The Mojave Extensional Belt, Southern California

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1992 Stratigraphic Architecture of an Extensional Orogen: The ojM ave Extensional Belt, Southern California. Christopher John Travis Louisiana State University and Agricultural & Mechanical College

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Stratigraphie architecture of an extensional orogen; The Mojave Extensional Belt, southern California

Travis, Christopher John, Ph.D.

The Louisiana State University and Agricultural and Mechanical Col., 1992

UMI 300 N. ZeebRd. Ann Arbor, MI 48106

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. S tratigraphic architecture o f a n E x t e n s io n a l O r o g e n : THE MOJAVE EXTENSIONAL BELT, SOUTHERN CALIFORNIA

A Dissertation

Submitted to the Graduate Faculty of the Louisiana State University and Agricultural and Mechanical College in partial fulfillment of the requirements for the degree of Doctor of Philosophy

The Department of Geology and Geophysics

by Christopher John Travis B.Sc., University of Wales, Swansea, 1982 M.S., University of South Carolina, 1984 May, 1992

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. After five and a half years in Baton Rouge and twelve months now in Houston, the

task of calling to mind all those who have had an impact on my life, my thinking and

this dissertation, is a monumental one. There are, however, several people who stand

out in my mind as having provided incalculable guidance, support and friendship. First

among these must be my wife, Denise, whose patience, restraint, and love are are without par. Similarly, my family have always supported and encouraged me in every

endeavour, and without them I could never have got anywhere. Among the non

relatives, first credit must go to Roy Dokka. He has been an excellent advisor: a strong

motivator, a gentle critic, a keen guide and, above all, a most true friend. Tim Ross, in

his role as room mate, office mate and field partner, has been a never-ending source of

stimulation and support. I value his friendship tremendously

Many friends and colleagues in Baton Rouge and, more recently, in Houston, have

provided inspiration and support. I am particularly grateful to Greg Riley, Jeff Nunn,

Kathy McManus, Darrell Henry, Steven Jones, Chad McCabe, David Wilensky, Tim

Fleming, Ellen Prager, Riley Milner, Bob Remy, Gill Apps, Ivo Bergsohn, Cindy

Yeilding, Chris Wager, Phil Behrman, Stephanie Moore, and Franklyn J. Peel.

My heartfelt thanks go also to those who have helped me to get through the

university System - Libby Holt, Sissie Gum, Lewis Nichols, Thea Burchiel, Karen,

Nancy Drew, and Wanda White. I am most grateful to the LSU Department of Geology

and Geophysics, Dr. Judith Schiebout of the LSU Museum of Geoscience, and BP

Exploration Inc. for providing financial support for my studies at LSU. Field work was

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. supported by Chevron, USA and the National Science Foundation (grants EAR-

8407136, and EAR-8721022 to Dr. Roy K. Dokka).

Finally, I would like to thank the Chair and the members of my committee, Drs. Roy

Dokka, Dag Nummedal, Jeff Nunn, Darrell Henry, Arnold Bouma, and Philip Larimore

for their guidance, their careful review, and their constructive criticism of this

dissertation. Although not a committee member. Dr. M. O. Woodbume of U.C.

Riverside had considerable influence on my work and his exhaustive reviews of

Chapters 2 and 3 were invaluable.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. T a b l e o f C o n t e n t s

Acknowledgements ...... ii

Table of Contents ...... iv List of Figures ...... ix

List of Tables ...... xiv

Abstract...... xv

• Prologue

1 Introduction: Tectonics and sedimentation 4

Introduction ...... 4

Studies of the source area ...... 5 Structural control on facies distribution ...... 6

Timing of tectonism ...... 9

Paleogeography ...... 10

Tectonic history ...... 19

Quantitative Models ...... 21 Geometric models...... 21

Dynamic models ...... 26 Creep processes ...... 27

Overland/channel flow ...... 29

Observational approach ...... 29

Conclusions ...... 30

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sedimentation in detachment-dominated extensional orogens: insights from the Mojave Extensional Belt, California 33

Introduction ...... 33

Terminology ...... 34

Geologic Background ...... 35 Stratigraphic architecture of the Mojave Extensional Belt ...... 40

Breakaway basins ...... 40

Upper plate fault basins ...... 61

Transfer zone basins ...... 74

Breakoff basins ...... 83

Core complex basins ...... 84 Discussion ...... 94

Paleogeographic development of the Mojave Extensional Belt 94 Textural evolution of the basin fill ...... 97

Comparison of Mojave Extensional Belt stratigraphy with other

extensional terranes ...... 99

Conclusions ...... 101

Sedimentation adjacent to an evolving metamorphic core complex, central Mojave Desert, California. ______

Introduction ...... 105

Geologic Background ...... 107

Need for Stratigraphic Revision ...... 112

Description of the Mud Hills Formation ...... 113

Lower Breccia Member ...... 114

Solomon Sandstone Member ...... 121

Middle Breccia Member...... 122

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Upper Sandstone Member ...... 123

Upper Breccia Member ...... 126

Tuff Breccia and sandstone tongues of the upper Pickhandle Formation.... 127

Age and Girrelation of the Mud Hills Formation ...... 129

Deposition of the Mud Hills Formation ...... 131

Basal part of Lower Breccia member ...... 131 Solomon Sandstone Member ...... 136

Upper Sandstone Member ...... 138

Coarser-grained deposits of the Breccia members ...... 146

Provenance ...... 153

Structure of the Mud Hills ...... 158

Discussion ...... 160

Paleogeography ...... 160 Tectonic implications ...... 168

Conclusions ...... 175

Stratigraphic architecture of extensional basins: insights from a 179 numerical model of sedimentation in evolving half graben ______

Introduction ...... 179

Description of the Model ...... 181

Faulting ...... 181

Isostatic compensation ...... 181

Erosion and sedimentation ...... 182

Grain Size Variation ...... 183

Results...... 184

Fault geometry and rate of slip ...... 188

Effective elastic thickness of the lithosphere ...... 197

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Coefficients of erosion and sediment transport ...... 200 Grain size distribution ...... 205 Discussion ...... 205

Comparison of model with observed stratigraphic relationships 205

Limitations of the model ...... 211 Conclusions ...... 213

Conclusions and recommendations 216

What is the regional stratigraphy in a continental extensional setting? ...... 216

What are the basin fill patterns associated with individual tectonic

elements of a continental extensional terrane? ...... 220

How can stratigraphic relationships be used to constrain tectonic models of rifting? ...... 222

e References cited 226

1 • Appendices 246

A. Measured sections of the Mojave Extensional Belt ...... 246

I. Measured section of the Mud Hills Formation in

Owl Canyon ...... 246

II. Measured section of the Mud Hills Formation in

Rainbow Canyon ...... 264

III. Measured section of the Mud Hills Formation in

Solomon Canyon ...... 270

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. IV. Measured section of the uppermost Pickhandle

Formation east of Copper City Road ...... 287 V. Measured section of the formations of Daggett

Ridge and Slash X Ranch at central Daggett Ridge ...... 294

VI. Measured section of the formations of Daggett

Ridge and Slash X Ranch near Azucar Mine ...... 302

B. Gravity survey along Fort Irwin Road, north of Barstow, California.... 320

C. Derivation of finite difference approximations for model of half graben sedimentation ...... 351

Sediment diffusion equation ...... 351

Flexure equation ...... 352

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. List of Figures

CHAPTER 1

1.1. Two dimensional modeling technique used by Pivnik (1990) to generate

hypothetical synorogenic sediment compositions ...... 7

1.2 . Compositional variation of sandstone suites derived from different generic

types of provenance terrane ...... 8

1.3. Thickness (a) and lithofacies distribution (b) of Lower Mississppian

(Kinderhookian and Osagean) and lowermost Upper Mississippian (lower

Meramecian) rocks in the western United States ...... 12

1.4. Patterns of sedimentation in alluvial basins ...... 14

1.5. Common rift margin structures and their geomorphic expression ...... 17

1.6. Facies models for continental and marginal marine half graben ...... 18

1.7. Effects of strike-slip faulting on sedimentation in the Ridge Basin,

California ...... 20

1.8. Computer simulation of axial-fluvial channel stacking patterns in a half

graben ...... 23

1.9. Geometric models of sedimentation in half graben ...... 24

1.10. Sequence stratigraphic model of Riley and others (in review, 1991) ...... 25

1.11. Modification of an initial step-like topography by erosion and deposition

according to the diffusion equation ...... 28

CHAPTER 2

2.1. Schematic representation of tectonic elements and associated basins of a

detachment-dominated extensional belt ...... 37

2.2. Location map of study area, highlighting extensional domains of the Mojave

Extensional Belt ...... 38

2.3. Geologic map and cross section of the Newberry Mountains ...... 42

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2.4. Miocene regional stratigraphy of the Daggett terrane ...... 44

2.5. Block diagrams illustrating the structural and paleogeographic evolution of

the breakaway zone of the Daggett terrane ...... 45

2.6. Palinspastic reconstruction of the central Mojave prior to the initiation of the

Eastern California Shear Zone ~10 Ma ...... 47

2.7. Measured section of the formation of Slash X Ranch at central Daggett

Ridge ...... 48

2.8. Photographs of distinctive rocks and textures at Daggett Ridge ...... 49

2.9. Stratigraphic columns showing Slash X Ranch formation at Azucar Mine

and at Daggett Ridge ...... 53

2.10. Detailed measured section of lower part of granitoid debris section at

Azucar Mine...... 54

2.11. Photograph of different facies in granitoid breccia section at Azucar Mine.... 55

2.12. Photographs of the Barstow Formation in the area of Daggett Ridge ...... 60

2.13. Seismic reflection profile from western Mojave Desert, showing an

extensional half graben and its syn- and posttectonic fill ...... 62

2.14. Miocene regional stratigraphy of the Waterman terrane ...... 65

2.15. Miocene regional stratigraphy of the Edwards terrane ...... 66

2.16. Photo of basement contact and overlying early Miocene volcanic strata in

the western Cady Mountains ...... 67

2.17. Composite geologic map of the Waterman Terrane ...... 69

2.18. Geologic map and idealized cross section of the Kane Springs area ...... 76

2.19. Photograph of well bedded breccias of basalt and basement clasts of the

upper section of the formation of Stoddard Valley unconformably

overlying basalts of the formation of Minneola Ridge in the vicinity of

Kane Springs ...... 77

2.20. Miocene stratigraphy along the Silver Bell Mine road, Rodman Mountains ..78

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2.21. Geologic map of the Mud Hills highlighting the Mud Hills Formation ...... 80

2.22. Models for the development of metamorphic core complexes ...... 85

2.23. Correlation chart for Miocene rocks in the Mud Hills-Calico Mountains

area...... 89

2.24. Paleocurrent directions measured in the Mud Hills Formation and

lowermost Barstow Formation ...... 91

2.25. Schematic models for the early Miocene tectonic development of the Mud

H ills...... 92

2.26. Conceptual model for the tectonostratigraphic development of the Mojave

Extensional Belt and other detachment-dominated extensional orogens ...... 95

CHAPTER 3

3.1. Location map of study area, highlighting extensional domains of the Mojave

Extensional Belt ...... 108

3.2. Composite geologic map of the Waterman Terrane highlighting upper plate

crystalline basement, lower plate rocks of the Waterman Metamorphic

Complex, and syntectonic rocks of the Mud Hills Formation ...... 109

3.3. Geologic map and cross section of the Mud Hills ...... 115

3.4. Correlation chart for Miocene rocks in the Mud Hills-Calico Mountains

area...... 119

3.5. E-W stratigraphic cross section of the Mud Hills Formation...... 120

3.6. Photograph of a folded lens of sandstone in the Middle Breccia Member of

the Mud Hills Formation in the western Mud Hills ...... 124

3.7. Photograph of unconformable contact relations of the Upper Sandstone

Member of the Mud Hills Formation in Owl Canyon ...... 125

3.8. Photograph of sheared gas segregation pipes of Pickhandle Formation tuff

breccia tongue in Ross Canyon ...... 128

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3.9. Composite sedimentary log of the basal sandstone and breccia interval of

the Lower Breccia Member of the Mud Hills Formation ...... 132

3.10. Photographs of the basal sandstone and breccia interval of the Lower

Breccia Member of the Mud Hills Formation ...... 133

3.11. Measured section of the Solomon Sandstone Member of the Mud Hills

Formation ...... 137

3.12. Measured section of the Upper Sandstone Member of the Mud Hills

Formation ...... 139

3.13. Photographs of general appearance and sedimentary structures of the

Upper Sandstone Member of the Mud Hills Formation in Owl Canyon ...... 140

3.14. Photographs of sedimentary textures and structures in the coarse-grained

breccia units of the Mud Hills Formation in Owl Canyon ...... 148

3.15. Paleocurrent directions measured in the Mud Hills Formation and

lowermost Barstow Formation ...... 155

3.16. Photograph of northwest-striking normal faults exposed in Owl Canyon ...... 161

3.17. Photograph of contact relations between Upper Breccia Member and upper

Pickhandle Formation tuff and sandstone tongue ...... 162

3.18. Cooling path for lower plate rocks of the Waterman Hills and Mitchel

Range ...... 169

3.19. Tectonic models for the development of the Waterman terrane ...... 171

3.20. Rolling hinge model for the tectonic development of the Waterman terrane... 172

3.21. Schematic models for the early Miocene tectonic development of the Mud

Hills...... 176

CHAPTER 4

4.1. Stages in the tectonostratigraphic evolution of two half graben ...... 185

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4.2. Geometric and textural trends associated with syn- and posttectonic

sedimentation in a single half graben ...... 187

4.3. Effect of sediment package geometry on facies architecture ...... 189

4.4. Differing basin geometries produced by varying the initial shape of the

major bounding faults ...... 190

4.5. Symmetrical basin fill patterns resulting from a single linear fault dipping at

60° and passing downwards into a flat detachment at 10 km ...... 191

4.6. Variations in basin width produced by varying depth to detachment ...... 193

4.7. Stratigraphic effects of varying the initial surface dip of a single, listric,

basin-bounding fault ...... 194

4.8. Effect of differential sediment flux on facies architecture ...... 196

4.9. Stratigraphic effects of varying extension rates ...... 198

4.10. Stratigraphic patterns associated with different values for the effective

elastic thickness of the lithosphere (Te) ...... 199

4.11. Effects of changing the ratio between rates of erosion and sediment

transport: I. ke=k...... 202

4.12. Effects of changing the ratio between rates of erosion and sediment

transport: H. ké= \W k...... 203

4.13. Effects of changing the ratio between rates of erosion and sediment

transport: III. ke= 0.0\kt...... 204

4.14. A. Seismic reflection profile from western Mojave Desert, showing an

early Miocene extensional half graben and its syn- and posttectonic fill. B.

Model output for two half graben ...... 206

4.15. Annotated line drawing of a regional deep seismic line from the North S ea.. 207

4.16. Geologic cross section of the northern Mesilla basin ...... 209

4.17. Sequence stratigraphic model for continental extensional half graben ...... 210

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. L IST OF T a b l e s

CHAPTER 1

1.1 Tectonic classification of alluvial basins ...... 15

XIV

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Field-based studies and quantitative models provide new insights into the

stratigraphic architecture of a developing continental extensional orogen. The early

Miocene age Mojave Extensional Belt of southern California (MEB) exhibits a tripartite

stratigraphy, consisting of (1) pre- to early synextension volcanic deposits,

unconformably overlain by (2) syntectonic basement-derived megabreccia and breccia

with local finer-grained units, overlain by (3) a posttectonic fining-upward sequence of gravel, sand, shale, and limestone. On a more local scale, stratigraphy and sediment

dispersal within the MEB reflect the evolution of the major tectonic elements of the belt:

(1) Breakaway Zone. Strata of the western Newberry Mountains record a complex

history of basin formation and dissection as the upper plate progressively broke up

during extension. Sediment dispersal was dominated by drainage from unextended

regions across the transfer boundary of the terrane. (2) Tranter Zone. Strike-slip

elements had a variable effect on patterns of sediment accumulation. Miocene

sedimentary rocks exposed near the Kane Springs transfer zone were mostly derived

from across the strike-slip boundary, but the Lane Mountain transfer zone appears to

have had little effect on sedimentation patterns in the adjacent Mud Hills. (3) Core

complex. Sedimentary rocks exposed in the Mud Hills record the topographic evolution

of the Mitchel Range-Hinkley Hills-Waterman Hills metamorphic core complex.

Syntectonic sedimentation in the Mud Hills was dominated by rock avalanches

composed of upper plate erosional debris. The largely posttectonic Barstow Formation,

however, is composed of alluvial fan, fluvial, and lacustrine strata and exhibits a minor

component of metamorphic clasts derived from the adjacent core complex. Integration

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of stratigraphic, structural, metamorphic, and thermochronologic data provides new

constraints on crustal-scale models for extension of this area.

Quantitative models based on the assumptions of a linear dépendance of sediment flux

on slope and perfect sorting of sediment during deposition, predict many of the

fundamental stratigraphic relationships observed in continental half graben. In addition,

these models generate new insight into the controls of fault geometry, rates of faulting,

and rates of erosion vs sediment transport on landscape evolution, sediment package

geometry, and facies distributions.

XVI

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. "... although sedimentation is affected

by many factors, the most fundamental is

tectonic."

Francis J. Petdjohn (1957)

Relief at the earth's surface is produced by tectonic processes associated with plate

boundary interactions and mantle heterogeneities. The surface manifestations of these

tectonic processes, mountain ranges and topographic basins, are continually levelled by

the natural phenomena of weathering, erosion, transport, and deposition acting at and

near the surface of the earth. Thinning of the continental cmst results in the

development of topographic and/or structural depressions due to faulting and to the

redistribution of mass within the lithosphere and asthenosphere. With time, these

depressions fill with sediments. Lithospheric thickening, on the other hand, results in

elevated regions which are gradually levelled by erosion; the sediments so generated fill

adjacent basins formed by flexural loading of the lithosphere and by backarc spreading,

forearc spreading, or subduction. The nature of the sediment fill of geologic basins

depends critically on the structural evolution of the basin, as well as on the lithology of

the source terrane, climate, and sea level. The size and geometry of basins produced as

a result of lithospheric thickness changes may vary as a function of lithosphere rheology

and the magnitude, distribution, and rate of change (in time or space) of applied load and

strain.

Of particular interest is the tectonostratigraphic development of basins associated with

the early stages of continental rifting, because these geologic features often remain

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. buried beneath a thick post-rift sedimentary cover. One region in which such rocks and

structures are exposed at the surface is in the southwestern United States of America.

The structural geology and, more recently, the metamorphic petrology and

thermochronology of detachment-dominated extensional orogens exposed in this region

have received much attention. Tectonic models developed as a result of outcrop-based

studies (e.g., Wernicke, 1981) have found widespread acceptance elsewhere. In

contrast to the abundant research on the geodynamics of extensional terranes of the

southwestern U.S., the stratigraphic evolution of these features has remained less well

studied. Correcting this imbalance may be important in light of the fact that the Basin

and Range Province is the paradigm for detachment-dominated extension.

The purpose of this dissertation is to assess the types of information that may be

obtained from the study of sedimentary rocks and which bear on the nature of

continental rifting. Three issues are addressed:

(1) What is the regional stratigraphy in a continental extensional setting, and what are

the geometric and textural trends associated with the syn- and posttectonic basin fill?

(2) What are the basin fill patterns associated with individual tectonic elements of a

continental extensional terrane?

(3) In what way may stratigraphic relationships be used to constrain tectonic models

of rifting?

These questions are approached through three overlapping studies. An introductory

chapter provides a brief review of previous work in sedimentation and tectonics.

Chapter 2 examines regional basin fill patterns within the Mojave Extensional Belt, an

early Miocene, detachment-dominated extensional orogen in southern California. This

chapter also provides an overview of stratigraphic patterns associated with the rnajor

individual tectonic elements of the belt, namely, breakaway, breakojf, and transfer

zones. Chapter 3 examines in greater detail the stratigraphic and structural events

associated with the evolution of a tectonic element unique to detachment-dominated

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. extensional orogens, the metamorphic core complex. This study demonstrates the utility

of local stratigraphic studies in constraining tectonic models of extension. Chapter 4

examines the insights to be gained from numerical modelling of sedimentation in

extensional orogens. Chapter 5 summarizes the findings of this thesis and suggests

future avenues of research. Supplementary data are contained in Appendices A-C.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INTRODUCTION

The fundamental control on sedimentation is tectonics (Pettijohn, 1957). Beneath

this all-encompassing umbrella are, however, many competing factors that directly and

indirectly influence the nature of sedimentary rocks. Source lithology, climate, and

transport affect the mechanical and chemical composition of sediments. Climate and

tectonics govern sediment supply. Eustacy and local to regional tectonics control the locus and style of deposition. All of these major factors, as well as the many more local

(but no less effective) influences on sedimentation and diagenesis, must be taken into

account in any attempt to explain the tectonic evolution of a region through study of the

sedimentary record. This complexity of controlling factors, and the potential for non

unique interpretations of sedimentary rocks, means that an integrated approach is

essential. Analysis of sedimentary rocks merely complements structural, geophysical,

thermochronologic, metamoiphic, and igneous studies; neither it nor these other sources

of information provide a complete picture alone.

The study of tectonics and sedimentation can be roughly divided into three

approaches: (1) field/laboratory studies of the source area, (2) field investigations of the

structural control on facies distributions, and (3) quantitative theoretical modeling. In all

cases there are a limited number of fundamental questions that are commonly addressed;

What is the tectonic origin of this basin? (e.g.. Is this a backarc basin or a foreland

basin?) Are the basin boundaries faulted or not faulted? Was this area uplifted, and by

how much? What was the timing of tectonic activity, both local and regional? What are

the implications of certain tectonostratigraphic patterns for the development of natural

resources?

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. STUDIES OF THE SOURCE AREA

One of the main controls on the minéralogie composition of detrital sedimentary

rocks is the lithology of the source area. Provenance studies involve the attempt to

locate source areas and to interpret tectonics based on the petrography of clastic rocks.

The technique has been used to infer plate tectonic setting and megageomorphology

(e.g., Krynine, 1942; Dickinson, 1970, 1988; Crook, 1974; Dickinson and Suczek,

1979), as well as the timing, location, and nature of specific uplift events (e.g.,

Ingersoll, 1978; DeCelles, 1988; Pivnik, 1990; Evans, 1990). The traditional basis of

the technique is comparison of the mineralogy of the sedimentary rock with the observed

or hypothetical composition of a potential source. Recent studies have attempted to

model the compositional evolution of debris progressively eroded from a polylithologic,

layered source (Fig. 1.1) (e.g., Graham and others, 1986; DeCelles, 1988; Pivnik,

1990). De Celles (1988) mixed the output from three theoretical models similar to

Figure 1.1 in an effort to reproduce the observed sediment composition of Late

Cretaceous synorogenic conglomerates in Utah. A similar, but far simpler, approach

was taken by Evans (1990) in a study of the Gulf of Suez rift basin. In this case, the

dominant sediment source was known to be the rift margins. Evans (1990) made some

(questionable) assumptions about the pre-erosion stratigraphy of the source terrane and

thereby used changes in the composition of syn-rift sedimentary rocks to infer timing

and rates of uplift of the rift margin. On a grander scale, Dickinson (1985,1988) has

continued the work of P. D. Krynine and produced a series of diagrams designed to aid

in the interpretation of plate tectonic setting from sandstone composition (Fig. 1.2).

Pétrographie studies are not the only method used to identify source terranes and

their tectonic history. Isotopic analyses of detrital minerals may yield distinctive ages

and thermal histories. For example, Baldwin and others (1986) used fission track ages

of detrital zircons to identify three distinct volcanic source teiranes for sandstones in

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Barbados. Cerveny and others (1988) cited zircon fission track ages, which were

consistently just 1 to 5 million years older than the sediments in which they occur, as

evidence that the Himalaya has undergone rapid uplift for at least the last 18 million

years. Interpretation of source terrane thermal history has been attempted by the study

of ^°Ar;^^Ar incremental release spectra (Harrison and Be, 1983), and source terrane

age has been determined using several other isotopic systems (see review in Heller and

Frost, 1988).

The limitations of pétrographie and isotopic methods are severe. Problems relate to

assumptions about original source area stratigraphy, the effects of basin thermal history

on isotopic systems, mixing of debris from multiple sources, and the strong secondary

influence of climate, vegetation, transport, depositional environment, and diagenesis on

the mineralogy and chemistry of sediments. Nevertheless, provenance studies play an

important role in the interpretation of tectonics from sedimentary rocks. Perhaps the

most valuable information to be gained is the timing of important tectonic events such as

uplift and erosion of a plutonic arc, unroofing of originally deep-seated crustal rocks in

the lower plate of an extensional terrane, or erosional dissection of a specific thrust sheet

in a foreland setting.

