Sedimentology, provenance, and tectonic setting of the Miocene Horse Camp Formation (Member 2),
east-central Nevada
by Brian Keith Horton
A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in
Earth Sciences
Montana State University
© Copyright by Brian Keith Horton (1994)
Abstract:
A study of the sedimentology, compositional trends, and modem tectonic setting of a 1600 m-thick
sedimentary succession provides information on the late Cenozoic paleogeography and tectonics of an
extensional basin and adjacent orogen in the Basin and Range province of east-central Nevada.
Interpretations of depositional environments and reconstructions of paleogeography utilize detailed
sedimentologic analyses of fourteen distinct facies and four facies associations from nine measured
stratigraphic sections. Conglomerate compositional data and considerations of modern tectonic setting
and local geologic structures provide constraints on the tectonic development of the extensional basin
and orogen.
Within the Miocene Horse Camp Formation (Member 2), four facies associations are defined by
specific combinations of fourteen potential facies and attributed to processes characteristic of particular
depositional environments. Subaerial and subaqueous fan-delta environments were characterized by
deposition of sediment gravity flows and an absence of fluvial processes. A nearshore lacustrine facies
association recorded wave-influenced sedimentation near a lake shoreline. Quiet-water deposition and
limited sediment gravity flows characterized an offshore lacustrine environment.
Lateral facies variations and stratal thinning and fining trends suggest a sediment source terrane to the
south/southeast in the northern Grant Range. Simulated unroofing of this source terrane generated a
hypothetical clast composition suite that is similar to actual conglomerate compositional data from
Member 2. Therefore, Member 2 strata recorded early to late Miocene unroofing of the northern Grant
Range portion of an extensional orogen. A lack of lower Paleozoic clasts in Member 2 conglomerates
requires at least 1 km of post-Miocene (post-Member 2)uplift in order to account for unroofing of the
modern northern Grant Range down to lower Paleozoic rocks.
The 3000 m-thick Horse Camp Formation is exposed in an uplifted footwall adjacent to a modern
extensional basin, Railroad Valley, to the west. The Horse Camp Formation was deposited in a
Miocene extensional basin prior to the development of Railroad Valley. Post-Miocene development of
Railroad Valley coeval with uplift and eastward tilting of Horse Camp strata is explained by a model
that requires a westward migration of the zone of active normal faulting and footwall uplift. SEDIMENTOLOGY, PROVENANCE, AND TECTONIC SETTING OF THE MIOCENE HORSE
CAMP FORMATION (MEMBER 2), EAST-CENTRAL NEVADA
by
Brian Keith Horton
A thesis submitted in partial fulfillment
of the requirements for the degree
of
Master of Science
in
Earth Sciences
MONTANA STATE UNIVERSITY
Bozeman, Montana
May 1 9 9 4
-fisng
ii
APPROVAL
of a thesis submitted by
Brian Keith Horton
This thesis has been read by each member of the thesis committee and has been
found to be satisfactory regarding content, English usage, format, citations, bibliographic
style, and consistency, and is ready for submission to the College of Graduate Studies.
Approved for the Major Department
T - </- f y
Date
Head, Major Department
Approved for the College of Graduate Studies
Date
Graduate Dean
iii
STATEMENT OF PERMISSION TO USE
In presenting this thesis in partial fulfillment of the requirements for a master's
degree at Montana State University, I agree that the Library shall make it available to
borrowers under rules of the Library.
If I have indicated my intention to copyright this thesis by including a copyright
notice page, copying is allowable only for scholarly purposes, consistent with "fair use"
as prescribed in the U.S. Copyright Law. Requests for permission for extended quotation
from or reproduction of this thesis in whole or in parts may be granted only by the
copyright holder.
Signature
Date
iv
ACKNOWLEDGMENTS
I would like to thank Dr. Jim Schmitt for support, encouragement, and advice
above and beyond the call of a graduate advisor. His expertise and enthusiasm were vital
to the completion of this project. I thank Drs. Dave Lageson and Joan Fryxell for
discussions of the structural complexities of the study area. Discussions with Dr.
Stephan Custer, Scott Singdahlsen, and Ted Campbell are greatly appreciated. Jessie
Smith provided invaluable encouragement, field assistance, financial assistance, and
culinary expertise. Her personal decision to pause her academic studies so that I could
finish my degree in a timely fashion is appreciated and will not be forgotten. I also thank
my parents, Steve and Cindy Horton, for their interest and encouragement throughout my
academic pursuits.
Field work was supported by research grants from the American Association of
Petroleum Geologists, Geological Society of America, and Sigma Xi. Other financial
support included funds from a MSU Graduate Presidential Scholarship and the 1993 D.L
Smith Memorial Scholarship. A National Science Foundation Graduate Research
Fellowship supported academic studies during the 19 9 3 -9 4 school year. This material
is based upon work supported under a National Science Foundation Graduate Research
Fellowship. Any opinions, findings, conclusions or recommendations expressed in this
publication are those of the author and do not necessarily reflect the views of the National
Science Foundation.
V
TABLE OF CONTENTS
Page
INTRODUCTION...................................................................................................................................
I
GEOLOGIC SETTING............................................................................................................................. 3
STRATIGRAPHY OF THE HORSE CAMP FORMATION............................................................ ... . 8
SEDIMENTOLOGY OF THE HORSE CAMP FORMATION (MEMBER 2 ) ........................................ 1 0
Facies Analysis........................................................................................... ....................
11
Facies G m u ..................... .................................. . . , ...................................
13
Ungraded, Matrix-Supported C onglom erate.................. ...
13
Interpretation........................................................................................13
15
Facies G m n .............................................................. ...........................
Normally Graded, Matrix-Supported Conglomerate . . . . . .
15
Interpretation........................................................................................ 15
Facies Gcu ........................................................................................................... 17
I 7
Ungraded, Clast-Supported Conglom erate.........................
Interpretation.............. ......................................................................... 17
Facies G c n ............................ ............................................................................... 19
Normally Graded, Clast-Supported Conglom erate......................
19.
Interpretation........................................................................................ 19
Facies Gh . , .................................................................. ..................................
21
Horizontally Stratified Conglom erate............................................... 21
Interpretation.........................................................................................21
Facies S m ............................................................................ ............................... 2 2
Massive Sandstone................................................................................2 2
Interpretation.......................................... ................. ............................2 2
Facies S n ............................................. ... ..........................................................
25
Normally Graded Sandstone . . . ................................ ...
25
Interpretation........................................................................................ 2 5
Facies S h ..................... ........................... ... , . .......................................; .
26
Horizontally Stratified Sandstone .............................
26
Interpretation . ..................................................................................... 2 6
Facies S r .............. .............................................................................................
28
Ripple Cross-Laminated Sandstone . , ......................... ................. 2 8
Interpretation................................................................ : .................... 2 8
Facies S t ............................................. ................................................................. 2 9
Trough Cross-Stratified Sandstone . ............................................... 2 9
Interpretation.........................................................................................2 9
Facies Fm . ........................................................ ................................................ 29
Massive to Poorly Laminated Mudstone . . . .................................29
Interpretation.............. ..........................................................................31
vi
TABLE OF CONTENTS— Continued
Page
Facies C lm ...................................................................................
Laminated or Massive C arbonate............................................
Interpretation........................................................................................... 34
Facies V t b ............................................................................................................... 34
Volcanic Tuff Breccia ....................................................
Interpretation...................................................................
Facies V m b .......................................................................................... ... . . . 3 6
Volcanic Bedrock Megabreccia.........................................
Interpretation........................................................................................... 3 9
Facies Associations and Depositional Environments.................................................
39
Subaerial Fan-Delta Facies A ssociation........................................ i . . .
41
Subaqueous Fan-Delta Fapies Association........................................................4 3
Nearshore Lacustrine Facies A ssociation...........................................
Offshore Lacustrine Facies Association............................................................ 4 6
Environmental Reconstruction..........................................................................................4 6
Subaerial-Subaqueous T ra n sitio n .................................................................... 4 8
Nearshore Environm ent........................................................................................ 51
Pyroclastic Flows and Megabreccias.........................................
PROVENANCE MODELING APPLIED TO THE HORSE CAMP FORMATION (MEMBER 2) . . 55
Source Terrane Evaluation . . . ...................................................................................... 57
Provenance Modeling of the Horse Camp Form ation................................................. 6 0
Data Collection........................................................................................
Procedure and Application................................................................................... 61
Implications..................... % .............................................................................
66
Provenance Modeling for Extensional B asin s...........................................
INVERSION OF THE MIOCENE HORSE CAMP B A S IN ............................. ' . . . . ..................
72
Tectono-Sedimentary Sequences..........................................................
Horse Camp Formation and Equivalent S tr a ta ...................................
Alluvial Deposits of the Grant Range . . ..........................................
Alluvial Deposits of Railroad V a lle y ................................................................. 74
Basin Inversion M o d e l.............................................................
Implications for Studies of Extensional B asin s..........................• . ........................ . 7 8
CONCLUSIONS . .................................................................................................................................8 0
REFERENCES C IT E D ........................................................................................................................... 8 2
3
31
3
35
36
45
53
60
70
7
73
74
vi i
LIST OF TABLES
Table
1. Lithofacies of Member 2, Horse Camp Form ation......................... ; .........................
Page
11
2. Conglomerate clast composition data from 1600 m section of Member 2 ,
Horse Camp Form ation.................................................................................’
3. Lithology of northern Grant Range stratigraphic source section ..............................
62
vi i i
LIST OF FIGURES
Figure
Rage
I • Index map of basin and range physiography of east-central N e v a d a ............................. 4
2. Geologic map of the northern Grant Range-Florse Range-southern White
Pine Range re g io n ............................................................................................................
5
3. Schematic stratigraphic cross section of the Horse Camp F o rm atio n .........................
8
4. Facies Gmn with top-only normal grading overlain by facies Gcu and
facies Gmu. Facies Sm and Sh separate conglomerate fa c ie s .............. ................. 1 4
5. Facies Gcu overlain by two beds of facies Gmu with thin interbeds of
facies Sm, Sb, and F m ..................................................................................................... 14
6. Facies Gmn: lower bed has poorly developed normal grading and load and
flame structure, upper bed has clast-rich base
andoutsized c l a s t .................
16
7. Facies Gcu interbedded with facies Sm and F m ......................................................................16
8 . Facies Gcu (lenticular shape and erosional base) within finer grained
conglomerate and sandstone fa c ie s ...............................................................................
18
9. Facies Gcn with upward increase in matrix c o n te n t............................. ... . . ' ...............18
10. Facies Gcn with basal inverse grading and thin interbeds of facies Sm and Fm . .
20
11. Facies Gh and interbedded facies Sh with uniform grain s i z e .......................................... 2 0
12. Facies Sm and Sn with interbedded facies Fm . . .
.....................
23
13. Facies Sn and Sm with thin bed of poorly laminated facies Sh. Basal
inverse grading and top-only normal grading present in Sn b e d s ...................... 2 3
14. Subvertical root casts in facies S m ..................................................................................
24
15. Flame structures in facies S h .............................................................................................
24
1 6. Ball and pillow structures in facies S h ...................... ......................................................
27
17. Facies Sn foresets dip l e f t ....................................................................................................... 2 7
18. Facies St and cap of facies Sh, Gh, and C lm .......................................... . ■
.............. ...
30
ix
LIST OF FIGURES— Continued
Figure
Page
19. Facies Fm overlain by facies S m ................................ ........................................................3 0
20. Facies Clm interbedded with facies Gh and S h .............. .................................................
32
21. Convex-upward laterally linked hemispheroid geometry of facies C l m ................... 3 2
22. Planar, subhorizontal, 0 .2 -5 mm-thick laminations of facies C l m ........................
33
23. Ooids and pisoids of facies Clm. Grains are well-rounded and
moderately spherical. Deposit is poorly sorted .......................................................3 3
24. Asymmetric fold in deformed substrate zone of facies Vmb . ....................................3 7
25. Mixed zone (MZ) and resistant, overhanging breccia sheet (BS) of
facies V m b .................................................................................................................. ... . 3 7
26. Jigsaw (JB) and crackle (CB) breccias of facies V m b .................................
38
27. Comminuted slip surface separates upper crackle breccia (CB) from
underlying jigsaw (JB) and crackle (CB) breccia z o n e .............................................. 3 8
28. Detailed stratigraphic section of subaerial fan-delta facies a ssociation...............
40
29. Detailed stratigraphic section of subaqueous fan-delta facies association................. 4 0
30. Detailed stratigraphic section of nearshore lacustrine facies association................. 4 0
31. Repetitive sequence of laterally continuous beds of subaerial fan-delta
facies association....................................................................................................... .
42
32. Environmental reconstruction in which relatively deep w ater in the
nearshore lacustrine environment promotes high-energy reworking
processes and prevents formation of a subaqueous fa n -d e lta ............................
49
33. Environmental reconstruction in which relatively shallow w ater near
the lake shoreline precludes reworking of subaqueous fan-delta
deposits by high-energy wave processes................................................................... 4 9
34. Steps involved in application of provenance m odeling.................................................. 5 6
35. Northern Grant Range stratigraphic source s e c tio n ......................................................
58
36. Comparison of hypothetical clast composition suites generated from an
unroofing model of the northern Grant Range and actual clast
composition data from Member 2 of the Florse Camp Form ation.........................6 5
X
LIST OF FIGURES— Continued
Figure
Page
37. Generalized tectonic model illustrating Miocene uplift of the northern
Grant Range as part of the footwall block of the down-to-the-north
Ragged Ridge f a u l t .................................................... ....................
. . . .................. 68
38. Schematic representation of modem tectpno-geomorphic setting of
Railroad Valley and deposits of the Horse Camp Formation .................................... 7 6
39. Generalized three-phase model depicting the tectonic development of the
Horse Range, Horse Camp Basin, and Railroad V a lle y ............................................
77
xi
LIST OF PLATES
Plate
I . Logs of measured stratigraphic sections of the southern exposures
of the Horse Camp Formation (Member 2 ) ......................... ... .
Page
back pocket
x ii
ABSTRACT
A study of the sedimentology, compositional trends, and modem tectonic setting of
a 16 00 m-thick sedimentary succession provides information on the late Cenozoic
paleogeography and tectonics of an extensional basin and adjacent orogen in the Basin and
Range province of east-central Nevada. Interpretations of depositional environments and
reconstructions of paleogeography utilize detailed sedimentologic analyses of fourteen
distinct facies and four facies associations from nine measured stratigraphic sections.
Conglomerate compositional data and considerations of modern tectonic setting and local
geologic structures provide constraints on the tectonic development of the extensional
basin and orogen.
Within the Miocene Horse Camp Formation (Member 2), four facies associations
are defined by specific combinations of fourteen potential facies and attributed to
processes characteristic of particular depositional environments. Subaerjal and
subaqueous fan-delta environments were characterized by deposition of sediment gravity
flows and an absence of fluvial processes. A nearshore lacustrine facies association
recorded wave-influenced sedimentation near a lake shoreline. Quiet-water deposition
and limited sediment gravity flows characterized an offshore lacustrine environment.
Lateral facies variations and stratal thinning and fining trends suggest a sediment
source terrane to the south/southeast in the northern Grant Range. Simulated unroofing
of this source terrane generated a hypothetical clast composition suite that is similar to
actual conglomerate compositional data from Member 2. Therefore, Member 2 strata
recorded early to late Miocene unroofing of the northern Grant Range portion of an
extensional orogen. A lack of lower Paleozoic clasts in Member 2 conglomerates requires
at least I km of post-Miocene (post-Member 2)uplift in order to account for unroofing
of the modfern northern Grant Range down to lower Paleozoic rocks.
The 3 0 0 0 m-thick Horse Camp Formation is exposed in an uplifted footwall
adjacent to a modern extensional basin, Railroad Valley, to the west. The Horse Camp
Formation was deposited in a Miocene extensional basin prior to the development of
Railroad Valley. Post-Miocene development of Railroad Valley coeval with uplift and
eastward tilting of Horse Camp strata is explained by a model that requires a westward
migration of the zone of active normal faulting and footwall uplift.
I
INTRODUCTION
Deposits of ancient extensional basins possess a wealth of information about
depositional processes and paleoenvironments during extension (e.g., Golia and Stewart,
1984; Hendrix and lngersoll, 1987; Cavazza, 1989; Fedo and Miller, 19 9 2 ). These
deposits also contain a record of the tectonic evolution of adjacent sediment source
terfanes. In fact, if an extensional orogen has been uplifted and completely eroded, the
only record of its temporal evolution may be the sediment derived from that orogen. In
order to gain as much information as possible from an extensional basin sequence, an
integrated study incorporating sedimentologic and stratigraphic data with sediment
compositional trends is required. This study utilizes these various data sources to
decipher the sedimentary processes, depositional environments, paleogeography, and
tectonic development of an extensional basin and adjacent orogen in the Basin and Range
province of western North America.
