highest peaks (3, 000 feet
high) in the northern Coast Range.
20
DESCRIPTIVE GEOLOGY OF THE THESIS AREA
Five rock units form the bedrock geology in the Tillamook Head
Necanicum Junction area. They are: the late Eocene to early Miocene
Oswald West mudstones, the Angora Peak sandstone and Silver Point
mudstone members of the middle Miocene Astoria Formation, and the
intrusives and extrusives of the middle Miocene Depoe Bay and Cape
Foulweather Basalts. Figure 5 is a correlation chart showing the
mapped units of the thesis area and their correlatives elsewhere in
the Oregon Coast Range. In addition, Pleistocene marine terraces,
Holocene beach and dune sands, river alluvium, and landslide debris
lie with angular unconformity over these older Tertiary units.
Oswald West Mudstones
Nomenclature
The informal name, Oswald West mudstones, was introduced by
Niem and Van Atta (1973) and Cressy (1974) for a 1600-foot thick
sedimentary unit that occurs along the seacliffs at Short Sands Beach
in Oswald West State Park, ten miles south of the thesis area. The
unit there consists of well-bedded, highly burrowed, light gray tuffaceous siltstones and dark silty mudstones interbedded with very
minor amounts of graded turbidite sandstones and submarine slump
deposits. The late Oligocene to early Miocene mudstones are overlain
21
with angular unconformity by the middle Miocene Angora Peak sandstone member of the Astoria Formation (C ressy, . 1974). The bottom
contact is not exposed in Oswald West State .Park.
Distribution
The Oswald West mudstones (informal) are exposed in the
eastern half of the thesis area between the north and south forks of
the Necanicum River, along State Highway 53, and possibly in the
area north of Tillamook Head near Seaside (Figure 6). The unit is
easily eroded and commonly forms lowlands partially covered by
colluvium. Areas underlain by the mudstones are characterized by
low hummocky topography caused by landslides.
Lithologies and Structures
The lower and middle parts of the Oswald West mudstones consist of a few thousand feet of predominantly dark-gray bedded mud-
stones interstratified with less abundant,. light gray sandy siltstones
and one-inch to three-feet thick tuff beds. The middle part also
contains local lenses of glauconitic sandstone. The upper portion of
the unit is composed of over 300 feet of laminated to structureless
burrowed, tuffaceous sandy siltstones and very fine-grained sandstones. A partial measured section of the upper portion of the unit
is described in Appendix III. The presence of thin tuff beds and
Seaside
w
U
O
EXPLANATION
Glauconitic sandstone
in middle member
Upper Oswald West
mudstone
Cannon
Beach
Miles
Figure 6. Map showing outcrop distribution of the Oswald West mudstones.
23
glauconite lenses and an absence of well-bedded, abundant carbonaceous and micaceous mudstones and turbidite sandstones help dis-
tinguish the Oswald West mudstones from the Silver Point mudstone
member of the Astoria Formation..
The dark gray, bedded, silty mudstones of the lower and middle
parts are volumetrically (about 70%) the most abundant lithology within
the unit. Mudstone beds range from one inch to several feet in
thickness.
Individual beds are laminated or structureless due to
In fresh
extensive burrowing which has obliterated stratification.
exposures, the mudstones are light olive gray (5Y5/2
-
5Y6/1) in
color. Many roadcut and streambank exposures, however, consist
of deeply weathered,. iron-stained, crumbly, yellowish orange
(10YR6/6) to pale reddish brown (1OR5/4) mudstones where bedding
is commonly obscured.
The mudstones are locally micaceous and
fossiliferous and contain scattered glauconite pellets and one-inch
to three-feet thick, very light gray (N7), pumiceous, "bentonite" tuff
beds. Calcareous concretionary horizons and scattered carbonized
plant fragment fossils also occur (Figure 7). The closely jointed,
well-bedded mudstones commonly weather to small chips. Slopes of
chippy talus are common at the bases of most exposures.
Invertebrate fossils are rare in the mudstones, however,
several specimens of scaphopods, Dentalium cf. D. pseudonyma
Pilsbry and Sharp (W.- Addicott, written commun. , 1974), were
24
Figure 7. Bedded medium gray Oswald West mudstones with
concretionary and lighter colored tuff horizons on northern
Tillamook Head near Seaside. (NE1/4, section 29, T.6
N., R.10 W.)
25
collected from the middle part of the mudstone overlying a thick
glauconitic sandstone bed near Little Joe Creek (see.Plate I and
Appendix IV for location).
Late Eocene to early Miocene Foraminifera are common in the
Oswald West mudstones (see locations and fauna list, Plate I and
Appendix IV).
Some of the more commonly occurring foraminifera
are Dentallna spp. , Gyroidina obicularis
anata Cushman, Cibicides
sp. , Cibicides elmaensis Rau, and Bulimina cf. B. alsatica Cushman
and Parker (W. Rau, written commun. , 1974).
A 40-foot thick glauconitic sandstone lense occurs within the
middle part of the Oswald West mudstones in the eastern part of the
thesis area (Plate I and Figure 6). The green sandstone is wellexposed in recent logging roadcuts and can be traced laterally over
three miles from the western half of section 26 along Little Joe Creek
to the basalt breccia-covered hills north of Kidder's Butte. Smith
also was able to map this glauconitic sandstone near Sugarloaf
Mountain and Onion Peak immediately south of the thesis area.
The
unit consists predominantly of thick-bedded, very coarse-grained,
glauconitic sandstones and several thin (less than one-foot thick),
grayish orange (1OYR7/4), tuffaceous mudstone interbeds (Figure 8).
Contacts between the mudstones and sandstones are sharp.
Fresh
exposures of the galuconitic sandstone are dusky yellow green
(5GY5/2), but rapidly weather to light olive brown (5Y5/6) to light
m
r
rt
G
.' E
'
i
erg
z
-rL
.-
'
_+
r
II
IAF
cam ..
-Vi
7-
Figure 8. Green glauconitic sandstone unit in the Oswald West
's r
ifi Tc & brown
EK, 1v'
[ill:'! . ':f1,lm13 mudstone
Note the orange
tuffaceous
interbed and calcareous concretions. (NE1/4 NW1/4,
section 34, T. 5 N. , R. 9 W. )
mudstones.
arm/ c
'I
A-
t-
a
a.r
.t
I
m 'al
it
ad
r
;
it
J44'
I
its
f
Figure 9. Close-up of the glauconitic sandstone showing characteristic spheroidal weathering pattern and hematite rims
(dark orange to brown). (Sample location same as Figure
YI
6)
5
27
brown (5YR5/6). In general, the well-indurated, calcite cemented
sandstone is a low ridge former. Calcareous concretions, up to,18
inches thick and four feet long, are commonly concentrated along
bedding planes in the sandstones (Figure 8). Glauconitic sandstone
exposures commonly exhibit characteristic nodular or spheroidal
weathering, containing exfoliation rims of reddish yellow hematite
and limonite (?) (Figure 9). Deeply weathered exposures are a dark
reddish brown (1OR3/4) color as a result of
the,complete oxidation
of the glauconite.
The upper part of the Oswald West mudstones consists of thick
sequences of laminated, dark mudstones and lighter burrowed,
tuffaceous, sandy siltstones and very fine-grained sandstone (Figure
10).
It is characteristically more resistant to erosion than the lower
and middle bedded mudstones, and as a result, commonly forms
ridges of moderate slopes and outcrops typified by blocky talus. A
300-foot section of this lithology is exposed just below the contact
with the Angora Peak sandstone along spur 14 of Summit Mainline
(logging road) in sections 19 and 20. of T. 5 N. ,. R. 9 W. near the
junction of the north and south forks of the Necanicum River. The
unit is correlative to similar exposures north of Little Joe Creek in
section 27 of T. 5 N. , R. 9 W. , and to exposures north of the map
area along Charlie Creek Road in the SW1/4 of section 14, T. 5 N. ,
R. 9 W. The sandy siltstones and very fine-grained sandstones
li
,2.8:
ti
0 ,1
y: L
'1
Win
ery thinly bedded tu:(rOceous and caicareous s -l tstotiesc
and darker silty mudstones of the upper Oswald Wrist
mudstones. Mottled appearance is due to burrowing and
loading. (SW1/4 SE1/4, section 19, T. 5 N., R. 9 W.)
Figure
-r
'11
eM
LI
Figure 11. Terebellina and Scalarituba burrows in the siltstones of
the Oswald West mudstones. (Sample location same as
Figure 8)
29
(feldspathic wackes) are composed of poorly sorted, subangular to
subrounded grains of quartz, feldspar, volcanic rock fragments, and
micas. Fresh exposures of the sandy siltstones are medium light
gray (N6) and weather to light olive gray (5YR7/4). The siltstones
are interbedded with one- to four-inch thick beds of dark yellowish
orange (10YR6/0) to dark gray (N3), finely laminated to massive
mudstones. In places, compaction has squeezed these mudstone
layers into sedimentary boudinages with the elongate axis parallel
to the stratification.
Thick bedded (up to 20 feet), massive, mottled and burrowed
layers of sandy siltstones also occur in the upper part of the Oswald
West mudstones. Burrows are composed of concentrations of dark-
gray fecal pellets in the lighter colored siltstones (Figure 11). Small
to large wispy forms of fecal ribbon Scalarituba (may be Helminthoida)
and elongate, up to 1/4 inch in diameter, agglutinated worm tubes
(some of which are irregular and branching) of Terebellina are the
most commonly occurring trace fossils (Chamberlain, written commun. , 1974).
The siltstones contain locally abundant calcareous
concretions (up to one inch in diameter). In addition, the unit is
sparsely fossiliferous containing only the pelecypod (Nucula sp. ) and
scaphopods (Dentalium cf. D. Pseudonyma Pilsbry and Sharp).
30
Petrology
Sandstones
of the upper Oswald West nudstones are very fine
grained and poorly sorted. Framework grains are s-ubangular to
subrounded. Modal analyses performed on four of the eight sandstone
thin sections indicate they are arkosic and feldspathic sackes (see
modal analyses Appendix VI and Figure 12).
The framework grains are dominantly, quartz (26-28%), plagioclase (17-20%), potassium feldspar (7--11%), rock fragments (7-11%),
and micas (4-6%). Minor constituents (less than 2%) include poly-
crystalline quartz., glauconite, heavy minerals, and chert (see modal
analyses Appendix VI).
Strained (undulatory) quartz is more abundant
than unstrained (normal) quartz; polycrystalline quartz and chert are
rare. Plagioclase feldspars range from labradorite (An65) to oligoclase (An18), but the most common type is andesine. Potassium
feldspar includes nearly equal amounts of orthoclase and mic rocline.
Plagioclase and potash feldspars are partially altered to finely disseminated kaolinite (?), chlorite, and white micas. Basic and intermediate volcanic rock fragments comprise the majority of the lithic
fraction and some brown calcareous siltstone and quartzite clasts also
occur. Detrital micas are dominantly biotite and smaller quantities
of muscovite. The micas are undeformed and are commonly aligned
parallel to stratification. This condition
not undergone extensive compaction.
suggests
that the strata have
31
Stable Grains
quartz, chert, quartzite
A: quartz wacke
B: feldspathic wacke
C : arkosic wacke
D: lithic wacke
Unstable Rock
Feldspars
Oswald West sandstones
Fragments
oAngora Peak sandstones
a Silver Point sandstones
Figure 12. Classification of point counted sandstone samples.
(Classification after Gilbert, 1954).
32
The most common heavy minerals from the 3. 50 to 4. O sand
size fraction are green hornblende, hypersthene, augite-, opaques,
and micas. Of the three Tertiary sedimentary units, these sandstones contained the most green hornblende (see Appendix VII). Other
less abundant heavy minerals include zircon, rutile, tourmaline, and
clear to pink garnet (possible grossularite).
A detrital matrix comprises approximately 30% of the sand
stones and is composed of clays, finely disseminated carbonaceous
material, and silt sized grains of quartz and feldspar. X-ray dif.-
fraction indicates that chlorite, chloriticintergrades montmorillonite, mica, and possibly kaolinite are the dominant clay matrix
minerals. The sandstones are tightly, cemented by these clay minerals and calcite.
Sandstones of the upper Oswald West mudstones are both com-
positionally and texturally immature. Steep, rugged, hilly source
areas in which rapid mechanical weathering predominated over
chemical weathering are reflected by the compositional immaturity.
The textural immaturity of these clay-rich, poorly sorted sandstones
suggests rapid deposition and burial with little or no current reworking.
The glauconitic sandstones in the middle part of the Oswald
West mudstones are composed predominantly of coarse- to very
coarse-grained sand- and small pebble-sized glauconite pellets
33
(0. 2mm to 8. 0mm in diameter). Other framework clasts include
basaltic volcanic rock fragments (8%), feldspars (4%), and quartz
(1%) (Figure 13)(see Appendix VI).
Plagioclase (3%), labradorite
and andesine, is more abundant than orthoclase-,. the potash feldspar.
The poorly sorted sandstone contains abundant matrix (40%) composed
of micrite, very tiny pellets of glauconite, silt sized angular grains
of quartz and feldspar, and rare shell fragments. A pore filling of
micro-sparry calcite formed from recrystallized micrite and shell
fragments acts as a cementing agent (Figure 11) along with hematite
formed by partial oxidation of the glauconite pellets.
X-ray diffractions of eight widely distributed samples indicate
that Oswald West mudstones consist dominantly of the clays chlorite,
montmorillonite, mica, vermiculite, possibly kaolinite, and chlori
tic intergrades. Heulandite and clinoptilolite, zeolites, also occur.
Silt-sized constituents are angular quartz and feldspar grains.
Treatment of the mudstones with hydrogen peroxide indicates that
between 1 and 5% by weight is composed of finely disseminated
organic material.
Contact Relations
The base of the Oswald West mudstones is not-exposed in the
map area. Stratigraphically, the lowermost exposure of Oswald
West mudstone occurs in the northeastern corner of the map area,
34
—
Figure 13. Photomicrograph of glauconitic sandstone of the Oswald
West mudstones showing well-rounded, green glauconite
pellets and less common andes.itic and basaltic rock
fragments cemented by micro-sparry calcite. Note very
small glauconite pellets in the matrix. (Plain light)
35
east of State Highway 53, where Dichter Road (logging road) crosses
an unnamed creek in NE1/4, SE1/4, section 23, T. 5 N. , R.9 W.
Microfossils collected from thinly bedded mudstones at this locality
were identified by Weldon Rau (written commun..,1974) as late
Eocene or early Oligocene in age.
The Oswald West mudstones are overlain with apparent angular
unconformity by the Angora Peak sandstones. The stratigraphic
relationship between the two units can be readily observed along Spur
14 of the Summit Mainline (logging road) in section 20 of T. 5 N. , R. 9
W. At this locality, thick coarse-grained, arkosic sandstones of the
Angora-Peak member of the Astoria Formation overlie a thick, wellbedded sequence of sandy siltstones and mudstones of the Oswald West
mudstones. The exact nature of the contact between these two map
units cannot be readily determined because the contact is buried by
thick talus. However, the dips and strikes of the two adjacent formations commonly do not conform. This disparity may be a result
of an angular unconformity separating the units as postulated by
Cressy (1974) in the Oswald West State Park area. On the other
hand, these discrepancies in strikes and dips could also be attributed to slumping of the Oswald West mudstones. However, in other
locations (see Plate I), Angora Peak sandstones are missing,, and the
Silver Point mudstones are either in fault contact or are overlying
the Oswald West mudstones.
36
Age and Correlation
A late Eocene to early Miocene age is assigned to the Oswald
West mudstones in the map area based upon megafossil, macrofossil,
and stratigraphic evidence. Foraminifera collected from several
widespread localities in the lower, middle, and upper parts of the
Oswald West mudstones suggest Refugian, Zemorrian, and early
Saucesian ages (latest Eocene to early Miocene, respectively) for the
strata (Rau, written commun. , 1974; see Plate I and Appendix IV).
In addition, molluscan fossils from directly above the glauconitic
sandstone indicate that the middle part of the Oswald West mudstones
is late Oligocene age (Warren Addicott, written commun. , 1974)
and belong to the Blakely Stage of the Pacific Coast standard megafaunal stages (see Plate I and Appendix IV).
The alternating dark gray bedded mudstones and lighter gray
laminated tuffaceous siltstones and the thick, structureless, extensively burrowed, tuffaceous siltstones of the upper part of the Oswald
West mudstones in the thesis area are lithologically similar to the
Oswald West mudstones at the type locality at Oswald West State Park
(six miles south of the thesis area). Foraminiferal suites and trace
fossil assemblages of the two areas correlate favorably. In addition,
the Oswald West lithology can be traced directly from Cressy's (1974)
map area northward through Smith's (1975) map area into this thesis
37
area. Rare graded "graywacke" sandstone beds and submarine slump
deposits,, described at the type locality by Cressy (1974), were not
recognized in this thesis area. In addition, the glauconitic sandstone
.in
unit mapped in this area was not recognized by Cressy (1974)
the
Angora Peak-Oswald West State Park area. Smith (1975) recognized
and mapped the glauconitic sandstone within the Oswald_West mud-
stones, in the Onion Peak area just south of this thesis area and adja-
cent to the map area of Cressy (1974). As originally defined by
Cressy (1974), the Oswald West mudstones are late Oligocene to
early Miocene in age. In the Tillamook Head-Necanicum Junction
area, strata mapped as the Oswald West mudstones are latest Eocene
to early Miocene. Thus, the Oswald West mudstones of this thesis
area comprise significantly more strata than does Cressy's (1974)
unit. C ressy did not, however, define a basal contact for the Oswald
West mudstones in the Onion Peak area. Because of deep weathering,
it is difficult to distinguish between the upper and.lower parts of the
Oswald West mudstones.
Therefore, for the sake of mapping con-
venience, all mudstones stratigraphically below the Astoria Formation were included in this enlarged definition of the Oswald West
mudstones. Perhaps when the regional late Eocene to early Miocene
stratigraphy is more thoroughly understood, the "Oswald West mud-
stone" term will be restricted to the late Oligocene-early Miocene
upper unit, and a new late Eocene to middle Oligocene unit will be
38
erected to include the lower and middle parts of the Oswald West
mudstones described in this report.
The outcrop distribution of the Oswald West mudstones near
State Highway 53 correlates in part with the undifferentiated late
Eocene and Oligocene sedimentary rocks that appear on the geologic
map of Oregon west of the 121st meridian (Wells and Peck, 1961).
The Oswald West mudstones cropping out between the north and south
forks of the Necanicum River, however, were incorrectly mapped
as middle Miocene "marine sedimentary rocks" of the Astoria
Formation on the geologic map of Oregon.
Depositional Environment
Sedimentary structures, lithologies, and fossils indicate that
the Oswald West mudstones were deposited in a moderate to deep
water, open marine environment. The upper part may have been
formed in a prodelta environment of a seaward extension of the
coeval deltaic Scappoose Formation to the east (Cressy, 1974), and
the lower and middle parts were probably deposited in an open outer
continental shelf environment.
Deep open marine conditions are suggested by foraminifera,
mollusca., scaphopods, and burrow forms. Benthonic foraminifera.l
suites, including Gyroidina obicularis planata Cushman, Cassidulina
bosa Hantken, Gassidu.lina cf. C., galvinens:is Cushman and
cf. C. globosa
39
Frizzel, and Quinqueloculina weaveri Rau, are indicative of cool
water in outer sublittoral to upper bathyal depths (500 to 2000 feet)
(Rau, written commun.
,
1974). Chamberlain (written commun.
,
1974) considers the burrow forms, Scalarituba and Terebellina, collected in the laminated mudstones and tuffaceous siltstones to be
typical of deep to intermediate water depths (outer shelf to abyssal).
The predominance of burrowed mudstones and siltstone beds
indicates that deposition consisted predominantly of hemipelagic
muds and silts in a relatively low energy environment. Local
laminated mudstones interstratified with thin sandy siltstones suggest
some size separations of detritus by deep fluctuating., gentle bottom
currents. An abundance of intricate feeding burrow patterns on
bedding planes suggests that deposition of elastics was very gradual
in a nutrient-starved environment. The coarse-grained glauconitic
sandstone unit on the other hand, suggests that higher energy, open
marine bottom current activity occurred at times. The presence of
glauconite and abundant disseminated organic material in the dark
mudstones, lack of benthic fossils, and minor authigenic pyrite
implies that a restricted, reducing environment existed during deposition of the Oswald West mudstones.
The Oswald West mudstones change lithologically from pre-
dominantly thin bedded mudstones in the lower part of the formation,
to massive burrowed siltstones and interstratified laminated, sandy,
40
tuffaceous siltstones and very fine-grained sandstone in the upper
part of the formation.. This change suggests an overall influx of
coarser detritus with time under slightly higher energy conditions.
The bedded mu.dstones and g.lauconitic sandstones in the lower and
middle parts of the Oswald West mudstones probably formed as
indicated by fossils, in an open, moderate to deep-water outer continental shelf and upper continental slope environment similar to that
which exists off of the present Oregon and California coasts. How-
ever, the extensively burrowed thick s.iltstones and the thin bedded
laminated sandy siltstones and very fine-grained sandstones in the
upper part of the Oswald West mudstones are similar in lithology,
thicknesses, and probable water depths to deposits of modern distal
prodelta environments described by Gould (1970) for the Mississippi
delta, by Allen (1970) for the Niger delta, and by Selley (1970) and
Visher (1965). Shallow marine and fluvial sandstones and coal beds
in the coeval Scappoose Formations, cropping out on the eastern
side of the northern Oregon Coast Range, have been interpreted as
having formed in a subaerial delta complex (Van Atta., 1971). The
upper Oswald West mudstones, which are an age equivalent of the
Scappoose Formations, might represent the deeper water pro-delta
facies of that delta system (Cressy, 1974). Heavy mineral suites of
the two units are very similar, and a westward pa.leocurrent dispersal
for the Oswald West mudstones at Oswald West State Park (Cressy,
41
1974) is in accordance with this deltaic model.
The bedded mudstones of the lower and middle part of the
Oswald West mudstones are -late Eocene to middle Oligocene correla-
tives with the Keasey, Pittsburg Bluff and Cowlitz Formations to the
east, and the Alsea and Nestucca Formations to the south (Figure 5).
Deposition of all these strata also occurred in moderate to deep,
open marine water (Snavely and Wagner, 1963).
Astoria Formation
The Astoria Formation crops out in a series of marine embay
ments along the Oregon coast from Newport to Astoria, the type area
(Wells and Peck, 1961), and as far north as Montesano in southwestern Washington (Pease and Hoover, 1957).
Since the first descrip-
tion of the Astoria Formation was published in 1880 by Cope, much
confusion regarding its stratigraphy and distribution has developed
because the formation has been correlated and mapped mainly on the
basis of middle Miocene fossil assemblages and not on lithologic
similarity to the type area (Moore, 1963). As a result, middle
Miocene strata mapped as Astoria Formation in western Oregon and
Washington exhibit rapid facies changes from shallow marine and
fluvial sandstones to deep marine siltstones and mudstones. The
formation has essentially become a time-stratigraphic unit referring
to middle Miocene strata and is deeply ingrained in the literature.
42
Cressy (1974) suggested that in order to avoid further confusion, the
name Astoria Formation should be retained to refer to middle Miocene
strata traditionally mapped as Astoria Formation.. However, he
suggested that different mappable facies within this unit in various
embayments be referred to as informal members in accordance with
the American Code of Stratigraphic Nomenclature (1961). Accordingly,
the Astoria Formation in this study consists of two informal mappable
members, 1) the Angora Peak sandstone and 2) the overlying Silver
Point mudstones.
Angora Peak Sandstone Member
Nomenclature.
The Angora Peak sandstone member of the
Astoria Formation was informally proposed by Niem and Van Atta
(1973) and by Cressy (1974) for a 1, 000-foot thick unit of shallow
marine sandstones and minor volcanic fluvial conglomerates, laminated micaceous and carbonaceous siltstones and rare coal seams.
The type section for this member was described near Angora Peak
in the extreme southwestern corner of Clatsop County, Oregon
(Cressy, 1974), approximately ten miles south of this thesis area.
The Angora Peak sandstone lies with angular unconformity over the
Oswald West mudstones and is unconformably overlain by the middle
Miocene Depoe Bay basaltic breccias which form Angora Peak.
43
Distribution. Small exposures of the Angora Peak sandstone
member occur along logging roads in the southern half of section 20,
T. 5 N. , R. 9 W. between the north and south forks of the Necanicum
River (Figure 14). The unit is probably more widespread in the
southeastern part of the map area north of Kidder's Butte but is
covered by landslide deposits of basaltic breccia, talus, and vegetation (see Plate I). Cressy (1974) and Smith (1975) have mapped
extensive outcrops of the Angora Peak sandstone in much of the 85
square miles adjacent to the southern boundary of the thesis area.
Small isolated outcrops of sandstones lithologically and petrograph-
ically similar to the Angora Peak sandstones are exposed in the
western part of the map area in the seacliffs of Tillamook Head
(NW1/4 of section 6, T. 5 N. ,R. 10 W. ) and at Crescent Beach in
Ecola State Park (Figure 14).