STRUCTURAL CONTROL ON FACIES DISTRIBUTION

The influence of geologic structures on the distribution of sedimentary facies is

evident at all scales from the regional down to that of an individual fault or short

wavelength fold. Some well known facies, such asflysch and molasse, actually

connote distinct tectonic settings, although these terms are at best confusing, since they

conflict with the more common usage of facies as a descriptive or genetic term (e.g.,

sandstone facies, turbidite facies, fluvial facies ) (cf. Reading, 1978; VanHouten,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. gate #1 100% C03 STEP 1 : Source stratigraphie gate #2 100% SS section is divided into 1000' gate #3 50% SS "gates". Percentages of 50% COS iithologies are calculated for gate #4 50% SS each gate. 50% Chert

STEP 2; Source stratigraphie section is uplifted in equal gate #2100% SS amounts through an erosional gate #3 50% SS profile. Areas of each gate eroded 50% COS (stippled) are assigned percentage values of the total area eroded, gate #4 50% SS presumably to form a synorogenic 50% Chert sediment at time t.

At time t, gate #1 is 75% of total eroded area gate #2 is 25% of total eroded area STEP 3: gate #1 has 100% C 03, so 75% of eroded material is C03 gate #2 has 100% SS, so 25% of eroded material is SS

erosional erosional profile debris

Cretaceous

Jurassic Triassic Upper Paleozoic

Fig. 1.1 Two dimensional modeling technique used by Pivnik (1990) to generate hypothetical synorogenic sediment compositions (modified from Pivnik, 1990).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CONTINENTAL BLOCK PROVENANCES with RECYCLED OROGEN sources on stbie cratons PROVENANCES (C) & In uplllled basement (B) Morgw of fields for mature rock# Incraatlng ratio oi with stable frameworks Decreasing Oceanic to Continental maturity or matenWs stability CONVNENTAL BLOCK RECYCLED OROGEN PROVENANCES PROVENANCES

(mixed) Increasing ratio of Chert to Quartz MAGMATICARC Merger of fields PROVENANCES for Plutonic basement and Arc roots Increasing ratio of Plutonic (P) to Volcanic (V) sources

Subduction complex sources

CIRCUM-PACIFIC VOLCANOPLUTONIC SUITES ARC OROGEN Increasing maturity/ SOURCES stability trom Continental Stock provenances

COLLISION SUTURE & FOLD-THRUSTBELT SOURCES Increasing ratio of Plutonic to Volcanic sources in Magamtic Arc provenances

Fig. 1.2 Compositional variation of sandstone suites derived from different generic types of provenance terrane (after Dickinson, 1985). Symbols for grain types: Qt, total quartzose grains (Qm+Qp); Qm, monocrystalline quartz; Qp, polycrystalline quartzose lithic fragments (including chert); F, total monocrystalline feldspar (P+K); P, plagioclase; K, K-feldspar; Lt, total polycrystalline lithic fragments (L+Qp); L, unstable lithic fragments (Ly+Ls); Lv, volcanogenic lithic fragments ; Ls, sedimentary and metasedimentary lithic fragments (except chert and metachert).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1981). Field mapping of geologic structures and depositional units, combined with the

interpretation of lateral and vertical facies changes, and paleocurrent data, typify the

facies approach to tectonics and sedimentation. The type of information to be gained

includes the timing and duration of tectonic activity and the paleogeographic/structural

evolution of the basin.

Timing of tectonism

Stratigraphie data have long been used to verify the existence and constrain the

timing of tectonic events through identification and dating of unconformities, overlap

assemblages, volcanic activity, and (presumed) syntectonic sedimentary rocks. A recent

example using all of these data is a study by Duebendorfer and Wallin (1991) on

Miocene sedimentary rocks in southern Nevada. A coarsening-upward

sandstone/conglomerate unit, interbedded with volcanic rocks, was tilted and cut by

normal faults which do not displace overlying formations. The sandstone contains

intraformational unconformities and an upward decrease in stratal dip. Duebendorfer

and Wallin (1991) interpret these relations to indicate coeval volcanism, sedimentation,

and tilt-block faulting during deposition of the sandstone (11.9-8.5 Ma).

At the plate tectonic scale, the presence of unconformities, and changes in the

distribution of depocenters and sedimentary facies, may be enough to determine the

timing and geometry of a major tectonic event with little reference to other structural data

(e.g., Poole, 1974; Poole and Sandburg, 1977). The credulity of this approach may be

stretched, however, in the case where tectonic events are inferred solely on the basis of

facies changes with no other supporting evidence (e.g., Kidder, 1988).

In smaller scale studies, the importance of establishing a direct link between

structural and stratigraphie data is even more important. The traditional view that the

onset of tectonism is marked by deposition of conglomerates and other coarse-grained

sedimentary facies, has recently been challenged by several authors (Blair and Bilodeau,

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1988; Heller and others, 1988; Paola, 1988). These workers assert that the immediate

result of uplift is the deposition of relatively fine-grained rocks, because uplift rates are

typically greater than erosion rates, and because lakes and axial river channels respond

virtually instantaneously to changes in topography, whereas prograding systems such as

alluvial fans have a slow response time. As uplift slows or ceases, braid plains and fans

prograde basinward and coarse sediments are transported far out into the basin. This

results in an overall syn- to posttectonic coarsening-upward sequence. This is not a

new idea, as episodic fault activity has long been associated with the development of

small-scale (tens to hundreds of meters) coarsening-upward depositional sequences on

alluvial fans (e.g.. Steel and Wilson, 1975; Steel, 1976; Reward, 1978). Similarly, it

has been shown that differential subsidence rates control the location of axial river

channels (Bridge and Leeder, 1979). Blair and Bilodeau (1988) and others have applied

these concepts to extensional basins at a much larger scale, however, including

cyclothems measuring hundreds to thousands of meters thick and representing periods

of millions of years. These concepts are discussed more fully below and in Chapter 2.

Paleogeography

The lateral distribution and thickness of sedimentary facies during any particular

interval of time may be used to infer paleogeography, which in turn helps constrain the

tectonic setting. Illustrative examples of large-scale application of this approach are

presented in a series of papers on the Paleozoic paleogeographic development of the

western United States (Stewart and others, 1977). These studies utilize lithofacies and

isopach maps which are interpreted in terms of tectonically significant paleogeographic

zones (Fig. 1.3). The example given in Figure 1.3 shows a large area of relatively thin

limestones and dolomites to the east, eroded areas to the west, and a thick sequence of

interbedded mudstone, siltstone, sandstone, and conglomerate in the center. Limestone,

phosphatic shale, and sandstone of intermediate thickness separates the thick clastic

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deposits from the thin carbonates. The thick clastic seqence is interpreted as flysch,

rhythmically bedded turbidites deposited in a trough generated by contractional

orogenesis. The limestones are considered to mark the undeformed craton of North

America, whereas the transitional unit lies on the continent side of an asymmetric

foreland basin. To the west of the inferred orogenic highland, the presence of volcanic

and sedimentary rocks is taken as tentative evidence of a backarc basin (Poole and

Sandberg, 1977). It should be noted that these interpretations are based on structural

and pétrographie considerations in addition to purely stratigraphie data.

The facies distribution approach has been taken furtlier in some continental tectonic

settings. At the largest scale, Miall (1981) has attempted to characterize various basin

types by style of sedimentation. Miall (1981) recognized nine fundamental patterns of

sedimentation within alluvial basins, based on the arrangement of alluvial fan, alluvial

plain, and coastal depositional systems relative to structural grain (transverse or

longitudinal) (Fig. 1.4). He documented occurrences of these patterns in sixty six

modem and ancient basins which he arranged according to the classification scheme of

Bally and Snelson (1980). This resulted in a list of sedimentation patterns likely to

occur in any tectonic setting (Table 1.1). The fact that each of Miall's nine patterns may

be found in many different settings serves to illustrate the point that, in continental

settings at least, sedimentary facies are probably more useful in elucidating structural

style in an individual basin than in typifying a certain plate tectonic class of basin.

On a more detailed level, the study of depositional systems in modem and ancient

rift and pull-apart basins has provided a potentially useful set of criteria for identifying

structural styles. Field studies of syntectonic sedimentary rocks indicate that certain

types of basin boundaries leave their signature in the stratigraphie record. The

characteristic depositional systems associated with various basin boundaries in the East

African Rift were described by Crossley (1984) (Fig. 1.5), and these features have since

been incorporated into a series of basin models for typical half graben (e.g., Alexander

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Fig. 1.3 Thickness (a) and lithofacies distribution (b) of Lower Mississppian (Kinderhookian and Osagean) and lowermost Upper Mississippian (lower Meramecian) rocks in the western United States (after Poole and Sandberg, 1977). Contours in hundreds of meters. Key to patterns: 1, Limestone of the cratonic platform; 2, Mostly craton-derived rocks deposited in foreland basin - limestone, phosphatic shale, siltstone, and sandstone, with minor reworked flysch sediments; 3, Antler-derived flysch deposited in foreland basin - mudstone, siltstone, sandstone, and conglomerate, with subordinate limestone; 4, Antler orogenic highland; 5, Allocthonous strata of inner-arc basin - mudstone, siltstone, chert, volcanic rocks, minor limestone, sandstone, and conglomerate.

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------0 #

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\ /

D 7

Fig. 1.4 Patterns of sedimentation in alluvial basins, according to Miall (1981). T=transverse, L=longitudinal. Scales are variable. Structural controls are given as examples, only. Most basin fill patterns can occur in several tectonic settings (Table 1.1).

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Miall Basin Bally/Snelson Basin Type Most Common

Classification Classification Fill (Fig. 1.4)

Divergent plate margins

1 111 pre-drift rift grabens 1,6

2 121 failed arms, aulocogens 7,8,9

3 114 passive oceanic margins 3,4,5

4 121 flanking transverse elements 1,2,3

Pre-collision convergent plate margins

5 311 forearc 1,3,4,5,6,7,8

6 312 backarc /marginal seas 1,2,3,4,5

7 22 retroarc/ftxeland 1,3,4,5,7,8

8 312.321.331 intermontane/successor 1.2,6

9 22 pseudo-foredeep (subduction and

suturing of passive margin)

Post-collision and suturing convergent margins

10 22 peripheral or foredeep (on 7,8

subducting plate)

Strike-slip plate margins

11 33 pull-apart and fault-flank 1.2,6

depression

Cratonic basin

12 121 broad downwarps or sags

Table 1.1 Tectonic classification of alluvial basins (after Miall, 1981)

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and Leeder, 1987; Leeder and Gawthorpe, 1987; Frostick and Reid, 1987) (Fig. 1.6).

It is now widely accepted that continental half graben are characterized by extensive

alluvial fan-braid plain deposits derived from the largely unfaulted hanging wall (dip

slope) of the basin, whereas footwall-derived fans adjacent to major fault scarps tend to

be both areally and volumetrically small (e.g., Crossley, 1984; Leeder and Gawthorpe,

1987; Denny, 1965; Cavazza, 1989; Gawthorpe and others, 1990). Furthermore, fault

activity plays a role in the facies architecture of alluvial fans. Steeper, more active basin

margins are characterized by the dominance of debris flow processes, whereas less

active boundaries exhibit mostly stream flow (e.g.. Steel, 1976). Longitudinal profiles

of modem fans in California and Nevada show segmentation, which is interpreted as a

response to changing depositional slopes caused by fault movements (Bull, 1964;

Hooke, 1972). The vertical textural trends theoretically associated with such processes

have been observed in ancient rocks (e.g.. Steel, 1976; Reward, 1978; Gloppen and

Steel, 1981), but the actual segmentation geometry has not.

Under subaqueous conditions, steep, faulted basin margins may be characterized by

the construction of fan deltas (e.g., Surlyk, 1984; Colella, 1988; Scholz and others,

1990). This footwall-derived debris may dominate sedimentation in some subaqueous

half graben (e.g., Surlyk, 1984), in contrast to its volumetrically subordinate role in the

subaerial realm.

The depositional characteristics of normal faulted basin margins discussed above

apply equally to the strike-slip margins of both rift and pull-apart basins. An added

factor in this case is that older fans are progressively removed from their original source

area as movement on the fault continues (e.g.. Steel and others, 1977; Steel and

Gloppen, 1980; Crowell, 1982; Nilsen and McLaughlin, 1985) (Fig. 1.7). The offset

of fans from their source area, upsection changes in source terrane, and the apparently

excessive stratigraphie thicknesses which may occur under such conditions are

distinctive signatures of strike-slip faulting.

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COMMON RIFT- RESULTING

M A R G IN GEOMORPHIC

STRUCTURES FEATURES

Single major boundary fault L an d slid es

Alluvial fan

Series of fault blocks

Tilted toward rift 7 lakes

Tilled away from hfi .G orge ' lak e s

G r.b e n ) lakes

M on o clin e Dissection throughout - warped area E xtensive alluvial fete^fans

Single major fault fault blocks monocline Riftward tilted Single bloc^ M o n o clin e

faultw '"*''^^'^

En echelon ' faults Ramp structure

Fig. 1.5 Common rift margin structures and their geomorphic expression (after Crossley, 1984).

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' 1

Fig. 1.6 Facies models for continental and marginal marine half graben (after Leeder and Gawthorpe, 1987).

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Tectonic history

The concepts discussed above relate depositional patterns at a single point in time to

the structural setting. Some workers have used basin fill architecture to reveal the

tectonic evolution of basins and of individual faults. The dominant controls on non

marine basin fill are the patterns of uplift and subsidence at the basin margins, the nature

of the source terrane, climate and climatic fluctuations, eustacy, drainage patterns, and

volcanism. As discussed above, the immediate sedimentary response to fault

movement, at both deposystem and basin scales, is thought to be the deposition of finer-

grained debris, followed by progradation as fault movements slow or cease. Heward

(1978) presented a detailed analysis of hypothetical alluvial fan behavior in response to

tectonic activity, and gave examples from the geologic literature. According to Heward

(1978), fault movements result in coarsening-upward textural trends, whereas erosional

scarp retreat and reduction of relief by basin infilling cause fining-upwards sequences to

be deposited.

In the subaqueous realm, fining-upwards trends and aggradation of fan deltas may

accompany fault movements, whereas progradation may characterize times of relative

tectonic quiescence (Gawthorpe and others, 1990). Colella (1988) hypothesized, based

on studies of the Crati Basin, Italy, a link between the type of normal fault motion

(continuous vs. stick-slip) and the internal geometry of fan delta deposits. The effect of

subsidence rates on the architecture of alluvial plain depositional systems has been

investigated by many workers. Overall subsidence rates influence the vertical stacking

of channel sand bodies, whereas differential subsidence controls the lateral distribution

of sands (Fig. 1.8) (e.g.. Bridge and Leeder, 1979; Alexander and Leeder, 1987; Allen,

1978; Blakely and Gubitosa, 1984). In addition, coarsening-upwards textural trends in

alluvial plain sediments that correlate directly with similar sequences in coeval basin

margin fan bodies have been documented in the Devonian basins of western Norway

(Steel and others, 1977). Channel stacking models have been applied mainly to foreland

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(/> z < UJ rj 00 o u UJ Ou Ü UJ Q O

ifili w v y i ï i i i UJ <( lU E v> I O I I

< < <

z 2 u o g w K > %

Fig. 1.7 Effects of strike-slip faulting on sedimentation in the Ridge Basin, California (after Crowell, 1982).

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basins (e.g., papers in Etheridge and others, 1987), although Alexander and Leeder

(1987) have speculated on the existence of such patterns in the axial-fluvial deposits of

externally drained rift basins.

Several factors other than fault movements may either control or obscure large-scale

coarsening-upward trends in basin fill. The effects of intemal vs. external drainage and

the associated variations of base level in basins of the Rio Grande rift were discussed by

DiGuiseppi and Bartley (1991), whereas Scholz and others (1990) investigated the

effect of lake level on East African Rift sedimentation. Ramp structures associated with

fault steps and accommodation zones between en échelon half graben control

volumetrically significant fans and fan deltas in the Gulf of Suez (Gawthorpe and

others, 1990) and the East African Rift (Crossley, 1984; Frostick and Reid, 1987). In

the case where local base level is sea level, eustacy may play an important role. In

addition, the effect of climatic variations on facies distributions may mimic that of

tectonics, although such fluctuations cannot produce the geometric manifestations of

differential subsidence. Finally, the enormous quantities of debris generated by synrift

volcanism may overwhelm all other controls on sedimentation in rift basins (e.g.,

Waresback and Turbeville, 1990).

QUANTITATIVE MODELS

Geometric models

Several varieties of quantitative sedimentation models have been constructed by

geologists in their effort to understand basin fill patterns and to predict the effects of

differing tectonic, sea level, and sediment supply conditions on stratigraphie

architecture. The simplest of these are geometric models in which the sediment surface

is assumed to be fixed and horizontal, or in which the form of the surface remains

constant. Examples of models based on the former assumption include theoretical and

analog models of sedimentation in half graben produced by domino-like tilting and

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extension of a series of rigid, fault-bounded blocks. For example, Vendeville and

Cobbold (1988) used sand box experiments to demonstrate that growth fault curvature

may result in part from progressive rotation of older fault segments and propagation of

new segments at a fixed angle relative to the surface (Fig. 1.9). Barr (1987) held the

instantaneous sediment surface fixed in space and investigated the effects of varying

extension, fault spacing, and detachment depth on fault and sediment fill geometries

(Fig. 1.9). Barr's (1987) geometric model yields results similar to those of Vendeville

and Cobbold (1987) for cases where the sediment surface remains higher than the crest

of the fault blocks. A more complex example of the fixed sediment surface model is the

analysis of Turcotte and Willeman (1983). These workers generated one- and two-

dimensional synthetic stratigraphies by combining harmonic sea level fluctuations,

laterally variable subsidence, and the alternative assumptions that sediment always fills

the basin or that it is limited by a fixed maximum accumulation rate.

The constant form assumption of geometric models is illustrated by the work of

Pitman (1978) and Riley and others (in review, 1991). In both cases, the depositional

slope of the coastal plain and shelf were assumed to remain in equilibrium. Pitman's

(1978) model maintained the coastal plain-shelf surface at constant slope despite

differential subsidence, by varying sedimentation rate laterally. Various subsidence and

absolute sea level histories were then applied in order to generate two-dimensional plots

of synthetic stratigraphy on a passive margin. In contrast, Riley and others (in review,

1991) segmented the profile into coastal plain, shoreface and shelf sections, then used

observational data on the rate of transgression, progradation, and subsidence of the

shoreface during a single sea level cycle to migrate the equilibrium profile through space

and time. The resulting chronostratigraphic-facies plots were compared directly to

outcrop and well data from the San Juan Basin in order to evaluate the chronologic

significance of various sediment surfaces (Fig. 1.10).

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Fig. 1.8 Computer simulation of axial-fluvial channel stacking patterns in a half graben. Models 1) through 3) reflect increasing differential subsidence. Stippled areas represent channel sand bodies, whereas overbank deposits are shown in white. Timelines (black) divide the stratigraphy, (after Alexander and Leeder, 1987).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 4

a' = 5 km

a' = 20 km

Fig. 1.9 Geometric models of sedimentation in half graben: a) Four steps illustrating the origin of growth fault curvature (modified after Vendeville and Cobbold, 1988); b) The effect of varying fault spacing (a') on sediment geometry, fault geometry, and tilt-block crest erosion (after Barr, 1987).

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A. MODEL

I I

Distance

B. CHRONOSTRATIGRAPHY

T-»

E-

Distance

Rcgrcssivc coastal plain field jia l Lowstand erosion Q Transgressive backbarriex field I I Regressive shoreface field l y.j Transgicssive erosion |Q Transgrcssive shelf field g Regressive shelf field 1- Phase of deposition

Fig. 1.10 Sequence stratigraphie model of Riley and others (in review, 1991).

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The computer simulations of fluvial channel stacking architecture discussed above

(Fig. 1.8) are hybrids of the geometric and the dynamic approaches. A constant form

sediment surface is not required, but channel-belt geometries are fixed. Results of this

type of modelling indicate that interconnectedness and areal density of channel-belt sand

bodies increase with decreasing flood plain width and mean avulsion period (Bridge and

Leeder, 1979). Differential subsidence caused by faulting may strongly affect both of

these factors (Alexander and Leeder, 1987).

Dynamic models

Dynamic models of sedimentation are based on approximations to the basic laws

governing sediment transport and deposition (Angevine and others, 1990). Those

investigating the stratigraphy of carbonate depositional environments have tended to

express these laws as an empirically-derived relationship between autocthonous

sedimentation rate and depth, combined with a geometric aproach to the dispersal of

erosional debris (e.g.. Read, 1986; Bice, 1988; Bosence and Waltham, 1990; Gildner

and Cisne, 1990). In contrast, modelling studies of clastic alluvial and deltaic systems

are based on an assumed linear relationship between sediment flux and topographic

slope (e.g.. Angevine and others, 1990; Kenyon and Turcotte, 1985; Flemings and

Jordan, 1989; Anderson and Humphrey, 1990). Marine depositional systems were

modelled by Lawrence and others (1990), who used a combination of empirically- and

theoretically-derived equations for erosion, transport, and deposition of different

materials.

The alluvial-deltaic models are the most relevant to continental rifts. The assumption

of a linear dépendance of sediment transport rate on slope may be expressed as

c _ (1)

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where h is elevation, x horizontal distance, and k the coefficient of sediment transport.

Substitution of (1) into the continuity equation,

a _ 1 dS à ■ (i.^)d x (2)

where t is time, and 0 porosity, leads to a diffusive relationship for the redistribution of

mass in two dimensions:

dh _ 1 d {, dh\ à~(I.^)SASx] (3)

If k is assumed to be independent of position (x), equation (3) simplifies to:

§ = (4) * ax '

where fc, the sediment diffusivity, is equivalent to kl(l-(j)). Equation (4) is similar in

form to other diffusion equations, such as that for heatflow. The physical effect of

applying this equation is that topographic slopes which are convex-up are eroded,

whereas deposition takes place in areas of concave-up topography (Fig. 1.11).

Several different approaches have been used in arriving at equations (1) and (4).

These approaches may be broadly divided in two types: a) those based primarily on

consideration of process (creep v^. overland or channel flow), which are investigated

through theory and short-term experiment; b) those based on observational data, either

of individual landforms or of geographic regions.

Creep nrocesses: Culling (1960) first proposed the diffusion model as an

explanation for the modification of landforms, but did not attempt to understand the

physical basis for his assumption of equation (4). In later papers, however. Culling

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developed a theory of soil creep based on random downslope movements of soil

particles which related mass displacement to hUl slope (Culling, 1963,1965).

Theoretical and experimental work by Mitchell (1976) on creep demonstrated that shear

stress is proportional to shear strain, whereas Schumm (1967) found that downslope

transport of shale debris by frost heave is proportional to the sine of the slope angle.

Both of these observations lead to equation (4) for small slopes. Kenyon and Turcotte

(1985) adopted equation (4) for deltas on the assumption that the effects of bioturbation

and wave-bottom interactions on delta front creep were analogous to the frost-heave and

biological activity which formed the physical basis of CuUing's (1963) analysis.

Fig. 1.11 Modification of an initial step-like topography by erosion and deposition according to the diffusion equation

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Overland!channel flow: Statham (1977) stated that rates of erosion by overland flow

should be related to slope angle because flow velocities and bed shear stress tend to

increase with slope, but recognized that this relationship may be obscured by the

tendency for sediment yield to increase with slope length in eroding environments. The

experimental rainfall simulations of Moseley (1974) indicate that sediment yield from

straight slopes is directly proportional to slope gradient.

Many workers have modelled the long term behavior of river systems using the

diffusion equation (e.g., Soni and others, 1980; Begin and others, 1981). According to

the analysis of Angevine and others (1990), the coefficient of sediment transport in

rivers is dependant mainly upon the type of river (gravelly v.y. cohesive bank), and on

long term water supply. These parameters are easily constrained in experimental

investigations and in studies of modem rivers, but are less easily arrived at for geologic

modelling purposes. In practice, geologic scale models may use empirical values of k

derived from studies of landform morphology and sediment yields from major rivers, or

from estimates of average basin fill rates (Flemings and Jordan, 1989).

Observational approach: Several workers have used the diffusion equation to

describe the topographic evolution of fault scarps and wave-cut features, having started

either with the premise that sediment transport is directly proportional to slope or from

the observation that degraded fault scarp profiles are similar in form to the error

function, which is the fundamental solution to the diffusion equation (Nash, 1980ab;

Colman and Watson, 1983; Hanks and others, 1984). These studies use known ages of

landforms and their present shape to derive values for the sediment diffusivity K, which

they then use to determine the ages of undated fault- and shoreline-controlled landforms.

Values of k derived in this way vary by two orders of magnitude. Nash (1980a)

attributes this to the differing influences of climate, vegetation, and grain

size/mineralogy.