The Miocene Horse Camp Formation (Member 2) is an extensional-basin
sequence in east-central Nevada which contains a continuous 15 0 0 -2 0 0 0 m-thick
/
succession of conglomerate and sandstone with minor amounts of mudstone, carbonate,
megabreccia and volcanic tu ff breccia (Moores, 19 6 8 ). This sequence is amenable to a
study th at evaluates the area's sedimentary and tectonic history. First, the well-exposed
deposits are in an area with a fairly well-defined structural geology. A previous study
by Moores and others (1 9 6 8 ) has defined probable basin-margin faults and the
stratigraphy of potential sediment source terranes. In addition, the Miocene tectonic and
sedimentary conditions recorded by the Horse Camp Formation can be coupled with the
2
area's modern tectonic setting to produce a more complete account of its late Cenozoic
history.
This study is divided into three primary sections. First, sedimentoiogic and
stratigraphic descriptions of the Horse Camp Formation (Member 2) provide a basis for
interpretations of the depositional environments and paleogeography of a Miocene
extensional basin. Second, compositional analysis of Member 2 conglomerates is
incorporated into a provenance model that defines the late Cenozoic tectonic evolution of a
particular source terrane within the adjacent extensional orogeh. Third, a model
depicting late Cenozoic tectono-geomorphic evolution of the area is developed based upon
constraints imposed by sedimentary and tectonic data preserved in the Miocene
extensional-basin sequence and present in the modern geologic environment.
3
GEOLOGIC SETTING
Outcrops of the Miocene Horse Camp Formation and the surrounding northern
Grant Range, Horse Range, and southern White Pine Range (Fig. I ) comprise the footwall
of the Railroad Valley fault, a modern, west-dipping normal fault (Fig. 2). A modern
extensional basin within the hanging wall of this fault, Railroad Valley (Fig. I ) , contains
. an eastward thickening, 3 0 0 0 m-thick sequence of late Miocene to Holocene sedimentary
rocks (Bortz and Murray, 1979; Effimoff and Pinezich, 1 98 1 ).
Exposures of the Horse
Camp Formation are separated from Railroad Valley by the Railroad Valley fault and from
the surrounding ranges by a north-striking, west-dipping normal fault (north segment
of Ragged Ridge fault) and two east-striking transverse fault zones (Currant Summit and
Ragged Ridge-Stone Cabin fault zones; Fig. 2).
Jurassic-Cretaceous c'ontractional deformation in the northern Grant Range and
southern White Pine Range consisted of minor east-vergent folding (Fig. 2) and
associated axial-planar cleavage development in the lower Paleozoic section (Camilleri,
1992; Walker and others, 1992; Lund and others, 199 3 ). In the Horse Range, eastvergent overturned folds in Devonian rocks (Fig. 2) and isoclinal folding and cleavage
development in the Cambro-Ordovician section originally attributed to Tertiary
extension (Moores and others, 1 96 8 ) probably record a regional Jurassic-Cretaceous,
east-west shortening episode (Taylor and others, 1 993). The overturned Paleozoic
section comprising most of the Horse Range (Fig. 2) is herein interpreted as the east
limb of a large, east-vergent anticline. Greater contraction within the Horse Range
(relative to the northern Grant Range and southern White Pine Range) may have been
accommodated by the east-striking Currant Summit and Ragged Ridge-Stone Cabin fault
4
zones (Fig. 2). These transverse structures may have acted as lateral ramps or tear
faults bounding a fold within the Horse Range. This scenario would account for the
overturned Paleozoic section defining the Horse Range and lack of such extensive
overturned sections in the northern Grant Range and southern White Pine Range (Fig. 2).
I
NEVADA
RANGE
WHITE
RIVER
VALLEY
HORSE
FIGURE 2
RANGE
RAILROAD
VALLEY
10
I I S 0SO1W
/
I I S 0W
Figure I . Index map of basin and range physiography of east-central Nevada.
5
Figure 2. Geologic map of the northern Grant Range-Horse Range-southern White
Pine Range region (a fte r Moores and others, 1 9 6 8 ). BSFZ1 Blind Spring fault zone;
CSFZ1Currant Summit fault zone; NRRF1 northern segment of Ragged Ridge fault; NWSF,
northern segment of Wells Station fault; RRF1 Ragged Ridge fault; RVF1 Railroad Valley
fault; SCFZ1 Stone Cabin fault zone; WSF1 Wells Station fault.
6
A Miocene phase of east-west extension is represented by down-to-the-west
normal faults. These faults formed at low angles to bedding, placed younger rocks over
older rocks, and attenuated or omitted strata. In the northern Grant Range, a distinct
stack of down-to-the-west normal faults within the Paleozoic section has been.uplifted
and tilted to shallow east-dipping orientations (Fig. 2; Camilleri, 1992; Lund and
others, 19 9 3 ). These faults arch over the northern Grant Range and converge westward
into a modern, west-dipping range-front fault (Railroad Valley fault).
Down-to-the-
west faults in the Horse Range (north segments of the Wells Station and Ragged Ridge
faults) place Oligocene or Miocene hanging wall rocks against lower Paleozoic footwall
rocks (Fig. 2). Motion along these normal faults probably downdropped the west limb of
a pre-existing, east-vergent anticline.
Enigmatic, down-to-the-west, low-angle
structures within the Paleozoic section of the western Horse Range (Fig. 2) are
characterized by extremely brecciated upper-plate rocks. These allochthonous upper
plate rocks have been interpreted as low-angle gravity slide blocks emplaced over
erosional surfaces (Moores and others; 1968; Moores, 1 968). The Horse Camp
Formation is interpreted to be deposits of an extensional basin that developed above the
west-dipping, north segment of the Ragged Ridge fault (Fig. 2). However, east-striking
transverse fault zones bounding the northern and southern margins of the basin (Currant
Summit and Ragged Ridge fault zones, respectively) were also fundamental structures
controlling basin subsidence (Fig. 2; Moores, 1968; Brown, 1993; Schmitt and others,
1 9 9 3 ).
These east-striking basin-margin structures are interpreted as transfer faults
or accommodation faults that separate areas of differential extension (Sengor, 1987;
Schmitt and others, 1 99 3 ). Paleocurrent patterns and strata! thickness trends within
the lower part of the Horse Camp Formation (Member I ) led Brown (1 9 9 3 ) to suggest
th at deposition of this sequence was coeval with initial uplift of the southern White Pine
Range to the north and northern Grant Range to the south.
7
Interpretations concerning the tectonic development of the Miocene extensional
basin are based exclusively on structural mapping by Moores and others (1 9 6 8 ). In
particular, north-striking faults are interpreted as down-to-the-west faults (Moores
and others, 19 6 8 ) although recent reconnaissance mapping of the area suggests that
previously unidentified structures may record a phase of down-to-the-east normal
faulting (J. Fryxell, personal commun., 19 9 4 ). The northern segment of the Ragged
Ridge fault is inferred to be a down-to-the west normal fault bounding the eastern
margin of the Miocene extensional basin (Fig. 2). However, there are no unequivocal
>
exposures of this fault and it is inferred in order to account for the juxtaposition of the
Miocene Horse Camp Formation against Cambro-Ordovician rocks and the east-dipping
orientation of Horse Camp strata (Fig. 2; Moores and others, 19 6 8 ). In addition, the
southern boundary of the Miocene extensional basin is interpreted to be the east-striking
Ragged Ridge fault (Fig. 2; Brown, 1993; this study). However, the east-striking Blind
Spring fault zone (Fig. 2) cannot be ruled out as a potential basin-margin structure
active during Miocene extension.
Late Miocene to Holocene extensional faulting included movement along the rangefront Railroad Valley fault forming the eastern boundary of Railroad Valley (Fig. 2). In
general, this west-dipping structure has been considered a high-angle fault (Effimoff
and Pinezich, 1981; Walker and others, 1 9 9 2 ).
However, a shallowly west-dipping
(approximately 30 ) reflector on seismic profiles has been interpreted as the
subsurface expression of this range-front normal fault (Potter and others, 1991; Lund
and others, 199 3 ). Footwall uplift along the Railroad Valley fault has uplifted and tilted
the Miocene Horse Camp succession to a moderately east-dipping orientation (Fig. 2).
8
STRATIGRAPHY OF THE HORSE CAMP FORMATION
The Horse Camp Formation is a 3 0 0 0 m-thick sequence of conglomerate and
sandstone with minor amounts of mudstone, carbonate, megabreccia, and volcanic tuff
breccia (Fig. 3; Moores, 196 8 ). The maximum possible age of the Horse Camp
Formation is constrained by the age of the youngest underlying Oligocene volcanic unit
( 40ArZ39Ar date of 26.2 ± .5 Ma for biotite and sanidine of the Shingle Pass Formation;
Taylor and others, 19 8 9 ). A camel skull of Barstovian age in the lower half of the
formation (T. Fouch, personal commun., 1991) and gastropods and ostracods of
Barstovian-Clarendonian age in the upper strata of the formation (Van Houten, 1956;
Moores, 1968; Kleinhampl and Ziony, 1 9 8 5 ) reveal a Miocene age, roughly 2 5 -2 0 Ma
to 10 -5 Ma, for the Horse Camp Formation.
|q
ojconglomerale
20 km
.3
km
|
I sandstone
Member 4
,0
- 2 km
o
o
M em ber0 I
0
^ _ -_ -_ jmudrock
II
I Icarbonatfi
j
I T l megabreccia
_ I km
unconformity
Figure 3. Schematic stratigraphic cross section of the Horse Camp Formation
Stratigraphic data are from Moores (1 9 6 8 ), Brown (1 9 9 3 ), and this study.
Moores (1 9 6 8 ) divided the Horse Camp Formation into four unconformitybounded members (Fig. 3). Member I contains a 3 0 0 -6 0 0 m-thick sequence of
conglomerate and sandstone deposited on a system of alluvial fans along the northern,
western, and southern margins of the basin (Brown, 1993). Member 2 contains a
15 0 0 -2 0 0 0 m-thick sequence of conglomerate, sandstone, mudstone, and carbonate
representing a fan-delta system along the southern basin margin and a nearshore to
offshore lacustrine system in the central and northern part of the basin. Measured
sections and conglomerate clast counts of this study are located within the well-exposed,
Member 2 sequence along the southern basin margin (Fig. 3). Members I and 2
comprise approximately 2 5 0 0 m of the Horse Camp Formation. Poorly exposed
conglomerate,, sandstone, and carbonate of Members 3 and 4 account for less than 5 00 m
of the Horse Camp Formation and represent alluvial deposition in the central and eastern
parts of the basin. The presence of a western sediment source area during deposition of
Member I (Moores, 1968; Brown, 1 99 3 ) suggests that the Railroad Valley region
(Figs. I and 2) was a positive topographic feature and not a sedimentary basin during
Miocene time. Therefore, the Horse Camp Formation is not correlative with the
sedimentary basin fill of Railroad Valley; rather, the Horse Camp Formation records an
older phase of extension and associated basin development.
10
SEDIMENTOLOGY OF THE HORSE CAMP FORMATION (MEMBER 2)
A fan-delta is an alluvial fan deposited directly into a standing body of water
(McPherson and others, 1 9 8 7 ). Although alluvial fan and lacustrine deposits are
commonly associated with each other (e.g., Link and Osborne, 1978; Tucker, 1978;
Ballance, 1984; Golia and Stewart, 19 8 4 ), documented examples of lacustrine fan-delta
deposits (e.g., Gloppen and Steel, 1981; Nemec and others, 1984; Billi and others,
1 99 1 ) are relatively uncommon. In fact, most lacustrine delta deposits of ancient and
modern systems are dominated by facies indicative of fluvial deposition (e.g., Link and
Osborne, 1978; Sneh, 1979; Dunne and Hempton, 1984; Link and others, 1985;
Frostick and Reid, 1986; Chough and others, 1990; Changsong and others, 1991; Glover
and O'Beirne, 19 9 4 ) and may be classified as deposits of braid deltas (McPherson and
others, 19 8 7 ). One reason for the lack of documented lacustrine fan-delta deposits,
particularly those dominated by facies indicative of mass-flow processes, may be related
to inherent difficulties in the distinction between subaerial and subaqueous fan deposits.
Without adequate fossil assemblages, recognition of an ancient subaqueous fan deposit
relies upon distinctions between facies of subaerial and subaqueous affinity. Some
workers suggest that a facies assemblage characteristic of a shoreline or beach
environment .may serve as an important stratigraphic marker between subaerial and
subaqueous fan-delta deposits (e.g., Wescott and Ethridge, 1983; Bourgeois and Leithold,
1984; Link and others, 1985; Colella, 1988; Pivnik, 1990; Renaut and Owen, 1 9 9 1 ).
This study focuses on interfingered coarse-grained siliciclastic alluvial fan and
finer grained carbonate and siliciclastic lacustrine strata of the Miocene Horse Camp
Formation of east-central Nevada (Fig. I ) . The excellent exposure of these units
provides an opportunity to document the sedimentary processes and paleoenvironments of
a coarse-grained alluvial fan and adjacent lake basin. In particular, because this close
association of subaerial fan and lacustrine deposits suggests the possibility of subaqueous
fan deposition, detailed facies analysis was undertaken in order to distinguish
characteristic facies associations of the constituent depositional environments. Four
defined facies associations are attributed to unique depositional processes within four
distinct environments (subaerial fan-delta, subaqueous fan-delta, nearshore lacustrine,
and offshore lacustrine environments). This investigation indicates that detailed facies
analysis facilitates a distinction between subaerial and subaqueous fan deposits in a
single stratigraphic sequence and, in general, provides a better understanding of the
depositional processes of subaerial fan environments, lacustrine environments, and
their transitions.
Facies Analysis
Facies analysis of the southern exposures of Member 2 of the Florse Camp
Formation utilizes variations in grain size, primary sedimentary structures, grading,
and matrix content. Table I presents the major features of each facies and
interpretations of their respective depositional processes. Facies codes are modified
from Miall (1 9 7 7 ) and Waresback and Turbeville (1 9 9 0 ).
These facies are
characteristic of fan-delta and lacustrine depositional environments dominated by
sediment gravity flows.
i
Table I . Lithofacies of Member 2, Florse Camp Formation
Facies
Gmu
Description
Poorly sorted granule-boulder conglomerate;
matrix support, no grading or poor inverse grading
Interpretation
Plastic debris flows
12
Table I . (continued)
Facies
Description
Interpretation
Gmn
Poorly sorted granule-boulder conglomerate;
matrix support, normal grading
Gcu
Poorly sorted granule-boulder conglomerate;
clast support, no grading or poor inverse grading
Gcn
Poorly sorted granule-boulder conglomerate;
clast support, normal grading
Hyperconcentrated flows;
high-density turbidity
currents
Gh
Moderately sorted granule-pebble conglomerate;
clast support, no grading, horizontal stratification
Sheetfloods; wave-driven
traction
Sm
Medium- to very coarse-grained sandstone;
no grading or poor inverse grading, no
stratification, outsized granule-pebble clasts
Hyperconcentrated flows;
high-density turbidity
currents
Sn
Fine- to very coarse-grained sandstone; normal
grading, no stratification, basal granulepebble clasts
Hyperconcentrated flows;
high- or low-density
turbidity currents
Sh
Fine- to very coarse-grained sandstone;
no grading, horizontal stratification
Sheetfloods; wave-driven
traction; high- or lowdensity turbidity currents
Sr
Very fine- to coarse-grained sandstone;
no grading, ripple cross lamination
Migrating wave and
current ripples
St
Very fine- to coarse-grained sandstone;
ho grading, trough cross stratification
Migrating dunes or bars
Fm
Massive or poorly laminated mudstone;
no grading, mudcracks
Waning flood flows;
suspension fallout
Clm
Massive or laminated carbonate; stromatolitic
hemispheroid laminations, secondary ooids
and pisoids, coarse sand-pebble lenses
Carbonate precipitation
and binding of calcareous
sediment by algal mats
Vtb
Rhyolitic-dacitic tu ff breccia; angular clasts,
no bedding or flow structures, rare xenoliths
Pyroclastic flows;
pyroclastic debris flows
Vmb
Megabreccia of Oligocene volcanic rock; jigsaw
and crackle brecciation, mixed with
substrate sand and mud, folded substrate
Rock avalanches
Pseudoplastic debris flows
Plastic or pseudoplastic
clast-rich debris flows;
• hyperconcentrated flows
13
Facies Gmu
Ungraded, Matrix-Support ed Conglomerate . This facies consists of ungraded to
weakly inversely graded, poddy to very poorly sorted, matrix-supported, granuleboulder conglomerate (Figs. 4 and 5). Typical clast sizes are 1 -3 0 cm in diameter, but
outsized clasts (> 5 0 cm) are common. Outsized clasts commonly project above the tops
of beds. The matrix consists of mud dr muddy sand. Facies Gmu lacks internal
stratification. Moderately to poorly developed, coarse-tail inverse grading is rare. Beds
of Gmu are 10 -2 0 0 cm thick and can be traced laterally for tens of meters. Most Gmu
boundaries are planar and non-erosional, although some basal load structures (similar
to Fig. 6 ) are present in intervals with interbedded mudstone (Fm).