Lithology and Structures. The Angora Peak member in the map
area is predominantly composed of thick beds (5 to 20 feet) of feld-
spathic and lithic sandstones. Calculations using regional dips and
outcrop distributions suggest that a minimum of 400 feet of section
exists in the map area. Fresh sandstones are yellowish gray (5Y7/2)
to greenish gray (5GY5/1). Weathered sandstones, typically forming
roadcut exposures, are iron-stained, friable, and range in color from
pale yellowish orange (10YR8/6) to moderate yellowish brown (10YR
Figure 14. Map showing outcrop distribution of the Angora Peak sandstones.
45
5/4). Sandstones are moderately to poorly sorted, medium- to very
coarse-grained, and are composed of subangular to rounded grains of
quartz and feldspar and volcanic rock fragments. The more resistant
Angora Peak member commonly forms intermediate slopes and hilly
topography surrounded by low, flat-lying areas of Oswald West mud-
stones and Silver Point mudstones. The sandstones form large, extensive exposures in roadcuts in the area.
Parallel laminations and rare large-scale planar cross-beds are
the dominant sedimentary structures. Laminations are formed by
the rapid alternation of very thin concentrations of very thin (less than
one inch thick) clayey siltstone and thicker sandstone layers and by
concentrations of carbonaceous material and micaceous minerals
(Figure 15).
Petrology. Angora Peak sandstones are fine- to very coarsegrained (0. 2 mm to 6. 0 mm in diameter), moderately to poorly sorted.,
and contain subangular to rounded clasts in grain support. Coarse-
grained sandstones are volcanic lithic wackes, and medium- to finegrained sandstones are arkosic or feldspathic wackes (Figure 12).
The dominant framework grains of three point-counted sand-
stones are rock fragments (32-40%), quartz (16-25%), potassium
feldspar .(7-11%), and plagioclase (4-8%). Less abundant constituents
include chert, polycrystalline quartz., micas and heavy minerals
46
Figure 15.
Outcrop of typical feldspathic sandstone of the Angora
Peak member. Laminations are due to concentrations of
carbonaceous plant fossil fragments and micas along
bedding planes. (SE1/4, section 20, T. 5 N. , R. 9 W.
47
(Figures 16 and 17) (see Appendix VI). Basalt and andesitic volcanic
rock fragments with intergranular, intersertal trachytic, pilotaxitic,
and hyalopilitic textures comprise the majority of the lithic constituents. The groundmass in many volcanic rock fragments has been
altered to green celadonite or nontronite (Figure 16). Pumice,
rhyolite, and/or dacite clasts and devitrified volcanic glass also are
present in small quantities.
Other rock fragments occurring in minor amounts (less than
5%) include sedimentary quartzites, carbonaceous laminated silt-
stones and mudstones, metaquartzites and rare granite and coarsely
crystalline diabase. Metamorphic quartzite clasts (up to
3%)
consist
of elongated, polycrystalline quartz with crenulated crystal boundaries,
strained extinction, and subparallel contorted micas defining foliation
(Figure 17).
Quartz grains are generally subangular to subrounded, however,
rare well-rounded grains with overgrowths occur. Strained monocrystalline quartz (9-17%) and unstrained monocrystalline quartz
(6-8%) account for most of the quartz types. Polycrystalline quartz
constitutes only a minor percentage of the quartz varieties. Monocrystalline chert clasts containing thin polycrystalline quartz veinlets
are also common in Angora Peak sandstones (Figure 17). Several
chert clasts contain small euhedral plagioclase phenocrysts, suggesting anorigin from devitrification of glassy rhyodacitic or dacitic lavas.
48
A
'o
1
S
0
e
,I
Figure
r ypical poorly sort
16.
ngora Peak sandstone consisting
of angular and subangular clasts of monocrystalline
quartz, polycrystalline quartz (A), microcline (B), orthoclase, plagioclase, and diagenetically altered volcanic
JFU
q
rock
fragments (C). Note zoning plin plagioclase grain (D)
and myrmekite (E). (Crossed nicols)
rO.
B
:.a
r,
t-W
0
Figure 17.
IM M
Note chert
si-ni'.,r ,,r
. 1T (A),
S. I
C 7 .it., polycrystalline
clasts containing
quartz veinlets
Photomicrograph of Angora Peak sandstone.
metamorphic quartzite fragment (B), dark siltstone (C),
and altered volcanic rock fragments. Yellow-orange
cement is vermiculite. (Crossed nicols)
49
In contrast to the sandstones of the Oswald West and Silver
Point units, potassium feldspar is more abundant than plagioclase -in
Angora Peak sandstones (see Appendix VI). Orthoclase and micro-
cline, are commonly unaltered or only partially altered to white mica
(sericite) and kaolinite (?). Rare myrmekite grains also were observed (Figure 16). Plagioclase grains are mainly andesine but vary
compositionally from labradorite (An65) to oligoclase (An18). They
range within the same sample from fresh, clear unaltered grains to
highly altered grains of chlorite, white mica, kaolinite (?), calcite,
and zeolites. This range in composition and alteration within the
same sample suggests mixing of feldspars from different source
areas and rock types under slightly different climatic conditions.
Folk (1968) has suggested that fresh and weathered feldspars of the
same composition can be derived from a source area of steep relief
where steep stream gradients mechanically erode fresh feldspars
from bedrock in channels and mix these with more chemically
weathered feldspars collected from nearby slopes. Biotite, minor
green biotite and muscovite comprise up to 1% of the framework
grains.
Heavy minerals range in abundance from a trace to 3% by weight
in the 3. 5V to 4:. oV size fraction. The most common are hypersthene,
zircon, green hornblende, clear to pink garnet (possibly grossularite),
and the opaque minerals pyrite, magnetite, hematite, ilmenite, and
50
leucoxene. Others include clinopyroxene, micas, basaltic hornblende,
epidote, monazite, apatite, tourmaline, and rutile (see Appendix
VIII).
Of the three Tertiary sedimentary units in the thesis area, the
Angora Peak sandstones contained the most zircon and the only epidote.
These sandstones are compositionally immature on the basis of
the overwhelming abundance of, chemically unstable rock and mineral
fragments. This immaturity suggests source areas from hilly or
mountainous terrains. Co-existence of both rounded and subangular
grains of one mineral species (particularly quartz) in the same
sample indicates a mixed provenance in which some grains were
recycled from sedimentary rocks.
Sandstone matrix varies from 18 to 26% and is composed of silt-
sized quartz, feldspars, and devitrified glass in a paste of carbonaceous clay (Figure 18). The clay matrix minerals determined from
X-ray diffraction are montmorillonite, chlorite, vermiculite, micas,
and possible traces of illite. The zeolite clinoptilolite also occurs.
Organic content of the matrix is 1. 15% by weight.
Sandstones are texturally immature based on the abundance of
clay matrix, poor sorting, and high degree of grain angularity (Folk,
1968). Much matrix, however, appears to have been formed by the
in-situ diagenetic alteration of volcanic rock fragments because in
thin section, many indistinct, altered, volcanic clast boundaries
51
k
Figure 18. Framework grains of Angora Peak sandstone including
altered volcanic rock fragments (A) and devitrified glass
(B) in a mixed layered vermiculite-chlorite clay matrix
(C). Same as Figure 17. (Plain light)
52
grade compositionally into the surrounding. matrix. In addition, the
.
presence of clinoptilolite, which is a common alteration product of
intermediate and silicic volcanic glass, also suggests that the mud
matrix is in part diagenetic in origin. Thus, the textural maturity
of the sediment during the time of deposition was probably submature
to mature (e. g. volcanic arenites), suggesting current winnowing and
sorting before deposition.
Angora Peak sandstones are characteristically friable because
they are loosely cemented by iron oxides (hematite and.leucoxene),
diagenetic clays, and zeolites.
Contact Relations. The lower contact of the Angora Peak sandstone with the underlying Oswald West mudstones has been discussed
previously in the Contact Relations section of the Oswald West mudstones. The -contact between the Angora Peak sandstone and the
overlying Silver Point member is not exposed. in the thesis area although mapped outcrop patterns and strikes and dips in the two mem-
bers suggest that the Angora Peak sandstone underlies the younger
Silver Point mudstone member. Two miles south of the thesis area,
near Hug Point State Park, in road cuts along the Hug Point mainline
logging road, the Silver Point member conformably overlies the
Angora Peak sandstone member (Smith, 1975). In addition,. Smith
mapped extensive areas of Angora Peak sandstone overlain by the
53
Silver Point member in the Onion Peak-Sugarloaf Mountain region
just south of the thesis area. In the Tillamook Head - Necanicum
Junction area, the Angora Peak sandstone dips northwestward under
Silver Point mudstones and thus does not crop out in most of the
region. The Angora Peak sandstones do crop out near the axis,of the
eastern syncline between the north and south forks of the Necanicum
River.
Although the Silver Point mu:dstone member regionally appears
to conformably overlie the Angora Peak sandstones in the Seaside-
Angora Peak region, the lower part of the Silver Point member may
interfinger on a small scale with the Angora Peak sandstone member
locally within the thesis area. Evidence for this interfingering rela-
tionship includes a large-, isolated, in-place
?). 20 feet high and 40
feet long boulder of pebbly, arkosic Angora Peak sandstone on
Crescent Beach in Ecola State Park. This boulder is surrounded by
outcrops of the lower part of the Silver Point mudstone member in the
adjacent seacliff and nearby hills. The nearest extensive exposure
of the Angora Peak sandstone is north of Hug Point State Park--more
than four miles to the south. Three possible -explanations are offered
for the presence of the Angora Peak sandstone boulder on Crescent
Beach: (1) the boulder is a,landslide block; (2) the boulder is actually
,part of an unusually thick sandstoneinterbed of the Silver Point member; or (3) the boulder is from a thick covered Angora Peak sandstone
54
bed which interfingers with the Silver Point member. The 100-foot
high seacliff above the boulder contains interbedded, thin.,. graded,
turbidite graywackes and mudstones of the lower part of the Silver
Point member. Extensive searching along this seacliff and vicinity
.
failed to uncover a sandstone bed as a source for the arkosic boulder,
thereby suggesting that the boulder is in place and is not a landslide
block.
The second hypothesis is possible but questionable. Silver
.
Point turbidite sandstones are fine grained., graded, micaceous and
carbonaceous "graywackes" with detrital clay matrix contents over
30%.
These turbidite sandstones occur as thin (two inches to eight
feet thick) interbeds in a dominantly mudstone member. In contrast,
the sandstone forming the isolated boulder is coarse-grained, arkosic,
clean, thick (20 feet), carbonate cemented, crossbedded and parallel
.
laminated, and contains cut-and-fill channel structures. These
lithological characteristics are much more typical of the Angora Peak
sandstones
mapped in the eastern part of the thesis area and mapped
by Smith (1975) at Hug Point and by Cressy (1974) at the type section,
suggesting that the boulder is actually a small exposure of a thin
tongue of Angora Peak sandstone in the Silver Point mudstones.
Another small isolated outcrop of Angora Peak (?) sandstone
lithology surrounded by extensive outcrops of the Silver Point mudstone
member occurs at the top of the precipitous 800-foot high
seacliff in the NW1/4 of section 6, T. 5 N., R. 10:W. on Tillamook
55
Head (Plate I and Figure 14). These inaccessible sandstones are
deeply weathered and cannot be reached from the Tillamook Head trail
near the top of the cliffs or from the beach. Lower Silver Point mudstones are exposed to the east in the NEI/4 of section 6 strat'igraphically
below the cliff sandstones. Additionally, in the north-central part of
the thesis area (north of Kidder's Butte, see Plate I), Silver Point
mudstones appear to overlie the Oswald West mudstones with no intervening Angora Peak sandstone. Thus, based on limited evidence,
it appears that local tongues of the Angora Peak sandstones interfinger with the lower part of the Silver Point mudstone member.
.Additional thesis studies are being conducted by Oregon State
University graduate students Peter Penoyer and Patrick Tolson
immediately north of the thesis area to further determine the nature
of the contact between the Angora Peak sandstone and Silver Point
mudstone members of the Astoria Formation.
Age and Correlation. The Angora Peak sandstone member is
assigned an early to middle Miocene age based on stratigraphic relationships. The sandstones overlie late Eocene to early Miocene
Oswald West mudstones, and they underlie and may partially interfinger with the middle Miocene Silver Point member of the Astoria
Formation. No fossils were found in the Angora Peak sandstones in
the thesis area, but early Miocene and middle Miocene fossils occur
56
in the underlying and overlying (respectively) stratigraphic units.
Therefore, the Angora Peak member is early Miocene to middle
Miocene in age. In addition, molluscan fossils collected by C ressy
(1974) and Smith (1975) in the Angora Peak sandstones in nearby
Angora Peak and Sugarloaf Mountain areas indicate a "Temblor" or
middle Miocene age.
The, Angora Peak sandstones in this map area correlate with
those of Cressy (1974) in the type area near Angora Peak based on
lithologic and stratigraphic similarities, and on similar mineralogy,
textures, and sedimentary structures. The member in both areas is
predominantly composed of thick bedded, laminated, fine- to coarsegrained feldspathic sandstone with thin interbeds of carbonaceous and
micaceous siltstone and mudstones. Cressy (1974) noted minor volcanic conglomerates, pebbly volcanic sandstones and coal beds in the
type Angora Peak sandstone, but these lithologies were not observed
in the limited exposures of Angora Peak sandstones in this thesis area.
The member cannot be traced laterally (due to landslide cover) from
the type section six miles to the south into the thesis area. However,
Smith (1975) traced and mapped extensive exposures of the Angora
Peak sandstones from the type area northward to within three miles
of the Angora Peak outcrops mapped in this thesis area.
In their geological engineering study of Clatsop County,
Schlicker and others (1972) did not differentiate the Angora Peak
57
sandstones but included them with other strata as undifferentiated
Oligocene through Miocene sedimentary rocks. They did, however,
map Astoria Formation, which they restricted in definition to the
Angoria Peak-like sandstones, along Canyon Creek east of Tillamook
Head.
However, a field check along Canyon Creek revealed only
bedded Silver Point sandstones and mudstones.
Niem and others (1973) and Cressy (1974) correlated the Angora
Peak sandstone as an age equivalent with the type Astoria Formation
at Astoria, Oregon, to the north, and with the Astoria Formation
mapped at Newport, Oregon, to the south on the basis of faunal
similarities.
Silver Point Mudstone Member
Nomenclature. The name Silver Point mudstone member of the
Astoria Formation was informally applied by Smith (1975) to a 650foot thick, well-bedded sequence of laminated, dark gray, micaceous
mudstones and siltstones with subordinate thin, calcareous, fine-
grained, graded turbidite sandstones. The type section is exposed
at Silver Point, approximately
1
1/2.miles south of Cannon Beach,
Oregon and 1/4 mile south of the thesis area.
The unit conformably
overlies the Angora Peak sandstone member and is overlain with
angular unconformity by middle Miocene Depoe Bay basaltic breccia
in the Silver Point-Hug Point State Park area (Smith, 1975).
58
Distribution. The Silver Point mudstone member is the most
widespread unit in the map area (Plate I and Figure 14). The mudstones are extensively exposed in the western and central parts of the
map area from Cannon Beach northward along the coast to West Point
near Seaside, and along U. S. 101, the lower Necanicum River, the
tributaries of Elk Creek, and the hills surrounding the rugged basaltic
mountains (unnamed) immediately north of Kidder's Butte and northeast of Sugarloaf Mountain (Figure 19). The member forms, low to
intermediate slopes and hilly landslide topography between ridges and
rugged peaks composed of resistant Depoe Bay or Cape Foulweather
Basalts. Storm waves have undercut many coastal cliff exposures
of the Silver Point mudstone member, resulting in many destructive
landslides along the coast such as at Ecola State Park (see Plate I).
Lithology and Structure. The lower part of the Silver Point
member in the thesis area consists of dark gray mudstones rhythmically interbedded with thin light gray siltstones and well-bedded,
laminated fine-grained, feldspathic, "graywacke" sandstones. The
upper part of the Silver Point member consists of well-bedded,
laminated,. dark gray, micaceous mudstones and.lighter gray siltstones. Within the upper Silver Point thin bedded mudstones are
isolated, dark gray, massive to thick-bedded, silty, very fine-grained
sandstones. Because only partial stratigraphic sections could be
60
measured (Appendices I and II), the total thickness of the Silver Point
member could be approximated only based on regional dips and out-
crop distribution of the strata between the Angora Peak sandstone
and middle Miocene basaltic breccia. The Silver Point member is
calculated to be from 600 to 1,000 feet thick.
The lower Silver Point member is well exposed between Indian
Beach and Crescent Beach in Ecola State Park where more than 400
feet of rhythmically alternating graded sandstones and mudstones are
exposed in seacliffs and landslide scarps (see measured section,
Appendix I). Mudstone interbeds in the lower part of the member are
commonly dark gray (N3) and composed of structureless to thinly
laminated, micaceous, carbonaceous and silty clay. Beds range from
an inch to 12 feet in thickness; thick beds commonly contain ellipsoidal
calcareous concretions (up to eight inches in diameter and two feet
in length). The interbeds also contain some one quarter- to two-inch
thick, light gray, micrbcr6ss=larffitiat6d siltstohe beds. Lower Silver
Point t-nudstbnes contain pelecypods, gastropbds,
and
forarninifers
(`see Appendix V).
Sandstones interbedded with these mudstones are olive gray
(5Y3/2) to yellowish gray (5Y7/2) when fresh, and they weather dusky
yellow (5Y6/4) to grayish brown (5YR3/9). They are commonly ironstained and
crumbly; The dirty, feldspathic wackes (up to 30%
matrix) are poorly to moderately sorted, medium- to very
61
fine grained, and composed dominantly of angular to subrounded
quartz, feldspars, micas, and carbonized plant fragments.
Well-indurated, carbonate cemented sandstones occur as resistant ledges: in the more easily eroded dark mudstones in seacliffs
at Ecola State Park (Figure 20). `Sandstones range from a few inches
to over eight feet in thickness and are uniform in thickness over tens
to hundreds of feet. Many sandstones are graded, being coarse- and
medium-grained sand at the bases, and medium- or fine-grained sand
in their upper parts. Basal contacts with the underlying mudstones
are usually sharp and/or undulatory; top contacts are generally gradational into the overlying siltstones and mudstones. Sedimentary
structures in individual graded sandstone beds include structureless
and/or parallel laminations, overlain by micro-cross-laminations and
convolute laminations, overlain by very fine parallel laminations.
This sequence of sedimentary structures is identical to the A, B, C,
and D divisions defined by Bouma (1962) for turbidite sandstones
(Figure 21). Parallel laminations defined by concentrations along
bedding planes of carbonized plant debris and very coarse grained
muscovite and biotite.are the most common internal sedimentary
structures. Other sedimentary structures include load coasts, flames,
pillow and ball structures, mudstone rip-ups (up to three inches long),
microfaults, and rate flute marks. Vertical and subparallel burrows
are found on the top surfaces:of several sandstone beds. A deep-water
62
Figure 20.
Well-bedded turbidite sandstones and mudstones of the
Silver Point mudstone member exposed south of Indian
Beach in Ecola State Park. (SW1/4 NW1/4, section 18,
T.5 N., R.10 W.)
Figure 21. Close-up of thin turbidite sandstones and mudstones in the
Silver Point member. Graded sandstones display basal
parallel laminations overlain by convolute laminations
(see pen). Flame and load structures are present above
pen cap. (Same location as Figure 13)
63
.marine fauna in the mudstones,, displaced pelecypod fragments and
forams in the sandstones, repeated graded bedding, ripped-up mud-
stones clasts, abundance of detrital clay matrix, sharp bottom con-
tacts and gradational upper contacts, the presence of A, B, C, and
D divisions, of the Bouma sequence suggest that these sandstones
were deposited by turbidity currents. The lower Silver Point mem-
ber is similar in lithology, sedimentary structures, and sandstone to
mudstone ratios to the "D" or distal turbidite facies defined by Walker
and Mutti (1973).
Turbidite sandstones interbedded with mudstones are also wellexposed in the SE1/4 of section 25, T. 5 N. , R. 10 W. These
weathered sandstones differ from the turbidites of Ecola State Park in
that they contain little or no mud matrix, have higher sandstone to
mudstone ratios, are more friable and iron stained, and are coarser
grained. The deposits show repeated graded bedding, units A, B, C,
and E of the Bouma sequence, and sharp bottom contacts with gradational upper contacts with the overlying mudstones (Figure 22). This
sequence is interpreted as a more proximal turbidite facies of the
lower Silver Point mudstones.
In the SE1/4 SW1/4 of section 6, T. 5 N. , R. 10 W. above
Canyon Creek on Tillamook Head, a.logging road exposure of interbedded thin sandstones and mudstones contains abundant complete
.leaf imprints in the sandstones. The leaves were identified by R.
64
Dennis, Department of Botany, Oregon State University, as Alpus B.
Ehrh.
,
Family Betulaceae (alder:); Quercus L. , Family Fagaceae
(oak); Ceanothus L. , Family Rhyamnaceae (buckthorn); and Acer L. ,
Family Aceraceae (maple).
The plants indicate a paleoclimate very
similar to the modern climate (R. Dennis, personal commun. , 1973).
The occurrence of abundant complete leaf imprints plus the lack of
graded bedding and other sedimentary structures indicative of turbidity currents suggests that not all of the interbedded sandstones in
the lower part of the Silver Point member are turbidites.
The rhythmically interbedded turbidite sandstones and mudstones of the. lower Silver Point member grade upward over a distance
of a few hundred feet into thinly bedded silty mudstones that character-
ize the upper Silver Point mudstones. This lithologic change can be
traced from the beach at Ecola Point to the top of the Ecola State Park
landslide scarp (section 18, T. 5 N. , R. 10 W. ). The thinly bedded
.
upper Silver Point mudstones are also well exposed at Haystack Rock
and in the seacliffs directly east of Haystack Rock.
The bedded silty
mudstones are light olive gray (5Y6/1) to grayish orange
(10R7/4)
and form beds from six inches to two feet in thickness. They are
interbedded with much less abundant,. very thin (one inch to two inches
thick), light gray, tuffaceous siltstones and very fine-grained sand-
stones. These siltstones are laminated and more rarely micro-crosslaminated. Individual mudstone beds are burrowed, laminated., or
65
structureless. Sparse gastropod and shell fragments occur throughout the upper Silver Point mudstones, and forams are common.
Several very thin, whole and fragmented pelecypod fossil horizons
were found near the top of the Ecola State Park landslide scarp east
of Ecola Point (see Appendix I).
Small (less than six inches in dia-
meter) calcareous concretions as well as scattered glauconite pellets
are also present.
Thick sequences (up to 100 feet) of structureless to finely
laminated, very fine--grained sandstones and coarse-grained siltstones
occur locally interstratified within the upper part of the Silver Point
mudstone member on Tillamook Head (Figures, 19 and 23).
The dark
siltstones form five-to thirty-foot thick beds and are predominantly
composed of quartz, potassium
feldspar, and mica in a clay-rich
matrix. Diagenetically altered glass shards, pumice, and decomposed volcanic rock fragments comprise much of the matrix. Fresh
rock exposures are yellowish gray (5Y8/1) to light bluish gray (5B7/1)
and weather to dusky yellow (5Y6/4) iron stained blocks (Figure 23).
The thick tuffaceous siltstones commonly contain a mottled texture
due to extensive burrowing which obliterated most bedding planes.
Small pyrite concretions, rare broken pelecypod fragments, fish
scales and
siltstone.
vertebrae, and fecal pellets are also contained within the
'6 6
Figure
2 2.
Graded turbidite sandstones in the Silver Point mudstonemember of the Astoria Formation. Divisions A, B, and
C. of the Bouma sequence are visible (arrow points up).
(SE1/4 SE1/4, section 25, T. 5 N. , R. 10 W. )
H-
Figure 23. Clastic dike of dark mudstone truncating blocky,; light
gray tuffaceous siltstones of the Silver Point member
(bedding is dipping slightly to right). (NE1/4, section 6,
T.S N., R.10 W.)
67
Petrology. Twelve thin sections of turbidite sandstones. in the
Silver Point member were examined microscopically. The sandstones are very fine- to medium-grained, clay-rich (up to 30%), and
moderately sorted; individual clasts are mainly elongate, angular to
subangular, and are commonly aligned parallel to bedding. Five
samples are feldspathic wackes, plotting on or near the feldspathic
sandstone - lithic wacke boundary on Williams and others, (1954)
ternary classification diagram (Figure 12).
The sandstones are composed dominantly of quartz (19-30%),
plagioclase (9 15%), potassium feldspar (5-10%), rock fragments
(11-20%), carbonized plant matter and foram tests (up to 5%). Minor
framework components (less than 4%) include micas, glauconite,
chalcedony, chert, shell fragments, and a variety ,of heavy minerals
(see modal analyses, Appendix VI). In thin section, individual sandstones are characterized by many light laminae of calcite cemented,
angular quartz, feldspar, and rock fragments alternating with darker,
thinner laminae of silty clay composed of abundant micas and comminuted carbonized plant fragments (Figure 24). Strained (undulatory)
quartz is slightly more abundant than unstrained (normal) quartz and
polycrystalline quartz is rare (less than 1%). Chert occurs in small
quantities (less than 4%). Much of the polycrystalline and micro-
crystalline quartz (chalcedony and chert) appears to be diagenetic in
origin, selectively infilling small molds of plant fragments.
