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 1994 -fisng

i i

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 -

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

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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 1993-94 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 ) ...... 10 Facies Analysis...... 11 Facies G m u ...... , ...... 13 Ungraded, Matrix-Supported Conglomerate...... 13 Interpretation...... 13 Facies G m n ...... 15 Normally Graded, Matrix-Supported Conglomerate ...... 15 Interpretation...... 15 Facies Gcu ...... 17 Ungraded, Clast-Supported Conglomerate...... I 7 Interpretation...... 17 Facies G c n ...... 19 Normally Graded, Clast-Supported Conglomerate...... 1 9 . Interpretation...... 19 Facies Gh . , ...... 21 Horizontally Stratified Conglomerate...... 21 Interpretation...... 21 Facies S m ...... 22 Massive Sandstone...... 22 Interpretation...... 22 Facies S n ...... 25 Normally Graded Sandstone ...... 25 Interpretation...... 25 Facies S h ...... , ...... ; . 26 Horizontally Stratified Sandstone ...... 2 6 Interpretation ...... 26 Facies S r ...... 28 Ripple Cross-Laminated Sandstone . , ...... 28 Interpretation...... : ...... 28 Facies S t ...... 29 Trough Cross-Stratified Sandstone ...... 29 Interpretation...... 29 Facies Fm ...... 29 Massive to Poorly Laminated Mudstone ...... 29 Interpretation...... 31 vi

TABLE OF CONTENTS—Continued

Page

Facies C lm ...... 31 Laminated or Massive Carbonate...... 31 Interpretation...... 34 Facies V t b ...... 34 Volcanic Tuff Breccia ...... 3 4 Interpretation...... 35 Facies V m b ...... 36 Volcanic Bedrock Megabreccia...... 36 Interpretation...... 39 Facies Associations and Depositional Environments...... 39 Subaerial Fan-Delta Facies Association...... i . . . 41 Subaqueous Fan-Delta Fapies Association...... 43 Nearshore Lacustrine Facies Association...... 45 Offshore Lacustrine Facies Association...... 4 6 Environmental Reconstruction...... 4 6 Subaerial-Subaqueous Transition...... 4 8 Nearshore Environment...... 51 Pyroclastic Flows and Megabreccias...... 53

PROVENANCE MODELING APPLIED TO THE HORSE CAMP FORMATION (MEMBER 2) . . 55 Source Terrane Evaluation ...... 57 Provenance Modeling of the Horse Camp Formation...... 60 Data Collection...... 60 Procedure and Application...... 61 Implications...... % ...... 66 Provenance Modeling for Extensional Basins...... 70

INVERSION OF THE MIOCENE HORSE CAMP BASIN...... ' ...... 72 Tectono-Sedimentary Sequences...... 73 Horse Camp Formation and Equivalent S tra ta ...... 73 Alluvial Deposits of the Grant Range ...... 7 4 Alluvial Deposits of Railroad V a lle y ...... 74 Basin Inversion M o d e l...... 75 Implications for Studies of Extensional Basins...... • ...... 7 8

CONCLUSIONS ...... 80

REFERENCES CITED...... 82 vii

LIST OF TABLES

Table Page

1. Lithofacies of Member 2, Horse Camp Form ation...... ; ...... 11

2. Conglomerate clast composition data from 1600 m section of Member 2, Horse Camp Formation...... ’

3. Lithology of northern Grant Range stratigraphic source section ...... 62 viii

LIST OF FIGURES

Figure Rage

I • Index map of basin and range physiography of east-central N evad a...... 4

2. Geologic map of the northern Grant Range-Florse Range-southern White Pine Range region...... 5

3. Schematic stratigraphic cross section of the Horse Camp Form ation...... 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 ...... 14

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 and outsized c la 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 facies...... 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 siz e ...... 20

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 ...... 23

14. Subvertical root casts in facies S m ...... 2 4

15. Flame structures in facies S h ...... 2 4

16. Ball and pillow structures in facies S h ...... 27

17. Facies Sn foresets dip l e f t ...... 27

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 ...... 30

20. Facies Clm interbedded with facies Gh and S h ...... 32

21. Convex-upward laterally linked hemispheroid geometry of facies C lm ...... 32

22. Planar, subhorizontal, 0 .2 -5 mm-thick laminations of facies C lm ...... 33

23. Ooids and pisoids of facies Clm. Grains are well-rounded and moderately spherical. Deposit is poorly sorted ...... 33