,
Interpretation . Facies Gmu is attributed to deposition by subaerial or subaqueous
plastic debris flows (Gloppen and Steel, 1981; Shultz, 1984; Nemec and Steel, 1984;
Ghibaudo, 19 9 2 ). The lack of basal scour and clast imbrication indicates laminar (nonturbulent) flow during sediment transport (Enos, 19 7 7 ). The sediment support
mechanisms of these viscous debris flows were matrix strength and dispersive pressure
(Lowe, 1979; 19 8 2 ). Debris was deposited en masse (cohesive freezing) when the
applied shear stress decreased below the yield strength of the material (Lowe, 197 9 ).
Gmu deposits that exhibit an upward increase in matrix content, basal load and flame
structures, and abundant interbedded mudstone (Fm) are interpreted as products of
water admixture and associated decrease in matrix strength during transport, rapid
escape of pore fluid from a saturated substrate during deposition, and post-debris flow
suspension fallout, respectively. These features suggest deposition in a subaqueous
environment (Nemec and Steel, 198 4 ).
14
Figure 4. Facies Gmn with top-only normal grading overlain by facies Gcu and facies
Gmu. Facies Sm and Sh separate conglomerate facies. Knife is 8 cm long.
15
Facies Gmn
Normally Graded, Matrix-Supported Conglomerate . Normally graded, moderately
to poorly sorted, matrix-supported, granule-boulder conglomerate (Figs. 4 and 6) is the
most abundant facies of the study area. Clasts 5 -2 0 cm in diameter are abundant.
Outsized clasts (> 5 0 cm), usually found at the base of individual beds, are uncommon.
The matrix is composed of mud or muddy sand. This facies lacks internal stratification
but contains coarse-tail normal grading. Beds of Gmn are 2 -5 0 cm thick, extend
laterally for tens of meters, and have planar, non-erosional bases. Some distinctive beds
of this facies (Fig. 6) are characterized by horizontally aligned clasts, an upward
increase in matrix content, and bases with load and flame structures. These beds are
commonly interbedded with mudstone (Fm).
Interpretation . Facies Gmn is interpreted as deposits of subaerial or subaqueous
pseudoplastic debris flows (Shultz, 1984; Pivnik, 19 9 0 ).
Non-erosional bases indicate
laminar flow (Enos, 19 7 7 ), but abundant normal grading and common horizontal
alignment of clasts suggest occasional turbulent flow during transport (Nemec and Steel,
1984; Rust and Koster, 1 9 8 4 ). These dilute (less viscous) debris flows exhibit lower
matrix strength and dispersive pressure during transport (Shultz, 1 9 8 4 ). Therefore,
the Gmn deposits of pseudoplastic debris flows are generally thinner, finer grained, and
better graded than the Gmu deposits of plastic debris flows. Gmn beds with horizontally
aligned clasts, an upward increase in matrix content, load structures, flame structures,
and interbedded mudstone are attributed to subaqueous deposition (Nemec and Steel,
1984; Pivnik, 19 9 0 ). These subaqueous deposits record the effects of increased
turbulence and decreased matrix strength associated with water admixture into moving
debris flows, syndepositional escape of substrate pore fluids, and post-debris flow
suspension fallout.
16
Figure 6 . Facies Gmn: lower bed has poorly developed normal grading, upper bed has
clast-rich base. Note load structure and flame structure (arrow) of lower bed and
outsized clast of upper bed. Hammer is 28 cm long.
Figure 7. Facies Gcu interbedded with facies Sm and Fm. Knife is 8 cm long.
17
Facies Gcu
Ungraded, Clast-Supported Conglomerate This facies comprises ungraded to
inversely graded, moderately to very poorly sorted, clast-supported, granule-boulder
conglomerate (Figs. 4, 5, 7, and 8 ). A muddy sand matrix (typically 2 0 -5 0 % of the
deposit) surrounds the clast-supported framework. Gcu deposits are unstratified and
lack clast imbrication. A few beds exhibit moderately to poorly developed, coarse-tail
inverse grading. Gcu beds are 2 -2 0 0 cm thick, extend laterally for tens of meters, and
commonly have planar, non-erosional bases. There are two distinct forms of facies Gcu:
( I ) a 2 -2 0 cm-thick, moderately sorted, granule-pebble conglomerate (Figs. 4, 5, and
7 ) commonly interbedded with ungraded to normally graded sandstone (Sm, Sn) and (2 )
a 20-200 cm-thick, poorly to very poorly sorted, cobble-boulder conglomerate rarely
exhibiting a lenticular geometry and erosional base (Fig. 8 ).
Interpretation . Facies Gcu is interpreted as deposits of subaerial, plastic or
pseudoplastic, clast-rich debris flows (Shultz, 1984; Waresback and Turbeville,
19 9 0 ) and subaerial hyperconcentrated flows (Smith, 1986; DeCeIIeS and others,
19 91a). Thick Gcu beds with cobble-boulder clasts are interpreted as plastic or
.
pseudoplastic, clast-rich debris flows. The sediment support mechanisms of these
debris flows include matrix strength, dispersive pressure (Lowe, 1979; 1 98 2 ) and,
due to high clast concentration, buoyancy (Hampton, 19 79). Thin Gcu beds dominated by
granule-pebble clasts are interpreted as the deposits of hyperconcentrated flows.
Turbulence, dispersive pressure, and buoyancy are the sediment support mechanisms in
hyperconcentrated flows (Smith, 19 8 6 ). The presence of considerable matrix, great
lateral extent of beds, and lack of traction-produced sedimentary structures in all Gcu
deposits suggests that the deposits are not the product of streamflow. The rare examples
of cobble-boulder Gcu deposits with lenticular geometries and erosional bases are
18
Figure 8 . Facies Gcu within finer grained conglomerate and sandstone facies. Note
lenticular shape and erosional base. Arrow points to hammer (2 8 cm long).
Figure 9. Facies Gen: note upward increase in matrix content. Knife is 8 cm long.
19
interpreted as the deposits of clast-rich debris flows which stopped flowing in stream
channels on a subaerial fan.
Facies Gcn
Normally Graded. Clast-Supported Conglomerate. This facies is composed of
normally graded, moderately to poorly sorted, clast-supported, granule-boulder
conglomerate (Figs. 9 and 10). Coarse-tail normal grading characterizes most deposits,
but distribution normal grading (clasts and matrix fine upward) occurs in a few beds. A
muddy sand matrix comprises 2 0 -5 0 % of individual Gcn deposits. Gcn beds are
unstratified and lack clast imbrication. Beds of Gcn are 2 -5 0 cm thick, can be traced
laterally for tens of meters, have planar, non-erosional boundaries, and are commonly
interbedded with ungraded to normally graded sandstone (Sm, Sn). Granule-pebble Gcn
beds commonly contain horizontal clasts and display a basal zone (lower 1-10 cm) with
inverse grading (Fig. 10).
Interpretation . Facies Gcn is attributed to deposition by subaerial
hyperconcentrated flows (Smith, 1986; Waresback and Turbeville, 1 9 9 0 ) and
subaqueous high-density turbidity currents (Lowe, 1982; Chough and others, 1 99 0 ).
Abundant normal grading, common basal inverse grading, and common horizontal clast
orientation are indicative of clast interactions within a fluid-rich or cohesionless flow.
However, the laterally persistent bedding, matrix-rich nature, and lack of tractionproduced sedimentary structures in facies Gcn suggests that the deposits are not the
product of streamflow. In hyperconcentrated flows, sediment is supported by
turbulence, dispersive pressure, and buoyancy (Smith, 19 8 6 ).
More turbulent
hyperconcentrated flows probably deposited normally graded Gcn beds, while less
turbulent hyperconcentrated flows resulted in ungraded Gcu deposits. Basal inverse
grading is the result o f dispersive pressure generated by abundant clast interactions
20
Figure 10. Facies Gcn with basal inverse grading and thin interbeds of facies Sm and
Fm. Knife is 8 cm long.
Figure 1 1 . Facies Gh and interbedded facies Sh. Note uniform grain size. Knife is 8
cm long.
21
(Lowe, 1 97 9 ).
Lowe (1 9 8 2 ) states th at gravelly high-density turbidity currents
carry sediment by turbulent suspension (vyhich accounts for normal grading throughout
most of the deposit) and dispersive pressure generated in a basal traction carpet layer
(which accounts for basal inverse grading). A Gcn bed directly overlain by ungraded to
normally graded sandstone (Sm, Sn) is interpreted as an ideal sequence of a sand-gravel
high-density turbidity current (see Fig. 1 1A of Lowe, 1 98 2 ). Such deposits are
uncommon because the sand within a turbidity current usually bypasses the site of
gravel deposition (Lowe, 19 8 2 ).
Facies Gh
Florizontallv Stratified Conglomerate . Ungraded, horizontally stratified,
moderately to well-sorted, clast-supported, granule-pebble conglomerate (Fig. 11) is a
relatively uncommon facies of Member 2. A sand matrix comprises 10 -4 0 % of
individual Gh deposits. Facies Gh exhibits horizontal to gently inclined (< 1 5°)
stratification and poor imbrication. Beds of Gh are I - 5 0 cm thick, extend laterally for
tens of meters, and have subplanar, slightly erosional to non-erosional boundaries. A
few distinctive beds of this facies have tightly packed, well-sorted (similar grain size
and shape), subhorizontally aligned clasts (Fig. 11 ). These distinct, texturally uniform
beds are always interbedded with low-angle or horizontally stratified sandstone (Sh),
ripple cross-laminated sandstone (Sr), trough cross-stratified sandstone (St), and
stromatolitic carbonate (CIm ).
Interpretation . Facies Gh is interpreted as deposits of subaerial sheetfloods and
wave-driven bedload traction. Sheetfloods were characterized by upper-flow regime
plane-bed conditions and deposited on low-relief fan surfaces (Waresback and
Turbeville, 1 99 0 ). Sheetfloods contained outsized clasts (> 2 0 cm diameter) and were
associated with debris flows and hyperconcentrated flows (facies Gmu, Gmn, Gcu, and
22
Gcn). The distinct, texturally uniform Gh beds (see description above) are similar to
wave-reworked, gravelly beach deposits of lacustrine systems (Tucker, 1978; Dunne
and Hempton, 19 8 4 ) and marine systems ( Wescott and Ethridge, 1983; Bourgeois and
Leithold, 1984; Nemec and Steel, 19 8 4 ) and represent tractional deposition beneath
shallow water, wave-driven currents. In contrast to Gh sheetflood deposits, these beds
lack outsized clasts, have very well-defined low-angle or horizontal stratification, and
are ihterbedded with sandstone (Sh, Sr, St) and carbonate (Clm).
Facies Sm
Massive Sandstone. This facies includes ungraded to poorly inversely graded,
massive (unstratified), moderately to poorly sorted, medium- to very coarse-grained
sandstone (Figs. 12 and 13) with a few granule-pebble outsized clasts. Sm beds are 0 .5 15 cm thick, can be traced laterally for several to tens of meters, and have subplanar,
non-erosional or slightly erosional boundaries. Some Sm deposits contain subvertical
root casts 0 .1 -1 cm in diameter and 2 -1 0 cm in length (Fig. 14). Uncommon
dewatering structures (similar to Figs. 15 and 16) include dish and pillar structures,
ball and pillow structures, and intrastratal contortions.
Interpretation . Facies Sm is attributed to deposition by subaerial
hyperconcentrated flows (Smith, 19 8 6 ) and subaqueous high-density turbidity currents
(Lowe, 1982; Chough and others, 1990; Higgs, 19 9 0 ). In both cases, sand was rapidly
deposited directly from turbulent suspension with insufficient time for bedform
development (Lowe, 1982; Smith, 19 8 6 ). This lack of bedform development accounts
for the absence of complete turbidite sequences that contain overlying horizontally and
ripple cross-laminated sandstone (divisions B and C of Bouma, 19 6 2 ). Inversely graded
beds are the result of freezing of a traction carpet particle layer maintained by
dispersive pressure (Lowe, 19 8 2 ). Sm beds vyith dewatering structures and interbedded
23
Figure I 2. Facies Sm and Sn with interbedded facies Fm. Coin is 2.1 cm in diameter.
Figure 13. Facies Sn and Sm with thin bed of poorly laminated facies Sb. Note basal
inverse grading and top-only normal grading in Sn beds. Coin is 2.1 cm in diameter.
24
Figure 14. Subvertical root casts in facies Sm. Knife (8 cm long) is parallel to
bedding.
Figure I 5. Flame structures in facies Sb. Coin is 2.1 cm in diameter.
25
relations with mudstone (Fm), sandstone (Sn, SM, Sr, St), and carbonate (CIm) are
interpreted as subaqueous deposits. Sm deposits with root casts are interpreted as
products of post-depositional homogenization by plant growth processes.
Facies Sn
Normally Graded Sandstone. This facies is characterized by normally graded,
unstratified, poorly to well-sorted, fine- to very coarse-grained sandstone (Figs. 12
and 13) with granule-pebble clasts commonly present at the base. Beds of Sn are 0 .5 15 cm thick, laterally continuous for several to tens of meters, and have subplanar,
non-erosional or slightly erosional boundaries.
Interpretation . Facies Sn is interpreted as deposits of subaerial
hyperconcentrated flows (Smith, 19 8 6 ) and subaqueous high- or low-density turbidity
currents (Lowe, 1982; Chough and others, 1990; Ghibaudo, 19 9 2 ). In
hyperconcentrated flows, sedimentation from turbulent suspension produces normal
grading, but high clast concentration generates dispersive pressure and buoyancy which
prevent bedform development. Therefore, deposition by hyperconcentrated flows
accounts for normal grading and lack of cross-stratification in Facies Sn (Smith, 19 8 6 ).
Deposition of Sn beds from sandy, high- or low-density turbidity currents occurs by
rapid, grain-by-grain suspension sedimentation with no development of bedforms or a
basal traction carpet (Lowe, 1982; Ghibaudo, 19 9 2 ). An absence of overlying
horizontally and ripple cross-laminated sandstone (divisions B and C of Bouma, 1 9 6 2 )
attests to the rapid deposition of the sand fraction prior to bedform development. Sn beds
associated with mudstone (Fm), sandstone (Sm, Sh, Sr, St), and carbonate (CIm) are
attributed to subaqueous deposition.
26
Facies Sh
Horizontally Stratified Sandstone. This facies is composed of ungraded, low-angle
(< 1 5 °) or horizontally stratified/laminated, poorly to well-sorted, fine- to very
coarse-grained sandstone (Figs. 1 1 -1 3 , 15, and 16). Facies Sh beds are 0 .5 -2 0 0 cm
thick and extend laterally for tens to hundreds of meters. Basal boundaries are erosional
and define shallow, broad scours (typically 5 -3 0 cm deep and 0 .5 -2 0 m wide).
Stratification is commonly at a very shallow angle (< 5 °) to the erosional base. Lowangle to concave-upward scours also are common within Sh beds. Some Sh beds pass
laterally into sandstone with poorly developed trough cross-stratification (St). Four
distinct varieties of facies Sh include: ( I ) a 2 0 -2 0 0 cm-thick, well-sorted sandstone
interbedded with mudcracked mudstone (Fm); (2 ) a 0 .5 -5 0 cm-thick, moderately
sorted sandstone with outsized granule-pebble clasts (Fig. 11) that is always interbedded
with subhorizontally stratified conglomerate (Gh); (3 ) a 0 .5 -1 5 cm-thick, moderately
sorted sandstone interbedded with Sm and Sn sandstones (Figs. 12 and 13); and (4 ) a I 50 cm-thick, well-sorted sandstone with abundant dewatering structures (Figs. 15 and
16).
Interpretation . Facies Sh is interpreted as deposits of subaerial sheetfloods,
wave-driven bedload traction, and high- or low-density turbidity currents.
Subhorizontal stratification in sheetflood deposits is the result of upper-flow regime
plane-bed conditions during deposition of sand (Sh; Fedo and Cooper, 19 9 0 ) and granulepebble gravel (Gh; Waresback and Turbeville, 19 9 0 ). Thick sheetflood deposits are
commonly interbedded with mudstones (Fm) that have abundant mudcracks. Sh beds
associated with carbonate (CIm) and subhorizontally stratified conglomerate (Gh) are
interpreted as shallow water, wave-reworked deposits (tractional deposition beneath
wave-induced currents) similar to those described for lacustrine and marine systems
27
Figure I 6. Ball and pillow structures in facies Sb. Knife is 8 cm long.