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Studies of regional and worldwide denudation rates are consistent with a linear

relationship between sediment transport and slope. Ruxton and McDougall (1967)

reconstructed dated eroded land surfaces in northeast Papua and found a linear

correlation between denudation rate and both slope and relief. Other studies have

reported a linear correlation of sediment yield from major rivers with slope and elevation

(Schumm, 1963; Ahnert, 1970; Pinet and Souriau, 1988).

In conclusion, use of the diffusion equation as a first order approximation to the

development of landforms appears to be justified by theoretical, experimental, and

empirical evidence. To date, there have been few applications of this technique to

stratigraphie modelling. Flemings and Jordan (1989) modelled foreland basin

stratigraphy using this approach, combined with a flexural loading model for basin

formation. Angevine and others (1990) constructed a simple basin model using the

diffusion equation and assumptions about selective sorting of sediment, in order to

investigate grain size variations caused by changing sediment flux, subsidence, water

supply, and initial gravel fraction.

CONCLUSIONS

The main purpose of this brief review of tectonics and sedimentation has been to

document the techniques commonly in use and to assess the errors and assumptions

attending each. Pétrographie studies are certainly of use in identifying potential source

terranes, but while it is undoubtedly true that certain source rock types may be found in

certain tectonic settings, the jump from tectonic origin of source lithology to tectonic

setting of sedimentary environment is a large one. Nevertheless, in cases where the

tectonic history of the source terrane is partly known or inferred, the composition of

sedimentary rocks may help constrain tectonic models for the region, as documented in

Chapter 3.

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The main drawbacks to isotopic studies of detrital minerals as a technique to identify

source terranes and their tectonic history are the potential effects of weathering,

transport, and diagenesis on the mobility of the relevant isotopes, and the fact that burial

and heating of sediments within a basin may ultimately lead to destmction of the isotopic

record. This is especially true of fission track studies because of the tendency for

fission tracks to anneal at quite low temperatures (Dokka and others, 1986). If the

isotopic system is fairly stable under conditions of weathering and mild heating, then it

might be possible to determine ages on certain minerals and relate these to potential

source terranes (Heller and Frost, 1988). If the kinetics of apatite fission tracks are well

understood, it is theoretically possible to remove the effects of basin thermal history and

thereby perhaps cast light on the thermal history of the source terrane (R. K. Dokka,

personal communication, 1991). No attempt is made in this study to utilize these

techniques. At the regional scale (Chapter 2), numerous, local sediment sources in the

study area preclude the attempt based on the balancing of time and effort vs. potential

importance of results. At a local level (Chapter 3), petrography, paleocurrent data, and

facies distributions firmly delineate the location of the local source terranes. It may be of

interest in the future, however, to compare fission track analyses of crystalline basement

clasts found in sedimentary rocks during this study to existing analyses of the inferred

source terrane.

The distribution of sedimentary facies in time and space may be important factors in

interpreting tectonics, provided the effects of climatic variations and eustacy can be

unravelled. Chapters 2, 3, and 4 are largely concerned with facies distributions.

Finally, quantitative stratigraphie models may provide insights into the effects of the

controlling variables on sedimentation. In non-marine settings, use of the diffusion

equation as a first order approximation to the development of landforms appears to be

justified by theoretical, experimental, and empirical evidence. The major problem in geologic modelling applications is the choice of values of the transport coefficient k.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 32

This topic is discussed in Chapter 4, where the stratigraphie evolution of an evolving rift

is modelled using the diffusion equation and a simple grain size sorting model.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sedimentation in detachment-dominated 2 extensional orogens: insights from the Mojave Extensional Belt, California______^

INTRODUCTION

The tectonic evolution of detachment-dominated rift systems has attracted much

interest in the decade since Wernicke's (1981) recognition of the importance of crustal- scale, low-angle normal faulting as a mode of extension. Popular issues have included

the documentation of upper and lower plate structural relationships (e.g., Miller and

others, 1983; Spencer, 1985; Davis and others, 1986; Pokka, 1986; Rosendahl, 1987;

John, 1987) and lower plate pressure-temperature histories (e.g., Dokka and others,

1986; Jones, 1990; Henry and Dokka, in press), as well as the geometrical evolution of

upper plate normal fault systems (e.g., Wernicke and Burchfiel, 1982; Barr, 1987;

McClay and Ellis, 1987; Axen, 1988; Jackson and others, 1988), the mechanical

feasibility of low-angle normal faults (e.g., Wernicke and Axen, 1988; Yin, 1989;

Buck, 1990), and the efficacy of simple v^. pure shear models in predicting regional

patterns of uplift, subsidence, magmadsm and metamorphism (e.g., England, 1983;

Wernicke, 1985; Mudford, 1988; Ruppel and others, 1988; White, 1989; Buck, 1990;

Latin and White, 1990). In contrast, the stratigraphie evolution of detachment-

dominated extensional orogens has remained less well documented. Many workers

have studied patterns of sedimentation in half graben (e.g., Surlyk, 1984; Alexander and

Leeder, 1987; Leeder and Gawthorpe, 1987; Frostick and Reid, 1987; Cavazza, 1989;

Ori, 1989; Morley, 1989; Mack and Seager, 1990; Scholz and others, 1990), but few

have addressed either the stratigraphy associated with other rift elements or the overall

^ paper submitted for publication as Sedimentation in detachment-dominated extensional orogens: insights from the Mojave Extensional Belt, California, by C. J. Travis, R. K. Dokka, and M. O. Woodbume.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 34

tectonostratigraphic framework of rifts. Important studies include the recent

paleogeographic-paleotectonic synthesis of the Rio Grande Rift by IngersoU and others

(1990), and the documentation of transfer zone sedimentation patterns in the east African

rift (Frostick and Reid, 1987) and the Devonian basins of western Norway (e.g.. Steel

and others, 1977).

In this paper we present a synthesis of stratigraphie relations in the Mojave

Extensional Belt (Dokka, 1989ab), an early Miocene, detachment-dominated extensional

orogen in southern California. We conclude that regional tectonostratigraphic patterns in

the Mojave Extensional Belt have a tripartite architecture, parts of which also

characterize several other extensional terranes. Stratigraphie patterns associated with

individual tectonic elements of the belt are complex, yet distinctive. Local stratigraphie

studies yield important constraints on models for the tectonic evolution of the region.

TERMINOLOGY

Detachment-dominated extensional orogens are composed of several discrete tectonic

elements (Fig. 2.1). Extension is facilitated by movement along low-angle, crustal-scale

normal faults (detachments) (Fig. 2.1) (e.g., Wernicke, 1981). The upper plate of these

faults breaks up during transport to form a series of similarly dipping, fault-bounded

blocks which constitute the main surface expression of rifting. Following the

nomenclature proposed by Dokka (1983a), the part of the extended terrane towards

which the detachment as well as the majority of the upper plate normal faults dip is

termed distal, whereas the direction in which upper plate strata are tilted is defined as

proximal (Fig. 2.1). The fault which separates the proximal part of the extended terrane

from adjacent unextended regions has been termed the breakawav (e.g., Howard and

others, 1982); we will refer to the basin formed by movement on this fault as the

breakaway basin (Fig. 2.1). Most of the surface of the extended terrane consists of

similarly oriented half graben {upper plate fault basins ) (Fig. 2.1), but antithetic faulting

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in the distal, or breakoff, part of the orogen may result in symmetric full graben or in

asymmetric basins of opposite polarity termed here, breakoff basins (Fig. 2.1).

Accommodation between adjacent extensional terranes or between extended and

unextended areas perpendicular to the tectonic transport direction is facilitated by zones

of strike-slip faulting known as tranrfer zones (Bally, 1981; Gibbs, 1984) (Fig. 2.1).

Similar structures which allow the transfer of strain between adjacent half graben in

regions which have undergone relatively minor extension have been named

accommodation zones (e.g., Rosendahl, 1987). A much studied surface element of

detachment-dominated extensional terranes is the metamoiphic core complex (Fig. 2.1)

(Davis and Coney, 1979; Crittenden and others, 1980). Rocks now exposed in these

complexes retain a long record of tectonism, culminating in rapid exhumation by

detachment faulting from mid-to-upper crustal levels to near the surface (e.g., Dokka

and others, 1986; Jones, 1990; Henry and Dokka, in press, 1992; Dokka, 1989a).

Basins associated with the development of metamorphic core complexes may initially be

located in the distal part of the extensional terrane (Fig. 2.1), but may subsequently

evolve into a new breakaway basin if extension in the proximal part of the terrane ceases

due to uplift of the core complex (e.g.. Lister and Davis, 1989). We will refer to basins

adjacent to metamorphic core complexes as core complex basins.

Throughout this paper, we refer to sedimentary and volcanic rocks as being of syn-

or posttectonic origin. Unless otherwise stated, syntectonic is taken to mean the period

in which there was substantial regional tectoitic activity in an extensional orogen; it does

not refer to individual intervals of discrete fault movement. Similarly, posttectonic is

understood to imply times after the cessation of regional extension.

GEOLOGIC BACKGROUND

The Mojave Extensional Belt stretches from the intersection of the San Andreas and

Garlock faults in the western Mojave Desert to the Granite Mountains fault in the east

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(Fig. 2.2) (Dokka, 1989ab). Additional elements of the belt may be present west of the

San Andreas fault in the Pinnacles area of west-central California (Dokka, 1989a) and

north of the Garlock fault in the San Joaquin Basin (Tennyson, 1989; Goodman and

Malin, in press). According to Dokka (1989a), strain in the Mojave Extensional Belt is

partitioned between four structural domains that each consist of one or more half-graben

(Fig. 2.2). Extension of each domain was accomplished primarily by movement along

low-angle detachment faults, above which deformation of the upper plate took place by

the formation, rotation, and abandonment of several generations of originally steeply-

dipping normal faults (Dokka, 1986; 1989a; Mathis, 1986). Differential extension

between the domains was accommodated along transfer zones (Dokka, 1989a).

The Mojave Extensional Belt began to open in early Miocene time, with the major

phase of extension occurring near 20 Ma (Dokka and others, 1991). High-angle normal

faults cut detachments locally and were active -20-17 Ma. The development of

metamorphic core complexes as positive topographic elements in the Mojave Extensional

Belt occurred during this latter stage of regional extension, after the main episode of

detachment faulting had ceased (Dokka, 1989ab; Travis and Dokka, in review).

The Mojave Extensional Belt originally opened in a ~N-S direction, but subsequent

rotations of parts of the region about vertical axes (Ross and others, 1989) have resulted

in a generally SW-NE present-day orientation of extensional kinematic indicators

(Dokka, 1989a). Much of the Mojave Extensional Belt is now cut by a family of

predominantly northwest-striking strike-slip faults associated with regional right shear in

the Eastern California Shear Zone, a major element of the Pacific-North American

transform plate boundary (Dokka and Travis, 1990ab). This late Neogene strike-slip

faulting, together with associated vertical axis block rotations, has altered the

distribution of the earlier formed extensional rocks and structures and is locally

responsible for their exposure at the surface.

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S (0

g 5

Fig. 2.1. Schematic representation of tectonic elements and associated basins of a detachment-dominated extensional belt.

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Fig. 2.2. Location map of study area, highlighting extensional domains of the Mojave Extensional Belt (modified after Dokka, 1989a). Heavy lines mark location of regional stratigraphie cross sections (Figs. 2.4,2.14, 2.15). Key to abbreviations: AM, Alvord Mountains; AB, Antelope Buttes; BM, Bullion Mountains; BrM, Bristol Mountains; BL, Bristol Lake; BWF, Baxter Wash Fault; CM, Cady Mountains; CoM, Calico Mountains; DR, Daggett Ridge; FP, Fremont Peak; GH, Gravel Hills; HL, Harper Lake; HH, Hinkley Hills; KSF, Kane Springs Fault; KH, Kramer Hills; LMF, Lane Mountain Fault; MR, Mitchel Range; MH, Mud Hills; NM, Newberry Mountains; CM, Ord Mountains; RM, Rand Mountains; RdM, Rodman Mountains; RgL, Rogers Dry Lake; RH, Rosamond Hills; RL, Rosamond Dry Lake; WH, Waterman Hills.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 39

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 0

STRATIGRAPHIC ARCHITECTURE OF THE MOJAVE EXTENSIONAL BELT

Breakaway basins

Breakaway zone rocks and structures of the Mojave Extensional Belt are best exposed

in the Newberry Mountains in the western part of the Daggett terrane (Figs. 2.2,2.3).

Here, upper plate pre- and syntectonic rocks are cut by three generations of rotated

normal faults and are separated from a lower plate of pre-Tertiary crystalline and

volcanic rocks by a gently arching detachment surface exposed in the Su Casa arch (Fig.

2.3)(Dokka, 1980,1986,1989a; Dokka and others, 1991). Rocks immediately adjacent

to the detachment show the effects of brittle deformation in the form of intense cataclasis

(Dokka, 1980,1986, 1989a). Extended rocks of the Newberry Mountains are separated

from unextended, pre-Tertiary basement to the west and south by a NW-striking

breakaway fault inferred to lie west of Daggett Ridge in Stoddard Valley and by an

ENE-striking transfer fault in the vicinity of Kane Springs (Fig. 2.3) (Dokka, 1980,

1986, 1989a). These faults are now covered by sedimentary rocks of post-extension

origin, but their presence is required based on structural and stratigraphie relations,

including projected truncation of hanging wall strata against basement, upward decrease

in dip of syntectonic strata, presence of landslide breccias, and hanging wall rollover

antiforms and other relations (Dokka, 1980,1986).

Overall, the lower Miocene stratigraphy of the Newberry Mountains follows a

regionally consistent pattern beginning with volcanic rocks that are unconformably

overlain by coarse, basement-derived deposits (Fig. 2.4). Alkali olivine basalt and

overlying andésite to rhyolite lava flows and tuffs of the formations of Minneola Ridge

and Newberry Springs comprise the basal -3.8 km of the Tertiary succession (Figs.

2.3,2.4) (Dokka, 1980,1986; Nason and others, 1979). Major normal faulting and

tilting in the Newberry Mountains began near the end of this dominantly volcanic phase,

and these rocks are unconformably overlain by more than 3 km of syn- to posttectonic.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 41

epiclastic breccia and conglomerate of the formations of Daggett Ridge, Slash X Ranch,

and Stoddard Valley (Dokka, 1980,1986) (Figs. 2.3, 2.4). Extension in the region

ceased before ~16 Ma, and a posttectonic sequence of breccia, sandstone, shale, and

limestone assigned to the Barstow Formation unconformably overlies older units (Figs.

2.3, 2.4). K-Ar and fission track data constrain the ages of the lower stratigraphie units

in the Newberry Mountains (Dokka, 1980; 1986; Nason and others, 1979), whereas

fossil mammals retrieved from the lowest rocks assigned to the Barstow Formation are

similar to those of the Rak Division Fauna in the Mud Hills (~16 Ma; Woodbume,

1991).

Detailed stratigraphie relationships at Daggett Ridge and near the Azucar Mine (Fig.

2.3) highlight the complexity of sedimentation processes in the breakaway zone. Not

only did multiple sources supply sediment to the basin at any one time, but the nature,

location, and relative dominance of these sources changed through time as high areas

were eroded and as the original breakaway basin became dissected and extended by

second and third generation normal faults. In the following paragraphs, we summarize

our present understanding of the stratigraphie, structural, and paleogeographic evolution

of the region. We should emphasize that our reconstructions (Fig. 2.5) are conceptual;

the broad paleogeography and the evolutionary style of the basin are considered

accurate, but some details of basin size and timing of individual fault initiation are still

not fully understood.

Tuff breccias, fluvialiy reworked Tertiary volcanic rocks, and minor lacustrine strata

of the formation of Daggett Ridge form the base of the syntectonic section throughout

the area (Figs. 2.3, 2.4). These rocks were derived from the northeast and east, based

on paleocurrent observations (Dokka, 1980). Clast lithologies include several varieties

of andesitic-dacitic material, as well as rare boulders of a distinctive, porous crystal lithic

tuff also found in the western Cady Mountains to the east (Figs. 2.1,2.4). This

similarity of distinctive clast types, together with paleocurrent data and the paucity of

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Fig. 2.3. Geologic map and cross section of the Newberry Mountains (after Dokka, 1989a).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ' L / 3(d generation m Azucar Mii %

I °- I QUATERNARY DEPOSITS

BARSTOW FORMATION

SYN- ni'MuAND ruoPOS'-TECTONIC -icviUMiv ucruoiDEPOSITS: i aiM- fttis. of Stash X Ranch, > d(laid Va«ey, Daagea Hidgo

UPPER PLATE, ROCKS: (ms, at Nawbeny Springs. Minneata Ridge UPPER PLATE CRYSTALLINE BASEMENT: m aM y granKe-quariz munzonKs

iv-yït&d LOWER PLATE ROCKS: Mesozoic granite qtz. monz. Ë iÿ jf'tl with Sidewinder Volcanic Series, gneiss, maitüs

NON-EXTENDED BASEMENT low-angle reverse fault

WSW

su o u w ar

«00,

Reprofjucetj with permission of the copyright owner. Further reproctuction prohibitect without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 43

5# m

3nl fleneration xiixemmmxtses« SSïî! 2nd oenetaBon' SSï!

m m m

Low>angle reverse fault Low-angle normal (autt Moderate-and Location of measured sections ' NMDF=Newberry Mountains high-angle normal faults detachment fault

s u CASA ARCH

AGE Newberry^ Newberry^ CadyMts^CadyMts^’'^C adyB ristol® (Ma) Mts (West) Mts (East) (west) (Central) (North) Mountains 12 -

1 4 -

16 -

18 - 20 — » ■ 22 - in 24 -

26 -

Legend for Figures 5,15, & 16

Mudstone ConglomeratB i i s Basalt flow

I I I Umestone Igneous Intrusive

Sandstone Tuff txecda Isotopically dated rock

Fig. 2.4. Miocene regional stratigraphy of the Daggett terrane. See Fig. 2.2 for localities. Key to abbreviations: Tmns, formation of Newbeny Springs; Tmmr, formation of Minneola Ridge; Tmdr, formation of Daggett Ridge; Tmsx, formation of Slash X Ranch; Tmsv, formation of Stoddard Valley; Tmb, Barstow Formation; Tmh, Hector Formation. Key to sources of data: 1, Dokka (1980); 2, Dokka and others (1991); 3, Woodbume and others (1982); 4, Dokka (1986); 5, Woodbume and others (1974); 6, C. J. Travis and T. M. Ross (unpublished data).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 45

_NM

SVS

KS7F-

A) B)

SVB SVB KW-

KSTF ENM

C) D)

L e g e n d

SV OR. Form alton d S to d d ard Valley KW AZI "sc Formation of Slash X Ranch

Formation of Daggett Ridge

Formation of Newberry Springs

Fig. 2.5. Block diagrams illustrating the structural and paleogeographic evolution of the breakaway zone of the Daggett terrane. Note orientation of north arrow. Key to abbreviations: AZM, Azucar Mine; DR, Daggett Ridge; ENM, Eastern Newberry Mountains; KSTF, Kane Spring transfer fault; KW, Kane Wash; NM, Newberry Mountains; SC, Su Casa dome; SR, Stoddard Ridge; SV, Stoddard Valley; SVB, Stoddard Valley breakaway; WCM, Western Cady Mountains.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 4 6

locally derived basement clasts, provide evidence of west-directed drainage during the

early stages of development of the breakaway basin. A palinspastic map based on

previously published tectonic reconstructions of the late Cenozoic Mojave Desert (Dokka

and Travis, 1991a), places the western Cady Mountains in a position adjacent to the

Newberry Mountains prior to initiation of the Eastern California Shear Zone -10-6 Ma

(Fig. 2.6). Based on this map, balanced stractural cross-sections through the Newberry

Mountains (Dokka and others, 1991), and the evidence from the sedimentary record

given above, we postulate a relatively wide (-10 km), asymmetric half graben with a

well developed hanging wall drainage system (Fig. 2.5b).

Basement-derived debris of the formation of Slash X Ranch unconformably overlies

the formation of Daggett Ridge with an angular discordance of as much as 40° (Dokka,

1980). In the central part of Daggett Ridge, the formation of Slash X Ranch contains

about 1150 m of pebble to boulder breccias of debris flow, landslide, and rock

avalanche origin, interbedded with minor streamflow deposits and several basalt flows

(Figs. 2.7,2.8). Strata in the lower part of the central Daggett Ridge section are

composed predominantly of Mesozoic granitoid clasts that were deposited by debris

flows, whereas the upper 900 m of the unit consists mostly of landslide and rock

avalanche breccias (Fig. 2.7). Individual landslide/rock avalanche units are lenticular in

shape, with a lateral extent of 0.3 km to >1.5 km. Each depositional unit is

monolithologic, and consists of Mesozoic volcanic detritus (Mesozoic Sidewinder Series

of Bowen, 1954) or, rarely, granitoid debris. In the northwestern part of Daggett

Ridge, the formation of Slash X Ranch is also dominated by material derived from

Mesozoic volcanic strata, but deposition was mainly by debris flows, and mixing of

different clast lithologies within depositional units is sometimes seen.

Present day basement outcrop patterns in the Stoddard Ridge area support the notion

that the landslide and rock avalanche breccias of older volcanic debris were shed from

the footwall of the breakaway southwest of Daggett Ridge. On the other hand, the

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area not modelled

:WH ^AM \Barstow.

;d r

10 mi

modelled

& ■

Fig. 2.6. Palinspastic reconstruction of the central Mojave prior to the initiation of the Eastern California Shear Zone ~10 Ma. A. Present day geology; B. 10 Ma configuration. Fault slips and rotations used in this reconstruction were taken from Dokka and Travis (1990a).

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LOG CLAST-TYPE INTERP. LOG CLAST-TYPE INTERP. f i S f i f |2 f i

700 UNEXPOSED 1 4 0 0 -I tOoatofffanÿoid tnccéêpmùtuy BafXowFm àtpitet) Fmo

6 0 0 1 3 0 0 - UWDSLgJe

5 0 0 • 1200 WOCPOSED

PROBABLE OEBRtSPLOWS 4 0 0 1100

rU M A U ■E AlWVlALFiAN Z PROBABLE DWGFUDWS 3 0 0 1000 BASAIT FUDW

oeefss FLOWS

Fm of SlaahX Ranch j Fm of Daggett I 200 ■ R d g e 9 0 0 H S 3 ROCK D^RtSFWWAlAUAAfCHEOR

100 - 8 0 0 •

L A H A ftS "■ I i j: I 7 0 0 a s c 8 a S C 8

H / /.y aandetono b re o d a tuM b re o d a

tnsatt horizontal tamtnatlona croaa-taminaWona

norm al gradino No/" aoourandfWI ^ fg a a w a n d o ra d d a tab rtc a

Fig. 2.7. Measured section of the formation of Slash X Ranch at central Daggett Ridge. Key to clast-type abbreviations: Tv, Tertiary volcanic rocks, Tb, Tertiary basaltic rocks; Mg, Mesozoic granitoids; Mv, Mesozoic volcanic rocks of the Sidewinder volcanic series.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 49

Fig. 2.8. Photographs of distinctive rocks and textures at Daggett Ridge. A. 'Jigsaw' fabric in breccia of volcanic debris derived from the Mesozoic Sidewinder series. B. 'Crackle' fabric in granitoid breccia - note quartz vein brecciated in place.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 50

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 51

decrease in importance of landslide/rock avalanche deposits to the northwest might be

taken as evidence that these deposits were derived from near the juncture of the

breakaway and the transfer zone to the south of Daggett Ridge (present day reference

frame). This ambiguity is enhanced by the fact that no direct indicators of transport

direction have been observed in breccias of the formation of Slash X Ranch at Daggett

Ridge. The general lack of mixing of different clast types in the formation of Slash X

Ranch at Daggett Ridge indicates the existence of at least two distinct source terranes.

The geographic and stratigraphie position of these sources relative to each other is

unknown, however, for Mesozoic volcanic rocks now exposed in the area both predate

and postdate emplacement of the shallow level quartz monzonite plutons (Schermer and

Busby-Spera, 1990).