24. Asymmetric fold in deformed substrate zone of facies Vmb ...... 37

25. Mixed zone (MZ) and resistant, overhanging breccia sheet (BS) of facies V m b ...... 37

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 ...... 38

28. Detailed stratigraphic section of subaerial fan-delta facies association...... 4 0

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...... 4 2

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 -d elta...... 49

33. Environmental reconstruction in which relatively shallow water near the lake shoreline precludes reworking of subaqueous fan-delta deposits by high-energy wave processes...... 49

34. Steps involved in application of provenance m odeling...... 56

35. Northern Grant Range stratigraphic source s ec tio n ...... 5 8

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...... 65 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 fa u lt...... 68

38. Schematic representation of modem tectpno-geomorphic setting of Railroad Valley and deposits of the Horse Camp Formation ...... 76

39. Generalized three-phase model depicting the tectonic development of the Horse Range, Horse Camp Basin, and Railroad V a lle y ...... 77 x i

LIST OF PLATES

Plate Page

I . Logs of measured stratigraphic sections of the southern exposures of the Horse Camp Formation (Member 2 ) ...... back pocket x ii

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. 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 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. 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, 1992). 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 tuff breccia (Moores, 1968). This sequence is amenable to a study that 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, 3000 m-thick sequence of late Miocene to Holocene sedimentary

rocks (Bortz and Murray, 1979; Effimoff and Pinezich, 1981). 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, 1993). In the Horse Range, east-

vergent 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, 1968) probably record a regional Jurassic-Cretaceous,

east-west shortening episode (Taylor and others, 1993). 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 /

IIS 0SO1W IIS 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 (after Moores and others, 1968). BSFZ1 Blind Spring fault zone; CSFZ1 Currant 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, 1993). 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, 1968). 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,

1993). 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, 1993). Paleocurrent patterns and strata! thickness trends within the lower part of the Horse Camp Formation (Member I ) led Brown (1 9 93 ) to suggest that 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, 1968) 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., 1994). 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, 1968). 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 range-

front 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, 1992). 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, 1993). 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 3000 m-thick sequence of conglomerate and

sandstone with minor amounts of mudstone, carbonate, megabreccia, and volcanic tuff

breccia (Fig. 3; Moores, 1968). 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, 1989). 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, 1985) reveal a Miocene age, roughly 25-20 Ma

to 10 -5 Ma, for the Horse Camp Formation.

|q ojconglomerale 20 km . 3 km | I sandstone Member 4

^ _ -_ -_ jmudrock , 0 o - 2 km

I I I Icarbonatfi o 0 _ I km j I T l megabreccia

Member0 I 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 (1 968) divided the Horse Camp Formation into four unconformity-

bounded members (Fig. 3). Member I contains a 300-600 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

1500-2000 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 2500 m of the Horse Camp Formation. Poorly exposed

conglomerate,, sandstone, and carbonate of Members 3 and 4 account for less than 500 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, 1993) 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, 1987). 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, 1984), documented examples of lacustrine fan-delta

deposits (e.g., Gloppen and Steel, 1981; Nemec and others, 1984; Billi and others,

1991) 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, 1994) and may be classified as deposits of braid deltas (McPherson and

others, 1987). 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, 1991).

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 (1977) and Waresback and Turbeville (1990). 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 Description Interpretation

Gmu Poorly sorted granule-boulder conglomerate; Plastic debris flows matrix support, no grading or poor inverse grading 12

Table I. (continued)

Facies Description Interpretation

Gmn Poorly sorted granule-boulder conglomerate; Pseudoplastic debris flows matrix support, normal grading

Gcu Poorly sorted granule-boulder conglomerate; Plastic or pseudoplastic clast support, no grading or poor inverse grading clast-rich debris flows; • hyperconcentrated flows

Gcn Poorly sorted granule-boulder conglomerate; Hyperconcentrated flows; clast support, normal grading high-density turbidity currents