Figure 17. Facies Sr: foresets dip left. Knife is 8 cm long.
28
(Link and Osborne, 1978; Reineck and Singh, 1980; Wescott and Ethridge, 1983; Dunne
and Hempton, 1984; Pivnik, 1990; Casshyap and Aslam, 1992). Sh beds associated
with sandstone (Sm, Sn) and containing dewatering structures are attributed to
tractional deposition at the base of low-density turbidity currents (Chough and others,
1990; Higgs, 19 9 0 ) and deposition by freezing of traction carpets at the base of highdensity turbidity currents (Lowe, 1982; Pivnik, 1990; Ghibaudo, 19 9 2 ).
Although
complete turbidite sequences are not present, these Sh occurrences are considered
analogous to division B of Bouma (1 9 6 2 ).
Facies Sr
Ripple Cross-Laminated Sandstone.
This ripple cross-laminated, well-sorted,
very fine- to coarse-grained sandstone facies (Fig. 17) is uncommon and exclusively
interbedded with sandstone (Sm, Sn, Sh, St), mudstone (Fm), carbonate (Clm), and
horizontally stratified conglomerate (Gh). Beds of Sr are 0 .5 -3 0 cm thick, extend
laterally for several to tens of meters, and have slightly erosional basal boundaries.
Climbing ripple geometries are common. Flaser bedding is developed where interbedded
siltstone (Fm) is common. Observed ripple foresets dip 15 -3 0 ° in a single direction or,
less commonly, in opposite directions (Fig. 17 ). Asymmetrical and symmetrical
straight-crested ripple marks are common on bedding surfaces. Sr deposits have an
average ripple wavelength of 5 cm and amplitude of 0.5 cm.
Interpretation . Ripple cross-lamination of facies Sr is attributed to ripple
migration and deposition beneath unidirectional storm currents and bidirectional wave
currents (Miall, 1977; Reineck and Singh, 1 9 8 0 ).
Unidirectional and bidirectional
currents produced foresets dipping in a single direction and opposite directions,
respectively. The small wavelength and amplitude of the ripple bedforms suggests
29
deposition in shallow w ater (Tucker, 1978; Martel and Gibling, 1 9 9 1 ).
Climbing
ripple geometries suggest a high sediment supply (Reineck and Singh, 198 0 ).
Facies St
Trough Cross-Stratified Sandstone. This trough cross-stratified, moderately to
well-sorted, very fine- to coarse-grained sandstone facies (Fig. 18) is uncommon and
typically occurs as a single set or coset within sandstone (Sm, Sn, Sh, Sr). St beds are
also interbedded with carbonate (CIra) and horizontally stratified conglomerate (Gh).
Gently curving, erosional bounding surfaces define 0 .4 -2 .2 m-thick sets of tangential
trough cross-strata that persist laterally for 5 -5 0 meters. Individual foresets dip 1 5 35° in the same direction (to the northwest) and can be traced laterally for 1 -8 m.
Interpretation . Facies St is interpreted as the depositional product of migrating
dunes under lower-flow regime conditions (Miall, 19 7 7 ). The consistency of foreset dip
direction is attributed to deposition by unidirectional currents. A common association
with ripple cross-laminated sandstone (Sr) and carbonate (CIm) is interpreted to be the
result of shallow water deposition. The large set thickness of trough cross-strata
suggests that migrating bedforms were dunes or bars several meters in height.
Facies Fm
Massive to Poorlv Laminated Mudstone. This facies is composed of ungraded,
massive or poorly laminated (0 .5 -1 0 mm-thick laminations) mudstone (Fig. 19).
Facies Fm commonly contains mudcracks. Individual Fm beds have non-erosional bases
and extend laterally for several to tens of meters. There are tw o distinct occurrences of
facies Fm: ( I ) individual, laterally discontinuous (< 1 0 m) Fm beds within
conglomerate- and coarse sandstone-dominated sequences (Figs. 5, 7, 10, 12); and (2 )
30
Figure 18. Facies St and cap of facies Sh1Gh, and Clm. Base of photo is approximate
base of set. Hammer is 28 cm long.
Figure 19. Facies Fm overlain by facies Sm. Hammer is 28 cm long.
31
\
numerous, laterally continuous (1 0 -3 0 m ) Fm beds within mudstone- and fine
sandstone-dominated sequences (Fig. 19).
Interpretation . Facies Fm is interpreted as deposits of subaerial waning flood
flows (Miall, 19 7 7 ) and subaqueous suspension (with minor traction) sedimentation
(Ghibaudo, 1 99 2 ). Fm deposits from waning flood flows are thinner (< 5 cm) and
laterally discontinuous (< 1 0 m). In contrast, Fm sequences deposited from suspension
fallout and traction are thicker (0 .1 -1 0 m) and have greater lateral continuity (1 0 -3 0
m). These subaqueous sequences may represent late-stage deposition by Iowrdensity
turbidity currents.
Facies Clm
Laminated or Massive Carbonate
Laminated (0 .2 -5 mm-thick laminations) or
massive carbonate facies commonly occurs as laterally discontinuous (0 .5 -2 0 m), 1 -5 0
cm-thick units within thin-bedded sandstone (Sm, Sr, St) and conglomerate (Gh)
sequences (Fig. 2 0 ). This facies also occurs as 5 -3 0 m-thick carbonate sequences with
minor interbedded clastic sediments. Laminations are more common in these thick
carbonate sequences. Carbonate laminae geometries include convex-upward laterally
linked hemispheroids (1 -1 0 0 cm wavelength, 0 .5 -4 0 cm amplitude) and crinkly to
planar, subhorizontal laminations (Figs. 21 and 22). Although most Clm beds lack
fossils, abundant pelecypod fragments and algal (charophyte) stems were found in a few
massive carbonate layers. A few irregular zones (approximately I m thick and 2 m
wide) containing ooids and pisoids are present within the 5 -3 0 m-thick carbonate
sequences. The well-rounded but moderately spherical ooids and pisoids are of variable
size (1 -2 0 mm in diameter), contain poorly defined laminations (< 3 mm thick), and
typically have carbonate intraclast nuclei (Fig. 2 3 ).
32
Figure 20. Facies Clm interbedded with facies Gh and Sh. Arrows indicate <1 cm
thick, subhorizontal beds of facies Clm. Knife is 8 cm long.
Figure 21. Convex-upward laterally linked hemispheroid geometry of facies Clm.
Coin is 2.1 cm in diameter.
33
Figure 22. Planar, subhorizontal, 0.2-5 mm-thick laminations of facies Clm. Coin
is 2.1 cm in diameter.
Figure 23. Ooids and pisoids of facies Clm. Note well-rounded, moderately spherical
grains and poor sorting. Coin is 2.1 cm in diameter.
34
Interpretation . Facies Clm is attributed to calcium carbonate precipitation in
shallow water due to photosynthetic uptake o f COg and/or nearshore wave agitation and
warming (Eugster and Kelts, 1983; Platt and Wright, 199 1 ).
Laminated Clm
structures are interpreted as stromatolites resulting from trapping and binding of
calcareous sediment by charophyte algal mats (Dean and Pouch, 1983). Stromatolites
associated with shallow water deposits (Gh, Sh, Sr, St), presence of fossils, lack of
evaporite facies, and lack of brecciation or displacive growth suggest that facies Clm is a
product of carbonate precipitation within a water column rather than pedogenic
(caliche) or vadose zone (calcrete) carbonate precipitation.(e.g., Goudie, 198 3 ).
However, the few irregular ooid-pisoid zones of facies Cfm (Fig, 2 3 ) are not associated
with scours or traction-produced sedimentary structures and, therefore, are
interpreted as the product of post-depositional carbonate precipitation (Read, 197 6 ).
Poor sorting in these zones (Fig. 23) may also suggest a secondary, vadose zone origin
for the ooids and pisoids (Dunham, 19 6 9 ). Thus, carbonate facies Clm contains a record
of initial carbonate precipitation in a nearshore environment with limited, postdepositional solution and reprecipitation, including ooid-pisoid formation, within the
vadose zone.
Facies Vtb
Volcanic Tuff Breccia. This massive rhyolitic-dacitic facies contains resistant
angular clasts (0 .1 - 4 0 cm diameter) within a less resistant matrix (<1 mm diameter
grains). Clasts and matrix have similar phenocryst compositions and abundances
(typically 5 0 -7 0 % quartz, 10 -3 0 % plagioclase, 1 0 -2 0 % sanidine, and <5% biotite).
Subhedral to euhedral phenocrysts generally comprise 20% of the breccia volume.
Quartz phenocrysts are commonly shattered. Although some clasts touch each other, most
are separated by a few millimeters of matrix, Facies V tb typically contains 60% matrix
35
and 40% clasts. Friable tu ff clasts, rounded Paleozoic carbonate clasts, and xenoliths
(4 0 -1 0 0 cm diameter) of bedded, Horse Camp (Miocene) sandstone (Sm) are present in
matrix-rich varieties of facies V tb (present only within 0 -1 0 0 m interval of sections I
and 2, Plate I ). Although outcrops of facies Vtb are laterally continuous for tens to
hundreds of meters and roughly parallel to underlying and overlying strata, the facies
lacks bedding structures and compaction or flow foliations resulting from flattened
pumice fragments, elongate vesicles, and aligned phenocrysts. Facies Vtb is uncommon
and typically associated with mudstone (Fm) and massive and normally graded sandstone
(Sm, Sn).
Interpretation . Facies Vtb is interpreted as tu ff breccia deposits emplaced during
volcanic eruptions. This facies lacks sedimentary detritus and contains only matrix and
clasts of rhyblitic-dacitic composition. These features suggest that debris was
transported by pyroclastic flows rather than sediment gravity flows (compare with Cole
and Stanley, 19 94 ). The presence of shattered crystals suggests emplacement at an
elevated temperature. The lack of stratification, bedding, or flow structures and
preservation of friable tu ff clasts and subhedral to euhedral crystals indicate limited
traction and turbulence within the flows. A high sediment concentration during flow may
have prevented tractive and turbulent suspension processes (Cole and DeCelles, 199 1 )
and promoted an en masse mode of deposition (Smith, 19 8 6 ). The few matrix-rich
deposits containing sandstone and carbonate clasts may reflect mixing with water and
entrainment of substrate material as pyroclastic flows transformed into pytoclastic
debris flows (Cole and DeCelles, 1 991). Intercalation with sandstone (Sm, Sn) and
mudstone (Fm) suggests that facies Vtb was typically deposited and preserved under lowenergy conditions in a subaqueous environment. Facies Vtb is rarely preserved within
sequences interpreted as subaerial deposits. Therefore, if deposited in subaerial
36
environments, this facies was probably reworked. Cole and DeCeIIes (1 9 9 1 ) have
documented similar pyroclastic deposits preserved in a submarine environment.
Facies Vmb
'
Volcanic Bedrock Meqabreccia. This megabreccia facies is composed of a resistant
sheet of brecciated Oligocene volcanic bedrock that is mixed to varying degrees with
underlying sediments. Facies Vmb contains: ( I ) a basal zone of deformed substrate (Fig.
24); (2 ) a mixed zone that incorporates both substrate material and clasts broken from
the base of the breccia sheet (Fig. 25); and (3 ) an upper allochthonous breccia sheet of
resistant Oligocene volcanic rock (Fig. 2 5 ). The breccia sheet consists of clasts (1 -3 0
cm diameter) of volcanic rock (Figs. 2 5 -2 7 ) separated by either minor amounts of sand
and mud matrix (clast-rich crackle breccia) or significant amounts of matrix (m atrixrich jigsaw breccia; Yarnold and Lombard, 1989; Brown, 1 9 9 3 ).
Comminuted slip
surfaces ( I -5 mm wide fractures along which there has been limited movement; Yarnold
and Lombard, 1989; Brown, 19 9 3 ) commonly separate these tw o breccia types and are
necessary for internal rotation and translation within the breccia sheet (Fig. 27).
Asymmetric to overturned folds (1 -1 0 m wavelength, <5 m amplitude) are common in
the basal zone of deformed substrate (Fig. 24). Facies Vmb has an irregular distribution
th at ranges from I - I O m thick and tens to hundreds of meters in lateral extent. This
facies is incomplete in some areas because the breccia sheet and mixed zone are missing
and only a disrupted zone is present. Although facies Vmb is uncommon, occurring only
at tw o stratigraphic levels in the southern Member 2 deposits (within lower 10 0 m of
sections 8 and 9 and approximately 12 0 0 m level of sections 5 and 6, Plate I ) , it is
associated with most facies identified in this study. Facies Vmb is similar to lower
megabreccia deposits within the Horse Camp Formation (Member I ) described by Brown
(1 9 9 3 ).
37
Figure 24. Asymmetric fold in deformed substrate zone of facies Vmb. Black lines
define folded bedding. Hammer is 28 cm long.
Figure 25. Mixed zone (MZ) and resistant, overhanging breccia sheet (BS) of facies
Vmb. Arrow points to hammer (28 cm long).
38
Figure 26. Jigsaw (JB) and crackle (CB) breccias of facies Vmb. Arrows define
contact. Hammer is 28 cm long.
Figure 27. Comminuted slip surface (arrows) from upper left to lower right
separates upper crackle breccia (CB) from underlying jigsaw (JB) and crackle (CB)
breccia zone. Hammer is 28 cm long.
39
Interpretation . Facies Vmb is interpreted as megabreccia deposits of pre­
existing volcanic bedrock emplaced during rock avalanches (Yarnold, 1 9 9 3 ). Therefore,
this facies is not directly related to volcanic eruptions. Rock avalanches moved
downslope as rapid inertial granular flows (Yarnold, 19 9 3 ).
Active particle layers th at
facilitated sliding along the base of the rock avalanche (Campbell, 1989; Brown, 1 99 3 )
are probably preserved as mixed zones that contain particles derived from the substrate
and breccia sheet (Fig. 25). Simple shear along the base of the allochthonous sheet
accounts for asymmetric to overturned folds in the deformed substrate zone of facies Vfnb
(Fig. 2 4 ). Jigsaw and crackle breccia reflect differing degrees of brecciation within the
landslide mass (Figs. 26 and 27). Comminuted slip surfaces separate jigsaw and crackle
breccias (Fig. 2 7 ) and may represent accommodation surfaces permitting differential
deformation within the breccia sheet (Brown, 19 9 3 ). The lateral discontinuity of
breccia sheets of facies Vmb suggests that the sheets broke up into discrete masses during
rock avalanche movement.
Facies Associations and Deoositional Environments
Analysis of the geometry, occurrence, and stratigraphic relations of individual
facies permits identification of four common facies associations (subaerial fan-delta,
subaqueous fan-delta, nearshore lacustrine, and offshore lacustrine facies associations)
within Member 2 of the Horse Camp Formation. Excellent exposures of Member 2 are up
to a thousand meters thick and contain beds that are laterally traceable for hundreds of
meters, These exposures permit identification of vertical and lateral variations among
the four facies associations. These variations are represented in generalized stratigraphic
sections of Plate I . Figures 28 to 30 show detailed stratigraphic sections up to 10 m
thick for the subaerial fan-delta, subaqueous fan-delta, and nearshore lacustrine facies
associations. A detailed stratigraphic section for the offshore lacustrine facies
40
G-
O o
0
CSiCfii
o \
\
o'
m MSPCB
Figure 28. Detailed
stratigraphic section of
subaerial fan-delta
facies association from
4 6 0 -4 7 0 m level of
section 6 (Plate I ) .
Key as in Plate I .
Figure 29. Detailed
stratigraphic section of
subaqueous fan-delta
facies association from
2 6 2 -2 7 2 m level of
section 6 (Plate I ).
Key as in Plate I .
Figure 30. Detailed
stratigraphic section of
nearshore lacustrine
facies association from
0 -7 m level of section
4 (Plate I ) . Key as in
Plate I.
41
association was not measured because of insufficient exposure. However, generalized
stratigraphic sections I and 2 of Plate I provide representative examples of the offshore
lacustrine facies association. Each facies association is defined by a restricted set of
interbedded facies that is readily distinguished from other facies associations. These
unique facies associations are attributed to processes characteristic of particular
depositional environments. The relations among the four depositional Environments are
discussed in an environmental reconstruction presented in a following section.
Subaerial Fan-Delta Facies Association
This facies association consists of 1 0 -2 0 0 cm-thick beds of ungraded (Gmu, Gcu)
and normally graded (Gmn, Gen) conglomerate and 1 -1 5 cm-thick beds of ungraded
(Sm) and normally graded (Sn) sandstone (Fig. 2 8 ).