Structural reconstructions indicate that upper plate rocks at Daggett Ridge and at

Azucar Mine were originally juxtaposed and that second and third generation normal

faults, along with movement along the detachment, have disrupted the region (Fig. 2.3)

(Dokka and others, 1991). Despite their original proximity, however, syntectonic rocks

at the two localities correlate poorly with each other on a lithologie basis (Fig. 2.9). The

lower 560 m of the formation of Slash X Ranch at Azucar Mine contains three

lithologies (Fig. 2.9). Basalt flows mark the base and top of this section, whereas the

middle of the section is composed of ~300 m of conglomerate of Mesozoic volcanic

debris. A 73 m thick sequence of laharic and fluvial reworked tuff breccias, similar to

those of the formation of Daggett Ridge, separates the conglomerate unit from the upper

basalt. Within the conglomerate of Mesozoic volcanic debris, an upward increase in

grain size and matrix content and a concomitant decrease in sorting, bedding, and

traction structures record a change from predominantly streamflow to debris flow

processes. Based on these criteria, we interpret the conglomerate unit as a prograding

alluvial fan sequence. Abundant pebble and cobble imbrication in the lower,

streamflow-dominated part of the unit indicates northerly to westerly transport (Fig. 2.9)

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and implies a source terrane in unextended regions southeast of Daggett Ridge adjacent

to the Kane Springs transfer zone.

The upper 1190 m of the formation of Slash X Ranch at Azucar Mine consists of

breccias composed of granitoid debris (Figs. 2.9, 2.10). These rocks range from well

bedded, clast-supported breccia containing imbricate pebbles and boulders to massive,

matrix-supported breccia (Fig. 2.11). We interpret these facies as streamflow and

debris flow deposits, respectively. Vertical grain size variations within the lower 380 m

of the granitoid breccia interval define two -110 m thick coarsening-upward sequences

and two -50 m thick fining-upward sequences (Fig. 2.10). These variations reflect

changes in the dominance of debris flow (coarser) vs. streamflow (finer) breccias and

record periods of fan progradation and quiescence. The upper -810 m of the granitoid

breccia interval is poorly exposed and occurs as low hills of rubble and grus. Local well

exposed intervals suggest that these deposits are mainly debris flow breccias; they

indicate an overall upward change from more distal to more proximal fan environments

throughout the granitoid breccia unit. Although breccias of this interval are dominated

by granitoid clasts, several other lithologies are found. The basal part of the unit fills a

55 m deep valley cut into the underlying basalt flows and contains abundant basaltic

clasts (Fig. 2.10). Clasts of basalt are compositionally similar to Miocene basalt flows

of the region and are also seen in the uppermost part of the Azucar Mine section. In

addition, clasts derived from Mesozoic volcanic strata are sometimes found in the

granitoid breccia interval and may form thin, monolithologic beds. In contrast to the

wide variety of Mesozoic volcanic debris found lower in the Azucar Mine section,

volcanic clasts from the granitoid breccia interval consist only of distinctive, hard, light

gray dacite-andesite which bears more affinity to lithologies at Daggett Ridge.

Imbricate fabrics within streamflow breccias of the granitoid breccia unit indicate

north-northeasterly to northwesterly paleocurrent directions (Fig. 2.9). Because rocks

of this interval are likely alluvial fan rather than alluvial plain deposits, these directions

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c A:

li CO CVJ II Î N (0 < j5 Ü If

m 6 iaii qi A i

m i': M

AM 6 m qi A1

p®CE 4 L

j-T ^i j-T r> 1 ' I ‘ I ‘ I ' I ' I o O O O o o o o o O O O o o o CO CD o 00 § CM SU 313W

Fig. 2.9. Stratigraphy, lithology/clast composition, and palecxurrent data for the Slash X Ranch formation at Azucar Mine and at Daggett Ridge. Key to clast-type abbreviations: Tv, Tertiary volcanic rocks, Tb, Tertiary basaltic rocks; Mg, Mesozoic granitoids; Mv, Mesozoic volcanic rocks of the Sidewinder volcanic series.

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0 60 W * 1200-

Mo*t>y

l _ #% P:(7-~ -^:

3 6 0 • 3

>4] <7

G m .Sh,

ÿzŸ/y/

G m /G I Î k

2 2 0 -

200 .pn^ I ■ y , ^ I Cl S P C B Cl 8 P C B Fig. 2.10. Detailed measured section of lower part of granitoid debris section at Azucar Mine. Facies designations after Rust and Koster (1984): Gm, clast-supported, commonly imbricate gravel with poorly defined subhorizontal bedding; Gms, muddy matrix-supported gravel without imbrication or internal stratification (note that in this paper we also use this designation for clay-poor deposits with a similar fabric); Sh, horizontally stratified sand; St, trough cross-stratified sand. Other facies abbreviations: G l, massive, clast-supported breccia; RA, Rock avalanche deposits.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 55

5'.-àiàfer

w m m m

Fig. 2.11. Photographs of different alluvial fan facies in granitoid breccia section at Azucar Mine. A. Imbricate fabrics in streamflow deposits. B. Debris flow deposits. C. Mixed streamflow and debris flow strata.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 5 6

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 57

are probably an accurate reflection of source terrane location. It would seem, therefore,

that the source of the majority of both the lower and upper basement-derived fan

deposits in the formation of Slash X Ranch at Azucar Mine was unextended basement

located to the southeast across the Kane Springs transfer zone. Given this

paleogeography, the upward change in clast type through the section from older volcanic

to dominantly granitoid could be due either to unroofing of the original basement

stratigraphy, or to changes in source terrane caused by lateral movement along the Kane

Springs transfer zone. The former interpretation is more likely based on present day

basement outcrop patterns, although the clast-type transition is remarkably abrupt.

Differences in facies and composition between strata of the formation of Slash X

Ranch at Daggett Ridge and near Azucar Mine suggest two different potential

paleogeographic reconstructions. One hypothesis is that rocks at each location were

deposited in different basins, separated by a divide formed by tilting of an upper plate

fault block. In favor of this model are the differences in the nature of the formation of

Slash X Ranch at Daggett Ridge and near Azucar Mine. Our second model considers

Daggett Ridge and Azucar Mine to have originally occupied the same basin, based on

structural reconstructions (Dokka and others, 1991). In this scenario, the differences in

composition reflect input from different sediment sources surrounding a single basin

(Fig. 2.5c). Mesozoic volcanic clasts were derived from the exposed footwall of the

breakaway (Daggett Ridge) and from unextended areas south of the Kane Springs

transfer zone (Azucar Mine). Granitoid debris was derived mainly from across the

transfer zone, whereas Tertiary volcanic detritus was probably eroded from a tilted

upper plate fault block to the northeast (Fig. 2.5c). Interfingering of the different

deposystems may be indicated by the similarity between older volcanic clasts in the

granitoid breccia interval at Azucar Mine and breccias of volcanic debris in the middle to

upper part of the section at Daggett Ridge.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 58

The paucity of hanging wall-derived Tertiary volcanic debris in the formation of

Slash X Ranch reflects both the development of significant cross-transfer zone drainage

systems and perhaps the destruction of the earlier regime by dissection of the hanging

wall of the breakaway into several smaller fault-bounded basins (Fig. 2.5c). Substantial

tilting of formation of Daggett Ridge rocks prior to Slash X deposition is consonant with

such an hypothesis. Given the tectonic setting of the basin, coarsening-upward

sequences of ~100 m thickness and fining-upward intervals of ~50 m thickness in

alluvial fan deposits of the formation of Slash X Ranch likely relate to incremental fault

movement, whereas coarsening-upwards sequences of 300 m and 1000 m thickness

were caused by major, fault-controlled reorganization of drainage systems (e.g.. Steel

and others, 1977; Heward, 1978).

At the southeastern end of Daggett Ridge, the formation of Slash X Ranch is

unconformably overlain by conglomerates of the formation of Stoddard Valley (Fig.

2.4) (Dokka, 1986). The formation of Stoddard Valley consists of at least 150 m of

conglomerate, breccia, and sandstone, and can be found along the southern margin of

the extensional terrane near the Kane Springs transfer zone. It is poorly exposed and

crops out as low hills composed of boulders of pre-Tertiary volcanic rocks, coarse

grained plutonic debris and minor schist. Based on the style and pattern of outcrop, it is

likely that these rocks are debris flow deposits derived predominantly from across the

Kane Springs transfer fault (Dokka, 1980). Equivalent rocks derived from the footwall

of the breakaway are likely buried beneath (Quaternary gravels in Stoddard Valley (Fig.

2.3). Debris flow deposits of the formation of Stoddard Valley range fi-om syn- to

posttectonic in timing (Dokka, 1980,1986). Stratigraphie relationships in Kane Wash

(discussed below) indicate that much of the presently exposed part of the unit was

deposited in a narrow, elongate basin adjacent to the Kane Springs transfer zone (Fig.

2.5d).

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Either before or shortly after deposition of the formation of Stoddard Valley, the

breakaway basin was again dissected by normal faults, and the Slash X Ranch

formation was tilted and extended such that the Daggett Ridge and Azucar Mine sections

became separated. By the time the posttectonic part of the Barstow Formation began to

accumulate in this area, earlier syntectonic units were exposed in the tilted fault block of

Daggett Ridge, and lower plate crystalline rocks had been unroofed in the Su Casa arch

(Fig. 2.5e). Strata of the Barstow Formation crop out on the southern flank of Daggett

Ridge and the northern side of the Su Casa arch (Dokka, 1980,1986,1989a). In the

central part of Daggett Ridge, these rocks unconformably overlie rocks of the formation

of Slash X Ranch with an angular discordance of 15-30°, whereas at the southeastern

end of the ridge they unconformably overlie fanglomerate of the formation of Stoddard

Valley. On the northern flank of Su Casa arch, gravels of the Barstow Formation were

deposited unconformably across the exhumed detachment, from which they were

derived (Fig. 2.12a) (Dokka, 1980). Rocks of the Barstow Formation fine rapidly

upwards to shales, marls, limestones and cherts of lacustrine origin (Fig. 2.12b), and

record infilling of the topography generated by regional extension. Limited paleocurrent

data from the Barstow Formation on the north side of Su Casa arch indicate northerly

dispersal directions, as does the northward thinning of coarser-grained intervals as they

interfinger with lacustrine shales and limestones (Dokka, 1980).

In conclusion, stratigraphie and structural relationships in the breakaway zone of the

Daggett terrane record a complex history of basin formation and dissection as the upper

plate of the terrane progressively broke up during extension. Uplift of the footwall of

the breakaway was relatively minor, judging by the present day occurrence of basement

lithologies similar to those of syntectonic rocks. Sediment dispersal patterns were

dominated by drainage from unextended areas across the Kane Springs transfer

boundary of the terrane. It is notable that similar dispersal patterns characterize the

extensional Homelen Basin of Devonian age in western Norway (Steel and others.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 60

Fig. 2.12. a) Photograph of Barstow gravels unconformably overlying detachment in Ord Mountain wash; b) Photograph of fluvial and lacustrine facies of the Barstow Formation on powerline road to Daggett Ridge.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 61

1977; Steel and Gloppen, 1980). In the Homelen Basin, tectonic transport of

depositional packages away from their point of origin by continued extension resulted in

onlap of successive sedimentary units on the detachment (Steel and others, 1977; Steel

and Gloppen, 1980). This overlapping fan geometry has also been recognized in the

strike-slip Ridge Basin of California, where it has been termed 'shingling* (Crowell,

1982). In the Daggett terrane of the Mojave Extensional Belt, shingling of individual

fan bodies has not been demonstrated; nevertheless, tectonic transport of older

syntectonic rocks northeastward along the detachment is evident based on structural

relationships and present day outcrop distributions (oldest rocks in northeast, youngest

in southwest) (Fig. 2.5). An important implication of this geometry of deformation and

sedimentation for breakaway basins is that, like strike-slip basins, total stratigraphie

thicknesses may be related less to the magnitude of vertical subsidence than to the

amount of lateral extension.

Upper plate fault basins

In contrast to proximal areas, upper plate extensional basins farther removed from

the breakaway zones of the Mojave Extensional Belt exhibit fairly straightforward early

Miocene structural and stratigraphie relations. In the Edwards terrane of the western

Mojave Desert, several simple half graben are visible at the surface and on seismic

reflection profiles (Fig. 2.13) (Dokka, 1989a). Similar structures are also observed in

the Cady Mountains (Dokka, 1986; Mathis, 1986; Ross and Dokka, 1990).

Sedimentation patterns in continental half graben are well studied (e.g., Alexander and

Leeder, 1987; Leeder and Gawthorpe, 1987; Frostick and Reid, 1987; Cavazza, 1989;

Mack and Seager, 1990; Scholz and others, 1990). Extensive alluvial fan-braid plain

deposits derived from the hanging wall dominate sedimentation in the basin, whereas

footwall-derived fans adjacent to major fault scarps tend to be both areally and

volumetrically small (e.g., Leeder and Gawthorpe, 1987; Cavazza, 1989; Denny, 1965;

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 62

West East B S'

1 Km

Fig. 2.13. Seismic reflection profile from western Mojave Desert, showing an extensional half graben and its syn- and posttectonic fill (after Dokka, 1989a).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 63

Crossley, 1984). Sedimentation along the axis of the basin is characterized by lacustrine

deposits or by fluvial systems, depending on whether the basin is internally or externally

drained (e.g., Leeder and Gawthorpe, 1987; DiGuiseppi and Bartley, 1991). Recent

models for the stratigraphy of half graben propose an overall coarsening-upward textural

trend for the basin fill (Mack and Seager, 1990; Blair and Bilodeau, 1988). Finer-

grained lacustrine and fluvial deposits are thought to typify sedimentation during active

extension, whereas coarse-grained prograding fan sequences characterize posttectonic

deposition (Mack and Seager, 1990; Blair and Bilodeau, 1988). A similar model has

been proposed for sedimentation in foreland basins (Heller and others, 1988).

Our studies of upper plate fault basins in the Mojave Extensional Belt have so far

dealt mainly with regional stratigraphie relationships and facies patterns. Few regions of

the Mojave Extensional Belt show the gross stratigraphie patterns predicted by the

coarsening-upward model of Blair and Bilodeau (1988). Instead, the Mojave

Extensional Belt exhibits a regional tripartite stratigraphie framework, consisting of (1)

pre- to early syn-extension volcanic deposits, unconformably overlain by (2)

syntectonic, basement-derived breccia and megabreccia, with local finer-grained

deposits, overlain by (3) a posttectonic fining-upward sequence of gravel, sand, shale,

and limestone. This pattern was first noted locally by Woodbume and others (1982),

who found that the Miocene stratigraphie record at some localities in the central Mojave

Desert began with a thick volcanic succession, and that a subsequent change to

predominantly fluvio-lacustrine deposition occurred ~22-19 Ma. Subsequent authors

recognized that the Miocene upper epiclastic section in the central Mojave could be

divided into syn- and posttectonic units in the Newberry Mountains (Dokka, 1986;

1989a; this paper) and Mud Hills (Dokka, 1989a; Dokka and others, 1988; Glazner and

others, 1989; Walker and others, 1990; Woodbume and others, 1990; Travis and

Dokka, in review, 1991). Several regional stratigraphie correlation charts, drawn sub

parallel to the extension direction, serve to illustrate the geographic extent of this

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 64

tripartite stratigraphy (Figs. 2.4,2.14,2.15). Each chart is arranged so that proximal

areas are shown on the left, whereas more distal sections are plotted to the right.

Localities shown on the correlation charts (Figs. 2.4, 2.14, 2.15) and discussed in the

text are plotted on Fig. 2.2.

Stratigraphie relationships similar to those of the Newberry Mountains are exposed in

the Cady Mountains to the east (Figs. 2.2,2.4). Andesitic and basaltic rocks

accumulated on eroded Mesozoic granitic basement in the western Cady Mountains ~24-

20 Ma, and were tilted by as much as 40° prior to deposition of middle Miocene

conglomerates and lacustrine rocks (Fig. 2.16) (Ross and Dokka, 1990; Dokka and

others, 1991). The southern and northern Cady Mountains expose the syn- to

posttectonic Hector Formation, which unconformably overlies older Miocene andésite

and rhyodacite lahars and breccias (Fig. 2.4) (Durrell, 1953; Dibblee, 1966; Woodbume

and others, 1982; Woodbume and others, 1974; Miller, 1980; Moseley and others,

1982; Glazner, 1988). Tilting of the volcanic section prior to Hector deposition is

constrained between 24.0±2.6 Ma and 21.6±1.1 Ma (Dokka, 1989a).

In the Bristol Mountains of the eastemmost Daggett terrane, the Tertiary section again

contains lower volcaniclastics and upper coarse, basement-derived breccias and

conglomerates (Fig. 2.4). However, this succession is not yet well dated. The base of

the section in the Bristol Mountains is marked by a thin unit of arkosic sandstone that is

similar to that found in the Cady Mountains (Woodbume and others, 1982; Dokka,

1986). These deposits are pre-extension weathering profiles and fluvial veneers.

Stratigraphie relations in the Waterman terrane also support the tripartite model,

although local conditions exerted additional control (Fig. 2.14). Tertiary upper plate

rocks are exposed as far west as the westem edge of the Hinkley Hills, where

unmetamorphosed limestone sits in low-angle fault contact (Mitchel detachment) on

lower plate rocks of the Mitchel Range-Hinkley Hills-Waterman Hills metamorphic core

complex (Fig. 2.17). Palynological analysis of this limestone revealed a number of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 65

O T 3 ü CO ■-S 0) O-

C*5 0 0) if

S i , CM CD OO O CM CM CM

-OCO JL 3 i i î ï l

Fig. 2.14. Miocene regional stratigraphy of the Waterman terrane. See Fig. 2.2 for localities. Key to abbreviations: Tj, Jackhammer Formation; Tmp, Pickhandle Formation; Tmmh, Mud Hills Formation; Tmb, Barstow Formation; Tmcf, Clewes Fanglomerate; Tmsc, Spanish Canyon Formation. Key to sources of data: 1, Dibblee, 1967; 2, Walker and others, 1990; 3, R. K. Dokka (unpublished data); 4, Woodbume and others (1990); Travis and Dokka (in review); 5, Dokka and Woodbume (1986).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 66

Q- yij— o v^ .

CO iinljljllllll'lllll i ii ,g: SSlilll : CM

.0 :

If o- o “ c^- I

Fig. 2.15. Miocene regional stratigraphy of the Edwards terrane. See Fig. 2.2 for localities. Key to abbreviations; Tniif, Fiss Fanglomerate (Upper Tropico Group); Tmgh, Gem Hill Formalin (Lower Tropico Group); Tmtl, Lower Tropico Group; Tmtu, Upper Tropico Group; Tsb, Saddleback Basalt; Trb, Red Buttes Quartz Basalt. Key to sources of data: 1, Dibblee, 1967; 2, Whistler, 1984; 3, Dokka and Baksi, 1989; 4, Woodbume and others, 1982; Woodbume, 1991.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 67

Fig. 2.16. Photograph of basement contact and overlying early Miocene volcanic strata in the westem Cady Mountains.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 68

extremely rare palynomorphs, mainly conifer pollen, along with specimens of

Compositae (possibly Ambrosial and Polemoniaceae pollen (R. A. Askin, written

communication, 1990). The Polemoniaceae suggest an age no older than late Miocene,

but may be a modem contaminant, whereas the Compositae indicate an age no older than

late Oligocene (R. A. Askin, written communication, 1990) and probably no older than

early Miocene (W. Gregory, personal communication, 1990). This contradicts previous

assertions that these rocks are pre-Tertiary (Walker and others, 1990). The occurrence

of probable middle Tertiary limestone in a belt along the proximal edge of the Waterman

terrane suggests that the early breakaway basin may have been the site of a large lake or

a series of smaller lakes, and has potentially important implications for models of fluid

interactions along the Mitchel detachment (cf. Chapin, 1990; Glazner, 1988).

Resting upon the Mitchel detachment farther to the east, in the Waterman Hills, are

undated rhyolite flows, tuffs and tuff breccias, overlain by basement-derived breccia,

conglomerate, sandstone and mudstone (Fig. 2.17) (Dokka, 1989a; Walker and others,

1990). These rocks have been correlated with early Miocene volcanic and syntectonic

sedimentary rocks in the Mud Hills, and with the Clews fanglomerate and Spanish

Canyon Formation at Alvord Mountain (Fig. 2.14) (Dibblee, 1967; Glazner and others,

1989; Walker and others, 1990). Similar rocks are exposed south of the Waterman

Hills, overlying the Mitchel detachment in the southern Mitchel Range (Fig. 2.17)

(Dokka, 1989a).

Miocene rocks of the Mud Hills, northeast of the Waterman Hills, clearly show a

threefold division of stratigraphy above the thin fluvial deposits of the Oligocene or

older Jackhammer Formation which form the local base of the section (Figs. 2.14,2.17)

(McCulloh, 1952; Dokka and Woodbume, 1986; Lambert, 1987). The Pickhandle

Formation (McCulloh, 1952; Dibblee, 1967) in the Mud Hills consists of 550-850 m of

mainly pyroclastic andésite tuff and tuff breccia (Travis and Dokka, in review, 1991).

The base of the Pickhandle Formation locally contains thick lenses of sandstone and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 69

Fig. 2.17. Composite geologic map of the Waterman Terrane highlighting upper plate crystalline basement (light shading), lower plate rocks of the Waterman Metamorphic Complex (medium shading), and syntectonic rocks of the early Miocene Mud Hills Formation (dark shading) (after Travis and Dokka, in review, 1991).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ’''ilylj/'p , - 2 ' ^ - W '

g;arag> 5%ak

x ; - ' ' Qo\ VBARSTOW

geology not Included 4 L Oo on this map

R2W I R1W RIWiRIE

5 MILES

5 KILOMETERS

SCALE

6o\ C ' n

TV 1% 1 N tv T IO N '1 %

^ Qo '=-‘\,^ ^ = y

îso^-Sl\ n

YERM a L

VBARSTOW

Included

R2W I R1W RIWtRIE

SMILES

5 KILOMETERS

SCALE

Fig. 2.17. contd.

EXPLANATION

Dashed where gradational or approximately located

Oo Qto Dashed where indefinite, dotted where concealed; ipteried wlure doubfful Ou, unJiff€/*miateds€dimtiUs Oa, albnitm Oo, oUtr aHuyium Ob, basaU Low-mgle noon ml fault (Mitchel detacbaest) Dashed where indefinite, dotted where concealed; UNCONFORUfTY queried where doubf/ul; nodes on hanging wall

10 Tsv Tv Tl Strike and dip of strata

Syp> lo post'exievioiuU aedimeDUry roc km 56 Pre- to eyn-extennonal vokcuc rocks, T s. spidastiessdimsHlary rocks Strike and dip of foliations Tsv, undifftrgHtiaUd vo/come and sadimtnlary rocks Tt, imruSTvt volcdiue rocks, mainly andtsùt Tv, votcanie rocks, mainly volcaniclasbc Trend and plunge of lineatioci UNCONFORMITY

'. u b . .M g d ' M qm . • Pzm . ■ 4.5 Ac'Tertigzyupperplaie buoneairoclu Ub, undifftratiaud upptr plaU bmtnuiu Mgd, gm w iorit*, dioril» (mapp*d or Maotoie) Mqm, auartt moKxoHiu 6.7 (mapptd ax Muoxoie) 7.8 Pzm. undiffxrtntiaUdimlwnorphic rocks (nxapptd as Palsoxde)

LOW ANGLE NORMAL FAULT (KtrrCHEL DETACHMENT)

Sources of map data

1.This sUJy 7. R. K. Dokka, unpubiishcd 2. DibUeon968) m apping(1985 1989) Pre*Teitiiry tower pUie reeka а.WooOtxjme and others (1980) 8.Dit)tmoe(1970) Mgd. granodicriis, dioriie 4 . MoCUlOh(1952) 9 . Dokka and Woodbume <1986) s. Santa Fo Mining, Inc., 10. Dokka (1989) (mappsd as Mtsosoie) unptifc*shod mapping (1957-1958) 1 1 .Lambert (198 7) wg, Wateman Mstamorpfde Con^Ux, б. 0kU0C(1967) 12. geology not indudod on map (protohlh PaUoxoie? mylotatixatioH xa/lyMiocsRs)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 2

conglomerate of granitic and dioiitic debris. These rocks may correlate with the Clews

fanglomerate at Alvord Mountain to the east (Figs. 2.1,2.14). The presence of these

basement-derived deposits low in the section in the Mud Hills attests to the potential for

local conditions to partially dominate over regional controls. Deposition of the

Pickhandle Formation in the Mud Hills is bracketed between 23.1 Ma and 18.1 Ma,

based on K-Ar age determinations on the underlying Lane Mountain quartz latite and an

overlying dacite flow (Burke and others, 1982). Partially overlying and interfingering

with the Pickhandle Formation in the Mud Hills is the Mud Hills Formation, a ~300 m

thick accumulation of syntectonic quartz monzonite breccia and megabreccia with

subordinate sandstone and shale which was deposited between ~19.8 Ma and -18.1 Ma

(Fig. 2.14) (Travis and Dokka, in review, 1991). The syn- to posttectonic Barstow

Formation unconformably overlies both Pickhandle and Mud Hills Formations and

consists of a fining-upward basin-fill sequence of conglomerate, sandstone, shale, and

limestone (Fig. 2.14). The Barstow Formation began to accumulate between 18.7 Ma

and 16 Ma and contains rocks as young as 13.4 Ma (Woodbume and others, 1990).