-
—
68
Figure 24. Large, dark mudstone rip-up (A) and Foraminifera test
in laminated, calcite cemented, clay-rich, feldspathic
wacke of the Silver Point member. Dark linear filaments
are crushed carbonized plant fossil fragments. (Plain
light)
-
Figure 25. Fine-grained, poorly sorted, clay-rich Silver Point turbidite sandstone. Note angular to sub rounded framework
grains, Foraminifera tests (A), and glauconite (B).
69
Unlike the coarse-grained Angora Peak sandstones, the finegrained Silver Point sandstones contain more plagioclase (9-15%) than
potash feldspar (5-10%) (see Appendix VI).
This difference in rela-
tive abundance of feldspar types is thought to be a function of the
different grain sizes in the two units. Detrital plagioclase grains
(labradorite), probably derived from finely crystalline basalts, are
consistently significantly smaller than potash feldspar grains
(possibly derived from coarser crystalline plutonic rocks). Plagioclase varies in composition from oligoclase (An37) to bytownite
(An
50-70
).
Potassium feldspar includes similar percentages of both
orthoclase and microcline. Plagioclase and potassium feldspars of
the same composition, respectively, range within each sandstone
sample from fresh grains to grains highly altered to calcite, finely
disseminated white micas (sericite?), and clay minerals. As discussed in the Angora Peak sandstone Petrology section, this range-in
alteration and composition suggests mixing of detritus from different
provenances and a source area of steep relief in a warm, humid
climate.
Altered microcrystalline basaltic rock fragments, with inter-
sertal and intergranular textures, and laminated, dark brown, carbonaceous mudstone and siltstone rip-ups comprise the majority of
the lithic constituents (Figure 25). Small quantities of pumice,
andesite clasts with pilotaxitic texture, denitrified glass shards, and
70
very rare metamorphic quartzite. and carbonate rock fragments
intrasparite (Folk, 1962) or grainstone (Dunham,
1962)]
are present.
Silver Point turbidite sandstones at Ecola State Park contain
pelecypod shell fragments and abundant foram tests (up to 5%) (Figure
22).
Solution of many of these fossils (Figure 25) has produced
moldic, reduced moldic, or enlarged moldic porosity (up to 2% of the
rock) (Choquette and Pray, 1970). Weldon Rau (written commun. ,
1974) identified the following Foraminifera from thin sections:
Bulimina cf. B. ovata d'Ordigny, Globigerina spp. , Bolivina advena
Cushman, Dentalina spp.
,
and arenaceous varieties.
Very coarse-grained micas, mainly biotite with minor muscovite, comprise up to 4% of the Silver Point sandstones. Commonly
aligned parallel to stratification, the micas are relatively undeformed,
indicating compaction was not severe. Glauconite pellets and rounded
aggregates of glauconite constitute up to 3% of the sandstones.
The most common heavy minerals are hypersthene, opaques,
zircon, and pink garnet (possibly grossularite or almandite). Grains
of monazite, augite, micas, and green hornblende were less abundant,
and trace amounts of tourmaline were found. Approximately half of
the heavy minerals are opaques: magnetite, hematite, pyrite, ilmenite, and leucoxene. Heavy minerals comprise less than 1% of the
sandstones by weight in the 3. 50 to 4. 0 size fraction.
71
Silver Point sandstones contain from 20 to 32% matrix (see
modal analyses, Appendix VI). The detrital clay matrix and high
degree of grain angularity makes these sandstones texturally im-
mature. Fine silt-sized quartz., micas, and feldspar
grains and
abundant, dark, carbonaceous clay minerals form the matrix. X-ray
diffraction of the -clay minerals indicates the presence of montmoril-
lonite, chloritic intergrades (mixed layer chlorite and mica), mica,
0
and a 9. 1 A zeolite, possibly clinoptilolite (see Appendix IX). Hydro-
gen peroxide treatment of the matrix of several sieved samples shows
an organic content from 2 to 4% by weight.
The sandstones are cemented by -detrital authigenic clays and
calcite cement (Figures 24 and 25). Calcite cement constitutes from
5 to
35%
of the samples and is associated with concentrations of foram
tests and shell fragments in the lighter laminae. The spatial relationship between the carbonate cement and the zones of shell con-
centrations, plus the-existence of secondary moldic porosity elsewhere in the samples, suggest that much of the carbonate -cement was
produced by dissolution of foraminiferal tests and shell fragments.
This Ca++ and CO3
enriched solution probably migrated into the
more permeable sandy laminae where it was precipitated as pore-
filling microspar and sparry calcite. Alteration of calcic plagioclases
could also have contributed calcite for the -cement.
72
The dark gray, silty mudstones that form the major portion of
the Silver-Point member consist of scattered Foraminifera, angular
silt-sized grains of quartz, feldspars, muscovite, chlorite, and up to
7% by weight of carbonized plant fragments and finely disseminated
organic matter in dark clay. X-ray diffraction of six samples reveals that the clay-size fractions consist of montmor-illonite, mixed
layer chlorite-micas, mica, and the zeolite, clinoptilolite (see
diffraction analyses,. Appendix IX).
Contact Relations. Evidence that the lower contact of the
Silver Point member may interfinger with the underlying Angora Peak
sandstone member has been discussed previously under Contact
Relations of the Angora Peak sandstones. The Silver Point member
in the thesis area is overlain with angular unconformity by middle
Miocene Cape Foulweather and Depoe Bay basaltic breccias. The
contact is commonly covered by twenty to several hundred feet of
colluvium. There are, however, two notable exceptions.
The sharp contact between the Silver Point mudstones and the
overlying Depoe Bay breccia is exposed at an elevation of 1, 200 feet
along a logging road in the mountains north of Kidder's Butte (section
33, T. 5 N.
,
R. 9 W. ).
In this small exposure, the basaltic breccia
appears to conformably overlie the Silver Point member. However,
the exact contact relationship between the units is unclear due to
73
partial cover by colluvium and deformation of Silver Point strata by
a nearby thick Depoe Bay Basalt sill.
An angular unconformable contact between the Silver Point
member and overlying Cape Fou.lweather basaltic pillow breccias is
well-exposed during low tide at Haystack Rock between the towns of
Tolovana Park and Cannon Beach.
Bedded upper Silver Point mud-
stones dip 400 to the west, and are truncated by nearly horizontal
pillow lavas of Cape Foulweather Basalt. Further discussion of this
angular unconformity is presented in the Contact Relations section
of the Cape Foulweather Basalt.
Age and Correlation.
The Silver Point member of the Astoria
Formation is assigned a middle Miocene age based on stratigraphic
relationships and microfossils. Foraminiferal suites collected at
scattered locations throughout the Silver Point member (see Plate I)
indicate Saucesian to possible Relizian ages (middle Miocene) for
the Silver Point member (Rau, written commun. , 1974). Some of the
more commonly occurring species are Bulimina advena Cushman,
Buliminella subfusiformis Cushman, Virgulina californiensis Cushman, Sigmoilina tenius (Cziek), and Clavulina sp. as well as radio-
laria, diatoms, and arenaceous Foraminifera. A complete checklist
of faunal assemblages is presented in Appendix V.
74
The Silver Point member overlies and possibly interfingers in
its lower part with the Angora Peak member. The Angora Peak member has been dated as middle Miocene from molluscan fossils col-
lected in the Angora Peak area (Cressy, 1974). Foraminifera
collected within 20 feet of the upper contact of the Silver Point
member with the overlying Cape Foulweather basaltic breccia at
Haystack Rock indicate a Saucesian (middle Miocene) age (minimum)
for the Silver Point mudstones. In addition, a mudstone interbed
within the overlying Depoe Bay basaltic breccias on the northwest
slope of Sugarloaf Mountain (see Plate I) contains middle Miocene
Foraminifera (Rau, written commun. , 1974). Middle Miocene radiometric dates of 14 ± 2. 7 m. y. and 16 ± 0. 3 m. y. from Depoe Bay
Basalt dikes and sills (Turner, 1970, and Niem and Cressy, 1973)
which intrude Silver Point strata at Ecola State Park also indicate
a minimum age of middle Miocene for the Silver Point member.
Ages of gastropods and pelecypods collected in the Silver Point
member at two different coastal locations were indeterminate, but
based on the appearance of Nucula Hannibali Clark (a pelecypod), the
strata may be tentatively as old as Blakely Stage (Addicott, written
common. , 1974).
The complete checklist of megafossils is presented
in Appendix V.
The Silver Point member can be traced from the thesis area
south to Silver Point, the type locality. The litholog'ies, textures,
75
mineralogy, and sedimentary structures for the Silver Point member
described in this study closely correspond to those described by
Smith (1975) at the type locality. On the basis of similar microfossil
assemblages, this member correlates in a time-stratigraphic sense
with Howe's (1926) .middle shale member of the type Astoria Formation
at Astoria, Oregon (Rau, personal commun.
,
1974).
Strata which comprise the Silver Point mudstone member-in this
report were mapped in part with the Oswald West mudstones and
Angora Peak sandstones in the thesis area as undifferentiated
"Oligocene to Miocene sedimentary rocks" by Schlicker and others
!972) and as "marine sedimentary rocks" of the Astoria Formation
by Wells and Peck (1961).
Depositional Environments of the Angora Peak
and Silver Point Members of the
Astoria Formation
Due to the limited exposures of Angora Peak sandstones in the
thesis area, only a few inferences about depositional environment can
be made. The sandstones are moderately sorted, lack detrital matrix,
and grains are subrounded, suggesting prolonged current reworking
and winnowing of fine detritus. Parallel laminations, sorting, and
rounding in Angora Peak sandstones are similar to those formed in
modern shallow marine shelf sands or in delta front sheets dominated
by high-energy wave and tidal currents (Selley, 1970). Extensive
76
exposures of Angora Peak sandstones immediately south of the thesis
area which contain abundant very shallow marine fossils were inter-
preted by Cressy (1974) as coastal spits, bars, and delta front sheet
sands. Cressy (1974) and Smith (1975) also recognized local channels,
cross-bedded, fluvial conglomerates, thin coal seams, and carhonaceous silty interbeds within these shallow marine Angora Peak sandstones. They interpreted this interfingering sequence of shallow
marine sandstones and fluvial conglomerates as distributary, channels,
swamps, and coastal marshes within a high energy, delta complex.
Coastal processes off the Oregon coast during the Miocene were
probably similar to those acting today. Perhaps wave, current, and
tidal action was too vigorous to allow seaward progradation of the
subaerial part of the delta. Delta plain sediments, may have been reworked by strong tidal currents and wave action, and redistributed by
longshore currents along the coast to build extensive coastal barriers
similar to the nearby beach sands that form Clatsop Spit associated
with the Columbia River. The delta front sheet- sands of Cressy
(1974) and Smith (1975) might conceivably have changed northward
into linear clastic shorelines (Selley, 1970) in the study area. This
depositional model compares favorably with the modern wave-domin-
ated, sand-rich Orinoco (van Andel, 1967), Niger (Allen, .1970), and
-
Rhone (Oomkens, .1967) deltas. The -sands in these deltas are re-
worked and concentrated by shallow marine -processes and
77
redistributed along the coastlines as a series of coastal spits, barrier
bars, and shallow marine shelf sands. The sands contain sorting,
rounding, and sedimentary structures similar to those in the Angora
Peak sandstones.
Fossils, sedimentary structures, and lithologies suggest that
the overlying and possibly inte rf inge ring Silver Point mudstone mem-
ber was deposited in an open marine, quiet-water environment of
moderate depths on the continental shelf. Foraminiferal suites from
the mudstones indicate that deposition of sediment occurred in cool
water of sublittoral to upper bathyal depths (200-1,000 feet) (Rau,
written commun.
,
1974).
Paleoecological interpretation of the
molluscan fossils from modern forms (Stanley, .1970) indicates that
these fossils lived in shallow, open marine waters of depths less than
800 feet. Addicott (written commun.
,
1974) considers the molluscan
fossils to be indicative of middle to outer sublittoral (middle to outer
neritic or shelf) depths. The abundance of unbroken thin molluscan
shells, burrows, laminations and general fine-grained nature of the
sediments (muds and silts) indicates continuous pelagic sedimentation
in a low energy environment. Fluctuating, weak currents probably
reworked pelagic and terrigenous silts into thin, micro-cross-laminated interbeds. Extensive burrowing produced massive s,iltstones and
mudstone:beds. Periodically, turbidity currents transported feld-
spathic sands into this low energy environment to form the graded
78
sandstones in the lower part of the Silver Point mudstones.
Medium- to fine-grained sandstones interbedded with the silty
mudstones contain parallel laminations, convolute laminations, micro-
cross-laminations, fossils, scattered burrows, grading, and rare
glauconite pellets; these are common features in modern upper delta
front platforms (Allen, 1970; Donaldson and others, 1970) and delta
slope deposits (Selley, 1970). Turbidite sands similar to those in
the lower Silver Point member at Ecola State Park have been found in
deep sea cores within modern prodelta environments (Selley, 1970);
Visher, 1965; Bouma, 1962). Burrowed,.deep marine, fossiliferous,
fine-grained, non-graded sandstones interbedded with laminated
mudstones of the Silver Point member are similar in lithologic character to the lower delta-platform or to the upper prodelta sediments
of the modern Niger.(Allen, 1970), Guadalupe (Donaldson and others,
1970), and Mississippi deltas (Gould, 1970). The bedded Silver Point
-
mudstones contain Foraminifera indicative of deposition in outer
shelf depths (Rau, written commun. , 1974). The thick, structureless
to thinly laminated, tuffaceous siltstones in the upper part of the
Silver Point mudstones may represent deposition in a delta-front
environment or a current produced shoestring sand or tidal current
ridge on the continental shelf (Off, .1963). The shifting of distributary
channels and temporary progradation and regression of the delta front
sands within the delta system could have produced the interfingering
79
relationship between the lower Silver Point mudstones and the upper
Angora Peak sandstones.
.An alternative depositional model for the Silver Point and Angora
Peak members could be the gradual infilling by pelagic muds and silts
of a marginal basin (now represented by the Silver Point member) on
the contental shelf adjacent to a linear continental coastal plain (represented by the Angora Peak sandstone).
Most Silver -Point and Angora
Peak lithologies and sedimentary structures (with the exception of
the turbidite sandstones) are found, for example, in a modern linear
clastic shoreline system (Selley, 1970). Turbidites could have been
formed by slumping of oversteepened sand and mud accumulations on
an irregular shelf topography. Subsequent sands transported by
turbidity flows funneled down local downwarped basins on the outer
shelf onto small submarine fans. Variations in subsidence and sedi-
mentation rates and/or fluctuations in sea .level may have resulted in
the interfingering and gradual transgression of deeper water muds
(Silver Point member) over the shallow coastal sands (Angora Peak
sandstones).
The turbidite sandstones in the lower Silver Point mudstones
may represent a gradual upward transition from the coarse-grained,
delta front or shallow marine shelf sandstones (Angora Peak sand-
stones) into the overlying, finer grained., interbedded silty sandstones
and bedded upper Silver Point mudstones of an open shelf environment
80
in a linear clastic shoreline system during a gradual marine transgression.
Depoe Bay Basalt
Nomenclature
Snavely and others (1973) defined two middle Miocene volcanic
petrologic types, the older Depoe Bay Basalts and the younger Cape
Foulweather Basalts in the Oregon Coast Range on the basis of differ-
ent ages, chemistries, lithologies, and stratig.raphic positions. The
type locality,for the Depoe Bay Basalt is at the coastal town of Depoe
Bay, located in the Central Oregon Coast Range approximately 80
miles south of the thesis area. The unit consists of submarine pillow
basalts and breccias, rare subaerial flows, and related dikes, sills
and irregular intrusive bodies unconformably overlying the middle
Miocene Astoria Formation. In hand specimen, Depoe Bay Basalt
is aphanitic to finely crystalline and non-porphyritic. Chemically, the
basalt is characterized by a high content of SiO2,, total iron oxide,
and alkalies. These basalts have similar chemical composition, age,
and petrology to the Yakima Basalts (Waters, 1961)' of the Columbia
River Group of eastern Oregon and Washington (Snavely and others.,
.1973).
Because of the localized eruptive nature of Coast Range vol-
canic rocks, the basalt units of this study are referred to in a
81
petrologic sense and are not strictly stratigraphic units. The petro-
logic correlation is based on similar chemistry, petrography, age,
and stratigraphic relationships to the type Depoe Bay and Cape
Foulweather Basalts defined by Snavely and others (1973).
Intrusive Rocks
Distribution,, General Features, and Contact Relations. Numer-
ous dikes, sills, and irregular intrusions of Depoe Bay Basalt intrude
the Oswald West mudstones and Angora Peak and Silver Point mem-
bers of the Astoria Formation throughout the thesis area. Due to
dense vegetation and landslide cover, the distribution and contacts of
the basaltic intrusions are in part approximated on the geologic map
(Plate I). Contacts between resistant basalts and the sedimentary
units were determined from quarry, roadcut, and stream bank exposures and traced laterally based on changes in slope (steep to
gentle) recognized in the field and on aerial photographs.
The most prominent Depoe Bay Basalt intrusion is the 900 -foot
thick Tillamook Head sill which dips 100 to
200
southeast and crops
out along the coast from West Point south to Indian Beach (Plate I).
The sill forms Tillamook Head, a sheer wave-cut basaltic headland
containing well developed vertical columnar -jointing (Figure 3). The
interior of the sill is slightly more coarsely crystalline than the
aphanitic to finely crystalline perimeter. The concordant bottom
82
contact is exposed in the seacliffs north of West Point (section 30,
T. 6 N. , R. 10 W.) where the sill irregularly overlies and-is in fault
contact with the Oswald, West mudstones. The top contact of the sill
with the, overlying Silver Point member is visible along Indian Beach
about four miles south of West Point. Many of the basalt sea stacks
in Ecola State Park are erosional remnants of the upper part of the
sill. The upper contact is sharp and highly irregular with several
large dike-like apophyses intruding and locally severely deforming
the overlying mudstones (Figure 26). The unnamed point north of
Ecola Point is an example of such an apophysis. Locally near the
contact, the intrusive is extensively brecciated and has incorporated
fragments and blocks of the overlying mudstone (Figure 27). The
breccia is composed of angular basalt fragments in an altered light
green clay groundmass consisting of mixed-layer montmorillonite--
mica chlorite, and a zeolite (possibly clinoptilolite).
Other isolated exposures of thick sills which are probably the
continuation of the Tillamook Head sill, are found over a several
square mile area east and north of Tillamook Head (Plate 1). The
more prominent exposures occur along U. S. Highway -101 in SEl/4
sec. 17, T. 5 N. , R. 10 W.., in the quarries along both sides of U. S.
Highway 26/101 in NE 1/4 sec. 4, T. 5 N., R. 10 W.
,
near Seaside,
and in numerous roadcuts and quarries along the Necanicum River
in T. 5 N. , R. 10 W. These thick finely crystalline sills with well
83
Figure 2
ike-like apophyses (bs) of the underlying Tillamook Head
sill intruding Silver Point mudstones along Indian Beach
in Ecola State Park. Note irregular contact and radiating
columnar joints.
Figure 27. Irregular contact between Silver Point mudstones (A) and
an overlying brecciated Depoe Bay Basalt intrusive (B) in
Ecola State Park south of Indian Beach. Breccia is composed of fragments of altered green and white basalt and
incorporated mudstone clasts.
84
developed columnar jointing dip approximately 150 south and south-
east. Although at isolated exposures the Tillamook Head sill appears
to have concordant contacts with the surrounding Silver Point strata,
the southeast dipping sill regionally cuts across gently- northwest
dipping Silver Point strata (see cross-sections, Plate II). Thus,
this tabular, nearly horizontal intrusion is really "sill-like" rather
than a sill defined in the classical geologic sense.
Numerous smaller dikes and thin sills of Depoe Bay Basalt are
well-exposed at Ecola State Park in seacliffs at Ecola Point and along
the unnamed beach between Bald Point and the unnamed point north of
Ecola Point. These dikes and sills intrude severely, deformed lower
Silver Point strata (Figure 38) and are locally brecciated along their
margins. Brecciation and folding were probably produced by steam
explosion and rapid quenching of hot magma during .intrusion and dis-
placement of water-saturated., semi-consolidated sediments (see
chapter on Structure). These smaller intrusions in Ecola State Park
are thought to be related to the thick, underlying Tillamook Head sill.
Schlicker and others (1961) mentioned lava flows in Ecola State Park,
but only intrusives were found by the author.
Other prominent Depoe Bay Basalt intrusives include a 2.00-foot
thick sill which underlies Depoe Bay basaltic breccia capping the
unnamed hills north of Kidder's Butte in the eastern part of the area
(Plate I), and a long resistant irregular intrusion (Figure 28) which
85
Figure 28. Depoe Bay Basalt intrusive exposed in a quarry northwest
of Sugarloaf Mountain. Note horizontal jointing near base
of exposure and vertical columnar jointing at top.
(NW 1/4 SW 1/4, section 36, T. 5 N. , R. 10 W. )
86
forms a linear ridge extending two miles northwest from the breccia
covered slopes of Sugarloaf Mountain (See Plate I).
Lithology and Petrology. Sills and dikes of the Depoe Bay
Basalt petrologic type are aphanitic to finely crystalline, medium
gray (N5) to greenish black (5GY2/1) in color, and weather to a
greenish black (5GY2/1) or iron stained light olive brown (5Y5/6).
These intrusions are characterized by well developed columnar
jointing and have very thin, red, baked, cooling contacts with the
adjacent rocks.
The basalts are composed of equigranular, calcic plagioclase
laths (45-55%), subhedral crystals of clino- and orthopyroxenes
(20-30%), opaque minerals (magnetite and ilmenite) (4-10%), and
trace amounts of apatite, and olivine. Mineral alteration products
include biotite (2-3%), actinolite of hornblende (0-22%), chlorophaeite
(0-4%) and a silicic dark, glassy residuum in the ground mass. The
basalts are texturally intergranular, intersertal, or sub--ophitic
depending on the degree of crystallization (Figure 29).
Modal analy-
ses of three typical Depoe Bay sills, including the Tillamook Head
sill, are displayed on Table 1.
Subhedral plagioclase laths (1 mm to 3 mm) range in composi-
tion from calcic andesine (An44-49) to sodic labradorite (An53-58)`
Many crystals are progressively zoned, containing sodic labradorite
87
Table 1. Modal analyses. of selected samples of Depoe Bay Basalt.
Db-1
Db-2
Db-3
Plagioclase
52%
47%
51%
Clinopyroxene
20%
20%
27%
O rthopy roxene
12%
-
Opaques
9%
5%
7°10
Apatite
--
2%
2%
Chlorophaeite
--
1%
4%
Hornblende
1%
22%
6%
Biotite
3%
2%
2%
Silicic Residuum
2%
1%
Chlorite
--
Olivine
tr
Glass
--
Sample locations are shown on Plate I.
tr
1%
-
--
tr
88
cores and calcic andesine rims. The plagioclase crystals have been
partly altered to finely disseminated sericite (white mica) and zeolites.
Rare phenocrysts of plagioclase (An46) (up to .3 mm wide and 5 mm
long) occur in some of the thick sills..
Py-roxenes consist of subhedral crystals of yellow orthopyroxene
(enstatite or bronzite) and light green clinopyroxenes (augite, subcalcic augite (2V=30-55), and pigeonite). A few augite crystals
exhibit twinning parallel to the 100 face which has intergrown with the
polysynthetic twins to form herringbone twinning (Figure 29).
Pyroxene crystals commonly are surrounded by reaction rims of,
or completely replaced by a deuteric blue-green pleochroic amphibole
(actinolite or hornblende), chlorite, and biotite. Unusual, small,
thin fracture fillings composed of large apatite crystals intergrown
with fibrous yellow-green actinolite cut the north end of the Tillamook
Head sill (Figure 30). These thin veinlets possibly represent a late
stage crystallization of magmatic fluids injected into newly formed
cooling fractures within the sill.
Small irregular 'patches of yellow to green chlorophaeite and a
silicic residuum composed of finely-intergrown quartz and alkali
feldspar commonly forms the groundmass between plagioclase and
pyroxene crystals in the basalts.
The above petrographic descriptions of basalt intrusives in the
thesis area are similar to those summarized by Snavely and others
I
Figure 29. Typical intergranular texture in weathered Depoe Bay
Basalt from Tillamook Head. Note euhedral crystal of
magnetite and poorly developed herringbone structure in
the augite (A). (Crossed nicols)
I
Figure 30. Fracture filling of actinolite or hornblende (A) with
hexagonal crystals of apatite (B) in Depoe Bay Basalt
from Tillamook Head near West Point. Opaque blebs in
the basalt are magnetite. (C rossed nicols)
90
(1973) for the Depoe Bay Basalt petrologic type found in the Oregon
Coast Range.
This similarity suggests a petrologic correlation be-
tween basalts in the two areas.
Extrusive Rocks
Distribution and General Features. The most extensive volcanic unit exposed in the map area is the Depoe Bay submarine ba-
saltic breccias. They cover more than seven square miles in the
southern part of the map area (Plate I) and form the highest and most
rugged topography.
The resistant basaltic breccias have an aggregate
thickness of over 800 feet in the Z.,000-foot high unnamed hills north
of Kidder's Butte (Figure 2).