Gh Moderately sorted granule-pebble conglomerate; Sheetfloods; wave-driven clast support, no grading, horizontal stratification traction

Sm Medium- to very coarse-grained sandstone; Hyperconcentrated flows; no grading or poor inverse grading, no high-density turbidity stratification, outsized granule-pebble clasts currents

Sn Fine- to very coarse-grained sandstone; normal Hyperconcentrated flows; grading, no stratification, basal granule- high- or low-density pebble clasts turbidity currents

Sh Fine- to very coarse-grained sandstone; Sheetfloods; wave-driven no grading, horizontal stratification traction; high- or low- density turbidity currents

Sr Very fine- to coarse-grained sandstone; Migrating wave and no grading, ripple cross lamination current ripples

St Very fine- to coarse-grained sandstone; Migrating dunes or bars ho grading, trough cross stratification

Fm Massive or poorly laminated mudstone; Waning flood flows; no grading, mudcracks suspension fallout

Clm Massive or laminated carbonate; stromatolitic Carbonate precipitation hemispheroid laminations, secondary ooids and binding of calcareous and pisoids, coarse sand-pebble lenses sediment by algal mats

Vtb Rhyolitic-dacitic tu ff breccia; angular clasts, Pyroclastic flows; no bedding or flow structures, rare xenoliths pyroclastic debris flows

Vmb Megabreccia of Oligocene volcanic rock; jigsaw Rock avalanches and crackle brecciation, mixed with substrate sand and mud, folded substrate 13

Facies Gmu

Ungraded, Matrix-Support ed Conglomerate . This facies consists of ungraded to

weakly inversely graded, poddy to very poorly sorted, matrix-supported, granule-

boulder conglomerate (Figs. 4 and 5). Typical clast sizes are 1 -3 0 cm in diameter, but

outsized clasts (>50 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, 1992). The lack of basal scour and clast imbrication indicates laminar (non-

turbulent) flow during sediment transport (Enos, 1977). The sediment support

mechanisms of these viscous debris flows were matrix strength and dispersive pressure

(Lowe, 1979; 1982). Debris was deposited en masse (cohesive freezing) when the

applied shear stress decreased below the yield strength of the material (Lowe, 1979).

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, 1984). 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 (>50 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, 1990). Non-erosional bases indicate

laminar flow (Enos, 1977), but abundant normal grading and common horizontal

alignment of clasts suggest occasional turbulent flow during transport (Nemec and Steel,

1984; Rust and Koster, 1984). These dilute (less viscous) debris flows exhibit lower

matrix strength and dispersive pressure during transport (Shultz, 1984). 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, 1990). 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 20-50% 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 2-20 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,

1990) and subaerial hyperconcentrated flows (Smith, 1986; DeCeIIeS and others,

1991a). 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; 1982) and, due to high clast concentration, buoyancy (Hampton, 1979). 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, 1986). 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 20-50% 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, 1990) and subaqueous high-density turbidity currents (Lowe, 1982; Chough and others, 1990).

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 traction- produced 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, 1986). 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 of 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 11. Facies Gh and interbedded facies Sh. Note uniform grain size. Knife is 8 cm long. 21

(Lowe, 1979). Lowe (1982) states that 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, 1982). Such deposits are uncommon because the sand within a turbidity current usually bypasses the site of gravel deposition (Lowe, 1982).

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 10-40% 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, 1990). Sheetfloods contained outsized clasts (>20 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, 1984) and marine systems (Wescott and Ethridge, 1983; Bourgeois and

Leithold, 1984; Nemec and Steel, 1984) 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, 1986) and subaqueous high-density turbidity currents

(Lowe, 1982; Chough and others, 1990; Higgs, 1990). In both cases, sand was rapidly deposited directly from turbulent suspension with insufficient time for bedform development (Lowe, 1982; Smith, 1986). 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, 1962). Inversely graded beds are the result of freezing of a traction carpet particle layer maintained by dispersive pressure (Lowe, 1982). 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, 1986) and subaqueous high- or low-density turbidity currents (Lowe, 1982; Chough and others, 1990; Ghibaudo, 1992). 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 86 ).