Beds are laterally continuous for
tens of meters and have planar, non-erosional boundaries defined by abrupt grain-size
differences or thin sandstone beds (Fig. 31). A detailed stratigraphic section (Fig. 2 8 )
measured within the lower part of section 6 (Plate I ) suggests that conglomerate facies
Gmn and Gcn are the primary facies of the subaerial fan-delta facies association.
Discontinuous, horizontally stratified conglomerates (Gh) and sandstones (Sh) are rare.
This facies association is distinctive in that it contains the largest clasts and only
channel-fill conglomerates of the study area.
This facies association is the result of deposition by sediment gravity flows on. a
subaerial fan-delta. Thick sequences (5 -1 0 0 m) composed of laterally continuous beds
of relatively uniform thickness suggest prolonged, episodic deposition of unconfined,
sediment gravity flows below the fan intersection point. Plastic and pseudoplastic debris
flows (Gmu, Gcu, Gmn), hyperconcentrated flows (Gen, Sm, Sn), and minor sheetfloods
(Gh, Sh) characterized deposition on the subaerial component of the fan-delta. The
subaerial fan-delta deposits lack features such as well-developed sorting, stratification,
imbrication, scours, and rounded clasts that are common in deposits of stream-dominated
42
Figure 3 1. Repetitive sequence of laterally continuous beds of subaerial
fan-delta facies association. Upper photo contains hammer (2 8 cm long).
Total stratigraphic thickness in lower photo is approximately 5 0 m.
43
alluvial fans (e.g., Evans, 1991; Ridgway and DeCelles, 1 99 3 ).
Lenticular
conglomerates are rare and limited to the southeastern exposures of the Horse Camp
Formation (sections 6 -9 , Plate I ) and probably represent deposition in stream channels
of the proximal fan-delta. Therefore, perennial streams and fluvial processes were rare
on the subaerial fan-delta (e.g., Ballance, 19 8 4 ).
The occurrence of only a few stream-
channel deposits, significant lateral extent and tabular nature of beds (Fig. 3 1 ), and
persistent repetitive sequence of beds (Fig. 3 1 ) representing individual sedimentation
events suggests that the subaerial fan-delta was dominated by episodic, relatively lowconcentration sediment gravity flows that were able to spread out into tabular sheets
over the surface of a smooth, non-channelized fan surface (Whipple and Dunne, 19 9 2 ).
Therefore, the subaerial fan-delta facies association is similar to deposits of mass floWdominated alluvial fans (e.g., Larsen and Steel, 1978; Gloppen and Steel, 1981; Nemec
and Muszynski, 1982; Nemec and others, 1984; Palmer and Walton, 1990; Whipple and
Dunne, 1 99 2 ). An abundance of large (several meters in diameter) angular clasts
suggests th at sediment underwent limited weathering and transport prior to deposition on
the fan-delta. A lack of mudstone (Fm), carbonate (Clm), dewatering structures, and
mudcracks indicates subaerial deposition of this facies association (Fig. 28).
Subaqueous Fan-Delta Facies Association
The subaqueous fan-delta facies association contains ungraded (Gmu) and
normally graded (Gmn, Gen) conglomerate, ungraded (Sm) and normally graded (Sn)
sandstone, and massive to poorly laminated (Fm) mudstone (see detailed stratigraphic
section in Fig. 29 measured within the lower part of section 6, Plate I ) . Beds are 0 .5 100 cm thick, persist laterally for tens of meters (similar to Fig. 3 1 ), and have nonerosional, subplanar to undulatory boundaries commonly defined by thin sandstones or
mudstones. Normally graded, matrix-supported conglomerate (Gmn) is the most
common facies (Fig. 29). Horizontally stratified conglomerate (Gh) and sandstone (Sb)
are rare. Matrix-supported conglomerate (Gmu, Gmn) and mudstone (Fm) commonly
have mudcracks. This facies association is distinguished from the subaerial fan-delta
facies association by the presence of syn- or post-depositional disruption (dewatering
structures and distorted beds) and abundance of thin interbeds of sandstone (Sm, Sn, Sb)
and mudstone (Fm).
,
Subaqueous sediment gravity flows dominated the depositional environment of the
subaqueous fan-delta facies association. The laterally continuous, uniformly thick beds
of this facies association are the product of episodic deposition of sediment gravity flows
on the unconfined surface of a subaqueous fan-delta. Plastic and pseudoplastic debris
flows, high- and low-density turbidity currents, and suspension fallout characterized
deposition on the subaqueous component of the fan-delta. Well-defined, non-erosionai
bed boundaries are the result of draping by mud (Fm) or sand (Sm, Sn, Sb) under
waning flow conditions. The common occurrence of conglomerates draped by thin (< 1 0
cm thick) sandstone or mudstone layers suggests that en masse deposition of subaqueous
debris flows was often followed by suspensional deposition by turbidity currents
(Ghibaudo, 1 99 2 ). A lack of complete Bouma sequences (ripple-cross laminations,
lower-flow regime plane beds, and mudstone caps are missing) suggests that these
turbidity currents, were deposited rapidly from turbulent suspension with insufficient
time for bedform development. Distorted beds and dewatering structures of this facies
association are typical of a subaqueous setting in which a saturated substrate was
commonly loaded by rapid deposition of an overlying bed (e.g., Postma, 1983; 1984).
The presence of mudcracks and interbedded mudstone (Fm) and sandstone (Sm, Sn, Sb)
indicates that successive sediment gravity flow events were punctuated by development of
relatively low-energy conditions in a shallow subaqueous environment susceptible to
periodic desiccation.
45
Nearshore Lacustrine Facies Association
Stratified sandstone (Sh, Sr, St) and granule-pebble conglomerate (Gh),
carbonate (Clm), and minor amounts of massive and normally graded sandstone (Sm, Sn)
and conglomerate (Gmu, Gmn, Gcu, Gen) characterize this facies association (see detailed
stratigraphic section in Fig. 3 0 measured within section 4 ,.Plate I ) .
Most beds are 1 -
50 cm thick, laterally continuous for several to tens of meters, and have subplanar,
non-erosional boundaries. However, trough cross-stratified sandstones (S t) have
erosional set boundaries and set thicknesses up to 2.2 m. Stratified coarse sandstones
(Sh, Sr, St) and conglomerates (Gh) are intimately associated with I -3 0 cm-thick
layers of stromatolitic carbonate (Clm). Carbonate layers exhibit extreme lateral
variations in thickness and clastic content. The nearshore lacustrine facies association is
distinctive in that it contains carbonate and stratified sandstone and conglomerate.
This facies association is representative of deposition in a wave-influenced,
shallow-water (several meters deep), nearshore lacustrine environment.
High-energy,
offshore-directed currents and oscillatory wave currents forced fractional transport and
deposition of well-sorted, texturally uniform, low-angle to horizontally stratified
coarse sandstone (Sh) and granule-pebble conglomerate (Gh) as well as migration of
ripples (Sr) and dunes (S t). Precipitation and binding of carbonate associated with
episodic terrigenous clastic deposition produced stromatolitic carbonate (Clm) and
interbedded, stratified sandstone (Sh, Sr, St) and conglomerate (Gh). Therefore, this
dynamic nearshore lacustrine environment promoted deposition of carbonate during lowenergy, quiet-water conditions and deposition of coarse clastic sediment during episodic,
high-energy, storm conditions. However, the presence of a few thick (5 -3 0 m)
carbonate units (approximately 10 0 0 m level of sections 5 -8 , Plate I ) suggests that
there were several prolonged intervals of limited clastic sedimentation in the nearshore
lacustrine environment.
46
Offshore Lacustrine Facies Association
Massive and normally graded very fine- to medium-grained sandstone (Sm, Sn)
and massive to poorly laminated mudstone (Fm) comprise this facies association. A
detailed stratigraphic section for the offshore lacustrine facies association is not
available due to inadequate exposure. However, sections I and 2 of Plate I are
representative of this facies association. Beds are 0 .5 -1 5 cm thick, laterally continuous
for tens of meters, and have subplanar, non-erosional boundaries. In general,
sandstones and mudstones of this facies association are well-sorted and lack sedimentary
structures. However, there are a few rare examples of ripple cross-laminated sandstone
(Sr). Normal grading in sandstones is poorly developed and uncommon. Volcanic tu ff
breccias (V tb ) are well-preserved in this facies association.
Suspension sedimentation and turbidity current deposition characterized the
depositional environment of the offshore lacustrine facies association. Massive and
normally graded sandstones (Sm, Sn) are depositional products of high- and low-density
turbidity currents. Massive to poorly laminated mudstone (Fm) was deposited during
suspension fallout With minor fractional transport. The preservation of pyroclastic flow
deposits (V tb ) attests to the lack of reworking processes in the environment. These
facies and inferred depositional processes are representative of low-energy conditions in
an offshore lacustrine environment.
Environmental Reconstruction
Prolonged deposition in fan-delta and lacustrine environments of a Miocene
extensional basin was recorded by facies associations within the southern exposures of
Member 2 of the Horse Camp Formation (Fig. 2). Simplified representations of
numerous stacked facies associations within southern Member 2 strata are shown in the
nine, dip-corrected, generalized stratigraphic sections of Plate I . Individual
47
occurrences of the four facies associations are typically tens of meters in thickness. The
generalized sections of Plate I provide a record of stratigraphic variations over
hundreds of meters and, therefore, do not record all stratigraphic occurrences of the
various facies associations. The stratigraphically lowest section correlation (0 -1 3 0 m
level) among sections 5 -9 and the correlation linking sections 3 -5 (Plate I ) are air
photo-based strike correlations projected across covered intervals. All other section
correlations are based upon laterally continuous exposures of distinctive
Iithostratigraphic units. More detailed versions of the nine generalized stratigraphic
sections (Plate I ) are presented in archival data that are available from the Montana
State University Department of Earth Sciences.
Different depositional processes in the subaerial and subaqueous fan-delta and
nearshore and offshore lacustrine environments resulted in unique facies associations
and sedimentary structures for their respective deposits within Member 2. Facies
associations indicative of deposition by numerous sediment gravity flows on subaerial
and subaqueous parts of a fan-delta are more common to the southeast (sections 5-9,
Plate I ). Typical occurrences of the subaerial fan-delta facies association include all of
section 9, 1 7 0 -3 5 0 m interval of section 8, and 4 0 0 -5 0 0 m interval of section 6
(Plate I ) . Examples of the subaqueous fan-delta facies association include the 6 2 0 -7 8 0
m interval of section 7 and 2 6 0 -3 6 0 m interval of section 6 (Plate I ) . The nearshore
lacustrine facies association, produced by alternating conditions of high-energy, wavedriven traction and low-energy carbonate precipitation, is more common to the
northwest (sections 3 -5 , Plate I ) and upper levels of measured sections (sections 5 -8 ,
Plate I ) . Typical nearshore lacustrine sequences are found in sections 3 and 4, and
8 3 0 - 1 13 0 .interval of section 7 (Plate I ) . The offshore lacustrine facies association,
representative of deposition from suspension and turbidity currents, is more common to
the northwest (Plate I ). In fact, deposits of the offshore lacustrine facies association
48
comprise most of sections I and 2 (Plate I ) . The distribution of the four facies
associations within Member 2 deposits suggests deposition in a fan-delta system to the
southeast and a nearshore to offshore lacustrine system to the northwest (Figs. 32 and
33). A northwestward grain-size fining trend (note the finer grain size and decreased
proportion of conglomerate units to the northwest, Plate I ) and abrupt northwestward
facies transitions (note the transition from conglomerate-rich section 5 to sandstone-,
mudstone-, and carbonate-rich sections 1-4, Plate I ) , indicate a close spatial
association of these depositional systems (Figs. 32 and 33) throughout Member 2
deposition (middle-late Miocene). Unique aspects of these ancient depositional systems
include the dynamic nature of the transition between the subaerial and subaqueous
environment and the rare deposition of pyroclastic flows and megabreccias.
Subaerial-Subaqueous Transition
Stratigraphic occurrences of the nearshore lacustrine facies association (Fig.
3 0 ) are exclusively situated between subaerial fan and offshore lacustrine facies
associations (e.g., 12 0 0 -1 6 6 0 interval of section 6, Plate I ). The nearshore lacustrine
facies association is not found interbedded with the subaqueous fan-delta facies
association. Therefore, during phases in which the nearshore lacustrine facies
association was deposited, the nearshore lacustrine environment existed as a transition
zone between subaerial fan and offshore lacustrine environments (Fig. 32). Stratified
coarse clastic sediments of the nearshore lacustrine facies association were
preferentially deposited during phases in which the lake margin was characterized by
high-energy wave-driven currents able to rework grains as large as 2 cm in diameter.
These nearshore high-energy wave conditions required relatively deep water or a steep
gradient near the lake margin (Fig. 3 2) in which the energy of onshore-directed waves
was focused along the shoreline.. A lack of mudcracks indicates few phases of desiccation.
49
00J
|:
conglomerate
sandstone
f~ _ ~ _ j sandstone, mudstone
T~ M carbonate
DEEP WATER PHASE
distorted bedding
subaerial fan
nearshore lacustrine
offshore
lacustrine
w a v e -re w o rk e d g ra v e l be a c h
subaqeous sandbar
Figure 32. Environmental reconstruction in which relatively deep water in the
nearshore lacustrine environment promotes high-energy reworking processes and
prevents formation of a subaqueous fan-delta.
SHALLOW WATER PHASE
subaerial fan
subaqueous fan-delta
offshore
lacustrine
t u r b i d i t y f lo w
/
/
/
Figure 33. Environmental reconstruction in which relatively shallow water near the
lake shoreline precludes reworking of subaqueous fan-delta deposits by high-energy
wave processes. Key as in Fig. 32.
50
Limited desiccation may have been related to relatively deep water and few shoreline
fluctuations in the nearshore environment.
The subaqueous fan-delta facies association is exclusively found interbedded with
subaerial fan and offshore lacustrine facies associations (e.g., 2 0 0 -7 0 0 m interval of
section 6, Plate I ). The nearshore lacustrine and subaqueous fan-delta facies
associations are not in close proximity to one another. Thus, during phases of subaqueous
fan-delta deposition, the subaqueous fan-delta environment served as a transition zone
between subaerial fan and offshore lacustrine environments (Fig. 3 3 ).
Relatively
shallow water or a shallow gradient near the lake margin during subaqueous fan-delta
sedimentation (Fig: 33) induced a dampening of wave energy resulting in a lack of wavedriven sediment transport along the shoreline. Common mudcracks in the subaqueous
fan-delta facies association suggest phases of desiccation that may have been related to
relatively shallow water depths and frequent variations in the shoreline position. Very
shallow water depths along the lake margin may have inhibited the development of
Gilbert-type foreset beds (e.g., Billi and others, 1 99 1 ).
The tw o environmental reconstructions depicted in Figures 32 and 33 represent
temporally distinct sedimentary phases during deposition of Member 2 of the Horse Camp
Formation. In general, Figure 32 represents conditions during deposition of the upper
half of Member 2 (sections 3 and 4 and above approximately 8 0 0 m level of sections 5 8, Plate I ) and Figure 33 represents conditions during lower Member 2 deposition
(sections I and 2 and below approximately 8 0 0 m level of sections 5 -9 , Plate I) .
. Possible controls on alternations from one sedimentary phase to the other include
tectonic-induced variations in sedimentation rates and/or climate-induced lake-level
variations. A phase characterized by the presence of a subaqueous fan-delta environment
(Fig. 3 3) may have been due to I ) higher sedimentation rate and resultant progradation
of the subaerial fan into the lake or 2) higher lake levels and an effective drowning of the
51
.subaerial fan. Conversely, a phase in which subaqueous fan-delta deposition was
supplanted by deposition of the nearshore lacustrine facies association (Fig. 3 2) may
reflect I ) reduced sedimentation rates and aggradation or retrogradation of the subaerial
fan or 2) lower lake levels and subaerial exposure of the entire fan.
This study identifies several distinct characteristics of subaqueous and subaerial
deposits of a mass flow-dominated fan-delta system. The subaqueous fan-delta facies
association of the Horse Camp Formation (Member 2) exhibits dewatering structures
(load, flame, and ball and pillow structures), intrastratal contortions, and distorted
bedding reflecting an inherent instability on subaqueous fan-delta surfaces related to
rapid deposition on a substrate characterized by high pore-fluid pressures (Postma,
1983; 19 8 4 ). Other indicators of subaqueous fan-delta deposition in the Member 2
sequence include an abundance of interbedded mudstone and matrix-supported
conglomerates with normal grading, upward increases in matrix content, and sandy caps.
These features have also been noted in other studies of subaqueous fan-delta deposits
(e.g., Larsen and Steel, 1978; Gloppen and Steel, 1981; Nemec and Steel, 1984; Nemec
and others, 1984; Pivnik, 1 9 9 0 ).