Contacts between the volcanic breccias and the underlying
Tertiary sedimentary rocks are commonly covered with a thick breccia talus or dense vegetation. On many mountain slopes in the eastern
part of the map area, differential erosion of underlying less resistant
mudstones has undercut support beneath the resistant breccia hills,
resulting in the development of extensive landslide deposits of breccia
boulders. As a result mapping the volcanic breccia-sedimentary
contacts in the landslide areas was very difficult and the contacts
were approximated along slope breaks.
91
Lithology and Petrology. Depoe Bay breccias consist of mas-
sive, thick, broken pillow and rare isolated pillow lava flows.
Fresh breccias are moderate olive brown (5Y4/4) to greenish black
(5GY2/1) in color, but iron-stained, pale yellowish orange (10YR8/6)
to dark yellowish orange (1OYR6/6) weathered breccias are most
common.
Breccias are composed of dark, angular lapilli- and coarse
ash-sized fragments of aphanitic basalt and basaltic glass (sideromelane and tachylyte) in a yellowish palagonitized fine ash matrix
(Figure 31). A few blocks three to five feet in diameter of incorF
porated mudstone from the underlying Silver Point mudstones are
found in the lower portion of the breccia unit. Small white cavity
fillings and veinlets of calcite, chalcedony, and zeolites occur.
The
breccias also contain rare, isolated elliptical pillows which range
from one to three feet in diameter and contain chilled black tachylyte
rinds up to one-half inch thick.
Pillow interiors are c-omposed of
dense aphanitic basalt. The presence of these dark-glassy borders
distinguishes pillows from more common exfoliation boulders in the
breccia which superficially resemble pillows (Figure-32).
Litho-
logically similar breccia flows have been described as "isolated-
pillow breccia" and "broken-pillow breccia" by Carlisle (1963) for
Triassic volcanics on Vancouver Island..
They are thought to have
it
-' $Ej
1
0
I
-
.'..
.V
'
i
to
j
.
6
.
.
.11
0
f- o
w
i
epoe Bay basaltic breccia. Darker fragutcrop
ments are basaltic glass and lighter colored chunks are
aphanitic basalt.
c
Figure
Figure 32. Exfoliation boulders of Depoe Bay basaltic breccia. This
weathering pattern is typical of the basaltic breccias.
93
formed by rapid quenching and shattering of freshly erupted submarine basalts when they came in contact with sea water.
Angular aphanitic basalt fragments in the breceias are petrographically similar to those described in the Depoe Bay Basalt intrusives. The basaltic glass fragments are hyalopilitic, containing
randomly oriented, scattered micro-phenocrysts of labradorite and
augite (up to 1 mm in length), microlites of plagioclase, and slender
prisms of augite in a groundmass of light yellowish brown to translucent basaltic glass (sideromelane) (Figure 33). Some sideromelane
has altered to light yellow brown or bright orange palagonite along
t
crystal borders and fractures. The matrix between angular aphanitic
basalt and glass rock fragments consist of fine ash-sized fragments
and slivers of palagonitized basaltic glass and minor aphanitic basalt.
Euhedral crystals of apophyllite, heulandite, chalcedony, and calcite
cement the breccia fragments together and also partially infill microcavities (Figure 33). The zeolites were identified by X-ray diffrac-
tion patterns.
Contact Relations. Depoe Bay basaltic breccia lies with angular
unconformity over the Silver Point member as previously discussed
in the Contact Relations section of the Silver Point member. The
lower breccia contact varies in elevation from 800 feet on the slopes
north of Kidder's Butte to about 300 feet near the mouth of the north
94
Figure 33. Photomicrograph of cavity fillings of apophyllite and
heulandite (A) between angular basalt glass fragments
(sideromelane) in Depoe Bay basaltic breccia. Note that
dark glass fragments with microlites of labradorite and
augite are extensively altered to orange palagonite around
fragment rims. (Crossed nichols)
95
fork of Elk Creek (See Plate I). Either submarine breccias extruded
over an uneven sea floor (e. g. abyssal hills and submarine canyons)
or later displacement by faults is thought to have produced this large
variation in elevation along the breccia-Silver Point mudstone contact.
No younger Tertiary units overlie the Depoe Bay basaltic breccias in
the thesis area.
Chemistry. Initial field and petrographic identifications of the
Depoe Bay Basalt petrologic type were further substantiated by whole
rock chemical analyses of basalt samples which were plotted on silica
variation diagrams. (Figure 34). The three samples plot within or
very close to the compositional variation limits determined by
Snavely and others (1973) for the Depoe Bay Basalts. The samples
are from the basaltic breccia capping Kidder's Butte, the sill underlying Kidder' s Butte, and from a sill in a state rock quarry near the
lower Necanicum River (geographic locations and chemical analyses
of samples are tabulated in Appendix X; locations are plotted on Plate
I).
Snavely and others (1973) chemically analyzed six basalt samples
from other locations in the thesis area and assigned five of them to
the Depoe Bay Basalt petrologic type.
The Depoe Bay Basalt in the thesis area is characterized by a
high content of SiO 2 (55.6%), iron oxides (13. 0%), and alkalies (4. 0%),
and a low percentage of alumina (13. 3%). These chemical percentages
96
12
16
I
.10
14
Ca0%
A1203%
12
8
6
N
16
4
Na20%
14
3
12
2
FeO%
2
10
K20%
0
8
6
MgO%
TiO2%
4
2
-
1
-
I
50
I
52
I
54
I
56(SiOg) 50
- Cape Foulweather Basalt
Depoe Bay Basalt
52
54
56
Compositional variation
limits defined by Snavely
and others (1973) for
middle Miocene coastal
basalts.
* Kidder's Butte Breccia (RHN-6)
o Sill underlying Kidder's Butte Breccia (RHN-1)
o Sill along lower -Necanicum River (RHN-5)
A Haystack Rock (RHN -7)
Sill on Tillamook Head (RHN-3)
Twin Peaks intrusive (RHN-4)
1, 3, 4, 5 from Snavely and others (1973) (samples in map area)
Precise sample locations are shown in Appendix X and Plate I.
Figure 34, Silica variation diagram of selected middle Miocene
basalt samples from the thesis area.
97
(plotted on Kuno's (1968) alkali-silica and alkali-alumina-silica
diagrams) support petrographic evidence that these basalts are
tholeiites.
Age and Correlation. Depoe Bay basaltic breccias in the thesis
area lie with angular unconformity. over the Silver Point member
which contains Foraminifera of Saucesian age (middle Miocene) (Rau,
written commun.,, 1974) suggesting a middle Miocene of younger age
for the Depoe Bay Basalt. A late Saucesian or middle Miocene age
for the Depoe Bay Basaltic breccia is also suggested by Foraminifera
collected in a five-foot thick dark gray bedded siltstone and mudstone
interbed in the lower part of the basaltic breccias which from Sugarloaf Mountain (section 35, T. 5 N. , R. 10 W.) (Weldon Rau personal
commun. , 1974). Sills and dikes of Depoe Bay Basalt intrude the
Astoria Formation at numerous localities. Depoe Bay Basalt dikes
in Ecola State Park have been dated by potassium argon radiometric
methods as 14 f Z. 7 m. y. (middle Miocene) by Turner (1970). Niem
and Cressy (1973) reported a radiometric date of 15.9 + 0. 3 m. y. for
the Tillamook Head sill exposed along Indian Beach in Ecola State
Park. The K/Ar ages for the Depoe Bay Basalt intrus;ives at Ecola
State Park and fossiliferous interbeds within the submarine breccias
strongly indicate a middle Miocene age for the Depoe Bay Basalt.
The age, chemistry, petrology, and stratigraphic relationship
98
of the Depoe Bay Basalt mapped in the thesis area is in agreement
with the same characteristics ascribed to the Depoe Bay Basalt
petrologic type elsewhere in the Oregon Coast Range by Snavely and
others (1973).
Cape Foulweather Basalt
Nomenclature
At the type locality near Cape Foulweather, 12 miles north of
Newport, Oregon, the Cape Foulweather Basalt is composed of
pillow lavas, submarine breccias, water-laid basaltic fragmental
debris, and related dikes and sills. Near the town of Depoe Bay,
Cape Foulweather Basalts and the underlying marine, arkosic sandstones of Whale Cove overlie the Depoe Bay Basalt. In hand speci-
men, Cape Foulweather Basalt is characterized by sparse, large
(up to 2 cm long), light yellow plagioclase phenocrysts in dark
aphanitic basalt. These sparse plagioclase phenocrysts distinguish
the Cape Foulweather Basalt petrologic type in the field from the
finely crystalline Depoe Bay Basalt. In addition, the Cape
Foulweather Basalt can be differentiated from the Depoe Bay Basalt
by distinctively higher chemical contents of total iron, TiO2, and
P205 and lower SiO 2 content and also by a younger radiometric age
(Snavely and others, 1973). These basalts correlate chemically,
99
chronologically, and petrologically with the late-Yakima Plateau
Basalt (Waters, 1961) of the Columbia River group of eastern Oregon
and Washington.
Intrusive Rocks
Distribution, General Features, and Contact Relations.
Intru-
sives of Cape Foulweather Basalt are less abundant than Depoe Bay
Basalt in the thesis area. Cape Foulweather Basalt crops out only
in the western half of the map area (see Plate I) as a few thin sills and
dikes in Ecola State Park and near Tillamook Head, and as a 600-
foot thick sill at Twin Peaks. Contacts between intrusives and the
encompassing sedimentary rocks are sharp, and irregular. The
basalt is, in most places, b;recciated near these :contacts.. The
shapes and contacts of the Cape Foulweather Basalt outcrops are
approximated on the accompanying geologic map (Plate I) because the
contacts with less resistant sedimentary rocks are commonly covered
with vegetation and blocks of basaltic talus. In many places only an
abrupt break in slope delineates the contact.
The most prominent intrusive unit of Cape Foulweather Basalt
forms the 1600-foot high Twin Peaks near the town of Seaside. The
lower contact of the intrusive unit is partially exposed in an abandoned, overgrown logging road which climbs the southeast flank to the
top of the peaks.
(Plate 10.
Due to lack of exposure, the geometric
100
form and contact relationships of the intrusion are largely inferred,
but the intrusion appears to be a tabular sill-like body approximately
600 feet thick. A small abandoned quarry near the top of Twin Peaks
exposes Cape Foulweather basalt characterized by an aphanitic tex-
ture with scattered feldspar phenocrysts. Since the contact rela-
tionships of the intrusion are obscured, there remains a possibility
that the intrusion is a volcanic plug or dike.
Other intrusions of Cape Foulweather Basalt occur one mile east
of Tillamook Head and in Ecola State Park. A thin (less than 150
feet thick) Cape Foulweather Basalt sill, stratigraphically overlying
the Depoe Bay Basalt Tillamook Head sill intruding Silver Point mud-
stones, occurs in sections 4 and 5, T. 5 N. , R. 10 W. A few, small,
isolated, Cape Foulweather dikes crop out elsewhere in the thesis
area (Plate I).
At Ecola Point and the unnamed beach between Ecola Point and
Bald Point in Ecola State Park, small dikes (less than 30 feet thick)
and thin sills (less than 60 feet thick) of Cape Foulweather Basalt
intrude both the interbedded lower Silver Point turbidite sandstones
and mudstones and cut across intrusions of Depoe Bay Basalt. The
sills consist of dense, finely crystalline basalt with sparse, scattered
plagioclase phenocrysts and have well-developed columnar joints.
They are locally brecciated near intrusive contacts with the host
sedimentary rocks. Contacts are generally conformable with the
101
bedded mudstones, but they are discordant in places.
Lithology and Petrology. Cape Foulweather sills are medium
gray (N5) to medium dark gray (N4) in color and weather to a medium
gray (N5) to light moderate brown (5YR5/6-5YR4/4). Cape
Foulweather Basalts are characterized by ubiquitous, but sparse,
scattered phenocrysts of plagioclase (up to 3 cm. in length and 1 cm.
wide) in an aphanitic to finely crystalline, dark, dense basalt. Tex
aurally, the basalt is vitrophyric, hyalopilitic, intersertal, or intergranular depending on the degree of crystallization. For example,
some intrusions consist of up to 47% glass (Figure 35;).
In thin section, crystalline Cape Foulweather Basalt is composed
of scattered, large euhedral phenocrysts of labradorite (An62) and
subophitic to ophitic clinopyroxenes, in a groundmass consisting of
micro-crystalline labradorite (An56) laths and interstitial clino- and
orthopyroxenes, (Figure 36) or dark, iron-rich, turbid basaltic glass
(tachylyte) (Figure 35). The clinopyroxenes (augite and pigeonite)
are commonly deuterically altered to chlorophaeite or hornblende.
Rare corroded micro-phenocrysts of olivine are also present. Modal
analyses of three Cape Foulweather Basalt samples from Twin Peak,
a sill near Tillamook Head, and asmall sill at Ecola,State Park range
from 35-41% calcic plagioclase,
2.7-36%
pyroxene,. 10% opaques.,
(magnetite and ilmenite) 5-10% chlorophaeite, and 1-13% chlorite
Figure
Hyalopilitic texture in Cape Foulweather Basalt.
phenocryst of andesine, microlites of plagioclase, and
deuteric chlorophaeite (A) in a groundmass of opaque,
iron-rich glass (tachylyte). (Plain light)
Figure 36. Photomicrograph showing intergranular texture in Cape
Foulweather Basalt. Large, faintly zoned labradorite
phenocryst (albite twinned) occurs in a groundmass composed of plagioclase laths and interstitial pigeonite, augite, and magnetite. (C rossed nicols)
103
(also see Table 2). Rare olivine, orthoclase, hornblende, and an
interstitial silicic residuum of intergrown feldspar and quartz are
also present.
In finely crystalline basaltic intrusives labradorite occurs both
as phenocrysts (up to 3 cm in.length), comprising less than 1% of the
rock, and as subhedral laths (1 mm to 3 mm in length). The phenocrysts are normally zoned and range from calcic andesine (An45)
near the rims to labradorite (An62) in the crystal cores (Figure 36).
The plagioclases are commonly partly replaced by sericite and zeo-
lites. Plagioclases in the hyalopilitic Cape Foulweather Basalts are
andesine (An31
-42)
in composition.
Pyroxenes include orthopyroxene and clinopyroxene varieties.
Subhedral non-pleochroic orthopyroxene, probably enstatite or bronz-
ite (2V>600) occurs mainly as small interstitial crystals in the
groundmass, but also appears as micro-phenocrysts. The clino
pyroxenes, augite (2V< 450) and pigeonite, are present both inter-
stitially in the groundmass and as mic rophenoc rysts. Pyroxenes are
partly deuterically altered to yellowish brown chlorophaeite or con-
tain a surrounding reaction rim of hornblende or chlorite. Olivine
forms a few micro-phenocrysts which are invariably altered to
iddingsite around the crystal periphery. Magnetite occurs as anhedral crystals up to 2 mm in diameter and needles up to 3 mm long
and 0. 25 mm wide. Near some intrusive contacts, the Cape
Table 2..
Modal analyses of selected samples of Cape Foulweather
Basalt.
Cf-1
Cf-7
Cf -8
Plagioclase
41%
37%
35%
Clinopyroxene
27%
4%
14%
O rthopy roxene
--
2%
--
Interstitial Pyroxene
--
30%
--
Opaques
11%
11%
--
6%
10%
10%0
Chlorophaeite
Hornblende
tr
--
2%
Silicic Residuum and
Orthoclase
tr
--
1%
Chlorite
--
1%
tr
Olivine
tr
3%
tr
Glass
Sample locations are shown on Plate I.
--
47%
105
Foulweather Basalts are amygduloidal, containing small vesicles
filled with white calcite, chalcedony, and/or zeolites.
These mineralogical and textural characteristics of Cape
Foulweather Basalt in the thesis area are very similar to those of
the Cape Foulweather Basalt petrologic type defined by Snavely and
others (1973) for the Oregon Coast Range.
Extrusive Rocks
Distribution and General Features. Extrusive Cape Foulweather
Basalt occurs at Haystack Rock, a prominent sea stack near Cannon
Beach. The sea stack consists of'200 feet of palagonitic pillow brec-
cia resting unconformably, on laminated siltstones and mudstones. of
the upper part of the Silver Poi-nt member. These breccias are
associated with small irregular Cape Foulweather dikes that form a
series of smaller stacks, known as the Needles, around Haystack
Rock.
Lithology and Petrology. The basaltic breccias consist of isolated and closely packed brecciated` pillow lavas.
composed of
Breccias are
lapilli-sized, angular, dark pillow fragments well-
cemented by a yellowish brown palag.onitic matrix. Pillow fragments
are dark basaltic glass of aphanitic basalt with scattered large
phenocrysts of yellowish plagioclase that typify Cape Foulweather
Basalt.
Several southwest striking brecciated Cape Foulweather
106
feeder dikes intrude the underlying Silver Point sedimentary beds and
continued upward through the breccia that form the sea stack. The
dikes become increasingly brecciated and irregular upward until they
merge with the extrusive breccia and lose their identity; this suggests
that the stack is an eroded remnant of a small volcanic center on the
Miocene Sea Floor.
Fresh surfaces of the breccia are greenish black (5GYZ/1) and
dark gray (N3) but rapidly weather to dark yellowish orange (10YR
8/6) after exposure. Pillows are elliptical in shape and range from
one to two feet in diameter. Chilled black tachylyte borders on the
pillows are up to one inch thick.
In thin section, breccia framework clasts are composed of
lapilli-sized angular basaltic glass and less common aphanitic basalt
fragments in an indurated fine ash-sized matrix of fragments and
slivers of palagonitized basaltic glass. The margins of the lapillisized glass fragments are altered to a golden brown clay which may
be either saponite or chlorophaeite, both of which are common alteration products of basaltic glass. Finely disseminated chlorophaeite
or saponite also infills small fractures in the breccias. A hyalopilitic texture of scattered euhedral labradorite phenocrysts, randomly oriented microlites of andesine-labradorite and slender
prisms of augite suspended in a brown, turbid, basaltic glass
(tachylyte) characterizes the basaltic glass fragments. Samples of
107
Cape Foulweather Basalts average 35% labradorite,.45% glass, 7%
augite, and 10% clay. Calcite, chalcedony, and euhedral zeolite
clinoptilolite crystals are locally present as small cavity fillings
between breccia fragments.
Contact Relations. Cape Foulweather pillow breccias, the
youngest Tertiary unit in the area, lie with angular unconformity
,over the upper Silver .Point mudstones at Haystack Rock. The contact
is well-exposed and accessible during low tide. An unconformity is
suggested by three lines of evidence.
1)
Thinly interbedded sandstones and mudstones of the Silver
Point member dip 180 to 400 southwest and are trucated by almost
horizontal, overlying Cape Foulweather breccias and pillow flows.
2) Foraminifera collected at Haystack Rock in the Silver Point
strata 20 feet below the contact with the Cape Foulweather breccias
were identified by Weldon Rau (written commun., .1974) as early
Saucesian Stage (early to middle Miocene). The Cape Foulweather
Basalt in the Oregon Coast Range is late-middle Miocene in age
0
(Snavely and others, 1973) and is associated with Relizian Stage microfauna. Possible Relizian Foraminifera have been also identified
in the Silver Point mudstones at Ecola State Park. The: lack of
possible Relizian Silver Point strata at Haystack Rock suggests some
period of erosion or non-deposition.
108
3) Absence-of the areally extensive. older,. 1000-foot thick
Depoe Bay basaltic breccias between the younger Cape Foulweather
basaltic breccia and the underlying Silver Point mudstones also
suggests the presence of an unconformity. However, the absence of
Depoe Bay breccias between Cape Foulweather basalt flows and, Silver
Point strata may be due to the local nature of Depoe Bay breccia
accumulation around vents on the sea floor.
The Depoe Bay breccia and the Silver Point mudstone contact
occurs several hundred feet above sea level in the map area whereas
the contact between the Cape Foulweather breccia and the Silver Point
member is at sea level. The elevation differences of the upper Silver
.Point contact with the overlying lavas suggest that significant erosional relief was developed on the Silver -Point mudstones prior to the
eruption of the basalts, perhaps by mid-Miocene uplift, subaerial
erosion, and late subsidence,. or possibly by formation of a very
irregular sea floor topography (e. g. Submarine canyons). A third
possibility is that post mid-Miocene faulting during the general coast
range uplift produced the differences in elevations now observed.
Chemistry
Whole-rock chemical analyses of three different intrusive and
extrusive samples plotted on silica variation diagrams (Figure 34)
further confirm field and petrographic identifications of Cape
109
Foulweather Basalts. The analyses fall within the chemical compositional ranges of the Cape Foulweather Basalts along the Oregon
coast determined by Snavely and others (1973).
The samples are from Haystack Rock breccia, the sill capping
Twin Peaks, and a thin sill two miles east of Tillamook Head (see
sample locations Appendix X and on Plate I). Snavely and others
(1973) also chemically analyzed a Cape Foulweather Basalt sill exposed on the unnamed beach between Bald Point and Ecola Point in
Ecola State Park (see analysis in Appendix X and plotted on Figure
34).
Major metal oxides in the Cape Foulweather Basalt samples
range f rom 50-53% SiO2, . 13% A1203, 14-15% total iron, 9% C aO,
and 4% total alkalies (K20 and Na2O). The Cape Foulweather Basalt
samples are characterized by a lower Si02 content but higher TiO 2
and total iron content than in Depoe Bay Basalt samples from the
thesis area (see table in Appendix X). These trends were also noted
by Snavely and others (1973) for the Depoe Bay and Cape Foulweather
Basalts elsewhere in the Oregon Coast Range. Metal oxides plotted
on Kuno's (1968) alkali-silica and alkali -alumina- silica diagrams and
mineralogic and petrographic evidence, indicate that the Cape
Foulweather Basalts, as well as the Depoe Bay Basalts, are tho-
leiitic in composition, suggesting a derivation from oceanic crust.
110
Age and Correlation
Cape Foulweather Basalt lies with angularunconformity over
the Miocene Silver Point member at Haystack Rock near Cannon
Beach. Foraminiferal assemblages from interbedded mudstones
and sandstones, immediately below the unconformity are lower
Saucesian in age (Rau, written communo , 1974), suggesting that the
overlying Cape Foulweather breccias are middle Miocene_ or younger.
Thin Cape Foulweather Basalt sills and dikes intrude dikes and sills
of Depoe Bay Basalt and the lower Silver Point member in seacliff
exposures near Ecola Point and Indian Beach'in Ecola State Park.
This relationship suggests the Cape Foulweather Basalts are younger
than middle Miocene Depoe Bay Basalts.
Quaternary Deposits
Unconsolidated Quaternary deposits in the map area are sub-
divided into beach deposits., marine terraces, river alluvium, and
landslide deposits.
Beach Deposits
Beach deposits include beach and dune sands composed of very
well-sorted, fine-grained, subangular to rounded, and quartzofeldspathic sand. Quartz, feldspars, basalt rock fragments,
hypersthene, mica, zircon, magnetite and hematite, ilmenite and
leucoxene are the primary, constituents. Less abundant minerals
include green and brown hornblende, apatite, clinopyroxenes
. (?
)
and tourmaline (Appendix VIII). The extensive-, straight coastal strip
of beach and dune sands forming the western boundary of the thesis
area is disrupted only at Tillamook Head by resistant basaltic head-
lands (Plate I). Isolated pockets of beach sand occur in the irregular
headlands. Due to differences in wave energy and intensity, beach
cover along the coast changes seasonally from thick sand summer
beaches to winter beaches composed dominantly, of flat, rounded,
basaltic cobbles and gravels.. A 40-foot high grass covered sand dune,
associated with a beach immediately north of Cannon Beach, stretches
from Chapman point to the mouth of Elk Creek (elate I).
Marine Terraces
Pleistocene marine terraces, occur as much as 30 feet above
sea level at south Cannon Beach and the southern part of Tolovana
Park. These terraces form nearly horizontal surfaces up to one-half
mile wide and one mile long (Plate I). The deposits consist of bedded
dark gray clays and silts, interbedded with fine-grained, well-sorted,
beach sands containing carbonized wood fragments, isolated pebbles,
and rare lenses of flat, rounded basaltic cobbles and pebbles.
112
Five to fifteen feet high beach berms of basaltic terrace gravels
formed by recent severe winter storm waves are common along the
upper parts of many beaches. Beach berms seen at Indian Beach and
along the northern part of Tillamook Head consist of interstratified
layers of sorted and rounded basaltic gravels (cobbles, boulders, and
pebbles) with intervening silty muds and tree trunks. An impressive
40-foot high deposit of storm berm gravels composed of rounded
six-foot basaltic boulders and cobbles occurs in the cove between
West Point and the nose of Tillamook Head (Plate I). Wave, energy
is funneled by refraction into the cove, producing breakers as much
as 30 feet high. Severe winter storms on Tillamook Head commonly
reach gale force winds and are reported to produce extremely large
waves. For example, the beacon at the 139 -foot level of Tillamook
lighthouse, located on a sea stack one mile west of Tillamook Head,
has been broken several times from basaltic cobbles thrown by
severe winter storm waves (Niem and others, 1973).