Syn- and posttectonic rocks of the Mud Hills were deposited in a core complex basin,

and the stratigraphie relations of the area are discussed further below.

In the Calico Mountains, ~6 km eastward along strike from the Mud Hills, local

conditions led to additional modifications of the regional stratigraphy (Figs. 2.14,2.17).

Deposition of volcaniclastic rocks around this early Miocene volcanic center

overwhelmed any other input. No basement-derived syntectonic sedimentary units are

observed, and the stratigraphie succession passes directly upwards from the volcanic

Pickhandle Formation to the largely posttectonic Barstow Formation (Fig. 2.14).

Our detailed knowledge of the Miocene stratigraphy of the Edwards terrane is less

complete than for those structural domains described above, but available data clearly

show a broad division between early volcanic rocks and later epiclastic deposits (Fig.

2.15). The angular unconformity separating these two stratigraphie divisions

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 73

corresponds to an episode of normal faulting and tilting bracketed between 21.1+0.5 Ma

and 20.3±0.7 Ma (Dokka and Baksi, 1989), and closely coincides with the time of rapid

unroofing of middle crustal rocks in the Harper Lake area to the northeast by detachment

faulting (Jones, 1990; Dokka and others, 1991, in press).

At Antelope Buttes and in the Rosamond Hills of the westem Edwards terane (Fig.

2.1), coarse clastic rocks of the Fiss fanglomerate (Upper Tropico Group)

unconformably overlie the volcaniclastic Gem Hill Formation (Lower Tropico Group),

from which they were derived (Fig. 2.15) (Dibblee, 1958,1967). Lithostratigraphic

correlations indicate an Oligocene to Miocene age for these units (Dibblee, 1967).

In contrast to the western Edwards terrane, the basal Upper Tropico Group in the

central part of the terrane (from Castle Butte to the Kramer Hills) includes lacustrine

shale and limestone, which are overlain by arkose and granitic fanglomerate (Fig. 2.15)

(Dibblee, 1967). This is the only area of the Mojave Extensional Belt in which Miocene

sedimentary rocks definitely exhibit the overall coarsening-upward trend predicted by

the model of Blair and Bilodeau (1988).

The Gravel Hills of the eastemmost part of the Edwards terrane occupy a similar

tectonic position to the Mud Hills in the Waterman terrane. Like this area, they expose a

lower sequence of predominantly pyroclastic rocks assigned to the Pickhandle

Formation and an upper, coarse-grained, basement-derived sedimentary succession

(Woodbume and others, 1982; Dibblee, 1967,1968). These units are separated by an

unconformity bracketed between 18.9+1.3 Ma and 16.54±0.4 Ma (Woodbume and

others, 1982; dates after Burke and others, 1982).

In summary, most upper plate fault basins and other extensional basin types of the

Mojave Extensional Belt exhibit a tripartite Miocene stratigraphy. Volcanic rocks of late

OIigocene(?) to early Miocene age overlie pre-Tertiary basement and thin, early Tertiary

fluvial deposits and weathering profiles throughout the region. The volcanic succession

is unconformably overlain by coarse, basement-derived clastic rocks associated with

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 4

extensional faulting which occurred ~21-17 Ma, and these rocks are in turn overlain by a

largely posttectonic, fining-upward succession of early to middle Miocene gravel, sand,

shale, and limestone. Important local variations include the early influx of basement-

derived debris (Mud Hills-Alvord Mountain), the overwhelming influence of volcanic

centers (Calico Mountains), and the overall coarsening-upward trends seen in the central

Edwards terrane.

Tranter zone basins

In areas of relatively small extension (e.g.. East African Rift, Gulf of Suez),

accommodation zones between en échelon half graben are the source of volumetrically

significant fans and fan deltas (Frostick and Reid, 1987; Crossley, 1984; Gawthorpe

and others, 1990). As extension increases, these structures develop into strike-slip fault

zones known as transfer zones (e.g.. Bally, 1981; Gibbs, 1984; Etheridge and others,

1984; Etheridge and others, 1985). Sediment dispersal across transfer zones may be of

great importance in the stratigraphie evolution of extensional belts (see above). The

nature of this dipersal is not simple, however, for the full suite of structures and

sedimentation patterns associated with strike-slip faulting is expected (e.g., Crowell,

1982; papers in Ballance and Reading, 1980; Biddle and Christie-Blick, 1985). In

addition, the juxtaposition of two extensional terranes (see Fig. 2.1) might be expected

to result in complex, changing patterns of sediment dispersal as connections between

adjacent basins are formed and destroyed.

In the Mojave Extensional Belt, we have examined sedimentary rocks adjacent to

two transfer boundaries: the Kane Springs transfer zone of the Daggett terrane and the

Lane Mountain transfer zone of the Waterman terrane (Fig. 2.2). In the vicinity of Kane

Springs in the Newberry Mountains, southwest-dipping basalts of the formation of

Minneola Ridge are unconformably overlain by more shallowly dipping to flat-lying

conglomerates of the formation of Stoddard Valley (Figs. 2.18, 2.19). The lower

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 7 5

section of the formation of Stoddard Valley near Kane Springs consists of massive,

matrix-supported conglomerates of Mesozoic volcanic detritus and subordinate granite-

quartz monzonite debris, which are interpreted as debris flow deposits derived from

unextended areas south of the Kane Springs fault (Dokka, 1980; Fig. 2.18). These

rocks are separated from a more gently dipping upper section of well bedded

conglomerates by a late eaiiy Miocene pyroclastic tuff unit (Dokka, 1980,1986).

Conglomerate and interbedded very coarse-grained sandstone of the upper section

exhibit fabrics indicative of both debris flow and braided stream deposition, and were

deposited in a mid-fan setting (Dokka, 1980). Two distinctive clast assemblages are

recognized within this interval: crystalline basement-derived materials and basement plus

basaltic debris of the formation of Minneola Ridge. Rocks containing the latter

assemblage are restricted to the eastern and northern part of the area shown on Fig.

2.18, and several measurements of imbricate clast orientations in these beds indicate

southeasterly paleocurrent directions. Strata bearing the basement clast assemblage dip

gently north, whereas those with mixed basement and Minneola Ridge clasts dip south

(Dokka, 1980). The two units are seen to interfinger about 1.5 km north of the main

wash shown on Fig. 2.18. The stratigraphie and structural relations described above

suggest the existence of a narrow, east-trending basin adjacent to the Kane Springs

transfer fault, into which alluvial fans shed basement debris derived from unextended

areas to the south and mixed clast material eroded from the crests of tilted fault blocks to

the north and northeast (Figs. 2.5d-e, 2.18) (Dokka, 1980,1986). This basin is

thought to have formed due to a component of oblique slip along the Kane Springs

transfer fault (Dokka, 1986).

Farther east, in the northeastern Rodman Mountains (Fig. 2.2), basalts equivalent to

the formation of Minneola Ridge are overlain by -220 m of sandstone, conglomerate,

and breccia containing clasts of quartz monzonite plus other crystalline basement rocks

and basalt. These sedimentary rocks form two sequences of 170 m and 50 m thickness

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 16

EXPLANATION Volcanic and Surficiat OeposKs Sodlm entary R ocks Basement Rocks

r o a r . .O f Mg . * • Younger Sedimeou Tmsv,/Of»»iflOo* of Stoddard YaUây Oal, oBitvium Mg, hornbUndt-diontt, graMê- Of, foAglomânu UNCONFORWTY iftiorQ moHiofiùt UNCONFORMfTY ■I.OTliIniII;'!!! ifTmns' -I |ÿJoalo o » o o k,'OoA V , » J niiiiiii;i;i!i Tmnsyormauo* of Msv. SideiviiuUr NtwUrry Spgs Voleanie S*ri*t Older Sedixnenn Goal. aJhMiM Oof, /anghnurait iM . Jmmr/OrmaiioHUit^MolaRidgt of

Cod (act Low-angle nomtal fault Dashtd wfurt gradational or apprOM/natrfy locaud Dashod wfurt ind^rito, dotttd whtrt eottctaJed; 10 qtaritd whtrt d o t é ^ ; nodts on hanging wall Strike and dip of atista

56 Dashtd whtrt iidtfinitt, dotttd whtrt eoncmltd; Strike ta d dip of foliatioDi qutritd whtrt dûub^

n o rth FOKM A JtO H OF SrODOABO VAU£Y

k«yilon« s . . t . ,

upper

Kona Sprinffs lauft

Fig. 2.18. Geologic map and idealized cross section of the Kane Springs area (after Dokka, 1986).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 77

Fig. 2.19. Photograph of well bedded breccias of basalt and basement clasts of the upper section of the formation of Stoddard Valley unconformably overlying basalts of the formation of Minneola Ridge in the vicinity of Kane Springs. View looking east.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 78

LOG DESCRIPTION INTERPRETATION Poorly sorted, mssslve, matrix- supported breccia of quartz monzonite cobbles In silty sand matrix. Wedga- shaped unit. Fades G ms.

2-90 cm thick, discontinuous bed* of 2 0 0 massive, graded, and cross-bedded sandstone and gravel. Composed of quartz monzonite (75%), basalt (20%), and MRPs (5%). Boarl-shaped unit. Fades Qm. Sh, St, G ms.

^'0C>0 I

1 5 0 -

Poorly sorted, massive, matrix- ‘r t e supported conglomerate of quartz C^v monzonite (36%) and basalt (24%) dasts In muddy-sllty-sandy matrix ■ ■'=435355*^ I (50%), Fades Gme. (Xmtalns sparse lenses of sandstone and bedded conglomerate (fades Sh. Gm). 100 - Tbbuiar-shaped unK. s m i l l

5 0 o:o■ yAAo.o%31 . .7f C..T- 2-60 cm thick, tabular beds of massive, graded, and horizontally laminated silty sandstone, coarse grained sandstone, and conglomerate. I Tabular-shaped unK. Fades Gm, Sh. Gm*.

1— I SI S a C o Fig. 2.20. Miocene stratigraphy along the Silver Bell Mine road, Rodman Mountains. Rocks depicted on this Figure overlie 1450 m of predominantly basaltic strata containing minor andésite and ~30 m of intercalated fanglomerate. Underlying the basaltic strata is 145 m of tuffaceous sandstone and fanglomerate. Measurements and descriptions by P. Frost and J, Lambert, unpublished data, 1985. See Figs. 2.7 & 2.10 for key to symbols.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 79

which coarsen upwards from thin, tabular to lenticular beds of normally graded

sandstone and granule conglomerate to massive, matrix supported cobble and boulder

conglomerate and breccia (Fig. 2.20). Distinctive Tertiary volcanic clasts, as well as

paleocurrent measurements on cross-bedded intervals within this section, indicate that

sediment dispersal patterns were subperpendicular to extensional axes, across the Kane

Springs transfer fault (Dokka and Baksi, 1986).

The Lane Mountain transfer zone separates the highly extended Waterman terrane

from unextended areas to the northwest (Fig. 2.17) (Dokka, 1989a). In the northern

Calico Mountains, the Lane Mountain fault separates moderately tilted, normal fault-

bounded blocks containing lower Miocene volcanic strata of the Pickhandle Formation

and pre-Tertiary metamorphic rocks from unextended pre-Tertiary rocks across a

subvertical contact (Fig. 2.17). To the west, the fault is truncated by the right-slip

Calico fault of late Cenozoic age. The offset strand of the Lane Mountain fault is

inferred to lie in Fossil Canyon in the western Mud Hills based on restoration of slip

along the Calico fault (Dokka, 1983b) and the distribution of syn- and posttectonic strata

of the Pickhandle, Mud Hills, and Barstow Formations (Figs. 2.17, 2.21) (Dokka,

1989a; Travis and Dokka, in review). No Pickhandle or Mud Hills Formation rocks are

seen west of Fossil Canyon, and the overlying, posttectonic part of the Barstow

Formation thins across the area (Woodbume and others, 1990). Despite these

indications of a transfer zone in the western Mud Hills, detailed studies of syntectonic

sedimentary rocks of the region yield no sedimentological evidence for a basin boundary

(Travis and Dokka, in review). Only during deposition of the upper part of the Barstow

Formation did the transfer zone manifest itself, in the form of easterly sediment dispersal

patterns (Woodbume and others, 1990).

To conclude, our studies to date of sediment dispersal near transfer boundaries of

the Mojave Extensional Belt confirm the importance of these tectonic elements in

supplying erosional debris to extending basins, but also serve to highlight their

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 80

Fig. 2.21. Geologic map of the Mud Hills highlighting the Mud Hills Formation (after Travis and Dokka, in review, 1991).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. \ m jJAM S PLAIN

Titibu CANYtJN ; s r ^

^iTmbuSOTpst %

-■Æ7 Ttm

^ sf' */ >y \ ^ Q o a ^

N>, _y s) An* • T ^ IT -L V ^ : / Qoa 60''X \ "J A TtC / y Qa

./ Mgd r '

WATERMAN H IU S ur<- 35°0'00" 1 MILE 8

1 KILOMETER

SCALE-.

WIUJAMS PLAIN

TptOa

mbu^%9)

Tpst GO \ 1,17 •nxn 4L ( 60 y J'' ”5> '\i“/ . \

^Ttc %\

3 5 * 0 '0 0 " 1 MILE

METER

Fig. 2.21. contd.

illl II I I

0I0Z0N30 oiozosaw

AyvNuaivno AHVIiWEL 3 UJ i L s llll .G S n 0 1 I 0

III!:

9U70Oiyi 7U900\yiJ0 m037Vi9J^ ( j o ) puv ouyootnoiJ 9U9X>8; i q j o D tr c v jn f

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 83

complexity. Simple sediment transport patterns peipendicular to extensional axes were

observed near the Kane Springs transfer zone, but such patterns were not observed

along the Lane Mountain transfer zone, where the basin boundary neither supplied

erosional debris (during active extension) nor acted as a sediment trap. Lateral offset of

clast lithologies from their source areas was not observed at either location, perhaps due

to the lack of suitably distinctive source terrane lithologies. Future studies will focus on

other transfer boundaries of the Mojave Extensional Belt, where we expect to find the

distinctive patterns of sedimentation commonly associated with strike-slip faults (e.g.,

Crowell, 1982; papers in Ballance and Reading, 1980; Biddle and Christie-Blick,

1985), as well as geometries unique to extensional terranes.

Breakoff basins

Antithetic faulting in the distal part of extensional terranes may create either

asymmetric basins oriented opposite to those of the majority of the terrane, or symmetric

graben. At Alvord Mountain, in the breakoff zone of the Waterman terrane, the base of

the Tertiary section exposes the Clews fanglomerate of presumed early Miocene age

(Fig. 2.14) (Woodbume and others, 1982; Dokka and Woodbume, 1986; Byers,

1960). Two phases of deposition have been recognized in this unit: northerly-derived

braid plain deposits in the lower part of the section are overlain by easterly-derived

deposits of a debris flow-dominated alluvial fan system (Fillmore and Miller, 1991).

Fillmore and Miller (1991) speculate that the upper alluvial fan deposits were derived

from the footwall of a NW-striking, W-dipping (antithetic) normal fault associated with

regional extension. The Clews fanglomerate is unconformably overlain by the volcanic

Spanish Canyon Formation, a correlative of the Pickhandle Formation (Fig. 2.14)

(Woodbume and others, 1982; Byers, 1960). The Spanish Canyon Formation is in tum

unconformably overlain by the Barstow Formation (Woodbume and others, 1982;

Byers, 1960). Deposits of the Clews fanglomerate may correlate with basement-derived

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 84

debris in the lower part of the Pickhandle Formation in the Mud Hills to the west. If

these correlations and the presumed age of the Clews fanglomerate are correct, the unit

may record an uncharacteristically early influence of extensional processes on surface

topography in the breakoff region. The apparent lack of syntectonic basement-derived

materials overlying the Spanish Canyon Formation at Alvord Mountain indicates that

upper plate faulting associated with the main period of extension and detachment faulting

in the Mojave Extensional Belt had little affect on this part of the breakoff zone.

Core complex basins

Metamorphic core complexes (Davis and Coney, 1979) have long been recognized

in association with detachment-dominated extensional orogens. These complexes

typically consist of a deformed metamoiphic core that lies beneath a carapace of

extended, unmetamorphosed to weakly metamorphosed rocks; a warped, brittle-ductile

shear zone separates the two tectonometamorphic assemblages. Petrologic, structural,

and thermochronologic studies suggest that metamorphic core complexes represent

middle crustal rocks exhumed along detachments faults (e.g., Dokka and others, 1986;

Jones, 1990); however, the structural development of the detachment faults remains

controversial. Many workers favor models which explain the arched geometry of core

complexes as a result of the isostatic response of the lower plate to unloading by tectonic

denudation (Fig. 2.22a) (e.g.. Lister and Davis, 1989; Spencer, 1984). Block and

Royden (1990) pointed out that these models cannot explain why core complexes form

positive topographic features, and showed that simultaneous isostatic uplift and lower

crustal flow can produce the observed geometries (Fig. 2.22b). Development of

metamorphic core complexes with short topographic wavelengths by isostacy-driven

arching is difficult to envisage unless the flexural rigidity of the lithosphere is very low.

Some short-wavelength core complexes may owe their geometry to late-stage, high-

angle normal faults which cut earlier detachments (Fig. 2.22c) (e.g., Holt and others.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 85

a) Simple bostaik rebound ( 0 4 . Sper>cer, 1984; b) Lower crustal flow (Block and Royden. 1990) c) Later high angk)le______normal fault_ (e.g.. Dokka Lister and Oavis. 1990} e ta l. 1^ : H oiietal. 1986) hdçioedUtÂ kteçmtiUuH

d) OriginaJ detachm ent morphopiogy «} RoSng hinge (Wernicke end Axen. 1968: Buck. 1908; Hamitton, 1988)

(w0^^ erp eantniCtdjftdt»"?) W ed o m #

Fig. 2.22. Models for the development of metamorphic core complexes.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 86

1986; Dokka and others, 1988; Dokka, 1989a). In some cases, the arched geometry of

core complexes may reflect the original shape of the detachment (Fig. 2.22d). This may

be the case for the Su Casa arch of the Daggett terrane (Fig. 2.3), which may be a

shallow level analog for a metamorphic core complex. Debate on the feasibility of low-

angle normal faults, and the observation that modem, large-scale, seismogenic faults

have medium to steep dips (Jackson, 1987), have focussed recent attention on

alternative models of crustal extension and core complex development. Several workers

have proposed that detachment faults initially form with steep dips, but are subsequently

rotated to gently dipping attitudes due to isostatic compensation for unloading during

extension (Fig. 2.22e) (Wernicke and Axen, 1988; Buck, 1988; Hamilton, 1988).

Although the general structure, metamorphic petrology, and thermochronology of

metamorphic core complexes is well studied, the surface history of core complex

development is, with few exceptions (e.g., Travis and Dokka, in review; Miller and

John, 1988), largely unstudied. As discussed below, rocks of the Mojave Extensional

Belt yield data on events both at the surface and in the lower plate which we can

correlate closely in time and space. These data allow us to trace the history of core

complex development and to constrain models for the tectonic evolution of the area.

Greenschist to granulite facies rocks of the Waterman Metamorphic Complex (Henry

and Dokka, in press) in the Mojave Extensional Belt are exposed in the Mitchel Range-

Hinkley Hills-Waterman Hills of the Waterman terrane, and near Harper Lake in the

Edwards terrane (Fig. 2.1) (Dokka and Woodbume, 1986; Dokka and others, 1988;

Dokka, 1989a; Dokka and others, 1991). These rocks contain a record of several

Mesozoic tectonometamorphic events, as well as the Miocene extensional orogenesis

which is of interest here (Henry and Dokka, in press; Henry and others, 1989; Henry

and Dokka, 1990; Dokka and others, 1991; Dokka and others, in review, 1991). A

well-developed early Miocene mylonitic fabric is superimposed on older structures in

rocks of the Waterman Metamorphic Complex below the Mitchell detachment of the

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 87

Waterman terrane and in the Harper Lake district (Dokka, 1989a). These ductile fabrics

are overprinted by slightly younger early Miocene-age brittle faults, joints, and

extension fractures filled with silicic dikes (Dokka, 1989a). Petrological data indicates

that lower greenschist facies metamorphism associated with this Miocene detachment

faulting and mylonitization developed at 3(X)-350°C and 3-5 kbar (Hemy and Dokka, in

press). Thermochronologic studies show that rocks of the Waterman Metamorphic

Complex cooled rapidly from near 350° C to below 100° C approximately 20 Ma as a

result of exhumation by detachment faulting (Jones, 1990; Dokka and Baksi, 1988;

Dokka and others, 1991, in review).

In the Waterman terrane, early Miocene, basement-derived breccias and overlying

finer-grained units exposed in the Mud Hills record events at the earth's surface during

unroofing of the Waterman Metamorphic Complex in the Waterman Hills -4.5 km to the

southwest (Fig. 2.17). Because the stratigraphy and sedimentology of these rocks is

addressed in detail elsewhere (Travis and Dokka, in review, 1991), we present only a

summary of the data and our interpretations below.

The Tertiary sedimentary record in the central Mojave Desert begins with

widespread, thin weathering profiles and fluvial deposits of the Oligocene or older

Jackhammer Formation (McCulloh, 1952; Dokka and Woodbume, 1986). The first

evidence of early Miocene deposition in the Mud Hills area is the local accumulation of

up to 200 m of fine- to coarse-grained, crystalline basement-derived sediments, basalt,

and tuff at the base of the Pickhandle Formation (Fig. 2.21). Walker and others (1990)

identified the provenance of the basement-derived rocks in this section as Alvord

Mountain, then northeast of the Mud Hills. The subsequent accumulation of

voluminous, predominantly pyroclastic volcanic rocks of the Pickhandle Formation

attests to the importance of volcanic activity during the early stages of development of

the Mojave Extensional Belt. The extent and geometry of the basin at this time are.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 88

however, difficult to define. It is not until deposition of the overlying Mud Hills and

Barstow Formations that we can recognize a distinct topographic and structural basin.

The Mud Hills Formation is a ~300 m thick unit of syntectonic breccia and

megabreccia with subordinate sandstone and shale that accummulated in a NW-trending

half graben (Travis and Dokka, in review, 1991). The unit both overlies and

interfingers eastward with volcaniclastic rocks of the Pickhandle Formation and is

unconfonnably overlain by the Barstow Formation, most of which was deposited

subsequent to regional extension (Figs. 2.21,2.23). The Mud Hills Formation contains

five mappable members (Fig. 2.21), which were deposited between -19.8 Ma and

-18.1 Ma (Travis and Dokka, in review, 1991). Several of the members are bounded

by angular unconformities, and most units thicken to the south (Fig. 2.21). Rocks of

the Mud Hills Formation are interpreted as rock avalanche and debris flow deposits,

with subordinate alluvial and lacustrine strata. Paleocurrent data from finer-grained

intervals indicate that basement debris was derived from a southern source early during

the accumulation of the unit (19.8-18.7 Ma), but that a northern source became

dominant after -18.7 Ma (Fig. 2.24). The presence of rock avalanche deposits, together

with southward-thickening geometries, intraformational unconformities, and the

paleocurrent data is considered good evidence that the Mud Hills Formation was

deposited in an active NW-trending half graben, with a major boundary fault located to

the southwest. Because southerly-derived breccias of the Mud Hills Formation are

petrologically distinct from equivalent rocks exposed above the Mitchel detachment in

the Waterman Hills (cf. Walker and others; Travis and Dokka, in review, 1991), this

fault must have been located between the Mud Hills and the present-day Waterman Hills

(Travis and Dokka, in review, 1991).

The Barstow Formation in the Mud Hills consists of 700-800 m of gravel, sand,

shale, and limestone (Fig. 2.23) (Woodbume and others, 1990). The unit

unconformably overlies the Mud Hills Formation on the north limb of the Barstow

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 73 ■DCD O Q. C g Q.

■D CD 3 OQ C/) C/)

■D8 If II

CD ! l Mud Hills (Barstow syndino) Mud Hills (north-central) Calico Mountains South North» West East 3. 3 " l3.4 t0 2 M a CD il CD (Upper Mmbti) VO ■D 3 BARSTOW FORMATION (undHefwUsded) O 16^i0.3 Q. C

a S' O Conghmèf^ ie.1±0.5 Ma 3 i (Middle U m im ) ' ((M dongbmerete -tmlh) ■o •*• •• ? O c MUDHULSFORMATION ^ CL .6tO.?Ma PCKHANDLE FORMA VON &

co ■o I CD

(/)(g I o' 3 ty> I 9 0

Syncline in the northern Mud Hills (Fig. 2.21), but likely interfingers with the

uppermost Mud Hills Formation in the subsiuface to the south (Fig. 2.23).