Intuitively, shoreline facies should serve as ideal
stratigraphic indicators separating subaerial and subaqueous deposits of a single fandelta (McPherson and others, 19 8 7 ). In this study, however, the nearshore lacustrine
facies association (Fig. 3 0 ) serves exclusively as a transition between facies
associations representative of the subaerial fan and the offshore lacustrine environment
(Fig. 3 2) and is not found situated between subaerial and subaqueous fan-delta facies
associations (Fig. 33).
Nearshore Environment
Facies characteristic of high-energy depositional processes are uncommon in
deposits of nearshore lacustrine environments (e.g., Tucker, 1978; Dunne and Hempton,
1984; Link and others, 1 9 8 5 ). However, the Horse Camp Formation (Member 2)
52
contains a facies association representative of a nearshore lacustrine environment (Fig.
3 2 ) in which high-energy wave-induced currents were responsible for fractional
reworking o f grains as large as 2 cm in diameter. In fact, low-angle to horizontally
stratified sandstone (Sh) and conglomerate (Gh) and large-scale (typically I -2 m
thick) trough cross-stratified sandstone (S t) of the nearshore lacustrine facies
association (Fig. 3 0 ) are more similar to nearshore marine deposits (e.g., DeCeIIes,
1987; Pivnik, 1990; Casshyap and Aslam, 1 9 9 2 ) than other lacustrine deposits (e.g.,
Hardie and others, 1978; Link and Osborne, 1 9 7 8 ). However, Member 2 nearshore
deposits lack the well-defined, 2 -2 5 m-thick, progradational sequences composed of
horizontally and cross-stratified sandstone and conglomerate commonly found in
nearshore marine systems (e.g., DeCeIIes, 1987; Pivnik, 1 9 9 0 ).
Frequent lake-level
fluctuations and associated reworking of previous deposits may account for the lack of
progradational lacustrine shoreline sequences (Renaut and Owen, 1 9 9 1 ). The highenergy conditions of the nearshore lacustrine setting (Fig. 3 2) depicted in this study are
probably the result of a large fetch and powerful, wind-driven wave currents. Traction
and bedform migration beneath these wave-induced currents account for deposition of
much of the nearshore lacustrine facies assemblage (including facies Gh, Sb, Sr, and St).
Despite the high-energy conditions required for reworking of coarse clastic sediment,
there is also evidence for intermittent low-energy conditions in the nearshore lacustrine
environment. Specifically, laminated and massive carbonate (CIm) are the result of
subaqueous precipitation and binding of carbonate under reduced energy conditions. The
stratified coarse clastic facies and interbedded carbonate facies of this ancient nearshore
lacustrine sequence are comparable to similar modern facies along the margin of Walker
Lake, a 2 0 0 km2 lake within a pull-apart basin in western Nevada (Link and others,
19 8 5 ). The margins of Walker Lake are bounded by alluvial fans and fan-deltas that are
fringed by carbonate and stratified coarse clastic deposits of a nearshore lacustrine
53
environment. These nearshore lacustrine facies are best developed on the deeper west
margin o f the lake (Link and others, 1 98 5 ), similar to the reconstruction in Figure 32
for Member 2 of the Horse Camp Formation.
Pyroclastic Flows and Meaabreccias
Unique volcanic tu ff breccias and megabreccias within facies associations of the
Horse Camp Formation (Member 2) record deposition of pyroclastic flows/debris flows
and non-volcanic rock avalanches, respectively, within fan-delta and lacustrine
environments typically characterized by terrigenous clastic sedimentation (Figs. 32 and
3 3 ). Transport and deposition of these rock avalanches and pyroclastic flows/debris
flows was rapid and chaotic. For instance, the megabreccia facies is invariably underlain
by a zone of deformed substrate in which beds of conglomerate and sandstone have been
asymmetrically folded. Some examples of the megabreccia facies also contain mixed
zones in which fragments broken from the breccia sheet are combined with substrate
material (Yarnold and Lombard, 1989; Yarnold, 1 9 9 3 ). Therefore, dry rock avalanches
composed of Oligocene volcanic bedrock were transported down the surface of a fan-delta,
shearing the underlying substrate, incorporating substrate material, occasionally
breaking into several discrete breccia sheets, and eventually coming to rest on the fandelta surface or lake floor. Similarly, pyroclastic flows/debris flows recorded by the
matrix-rich varieties of the volcanic tu ff breccia facies were transported across the
surface of the fan-delta and lake floor in a chaotic fashion. The flows entrained rounded
gravel clasts located on the fan surface and ripped up boulders of Iithified Horse Camp
substrate; Most examples of the volcanic tu ff breccia facies are preferentially preserved
within the offshore lacustrine facies association and, therefore, were deposited in a
quiet-water lacustrine environment (Figs. 32 and 33). A lack of tu ff breccias in most
deposits of the fan-delta facies associations suggests non-deposition or remobilization of.
54
most pyroclastic flows/debris flows in the fan-delta environment. The stratigraphically
lowest strike correlation (0 -1 3 0 m level) linking sections I , 2, and 5 -9 (Plate I )
shows a.coexistence of megabreccia and volcanic tu ff breccia facies. This correlation
suggests that discrete phases of volcanic activity were also characterized by mobilization
and deposition of rock avalanches.
55
PROVENANCE MODELING APPLIED TO THE HORSE CAMP FORMATION (MEMBER 2)
The conglomerate clast compositions of a basin-margin alluvial fan or fan-delta
sequence often record the unroofing history of a sediment source terrane. Temporal
variations in the volumetric distribution of a particular resistant (i.e., gravelyielding) source lithology within the drainage area are the result of continued erosion of
the source terrane. Such temporal changes are preserved in the basin fill as
stratigraphic variations in the relative percentage of clasts derived from that source
lithology. Over significant time intervals, continued erosion may produce an unroofing
sequence in which stratigraphically higher (younger) lithologies dominate clast suites
within the lower part of the basin-fill sequence and stratigraphically lower (older)
lithologies are more common in clast suites in the upper part of the deposit.
Provenance modeling (see Fig. 3 4 and steps summarized below) compares a
hypothetical clast composition suite generated from simulated unroofing of a particular
source terrane with the actual clast composition data from a fan sequence (Graham and
others, 19 8 6 ). In this manner, the actual unroofing process is approximated by the
model that creates a hypothetical clast composition suite most similar to the actual data.
Specifically, this technique may be used to identify the location of a source area for a
particular stratigraphic sequence when given a choice among several potential source
areas (e.g., DeCeIIes, 19 8 8 ). Comparison of the actual clast composition data with
different hypothetical clast composition suites (each derived from a distinct, potential
source terrane) facilitates the selection of the most appropriate source area. In addition,
provenance modeling may be used to document the uplift history, of a single source
terrane through comparison of actual clast composition data with distinct hypothetical
56
clast composition suites generated by different unroofing models for that source terrane
(e g., Pivnik, 1990; DeCeIIes and others, 1991b). The actual unroofing history is
approximated by the unroofing model that generates the hypothetical clast composition
suite most similar to the actual clast composition data.
I ) Compile
source
section
5 0 0 0 |---------
2) Tabulate lithology
percentages for each
3) Choose model's
source section interval
exposure gates
5000-4800
4800-4600
200-0
C D E
4) Calculate normalized clast compositions
for each exposure gate (gates A through E)
100 i
5) Compare modeled clast composition sequence
with actual clast composition sequence and
check modeled sequence for geological validity
Figure 34. Steps involved in application of provenance modeling (modified from
DeCeIIes 1988). See text for explanation.
57
Provenance modeling has proven to be a useful tool in studies of proximal
foreland basin conglomerates. The technique has helped constrain the erosional history
of individual thrust sheets, providing a basis for time-slice tectonic reconstructions of a
fold-thrust belt (e.g., DeCelles, 1988; 1994; Pivnik, 1990; DeCeIIes and others,
1991b; 19 9 3 ). Although utilized in studies of proximal foreland basin deposits,
provenance modeling has not been applied to conglomerates of extensional or strike-slip
basins. This study examines the potential usefulness of provenance modeling in an
evaluation of source terrane evolution for coarse-grained deposits derived from an
extensional orogen. Specifically, clast composition data from fan-delta conglomerates of
the Miocene Horse Camp Formation of east-central Nevada are compared to a hypothetical
clast composition suite generated from an unroofing model in an attem pt to characterize
the late Cenozoic uplift history.of the adjacent northern Grant Range. Exposure of as
much as 3 0 0 0 m of conglomeratic basin-margin deposits in the Horse Camp Formation
provides an excellent opportunity to apply provenance modeling in an attem pt to better
understand the uplift history of an extensional orogen.
,
Source Terrane Evaluation
The northern Grant Range, Horse Range, and southern White Pine Range contain
similar, nearly 8 0 0 0 m-thick stratigraphic sections of Paleozoic sedimentary rocks and
Paleogene volcanic and sedimentary rocks (all three sections are similar to that
presented in Fig. 35; Moores and others, 1 96 8 ). The Cambrian through Middle
Ordovician sequence includes up to 2 8 0 0 m of limestone, shale, and quartzite. A 1200
m-thick Upper Ordovician through Middle Devonian sequence is dominated by dolomite.
Upper Devonian through Pennsylvanian limestone, shale, and sandstone attain a thickness
of 17 0 0 m. Less than 2 00 m of Eocene shale and limestone and up to 2 10 0 m of
Oligocene rhyolitic ash-flow tu ff overlap Paleozoic rocks with slight angularity (Scott,
58
LEGEND
7800
[>>>>•[ Volcanic Rock
7500
Shingle Pass Formation
Needles Range Formation
Windows Butte Formation
Friable Limestone
7000 -
OUGOCENE
Shale
Stone Cabin Formation
Limestone
6500
-
Calloway Well Formation
Quartz Sandstone
Railroad Valley Rhyolite
6000 -
Dolomite
.\
Quartzite
5500
Sheep Pass Formation
EO&NE
Ely Limestone
PENNSYLVANIAN
i
Unconformity
5000 -
Chainman Formation
T
MISSISSIPP1AN
Joana Formation
4500
Guilmette Formation
4000
DEVONIAN
Simonson Dolomite
3500 -
Sevy Dolomite
Laketown Dolomite
SILURIAN
Fish Haven Dolomite
i
3000
ORDOVICIAN
Eureka Quartzite
2600
meters
p , \ \ \ \ \ ^ ' ' ' ' ' \ vI
Undifferentiated Units
I
+
CAMBRIAN
I
Figure 35. Northern Grant Range stratigraphic source section (based on data from
Moores and others, 1968).
59
1965; Moores and others, 19 6 8 ). The overturned Paleozoic section composing the Horse
Range (Fig. 2) probably represents the overturned limb of a large, east-vergent
anticline formed during late Mesozoic shortening. In the northern Horse Range, the
upper part of this anticline was eroded down to the Devonian section prior to
unconformable overlap by Oligocene volcanic rocks. Folds in the Cambrian section of the
southern White Pine Range (Fig. 2; Walker and others, 19 9 2 ) suggest a pre-extensional
history that may have included erosion of part of the upper Paleozoic section. However,
the lack of large folds or thrust faults in the northern Grant Range (Camilleri, 19 9 2 )
suggests that its pre-extensional geology consisted of an upright, relatively intact
section of Paleozoic to Oligocene rocks.
Nine stratigraphic sections measured in the southern exposures o f Member 2
(Plate I ) and general stratigraphic data from the Horse Camp Formation (Fig. 3)
provide information on the sediment dispersal patterns within the southern half of the
basin. The reader is referred to the sedimentology chapter for an analysis of the
stratigraphy and sedimentology of these deposits. Member 2 strata thin to the northnorthwest and display an abrupt decrease in grain size to the north-northwest (Fig. 3,
Plate I ) .
These trends are evidenced by a correlative time^stratigraphic interval in
sections I and 2 (based on correlation of volcanic tu ff breccias) th a t contains a thinner
strata! sequence to the northwest and an abrupt transition from conglomerate-rich
section 5 to sandstone-, mudstone-, and carbonate-rich sections 1 -4 (Plate I ) . The
stratigraphic sequence also records a transition from fan-delta facies (matrix- and
clast-supported conglomerate) to nearshore lacustrine facies (horizontally stratified
conglomerate and sandstone and stromatolitic carbonate) to offshore lacustrine facies
(massive and laminated mudrock and sandstone) from south-southeast to northnorthwest (Figs. 3, 32 and 33 and Plate I ) . These trends, as well as sparse
paleocurrent data (including ripple mark paleocurrents presented in Plate I and
60
northwest dipping cross-stratification foresets), indicate a sediment transport direction
to the north-northwest. These paleoflow trends suggest that Member 2 strata of the
southern part of the extensional basin were derived from the northern Grant Range. In
addition, the abundance of angular clasts, clasts several meters in diameter, and
carbonate clasts susceptible to chemical weathering suggests that sediment underwent
limited weathering and transport prior to deposition.
Provenance Modeling of Horse Camp Formation
Data Collection
Conglomerate clast composition data from Member 2 of the Horse Camp Formation
were collected from 19 clast count sites within a 16 0 0 m-thick measured stratigraphic
section (Table 2; section 6 of Plate I ) . An approximately 7 5 m stratigraphic sampling
interval was employed, but a lack of conglomerate within some parts of the section
prevented continuous sampling. Each clast count utilized a 10 cm square grid for
sampling of a population of 10 0 pebble- to boulder-sized clasts. All clasts underlying
grid nodes were counted. However, if a single large clast was encountered by several grid
nodes, that clast was counted only once. This point counting technique is biased in that it
emphasizes the importance of large clasts (that have an increased probability of being
touched by a grid point) over smaller clasts (Howard, 19 9 3 ). Therefore, the tabulated
data reflect volumetric percentages of clast types rather than clast type frequencies. A
population of 100 clasts is a commonly accepted total for clast counts (e.g., Blatt,
19 9 2 ), although a greater population (up to 4 0 0 clasts) may be more desirable
(Howard, 1 9 9 3 ). Each clast was classified as a single fragment of volcanic rock,
limestone, dolomite, quartz sandstone, or quartzite. Lithologic similarities among some
formations of the sediment source terrane precluded identification of the specific source
formations for all clasts. However, this evaluation of clast lithologies as opposed to clast
61
source formations prevents errors caused by misidentification of lithologically
indistinguishable clasts derived from different formations (DeCelles, 1988).
Table 2. Conglomerate clast composition data from 1600 m section of Member 2, Horse
Camp Formation (section 6, Plate I ). Data are expressed in percentages (VC—
volcanic rock; L S - limestone; D L - dolomite; S S -sandstone; Q Z - quartzite).
Stratigraphic
Level (m)
1534
1477
1444
1358
1322
1197
I 145
1034
889
791
703
639
534
465
395
313
239
125
69
14
VC
100
100
100
94
100
62
74
80
78
72
64
18
62
67
82
93
88
96
84
72
LS
DL
SS
QZ
6
16
14
2
I I
14
20
47
24
22
14
6
10
3
16
28
22
I I
18
I I
12
16
35
12
8
2
I
I
2
I
3
I
2
2
I
Procedure and Application
Step I in provenance modeling is the compilation of a stratigraphic section for
the sediment source terrane (Fig. 34). As previously discussed, the conglomerates
analyzed in this study represent fan-delta deposits sourced from the northern Grant
Range to the south-southeast. Compilation of the northern Grant Range stratigraphic
source section based on data from Moores and others (1968) is summarized in Figure
35. The base of the source section is defined as the base of the Cambrian Prospect
62
Mountain Quartzite, but the absence of Cambrian and Ordovician clasts in the Horse Camp
conglomerates suggests that the lowest 2600 to 2800 m of the source section (Fig. 35)
was not unroofed during Horse Camp deposition.
Table 3. Lithology of northern Grant Range stratigraphic source section shown in Figure
35. Data are expressed in percentages (VC—volcanic rock; L S - limestone; DL—
dolomite; S S -sandstone; QZ—quartzite; S H -shale; F l - fissile limestone).
Stratigraphic
Level (m)
VC
7 8 0 0 -5 8 0 0 *
5 8 0 0 -5 6 0 0
5 6 0 0 -5 4 0 0
5 4 0 0 -5 2 0 0
5 2 0 0 -5 0 0 0
5 0 0 0 -4 8 0 0
4 8 0 0 -4 6 0 0
4 6 0 0 -4 4 0 0
4 4 0 0 -4 2 0 0
4 2 0 0 -4 0 0 0
4 0 0 0 -3 8 0 0
3 8 0 0 -3 6 0 0
3 6 0 0 -3 4 0 0
3 4 0 0 -3 2 0 0
3 2 0 0 -3 0 0 0
3 0 0 0 -2 8 0 0
2 8 0 0 -2 6 0 0
100
20
LS
DL
28.5
95
95
14.25
59
100
88.5
30.5
SS
QZ
1.5
5
5
0.75
1 1.5
69.5
100
93.5
100
100
100
30
SH
FL
25
25
85
100
100
20
21
6.5
70
*Each 200 m interval within the 7800-5800 m range consists of 100% volcanic rock.