River Alluvium
Flood plain alluvium (10 to 20 feet thick) is exposed along the
lower reaches of Elk Creek and the Necanicum River (Plate I). The
alluvium consists of very poorly sorted, indistinctly bedded, basalt
gravels and interbeds and lenses of poorly sorted sand, massive stilts:,
and gray, muds. A linear ridge of basalt gravels extends northward
113
from Tillamook Head, molded by longshore drift. The gravel is
overlain by five feet of sand along the beach and by ten feet of alluvial
silts and clays near the Necanicum River. Apparently the longshore
growth of the elongate gravel terrace has diverted the mouth of the
Necanicum River one-half mile to the north (Schlicker and others,
1972).
Dark gray (N3) tidal flat muds and carbonaceous, estuarine
muds and silts with woody debris interfinger with river alluvium in
the lower reaches of the Necanicum River near Seaside.
Landslide Deposits
Landslide deposits along the coast consist of unconsolidated,
very poorly sorted., chaotic masses of rubble. The rubble is composed
of blocks of weathered mudstone-, sandstone,, and intrusive basalt
boulders suspended in a medium gray, sheared mud matrix. Slides
are particularly abundant in Ecola State Park (Plate I and Figure 37).
Recent movements on landslides are recognizable by the tilt of live
trees in different directions, and cracks and displacements of pavement and other man-made structures. Outlines of four older coastal
landslides were delineated on aerial photos and in the field by recog-
nition of a series of scarps and by low, hummocky, poorly drained
topography surrounded by steeper, resistant hills. Geologic mapping
suggests that landslides result from undercutting by winter storm
waves of steep coastal hills composed of seaward dipping, deeply
1.14
weathered, lower Silver Point mudstones and sandstones. The
deeply weathered mudstones have a very low shear strength com-
pared to the basaltic rocks (Schlicker and others, 1972). As a re
sult, basalt headlands are generally resistant to landslide movement.
Limited field and historical data suggests that slide movements
are most common in the winter and spring during and soon after prolonged periods of precipitation. Presumably, the infiltration of
heavy rainfall along slump scarps and fractures increases the-weight
of the upper landslide slopes and may lubricate planes of weakness
in the bedrock where movement has occurred before.. The Silver
Point mudstones also contain abundant expandable montmorillonite
clays which characteristically will absorb ground water -into their
crystal structure, thus increasing the weight of the slide.
A well documented coastal landslide undercut by wave erosion
occurred in Ecola State Park during February, 1961 (Schlicker and
others, 1961). The one-half mile long slide virtually destroyed the
parking lot and picnic facilities. The toe of the landslide extended
100 feet into the wave zone, and a forty-foot vertical displacement
occurred along the scarp at the head of the slide over several days.
The total scarp height is eighty feet; suggesting that the landslide
also had moved prior to 1961. No new activity on this slide was
recognized in the field.
115
However, during February and March of 1974, renewed move-
ment occurred along another landslide at Crescent Beach south of
Ecola Point. The one-quarter mile -long slide (the result of active
undercutting by winter storm waves) destroyed the lower section of
the Crescent Beach trail and displaced the Ecola State Park road
several feet. The southern part of Ecola State Park along the beach
is characterized by numerous old scarps and small active landslides
(Plate I).
Due to the continuous dynamic nature of wave erosion by winter
storms, and the general ins-tability of the deeply weathered Silver
Point strata, additional movement along existing coastal scarps is to
be expected in the future. It is suggested that the ,landslide outlines
on Plate I be used in a general way by land planners as a guide to
sites where further construction should be avoided.
Extensive areas of landslide breccias occur in the central and
eastern parts of the thesis area (Plate I and Figure 37). These
slides produce chaotic masses of blocks and weathered boulders of
basaltic breccias and medium gray mudstones in a mud matrix.
When these deposits bec-ome further iron-stained by, oxidation of
breccias during weathering, and covered with a dense forest and
undergrowth, they are difficult to distinguish from in-place breccia
(Figure 31); thus, the contacts between these two units shown on the
geologic map (Plate I) are approximate. The presence of tree limbs,
116
roots, and other organic debris in the deposits and hummo:cky topography are useful in distinguishing recent landslide breccias from
in-place breccia flows. These landslides are caused in part by
slope and stream erosion undercutting the less resistant Silver Point
mudstones and Oswald West mudstones which underlie and support
the topographically higher, more resistant basaltic volcanic breccias.
117
SIZE ANALYSES
No sieve analyses were performed on samples of Oswald West
sandstones because of the difficulty in disaggregating these tightly
cemented., very fine grained sandstones. Grain sizes of these sandstones were determined in thin section by using a calibrated scale
inscribed on an eyepiece. Matrix percentages were estimated from
modal analyses. Sieving and pipette grain size analyses were per-
formed on several disaggregated samples of Angora Peak and Silver
Point sandstones. Angora Peak sandstones average 85% sand, 11%
silt, and 4% clay by weight. Silver Point sandstones are composed
of 76% sand,. 19% silt, and 5% clay. When plotted on Folk's (1954)
ternary diagrams (sand, silt, and clay), these sandstones are silty
sands.
Folk and Ward's (1957) statistical size parameters were calculated from graphical plots of cumulative weight percent versus
.grain size. The statistical data are presented in Appendix VII and
summarized in Table 3. In general, Angora Peak sandstones are
coarser grained than Silver Point sandstones. The median grain size
of the Angora Peak sandstones is medium sand (1. 9'). The mean is
fine sand (2.. 27$), whereas mean grain sizes for. Silver Point turbidite
sandstones of Ecola State Park range from 3.53$ to 3.60$ (very fine
sand). The median grain size is also very fine sand (3-. 40$). The
118
most common sandstone in the measured section of Silver Point mud-
stone (Appendix I) is also very fine-grained. Quaternary marine
terrace sands south of Tolovana.Park have a median grain size of
fine sand (2. 500) and a mean of fine sand (2. 480).
Table 3. Summary of size parameters.
Angora Peak
Silver .Point
Quaternary
sandstone
sandstone
sands
Mean=t
2.27
3.57
2..48
Sorting*
1.70
1.53
0.30
Skewness
+0.49
+0.30
-0.02
""Folk and Ward's (1957) size parameters in phi (0).
Standard deviation (sorting) and skewness values were also
calculated for these sandstone grain size distributions and interpreted
according to the terminology and phi limits defined by Folk and Ward
(1957). Angora Peak sandstones are poorly sorted (1. 700') and are
positively skewed (+0.4901) (enriched in coarse grains), suggesting
that the fine clay and silt fraction was partially removed by winnowing
during transportation by currents and waves. As mentioned in the
petrology section, much of the Angora Peak sandstone matrix is
diagenetic. Thus, when the Angora Peak sands were originally de-
posited, they were probably much better sorted and more positively
skewed. The Silver Point turbidite sandstones are slightly better
119
sorted (1.430 to 1.630) and less positively skewed (+0. 300) than the
Angora Peak sandstones. These lower skewness values reflect the
greater abundance of detrital clay- and silt-sized matrix in the
Silver Point sandstones which is typical of turbidite deposition.
Beach and dune sands from the Quaternary marine terraces are very
well sorted (0. 300) indicating prolonged wave abrasion and winnowing.
They are almost normally distributed or very slightly skewed (-0.020)
(enriched in fine grains).
These statistical size parameters (mean, median, skewness,
kurtosis, etc.) were plotted on Friedman's (1962) binary graphs and
Passega's (1957) chart (see Appendix VII) to aid in the interpretations
of their probable depositional environments. Diagenetic alteration of
volcanic rock fragments, ferromagnesian minerals, and feldspars to
a secondary clay matrix in the Angora Peak sandstone samples may
have strongly affected the interpretative value of these graphical plots
about the original depositional environments. Angora Peak statistical
size data plotted on Friedman! s graphs,indicate deposition by tractive
currents (see Appendix VII); plotted on Passega's chart, they suggest
deposition by rivers or tractive currents with maximum turbulence
at the bottom of the current (see Appendix VII). Thus, the sorting
and enrichment of coarser sediments in the Angora Peak sandstones
probably are originally the result of rapid transportation in the bed
load of a river.
120
Statistical size data of Silver Point turbidite sandstones also
plotted on Friedman's graphs indicate tractive current deposition for
these sediments (see Appendix VII). On Passega's chart, these
detrital clay-rich sandstones plot closest to the range which indicates
deposition from turbidity currents (see Appendix VII). These results
further support a turbidite origin hypothesized from field observations. Presumably, rapid deposition from dense, muddy turbidity
currents precludes size separation of fine clays and silts from the
sand size detritus.
The well sorted, fine-grained Quaternary marine terrace sands
were deposited in the surf and wind environments according to sta-
tistical data plots on Friedman's graphs (see Appendix VII). The plot
is also well within the confines of the beach environment on Passega's
chart. The well sorted and rounded character of these Quaternary
sands and their proximity to the Pacific Ocean also suggest that
deposition occurred in a high energy beach environment.
121
STRUCTURAL GEOLOGY
Folds
Four northeast -trending folds involve the Oswald West mud-
stones, Angora Peak sandstones, and Depoe Bay basaltic breccia in
the central and eastern part of the thesis area (Plate I and Figure 37).
The folds were discerned by consistent patterns of dip reversals in
sedimentary strata over large areas and by outcrop patterns. The
location of fold axes on the geologic map (Plate I) and Figure 37 are
approximate due to thick soil and forest cover and incomplete rock
exposures. The broad, open folds appear to be symmetrical (the
limbs dip from 100 to 300). The westernmost anticline in sections 19
and 30, T. 5 N. , R. 9 W. near the junction of the north and south forks
of the Necanicum River is, however, asymmetrical with the steeper
western limb dipping 450. The steep dip may be the result of drag by
an adjacent north-south striking fault that roughly parallels the lower
stretch of the south fork of the Necanicum River (Plate I and Figure
37). Neither this anticlinal fold axis nor the adjacent fault and nearby
synclinal axis can be traced more than four miles due to extensive
cover by breccia landslide deposits to the south and river alluvium to
the north. The easternmost anticline plunges northward based on out-
crop patterns.
D
Fault
Syncline
Anticline
Landslide scarp
Landslide colluvium
(blocks of breccia)
D
g
Cannon
Beach
Figure 37.
Structural map of the Tillamook Head-Necanicum Junction area.
123
The westernmost syncline near,Elk Creek-continues for several
miles south of the thesis area. Smith (1975) mapped this northward
plunging syncline as a major structure based on outcrop patterns and
persistent reversals in dips. The nose of the syncline is exposed in
the Neahkahnie-Angora Peak area mapped by Cressy (1974). Based on
dips, this fold continues for approximately one mile into the southern
part of this map area (section 35, T. 5 N.
,
R. 10 W. ).
Synsedimentary Folds
Intensely deformed lower Silver Point turbidite sandstones and
mudstones locally occur in association
with basalt sills and dikes
along the coast from Indian Beach south to Crescent Beach in the
southern part of Ecola State Park. Disharmonic chevron folds are
well-exposed along Crescent Beach trail (Figure 38). Spectacular
isoclinal anticlines and synclines with amplitudes up to 100 feet form
the sea cliffs at Ecola Point (Figure 39). Areas containing these
highly deformed folds are shown on the geologic map (Plate I) as zones
of contorted strata. These synsedimentary folds reflect a severe
degree of deformation which is atypical of the broad regional struc-
tural setting of the Oregon Coast Range.
The genetic relationship of the folds to the basaltic intrusions
is well displayed at Indian Beach and Ecola :Point (Figure 39) where
sedimentary folds appear to be "squeezed'' or incorporated between
Figure 38. Synsedimentary chevron folds of well-bedded turbidite
sandstones and mudstones of the lower Silver Point
member. Fold is associated with an underlying basalt
sill (bs). (North end of Crescent Beach, Ecola State
Park).
.'.
Figure 39.
Isoclinal syncline in Silver Point strata at Ecola Point in
Ecola State Park. These strata were squeezed between
apophyses of intrusive basalt (bs) (lower left and lower
right).
126
apophyses of the upper part of the Tillamook Head still. All the in-
tensely folded sedimentary strata are in close spatial proximity to
Depoe Bay intrusions; strata a few hundred feet above the intrusions
exhibit little or no deformation. Blocks of lower Silver .Point strata
several feet in diameter are also incorporated in the upper part of the
basalt sills. The laminated strata are microfaulted and contorted,
suggesting the strata were semi-consolidated and plastically deformed
when engulfed by the intrusion. The upper parts of many sills and
apophyses also are locally brecciated and altered in zones up to 25
feet thick near contacts with the deformed sedimentary rocks indicating the basalt was fragmented due to steam blasting when hot mag-
mas came in contact with water-saturated sediments.
A middle Miocene (16.0 + 0. 3 m. y.) age has been radiomet-
rically determined by Potassium-Argon dating methods for the
Tillamook Head sill in Ecola State Park (Niem and Cressy, 1973).
The Silver Point member which is intruded by these basalts is also
middle Miocene (late Saucesian) in age based on Foraminifera recovered from these deformed strata (Rau, personal commun.. , 1974).
This close proximity, of ages for the two units strongly suggests that
the intrusions occurred soon after deposition of the sediment and prior
to complete lithification of the sediments. The sills and dikes ap-
parently intruded these strata at extremely shallow depths because
related submarine basaltic breccias occur only several hundred
127
stratigraphic feet above these dikes and sills.
From the field and age relationships, it is concluded that the
severely deformed Silver Point strata in Ecola State Park were produced by thick sills, dikes, and apophyses forcefully intruding and
displacing a veneer of semi -consolidated, water- saturated sediments
at extremely shallow depths. The resultant steam blasting and rapid
quenching of hot magma brecciated the upper parts of the intrusives,
and incorporated blocks of semi-consolidated mudstones. Niem (1974)
proposed a similar origin for these folds.
Faults
High angle normal and reverse faults with displacements ranging
from a few feet to ten feet are exposed throughout the thesis area.
Five major high angle faults with displacements up to several hundred
feet are delineated on Figure 37 and Plate I. One fault strikes eastwest near West Point on northern Tillamook Head. The largest fault
trends northwest and parallels the lower Necanicum River. Two
smaller faults criss-cross U. S. Highway 101. A fifth small fault
strikes north and parallels the lower reach of the south fork of the
Necanicum River. Faults are probably more prevalent than shown
in Figure 37 and Plate I but are hidden by thick soil, landslides, and
dense forest cover.
128
The east-west, nearly vertical fault near West Point on the
northern part of Tillamook Head has a minimum displacement of 200
feet.
Oswald West mudstones (?) in the north block are faulted against
the Tillamook Head Depoe Bay sill and Silver Point member in the
The sharp fault contact and sill displacement can be
south block.
readily observed on the seacliff at West Point.
The inclusion of
bedded and burrowed mudstones (in the northern block) within the
Oswald West mudstones is, however, only based on lithologic simi-
larities to the typeOswald West mudstones exposed at Short Sands
beach, and the identification of a few poorly preserved Oligocene (?)
Foraminif era (Rau, personal commun. , 1974). These strata may
actually be part of the lithologically similar upper part of the Silver
Point member, in which case, the relationship observed at West
Point would become a disconformable contact of an apophyses.of the
Tillamook Head sill.
The straight nature of the Necanicum stream valley and the
vertical offset of strata on opposite sides of the river suggest the
presence of a major six mile long fault covered by stream alluvium.
Due to the inferred nature of the fault, it is shown on the geologic
map (Plate
I)
and Figure 37 as a dashed line. Taking the dip of the
sill into account, a minimum of 300 feet displacement on this fault
is suggested by comparison of the altitudes of the upper contact of the
Tillamook Head sill in two quarries (SW1/4 SW 1/4 of section 10, and
129
SW 1/4 SW 1/4 of section 11, T. 5 N.
R. 10 W. ) on opposite sides of
the Necanicum River.
At the southern end of a 100-foot high, one-half mile long cut
along both sides of U. S. 101 in the SW 1/4 of section 17, T. 5 N. ,
R. 10 W.., there are exposed two, small, intersecting normal faults
which have displaced 50 to 100 feet of Silver Point mudstones against
the thick columnar jointed Tillamook Head sill. They are not trace-
able very far due to cover and vegetation. The two faults strike north
and northwest, respectively.
The north-south trending fault that parallels the south fork
of
the Necanicum River in sections 19 and 30 of T. 5 N. , R. 9 W. , occurs
on the western limb of a small anticline (Figure 37). The fault downdropped middle Miocene Silver Point mudstones found on the west
side of the narrow river against bedded Oligocene Oswald West mud-
stones and siltstones on the east side.
130
GEOLOGIC HISTORY
Transport Directions
No paleocurrent dispersal patterns or paleoslopes were determined for the Oswald West mudstones in the thesis area due to a
,lack of current-formed sedimentary structures. Extensive bioturbation in the interbedded siltstones and mudstones has obliterated any
current formed primary sedimentary structures. Cressy (1974)
determined a paleoslope and paleocurrent direction to the south or
southwest for micro-cross laminated, tuffaceous Oswald West silt-
stones exposed six miles south of the thesis area. A few planar
crossbeds in limited exposures of the Angora Peak sandstones indi-
cate a generally westward sediment dispersal pattern. This trend
corresponds with the findings of Cressy (1974) and Smith (1975)
who determined dispersal patterns from the east to the southwest,
west, and northwest for the Angora Peak sandstones.
A total of 38 paleocurrent directions in the Silver Point sand
stones were determined from micro-cross laminations, flutes and
grooves along the unnamed beach immediately south of Indian Beach
in Ecola State Park (in lower Silver Point turbidite sandstones) and at
Haystack Rock south of Cannon Beach (in upper Silver Point sandstones). Measurements were rotated on a stereonet to correct for
tectonic distortion of the bedding. The two locations show a
131
sandstone dispersal pattern from east to west in lower Silver Point
turbidite sandstones, and from east to west-southwest in upper-Silver
Point sandstones (Figure 40). The cumulative mean paleocurrent
direction is west-southwest (260 + 360). The individual means are
2620 at Ecola State Park and 2580 at Haystack Rock.
The standard
deviation of the grand mean is 360,.. suggesting only a small dis-
persion or fluctuation from the westward mean. The distribution
at Haystack Rock is distinctively bimodal with major directions to
the west and northwest. The northwest paleocurrent direction re-
flects a source from the southeast, possibly the Angora Peak sand
delta described at Angora Peak by Cressy (1974) as these turbidite
sandstones have the same general heavy mineral suite and mineralogy
as do the Angora Peak sandstones. The west - southwest dispersal
pattern indicates different sources from the east or northeast, perhaps reflecting meandering distributary channels on lobes of the
Angora Peak sandstone delta.
Provenance
Most of the detritus for Oswald West mudstones, the Angora
Peak sandstones, and the Silver Point mudstones was probably derived from local highs in the ancestral Coast Range and from the
western Cascades. However-, heavy mineral analyses and petrographic examinations also indicate that small quantities of sediment
132
3s
U
P-1
25
8
Standard
deviation = 61°
Grand mean
cumulative rose
0
1
2
I
Miles
Figure 40. Rose diagrams of paleocurrent measurements from the
Silver Point sandstones and siltstones. Number of
measurements and mean azimuth directions (arrows)
are given with roses.
J33
were derived from acid plutonic and metamorphic terrains of northcentral Washington, eastern Oregon, southeastern British Columbia.,
and Idaho (see modal and heavy mineral analyses, Appendices VI and
VII).
Intermediate and basic volcanic rock fragments occur in the
coarse glauconitic sandstones in the Oligocene Oswald West mudstones. The rock fragments are generally very altered. Discernible
igneous textures (e. g. intersertal and pilotaxitic) are typical of
basalts, diabases, and andesite lava flows. The majority of rock
fragments are basalt and some contain partly altered augite pheno-
crysts. These fragments were probably derived from nearby Eocene
Siletz River and Tillamook Volcanic highlands. Snavely and Wagner
(1963) envisioned local structural highs of folded Eocene rocks east
and southeast of the thesis area on their paleographic map for the
Oligocene epoch of western Oregon and Washington. Andesitic rock
fragments may have been derived from the Eocene-Oligocene Little
Butte Volcanics of the "ancestral" western Cascades. Contemporaneous volcanic activity in the Cascades is suggested by intercalated
one-to three-inch thick tuff and pumice beds in the Oswald West mudstones.
The heavy minerals hypersthene, zircon, green hornblende,
and augite also reflect sediment contribution from these basaltic and
andesitic volcanic provenances. Angular grains of garnet, muscovite,
134
microc.line, orthoclase, and tourmaline suggest some detritus was
derived from metamorphic (e. g.. gneisses and schists) and acid plu-
tonic sources (e. g. granodiorite and granite) of eastern Oregon,
Washington, and Idaho.
The heavy mineral suite from the Oswald West mudstones is
similar to the suite described by Van Atta (1971) from the partly
coeval late Oligocene Scappoose Formation of the northeastern Coast
Range. Van Atta hypothesized from Scappoose sandstone mineralogy
that an ancestral Columbia River which drained the ancestral volcanic
Cascades and the metamorphic and plutonic terrains of eastern
Oregon, Washington, and Idaho deposited the sediments that now form
the Scappoose Formation. The similarity of the heavy mineral suites
suggests
that the Oswald West mudstones may represent in part a
deeper water or prodelta facies of the Scappoose delta. Cressy (1974)
made similar hypotheses for the type Oswald West mudstones in the
Angora Peak - Neahkanie Mountain area 10 miles to the south.
Rock fragments and heavy mineral suites of both the Angora
Peak sandstone and Silver Point mudstone members are very similar.
Therefore, their source areas were probably very similar. Rock
fragments common in both members are overwhelmingly intermediate
and basic volcanic, but some metamorphic and sedimentary fragments
occur (see comparative modal analyses, Appendix VI). The domi-
nance of basalt lava clasts in the Angora.Peak sandstones reflects the
135
continuation in middle Miocene of the supply of large amounts of
detritus from local uplifted highlands of Tillamook and Siletz River
Volcanics to the east and southeast. Andesite, dacite, pumice and
tuff fragments were probably, derived from erosion of freshly erupted
pyroclastics and the Little Butte Volcanic lava flows in the "ancestral"
western Cascades. The heavy mineral suites in the Angora Peak and
Silver Point sandstones contain abundant euhedral hypersthene,
augite, zircon, basaltic (lamprobolite) and green hornblende suggesting an andesitic and dacitic source. The probable provenance, the
Oligocene-early Miocene Little Butte Volcanics, contains abundant
hypersthene and green hornblende phenocrysts in andesite flows,
augite and, lamprobolite in flows of olivine basalt and basaltic ande-
site, and tuff beds (Peck and others,
1964).
Euhedral augite and
zircon could also have been supplied by erosion of the Eocene Colestin
Formation, predominantly augite-rich andesite flows, which stratigraphically underlies the Little Butte Volcanics (Peck and others,
1964).
Rarer grains of garnet (grossularite), epidote, muscovite,
chlorite, biotite, orthoclase and strained quartz, and metamorphic
quartzite and schist fragments indicate some contribution from a low-
grade metamorphic terrain. There are no metamorphic source rocks
known in the Oregon Coast Range or the nearby Cascades thereby
presenting an enigma as to the origin of these minerals and rock
136
fragments. In addition, the heavy minerals euhedral, dark gray
tourmaline, zircon, rutile, muscovite, biotite, monazite, microcline, and orthoclase strongly suggest minor contribution of detritus
from an acid igneous source such as granodiorites or diorites. The
present day Columbia River and its tributaries drain granitic, volcanic, and sedimentary areas which conceivably could have supplied
the same kind of detritus in the past. Plutonic and metamorphic
minerals and rock fragments could have been transported from the
extensive granodiorites and pre-Tertiary metamorphic rocks (schist,
marble, quartzite, and metasediments) in north-central Washington
and southeastern British Columbia during the middle Miocene
(Snavely and Wagner, 1964). Granodiorite intrusions and metamorphic
rocks in eastern Oregon, rocks of the Precambrian Belt series in
Idaho, and the Idaho Batholith recognized by Ross and Forrester
(1947) and Huntting and others (1961) also could have acted as source
areas for these sandstones. Metamorphic rock fragments and
associated heavy minerals could also have been contributed from the
Klamath Mountains and granitic minerals from the small granodiorite
plutons in the western Cascades (Wells and Peck, 1961).
Very rare sedimentary quartzite clasts, detrital quartz grains
.
with overgrowths, and rounded zircon grains in Silver Point and
Angora Peak sandstones suggest a recycled origin from orthoquartz-
ites and supermature siliceous quartz sandstones. The Snake River,
137
a tributary of the Columbia River, drains a sedimentary terrain in
Idaho containing sedimentary quartzites and supermature quartz sandstones
(Ross and Forrester, 1947). The modern Columbia River
also drains Cambrian quartzites and Paleozoic strata (quartzites,
quartz sandstones, and shales) in north-central Washington (Huntting
and others, 1961). The absence of sedimentary quartzites and supermature quartz sandstones in the Oregon and Washington Cascades,
Coast Range, and Klamath Mountains necessitates an appeal to
Paleozoic sedimentary rocks of eastern Oregon and Washington, and
Idaho as source areas.
Therefore, the sandstone mineralogy suggests that a drainage
system similar to the present day Columbia River supplied the variety
of minerals and rock fragments that occur in the Angora Peak and
Silver Point members of the Astoria Formation as well as in. the
Oswald West Formation. The east to west paleocurrent dispersal
pattern in the Angora Peak and Silver Point sandstones is consistent
with an ancestral Columbia River system. Cressy (1974) and Smith
(1975) reached similar conclusions from the mineralogy of Angora
Peak sandstones in the Angora Peak-Onion Peak areas immediately
to the south. They also located fluvial conglomerates (within the
sandstone member) consisting of well rounded exotic basalt and rarer
granodiorite, andesite, quartzite, and gneiss pebbles and boulders.