Sedimentary rocks of the Barstow Formation fine upwards from alluvial fan gravels to

sandstone, shale, and limestone of marginal lacustrine origin. Barstow Formation rocks

exposed in the northern Mud Hills exhibit north-to-south dispersal patterns, whereas

those exposed on the southern limb of the Barstow Syncline were derived from the

south (Fig. 2.24). In the southern Mud Hills, evidence for a major change in the nature

of the southern source terrane at ~16.5 Ma is provided by the first appearance of clasts

of mylonite and other metamorphic rocks of the Waterman Metamorphic Complex in

gravel and sandstone of the lower Barstow Formation (Owl Conglomerate).

Early Miocene sedimentary events in the Mud Hills correspond closely in time with

the uplift of the lower plate in the Waterman Hills and Mitchel Range discussed above.

Lower plate rocks now exposed in the Waterman Hills cooled rapidly from 350°-300°C

to below 100°C near 20 Ma, about the time at which deposition of the Mud Hills

Formation began. Pressure constraints from metamorphic petrology indicate that rocks

of the shear zone occupied depths of 10-17 km just prior to extension (Henry and

Dokka, in press; Henry and Dokka, 1990), suggesting an equilibrium geothermal

gradient of 20°-35°C/km before extension and a final burial depth of ^ km at ~20 Ma.

The occurrence of clasts of mylonite and other metamorphic rocks in southerly-derived

gravel and sandstone of the lower Barstow Formation provides evidence that lower plate

rocks of the Waterman Hills were exposed at the surface by -16.5 Ma.

Integration of data from the upper and lower plates of the Waterman terrane provides

new constraints on models for extension of this area. These data support tectonic

models for the region that invoke extension by low-angle, crustal-scale normal faulting

(Figs. 2.22a-d), but cannot further differentiate between proposed mechanisms of core

complex development. Alternative models for development of the basin adjacent to the

evolving Waterman Metamorphic Complex based on the concepts of Dokka and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 91

WEST EAST

Coon Canyon Rainbow Canyon Owl Canyon Solomon Canyon

n=4 n=5 n=5 I n=7 i i

n=15 V I n=6 n=8 n=12

n=17 n=24 n=6

Fig. 2.24. Paleocurrent directions measured in the Mud Hills Formation and lowermost Barstow Formation, a) Pebble/cobble imbrication in Lower Breccia Member near Coon Canyon: n=12; vector mean=334°; b) Pebble/cobble imbrication in Lower Breccia Member near Owl Canyon: n=17; vector mean=059°; c) Cobble imbrication in Owl Conglomerate in Owl Canyon: n=5; vector mean=200°; d) Minor fold axes in Upper sandstone Member in Owl Canyon: Dots, fold axes; Semicircular arrows, sense of asymmetry; Black arrow, generalized slip line; e) Cross bedding in Lower Breccia Member: n=24; vector mean=009°; f) Cobble imtnication in Owl Conglomerate in Solomon Canyon: n=5; vector mean=200°; g) Cross bedding in Solomon Sandstone Member in Solomon Canyon: n=6; vector mean=062°; h) Cross bedding in Barstow Formation east of Copper City Road: n=4; vector mean=265°; i) Imbrication and cross bedding in Pickhandle Formation upper tuff breccia and sandstone tongue east of Solomon Canyon: n=7; vector mean=209°; j) Cross bedding in "Red T u ff marker bed of Pickhandle Formation middle tuff breccia tongue east of Solomon Canyon: n=8; vector mean=285°; k) Pebble/cobble imbrication in Lower Breccia Member east of Solomon Canyon: n=6; vector mean=284° (after Travis and Dokka, in review, 1991).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 2

Watomwi KIb MudHlb WlKams P bin iffiptfpntttmjnuoeifs.

d tta c hm mit

FottiMfkmdM

l£G£NO

I ^ I PicM\ndl« FonMfon 6VM0IV Formafton Ir :1 upper pkW(%y#mkn#b#wm#ni ShMTtooerocM Mud HM Formtlcn

Fig. 2.25. Schematic models for the early Miocene tectonic development of the Mud Hills, a) based on the tectonic model of Dokka and Woodbume ( 1986); b) based on the model of Dokka and others (1988) (after Travis and Dokka, in review, 1991)

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 93

Woodbume (1986) and Dokka and others (1988) are shown in Fig. 2.25. Bartley and

others (1990) proposed a "rolling hinge" model of extension by high-angle, crustal-scale

normal faulting for the Waterman terrane on the basis of structural evidence. The

"rolling hinge" model involves the sequential removal of fault-bounded slices from the

hanging wall of the detachment (Wernicke and Axen, 1988; Buck, 1988; Hamilton,

1988) (Fig. 2.22d). This model implies that sedimentary rocks originally deposited in

the same basin should now be exposed in different upper plate fault blocks. Data from

the Waterman terrane do not support the "rolling hinge" model of extension for this area,

because upper plate rocks now exposed in the Waterman Hills exhibit petrology and

sediment dispersal patterns different from correlative strata in the Mud Hills. Structural

evidence for both extension and contraction of shear zone rocks in the Waterman Hills

presented by Bartley and others (1990) may indeed reflect flexural rotation of the

detachment in response to unloading, but the magnitude of that rotation is unknown.

The surface history of core complex development recorded in rocks of the Waterman

terrane results from the simplest of multiple potential scenarios for core complex

evolution. For example, with continued extension a core complex basin might develop

into a secondary breakaway basin and undergo extensive fragmentation such as seen in

the proximal part of the Daggett terrane. Alternatively, one might hypothesize that

incisement of the detachment into the lower plate (Lister and Davis, 1989) could transfer

the core complex to the upper plate as an extensional nappe and remove it as a significant

surface feature. Integration of detailed structural, metamorphic, thermochronologic, and

stratigraphic-sedimentologic data is necessary to unravel the complexities of core

complex basin evolution.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 4

DISCUSSION

Paleogeographic development of the Mojave Extensional Belt

Figure 2.26 depicts a simple model for the paleogeographic evolution of the Mojave

Extensional Belt based on data presented above and previous work. The early Cenozoic

Mojave Desert was a highland which supplied sediment to neighboring regions (Hewett,

1954; Nilsen and Clarke, 1975) (Fig. 2.26a). Weathering profiles beneath the basal

Tertiary contact and local, thin fluvial deposits constitute the entire stratigraphie record

of the period. Relief was generally low, but valleys up to 150 m deep and several

hundred meters wide have been identified in the western Cady Mountains (Dokka and

others, 1991; Ross and Dokka, in prep., 1991).

Opening of the Mojave Extensional Belt was preceded by extensive basaltic to

rhyolitic volcanism which began near the beginning of the Miocene (Dokka, 1986,

1989a). Evidence for faulting of this age is found only locally and the volcanic rocks

may have been deposited in broad sag basins similar to those identified in the Rio

Grande Rift (Ingersoll and others, 1990). The observation that the most extensive and

volumetrically important volcanic activity in the Mojave Extensional Belt occurred within

a few million years prior to faulting may imply that detachment faulting and the surface

manifestations of extension were preceded by related events at depth. A discussion of

these relationships is, however, beyond the scope of paper and must await better

documentation of the volcanic succession.

Volcanism dominated deposition until ~20 Ma, when normal faulting and block

rotations accompanying regional detachment faulting resulted in tilting of the volcanic

succession and the accumulation of coarse, basement-derived clastic rocks. Rock

avalanche, landslide, and alluvial fan breccias dominate the sedimentary record at most

locations, although lacustrine deposits may have been more widespread in some parts of

the Waterman and Edwards terranes. Rough calculations of rock avalanche drop heights

from estimated volume and runout distance, based on empirical relationships given in

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 5

A*I

fîi - ® ° sSeô -? liJi ■ lâ till-?i l ï l t içll Sis il §11 Egas ââissêU S ë liâ is liJ IHi 1 lAII ilHffiiluiil

Fig. 2.26. Conceptual model for the tectonostratigraphic development of the Mojave Extensional Belt and other detachment-dominated extensional orogens.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 6

Hsü (1975), indicate possible fault scarp relief of several hundred meters. The

syntectonic period saw the development of several fault-bounded basins within the

Mojave Extensional Belt, most of which were probably isolated from each other and

received erosional debris only from local sources. Proximal basins were tilted and

dissected into smaller sub-basins as extension proceeded, whereas more distal basins

record a simpler history. One might speculate that this difference is due to a concomitant

decrease in the amount of extension within the upper plate as its thickness increases

away from the breakaway.

Major faulting appears to have ceased by ~17 Ma (Dokka, 1989a), after which the

Miocene sedimentary record is one of basin infill by a fining-upward sequence of

gravel, sand, shale, and limestone. Widespread lacustrine deposits in this section

suggest the continuation of dominantly interior drainage patterns, although adjacent

basins may have amalgamated as they became filled. During the latter stages of regional

extension and early postextension times, lower plate rocks exhumed by detachment

faulting became exposed in arched domes and metamorphic core complexes. In the

Waterman and Edwards terranes, the formation of northwest-trending metamorphic core

complex ranges divided the Mojave Extensional Belt into two major basins or groups of

basins, the Barstow and Boron Basins (Dokka and Woodbume, 1986; Dokka, 1989a;

Dokka and others, 1991). In the Daggett terrane, lower plate rocks exhumed from

shallower depths formed the Su Casa arch which, together with the tilted fault block of

Daggett Ridge, partially separated the Barstow Basin from the Stoddard Valley area to

the west and southwest. This posttectonic topography remains visible to this day, with

modifications the result of strike-slip faulting and block rotations about vertical axes

associated with development of the late Cenozoic Eastern California Shear Zone (Dokka

and Travis, 1991a).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 9 7

Textural evolution o f the basin fill

Several authors recently have proposed that widely distributed, coarse-grained,

progradational facies characterize the posttectonic stage of basin fill in both foreland and

rift basins, whereas syntectonic sediments are predominantly finer-grained lacustrine or

axial-fluvial deposits, with transverse fan systems restricted to the area immediately

adjacent to the basin margins (Mack and Seager, 1990; Blair and Bilodeau, 1988; Heller

and others, 1988; Heller and others, 1989). These published models contrast with

much of the data from basins of the Mojave Extensional Belt, which indicate that the

coarsest sedimentary rocks were deposited concurrent with regional extension and that

posttectonic detrital rock units fine upwards overall. These differences in textural trends

between the models and examples of Blair and Bilodeau (1988) and Mack and Seager

(1990) and our data from the Mojave Extensional Belt may stem in part from differences

in (1) local style of sedimentation; (2) basin dimensions; and (3) the evolution of

drainage patterns. Local climatic variations may also be important, but are difficult to

assess.

Episodic fault activity has long been associated with the development of coarsening-

upward depositional sequences (e.g., Heward, 1978; Steel and Wilson, 1975; Steel,

1976). These vertical textiu’al trends are thought to be controlled by the interplay

between uplift rate and erosion rate, and by differences in the response time of

prograding fan systems v.ï. axial-fluvial and lacustrine systems to rapid subsidence

(Steel and others, 1977; Heward, 1978; Leeder and Gawthorpe, 1987). Blair and

Bilodeau (1988) and Mack and Seager (1990) have applied these concepts to extensional

basins at a much larger scale; i.e., to cyclothems measuring hundreds to thousands of

meters thick and representing periods of millions of years. Within the syntectonic

section in the Mojave Extensional Belt, lacustrine deposits are dominant only in the

central Edwards terrane and along the southwestern edge of the Hinkley Hills in the

Waterman terrane; coarse-grained rocks are more common at most locations. Small to

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medium scale syntectonic coarsening-upward sequences (tens to hundreds of meters

thick) have been identified in some areas (e.g., this paper; C. J. Travis and R. K.

Dokka, manuscript in review, 1991), but large-scale coarsening-upward of syntectonic

(to posttectonic?) units is recognized only in the central Edwards terrane (Fig. 2.12).

One possible reason for the apparent paucity of syntectonic fine-grained rocks and

overall coarsening-upward trends in the Mojave may be the volumetric importance of

syntectonic rock avalanche deposits at the better studied locations, which would

overwhelm lacustrine deposition and obscure slow progradational trends.

While it does seem likely that low uplift:erosion rate ratios and subsidence rates favor

the development of progradational coarsening-upwards sequences between periods of

active fault movement, we propose, on the basis of our observations in the Mojave

Extensional Belt, that these depositional systems may equilibrate fairly quickly (<1

m.y.?) after fault movement ceases or slows. It is possible, however, that the difference

in scale between basins of the Mojave Extensional Belt (<10 km wide) and those used in

the studies cited above (IQ's of km in some cases) is partially responsible for differences

in our results.

Finally, the evolution of drainage patterns during basin development may strongly

affect grain size trends. Posttectonic fining-upward is caused by the

erosional/depositional destruction of topography as source areas retreat and basins

become full (e.g.. Steel and others, 1977; Heward, 1978; Steel and Wilson, 1975; Mack

and Rasmussen, 1984). Such a trend might only be expected, however, in the case of

internally drained basins similar to those commonly formed during extension. If basins

coalesce as a result of overfilling or erosion of basin divides, it is likely that the

development of exterior drainage patterns will preclude such trends. Development of

throughgoing axial drainage by the Colorado River in the southern Basin and Range

province has resulted in coarse-grained, posttectonic fluvial strata overlying fine- to

coarse-grained syntectonic deposits of the earlier, interiorly drained basins (Eberly and

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Stanley, 1978; DiGuiseppi and Bartley, 1991). DiGuiseppi and Bartley (1991) present

a lucid discussion of the potential effects of changing drainage patterns on extensional

basin stratigraphy.

Comparison of Mojave Extensional Belt stratigraphy with other extensional terranes

If the tripartite nature of the regional stratigraphy of the Mojave Extensional Belt and

the local stratigraphies associated with individual tectonic elements of the belt are

fundamentally related to extensional processes, one might expect to observe similar

stratigraphie architectures in other extensional zones. Detailed studies of local tectono-

stratigraphic relationships are sparse, and relevant examples are given in the appropriate

sections above. Perusal of the geologic literature suggests that parts of the regional

tripartite model of stratigraphy may be applied beyond the confines of the Mojave

Extensional Belt, in the Tertiary extensional terranes of the southern Basin and Range

and elsewhere. In southwestern Arizona, voluminous late Oligocene to early Miocene

rhyolitic to andesitic volcanic rocks with subordinate coarse- and fine-grained

continental deposits are found in outcrop and in the subsurface (e.g., Eberly and

Stanley, 1978). These rocks were steeply tilted prior to 16.4 Ma and are unconformably

overlain by middle Miocene fanglomerate, sandstone, and lake beds (Eberly and

Stanley, 1978). A second episode of tilting and faulting occurred -13-10 Ma and

resulted in the deposition of both coarse-grained elastics and thick evaporites in interior-

drained fault basins. With the waning of fault activity -10 Ma, these basins filled and

coalesced and exterior drainage patterns began to develop (Eberly and Stanley, 1978),

thus preventing the development of a fining-upwards posttectonic sequence similar to

that in the Mojave Extensional Belt. Other areas of the Basin and Range apparently

exhibit similar stratigraphie relationships. Rhyolitic to andesitic volcanism shortly

predates faulting, tilting, and deposition of syntectonic sedimentary rocks in

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southeastern California (e.g., Crowe, 1978), the Great Basin (e.g., Proffett, 1977;

Oldow and others, 1989), and the Rio Grande rift (e.g., Seager and others, 1984).

Comparisons to basins further afield have mixed results. An early volcanic stage is

not seen in the few published examples we have been able to access of basins associated

with detachment-dominated extensional belts containing metamorphic core complexes -

the Spanish Guadalquivir Basin (Rfos, 1978; Ager, 1980) associated with Betic-Rif

Neogene extension (Doblas and Oyarzun, 1989; Platt and Vissers, 1989), and Devonian

basins of Western Norway (e.g.. Steel, 1976). Nor is a discreet volcanic stage

observed consistently in other extensional basins, although volcanism is associated with

rifting in many of them. A volcanic stage is seen in the Punta del Este Basin of Umguay

(Stoakes and others, 1991) and the Midcontinent Rift System (e.g., Woelk and Hinze,

1991). A discreet early volcanic stage is not seen, however, in the North Sea (e.g.,

Ziegler and van Hoorn, 1989), the Bass and Otway Basins (Williamson and others,

1987), the East African Rift (e.g., Rosendahl, 1987), the Gulf of Corinth (Ori, 1989),

the Gulf of Suez (e.g.. Said, 1962), the east coast USA Triassic rift basins (e.g.,

Manspeizer, 1985), or the Gulf Coast USA (e.g., Scott and others, 1961). Upward

fining of the overall basin fill is present in several extensional basins worldwide

(although the precise timing of grainsize variations relative to tectonic activity is not

known). The best examples of this trend are in the Triassic basins of the east coast USA

(e.g., Manspeizer, 1985), the Bass Basin of Australia (Williamson and others, 1987),

and the Punta del Este Basin offshore Uruguay (Stoakes and others, 1991).

In summary, we have had mixed success in applying the tripartite model of regional

stratigraphy outside the Mojave Extensional Belt; only the Punta del Este Basin offshore

Uruguay exhibits all three divisions. Discreet early volcanic intervals are seen in some

basins, but not in others, and there does not seem to be a relationship between the

occurrence of this interval in the detachment-dominated Basin and Range province and

the stratigraphy of other detachment-dominated extensional orogens. Overall textural

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trends within the epiclastic part of the basin fill are influenced by several factors, as

discussed above. It is not surprising, therefore, that some continental extensional basin

fills mimic that of the Mojave Extensional Belt, whereas some do not. We conclude that

whereas the occuixence of a discreet early volcanic interval appears to have only local

(western USA) significance, the regional syn- to posttectonic stratigraphie patterns are

important, since they reflect a common paleogeographic evolutionary sequence within

continental extensional belts. In addition, our more detailed stratigraphie studies in the

Mojave Extensional Belt may provide insights into the evolution of individual tectonic

elements of extensional belts.

CONCLUSIONS

The following conclusions were reached during this study:

(1) The regional stratigraphie framework of the Mojave Extensional Belt is tripartite,

consisting of (i) pre- to early synextension volcanic deposits, unconformably overlain

by (ii) syntectonic, basement-derived breccia and megabreccia with local finer-grained

units, overlain by (iii) a posttectonic fining-upward sequence of gravel, sand, shale, and

limestone. Important local variations from this theme include the early influx of

basement-derived debris (Mud Hills-Alvord Mountain ), the locally overwhelming

influence of volcanic centers (Calico Mountains), and the overall coarsening-upward

trends seen in the central Edwards terrane.

(2) Widespread pre- to early syntectonic volcanic deposits similar to those of the

Mojave Extensional Belt are found in several other Cenozoic extensional terranes in the

western United States, suggesting a causal link between early volcanism and later

surface expressions of rifting. Discreet early volcanic stages are not observed,

however, in similar, detachment-dominated extensional terranes in other parts of the

world. A volcanic stage is seen in at least two other extensional belts that do not expose

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the detachment faults and metamorphic core complexes characteristic of the western

United States.

(3) Textural trends in syn- and posttectonic stratigraphie units of the Mojave

Extensional Belt record the creation and subsequent destruction of landforms associated

with deformation of the upper plate and unroofing of lower plate rocks during regional

extension. Rocks of the Mojave Extensional Belt do not exhibit the overall coarsening-

upwards syn- to posttectonic basin fill patterns predicted by the models of Blair and

Bilodeau (1988) and Mack and Seager (1990), but observed coarsening-upward

sequences of tens to hundreds of meters thickness within the syntectonic section are

likely related to discrete episodes of fault movement followed by inactivity. The fining-

upwards nature of posttectonic rocks in the Mojave Extensional Belt is likely a signature

of internally drained basins. Post-extension development of throughgoing drainage may

result in coarser-grained rocks being deposited, as documented in the southern Basin

and Range province by Eberly and Stanley (1978) and DiGuiseppi and Bartley (1991).

Regional vertical textural trends similar to those of the Mojave Extensional Belt are

exhibited by several other extensional basins worldwide.

(4) Distinctive stratigraphie patterns are associated with individual tectonic elements

of the Mojave Extensional Belt. A complex paleogeographic history of basin formation

and dissection associated with the progressive breakup of the upper plate during

extension is recorded by rocks now exposed in the breakaway zone of the Daggett

terrane. Sediment dispersal patterns in this area were dominated by drainage from

unextended regions across the Kane Springs transfer boundary of the terrane, although

erosional debris was also derived from the fault scarp of the breakaway. Similar

dominant sediment dispersal patterns, subperpendicular to extensional axes, characterize

the extensional Homelen Basin of Devonian age in western Norway (Steel and others,

1977; Steel and Gloppen, 1980) and the strike-slip Ridge Basin of California (Crowell,

1982). In these basins, tectonic transport of depositional packages away from their

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point of origin by continued extension resulted in shingling of successive sedimentary

units. An important implication of this geometry of deformation and sedimentation for

breakaway basins is that, like strike-slip basins, total stratigraphie thicknesses may be

related less to the magnitude of vertical subsidence than to the amount of lateral

extension.

(5) Despite the dominance of cross-transfer zone sediment dispersal patterns near the

breakaway of the Daggett terrane, strike-slip elements elsewhere in the Mojave

Exensional Belt had a variable effect on patterns of sediment accumulation. Syn- and

posttectonic sedimentary rocks exposed near the Kane Springs transfer zone in the

eastern Newberry and Rodman Mountains were mostly derived from across the strike-

slip boundary, with additional components eroded from tilted upper plate fault blocks

within the extensional terrane. In the Waterman teirane, however, the Lane Mountain

transfer zone appears to have had no effect on sediment accummulation in the Mud Hills

until deposition of the posttectonic part of the Barstow Formation. Lateral offset of clast

lithologies away from their source areas was not observed at either location, perhaps due

to the lack of suitably distinctive source terrane lithologies.

(6) In contrast to proximal areas, upper plate extensional basins farther removed

from the breakaway zones of the Mojave Extensional Belt exhibit fairly straightforward

early Miocene structural and stratigraphie relations. Sedimentation patterns in simple

continental half graben are well documented and we have made no attempt to further

characterize them.

(7) Syntectonic sedimentation associated with the topographic evolution of the

Mitchel Range-Hinkley Hills-Waterman Hills metamorphic core complex in the

Waterman terrane was dominated by rock avalanche deposits composed of upper plate

erosional debris. The largely posttectonic Barstow Formation, however, is composed

of alluvial fan, fluvial, and lacustrine strata and exhibits a minor component of

metamorphic clasts derived from the adjacent core complex. Integration of stratigraphic-

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sedimentologic, structural, metamorphic, and thermochronologic data from the upper

and lower plates of the Waterman terrane provides new constraints on models for

extension of this area. These data support tectonic models for the region that invoke

extension by low-angle, crustal-scale normal faulting, but cannot further differentiate

between proposed mechanisms of core complex development.

(8) Sediment dispersal patterns in some parts of the Mojave Extensional Belt may

reflect tectonic elements that are less well understood. For example, little work has been

published on sedimentation patterns in the breakoff zones of the Mojave Extensional

Belt; at Alvord Mountain in the Waterman terrane, Fillmore and Miller (1991) postulate

the presence of an early antithetic fault on the basis of alluvial fan deposits.

(9) Although we have gained valuable insights into the development of the Mojave

Extensional Belt through our own and previously published stratigraphic-sedimentologic

studies, much remains to be done as shown by the rather superficial treatment given

here. With this overview paper, we hope that we have provided a loose framework and

enough information so that future, more detailed studies can be planned to further test

some of the concepts presented here and elsewhere and that better comparisons to other

extensional terranes can be made.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Sedimentation adjacent to an evolving 3 metamorphic core complex, central Mojave Desert, California ^

INTRODUCTION

Recognition of the significance of crustal-scale, low-angle, normal faulting as a mode

of extension has stimulated a fundamental réévaluation of how continental rift zones

evolve (Wernicke, 1981). Much effort has been directed at documenting the structure of detachment systems and understanding their geodynamic context (e.g., Mayer, 1986;

Coward and others, 1987). In contrast, the stratigraphie development of detachment-

dominated extensional orogens has remained less well studied. Most recent interest in

the stratigraphy of extensional terranes has been focussed on the basin fill patterns and

sedimentology of half-graben (e.g., Alexander and Leeder, 1987; Leeder and

Gawthorpe, 1987; Blair and Bilodeau, 1988; Cavazza, 1989; Ori, 1989; Morley, 1989;

Mack and Seager, 1990; Scholz and others, 1990). Stratigraphie patterns associated

with other tectonic elements and the constraints that such data may place on tectonic

models of rifting have received less attention. For example, stratigraphie developments

associated with the evolution of metamorphic core complexes have been treated only

briefly (Miller and John, 1988), whereas studies of sedimentation adjacent to transfer

zones are restricted to a few good examples (e.g.. Steel, 1976; Gloppen and Steel, 1981).