Step 2 is the tabulation of lithology thicknesses and calculation of lithology
relative percentages for each 200 m interval within the stratigraphic source section
(Fig. 34; Table 3). Note that the northern Grant Range source section intervals do not
extend below the 2600 m level since no Cambrian or Ordovician clasts were found in the
Horse Camp conglomerates. Step 2 is a one-dimensional analysis that assumes intact,
flat-lying units in the source terrane. A reconstruction of the northern Grant Range
suggests only minor faulting of the approximately flat-lying rocks prior to extension and
63
basin development (Camilleri, 19 9 2 ). Modeling of a folded source section would require
lithologic data generated from simulated erosion of a two-dimensional, structural cross
I
section through the source terrane (e.g., Pivnik, 19 9 0 ). In addition, step 2 does not
consider the differential durabilities of the various lithologies in the northern Grant
Range source section. If feasible, each lithology thickness value for every 2 00 m
interval may be multiplied by a different durability coefficient th at reflects the
lithology's likelihood of preservation under specific climatic and transport conditions
(e.g., DeCeIIes and others, 1991b). A lack of pateoclimatic data precludes the derivation
and utilization of such coefficients in this study. However, shale and thin-bedded, fissile
limestone source rocks are excluded from future steps of the model (see Fig. 36) because
these less durable lithologies do not yield gravel-sized clasts in the Horse Camp deposits.
Step 3 is the selection of the stratigraphic intervals (so-called "exposure gates")
exposed at different times during unroofing of the compiled source section and step 4 is
the calculation of the simulated clast composition data for each of these incremental
exposure gates (Fig. 3 4 ). The stratigraphically lowest clast composition count in this
study (1 4 m clast count of Table 2) contains only clasts of Oligocene volcanic rocks,
Pennsylvanian Ely Limestone, and Mississippian Joana Limestone. Therefore, the lower
limit of exposure during this initial deposition is the base of the Joana Limestone
(approximately 4 4 0 0 m level of Fig. 3 5 ) and the upper limit is the top of the volcanic
sequence (7 8 0 0 m level of Fig. 35). The initial exposure gate (step 3A; Fig. 34) is thus
chosen as the stratigraphic range from 4 4 0 0 m to 7 8 0 0 m in the northern Grant Range
source section (Fig. 3 5 ). Each successive exposure gate incorporates an additional 8 0 0
m of underlying strata. A simulated clast composition suite for each exposure gate (step
4; Fig. 34) is depicted in Figure 36. Note th at complete erosion of the uppermost part of
the source section (step 3D, E; Fig. 3 4 ) is not initiated because Oligocene volcanic rocks
remained a dominant clast type throughout Horsd Camp deposition and are presently
64
exposed in the modern northern Grant Range (i.e., the volcanic sequence was not
completely eroded from the source terrane).
Step 5 is the comparison of the modeled clast composition suite with actual clast
composition data and assessment of the geological validity of this modeled unroofing
sequence (Fig. 34). Figure 36 compares the modeled clast composition suite derived
from the source section of the northern Grant Range (depicted in Fig. 3 5 ) with the actual
clast composition data of the Horse Camp Formation. Table 2 displays the 19 clast counts
(from the southern exposures of Member 2 of the Horse Camp Formation) and Figure 36
reduces these data to four sets of two consecutive clast counts (sets A, B, C, and D) that
sample the measured section at approximately 5 0 0 m intervals. Modeled clast
composition histograms of Figure 36 depict a sequence generated by simulated unroofing
of the northern Grant Range source section. The abundance and distribution of clast
lithologies depicted in modeled clast composition histograms A, B, and C are quite similar
to clast data shown in histogram sets A, B, and C of the lower 12 0 0 m of Member 2 (Fig.
3 6 ). Therefore, the model appears geologically valid and the lower 12 00 m of Member 2
is a faithful record of the unroofing of the northern Grant Range. However, the modeled
compositions for exposure gate D are quite different from the actual clast compositions at
J
the highest levels of Member 2 (Fig. 36, set D). The model predicts an upsection
decrease in volcanic debris, but the actual clast compositions of uppermost Member 2
are entirely volcanic (Fig. 36, set D; Table 2). The abrupt increase in volcanic clasts
(Fig. 36; Table 2) coincides with the lowest stratigraphic appearance of two Cenozoic
clast types: ( I ) . a volcanic breccia with gray clasts within a yellow or brick red matrix
and (2 ) a carbonate th at occurs both as a single clast and as fracture-fill in other clasts
of volcanic rocks. Prolonged exposure of Tertiary volcanic rocks and unroofing down to
the upper Paleozoic section in the northern Grant Range source terrane prior to the
appearance of these new clasts suggest that these clasts are not simply the result of
65
MODEL
ACTUAL
100
WV
7 8 0 0 -2 6 0 0
I 444 m
V aV
I 534 m
X -X
1145m
M
7 8 0 0 -3 6 0 0
m
X V X
% 0
-
465 m
B
534 m
v v x
X '
lTl TlT l
1197 m
vX
100
z v-.l
7 8 0 0 -4 4 0 0
m
69 m
- X--I
-X*-
% 0
VC LS DL SS QZ
VC LS DL SS QZ
Figure 36. Comparison of hypothetical clast composition suites generated from an
unroofing model of the northern Grant Range (see Fig. 35 and Table 3) and actual clast
composition data from Member 2 of the Horse Camp Formation (see Table 2). Modeled
clast composition suites A, B, C, and D depict simulated erosion of four different
stratigraphic ranges, or exposure gates, selected for the northern- Grant Range source
section. Actual clast composition data sets A, B, C, and D are derived from clast counts
within a 16 00 m measured section of Member 2.
66
erosional breaching of a new Cenozoic formation within the source terrane. Rather, the
presence of these clasts and the departure of actual clast compositions from model
predictions (Fig. 36, set D) are interpreted as indicators of a change in sediment
provenance (i.e., a source terrane other than the northern Grant Range was supplying
sediment to the basin). This new source terrane contained the two distinctive clast types
mentioned above (volcanic breccia and carbonate clasts of Cenozoic age) while the
northern Grant Range did not. As suggested by Moores (1 9 6 8 ), the new source area was
probably located in the Horse Range to the east (Figs. I and 2). This source terrane fed
Paleozoic carbonate-rich sedimentary breccias to the central part of the basin during the
late stage of Member 2 sedimentation (Moores, 19 6 8 ).
Implications
A provenance model based on unroofing of the northern Grant Range is similar to
the lower 1 20 0 m of the southern Member 2 sequence of the Horse Camp Formation (Fig.
36, sets A, B, and C). Therefore, this sequence of the Horse Camp Formation is
interpreted as a synorogenic stratigraphic sequence that was deposited along the southern
boundary of an extensional basin during uplift of the northern Grant Range. Indeed, the
facies trends and overall northward fining trend discussed above (Fig. 3) are in
agreement with this interpretation. Thus, the Horse Camp clast composition sequence
records progressive unroofing of the northern Grant Range during uplift and can be used
to help assess the timing and character of this uplift.
The Horse Camp Formation records the early evolution of the northern Grant
Range. The dominance of Oligocene volcanic and upper Paleozoic carbonate clasts within
the Horse Camp Formation requires that deep levels of the northern Grant Range (i.e.,
lower Paleozoic rocks) were not breached during deposition of the Horse Camp strata. In
fact, the dominance of Oligocene volcanic clasts throughout the Horse Camp Formation
(Fig. 36; Moores, 1968; Brown, 19 9 3 ) suggests the existence of a nascent orogen in
67
which the upper carapace of the source terrane (Oligocene volcanic sequence) was not
(.
completely eroded from the source terrane (Fig. 37). The age of the lower part of
Member 2 of the Horse Camp Formation is thus an accurate approximation of the timing
of the early stage of uplift of the northern Grant Range. This section of the Horse Camp
Formation is poorly dated between 20 and 10 Ma (early to middle Miocene; see
stratigraphy chapter). Isotopic age determinations for several ash-flow tuffs within the
Horse Camp strata are underway and will provide more precise age constraints.
Nonetheless, the inferred early to middle Miocene age of initial uplift for the northern
Grant Range is in agreement with structural studies that identify down-to-the-west
normal faults of Miocene age in the northern Grant Range (Camilleri, 1992-r Lund and
others, 1 9 9 3 ).
The lack of lower Paleozoic clasts in the Horse Camp clast composition suite and
the presence of a lower Paleozoic section exposed in the northern Grant Range at
elevations approximately I km above the modem valley floor suggest that at least I km
of uplift of the northern Grant Range occurred from approximately 10 -5 Ma to the
present (i.e., post-Horse Camp deposition).
This inference assumes that the fan-delta
sequence of this study had a drainage area large enough to adequately sample all areas of
the uplifted source terrane (Fig. 3 7 ) rather than a small drainage catchment in a limited
portion of the orogen. Two considerations suggest that the studied sequence was
characterized.by a large drainage area: ( I ) the exceptional thickness (1 2 0 0 m) of the
fan-delta sequence and presence of upper Paleozoic through Oligocene clasts are the
result of a long-lived depositional environment th at contained detritus sourced from a
well-developed drainage network that had sufficient time to expand over much of the
northern Grant Range (e.g., Fraser and DeCeIIes, 1 99 2 ) and (2 ) the source terrane was
bounded by a transverse structural zone that linked two normal faults, a common setting
for large drainage networks in extensional orogens (Leeder and Jackson, 19 93 ).
68
p^TTj Miocene Horse Camp
L’ '
Formation
Oligocene
“
|
volcanic rocks
j Devonian-Pennsylvanian
sedimentary rocks
F = I Cambrian-Ordovician
^
sedimentary rocks
5 km
Figure 37. Generalized diagram illustrating Miocene uplift o f the northern Grant
Range as part of the footwall block of the down-to-the-north Ragged Ridge fault (see Fig.
2). Oligocene volcanic rocks and Devonian to Pennsylvanian sedimentary rocks are the
sediment sources for the Miocene Horse Camp conglomerates. Cambrian and Ordovician
rocks (including Eureka Quartzite) are not exposed in the northern Grant Range drainage
area during Horse Camp deposition.
Clast counts of the Horse Camp Formation record an abundance of Oligocene
volcanic rocks (Table 2) and absence of most Cambrian and Ordovician rocks. This
suggests that the lower Paleozoic section in the northern Grant Range remained
unbreached during most of the Miocene. However, the mapped geology of the northern
Grant Range (Fig. 2; Moores and others, 1968; Camilleri, 1992; Lund and others,
1993) reveals widespread present-day exposure of lower Paleozoic rocks. In fact,
69
modern alluvial fans sourced from the northern Grant Range contain many lower
Paleozoic clasts and are often dominated by resistant quartzite clasts of the Ordovician
Eureka Quartzite. The enhanced durability of quartzite may lead to over-representation
of this lithology in clast counts (e.g., Schmitt, 1 99 2 ). Thus, if the Eureka Quartzite
>
were present in the source area during Horse Camp deposition, the relative percentage of
this lithology should well exceed the model prediction D of Figure 36. However, because
quartzite clasts are extremely rare in the Horse Camp conglomerates (Fig. 36; Table 2),
the Eureka Quartzite and other Ordovician rocks were not exposed in the northern Grant
Range during Horse Camp deposition (Fig. 37), but were exposed a fte r Horse Camp
deposition (Fig. 2). This relationship suggests th at uplift responsible for exposure of
the lower Paleozoic section in the northern Grant Range is of Pliocene to Holocene age
(i.e., post-Member 2 deposition). In fact, much of this uplift may be a Quaternary
phenomenon. For example, relict Pleistocene pediments and alluvium in the Grant Range
are found at elevations of 2 4 4 0 m, approximately 9 0 0 m above the elevation of modern
alluvial fans in Railroad Valley (Scott, 1965; Kleinhampl and Ziony, 198 5 ). These
high-level deposits are considered to be uplifted remnants of alluvial fans formed
adjacent to the valley floor. Therefore, approximately 9 0 0 m of structural relief in the
Grant Range was generated during the Quaternary.
Although unroofing of the Cambro-Ordovician section is probably a post-Miocene
event in th e northern Grant Range, that is not the case less than 5 0 km to the south in the
central Grant Range. An unnamed Miocene conglomerate (40ArZ39Ar age of 14.2 Ma for
interbedded ash-fall tu ff) of the central Grant Range contains abundant Ordovician clasts,
including clasts of Eureka Quartzite, and several slide blocks of Ordovician and Devonian
lithologies (Fryxell, 1 9 8 8 ).
Therefore, erosional breaching of the lower Paleozoic
section occurred by the middle Miocene in the central Grant Range, approximately I Q
m.y. prior to such breaching in the northern Grant Range. This discrepancy probably
70
reflects differences in the stratigraphic sections of the two areas. A nearly 5 0 0 0 rrithick section overlies Ordovician rocks in the northern Grant Range source section (Fig.
3 5 ) while only 2 0 0 m of post-Ordovician rocks are found in the central Grant Range
source section (Fryxell, 19 8 8 ). This may be due to erosion of the upper Paleozoic rocks
in the central Grant Range during Mesozoic contraction and uplift or Cenozoic tectonic
denudation of these rocks during extensional faulting (Fryxell, 1 9 9 1 ). The northern
Grant Range was spared from such pre-Miocene unroofing of upper Paleozoic rocks.
Provenance Modeling for Extensional Basins
Provenance modeling (Graham and others, 19 8 6 ) has been utilized exclusively in
studies o f proximal foreland basin conglomerates in order to ( I ) link a stratigraphic
sequence to a particular source terrane (e.g., DeCeIIes, 198 8 ) and (2 ) constrain the
kinematic history of a contractional uplift (e.g., Pivnik, 1990; DeCeiIes and others,
1991b ). However, provenance modeling may be applied to extensional basin
conglomerates and is potentially useful for ( I ) selection of a basin's appropriate source
terrane when given a choice among several lithologically distinct source terranes and
(2 ) identification of the unroofing and uplift history of an adjacent source terrane in an
extensional orogen (this study).
Provenance modeling applied to extensional basin conglomerates can distinguish
between individual source terranes only if the potential source terranes have differing
source sections. For example, this approach is futile for the Horse Camp Formation
because all potential sediment source areas (i.e., the northern Grant Range, Horse Range,
southern White Pine Range, and subsurface Railroad Valley area; Fig. 2) have
approximately similar geologic sections (Moores and others, 1 9 6 8 ). In addition, the
relatively small size of the Horse Camp depositional basin increases the likelihood of a
homogeneous source section on all sides of the basin. This approach is better suited for
71
large extensional basins surrounded by lithologically distinct areas that possibly served
as sediment source terranes.
Provenance modeling can help independently constrain the character and timing of
an extensional orogen source terrane only after the conglomerates are documented as the
product of that particular source terrane. For this study, the southward thickening
basin-fill sequence exhibits an abrupt decrease in grain size and a subaerial fan-delta to
subaqueous fan-delta to lacustrine facies transition to the north-northwest. These
relations thus suggest that sediment was derived from the northern Grant Range to the
south. After the location of the source area is determined, the clast composition data may
be used to document the unroofing history of the particular source area (this study).
This approach is useful for conglomerates of proximal alluvial fan or fan-delta
environments th at contain sediment from an integrated drainage network within the
source terrane. The approach is probably less useful for terrigenous clastic sediment
deposited in more distal environments because of the possibility o f significant mixing of
sediment derived from drainage areas of different source terranes.
72
INVERSION OF THE MIOCENE HORSE CAMP BASIN
The process of inversion is defined simply as a reversal o f uplift or subsidence
(Coward, 1 9 9 4 ).
Inversion of a sedimentary basin (e.g., Mather, 1 9 9 3 ) thus involves
uplift of a formerly subsiding region (the basin) and resulting erosion of previously
deposited basin-fill sediments. Inherent with this uplift is potential obliteration of a
portion of the record of basin evolution as well as development of complex provenance
relations by subsequent sediment recycling. In contractional orogens, basin inversion
typically occurs by reactivation of pre-existing basin-margin normal faults as reverse
faults during a later phase of contraction (e.g., inversion of Mesozoic rift-margin faults
during Cenozoic Alpine contraction; Ziegler, 1983; Munoz, 19 9 2 ) or uplift of proximal
foreland basin deposits due to continuous basinward migration of the locus of thrusting
during prolonged contraction (e.g., DeCeIIes and others, 1987; Burbank and Reynolds,
1988; Royse, 19 9 3 ).