138
Perhaps these conglomerates formed in a channel of this river system.
Heavy minerals in the Quaternary marine terrace sands consist
of varying amounts of hypersthene, clinopyroxenes, opaques, biotite,
and green hornblende. All mineral grains could have been supplied
by longshore drift from the modern Columbia River and/or by Coast
Range rivers draining the Eocene Tillamook Volcanics, middle
Miocene Depoe Bay and Cape Foulweather basalts, and Eocene to
Miocene sedimentary strata.
Summary and Conclusions
Five mappable Tertiary rock units occur in the thesis area.
From oldest to youngest, these units are the Oswald West mudstones,
the Angora Peak sandstone and Silver Point mudstone members of
the Astoria Formation, and intrusive and extrusive Depoe Bay and
Cape Foulweather Basalts. In addition,, Quaternary deposits con-
sisting of marine terraces, landslides, beaches, and stream alluvium
also occur in the thesis area.
Late Eocene to early Miocene Oswald West mudstones consist
of more than 2, 000 feet of medium gray, deep marine mudstone interstratified with thin to thick beds of light gray tuffaceous siltstones,
rare tuff beds, and glauconitic sandstones. This abundant tuffaceous
material indicates extensive contemporaneous acidic volcanism in the
.139
ancestral Cascade Mountains during Oswald West mudstone deposition. Volcanic rock fragments and heavy mineral suites suggest that
the Oswald West coarse clastics were dominantly derived from nearby
Coast Range Eocene basalt highlands and from andesitic and dacitic
lavas
of the "ancestral" western Cascades. Open marine, .low energy
reducing conditions in outer shelf and slope depositional environ-
ments are implied by the dominance of organic rich mudstone, silt-
stone, and glauconite lithologies, lack of current formed structures,
pyrite, and molluscan, foraminiferal, and trace fossils. Based on
similar ages, mineralogy, and stratigraphic position, itis postulated
that the Oswald West mudstones are in part a prodeltaic or deeper
water facies of the late Oligocene coeval deltaic Scappoose Formation
of the northeastern Oregon Coast Range.
During early to middle Miocene time, the Oswald West mudstones were uplifted..
This shallowing of the depositional basin is
reflected in the geologic record by the deposition of the shallow water
marine Angora Peak sandstones immediately overlying burrowed,
deep-water Oswald West mudstones. An unconformable relationship
between the two units has been postulated in the nearby Angora Peak
area by Cressy (1974).
The middle Miocene Astoria Formation consists of two informal
members, the Angora Peak sandstones and the overlying Silver Point
mudstones. The exposed Angora Peak sandstones consist of volcanic
140
wackes and arkosic sandstones. Moderate sorting (disregarding
diagenetic matrix), laminations, and cross laminations in the sandstones are suggestive of shallow marine deposition (e. g. middle to
inner shelf and/or beach). The overlying 600-foot thick Silver Point
mudstones are composed predominantly of well bedded dark gray,
micaceou's mudstones and thin light gray siltstones.
Thin, carbona-
ceous, feldspathic, turbidite sandstones are rhythmically inter
bedded with the mudstones in the lower part of the Silver Point
member. Foraminifera and molluscan fossils indicate the Silver
Point mudstones were deposited in sub-littoral to outer shelf depths
(600-1, 000 feet).
In the Angora Peak area, Cressy (1974) interpreted the Angora
Peak sandstone as forming in a deltaic environment based on the
occurrence of 1, 000 feet of interfingering coal beds, fluvial channel
conglomerates and sandstones with laminated shallow marine fossili-
ferous sandstones. Assuming that vigorous coastal processes during
the middle Miocene were similar to those operating today along the
Oregon Coast, it is hypothesized that the shallow marine delta front
sheet sands were transported in part northeast by longshore drift to
form coastal bars and chenier plains like in the Mississippi delta
today.
The sedimentary structures, sorting, limited distribution and
thickness (less than 100 feet) of the Angora Peak sandstones in the
thesis area suggest that the unit exposed here probably represents
141
these redistributed sands transported north of the main body of the
subaerial delta described by Cressy (1974). The Silver Point turbidite sandstones and deep water burrowed mudstones in the area
which may interfinger with Angora Peak sandstones are interpreted
as prodelta sediments derived from the prograding Angora Peak delta
and barrier bar complex. The upper Silver Point mudstones represent an outer shelf depositional environment that eventually transgressed over the rapidly subsiding Angora Peak delta.. Paleocurrent
measurements in lower Silver Point turbidites indicate a sediment
dispersal pattern from the east to the west which is consistent with
the prodelta hypothesis.
Sandstone composition and heavy mineral assemblages suggest
that most of the Angora Peak and Silver Point strata were derived
from erosion of local Coast Range Eocene basaltic highs and from
Oligocene to early Miocene andesite flows and dacitic tuffs of an
ancestral western Cascades. In addition, small quantities of "exotic"
mineral and rock clasts were derived from granitic plutons, schists,
gneisses, quartzites, and supermature Paleozoic quartz sandstones
of southeastern British Columbia, eastern Washington and Oregon,
and Idaho. A river drainage system similar to the modern Columbia
River probably existed in the middle Miocene based on the mineralogic
composition of these members of the Astoria Formation.
142
Based on similar lithologies, mineralogies, textures, dispersal
patterns, and spatial relationshups, it is hypothesized that the sediments which formed the middle Miocene Astoria Formation and the
late Oligocene to early Miocene Scappoose Formation were trans-
ported by the same ancestral "Columbia River" drainage system and
that the mouth of this river prograded during early Miocene from the
location of the Scappoose Formation in the northeast Coast Range 40
miles westward to deposit the Angora Peak sandstones in the northwest Coast Range. The westward shift of the site of deposition was
probably produced by normal delta progradation coupled with early
Miocene uplift and/or accompanying regression of the Pacific Ocean.
Basin subsidence rates once again surpassed deltaic sedimentation
rates, resulting in the gradual transgression of the deep water Silver
Point muds over the Angora Peak sands.
Deposition of the Silver Point mudstone member was interrupted by
uplift producing an erosional unconformity. During the late middle
Miocene, Depoe Bay basalt dikes, sills, irregular intrusives, and
more than 1, 000 feet of related tholeiitic submarine breccias were
emplaced in the rapidly subsiding basin. These flows and intrusions
were soon followed by another volcanic episode that produced the less
extensive sills, dikes, and pillow flows and breccias of the porphyritic
Cape Foulweather Basalt. The largest of the Depoe Bay intrusives
is the 900-foot thick Tillamook Head sill which intruded thick,
143
semi-consolidated, water-saturated lower Silver Point sediments at
shallow depths producing locally intensely deformed strata (chevron
folds and isoclinal folds in Ecola State Park).
Late Miocene to Pleistocene deformation and uplift formed a
series of northeast trending folds and north to northwest trending,
high-angle, normal and reverse faults in the Cannon Beach Necanicum Junction area. Submarine pillow lava flows capping
Sugarloaf Mountain south of the thesis area suggest that a minimum
uplift of 2, 900 feet has occurred. Uplift and accompanying erosion
of the Coast Range are hypothesized to have begun during middle to
late Miocene because no Tertiary strata younger than middle Miocene
occur in the thesis area, and a regional erosional unconformity of
this age has been reported on the nearby continental slope (Braislin
and others, 1971; Kulm and Fowler, 1974a). Quaternary marine
terraces over 30 feet high in the thesis area and elevated PliocenePleistocene sediments on the adjacent continental shelf (Kulm and
Fowler, 1974a, b) suggest that periodic uplift and subsidence have
continued into the present.
In conclusion,.the Tertiary strata in the Cannon Beach Necanicum Junction area record a long history of subsidences and
varied uplifts accompanied by changing deep marine and shallow marine environments, erosion, and basaltic volcanism. A possible
mechanism to explain these periods of uplift and subsidence is changes
144
in the rates of subduction between the converging North American
and Juan de Fuca plates in this reg-ion during the Tertiary. At some
subduction rate., subsiding factors and uplift forces will be in dynamic
equilibrium. A rapid rate of sea floor spreading and accompanying
subduction might produce subsidence along the trench and bordering
continental margin. A slower spreading rate would produce less
subduction, and by isostatic rebound, uplift of this portion of the
continental crust.
145
HYPOTHESES FOR THE ORIGIN OF THE
OREGON COAST RANGE UPLIFT
A variety of possible uplift mechanisms for the present day
Oregon Coast Range exist. Uplift could have been caused by a termin-
ation or reduction of sedimentation that destroyed a balance between
basin subsidence versus uplifting forces created by the heating and
expansion of sediments at depth. Christiansen and Lipman (1972)
hypothesized that termination of subduction between the North
American plate and the Pacific plate probably resulted in regional
uplift of the western United States by isostatic readjustments. The
presence of a deep-seated slab of lithosphere no longer in dynamic
equilibrium with subduction, but being heated and expanding in volume,
may have caused this uplift of the overlying continental crust.
A
similar interpretation on a smaller scale might be applicable to the
general uplift of the Oregon Coast Range in late Miocene to Pleistocene. The presence of a widespread late Miocene unconformity on
the Oregon continental shelf may be related to the termination of subduction and a change in plate motion 10 million years ago (Kulm and
Fowler, 1974a). This time was also the approximate beginning of
significant uplift and erosion of the Oregon Coast Range.
Byrne and others (1966) attributed this uplift of the Oregon
continental shelf to an early stage of continental accretion., possibly
as a result of the convergence of the Juan de Fuca and American
146
plates. Kulm and Fowler (1974b) hypothesized that a compressional
thrust model offered the best explanation for the Cenozoic evolution
and uplift.of the Oregon continental margin. Through repeated thrust
faulting (possibly due to the American plate overriding the Juan de
Fuca plate) at the base of the continental slope, wedge-shaped slices
of younger sediments are accreted beneath the continental margin.
The underthrusting of these younger sediments elevated the older
Tertiary strata on the continental shelf of the overriding plate.
Kulm and Fowler (1974b) calculated that imbricate thrusting of
unconsolidated abyssal deposits presently uplifts the lower continental
slope at an average rate of 1, 000 meters/m. y.., but uplifts the older
Tertiary strata on the outer shelf at a rate of only 100 meters/m. Y.
Foraminifera assemblages in mudstone interbeds indicate that the
middle Miocene Depoe Bay basaltic pillow breccias were erupted in
middle to outer shelf depths (approximate-ly 1, 000 to 1, 500 feet deep).
These breccias are now elevated to 1, 500 feet above sea level, reflecting a maximum total of about 3, 000 feet (900 meters) of uplift.
Assuming plate tectonic processes and average rates of uplift during
the late middle Miocene (10 m. y. ago) were similar to those postulated
by Kulm and Fowler for the modern outer shelf, a maximum uplift
of 1, 000 meters would be predicted for the Depoe Bay breccias in the
thesis area (using Kulm and Fowler's average uplift value of 100
147
meters/m. y. ). This predicted value compares very favorably with
the 900 meters of uplift estimated for the middle Miocene Depoe Bay
Basalt breccia.
148
MINERAL RESOURCES
Crushed Rock Resources
Crushed rock and riprap are presently the most important
mineral resource in the study area. Plate I shows the distribution of
rock quarries in the thesis area. Crushed rock from these quarries'
is used nearby for road bases and fill. All the quarries are in dikes
or sills of either Depoe Bay or Cape Foulweather Basalt. According
to Schlicker and others (1972), nearly all the rock products for
Clatsop County are derived from quarries in basalt. Other rock units
in the map area, with the exception of basaltic fluvial gravels adjacent
to the Necanicum River, are not suitable for road material. Oswald
West mudstones, Angora Peak and Silver Point sandstones, and
basaltic breccias lack the strength and resistance to prolonged abrasion of the intrusive basalts. Crushed sedimentary rocks noted along
a few logging roads are easily pulverized to sand and mud after short
exposures to weathering and abrasion by logging trucks.
Most quarries in the thesis area are small and belong to Crown
Zellerbach Corporation. A few of these quarries are located in thick
extensive sills which represent considerable reserves. Several large
quarries belonging to the State of Oregon, Crown Zellerbach Corporation, and independent logging and gravel companies are -located in the
Tillamook Head basalt sill along the lower Necanicum River in sections
149
3, 4, 9, 10, and 11, T. 5 N.
R. 10 W.
,
R.10 W.
,
and section 33, T. 6 N.,,
Potential basalt quarry locations are abundant in the region
due to the many intrusive dikes and sills. Applying the factors of
accessibility,. minimum depth of overburden (less than ten feet),
fractures and hardness, freshness, and thickness (over 50 feet), the
most promising future quarry sites are Depoe Bay Basalt sills cropping out in NW 1/4 section 4, and NW 1/4 section 9, T. 5 N., R.. 10 W.
(Plate I).
Excellent alluvial sand and basaltic gravel deposits for road
bases and fill are located along the Necanicum River near Seaside
in section 4, T. 5 N.
,
R. 10 W. and in section 33, T. 6 N. , R. 10 W.
Although the total thickness of the sands and gravels is not known,
a minimum thickness of 15 feet is exposed along the banks of the
river.
Petroleum
Presently, no exploratory oil or gas wells have been drilled
within the boundaries of the map area. Shell Oil Company drilled a
wildcat well approximately 18 miles west of Tillamook Head in the
Nehalem Bank on the continental shelf. The well penetrated only
siltstone and mudstone facies which had low porosity and permeability
(Braislin and others, 1971). Oil and gas have been reported, however, from northwestern Oregon and southwestern Washington in
150
Tertiary formations correlative to the ones in the thesis area. Sunshine Mining Company's Medina No. J well produced 175 bbl/day of
38.90-
gravity paraffin-base oil from Oligocene and Miocene strata
(equivalent to the Oswald West mudstones and Astoria Formation
of this report) in Grays Harbor County, Washington, approximately
70 miles north of the thesis area (Braislin and others,-1971). The
well produced more than 12, 000 bbl of oil before it was plugged and
abandoned in August, 1962. A 15,000-foot exploratory oil well drilled
near Nehalem (ten miles south of the thesis area) by the Necarney
Hydrocarbon Oil Company produced gas (methane) shows in the
Oswald West mudstones (Cressy, 1974). Braislin and others (1971)
suggested that sandstones of the middle Miocene Astoria Formation
offer excellent offshore reservoir objectives.
The basic geologic requirements for hydrocarbon accumulation
(combination of source beds, reservoir rocks, cap rock, entrapment
structures, and time) are present to a limited extent in the thesis
area. Potential source beds are the Oswald West mudstones and
Silver Point mudstones. The dark gray color of the 2,000 feet of
deep marine mudstones suggest an abundance of finely disseminated
organic matter. Organic matter comprises from one to seven percent
of the mudstones as indicated by weight losses.in over two dozen
widely distributed samples when treated with a 30% hydrogen peroxide
(H.,O,) solution.
151
Potential reservoir rocks in the thesis area are the middle
Miocene Angora Peak sandstones. These medium- to coarse-
grained, feldspathic sandstones are intercalated between the two
potential thick source beds and possibly interfinger with the overlying
Silver Point mudstones which could also act as a thick, impermeable
cap rock. The Angora Peak sandstones attain thicknesses of over
1, 800 feet in the type area (Cressy, 1974) and a few hundred feet in
the thesis area. Sedimentary structures, facies patterns, and fossils
indicate that these shallow marine sandstones may have been formed
as linear beach bars, barrier island sands; distributary, channels,
and/or delta front sands. Such shallow water, deltaic sandstones act
as numerous reservoirs in many oil fields. Unfortunately, the low
porosities and permeabilities in surface exposures of the Angora
Peak sandstones diminish its potential as a suitable reservoir.
In
surface exposures, chemical weathering and diagenesis has disinte-
grated unstable ferromagnesian minerals, feldspars, and volcanic
rock fragments in the compositionally immature sandstones. This
decomposition resulted in the production of abundant clay matrix,
.iron oxides, and calcite cement which have filled most pore spaces.
At depth below the zone of surface weathering, the Angora Peak
sandstones
may be less altered and thus be more porous and perme-
able, and as a result, would make potential reservoir rocks. More
friable, porous, and "clean" Angora Peak
sandstone beds have been
152
noted by Smith (1975) and Cressy (1974) in the nearby areas.
Perme-
ability tests of fresh cored samples will be needed to fully evaluate
the reservoir potential of the Angora Peak sandstones.
Turbidite sandstones of the Silver Point member are not prom-
ising reservoirs due to thinness (beds less than eight feet thick),
limited aerial extent, and low permeability and porosity. The low
porosity (less than 0-. 5% in thin section) is due to extensive carbonate
and hematite pore-filling cements, and abundant impermeable clay
matrix and disseminated, carbonized plant fragments.
Two northeast-southwest trending anticlines in the northeast
part of the area and several east-west trending vertical faults provide
structural traps for hydrocarbons. However, the folds have been
breached and the potential reservoir rocks (Angora Peak sandstones)
either are exposed at the surface (and hydrocarbons have been lost)
or have been eroded away. Potential fault traps exist in the central
and western parts of the map area where vertical faulting has juxtaposed west dipping Angora Peak sandstones against impermeable
Silver Point or Oswald West mudstones to the east (see Cross Section,
Plate II). Retention and preservation of hydrocarbons in these struc-
tural traps is improbable due to abundant, major basaltic intrusions
which might have vaporized the oil.
Petroleum prospects in the deep subsurface of the thesis area
are poor. The Oswald West mudstones are probably, underlain by
15.3
Eocene basalts (Tillamook and Siletz River Volcanics) which are the
acoustic and economic basement in northwest Oregon (Braislin and
others, 1971). No older rocks are known in the region. Modern,
sophisticated seismic surveys are necessary to further determine the
petroleum potential in the subsurface of the map area, but commercial
petroleum accumulations are not likely due to extensive faulting,
abundant basaltic intrusions, and lack of reservoirs at depth.
However, there is a stronger potential for petroleum in the
nearby continental shelf adjacent to the thesis area. Field mapping in
this thesis area and Smith's (1975) thesis area suggest that thick
Angora Peak sandstones, confined between two potential source beds
(Oswald West mudstones and Silver Point mudstones) dip northwest-
ward under the continental shelf. These offshore Angora Peak sand-
stones are potential petroleum reservoirs. The Silver Point
mudstones,, which could act as an impermeable cap rock, also
possibly interfinger with the Angora Peak sandstones in the thesis
area and offshore. This interfingering relationship could produce
stratigraphic traps. Seismic studies suggest that the nearby Oregon
continental shelf contains numerous folds and vertical faults, and that
up to 3, 000 feet of Pliocene strata overlies the Miocene strata (Kulm
and Fowler, 1974a). Therefore, both structural and stratigraphic
traps are likely on the continental shelf.
154
Shell Oil Company's Ex Test 2 well, .located near the Nehalem
Bank 27 miles southwest of the mouth of the Columbia River and
approximately 18 miles west of Tillamook Head, penetrated only
impermeable Tertiary mudstones. Braislin and others (1971) reported
that deep oil tests 18 - 20 miles offshore on the central and northwestern Oregon continental shelf also encountered only finer-grained,
deep-marine mudstones and siltstones. These offshore well data
suggest that the sandy, shallow-marine deltaic wedge of Angora Peak
sandstone thins rapidly and undergoes a change westward to a deep-
marine mudstone facies under the nearby continental shelf (less than
14 miles). It suggests that better petroleum prospects occur in these
shallow-marine Angora Peak sandstones closer to shore, but numerous basalt intrusions with accompanying faulting in the near-shore
area diminish the likelihood of oil preservation. However, Snavely
and others (1973) report from seismic studies that these intrusions
are confined to a narrow belt on the continental shelf within two or
three miles of shore. It is concluded from depositional environments,
stratigraphic relationships, regional dips, associated structures, and
spatial relationships of the Astoria Formation and Oswald West mud-
stones that hydrocarbon accumulations are possibly present along the
inner continental shelf of northwest Oregon in an offshore area a few
miles west of the intrusion belt but east of the finer-grained mudstone
155
facies. Seismic surveys and test wells will be necessary to further
evaluate the nearby offshore potential.
Coal
Cressy (1974) reported that several subbituminous to bituminous
coal seams, ranging in thickness from nine inches to two feet, occur
in the lower part of the Angora Peak sandstone member southeast of
Angora Peak, about seven miles south of this thesis area. B. T. U. s
are reported to range from 11, 280 to 12,080 (U. S. Bureau of Mines,
Coal Laboratory, Pittsburg). No coal seams were observed in the
Angora Peak sandstones of the Tillamook Head-Necanicum Junction
area. In addition, outcrops of Angora Peak sandstones in this thesis
area are very limited (Plate I). Therefore, if coal seams are present,
they are very localized and probably uneconomical.
15 6
REFERENCES CITED
Addicott, W. O. , Paleontologist, U. S. Geological Survey, Written
communication, 1974.
.Allen, J. R. L. ,
1970, ,Sediments of the modern Niger delta: a
summary and review, in Morgan, J. P.., ed., Deltaic sedimentation-modern and ancient: Soc. Econ. Paleontologists and
Mineralogists Spec. Pub. 15, p. 138-151.
, 1964,. Geology of Oregon, 2nd ed.: University of
Oregon Cooperative Bookstore, Eugene, 165 p.
Baldwin, E. M.
Beaulieu, J. D. , 1971, Geologic formations of western Oregon;
Oregon Dept. Geol. Mineral Indus.., Bull. 70, 72 p.
Berry, L. G.
, ed.., 1971, Inorganic index to the powder diffraction
file: Joint Committee. on Powder Diffraction Standards,
Swarthmore,. Pennsylvania.
Bouma, A. H. , .1962, Sedimentology of some flysch deposits:
Elsevier Publishing Co.., Amsterdam, 168 p.,
, and Brouwer, A. , 1964, Developments in sedimentology, v. 3; Turbidites: Elsevier Publishing Co. ,
Amsterdam, 264 p.
Braislin, D. B. , Hastings, D. D.., and Snavely, P. D.., Jr.,, 1971,
Petroleum potential. of western Oregon and Washington and
adjacent continental margin: Am. Assoc. Petroleum Geologists
Mem. 15, p. 229-238.
Brown, G. ed. , 1969, The x-ray identification and crystal structures
of clay minerals: Mineral Soc. .Great Britain, Monograph,
544 p.
Byrne, J. V. , Fowler, G. A., and Maloney, N. J.. , 1.966, Uplift of
the continental margin off Oregon: Science, v. 154, p. 16541656.
Cannon Beach Quadrangle, Oregon, 1955, U. S. Geological Survey
15-minute topographic map, scale 1:62,.500,.
157
Carlisle, Donald, .1963, Pillow breccias and their aquagene tuffs,
Quadra Island, British Columbia: Jour. Geol., v. 71, no. 1,
p. 48-71.
Carroll, Dorothy, 1970, Clay minerals: a guide to their X-ray
identification: Geol. Soc. America Spec. Paper 126, 80 p.
Chamberlain, K. C.. , Paleontologist, Ohio University, Written
communication,. 1974.
Choquette,, P. W. and Pray, L. C. , 1970, Geologic nomenclature and
classification of porosity in sedimentary carbonates: Am.
Assoc. Petroleum Geologists Bull. , v. 54, no. 2, p. 207-250.
Christians en, R. L. and Lipman,, P. W. , 1972:, Cenozoic volcanism
and plate-tectonic evolution of the western United, States:
Late Cenozoic: Royal Soc. (London) Philos, Trans.., v. 271,
p. 249-284.
Cressy, F. B.., Jr. , 1974, Geology of the Angora Peak-Neahkahnie
Mountain area, Tillamook and Clatsop Counties, Oregon:
unpublished master's thesis Oregon State Univ. , Corvallis,
148 p.
Dennis, R. L.
,
Paleobotanist, Oregon State University, Personal
communication, 1973.
Diller, J. S. , 1896,,A geological reconnaissance in northwestern
Oregon: U. S. Geol. Survey, 17th Annual Report, p. 1-80.
Dodds, R. K.., 1969, The age of the "Columbia River Basalts" near
Astoria, Oregon, in Proceedings of Second Columbia River
Basalt Symposium, Cheney,.Wash.: Eastern Wash. State
College Press, p. 239-270.
Donaldson, _A. C. , Martin, : R. H.. , and Kanes, W. J.., 1970, Holocene
Guadalupe delta of the Texas Gulf Coast, in Morgan, J. P. ,
ed. , Deltaic sedimentation-modern and ancient: Soc. Econ.
Paleontologists and Mineralogists Spec. Pub.. 15, p. 107-137.
R. J.., 1962, Classification of carbonate rocks according to
depositional texture: Am. Assoc. Petroleum Geologists Mem.
1, p. 108--121.
Dunham,
158
Folk, R. L. , 1954, The distinction between grain size and mineral
composition in sedimentary-rock nomenclature: Jour. Geol. ,
v. 62, p. 344--359.
,
1962., Spectral subdivision of. limestone types:
Am.. Assoc. Petroleum Geologists Mem. 1, p. 62-84.
, 1968, Petrology of sedimentary rocks: Hemphill's,
Austin, Texas, 170 p.
and Ward, W. C. , 1957, Brazos River bar: a
study in the significance of grain size parameters: Jour. Sed.