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, 1992). An absence of overlying horizontally and ripple cross-laminated sandstone (divisions B and C of Bouma, 1962) 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

(<15°) or horizontally stratified/laminated, poorly to well-sorted, fine- to very coarse-grained sandstone (Figs. 11-13, 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. Low- angle 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 20-200 cm-thick, well-sorted sandstone interbedded with mudcracked mudstone (Fm); (2) a 0.5-50 cm-thick, moderately sorted sandstone with outsized granule-pebble clasts (Fig. 11) that is always interbedded with subhorizontally stratified conglomerate (Gh); (3) a 0.5-15 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, 1990) and granule- pebble gravel (Gh; Waresback and Turbeville, 1990). 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, 1990) and deposition by freezing of traction carpets at the base of high- density turbidity currents (Lowe, 1982; Pivnik, 1990; Ghibaudo, 1992). Although complete turbidite sequences are not present, these Sh occurrences are considered analogous to division B of Bouma (1962).

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 0.5-30 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 15-30° 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, 1980). 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 water (Tucker, 1978; Martel and Gibling, 1991). Climbing

ripple geometries suggest a high sediment supply (Reineck and Singh, 1980).

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 15 -

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, 1977). 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 two 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 Sh1 Gh, 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 77 ) and subaqueous suspension (with minor traction) sedimentation

(Ghibaudo, 1992). 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 (0.1-10 m) and have greater lateral continuity (10-30 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. 20). 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. 23). 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 of COg and/or nearshore wave agitation and warming (Eugster and Kelts, 1983; Platt and Wright, 1991). 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, 1983).

However, the few irregular ooid-pisoid zones of facies Cfm (Fig, 23) are not associated with scours or traction-produced sedimentary structures and, therefore, are interpreted as the product of post-depositional carbonate precipitation (Read, 1976).

Poor sorting in these zones (Fig. 23) may also suggest a secondary, vadose zone origin for the ooids and pisoids (Dunham, 1969). Thus, carbonate facies Clm contains a record of initial carbonate precipitation in a nearshore environment with limited, post- depositional 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 (0.1-40 cm diameter) within a less resistant matrix (<1 mm diameter grains). Clasts and matrix have similar phenocryst compositions and abundances

(typically 5 0-70% quartz, 10-30% plagioclase, 10-20% 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 Vtb typically contains 60% matrix 35 and 40% clasts. Friable tuff 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 Vtb (present only within 0-100 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 tuff 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, 1994). 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, 1991) and promoted an en masse mode of deposition (Smith, 1986). 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, 1991). Intercalation with sandstone (Sm, Sn) and mudstone (Fm) suggests that facies Vtb was typically deposited and preserved under low- energy 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 91 ) 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. 25). 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 (matrix- rich jigsaw breccia; Yarnold and Lombard, 1989; Brown, 1993). Comminuted slip surfaces ( I -5 mm wide fractures along which there has been limited movement; Yarnold and Lombard, 1989; Brown, 1993) commonly separate these two breccia types and are necessary for internal rotation and translation within the breccia sheet (Fig. 27).

Asymmetric to overturned folds (1-10 m wavelength, <5 m amplitude) are common in the basal zone of deformed substrate (Fig. 24). Facies Vmb has an irregular distribution that ranges from I - IO 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 two stratigraphic levels in the southern Member 2 deposits (within lower 100 m of sections 8 and 9 and approximately 1200 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, 1993). Therefore,

this facies is not directly related to volcanic eruptions. Rock avalanches moved

downslope as rapid inertial granular flows (Yarnold, 1993). Active particle layers that

facilitated sliding along the base of the rock avalanche (Campbell, 1989; Brown, 1993)

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. 24). 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. 27) and may represent accommodation surfaces permitting differential

deformation within the breccia sheet (Brown, 1993). 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

0 o \ G- CSiCfii \

O o o'

m MSPCB

Figure 28. Detailed Figure 29. Detailed Figure 30. Detailed stratigraphic section of stratigraphic section of stratigraphic section of subaerial fan-delta subaqueous fan-delta nearshore lacustrine facies association from facies association from facies association from 4 6 0 -4 7 0 m level of 2 62 -2 7 2 m level of 0 -7 m level of section section 6 (Plate I). section 6 (Plate I ). 4 (Plate I) . Key as in Key as in Plate I . Key as in Plate I . 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 -20 0 cm-thick beds of ungraded (Gmu, Gcu) and normally graded (Gmn, Gen) conglomerate and 1-15 cm-thick beds of ungraded