The early Miocene Mojave Extensional Belt of southern California (Dokka, 1989a) is

a detachment-dominated extensional orogen which provides excellent exposures of strata

deposited during and immediately after extension. The structural geology of the Mojave

Extensional Belt is well documented (Dokka, 1980,1986,1989ab; Dokka and

^ paper submilled for publicalion as Sedimentation adjacent to an evolving metamorphic core complex, central Mojave Desert, California, by C. J. Travis, and R. K. Dokka.

105

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Woodbume, 1986; Mathis, 1986; Dokka and others, 1988; Glazner and others, 1988,

1989; Walker and others, 1990; Bartley and others, 1990), as are some aspects of the

regional and local stratigraphy (e.g., Dibblee, 1967; Woodbume and others, 1974;

Dokka, 1980,1986; Moseley and others, 1982; Woodbume and others, 1982; Dokka

and Woodbume, 1986; Lambert, 1987; Woodbume and others, 1990). Recent work in

the region has focussed on the structure (Dokka and Woodbume, 1986; Dokka and

others, 1988; Glazner and others, 1988,1989; Dokka, 1989a; Walker and others, 1990;

Bartley and others, 1990) and presssure-temperature history (Henry and others, 1989;

Jones, 1990; Jones and others, 1990; Henry and Dokka, 1990, in press) of lower plate

rocks exposed in metamorphic core complexes. One of the more important results of

these studies has been the recognition that these originally deep-seated rocks were

exhumed rapidly in early Miocene time as a result of detachment faulting (Dokka and

others, 1991).

In this paper we present results of recent stratigraphie and sedimentologic studies in

the Mojave Extensional Belt. Specifically, we describe the stratigraphy of a part of the

Mud Hills area in the central Mojave Desert north of Barstow and propose revision of

the local stratigraphie nomenclature to include the newly named Mud Hills Formation.

We examine the relationship between the stratigraphy and the tectonic evolution of the

area and conclude that the Mud Hills Formation is a syntectonic unit which was

deposited on the hanging wall of a major, northwest-striking normal fault situated

northeast of the Mitchel Range-Hinkley Hills-Waterman Hills metamorphic core

complex. This fault was active during the late stages of extension in the Mojave

Extensional Belt, shortly after the main episode of detachment faulting. Our results also

provide new constraints on the timing and geometry of uplift of the lower plate in the

Waterman Hills which may be used to evaluate previously published tectonic models of

rifting in this area.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 107

GEOLOGIC BACKGROUND

Although now disrupted by late Cenozoic right-slip faulting associated with Pacific-

North American transform motion (Dokka and Travis, 1990ab), the early Miocene

Mojave Extensional Belt can be traced eastward from the intersection of the San Andreas

and Garlock faults to the eastern Mojave Desert near the Granite Mountains fault

(Fig.3.1) (Dokka, 1989a). The belt likely continues to the northwest and includes the

southern San Joaquin basin (Tennyson, 1989; Goodman and Malin, 1990) and the

Pinnacles area of central California (Dokka, 1989a). According to Dokka (1989a),

strain in the Mojave Extensional Belt is partitioned between four structural domains that

each consist of one or more half-graben, in which similarly oriented, tilted, normal fault-

bounded blocks lie above rooted, low-angle, crustal-scale normal faults (detachments)

(Fig. 3.1). Differential extension between the domains was accommodated by strike-

slip faults (transfer zones). The Mojave Extensional Belt initially began to open in

earliest Miocene time, with the major phase of extension occurring near 20 Ma (Dokka,

1989a; Dokka and others, 1991). Local, high-angle normal faults cut the older

detachments and were active -20-17 Ma.

This paper deals with stratigraphie relationships in the Waterman terrane, areally the

smallest structural domain of the Mojave Extensional Belt (Figs. 3.1,3.2). Although

limited in size, this domain is important because geologic relations exposed there

provide crucial information on the sequence of events and on the processes involved in

extension. The Waterman terrane contains excellent exposures of both upper and lower

plate rocks, as well as post-extension deposits. Predominantly southwest-dipping

Miocene sedimentary and volcanic rocks that rest unconformably on pre-Tertiary

crystalline complex are exposed in the northwestern part of the terrane (Fig. 3.2). A

low-angle fault (Mitchel detachment) separates these upper plate rocks from a lower

plate of mylonitic, granulite to greenschist facies polymetamorphic rocks in the Mitchel

Range-Hinkley Hills-Waterman Hills metamorphic core complex of the central part of

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Fig. 3.1. Location map of study area, highlighting extensional domains of the Mojave Extensional Belt (after Dokka, 1989). Key to abbreviations: AM, Alvord Mountains; AB, Antelope Buttes; BM, Bullion Mountains; BrM, Bristol Mountains; BL, Bristol Lake; BWF, Baxter Wash Fault; CM, Cady Mountains; CoM, Calico Mountains; DR, Daggett Ridge; FP, Fremont Peak; GH, Gravel Hills; HL, Harper Lake; HH, Hinkley Hills; KSF, Kane Springs Fault; KH, Kramer Hills; LMF, Lane Mountain Fault; MR, Mitchel Range; MH, Mud Hills; NM, Newberry Mountains; CM, Ord Mountains; RM, Rand Mountains; RdM, Rodman Mountains; RgL, Rogers Dry Lake; RH, Rosamond Hills; RL, Rosamond Dry Lake; WH, Waterman Hills.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 109

Fig. 3.2. Composite geolo^c map of the Waterman Terrane highlighting upper plate crystalline basement (light shading), lower plate rocks of the Waterman Metamorphic Complex (medium shading), and syntectonic rocks of the Mud Hills Formation (dark shading).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I

< !-J0 ^ j \(''~ Ll/Tv r - y QotVBARSTOW

geology not Included

Qo on this

R2W I R1W R IW iR IE

5 MILES

5 KILOMETERS

SCALE

. T l t N a . \ % Tv TION 'U t?

f^Qo

STOW U/T»

RIWiRie

S MILES

5 KILOMETERS

SCALE

Fig. 3.2. contd.

EXPLANATION

Dashed where gradadonal or i^projimaiely located

Fault Dashed vrhere indefinite, dotted where concealed; queried where doubtful Q u , u/uHfftrtiuialidstdi/tuHts Q a , alluvium Q o, oldtr alluvium O b , basalt Low'fDgle Qotmal fault (Mitchel detachment) Dashed tvhere ind^nite, dotted where concealed; UNCONFORMÏÏY queried where dmétful; nodes on lunging wall

10 Tv Tl Strike and dip of strata

Syp. to pos l-oxKiuioDil sedimentuy rocks 56 Tie» losyn-oitensicDs] volcmicrocks, Strike and dip of foliations T s, epiclastic sedimentary rocks T sv , undifferentiated volcanic aid sedimentary rocks Ti, intrusive volcanic rocks, iruinly andésite T v, volcanic rocks, mainly volcanicLsstie x ' Trend and plunge of lineatioos UNCONFORMITY

'.ub‘. .Mgd* Mqm.'Pzm. 4.5 Ac-Tcnicy upperpliie buemeol roclu 2.3 Ub, undifferentiated upptr plate basement M gb, granodiorite, dioriSe (mapped as Mesùioic) M qm . quarts monsonite 6.7 {mapped as Mesosoic) P zm , undifferentiatedrrutmorphicroeh (mapped as Paleozoic) 9,10,11

LOW ANGLE NORMAL FAULT (M ITC H EL D ETACH M ENT)

Sources of data 1.This sudy 7. R. K. Dokka. unpubi&hod 2. DfcUce(t9«8) mapping (1985-1989) Pic-Tcrtiiry lower pUie rocks 3. WoodtKino and others (1990) 8. DibWeo (1970) 4.McCultoh(l932) 9 . Dokka and W oodbume (1986) M gd, granodiorite, diorite 5. Santa Fe Mining, inc., 10.Dokka(1989) (mapped as Mesozoic) unoubishod m9ping(1957-19Sa) 11. Lambert (1987) w g. Waterman MelamorpMe Cor^lex, 6. D tw e e (1967) 12. geology not indudod on map (proloEth Paleozoic? myionitization earlyMiocene)

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the terrane (Fig. 3.2) (Dokka and Woodbume, 1986; Dokka and others, 1988; Dokka,

1989a; Henry and others, 1989; Henry and Dokka, 1990). Metamorphic petrology

(Dokka and Woodbume, 1986; Dokka, 1989a; Henry and others, 1989; Henry and

Dokka, 1990), stmctural relationships (Dokka and Woodbume, 1986; Dokka and

others, 1988; Glazner and others, 1988,1989; Dokka, 1989a; Walker and others, 1990;

Bartley and others, 1990), and thermochronologic data (Dokka, 1989a; Henry and

others, 1989; Jones, 1990; Jones and others, 1990) suggest that rocks of the Mitchel

Range-Hinkley Hills-Waterman Hills core complex occupied middle cmstal levels

immediately prior to extension and were rapidly exhumed by tectonic denudation near 20

Ma.

Thick deposits of early Miocene, basement-derived breccias are exposed in the Mud

Hills, about 4.5 km northeast of the Waterman Hills. These syntectonic breccias and

overlying finer-grained deposits record events at the earth's surface during unroofing of

the Mitchel Range-Hinkley Hills-Waterman Hills metamorphic core complex. Until

now, these basement-derived breccias have been assigned to the Pickhandle Formation

of Dibblee (1968). In this paper we present a revised stratigraphy of the Mud Hills

which treats these breccias as a new, separate formation.

NEED FOR STRATIGRAPHIC REVISION

The name "Pickhandle Formation" was first used by McCulloh (1952) to designate

volcaniclastic rocks in the westem Calico Mountains near Pickhandle Pass,

approximately 6 km east of the Mud Hills (Fig. 3.2). Dibblee (1967) formalized the unit

and correlated it with rocks in the Mud Hills, Opal Mountain-Black Canyon and the

Gravel Hills (Dibblee, 1967,1968); he did not, however, define a stratotype. At

Pickhandle Pass, the Pickhandle Formation consists of ~15(X) m of pyroclastic to

epiclastic andesitic tuff, tuff breccia and breccia (Dibblee, 1968), and is intruded by

dacite and andésite dikes, sills and plugs. A dacite flow that unconformably overlies the

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Pickhandle Formation near Jackhammer Gap, at the northern edge of Pickhandle Pass,

has been dated as 18.1+0.5 Ma (Burke and others, 1982). In its type area, the

Pickhandle Formation is entirely volcaniclastic, whereas at Opal Mountain and in the

Gravel Hills it contains lenses up to 10 m thick of conglomerate composed of granitic

and dioritic debris (Dibblee, 1968). In the Mud Hills, however, more than half of

Dibblee's (1968) Pickhandle Formation is composed of coarse-grained, crystalline

basement-derived sedimentary rocks. The lowest part of the formation locally contains

up to 200 m of sandstone and conglomerate of tuffaceous and granitic composition

(Dibblee, 1968). In contrast, the upper third of the unit is dominantly a cliff-forming

sequence of monolithologic breccias of quartz monzonite and felsite debris. The

distinctive lithologie character of these breccias and their substantial thickness warrants

their differentiation from the predominantly volcaniclastic Pickhandle Formation and

their designation as a separate unit, the Mud Hills Formation.

DESCRIPTION OF THE MUD HILLS FORMATION

The Mud Hills Formation consists of ~300 m of quartz monzonite breccia with

subordinate felsite breccia and sandstone that crops out on the north limb of the Barstow

syncline (Fig. 3.3). The unit conformably overlies the predominantly pyroclastic

Pickhandle Formation to the north and is unconformably overlain to the south, in the

westem and central Mud Hills, by the Owl Conglomerate and Middle Members of the

Barstow Formation (Fig. 3.3) (Dibblee, 1968; Woodbume and others, 1990). To the

east, the Mud Hills Formation interfmgers with volcaniclastic rocks of the uppermost

Pickhandle Formation (Figs. 3.3, 3.4). A thin, discontinuous tongue of Pickhandle tuff

breccia bisects the Mud Hills Formation east of Coon Canyon, whereas a thicker tongue

of tuffaceous sandstone and tuff breccia overlies the unit east of Solomon Canyon. As

defined here, the Mud Hills Formation comprises all of the "granitic breccia" and

"rhyolitic breccia" mapped by Dibblee (1968) and assigned by him to the upper part of

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the Pickhandle Formation in the Mud Hills. The Mud Hills Formation does not crop out

on the south limb of the Barstow syncline (Fig. 3.3), but a tuffaceous sandstone marker

bed of equivalent age is found near the base of the section there (Woodbume and others,

1990). It seems likely, therefore, that the upper part of the Mud Hills Formation is

replaced to the south by rocks assigned to the lowermost Barstow Formation

(Woodbume and others, 1990) (Fig. 3.4).

The Mud Hills Formation is divided into five mappable members (Fig. 3.3). Four

members crop out in Owl Canyon, the stratotype for the formation; the fifth member is

exposed in Solomon Canyon. The stratigraphie character of each member of the Mud

Hills Formation is described below and is illustrated in Figure 3.5.

Lower Breccia Member

The basal member of the Mud Hills Formation is a 10-81 m thick unit of cobble to

boulder breccia and sandstone. It crops out as far west as 0.75 km east of Fossil

Canyon and continues eastward to 0.9 km east of Solomon Canyon, where it is

truncated by the Mud Hills fault of Dibblee (1968) (Fig. 3.3). The Lower Breccia

Member conformably overlies pyroclastic tuff breccia of the Pickhandle Formation

everywhere but in Owl Canyon, where the contact may be a fault. The unit is

unconformably overlain by the Middle Breccia Member west of the Ross Canyon-

Solomon Canyon drainage divide and by the Solomon Sandstone Member or by the

Pickhandle tuff breccia tongue east of the divide (Fig. 3.3).

The basal 10-24 m of the Lower Breccia Member consists of 5 cm to 1.4 m thick

beds of grayish orange, fine- to coarse-grained sandstone and pebble to cobble breccia

(Fig. 3.5). Although absent at the type section on the floor of Owl Canyon, this

sequence may be seen high up on the flanks of the canyon. In the east-central Mud Hills

between Ross and Solomon Canyons the basal part of the unit is interbedded with

underlying tuff breccias of the Pickhandle Formation. In the westem part of the Mud

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Fig. 3.3. Geologic map and cross section of the Mud Hills.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. W ttJ U A M S P L A I N

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saaxaw

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Fig. 3.3. contd.

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X Mud Hills (SarsUNV syndine) Mud Hits (nonh-centrall Calico Mountains South North» West East ItUt02Us Xiil^air^^iBARSJOW POaMnON « e e e e e e e e s e e

BAltàfbWfOfOAnON ia it0 3 M s ia7t0.7

UUOHlUSFOfiMATKN ? 19Xt0.7Ma 2 % . PCWANOŒFOfWÀTKN

Fig. 3.4. Correlation chart for Miocene rocks in the Mud Hills-Calico Mountains area. Based on data from this study, Dokka and others (1991), Woodbume and others (1990), Dibblee (1968), and Burke and others (1982).

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te 55

SJ0I9UJ

wy.

te LU

Fig. 3.5. E-W stratigraphie cross section of the Mud Hills Formation.

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Hills, this sandstone and breccia sequence constitutes the entire Lower Breccia Member.

The basal sandstone and breccia are composed of angular clasts of quartz monzonite,

minor felsite and purplish porphyritic andésite fragments, and trace amounts of small,

brownish volcanic rock fragments, schistose rock fragments and chert fragments.

Pebble-boulder breccias may be clast- or matrix-supported and contain a matrix of

arkose and clay. Medium- to very coarse-grained and pebbly sandstones are fairly

abundant in this sequence, although finer-grained sedimentary rocks are rare.

The upper 58-64 m of the Lower Breccia Member is a grayish orange, matrix- to

clast-supported breccia of angular, subequant to brick-shaped cobbles and boulders

<2.5 m in diameter (Fig. 3.5). Framework grains are predominantly quartz monzonite,

with minor light gray felsite, graphic granite, aplite and pegmatite, and rare diorite and

volcanic rock fragments. Diorite is also found as small xenoliths within quartz

monzonite clasts. The matrix of the breccia consists of granule- to sand-size fragments

of the framework grains and clay. Bedding is difficult to recognize in this unit, although

a crude dip-parallel fabric related to the presence of more resistant, calcite cemented

layers is visible at 2-5 m intervals in the lower 40 m of section in Owl Canyon and

Rainbow Canyon. Thin (<2 cm), finer-grained horizons with a vertical spacing of 2-10

m are also seen locally.

Solomon Sandstone Member

The Solomon Sandstone Member is exposed only in Solomon Canyon, where it

consists of 27 m of light brown shale and fine- to very coarse-grained sandstone (Fig.

3.5). Shales are 1 cm to 3 m thick and contain stringers of very fine-grained sandstone

and occasional volcanic rock fragments. The shales are interbedded with thin- to

medium-bedded arkosic sandstone composed of quartz monzonite debris and volcanic

rock fragments. The Solomon Sandstone Member unconformably overlies the Lower

Breccia Member, and is conformably overlain by the Pickhandle tuff breccia tongue

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(Figs. 3.3, 3.5). The stratigraphie relationship between the Solomon Sandstone and the

Middle Breccia Member is not fully understood. On the east flank of Owl Canyon,

massive breccias of the Lower Breccia Member and the overlying Middle Breccia

Member are separated by a ~9 m thick sandy interval which may be correlative with the

Solomon Sandstone Member (Fig. 3.5). The base of the sandy interval contains a

discontinuous, 0.3-1 m thick unit of very pale gray laminated limestone and light gray to

brown, stromatolitic limestone and mottled pelmicrite containing ostracode and

gastropod remains. These limestones are overlain locally by <8 m of grayish orange

sandstone and breccia. The sandy interval between the Lower and Middle Breccia

Members in Owl Canyon is tentatively correlated with the sandstone and shale of

Solomon Canyon, suggesting that the Solomon Sandstone may be older than the Middle

Breccia.

Middle Breccia Member

The Middle Breccia Member of the Mud HUls Formation crops out from 0.75 km east

of Fossil Canyon in the west to 0.5 km west of Solomon Canyon in the east (Fig. 3.5).

It is distinguished from the Lower and Upper Breccia Members by its reddish color and

by the presence of megabreccia. The Middle Breccia Member unconformably overlies

the Lower Breccia. At some locations it is conformably overlain by the Pickhandle

Formation tuff breccia tongue; at other localities it is unconformably overlain by the

Upper Breccia Member of the Mud Hills Formation (Figs. 3.3, 3.5, 3.7). The Middle

Breccia Member attains its maximum thickness in the western Mud Hills (-150 m in

Coon Canyon), but is best exposed in Owl Canyon, where it is 81 m thick. The unit

consists predominantly of framework-supported, angular cobbles and boulders of quartz

monzonite containing scarce aplite veins and xenoliths of fine-grained diorite, with a

medium sand-granule matrix of similar composition. The uppermost 20 m of the

member in Owl Canyon, however, contains several 0.05-2 m thick monolithologic

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lenses of distinctive, light gray felsite breccia of rhyolitic composition. The Middle

Breccia Member contains clasts that are more than 30 m in diameter, average clast-size is

^ m. Bedding is not recognized within the Middle Breccia Member, although the

upper part of the unit in Owl Canyon, below the felsite breccia interval, exhibits thin,

dip-parallel, clay-rich zones with a vertical spacing of 1.5-4 m. West of Owl Canyon,

the Middle Breccia Member is an unbedded interval of reddish brown (biotite-rich) to

white monolithologic breccia of quartz monzonite composition, with a patchy

appearance caused by color variations on a 5-15 m scale. In this area the unit may also

contain folded pods of well bedded sandstone and breccia up to 7 m by 20 m in size

(Fig. 3.6).

Upper Sandstone Member

The Upper Sandstone Member of the Mud Hills Formation crops out only in Owl

Canyon, where it unconformably overlies the "Red T u ff marker bed of the Pickhandle

Formation tuff breccia tongue. The unit is in turn unconformably overlain by the Upper

Breccia Member of the Mud Hills Formation (Figs. 3.3, 3.5, 3.7). The Upper

Sandstone consists of 22 m of light red to grayish orange, well bedded (3-60 cm), very

fine-grained to pebbly sandstone and granule breccia of arkosic to litharenite (quartz

monzonite) composition. The basal 5 m contains 0.8-1.7 m thick beds of very pale

orange, muddy, tuffaceous pebble breccia of volcanic composition.

Woodbume and others (1990) recently assigned the Upper Sandstone Member (this

paper) to the Owl Conglomerate Member of the Barstow Formation, based on

correlation of their "Red T uff marker bed between the south and north limbs of the

Barstow syncline (Figs. 3.3, 3.4). Specifically, Woodbume and others (1990) correlate

the sandstone overlying the "Red T uff in Owl Canyon (north limb) with rocks mapped

as Owl Conglomerate in a similar stratigraphie position on the south limb. They

therefore conclude that the base of the Barstow Formation interfingers with the top of

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X'ifS-'

a s #

Fig, 3.6. Photograph of a folded lens of sandstone in the Middle Breccia Member of the Mud Hills Formation in the western Mud Hills. Other such lenses may retain original internal bedding, and attain thicknesses of 7 m and lengths of ~20 m.

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Fig. 3.7. Photograph of unconfoimable contact relations of the Upper Sandstone Member of the Mud Hills Formation in Owl Canyon. Key to abbreviations: Tmbm, Middle Breccia Member, Tmsu, Upper Sandstone Member, Tmbu, Upper Breccia Member, Rtm, "Red Tuff" marker

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the Mud Hills Formation (Pickhandle Formation in their parlance). We prefer Dibblee’s

(1968) original formational boundaries for the Barstow Formation on the north limb of

the Barstow syncline for the following reasons: (1) The Upper Sandstone contains no

metamorphic clasts, as is characteristic of the southerly-derived Owl Conglomerate on

the south limb of the syncline; (2) The unit was probably derived from the north rather

than from the south, as discussed below; (3) The Owl Conglomerate on the north limb

of the syncline differs markedly in texture, structure and color, from the Upper

Sandstone and is separated from it in Owl Canyon by an angular unconformity of ~20°,

as well as by 80 m of quartz monzonite breccia. At Solomon Canyon (north limb), the

"Red T uff is separated from the Owl Conglomerate by quartz monzonite breccia and by

nearly 50 m of Pickhandle Formation tuff breccia and tuffaceous sandstone (Fig. 3.5).

Upper Breccia Member

The Upper Breccia Member crops out from 0.75 km east of Fossil Canyon to ~1 km

east of Copper City Road (Fig. 3.3). The unit unconformably overlies both the Upper

Sandstone and the Middle Breccia Member west of Owl Canyon, whereas to the east it

overlies the tuff breccia tongue of the Pickhandle Formation. The Upper Breccia

Member is unconformably overlain by the Owl Conglomerate or the Middle Member of

the Barstow Formation west of Solomon Canyon. In the eastern Mud Hills, however, it

interfingers with, and is conformably overlain by, the upper tongue of tuffaceous

sandstone and tuff breccia belonging to the uppermost Pickhandle Formation (Fig. 3.3).

The Upper Breccia attains its maximum thickness (~130 m) in the vicinity of

Rainbow Canyon; it is 81 m thick in the type section in Owl Canyon. The unit consists

of massive, grayish orange breccia composed of angular, subequant to brick-shaped

pebbles-boulders of quartz monzonite, with minor leucocratic material and fine-grained

diorite. Framework grains are set in a matrix of fine sand- to pebble-size quartz

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monzonite debris and clay. Textures vary from matrix-rich, clast-supported to clast-

rich, matrix-supported. Maximum clast size is ~1.5 m.

TUFF BRECCIA AND SANDSTONE TONGUES OF THE UPPER PICKHANDLE

FORMATION

Crystalline basement-derived rocks of the Mud HiUs Formation interfinger eastward

with volcaniclastic strata of the upper Pickhandle Formation (Fig. 3.3). The Pickhandle

Formation forms two westward-thinning tongues within the Mud Hills Formation. A

dominantly pyroclastdc sequence bisects the unit (tuff breccia tongue), whereas an upper

unit containing more abundant epiclastic volcanic debris overlies the Upper Breccia

Member (tuff breccia and tuffaceous sandstone tongue).