Petrology, v. 31, no. 4, p. 514-529.
Friedman., G. M. , 1962, Comparison of moment measures for sieving and thin-section data in sedimentary and petrological studies:
Jour. Sed. Petrology, v. 32, p. 15-25.
Gould, H. R.
1970, The Mississippi delta complex, in Morgan,
, Deltaic sedimentation - modern and ancient: Soc.
Econ. Paleontologists and Mineralogists Spec. Pub. 15, p. 3,
J. P. ,
ed..
30.
Grim, R. E., .1968, Clay mineralogy, .2nd ed., McGraw-Hill, New
York, 596'p.
1926, Astoria: mid-Tertic type of the Pacific Coast:
Pan. Amer. Geologist, v. 45, p. 295 -306.
Howe-, H. V.
,
Huntting, M. T. , Bennett, W.A. G. , Livingstone, V. E. , Jr., and
Meon, W. S. , 1961, Geologic map of Washington: Washington
State Dept. Conserv. and Div. Mines and Geology.
Inman, D. L.., .1952, .Measures for describing the size distribution of
sediments: Jour. Sed. Petrology, v. 22., p. 125-145.
Kulm, L. D. and Fowler, G. A,, 1974a, Cenozoic sedimentary frame
work of the Gorda-Juan de Fuca Plate and adjacent continental
margin - a.review: Geosyncline Symp.
1974b, Oregon continental margin structure: a test
of the imbricate thrust model, in Continental Margins, Burke,
C. , and Drake, C. , eds.
,
159
Kuno, Hisashi, .1968, Differentiation of basalt magmas, in Hess, H.
and Poldervaart, Arie, eds. , Basalts: the Poldervaart treatise
on rocks of basaltic composition: Interscience Publishers, New
York, 862 p.
Laniz, R. V.., Stevens, R. E.., and Norman, M. B.,, 1964,. Staining
of plagioclase feldspar and other minerals: U. S. Geological
Survey, Prof. Paper 501B, p. 152-153.
Lowry, W. D. and Baldwin, E. M.. , 1952, Late Cenozoic geology of
the lower Columbia River valley, Oregon and Washington:
Geol. Soc. America Bull. , v. 63, p. 1-24.
McKee, E. D.., and Weir., G. W. , 1953, Terminology for stratification and cross-stratification in sedimentary rocks: Geol. Soc.
America Bull.., v. 64, article p. 381 -390.
Milner, H. B.., 1940, Sedimentary Petrography, 3rd ed.: Nordeman
Publishing Co. , New York, 666 p.
Moore, E. J.., 1963, Miocene marine mollusks from the Astoria
Formation in Oregon: U. S. Geol. Survey Prof. Paper 419,
109 p.
Niem, A. R. , 1974, Formation of large-scale penecontemporaneous
folds and intraformational breccias in the Astoria Formation,
northwestern Oregon: Geol. Soc. America Abstracts with
Programs, v. 6, no. 3, p. 229.
and Cressy, F. B. , Jr. , .1973, K-Ar Dates for sills
from the Neahkanie Mountain and Tillamook Head areas of the
northwestern Oregon coast: Isochron/West, v. 7, no. 3, p.
13-15.
and Van Atta, R. 0.., 1973, Cenozoic stratigraphy
or northwestern Oregon and adjacent southwestern Washington,
in Beaulieu, J. D. , ed. , Geologic field trips in northern
Oregon and southern Washington: Oregon Dept. Geol. and
Mineral Indus..., Bull. 77, p. 75-92.
Niem, A. R., Van. Atta, R. 0.., Livingston, Vaughn, and Rau, W. W..,
1973, Cenozoic geology of northwestern Oregon and adjacent
southwestern Washington, in Beaulieu, J. D. , ed. , Geologic
field trips in northern Oregon and southern Washington; Oregon
Dept. Geol. and Mineral Indus. , Bull. 77, p. 93-132..
160
Off, T., 1963, Rhythmic linear sand bodies caused by tidal currents:
Am. Assoc. Petroleum Geologists Bull., v. 47, p. 324-341.
Oomkens, E. , 1967, Depositional sequences and sand distribution in
a deltaic complex: Geologie en Mijnbouw, v. 46e, p. 265 -27 8.
Passega, R. , 1957, Texture as a characteristic of clastic deposition:
Am. Assoc. Petroleum Geologists Bull. v. 41, p. 1952-1984.
Pease, M. H. , Jr. , and Hoover, Linn, 1957, Geology of the DotyMinot Peak area, Washington: U. S. Geol. Survey Oil and Gas
Inv. Map OM-188-, scale 1:62,300.
Peck, D. L., Griggs,, A. B., Schlicker, H. G. Wells, F. G., and
Dole, H. M. , 1964, Geology of the Central and Northern parts
of the Western Cascade Range in Oregon: U. S. Geol. Survey
Prof. Paper 449, 56 p.
Potter, P. E. and Pettijohn, F. J. , 1963, Paleocurrents and basin
analysis: Berlin, Springer-Verlag, 296 p.
Rau, W. W. , Biostratigrapher, Washington Department of Natural
Resources,, Geology and Earth Resources Division,. Written
communication, 1974.
Ross, C. P. and Forrester, J.
1947, Geologic map of the State
Idaho: U. S. Geol. Survey and Idaho Bureau of Mines and
S. ,
Geology.
Royse, C. F. , Jr.., 1970, An introduction to sediment analysis:
Tempe, Arizona, 180 p.
Schlicker, H. F. , Corcoran, R. E.
and Bowen, R. G. , 1961,
Geology of the Ecola State Park landslide area, Oregon: The
,
Ore Bin, v. 23, no. 9, p. 85-90.
, Beaulieu, J. D.., and Olcott, G.
1972, Environmental geology of the coastal region of
Tillamook and Clatsop Counties, Oregon: Oregon Dept. Geol.
and Mineral Indus. Bull. 74, 164 p.
Schlicker, H. F.., Deacon, R. J.
W.
,
Selley, R. C. , 1970,,Ancient sedimentary, environments: Cornell
University Press, New York, 237 p.
161
Smith, T. N-, 1975, Stratigraphy and sedimentation of the Onion Peak
area, Clatsop County, Oregon: Master of Science thesis,
Oregon State University, Corvallis.
Snavely, P. D. , Jr. and Baldwin, E. M. , 1948, Siletz River volcanic
series, northwestern Oregon: Am. Assoc. Petroleum Geolo-
gists Bull., v.
32_, p. 805 -812,
Snavely, P. D., Jr.., MacLeod, N. S., and Rau, W. W.., 1969b,
Geology of the Newport area, Oregon: The Ore Bin, v. .31, no.
2, p. 25-71.
Snavely, P. D., Jr., MacLeod, N. S., and Wagner, H. C.,, 1968,
Tholeiitic and alkalic basalts of the Eocene Siletz River Volcanics, Oregon Coast Range: Am. Jour.. Sci. , v. 266, no. 6,
p. 454-481.
Snavely, P. D., Jr., MacLeod, N. S., and Wagner, H. C., 1973,
Miocene tholeiitic basalts of coastal Oregon and Washington and
their relations to coeval basalts of the Columbia Plateau: Geol.
Soc. America Bull., v. 84, no. 2, p. 387 -424.
Snavely, P. D. , Jr. , and Wagner, H. C.,, 1963, Tertiary geologic
history of western Oregon and Washington: Washington State
Div. Mines and Geol. Rpt. Invest. 22, 25 p.
Snavely, P. D.., Jr. and Wagner, H. G. , 1964, Geologic sketch of
northwestern Oregon: U. S. Geol. Survey Bull. 1181-M, p.
.
1-17.
and MacLeod, N. S. , 1969a, Geology of western
Oregon north of the Klamath Mountains, in Mineral and Water
Resources of Oregon: Oregon Dept. Geol. and Mineral Indus..,
,
Bull. 64, p. 32-46.
Stanley, S. M. , 1970, Relation of shell form to life habits of theBivalvia (Mollusca): Geol. Soc. America Mem..125, 296 p.
Taylor, E. M. , 1974, Taylor's cookbook for standard chemical analysis: unpublished lab manual, Oregon State Univ. Dept. of Geol..,
Co rvallis.
162
Turner, D. L.., 1970, Potassium-argon dating of Pacific Coast
Miocene Foraminiferal Stages, in Bandy, 0. L.., ed.,, - Radiometric dating and paleontologic zonation: Geol. Soc. America
Spec. Paper 124, p. 91-129.
U. S. Department of Commerce, 1972, Climatological Data, Oregon:
National Oceanic and Atmospheric Administration.
van Andel, Tj. H. , 1967, The Orinoco delta: Jour. Sed. Petrology,
v. 37, no.
2,,
p. 297-310.
Van Atta, R. 0. , 1971, Sedimentary petrology of some Tertiary
formations, upper Nehalem River basin, Oregon: Ph. D. thesis
Oregon State Univ. , Corvallis, 245 p.
Visher, G. S. , .1965, Use of vertical profile in environmental reconstruction: Am. Assoc. Petroleum Geologists Bull. , v. 49,
p. 41-61.
Walker, R.. G. and Mutti, E.., 1973, Turbidite facies and facies
associations, in Turbidites and deep-water sedimentation:
Soc. Econ. Paleontologists and Mineralogists, Pacific Section,
short course, Anaheim, p. 119-158.
Warren, W. C. and Norbisrath, Hans, 1946, Stratigraphy of the
upper Nehalem River basin, northwestern Oregon: Am. Assoc..
Petroleum Geologists Bull. , v. 30, p. 213-237.
Warren, W. C. , Norbisrath, Hans, and Grivetti, R. M. , 1945,
Geology of northwestern Oregon, west of the Willamette River
and north of latitude 450-15' : U. S. Geol. Survey Oil and Gas
Inv. Map OM 42.
Washburne, C. W. , 1914, Reconnaissance of the geology and oil
prospects of northwestern Oregon: U. S. Geol..Survey Bull.
590, .111 p.
Waters, A. C. , 1961, Stratigraphic and lithologic variations in the
Columbia River Basalt: Amer. Jour. Sci., v. 259, no.. 8, p.
583-611.
Wells, F. G. and Peck, D. L., 1961, Geologic map of Oregon west
of. the 121st meridian: U. S. Geol. Survey Misc. Geol. Inv.
Map 1-325, in coop. with Oregon Dept. Geol. and Mineral
Indus.
163
Wilkinson, W. D., Lowry, W. D., and Baldwin, E. M.., 1946,
Geology of the St. Helens quadrangle, Oregon: Oregon Dept.
Geol. and Mineral Indus., Bull. 31, 39 p.
Williams, H. , Turner, F. J., and Gilbert, C. M.., 1954, Petrology;
an introduction to the study of rocks in thin sections: W. H.
Freeman and Company, San Francisco, 406 p.
APPENDICES
164
APPENDIX I
Reference Section A-B: Landslide Scarp, Ecola State Park
Silver Point Member of Astoria Formation
Initial Point (A): center of section 18, T. 5 N. , R. 10 W.
Section starts at top of an excellent exposure along a 150-foot
high scarp produced by the Ecola State Park landslide of 1961.
The scarp is due east of Ecola Point. The top of the section
is a Depoe Bay Basalt sill which caps the ridge.
The section trends down the northern face of the landslide
scarp, across a covered area produced by landslides and down
to the unnamed point north of Ecola Point. From this point,
the section is offset due north to the unnamed beach south of
Indian Beach. Accessibility to this part of the section is only
by walking via Indian Beach south across Bald Point.
Terminal Point (B): SW 1/4 NW 1/4 of section 18, T. 5 N., R. 10 W.
Section ends at the southern margin of a landslide toe in the
center of the unnamed beach between Bald Point and Ecola Point.
165
APPENDIX I (continued)
Reference Section A -B
Unit
33
Description
Thickness (feet)
Unit
Total
30.0
625.8
17, 7
595.8
4.0
591.8
Depoe Bay Basalt sill, poorly developed
columnar jointing.
Contact: baked, sharp, brecciated.
32
Silty mudstone: light olive gray (5Y6/1)
to greenish black (5GY2/1); massive to
irregularly bedded; micaceous; fossiliferous
with shell fragments and forams; contains
light olive brown (5Y5/6) calcareous, one to
two-inch thick sandstone lenses.
31
Covered by colluvium.
30
Silty mudstone: olive gray (5Y3/2) to
dark yellowish orange (10YR6/6); well-
bedded; micaceous; contains randomly
interbedded 1/2-to two-inch thick shell
layers of clams, gastropods,
and forams,;
contains very fine-grained sandstone
lenses up to two inches thick and local
yellowish gray (5Y7/2) calcareous
concretions up to six inches long;
166
Appendix I (continued)
Unit
Description
Thickness (feet)
Total
Unit
non-resistant, breaks into small
angular chips. Mudstone character
changes vertically. Lower mudstones;
olive gray (5Y3/2); silty; micaceous; locally
calcareous; contain forams and shell
fragments. Upper mudstones; grayish
olive (10Y4/2); very thinly laminated
(10 laminae/inch) in beds up to six inches
thick; micaceous. Capped by dark
yellowish orange (10YR6/6) thinly bedded
(up to one inch thick) mudstone.
Contact: Covered. Offset 200 feet due
north to northern face of landslide scarp.
Large chevron fold associated with a sill
is exposed. Fold axis is cut by a sixinch thick Depoe Bay Basalt dike. Begin
measuring below dike.
29
Mudstone: moderate yellowish brown
(10YR5/4); bedded; contains moderate
brown (10YR4/4) to dark yellowish
23.0
574..1
167
Appendix I (continued)
Description
Unit
Thickness (feet)
Total
Unit
orange (1OYR6/6) calcareous concretions
up to one inch in diameter and three.inches
long; weathers to angular chips and nodules.
Contact: gradational over several feet.
28
Interbedded sandstones and mudstones;
sand/mud=1/10. Sandstones are light
gray (N7) to olive gray (5Y4/1); medium-
to very fine-grained; poorly sorted; well
bedded in layers up to six inches thick;
thinly laminated; calcareous, well cemented;
forms resistant ribs; contacts with mudstones are sharp. Mudstones are olive
gray (5Y4/1) to dark greenish gray
(5 GY4/1);
thinly laminated in beds from
four inches to one foot thick; local fossil
horizons of clams, gastropods, and
forams occur; shell fragments are
ubiquitous; contain one inch interbeds
and stringers of dusky yellow (5Y6/4)
silty mudstone.
21.5
551.1
168
Appendix I (continued)
Description
Unit
Contact: baked, brecciated, irregular.
27
Unit
Total
57.0
529.6
1.0
472.6
110.0
452.6
8.0
342.6
Depoe Bay, dike: "peperite"
Contact: baked, brecciated, irregular.
26
Thickness (feet)
Interbedded sandstones and mudstones:
as unit 23.
25
Covered; offset due west to seacliff on
unnamed point north of Ecola Point.
24
Depoe Bay Basalt sill:
Contacts: brecciated, conformable to
bedding..
23
Interbedded sandstones and mudstones;
sand/mud=2/5,. Sandstones are olive
gray (5Y3/2) to yellowish gray (5Y7/2).
weather dusky yellow (5Y6/4) to grayish
brown (5YR3/2); medium-to very fine-
grained; subangular to sub rounded grains;
very poorly sorted; micaceous; carbonaceous; beds are from one inch to one
foot thick; contain shell fragments, forams,
and angular carbonate rock fragments up to
169
Appendix I (continued)
Description
Unit
16 mm in diameter; sedimentary struc-
tures include parallel and small-scale cross
laminations, massive to normally graded
units, divisions a, b, and c of the Bouma
sequence, load casts, flute casts, flames,
and mic rofaulting.
Mudstones are olive gray (5Y3/2) to
moderate brown (5YR4/4); individual beds
are from one to ten inches thick; thinly
laminated; carbonaceous and micaceous;
fossiliferous with shell fragments and
forams; contain small calcareous concretions (less than one inch in diameter
and less than two inches long) and thin
calcareous sandstone lenses; nonresistant,
weathers to small angular ships and
nodules.
Contacts (within the unit): Generally
sharp, planar basal contacts of the
sandstones with the mudstones. Sandstones
Thickness (feet)
Total
Unit
170
Appendix I (continued)
Thickness (feet)
Description
U nit
Unit
Total
73.0
334.6
typically grade upward into overlying
mudstones. Gradational lower contacts
and sharp upper contacts locally exist.
Representative bed thicknesses in
unit:
8T
sandstone bed
9 1/2" mudstone
7 1/2" sandstone bed
1"
sandstone bed
mudstone
l" sandstone bed
2!'
mudstone
4" mudstone
311
sandstone bed
1" sandstone lens
2"
mudstone
7" mudstone
sandstone bed
1" sandstone lens
II
10" mudstone
2"
mudstone
6"
sandstone bed
6" mudstone
311
mudstone
l" sandstone bed
sandstone bed
2" mudstone
mudstone
5" sandstone bed
1/211
3"
sandstone lens
Contact: gradational; inaccessible.
171
Appendix I (continued)
Description
Unit
22
Thickness (feet)
Unit
Total
27.0
261.6
41.0
234.6
20.0
193.6
Contorted zone: interbedded sandstones
and
mudstones: same as unit 23
Contact: baked, irregular; sediment
incorporated into sill along contact.
21
Depoe Bay Basalt sill: irregular,
apophyse-like intrusion.
20
Covered by ocean: offset 100 feet
due north to unnamed beach south of
Indian Beach. Accessibility is only, via
Indian Beach
19
Interbedded sandstones and mudstones;
sand/mud=3/2.
Sandstones are very
light gray (N8), weather light olive gray
(5Y6/1); medium-to very fine-grained;
very poorly sorted; micaceous and carbonaceous; well cemented with carbonate;
beds are up to 10 inches thick; structures
include massive, parallel, and cross
laminated beds, load structures, flames,
pillow and ball structures, vertical and
172
Appendix I (continued)
Description
Unit
sub-parallel burrows.
and 22).
Thickness (feet)
Unit
Total
32.0
173.6
20.0
141.6
32..0
121.6
17.0
89.6
(See Figure 21
Flames indicate a paleoslope
to the southwest.
Mudstones are dark
gray (N3); micaceous and carbonaceous;
beds up to six inches thick; fossiliferous
with forams and pelecypod fragments.
Contacts (within unit): Sharp, planar,
commonly undulating bottom contacts of
sandstone layers with underlying mudstones.
Sandstones grade normally upward into silty
mudstones.
18
Covered by landslide debris; lithology
thought to be same as unit 19.
17
Depoe Bay Basalt sill
Contact: baked, brecciated, sharp.
16
Cape Foulweather Basalt sill
Contact: baked, sharp, brecciated.
15
Interbedded sandstones and mudstones:
lithology same'as unit 19. Thicker sandstone units are massive in lower portions,
173
Appendix I (continued)
Unit
Description
,
Thickness (feet)
Unit
Total
18.0
72.6
9.0
54.6
finely laminated in middle portion, and
cross laminated (small scale) in upper
parts.
Contact: sharp, planar.
14
Sandstone: yellowish gray (5Y7/2),
weathers dusky yellow (5Y6/4); poorly
sorted; normally graded from basal
coarse-grained sandstone to fine-grained
sandstone; well-indurated, calcareous,
ledge former; contains abundant mud rip-
ups, calcareous concretions several inches
in diameter, .load structures, and burrows
parallel and subparallel to bedding.. Two
3-inch thick interbeds of olive gray mudstone occur.
Upper and lower contacts
with the mudstones are sharp and planar.
Contact: sharp.
13
Mudstone: dark gray (N3); thinly
laminated; fossiliferous with sparse
shell fragments and forams; micaceous;
174
Appendix I (continued)
Description
Unit
Thickness (feet)
Unit
Total
0.6
45.6
2..0
45.0
1.5
43.0
carbonaceous.
Contact: gradational over three inches.
12
Sandstone: very light gray (N8), weathers
light olive gray (5Y6/1); medium- to very
fine-grained; very poorly sorted; micaceous
and carbonaceous; calcareous, well cemented;
lower portion is massive, upper parts are
thinly laminated. A and B divisions of
Bouma sequence are present.
Contact: sharp and planar.
11
Mudstone: dark gray (N3): micaceous;
carbonaceous; finely, laminated; contains
a 3-inch thick interbed of fine-grained,
light olive gray (5Y6/1) laminated sandstone
with a sharp bottom contact and
gradational upper contact.
Contact: sharp.
10
Sandstone: very light gray (N8); massive;
medium-grained; matrix-rich, poorly sorted;
micaceous; calcareous, well cemented.
175
Appendix I (continued
Description
Unit
Contact: sharp.
9
Thickness (feet)
Total
Unit
1.5
41.5
0.5
40.0
2.0
39.5
Mudstone: dark gray (N3); finely laminated;
micaceous; carbonaceous; non-resistant,
weathers to small angular chips.
Contact: gradational.
8
Sandstone: very light gray (N8); poorly
sorted; calcareous, well cemented;
normally graded from basal mediumgrained sandstone to fine-grained sand-
stone; lower portion is finely laminated
(5 lam/cm); middle portion is convolutely
laminated; top portion is massive; calcareous concretions and vertical burrows are
common.
Contact: sharp, planar.
7
Mudstone: dark gray (N3); micaceous,
carbonaceous; fossiliferous with pelecypod
fragments and forams; contains thin 1 to
2-inch calcareous sandstone lenses; non-
resistant, weathers to chips and mud.
176
Appendix I (continued)
Description
Unit
Thickness (feet)
Total
Unit
Contact: gradational over 1 foot.
6
9.0
37.5
2.8
28.5
7.0
25.7
1.6
18.7
Sandstone: light gray (N7) to light olive
gray (5Y6/1); fine-grained; micaceous;
carbonaceous, abundant matrix; calcareous,
.well cemented, ledge former; bottom portion
of bed is massive and grades normally upward
into very fine-grained, thinly laminated
sandstone in the upper foot of the unit.
Contact: sharp
5
Mudstone: dark gray (N3); carbonaceous;
micaceous; massive; non-resistant, weathers
to small angular chips.
Contact: gradational over six inches.
4
Sandstone: very light gray (N8), weathers
light olive gray (5Y6/1); fine-grained;
micaceous, carbonaceous; finely laminated;
numerous loading structures and small
calcareous concretions are present.
Contact: sharp.
177
Appendix I (continued)
Description
Unit
Thickness (feet)
Unit
3
Total
Mudstone: dark gray (N3); carbonaceous,
micaceous; foram bearing; massive;
contains thin, 1 to 3-inch thick, light
olive gray (5Y6/1) sandstone lenses and
beds with sharp bottom contacts and gradational upper contacts. Numerous clastic
dikes and other loading structures are
pres ent.
Contact: gradational.
2
11.5
Sandstone: very, light gray (N8), weathers
light olive gray (5Y6/1); micaceous,
carbonaceous; poorly sorted; normally
graded from medium-grained basal sandstone to very fine-grained upper sandstone;
calcareous, well cemented; contains sparse
flutes and loading structures.
Contact: sharp and planar.
1
Mudstone: dark gray (N3); carbonaceous,
micaceous; very thinly laminated to massive;
4. 3
5.6
178
Appendix I (continued)
Description
Unit
Thickness (feet)
Total
Unit
contains dispersed shell fragments and
forams; weathers to small angular chips
non-resistant.
Contact: covered by landslide debris.
1.3
1.3
179
APPENDIX II
Reference Section C -D: Haystack Rock
Silver Point Member of the Astoria Formation
Initial Point: Center of south half section 30, T..5 N. , R. 10 W.
Section starts on the northeastern face of Haystack Rock at the
distinctive contact between the overlying basaltic breccia and
the sediments beneath. This contact represents the strati-
graphic top of the Astoria Formation at this locality. Evidence
for an unconformity is discussed in the text.
Terminal Point: The section trends southeast from the initial point,
.goes over a small hill, and then drops down to the beach where
it is covered with beach sand and basaltic boulders derived
from Haystack Rock.
180
APPENDIX II
Reference Section C-D; Haystack Rock
Description
Unit
11
Thickness (feet)
Total
Unit
Cape Foulweather basaltic isolated
pillow breccia.
Contact: irregular; baked; brecciated;
Bits of sediment incorporated into basaltic
250.0
378.0
5.0
128.0
burrowed.
2.0
123.0
8
Covered by beach cobbles.
3.0
121.0
7
Interbedded sandstones and mudstones:
breccia; unconformity.
10
Sandstone: yellowish gray (5Y7/2),
weathers light olive (lOY5/4) to pale
olive (10Y6/2); fine-grained; subangular;
well sorted; parallel laminations and planar
cross bedding are present; well indurated,
carbonate cemented.
Contact: sharp and undulatory; characterized
by loading structures, flames, and clastic
dikes.
9
Muddy
siltstone: light gray (N7), weathers
light gray (N6); calcareous; mottled, well
181
Appendix II (continued)
Description
Unit
Thickness (feet)
Total
Unit
sand/mud=1/4 sandstones are yellowish
gray (5Y7/2), weathers light olive (1OY5/4);
calcareous, well cemented; thinly laminated
(8 laminations/inch); very micaceous; high
matrix content, dirty; beds are less than one
inch thick; contains local small scale cross
laminations. Mudstones are olive gray
(5Y3/2); micaceous; silty; massive to very
thinly laminated (12 laminations/inch);
fossiliferous with shell fragments and
forams; contains small, flattened calcareous
nodules and calcareous sandy lenses.up to
1/2 inch in diameter and three inches long.