(Sm) and normally graded (Sn) sandstone (Fig. 28). 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. 28) 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 31. Repetitive sequence of laterally continuous beds of subaerial fan-delta facies association. Upper photo contains hammer (28 cm long). Total stratigraphic thickness in lower photo is approximately 50 m. 43 alluvial fans (e.g., Evans, 1991; Ridgway and DeCelles, 1993). 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, 1984). The occurrence of only a few stream- channel deposits, significant lateral extent and tabular nature of beds (Fig. 31), and persistent repetitive sequence of beds (Fig. 31) representing individual sedimentation events suggests that the subaerial fan-delta was dominated by episodic, relatively low- concentration 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, 1992).

Therefore, the subaerial fan-delta facies association is similar to deposits of mass floW- dominated 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, 1992). An abundance of large (several meters in diameter) angular clasts suggests that 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. 31), and have non- erosional, 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 (<10

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, 1992). 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 (St) 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 (St). 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 low- energy, 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 1000 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 tuff breccias (Vtb) 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 (Vtb) 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 4 7

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, 170-350 m interval of section 8, and 400-500 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, wave-

driven 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 4 8

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.

30) 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. 32) 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

distorted bedding DEEP WATER PHASE

subaerial fan nearshore lacustrine offshore

wave-reworked gravel beach lacustrine

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

turbidity flow

///

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. 33). 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 wave-

driven 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, 1991).

The two 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 800 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 800 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. 33) 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. 32) 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; 1984). 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, 1990). Intuitively, shoreline facies should serve as ideal stratigraphic indicators separating subaerial and subaqueous deposits of a single fan-

delta (McPherson and others, 1987). In this study, however, the nearshore lacustrine facies association (Fig. 30) serves exclusively as a transition between facies

associations representative of the subaerial fan and the offshore lacustrine environment

(Fig. 32) 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, 1985). However, the Horse Camp Formation (Member 2) 52 contains a facies association representative of a nearshore lacustrine environment (Fig.

32) in which high-energy wave-induced currents were responsible for fractional reworking of 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 (St) of the nearshore lacustrine facies association (Fig. 30) are more similar to nearshore marine deposits (e.g., DeCeIIes,

1987; Pivnik, 1990; Casshyap and Aslam, 1992) than other lacustrine deposits (e.g.,

Hardie and others, 1978; Link and Osborne, 1978). 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, 1990). Frequent lake-level fluctuations and associated reworking of previous deposits may account for the lack of progradational lacustrine shoreline sequences (Renaut and Owen, 1991). The high- energy conditions of the nearshore lacustrine setting (Fig. 32) 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 200 km2 lake within a pull-apart basin in western Nevada (Link and others,

1985). 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 of the lake (Link and others, 1985), similar to the reconstruction in Figure 32

for Member 2 of the Horse Camp Formation.

Pyroclastic Flows and Meaabreccias

Unique volcanic tuff 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

33). 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, 1993). 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 fan- delta surface or lake floor. Similarly, pyroclastic flows/debris flows recorded by the

matrix-rich varieties of the volcanic tuff 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 tuff 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 (0-130 m level) linking sections I, 2, and 5-9 (Plate I) shows a.coexistence of megabreccia and volcanic tuff 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., gravel- yielding) 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. 34 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, 1986). 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, 1988). 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 2) Tabulate lithology source percentages for each 3) Choose model's section source section interval exposure gates

5 0 0 0 |------5 0 0 0 -4 8 0 0 4 8 0 0 -4 6 0 0

2 0 0 -0 CDE 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; 1993). 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 attempt to characterize the late Cenozoic uplift history.of the adjacent northern Grant Range. Exposure of as much as 3000 m of conglomeratic basin-margin deposits in the Horse Camp Formation provides an excellent opportunity to apply provenance modeling in an attempt 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 00 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, 1968). The Cambrian through Middle

Ordovician sequence includes up to 2800 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 1700 m. Less than 200 m of Eocene shale and limestone and up to 2100 m of