In extensional settings, reactivation of pre-existing reverse or
thrust faults as normal faults during a younger phase of extension is fairly common
(e.g., Royse and others, 19 7 5 ), but uplift of extensional basin deposits due to inherent
processes during prolonged extension is rarely documented (see Leeder and Gawthorpe,
1987, p, 1 4 9 -1 5 0 ). This study presents an example of basin inversion in which a
portion of an extensional basin experienced initial subsidence and subsequent uplift
during extension and associated footwall uplift.
A synextensional sedimentary succession of the Miocene Horse Camp Basin is .
exposed and tilted as much as 80° in an uplifted footwall of a seismically active, west­
dipping normal fault in east-central Nevada. The hanging wall to the west is defined by
Railroad Valley, a modern extensional basin containing a late Miocene to Holocene
73
sedimentary sequence. Regional late Cenozoic extension and accompanying footwall uplift
induced initial Miocene subsidence and subsequent late Miocene to Holocene uplift
(inversion) of the Horse Camp Basin. The evolution and inversion of such an extensional
basin-fill sequence under a single, relatively short lived (< 2 0 m .y.) tectonic regime
suggests that provenance characteristics and basin subsidence histories for extensional
basins may be more complex than previously envisioned.
Tectono-Sedimentarv Sequences
Horse Camp Formation and Equivalent Strata
The Miocene Horse Camp Formation is a 3 0 0 0 m-thick sequence of conglomerate
and sandstone with minor amounts of mudstone, carbonate and volcanic flows (Fig. 3; see
stratigraphy chapter). The presence of a western sediment source area during deposition
of the lowermost part of the Horse Camp Formation (Member I ) suggests that the
Railroad Valley region (Fig. I ) was a positive topographic feature and not a sedimentary
basin during Miocene time (Moores, 1968; Brown, 1 99 3 ). Therefore, the Horse Camp
Formation is not correlative with the sedimentary fill of Railroad Valley; rather, the
Horse Camp Formation records an earlier period of basin development. However, it is
likely th at the Horse Camp Formation exists in places in the subsurface beneath Railroad
Valley, where it was preserved in the hanging wall of the Railroad Valley fault (e.g.,
Johnson, 19 9 3 ). Uplift (inversion) of the Horse Camp Formation has brought
exposures to elevations over I OO m above the floor of Railroad Valley.
A coarse-grained sequence equivalent to the Horse Camp Formation is exposed less
than 50 km to the south in the central Grant Range (Fryxell, 19 8 8 ). This sequence,
informally named the Little Meadows conglomerate, is similar to the Horse Camp
Formation in that it is exposed in the uplifted footwall of a west-dipping range-front
fault. A dated ash-fall tu ff within the Little Meadows conglomerate ( 40ArZ^^Ar age of
74
14.2 Ma) suggests Miocene sedimentation, coeval with Horse Camp deposition (Fryxell,
19 88 ). Thus, the Horse Camp Formation and Little Meadows conglomerate are broadly
equivalent in age and modem tectonic setting.
Alluvial Deposits of the Grant Range
A sequence less than 100 m thick of poorly consolidated, PIeistocene(T) conglomerate
and sandstone is exposed within the Grant Range, approximately 9 0 0 m above the elevation of
modern alluvial fans in Railroad Valley (Scott, 1965; Kleinhampl and Ziony, 1 9 8 5 ).
This
alluvium is interpreted as deposits of alluvial fans (Fryxell, 19 8 8 ) originally formed on
the western margin of Railroad Valley. Therefore, these deposits have been uplifted
(inverted) at least 9 00 m during Quaternary tectonism.
Alluvial Deposits of Railroad Valley
Railroad Valley contains a late Miocene to Holocene sequence of sandstone,
conglomerate, mudstone, and basalt flows. These deposits record deposition in basinmargin alluvial fan environments and basin-center lacustrine and fluvial environments
(Kleinhampl and Ziony, 19 8 5 ). The stratigraphic sequence of Railroad Valley has an
average thickness of approximately 3 0 0 0 m and thickens eastward toward the west­
dipping Railroad Valley fault ( Bortz and Murray, 1979; Effimoff and Pinezich, 19 8 1 ).
Basalt flows in the upper half of the basin-fill sequence (Johnson, 1 9 9 3 ) may be
correlative to Pliocene and younger basalt flows along the western margin of Railroad
Valley (Kleinhampl. and Ziony, 1 9 8 5 ).
Despite previous stratigraphic assignments
(e.g., Effimoff and Pinezich, 1 98 1 ), the basin fill of Railroad Valley cannot be
considered simply as correlative to the Horse Camp Formation because the two sequences
are of different ages.
75
Basin Inversion MoHp I
The stratigraphic successions of the Miocene Horse Camp Basin and Railroad
Valley are spatially and temporally distinct products of regional late Cenozoic extension
in east-central Nevada. The Miocene basin-fill sequence of the Horse Camp Basin is
exposed in the footwall of the Railroad Valley fault, which bounds the eastern margin of
the late Miocene to Holocene basin, Railroad Valley (Fig. 38). Thus, the youngest basing
fill sequence (alluvial deposits of Railroad Valley) lies to the west of the older basin-fill
sequence (Horse Camp Formation), which has been uplifted as part of the modem .
footwall (Fig. 38). These observations suggest that both basins formed within an
extensional orogen characterized by prolonged footwall uplift and westward migration o f
the zone of active faulting (Fig. 39; Lucchitta and Suneson, 1 9 9 3 ). The alluvial deposits
of the Grant Range most likely represent the easternmost basin-fill sequence of Railroad
Valley that was uplifted as the locus of footwall uplift migrated westward. A three phase
model has been developed which accounts for the subsidence and subsequent uplift
(inversion) of the Horse Camp Basin and evolution of Railroad Valley (Fig. 39); The
pre-extension geology of the area consisted of a large anticline (see geologic setting
chapter) overlapped by Oligocene volcanic rocks (phase A, Fig. 3 9 ). Fault-controlled
subsidence along west-dipping normal faults of the Horse Range dropped the west limb of
the pre-existing anticline and formed the Horse Camp Basin in a hanging wall setting
(phase B, Fig. 3 9 ). Westward migration of the locus of active normal faulting resulted
in subsequent initiation of motion on a younger fault to the west, the west-dipping
Railroad Valley fault, abandonment of motion on the older normal faults of the Horse
Range, and effective transfer of the Horse Camp deposits to a footwall position (phase C,
Fig. 3 9 ), Subsidence of the hanging wall formed Railroad Valley while footwall motion
uplifted and tilted the Horse Camp sequence (phase C, Fig. 39). A potential mechanism
76
that explains footwall uplift and inversion of the deposits of the Horse Camp Basin is
isostatic compensation of a thinned crust via either subvertical simple shear or flexural
rotation of the footwall (Buck, 1988; Wernicke and Axen, 1988; Coleman and Walker,
1994). Alternatively, extensional faulting controlled by pre-existing tectonic elements
(faults, fractures, or cleavages) formed during Mesozoic shortening may account for the
variable fault geometries depicted in Figure 39 (i.e., low-angle faults to the east and a
high-angle range-front fault to the west; D. Lageson, personal commun., I 994).
Horse Camp
exposures
Railroad
V a lle y
5 km
3 km
O
°
Late Miocene-Holocene Railroad Valley alluvial deposits
\ ; • Miocene Horse Camp Formation
Cambrian-Oligocene sedimentary and volcanic rocks
Figure 38. Schematic representation of modern tectono-geomorphic setting of
Railroad Valley and deposits of the Horse Camp Formation. Note that Horse Camp
exposures are a sediment source for Railroad Valley alluvial deposits. Closed arrows
depict offset on modern active normal fault. Open arrows depict offset on normal fault
that was active during the Miocene.
77
L—— I
Latest Miocene-Holocene
basin fill (RVB)
N WS F
/ o6c-i Miocene
I basin fill (HCB)
Oligocene
3
volcanic rocks
v7»
E
I : I Ordovician-Pennsylvanian
'
I
sedimentary rocks
I Precambrian(T)-Cambrian
sedimentary rocks
NRRF
NWSF
^r 7
* ►v > v T
-< < > «■ V T A « ■> >
4. A ►
• O
O •
HCB
NRRF
NWSF
RVB
» <*
* T
Figure 39. Generalized three-phase model depicting the tectonic development of the
Horse Range, Horse Camp Basin(HCB), and Railroad Valley basin(RVB) along a 3 8 04 5 ’N
latitude transect. Scales are approximate. NWSF1 north segment of Wells Station fault;
NRRF1 north segment of Ragged Ridge fault; RVF1 Railroad Valley fault. Phase A1 postvolcanic and pre-extensional stage (earliest Miocene). Phase B1 development of Horse
Camp Basin (Miocene). Phase C1 development of Railroad Valley basin (late Miocene to
Holocene).
78
implications for Studies of Extensional Basins
Basin inversion during a single phase of prolonged extension is a potentially
important control on the tectono-sedimentary evolution of extensional basins. In
particular, basin inversion will lead to complex provenance trends and subsidence
patterns. Provenance studies of extensional basin sequences (e.g., Miller and John,
19 8 8 ) have implicitly assumed that extensional basin deposits themselves are not
potential sediment sources. Cannibalization or recycling of proximal basin sequences
commonly recorded in contractional settings (DeCelles, 1987; Colombo, 19 9 4 ) and
related to basinward propagation of the locus of tectonism (Burbank and Rayholds, 1988;
Steidtmann and Schmitt, 19 8 8 ) is not considered to be an important process in
extensional basins. However, this study describes a sediment recycling, situation,
probably due to migration of the locus of faulting, in which a Miocene synextensional
sequence (Horse Camp Formation) exposed in a footwall setting is providing sediment to a
modern extensional basin, Railroad Valley (Fig. 3 8 ). Contemporaneous erosion of the
Horse Camp Formation and Paleozoic carbonate and Oligocene volcanic rocks in the
surrounding ranges is contributing to complex sediment compositional relations in the
alluvium of Railroad Valley.
The Horse Camp Formation is 3 0 0 0 m thick, exposed over
a 50 km2 area, and its basal contact with Oligocene volcanic rocks is exposed throughout
the area (Fig. 2). Thus, the entire thickness of the Horse Camp Formation is exposed and
the sequence is an important contributor of detritus to Railroad Valley. Basin
development and subsequent inversion and associated cannibalization of the Horse Camp
sequence occurred relatively rapidly (< 2 0 m.y.) during regional extension. Although
this extension is certainly heterogeneous in time and space, there is no Neogene
contractional deformation or reversal of motion on normal faults recorded in eastern
Nevada to account for basin inversion. Therefore, the basin inversion described here
\
79
must be a product of an extension-related process such as footwall uplift (see Fig. 39).
Relatively rapid evolution, inversion, and erosional recycling of basin sequences during
extension suggests that continental extensional basins are dynamic systems that do not
conform to simple tectono-sedimentary models (e.g., Leeder and Gawthorpe, 19 8 7 )
depicting prolonged subsidence of a half graben basin throughout an interval of regional
extension. In fact, extensional basins are similar to foreland basins in that they may be
characterized by uplift and recycling of the proximal basin deposits.
Subsidence histories of extensional basins must reflect the complex interplay
between basin subsidence and basin uplift. Net subsidence in extensional basins may be
viewed as the difference between fault-controlled, hanging wall subsidence and isostatic
footwall uplift. In foreland basins, a decrease in net subsidence may be the result of
increased uplift as proximal parts of a basin are incorporated into a fold-thrust orogen
(e.g., Burbank and Raynolds, 1 9 8 8 ).
Similarly, inversion of a proximal extensional
basin-fill sequence due to footwall uplift will cause a decrease in net subsidence.
However, in distal settings, away from the rising footwall, net subsidence may be
unaffected. Therefore, the subsidence history of an exposed, proximal basin-fill
sequence th at underwent subsidence and subsequent uplift (inversion) is probably not
indicative of the subsidence history of more distal parts of the basin. In addition, young
uplifted stratigraphic sequences exposed along the margins of modern extensional basins
(e.g., Gordon and Heller, 1993; Grier, 19 8 3 ) recorded different subsidence histories
than adjacent modern basins (Van Houten, 1956, p. 282 3 ).
80
CONCLUSIONS
This study utilizes several field research techniques in order to assess the
sedimentary and tectonic history of a Miocene extensional basin and adjacent orogen of
the Basin and Range province. Sedimentologic and stratigraphic analyses of the Miocene
Horse Camp Formation (Member 2) of east-central Nevada provide constraints on the
paleogeography, depositional processes, and depositional environments along the
southern margin of an extensional basin. Specifically, analyses of distinct facies and
different associations of these facies define a subaerial and subaqueous fan-delta
environment dominated by sediment gravity flow processes, a wave-influenced
nearshore lacustrine environment, and a quiet-water offshore lacustrine environment.
Walker Lake of western Nevada may provide a modern analogue for Member 2
depositional environments. However, the dynamics of modern and ancient nearshore
lacustrine environments characterized by coarse clastic and carbonate deposition remain
poorly understood. Facies trends and grain-size trends show that the terrigenous clastic
sediment of Member 2 was derived from a part of the extensional orogen to the southsoutheast, the northern Grant Range.
Compositional analysis of conglomerates of Member 2 yields information on the
tectonic history of the sediment source area within the adjacent extensional orogen. A
provenance model constructed for the source area, the northern Grant Range, predicts a
compositional distribution of sediment th a t is remarkably similar to actual
compositional data from the lower 12 00 m of Member 2. Therefore, conglomerates
within the lower 12 00 m of Member 2 serve as a record of the Miocene uplift of the
northern Grant Range. Comparison between Member 2 compositional data (diagnostic of
81
the lithologic composition of the northern Grant Range during Miocene time) and the
lithologic composition of the northern Grant Range today reveals significant postMiocene (post-Member 2) unroofing.
Exposure of lower Paleozoic rocks in the
northern Grant Range today requires at least I km of post-Miocene uplift.
Considerations of the modern structural setting of the area containing Horse Camp
exposures help decipher the past setting of the Miocene basin in which these rocks were
deposited. Current tectonic models depicting prolonged evolution and abandonment of
several generations of basins and basin-margin faults during regional extension (e.g.,
Buck, 1988; Wernicke and Axen, 19 8 8 ) predict several distinctive tectonic and
sedimentary features of the Horse Camp Formation and the adjacent modern basin,
Railroad Valley. In particular, such models predict a younging of basin sequences in a
single direction and uplift .and tilting of older faults and basin sequences as part of the
footwall. Late Cenozoic westward migration of the zone of active faulting, footwall uplift,
and basin formation accounts for the differences in age, location, and modem structural
setting of Railroad Valley and the Horse Camp Formation. However, models of continental
extensional tectonics based upon studies in the Basin and Range province (e.g., Buck,
1988; Wernicke and Axen, 19 8 8 ) fail to assess the effect of pre-existing, contractional
structures developed during Mesozoic contraction throughout much of the Basin and
Range province. These older structures may have controlled Cenozoic extensional
geometries on a local and regional scale.
.
82
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r
Plate I . Logs of measured stratigraphic sections
of the southern exposures of the Horse Camp
Formation (Member 2). Inset (see Fig. 2) shows
location of dip-corrected sections.
All
correlations are based upon laterally continuous
Iithostratigraphic units except the lowest
correlation between sections 5-8 and the
correlation linking sections 3 -5 which are air
photo-based strike correlations projected across
covered intervals.
More detailed versions of the
nine stratigraphic sections are presented in
archival data that are available from the Montana
State University Department of Earth Sciences.
FACIES
?
<3
Horse Camp Formation
Member 2
Gmu
1600
Gmn
X
Gcu
1500
Gcn
/R e d H iII A
Slide Block
6
I
I
( 6)
i
i I
Gh
eccc?
Horse Camp Formation
<
Member I
EX
\
\
.
y
| — I Sh
Pliocene-Quaternary
alluvium
.
Sn
■
p > 7 | Sr
V
Oligocene yI X
^7 volcanic rocks
; :
-7 JX :
/ ^
//\
V
\
Y
r'
GRAIN SIZE
distorted bedding
M = mud
mudcracks
S = sand
&
P = pebble
O
C = cobble
yv
B = boulder
X
Clm
I
1100-
I
I v v I Vtb
pelecypods
|t>
a
| Vmb
pisoids
1000
I X C I Covered
dewatering structures
ripple mark paleocurrent
CA
m
MSPCB
Section 3
c5 l
50- I I %
V
m
I
i i i
M<B P C B
Section 4
C \
V V V V
yX
o
m
T
■ •• x ______
I T l
I I
Section 8
MSPCB
Section I
I
MSPCB
Section 2
I
I
I
I
m MSPCB
m
MSPCB
Section 5
Section 6
I
m
i
I
I
MSPCB
Section 7
I
rn
i
m MSPCB
Section 9
I
-----