Pervasive burrowing, vertical and subparallel to bedding, obscures bedding and
creates mottled appearance of unit.
Small scale, (less than one foot displacement), high angle, normal faults are
common.
Contact: sharp, baked.
27.0
118.0
182
Appendix II (continued)
6
Thickness (feet)
Total
Unit
Description
Unit
Cape Foulweather Basalt dike: "peperite"
texture; contains incorporated Silver Point
member rock fragments.
Contact: sharp; baked;
5
irregular.
10.0
91.0
46.0
81.0
2.0
35.0
8.0
33.0
12.0
25.0
13.0
13.0
Interbedded sandstones and mudstones:
as unit 7; forams from this unit are
Sauc e s i an St age.
Contact: sharp; baked; brecciated.
4
Cape Foulweather Basalt dike.
Contact: sharp; baked; brecciated.
3
Interbedded sandstones and mudstones:
as unit 7, but sand/mud-110.
Contact: sharp; brecciated; contorted.
2
Cape Foulweather Basalt dike: parallels
unit 6. Both units extend into the sea.
Perhaps are feeder dikes.
Contact: sharp; brecciated.
1
Interbedded sandstones and mudstones:
as unit 3.
Covered: Beach sand and cobbles.
183
APPENDIX III
Reference Section E-F: near Seaside on Tillamook Head
Oswald West Mudstones
Initial Point: NW 1 /4 SW 1 /4 of section 29, T. 6 N., R. 10 W. The
section commences at the top of a mudstone seacliff which is
reached by walking south on the beach from Seaside toward
Tillamook Head. Walking distance from the sand beach at
Seaside across a cobble and boulder beach to the section is
approximately one mile.
Terminal Point: Section terminates at the base of the seacliff. The
lower section is covered by landslide debris and recent gravel
terraces.
184
APPENDIX III
Reference Section E-F
Unit
Description
5
Mudstone: light olive gray (5Y6/1) to
Thickness (feet)
Total
Unit
grayish orange (10YP7/4); well bedded;
burrowed: contains sparse one inch to
three-inch thick, moderate brown
(5YP4/4) glauconitic sandstone beds: one
to two-inch thick beds are common.
Contact: gradational over one foot.
4
20.0
221.0
Mudstone: light olive gray (5Y6/1),
weathers light olive gray (5Y5/2) to
pale reddish brown (10R5/4); well bedded;
contains vertical burrows and burrows
sub-parallel to bedding; foram bearing;
non-resistant, weathers to small angular
chips; sparse interbedded one to four-inch
thick burrowed pumiceous dusky yellow (5Y6/4)
medium-grained sandstones.
Contact: gradational over several feet.
3
Mudstone: grayish green (1OG4/2);
massive, non-laminated; contains
32.0
201.0
185
Appendix III(continued)
Description
Unit
Thickness (feet)
Total
Unit
numerous calcareous concretions up to
two feet long; non-resistant, weathers
to small angular chips..
Contact: gradational over several feet.
2
Interbedded sandstones and mudstones;
sand/mud ratio varies from 1/4 in the
lower section to 1/20 in the upper section.
Sandstones are dark greenish gray (5GY4/1)
to yellowish gray, weathers dusky yellow
(5Y6/4) to greenish black (5GY2/1); mediumto coarse-grained; subangular to rounded;
poorly sorted, high matrix content; locally
glauconitic; calcareous, well cemented,
forms ribs; some beds normally graded;
beds one inch to six inches thick, rhythmically
spaced
every six inches to one and one half
feet. Bottom contacts are sharp; upper
contacts gradational into overlying
mudstones.
Mudstones are light olive gray (5Y6/1),
33.0
169,0
186
Appendix III (continued)
Unit
Description
Thickness (feet)
Total
Unit
weather light olive gray (5Y5/2) to pale
reddish brown (lOR5/4); massive; well
burrowed; foram bearing; beds up to
two feet thick; one to two inch tuff beds
are common; non-resistant, weathers
to small angular chips.
Contact: covered by talus.
87.0
136.0
49.0
49.0
Mudstone: talus slope.
Contact: covered by cobble beach and
gravel terraces.
APPENDIX IV
CHECKLIST OF FOSSILS FROM THE OSWALD WEST MUDSTONES
Locality
Fossil
A
B
C
D
E
F
G
H
I
J
K
-
-
-
-
-
-
-
-
x
-
-
-
-
-
-
-
-
-
-
-
-
x
-
x
-
-
-
-
?
-
-
-
-
?
-
-
-
-
-
-
-
-
-
-
x
L
BIVALVIA
Nucula sp.
-
SCAPHOPODA
Dentalium cf. D. Pseudonyms Pilsbry and Sharp
FORAMINIFERA
Cassidulina crassipunctata Cushman & Hobson
Gyroidina sp.
Virgulina californiensis Cushman
Sigmoilina tenius (Czjzek)
Clavulina sp.
Bagging (?) sp.
Pseudoglandulina sp.
Cibicides sp.
Uvigerina sp.
Dentalina spp.
Dentalina dusenburyi Beck
Gyroidina obicularis lp anata Cushman
Pullenia salisbury R. E. and K. C. Stewart
Cibicides elmaensis Rau
Plectofrondicularia packardi packardi:
-
x
-
x
-
x
-
-
x
-
-
-
-
x
-
-
-
x
-
-
-
x
-
-
-
-
-
-
-
-
-
-
-
-
Robulus inornatus d'Orbigny
-
-
-
Lagena sp.
-
-
Quingueloculina imperialis Hanna and Hanna
-
-
Uvigerina cocoaensis Cushman
-
-
-
-
x
-
-
-
x
x
-
x
-
-
-
-
-
x
-
-
-
-
x
-
?
-
-
-
-
x
-
x
-
x
-
-
-
-
-
-
-
x
-
-
-
x
x
-
-
-
x
-
-
-
-
-
-
Cushman and Schenck
Cassidulina cf. C. globosa Hantken
?
-
x
x
-
-
-
-
M
N
APPENDIX IV (continued)
Locality
Fossil
A
B
C
D
E
F
G
H
I
J
K
L
M
N
FORAMINIFERA (continued)
x
Quinqueloculina weaveri Rau
Anomalina californiensis Cushman & Hobson
Cassidulina cf. C. balvinensis Cushman & Frizzell
Cassidulinoides sp. (?)
x
x
x
x
Gandryina alazanensis Cushman
Bulimina cf. B. alsatica
x
Arenaceous Foraminifera
Diatoms
Radiolaria
Spicules
x
-
x
-
-
x
x
-
x
x
x
x
-
-
x
x
TRACE FOSSILS
Scalarituba (Helminthoida)
(fecal ribbon form)
Terebellina (agglutinated worm tube)
x
LEGEND
Locality
Field No.
USGS Cenozoic Loc.
Location
SE 1/4, SE 1/4, section 34, T.S N. , R. 9 W.
"
SE 1/4, SW 1/4, section 27,
"
NW 1/4, SE 1/4, section 27,
A
f-7
B
f-20
C
D
f-29
E
f-35
F
9/33
G
f-65
SW 1/4, NW 1j4, section 27,
SW 1/4, SW 1/4, section 22,
center, section 34,
SW 1/4, SE 1/4, section 27,
f-57
NE 1/4, SE 1_/4, section 23,
H
f-30
M5986
"
"
"
"
APPENDIX IV (continued)
Locality
Field No.
USGS Cenozoic Loc.
Location
I
f-63
SW 1/4, NE 1/4, section 34, T. 5 N., R. 9 W.
J
f-64
SW 1/4, SE 1/4, section 27,
It
K
f-6
it
L
tf-1
tf-2
f-12
NE 1/4, NE 1/4, section 23,
SW 1/4, SE 1/4,.section 19,
NW 1/4, SE 1/4, section 27,
SW 1/4, SE 1/4, section 28,
M
N
M5985
APPENDIX V
CHECKLIST OF FOSSILS FROM THE SILVER POINT MEMBER
Locality
Fossil
A
B
C
D
E
F
G
H
K
I
L
M
N
O
P
BIVALVIA
Lucinoma sp.
Nucula sp.
Tellinid
Nucula Hannibali Clark
fragment
x
x
x
x
-
x
FORAMINIFERA
Bulimina cf. B. ovata d' Orbigny
Valvulinera araucana d'Orbigny
x
x
-
Globigerina spp.
x
x
-
Bolivina advena Cushman
Buliminella subfusiformis Cushman
Nodogenerina advena Cushman and Laiming
Si±ogenerina sp.
Nonion coastiferum Cushman
UviQerinella obesa impolita Cushman and Laiming
x
x
x
x
-
-
-
-
x
x
x
x
x
?
x
x
-
-
-
x
-
-
-
-
-
-
-
x
-
-
-
x
x
-
?
?
x
x
?
Bulimina inflata alligata Cushman and Laiming
Epistominella parva (Cushman and Laiming)
UviQerina californica Cushman
x
-
-
?
-
-
x
x
-
Nonion incisum kernensis Kleinpell
-
-
-
Cassidulina crassipunctata Cushman n-and Hobson
Cassidulina sp.
Dentalina sp.
Plectofrondicularia sp.
Gyroidina sp.
Virgulina californiensis Cushman
-
?
-
-
-
-
-
x
x
x
x
x
-
-
-
-
-
x
x
-
-
x
-
x
-
x
-
x
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
x
x
-
-
-
-
-
-
-
-
-
-
?
-
-
x
-
?
-
x
x
-
x
x
x
-
-
-
?
x
-
-
x
-
-
-
x
x
-
-
-
-
x
-
-
-
x
-
x
x
-
-
x
-
x
x
-
-
-
-
-
-
-
-
-
-
-
-
?
-
-
x
x
-
-
x
-
-
-
-
-
F,
o
APPENDIX V (continued)
Locality
Fossil
A
B
C
D
E
F
H
G
I
K
L
M
N
O
P
x
Epistominell a cf. E. subperuviana Cushman
Sigmoilina miocenica Cushman
Amphimorphina. californica (Cushman and
x
R. E. Stewart)
x
Bolivina marginata adelaidana Cushman and
Kleinpell
Buliminella ellegantissima d'Orbigny
Robulus inornatus d'Orbigny
Siphogenerina cf. S. Kleinpelli Cushman
x
Arenaceous Foraminifera
x
x
Radiolaria
Diatoms
-
-
-
x
x
x
x
Fish vertebra
TRACE FOSSILS
x
A sterosoma/ T eichichnus
LEGEND
Locality
Field No.
A
f-2
B
f-62
f-51
f-52
f-53
C
D
E
F
G
H
USGS Cenozoic Loc.
M5982
SW 1/4, NE 1/4, section 18, T. 5 N. , R. 10 W.
SW 1/4, SE 1/4, section 30,
SE 1/4, SE 1/4, . section 17,
NW 1/4, NE 1/4, section 20,
NW 1/4, NE 1/4, section 20,
center, section 18,
SW 1/4, NW 1/4, section 18,
M5983
SE 1/4, SE 1/4, section 30,
f-54
f-55
f-3
Location
APPENDIX V (continued)
Locality
Field No.
0
f-32
f-45
f-48
f-1
f-17
f-43
f-66
P
f-4
I
K
L
M
N
Location
USGS Cenozoic Loc.
NE 1/4, section 31, T. S N. , R. 9 W.
SW 1/4, NE 1/4, section 5, T. 6 N. , R. 10 W.
NE 1/4, SE 1/4, section 31, T. 6 N., R. 10 W.
SW 1/4, SE 1/4, section 35, T. 5 N. , R. 10 W.
NE 1/4, NE 1/4, section 30, T.5 N., R. 9 W.
NE 1/4, NW 1/4, section 4, T. 5 N., R 10 W.
NW 1/4, NW 1/4, section 18, T. S N. , R. 10 W.
SE 1/4, section 30, T. 6 N., R. 10 W.
NW
APPENDIX VI
MODAL ANALYSES OF SANDSTONE SAMPLES (600 points)
(See Plate I for sample locations)
Sample:
Cement (CaCO 3 )
OW-1
1%
OW-2
tr
Silver Point member
Angora Peak sandstones
Oswald West mudstones
A-26
T-8
EcPt
T-10
Tb-i
As-20
AP-1
AP-2
AP-3
-
-
-
8%
25%
5%
6%
?
22%
319/6
27%
499/6
53%
64%
67%
51%
19%
19%
24%
21%
tr
tr
10%
Matrix
32%
28%
30%
18.29/6
21%
26%
33%
Grains
67%
72%
60%
81.8%
79%
76%
59%
Quartz
27%
24%
1%
Quartzite
tr
Stable Grains:
Chert
299/0
21%
11%
14%
-
3%
2%
2%
tr
-
1%
3%
1%
1%
2%
tr
3%
8%
7%
4%
12%
9%
12%
15%
3%
7%
20%
11%
1%
13%
12%
9%
8%
5%
9%
6%
12%
11%
7%
6%
25.5%
32%
33%
13%
11%
18%
20%
-
-
tr
1.9%
4%
-
-
-
tr
-
5%
5%
5%
1%
1%
1%
1%
-
6%
-
5%
1%
1%
1%
1%
4%
1%
-
3%
-
-
1%
1%
1%
1%
2%
1%
1%
-
2%
1%
1%
t
1%
Feldspar:
Plagioclase
K-spar
17%
Rock Fragments:
Volcanic
Metamorphic
Sedimentary
Mica
Mafic
Opaques
Glauconite
4%
tr
-
-
tr
Others
-
2%
Porosity
-
-
tr. - less than 0.5%
48%
tr
1%
tr
3%
2%
tr
3%
tr
-
tr
1%
tr
-
tr
1%
tr
tr
tr
tr
-
3%
1%
3%
5%
1%
1%
-
3%
1%
1%
1%
-
-
1%
APPENDIX VI (continued)
Location
Sample
Oswald West mudstones:
OW-1
Roadcut: SW 1/4 SE 1/4, section 19, T. 5 N., R. 9 W.
OW-2
Roadcut: SE 1/4 SE 1/4, section 27, T. 5 N. , R. 9 W.
A -26
Roadcut: SW 1/4 SE 1/4, section 27, T. 5 N. , R. 9 W.
Angora Peak sandstones:
AP-1
Roadcut: SE 1/4 SW 1/4, section 20, T. 5 N., R. 9 W.
AP-2
Roadcut: NE 1/4 SW 1/4, section 20, T. S N., R. 9 W.
AP-3
Roadcut: NW 1/4 SE 114, section 20, T. 5 N. , R. 9 W.
Silver Point mudstones:
T-8
Seacliff south of Indian Beach: SW 1/4 NW 1/4, section 18, T. SN., R. 10 W.
EcPt
South face of unnamed point north of Ecola Point: SW 1/4 NW 1/4,, section 18, T. 5 N., R. 10 W.
T-10
Seacliff south of Indian Beach: SW 1/4 NW 1J4, section 18, T. 5N, R. 10 W.
T6-1
Near foot of Crescent Beach Trail, Ecola State Park: NE 1/4 SW 1/4, section 18, T. S N. ,
R. lO W.
As-20
Roadcut southeast of Tillamook Head: SE 1/4 SW 1/4, section
6, T. 5 N., R. 10 W.
APPENDIX VII
SELECTED ANALYSES OF SELECTED SANDSTONES
Statistical parameters are those of Folk and Ward (1957). (See Plate I for sample locations)
Samples:
Sand
Silt
Clay
%
%
%
Coarsest
1% mm
Median'
Median'
phi
Mean
phi
Sorting
phi
Skewness
mm
phi
Kurtosis
phi
,Oswald West mudstones
A-26
91.38
7.01
1.61
1, 1
0.26
1.90
1.97
1.16
0.18
1.48
AP-1
85.24
10.S4
4.22
0.98
0.26
1.90
2.27
1.70
0.49
1.96
AP-12
98.30
1.70
--
0.95
0.29
1.80
1.82
0.85
0.04
1.10
T-18
77.26
17.94
4.80
0.55
0.10
3.40
3.S3
1.43
0.30
2.86
T-14
75.14
20.45
4.41
0.52
0.10
3.40
3.60
1.64
0.30
2.43
96.84
3.26
--
0.30
0.18
2.50
2.48
0.33
-0.02
1.17
Angora Peak sandstones
Silver Point member
Quaternary Marine Terrace
Qmt
1 From Inman (1952).
2 Recalculated matrix free.
196
APPENDIX VII
(Continued)
STATISTICAL PARAMETERS FROM SIZE ANALYSES
PLOTTED ON PASSEGA'S (1957) C-IART
C:--4,.,000-
Irrrs
x,=,r: rivers, tractive currents
I
turbidity currents
X.,
quiet water deposits
beaches
C=1, 000-
C=100
M=10
M=100
M = median in microns
C = size with 1% coarser in microns
M=1, 000
197
APPENDIX VII
(Continued)
STATISTICAL PARAMETERS FROM SIZE ANALYSES
PLOTTED ON FRIEDMAN' S (1962) GRAPHS
+2
\
T-14
T-18
AP-1
+1
O(
O AP-1
O Qmt
0
A -26
-1
surf
-2
I
T
0.2
0.4
0.6
0.8
1.2
1.0
Standard Deviation (sorting)
1
.
0
. 8
0. 6
-
A-26
AP-1
AP-1 2 O
!
I
T-14
T-18
tractive
currents /
-i
mixed
0.4 -
0 2-I
,i
11
wind
-1
Mean Size (in V)
O Qmt
APPENDIX VII (continued)
Sample
Location
Oswald West mudstones
A-26
Roadcut: SW 1/4 SE 1/4, section 27, T. 5 N., R. 9 W.
Angora Peak sandstones
AP-1
Roadcut: SE 1/4 SW 1/4, section
20, T.5 N., R.9 W.
Silver Point mudstones
T-18
Seacliff south of Indian Beach: SW 1/4 NW 1/4, section 18, T. 5 N., R. 10 W.
T-14
Same location as T-18.
Quaternary Marine Terrace
Qmt
Beach exposure south of Tolovana Park (town): SE 1/4 SE 1/4, section 31, T. 5 N., R. 10 W.
199
APPENDIX VIII
HEAVY MINERALOGY OF SELECTED SAMPLES*
(A = abundant, C = common, P = present)
Samples
Quaternary
Marine Terrace
Glauconitic
Silver Point
Member
Angora Peak
Sandstone
Sandstone
Burrowed
Sandstone
A
A
A
P
P
P
A
A
Minerals
Glass
C
Monazite
Hypersthene
A
Clinopyroxene
C
P
Micas
A
P
P
C
Zircon
A
C
A
P
Hornblende:
Basaltic
Green
*
A
P
C
P
Epidote
P
Rutile
P
A
C
Apatite
P
P
Tourmaline
-
P
P
Garnet
-
C
C
P
Opaques
A
C
A
P
List compiled according to individual formations from sandstone samples listed in Appendix VI.
P
200
APPENDIX IX
CLAY MINERALOGY OF SELECTED SAMPLES
Procedure
Pretreatment of mudstones and clays from the matrix of the
sandstone samples included disaggregation by gentle grinding and
soaking in water, wet sieving through a 40 sieve, soaking in 0. 1 N
hydrochloric acid to remove calcium carbonate, addition of 30%
hydrogen peroxide to remove organic matter, and soaking with
sodium dithionite (Na2S2O4) to remove iron.
The fine-grained dis-
aggregated samples were then dispersed in a calgon solution and
centrifuged to separate the clays from the silt fraction. The clay size
fraction was smeared onto glass slides for X-ray analysis. The runs
were made on each sample 1) dried at room temperature; 2) treated
with ethylene glycol overnight; 3) heated to 4000C for one hour; and
4) heated to 6000C for one hour.
Fine-grained aggregates of zeolite and clay, which were difficult
to identify from thin sections
of
sandstones and basalt, were X-rayed
in a powder camera. These minerals were identified by comparing
diffraction data obtained from the powder photographs with known
diffraction patterns of minerals published in the Inorganic Index to the
Powder Diffraction File (Berry,.1971).
APPENDIX IX (continued)
CLAY MINERALOGY OF SELECTED SAMPLES
(Locations are shown in Plate I)
Chloritic
Sample
Montmorillonite
Chlorite
Intergrades
Kaolinite
Mica
Vermiculite
A eolite
Oswald West mudstones:
XR-3-D
X
X
XR-3-C
X
XR-3-B
X
XR-3-A
X
A-26
X
XR-2
X
X
X
X
-
X
X
X
X
?
X
X
X
?
X
XR-1-C
XR-1-B
X
XR-1-A
X
X
X
X
Silver Point mudstones
X
X
X
X
X
XR-4-B
X
X
f-32
X
Ecola landslide
XR-4-A
X
?
X
-
X
APPENDIX IX (continued)
Chloritic
Sample
Montmorillonite
Chlorite
Intergrades
X
X
X
X
X
-
X
X
Kaolinite
Mica
Vermiculite
Zeolite
Sandstone matrices:
Oswald West sandstones:
OW-1
X
OW -2
A-26
X
Angora Peak sandstones
AP-1
X
-
?
X
X
X
Silver Point sandstones
T-8
X
-
X
-
X
-
X
T-10
?
-
?
-
X
-
X
APPENDIX IX (continued)
Location (See Plate I)
Sample
Oswald West mudstones:
XR-3
Mudstone outcrop in roadcut: NW 1/4 SW 1/4, section 26, T. 5 N., R. 9 W.
A-26
Massive mudstone above glauconitic sand: SW 1/4 SE 1/4, section 27, T. S N. , R. 0 W.
XR-2
Bentonite bed in roadcut: SW 1/4 NW 1/4, section 34, T. 5 N. , R. 9 W.
XR-1
Mudstone outcrop in roadcut: SE 1/4 SE 1/4, section 23, T. 5 N., R. 9 W.
Oswald West sandstones:
OW-1
Roadcut: SW 1/4 SE 1/4, section 19, T.5 N., R. 9 W.
OW-2
Roadcut: SE 1/4 SE 1/4, section 27, T. S N., R. 9 W.
Angora Peak sandstones:
AP-1
Roadcut: SE 1/4 SW 1/4, section 20, T. 5 N. , R. 9 W.
Silver Point mudstones:
Ecola landslide
Near top of Ecola State Park Landslide scarp: SW 1/4 NE 1/4, section 18, T. 5 N. , R. 10 W.
XR-4
Center of Ecola State Park Landslide scarp: SW 1/4 NE 1/4, section 18, T.5 N. , R. 10 W.
f-32
Roadcut: NW 1/4 NE 1/4, section 31, T. 5 N. , R. 9 W.
Silver Point sandstones:
T-8
Seacliff south of Indian Beach: SW 1/4 NW 1/4, section 18, T.5 N. , R. 10 W.
T-10
Same location as T-8.
APPENDIX X
CHEMICAL ANALYSES OF SELECTED BASALT SAMPLES
MTL68-39*
MTIL68-40*
SR61-30*
Sample
RHN-1
RHN-3
RHN-4
RHN-5
RHN-6
RHN-7
Si02
56.5
53.0
53.0
54.5
56.0
49.8
55.4
59.8
53.8
55.3
52.6
A12 3
13.7
13.4
13.4
12.7
13.5
12.7
14.0
13.0
14.8
14.1
14.4
FeO
12.8
15.0
14.4
12.9
13.2
13.7
12.6
11.9
11.7
13.2
13.9
MgO
2.2
3.2
3.4
4.2
2.6
4.8
4.0
1.9
5.1
3.4
3.9
CaO
7.0
8.6
8.5
9.0
6.2
9.2
7.S
5.2
8.7
7.0
7.8
Nat
3.4
3.3
3.1
2.9
2.2
3.1
3.1
3.3
3.1
3.1
2.9
K20
1.61
0.78
1.08
0.68
1.39
0.81
1.2
2.4
0.53
1.5
1.2
TiO
1.98
3.12
3.01
1.88
1.79
2.61
1.9
1.8
1.7
1.9
2.6
2
* From: Snavely and others, 1973.
Sample Locations:
Depoe Bay Basalts:
RHN-1: Breccia north of Kidder's Butte: NW 1/4 SW 1/4, section 34, T.5 N., R. 9 W.
RHN-5: Sill: SE 1/4 NE 1/4,. section 10, T. 5 N., R. 10 W.
RHN-6: Sill below RHN-1: SW 1/4 NW 1/4, section 34 , T. 5 N. , R. 9 W.
MTIL68-39: Sill: SW 1/4, section 31, T. 6 N., R. 10 W.
SR61-30: Dike: SW 1/4, section 17, T. 5 N., R. 10 W.
SR61-32: Dike: SE 1/4, section 17, T. S N. , R. 10 W.
Cape Foulweather Basalts:
RHN-3: Sill: SE 1/4 NE 1/4, , section 5, T. 5 N. , R. 10 W.
RHN-4: Intrusive top of Twin Peaks: SW 1/4 NW 1/4, section 1, T. S N. , R. 10 W.
RHN-7: Pillow Breccia on Haystack Rock: SW 1/4 SE' I/4, section 30, T. 5 N. , R. 10 W.
MT168-37: Dike: NW 1/4, section 18, T. 5 N. , R. 10 W.
SR61-32*
MR168-37*