Oligocene rhyolitic ash-flow tuff overlap Paleozoic rocks with slight angularity (Scott, 58

LEGEND 7800 Shingle Pass Formation

Needles Range Formation [>>>>•[ Volcanic Rock 7500

Windows Butte Formation Friable Limestone 7000 -

Shale OUGOCENE Stone Cabin Formation

Limestone 6500 - Calloway Well Formation

Quartz Sandstone 6000 - Railroad Valley Rhyolite

Dolomite Sheep Pass Formation EO&NE

5500 . \ Quartzite Ely Limestone PENNSYLVANIAN i Unconformity 5000 - T Chainman Formation MISSISSIPP1AN

4500 Joana Formation

Guilmette Formation

4000 DEVONIAN

Simonson Dolomite

3500 - Sevy Dolomite

Laketown Dolomite SILURIAN 3000 Fish Haven Dolomite i ORDOVICIAN Eureka Quartzite 2600 I + CAMBRIAN Undifferentiated Units p, \ \ \ \ \ ^ ' ' ' ' ' \ vI meters I

Figure 35. Northern Grant Range stratigraphic source section (based on data from Moores and others, 1968). 59

1965; Moores and others, 1968). 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, 1992) 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, 1992) 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 of 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 north- northwest 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 tuff breccias) that 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 north- northwest (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 1600 m-thick measured stratigraphic section (Table 2; section 6 of Plate I). An approximately 75 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 100 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, 1993). 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,

1992), although a greater population (up to 4 00 clasts) may be more desirable

(Howard, 1993). 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 VC LS DL SS QZ Level (m)

1534 100 1477 100 1444 100 1358 94 6 1322 100 1197 62 16 22 I 145 74 14 I I I 1034 80 2 18 889 78 I I I I 791 72 14 12 2 703 64 20 16 639 18 47 35 534 62 24 12 I I 465 67 22 8 3 395 82 14 2 2 313 93 6 I 239 88 10 2 125 96 3 I 69 84 16 14 72 28

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 VC LS DL SS QZ SH FL Level (m)

7 8 0 0 -5 8 0 0 * 100 5800-5600 20 28.5 1.5 25 25 5 600-5400 95 5 5 400-5200 95 5 5 200-5000 14.25 0.75 85 5 000-4800 100 4 8 00 -4 6 00 100 4 6 00 -4 4 00 59 20 21 4 4 00 -4 2 00 100 4 2 00 -4 0 00 88.5 11.5 4 0 00 -3 8 00 30.5 69.5 3800 -3 6 00 100 36 00 -3 4 00 93.5 6.5 3 400-3200 100 3200-3000 100 3 000-2800 100 28 00 -2 6 00 30 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, 1992). 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, 1990). 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 200 m

interval may be multiplied by a different durability coefficient that 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. 34). 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 40 0 m level of Fig. 35) 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 40 0 m to 7800 m in the northern Grant Range source section (Fig. 35). Each successive exposure gate incorporates an additional 800 m of underlying strata. A simulated clast composition suite for each exposure gate (step

4; Fig. 34) is depicted in Figure 36. Note that complete erosion of the uppermost part of the source section (step 3D, E; Fig. 34) 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. 35) 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 500 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 1200 m of Member 2 (Fig.

36). Therefore, the model appears geologically valid and the lower 1200 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 that 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 4 4 4 m I 5 3 4 m V aV

X-X

1145m 1 1 9 7 m

M-

7800-3600 m 4 6 5 m 5 3 4 m XVX B v v x

v X X' % 0 lTlTlTl 1 0 0

z v-.l 7800-4400 m 6 9 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 1600 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 (1968), 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, 1968).

Implications

A provenance model based on unroofing of the northern Grant Range is similar to the lower 1200 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, 1993) 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, 1993).

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. 37) 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 that 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, 1992) 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, 1993). 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 of 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, 1992). 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 after Horse Camp deposition (Fig. 2). This relationship suggests that 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 2440 m, approximately 900 m above the elevation of modern alluvial fans in Railroad Valley (Scott, 1965; Kleinhampl and Ziony, 1985). These high-level deposits are considered to be uplifted remnants of alluvial fans formed adjacent to the valley floor. Therefore, approximately 900 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 the northern Grant Range, that is not the case less than 50 km to the south in the central Grant Range. An unnamed Miocene conglomerate (40ArZ39Ar age of 14.2 Ma for interbedded ash-fall tuff) of the central Grant Range contains abundant Ordovician clasts, including clasts of Eureka Quartzite, and several slide blocks of Ordovician and Devonian lithologies (Fryxell, 1988). 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 5000 rri- thick section overlies Ordovician rocks in the northern Grant Range source section (Fig.

35) while only 200 m of post-Ordovician rocks are found in the central Grant Range source section (Fryxell, 1988). 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, 1991). 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, 1986) has been utilized exclusively in studies of proximal foreland basin conglomerates in order to ( I ) link a stratigraphic sequence to a particular source terrane (e.g., DeCeIIes, 1988) 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, 1968). 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 that 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 of 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 of uplift or subsidence

(Coward, 1994). Inversion of a sedimentary basin (e.g., Mather, 1993) 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, 1992) 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, 1993). 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, 1975), 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 (< 20 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 3000 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, 1993). 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 that 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 93). 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 88). 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 tuff within the Little Meadows conglomerate (40ArZ^^Ar age of 74

14.2 Ma) suggests Miocene sedimentation, coeval with Horse Camp deposition (Fryxell,

1988). 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 900 m above the elevation of

modern alluvial fans in Railroad Valley (Scott, 1965; Kleinhampl and Ziony, 1985). This alluvium is interpreted as deposits of alluvial fans (Fryxell, 1988) originally formed on the western margin of Railroad Valley. Therefore, these deposits have been uplifted

(inverted) at least 900 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 basin- margin alluvial fan environments and basin-center lacustrine and fluvial environments

(Kleinhampl and Ziony, 1985). The stratigraphic sequence of Railroad Valley has an average thickness of approximately 3000 m and thickens eastward toward the west­ dipping Railroad Valley fault ( Bortz and Murray, 1979; Effimoff and Pinezich, 1981).

Basalt flows in the upper half of the basin-fill sequence (Johnson, 1993) may be correlative to Pliocene and younger basalt flows along the western margin of Railroad

Valley (Kleinhampl. and Ziony, 1985). Despite previous stratigraphic assignments

(e.g., Effimoff and Pinezich, 1981), 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 MoHpI

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, 1993). 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. 39). 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. 39). 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. 39), 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).

Railroad Horse Camp V alley exposures

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

Latest Miocene-Holocene L—— I basin fill (RVB) NWSF / o6c-i Miocene I basin fill (HCB) v 7 » Oligocene E 3 volcanic rocks I : I Ordovician-Pennsylvanian ' I sedimentary rocks I Precambrian(T)-Cambrian sedimentary rocks NWSF NRRF ^ r 7

* ► v > v T • O O • -< < > «■ V T A « ■>> 4. A ►

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 3804 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 post- volcanic 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,

1988) 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, 1994) and related to basinward propagation of the locus of tectonism (Burbank and Rayholds, 1988;

Steidtmann and Schmitt, 1988) 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. 38). 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 3000 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 (<20 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, 1987) 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, 1988). 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 that 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, 1983) recorded different subsidence histories than adjacent modern basins (Van Houten, 1956, p. 2823). 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 south- southeast, 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 that is remarkably similar to actual compositional data from the lower 1200 m of Member 2. Therefore, conglomerates within the lower 1200 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 post-

Miocene (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, 1988) 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, 1988) 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 FACIES Formation (Member 2). Inset (see Fig. 2) shows location of dip-corrected sections. All

? <3 Gmu correlations are based upon laterally continuous Horse Camp Formation 1600 Iithostratigraphic units except the lowest Member 2 Gmn X correlation between sections 5-8 and the correlation linking sections 3-5 which are air Gcu photo-based strike correlations projected across 6 (6) 1500 I i covered intervals. More detailed versions of the Gcn I i I nine stratigraphic sections are presented in /R e d H iII A e c c c ? G h archival data that are available from the Montana Slide Block State University Department of Earth Sciences. Horse Camp Formation < Member I

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