The lower, tuff breccia tongue is discontinuously exposed as far west as Coon

Canyon, and consists of 0-20 m of pyroclastic, light greenish gray to yellowish gray

lapilli tuff breccia, capped by a distinctive moderate reddish orange tuffaceous sandstone

marker bed ("Red T uff of Woodbume and others (1990)). This unit is composed of

two 10-15 m thick, massive to reverse graded, matrix-supported lapilli tuff breccias of

angular white tuff clasts, pumice relics, purplish andésite fragments, basalt pebbles and

rare quartz monzonite and schistose rock fragments, set in a matrix of clay, quartz,

feldspar and biotite grains (Fig. 3.8). Each breccia is capped by well-bedded, reworked

volcaniclastic sedimentary rocks. The epiclastic "Red T uff is the only part of the tuff

breccia tongue represented at the type section of the Mud Hills Formation in Owl

Canyon (Fig. 3.7).

The upper, tuffaceous sandstone and tuff breccia tongue of the Pickhandle Formation

is exposed as far west as midway between Solomon Canyon and Ross Canyon. It is

~45 m thick in Solomon Canyon and thickens rapidly to the east, reaching ~450 m in the

area east of Copper City Road. The basal 10.5 m of the unit in Solomon Canyon is

similar in character to the Solomon Sandstone and contains 1-25 cm thick, tabular beds

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m m

Fig. 3.8. Photograph of sheared gas segregation pipes of Pickhandle Formation tuff breccia tongue in Ross Canyon, indicating pyroclastic origin and (possibly) flow direction —southwest quadrant.

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of very fine-grained to very coarse-grained sandstone, interbedded with <3 m thick

shales. The sandstones are composed of angular to subangular, relatively poorly sorted

clasts of quartz monzonite debris and tuffaceous material, and exhibit small scour and

fill structures, normal grading with basal pebbly lags, and parallel laminations. Shales

are more abundant in the upper 4 m of the sequence. The remainder of the unit in

Solomon Canyon consists of 3-7 m thick beds of massive, matrix-supported lapilli tuff

breccia of laharic and possibly pyroclastic origin, along with 0.15-2.5 m thick beds of

ungraded to graded and parallel laminated, very fine-grained to pebbly tuffaceous

sandstone. These sandstones may form fining-upwards sequences up to 8 m thick and

may exhibit channel-like geometry; they are interpreted as fluvially reworked volcanic

deposits. The section in Solomon Canyon is cut by many small down-to-the-north

normal faults. East of Solomon Canyon, the upper Pickhandle tuff breccia and

sandstone tongue is easily divided into a lower tuffaceous sandstone and an upper tuff

breccia section (Fig. 3.3). At least two lenses of granitoid breccia similar to the Upper

Breccia are seen within the tuff breccia and sandstone tongue (Fig. 3.3). The top of the

sequence east of Copper City Road is dominated by a ~ 145 m thick, grayish pink

andésite flow.

AGE AND CORRELATION OF THE MUD HILLS FORMATION

The age of the Mud Hills Formation is constrained by '•^Ar/^^Ar, K-Ar, and other

chronostratigraphic data from underlying and overlying rocks. Single-crystal laser

fusion 40Ar/39Ar dating of feldspar from a pumice of the Pickhandle Formation

immediately underlying the Mud Hills Formation yields an age of 19.8+0.2 Ma (C. C.

Swisher, written communication, 1990), whereas the biostratigraphic,

magnetostratigraphic, and isotopic studies of Woodbume and others (1990) and

MacFadden and others (1990) in the Mud Hills suggest that accumulation of Owl

Conglomerate on the north limb of the Barstow syncline began before ~16 Ma. The age

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of the top of the Mud Hills Formation is further constrained by dates in and overlying

the Pickhandle Formation. ^Ar/^^Ar dating of a biotite separate from the pyroclastic

tuff breccia tongue of the Pickhandle Formation which bisects the Mud Hills Formation

yields an age of 18.710.2 Ma (Dokka and others, 1991). A dacite flow that

unconformably overlies the Pickhandle Formation in its type area has been dated at

18.110.5 Ma by the K-Ar method (Burke and others, 1982). Deposition of the Mud

Hills Formation is, therefore, constrained to between -19.8 Ma and-18.1 Ma.

Correlation of the Mud Hills Formation with other rocks of the Mojave region is

hampered by the lack of well dated sedimentary and volcanic strata. In addition,

lithostratigraphic correlations may be misleading because most basins of the Mojave

Extensional Belt are of local extent. Known and suspected correlatives include the

upper Tropico Group in the western Mojave Desert (Dibblee, 1967), the upper

Pickhandle Formation in the Gravel Hills, Opal Mountain-Black Canyon, Lead

Mountain and the Calico Mountains (Dibblee, 1967; Dokka and Woodbume, 1986), the

Spanish Canyon Formation at Alvord Mountain (Dokka and Woodbume, 1986), the

formations of Stoddard Valley and Slash X Ranch in the Newberry Mountains (Dokka,

1980, 1986), the Hector Formation in the Cady Mountains (Woodbume and others,

1974; Miller, 1980), sedimentary and volcanic rocks of the Pickhandle Formation

exposed in the Waterman Hills (Dibblee, 1967; Walker and others, 1990), and unnamed

sedimentary and volcanic rocks in and north of the Mitchel Range and in the Bristol

Mountains. Regional stratigraphie correlation charts have been published by Dibblee

(1967), Woodbume and others (1982), Dokka and Woodbume (1986), Dokka and

others (1988), and Travis and Dokka (1991, in prep.). The reader is referred to those

articles for further details.

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DEPOSITION OF THE MUD HILLS FORMATION

Basal part of Lower Breccia member

The basal 10-24 m of the Lower Breccia Member contains several lithofacies which

may be classified according to the scheme for coarse alluvial sediments outlined in Rust

and Koster (1984). Massive, matrix-supported pebble-boulder breccia forms 0.9-1.4 m

thick beds with sharp to gradational basal contacts and sharp, irregular upper contacts.

This breccia is the least common of the coarser facies in the sequence (Fig. 3.9) and

corresponds to Facies Gms of Rust and Koster (1984). The massive, matrix-supported

fabric, as well as the bedding contacts of Facies Gms strata suggests a high viscosity

cohesive debris flow origin (e.g., Middleton, 1973; Nemec and Steel, 1984). Rare,

massive, clast-supported pebble breccias with a muddy matrix within the sandstone and

breccia sequence are interpreted as low viscosity debris flows (Fig. 3.9) (e.g., Shultz,

1984); this lithofacies (Gl) does not appear in the Rust and Koster (1984) scheme.

Clast-supported, pebble-boulder breccia with a sandy, clay-poor matrix corresponds

to Facies Gm of Rust and Koster (1984). This breccia forms 5-60 cm thick beds with a

5-10 m wide channel-like geometry (Fig. 3.10). Fabrics range from massive to

normally graded with horizontal stratification and rare cross bedding. Basal bedding

contacts are planar to scoured (10-30 cm relief), whereas upper contacts are sharp to

gradational. We interpret these breccias as braided channel deposits on the basis of

geometry and fabric.

Medium- to very coarse-grained and pebbly sandstones form 3-60 cm thick, lenticular

to laterally continuous beds with sharp, planar or scoured basal contacts and sharp to

gradational upper contacts. These sandstones are designated as Facies Sh, Sp, or St of

Rust and Koster (1984), depending on their sedimentary structures (Fig. 3.9). The

sandstones may be normally graded, with a basal lag of imbricate pebbles and cobbles,

or they may exhibit horizontal, wavy or ripple cross laminations and planar tabular or

trough cross bedding. These rocks may form facies associations with rocks of either

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24 1

FACIES Fm Light bfownshalo and yellowish gtaymudsBoe. Ovetbank deposits

FACIES Gms 0 .3^.S m thick beds d massive, matrix-supported pebble breccia and pebble-bouldef breccia. Lateral oxttinuity-50 m. Lower contacts planar, sharp to gradational; upper contacts sharp, sometimes kiegUar. Composed predrxninantly of angular quartz monzrxrite debris, with (etshe-iich beds seen locally (e.g., Solomon Canyon). Matrix modkan to coarse sand-size with tiighly variable day content Cohesive debris How.

FACIES Sh(SLSp) 2-50 cm thick beds of poorly sorted, medium- to very coarse-grained and pebbly sandstone. B a ^ contacts sharp, planar to scoured with-5 cm relief; imperrxxitacts sharp to gradational. Lateral continutty 1 m to 10% of meters. Massive to nomially graded with or whttout parallel laminations, wavy lattiinalions, trough cm ss g bedding (St), planar cross bedding (Sp), and EcatterMknbncate pebbles. Composed of angular quartz monzrxxte debris with sparse volcanic and schistose rock fragments. May forms Facies Associations Sh/B and Gm/Sh. Braided channel and sheedlood deposits.

FACIES Gm(Gp) 10-60 cm thick beds of poorly sorted, dast-supported, granule-boulder breccia; max. dastsize HO cm. Basal crcntacts scoured, erosive; upper contacts predominantly sharp. Channel geometries evident NomiaS grarfmg, basal boulder lag, and imbrication common, planar cross bedding rare (Gp). May grade laterally to Facies Sh. Composition as above. Matrix niêdium- to coarsegrained, with variable d ay content Ctiarmel deposits.

FACIES FI 2-5 cm thick, tabular, parallel laminated beds of well sorted, hrie- to very finegrained sandstone. Sharp, pkuiar contacts. May be intertiedded with ttiin, medium-grained, parallel laminated sandstones of Fades Sh and day laminae (Facies . assodah'on Sh/Fm). Commonly grayish orange, basement-derived; rarely red colored and tuffaceous. Sheetilood/overtiank deposits Tun bm ooa

FACIES G l 15-70 cm thick, massive, dast-supported, poorly packed, pebble brecda of poorly sorted, angular quartz monzonite debris. Sharp to gradational contacts. Matrix muddy, with day- to coarse sand-size debris. Low viscosity debris How?

Cl V F F M C lS rP C o B

Fig. 3.9. Composite sedimentary log of the basal sandstone and breccia interval of the Lower Breccia Member of the Mud Hills Formation. See Figure 3.11 for key to symbols.

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Fig. 3.10. Photographs of the basal sandstone and breccia interval of the Lower Breccia Member of the Mud Hills Formation, a) Sheetflood deposits between Owl Canyon and Solomon Canyon; b) Debris flow deposit between Owl Canyon and Rainbow Canyon (note basal load structure); c) Braided channel and sheetflood deposits at Solomon Canyon (note channel geometry of the two most distinctive breccia beds).

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Î^ArîîU F-Ç.r" %-"^S

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C) y

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Faciès Gms or Faciès Fm (Fig. 3.9) and are interpreted as braided channel and

sheetflood deposits.

Fine-grained rocks comprise Facies Fm and FI of Rust and Koster (1984). Such

rocks occur infrequently in the basal part of the Lower Breccia Member, and are parallel

laminated, very fine-grained sandstone and yellowish mudstone. These finer-grained

rocks are probably overbank deposits.

We interpret the sandstone and breccia sequence as a whole to be the result of

deposition in an alluvial fan setting, based on textural criteria and the presence of debris

flows with braided channel and sheetflood deposits (e.g., Blissenbach, 1954; Bull,

1972; Nilsen, 1982). The dominance of streamflow deposits at most localities, together

with the fact that debris flows were mostly of the viscous type suggests a distal mid-fan

environment (e.g., Gloppen and Steel, 1981; Rust and Koster, 1984. cf. Shultz, 1983).

At several localities, the upper part of the sandstone and breccia sequence and the lower

part of the overlying, coarser-grained breccia are dominated by breccias interpreted as

debris flow deposits. This coarsening-upward trend may indicate local fan

progradation.

Solomon Sandstone Member

In contrast to the sandstone and breccia sequence described above, the Solomon

Sandstone is dominated by shale and thinly bedded sandstone (Fig. 3.11). This 27 m

thick unit consists of fine-grained and medium- to very coarse-grained, calcite cemented

sandstone and shale with stringers of very fine-grained sandstone. Sandstones form 2-

25 cm thick beds, which are stacked in <1 m thick packages and separated by up to 3 m

of shale. Coarser-grained sandstones are more common in the upper two-thirds of the

section (Fig. 3.11). Sandstone beds have sharp, planar basal contacts and abruptly

gradational upper contacts; some beds are of limited lateral extent (1-10 m), whereas

others continue for distances of >20 m. The rocks contain graded beds, parallel

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2-25 cm thick beds of fine-grained and medium- to very coarse-grained sandstone arranged in packages s i m thick. Basal contacts abrupt and planar, upper contacts k) shale sharply gradational. Lateral continuity of individual beds varies between 1-10 m and tens of meters. Coarse-grained sandstone mote abundant in upper two llfirris of unit Sandstone composed of poorly packed, angular, sand- to pabWe-size (S16 mm) clasts of quartz, felœpar. biotite. and volcanic rock fragments. Abundant calcile cem enl S ed im en t^ structures include normal grading (in coarser-grairved rocks), parallel laminadorrs. rif^ e trough cross laminations IS cm thicksets), low angle, gkuw cross bedding, and discontinuous, flaser-fike

0.01-3 m thick beds of light brown shale with discontinuous stringers of very fine-grained sandstone and scattered volcanic rock fragments.

KEY TO SYMBOLS

Graded ftedrfmg

Inverse graded bedding = Parallel laminations Wavy laminations s c Scour and fill

e=* Asymétrie ripple marks Raser bedding Trough cross bedding A. Tabular, planar cross tredding i r Load structure

_ c Pull-over flame structure

Slump Burrows

• < Pebtjies a ? Imbrication

VTF MCGrPCoB

Fig. 3.11. Measured section of the Solomon Sandstone Member of the Mud Hills Formation.

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laminations, ripple trough cross-laminations (5 cm sets), low angle planar cross-bedding

and lenticular bedding. The abundance of shale, grading, and tractive sedimentary

structures in the sandstone indicate periods of slow deposition by sediment fallout,

punctuated by a number of brief, high energy waning flow events. In non-marine

settings such conditions are found in an alluvial floodplain/distal fan or a (lacustrine)

delta front environment (e.g., Collinson, 1978; Elliott, 1978). The absence of

pedogenic features and the abundance of shale in this member relative to other units of

the Mud Hills Formation might favor a lacustrine environment, but no direct evidence is

seen (e.g., wave ripples, fossils). The presence of possibly coeval limestones to the

west in Owl Canyon is evidence for the existence of at least local ponds during this time

period, and lacusuine deposits are widespread in the overlying Barstow Formation

(Woodbume and others, 1990).

Upper Sandstone Member

Rocks of the Upper Sandstone Member are divided into five lithofacies (A-E) on the

basis of bedding, texture, and sedimentary structures (Fig. 3.12). Synsedimentary

folding of strata is common and affects all facies (Fig. 3.13a). The Upper Sandstone

includes rocks of two different compositions. The lowest 5 m of the unit contains light

red and very pale orange sandstone and breccia of predominantly volcanic composition.

The remainder of the member consists of grayish orange sandstone composed of quartz

monzonite debris. This compositional variation does not mark a fundamental change in

sedimentation but, instead simply reflects the reworking of the underlying volcaniclastic

rocks of the Pickhandle Formation tuff breccia tongue, followed by a return to a

basement source.

Clast-supported, poorly packed granule-pebble breccia containing tuff and granitoid

clasts in a muddy-sandy matrix is assigned to Facies A of the Upper Sandstone

Member. Breccia of this facies forms 0.15-1.2 m thick beds with gradational to sharp

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FACIES E SIO cm thick, tabular, discontinuous beds ot fine- (o very fine-grained, we# scried sar>ds(one. inlercalaled with <1 cm thick muds. Sharp contacts. Thicker t)eds may be normafly graded. wHh a basal tag of granules. Sedrnonlary structures irtdude parallel at>d vravy laminations, ripple cross-laminalions, (laser beddng, loaded microrTOles. folded lainmatiofts. and horizontal burrows. Abundant bloflte aligned parallel to bedding. Alternating slow deposition and high energy, rapid deposition.

FACIES D 5-10 cm. lateraly continuous, normally graded beds of ooerse-grairted lo pebbly sandstorm. Predominantly sharp contacts: some gradational upper contacts In lower, volcanic section. SedrmerCary structures include parallel laminations and ripple ooss^amlnations (S5 cm thick sets). Abundant biotite oriented parafiet (o bedding. 1 Deposition from turbulent, waning flows.

FACIES 0 2-5 cm thick beds of coarse-grained to pebbly sandstone containing abundant biotite aligned parallel to bedding. Sharp contacts. Inverse grading. 1 Grain flow or traction carpet.

FACIES B 2 0 -5 0 cm thick beds of dast-supported granule brecda containing randomly oriented, thick sheaves biotite. Sharp, norverosive contacts. Mostly massive. Some basal inverse grading and upper normal grading, both with abundant biotite flakes oriented paralie to bedding. Clay-poor, cohesive debris flow

FACIES A 40-120 cm Ihdtbeds of dast-supporied, poorly packed, muddy, granule and pebble brecdas. Sharp to gradational contacts. Modly massive. Some basal inverse grading and upper rxxmal grading. Volcanic composition (reworked). Low viscosity debris flow.

Fig. 3.12. Measured section of the Upper Sandstone Member of the Mud Hills Formation. See Figure 3.11 for key to symbols.

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Fig. 3.13. Photographs of general appearance and sedimentary structures of the Upper Sandstone Member of the Mud Hills Formation in Owl Canyon, a) Soft-sediment fold in the Upper Sandstone Member - note hammer for scale; b) sheared flame structure in sandstone of Facies E, and granule breccia of Facies B (note randomly oriented sheaves of biotite in Facies B ^ d ); c) thinly bedded, biotite-rich sandstone and mudstone of Facies E; d) burrows in sandstone of Facies E - curved structures in burrow to right of lens cap result from preferential alignment of biotite grains by burrowing organism.

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BBk

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ïr.i'?

W a r n gp;. ' ' -

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contacts, and may exhibit either a massive fabric or weak basal reverse grading and

upper normal grading. The inverse-massive-normally graded fabric, sheet-like

geometry, non-scouring bases and poor sorting of Facies A rocks suggest debris flow

deposition (e.g., Middleton and Hampton, 1973; Nemec and Steel, 1984). Although

the abundance of clay in these breccias may indicate cohesive transport processes,

several authors propose that clast-supported, inverse-normally graded debris flows are

dominated by turbulence and grain-grain interactions, and are transitional between

cohesive debris flows and high density turbidity currents (Lowe, 1982; Shultz, 1984;

Nemec and Steel, 1984).

Poorly sorted breccia containing loosely packed angular granules and small pebbles

of granitoid material in a matrix of medium-grained sandstone and clay is designated as

Facies B. These rocks form tabular beds ^ 0 cm thick and 5 m to tens of meters in

width. Breccia beds of Facies B are predominantly massive, but some may exhibit

inverse-massive-normal grading. These rocks are interpreted as debris flow deposits on

the basis of texture, fabric, and bedding, but differ from those of Facies A in terms of

their dominant transport processes. The unusual preservation and random orientation of

thick sheaves of biotite in massive parts of Facies B beds suggest that turbulence and

grain-to-grain interaction during transport were not important (Fig. 3.13b). This

indicates that cohesive forces dominated over frictional forces within the main part of the

debris flows and implies transport as a rigid plug or as a high viscosity flow (cf.,

Johnson, 1970; Lowe, 1982; Nemec and Steel, 1984). Cohesive debris flows

commonly contain abundant muddy matrix, whereas breccias of Facies B are clay-poor.

Clay contents of as little as ~1% have been reported, however, in cohesive debris flows

(Curry, 1966); in such cases the clay acts as a lubricant to prevent frictional freezing of

the flow (Rodine and Johnson, 1976). The basal inverse grading of Facies B beds

indicates higher rates of shear at the base of the flow and the operation of dispersive

pressure within a basal density-modified grain flow (Enos, 1977; Lowe, 1979; Nemec

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and Steel, 1984). These processes are reflected by the occurrence of thin flakes of

biotite oriented parallel to bedding within the inverse graded intervals, rather than the

thick, randomly oriented sheaves of the massive intervals. The normally graded

uppermost portion of Facies B beds likely resulted from tractive deposition from an

overlying stream or fluidal flow (e.g., Hampton, 1972; Nemec and Steel, 1984) or by

the frictional and diluting effects of overlying still water (Johnson, 1970; Hampton,

1975).

Facies C includes coarse-grained to pebbly sandstone of similar composition to that

of Facies B. Beds are 2-5 cm thick, reverse graded, and contain abundant flakes of

biotite oriented parallel to bedding. The occurrence of reverse grading in rocks of Facies

C suggests that, like Facies A and B, they are the product of mass flow. Reverse

grading in sandstones is commonly interpreted to be the result of dispersive pressure in

traction carpets, true grain flows, or density-modified grain flows (Lowe, 1976,1979,

1982).

Coarse-grained to pebbly sandstone beds with normal grading, parallel laminations,

ripple cross-laminations ( ^ cm thick sets), and abundant aligned biotite flakes are

assigned to Facies D. These 1-15 cm thick beds are laterally continuous and exhibit

sharp upper and lower contacts. Textures, grading, and tractive sedimentary structures

of Facies D beds suggest deposition by turbulent waning flows. Whether such currents

were streamflows or sediment gravit)' flows is not clear.

Tabular to lenticular, 1-10 cm thick beds of well sorted, fine- to very fine-grained

sandstone intercalated with <1 cm thick muds comprise Facies E (Fig. 3.13c).

Sedimentary structures of Facies E strata include parallel and wavy laminations, biotite-

rich ripple cross-laminations, flaser bedding, loaded microripples, flame structures, and

horizontal burrows (Fig. 3.13b, d). Thicker sandstone beds commonly exhibit normal

grading, with a basal lag of granule-size clasts. Rocks of Facies E record lower

depositional energies than those of other facies. The thin, tabular to discontinuous

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bedded sandstones were deposited rapidly, as indicated by abundant load casts and

liquefaction structures (Fig. 3.13b). Times of rapid sand deposition alternated with

quiet periods of clay fallout from suspension, evidenced by shale interbeds and drapes.

The depositional environment of the Upper Sandstone Member is not immediately

obvious. The occiurence of debris flows within the Upper Sandstone (Facies A, B), as

well as the dominance of alluvial fan and other coarse-grained "proximal" rocks

throughout the Mud Hills Formation, suggests a (distal) alluvial fan setting. A number

of features, however, provide evidence against an alluvial origin: (1) There is no

evidence of the pedogenic processes characteristic of distal fan settings; (2) The tabular

geometry and lateral continuity of beds in the Upper Sandstone Member contrasts

strongly with the irregular scour and fill of typical pebbly braided channel/sheet flood

deposits, such as seen in the Lower Breccia Member (cf. Figs. 3.7, 3.10); (3)

Burrowing, which is common in the Upper Sandstone, is not widely recognized in the

alluvial fan environment; (4) Slumping is probably rare in the distal fan environment

where slopes are low; (5) If the Upper Sandstone represents a distal fan setting, one

might expect gradational vertical transitions to more proximal fan facies; these are not

observed.

Interpretation of the unit as a fluvial deposit suffers from many of the same

drawbacks as those outlined above. Bedding geometries in the Upper Sandstone differ

from typical pebbly braided geometries. The deposits of meandering streams may

preserve cut-bank slumps (e.g., Laury, 1971) and point bar burrowing (R. Remy, pers.

comm., 1990), but point bar facies sequences are not recognized in the Upper

Sandstone. In addition, the abundance of debris flows in the Upper Sandstone

precludes a strictly fluvial setting.

An alternative inteipretation that appears to satisfy the available data is that the Upper

Sandstone is of lacustrine origin. The association of mass flows, rapid deposition,

slumping, and abundant burrowing is consistent with deposition on a subaqueous fan

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delta as described by numerous authors (e.g., papers in Koster and Steel, 1984; Nemec

and Steel, 1988). The Upper Sandstone does not, however, exhibit any features

diagnostic of subaqueous deposition. According to Nemec and Steel (1984), such

criteria include associations of diagnostic subaqueous facies and structures, such as

turbidites, fossiliferous interlayers, bioturbation, wave-generated structimes, mud/silt

interbeds or partings, pebbly mudstones, and slump-contorted muddy units. The

evidence for turbidite processes in the Upper Sandstone is equivocal and fossil remains

have not been recognized within the sequence, although burrowing is common. Small-

scale ripples in Facies E may be either wave- or current-formed, and muddy partings,

while present, are common in many sedimentary environments. Pervasive

synsedimentary slumping of the Upper Sandstone (Figs. 3.13a,b) is the strongest

supporting evidence for a subaqueous fan delta setting because it indicates a substantial

depositional slope. In addition to the facies association criteria outlined above, Nemec

and Steel (1984) proposed that subaqueous debris flows may be characterized by certain

viscosity-dependant fabrics. Fabrics appropriate to subaqueous debris flow deposition

are observed in rocks of Facies A and B, but such features may also be seen in the

subaerial environment (e.g., Shultz, 1983). In summary, evidence for subaqueous

deposition of the Upper Sandstone is ambiguous, but interpretation of the unit as part of

a small lacustrine fan delta, perhaps similar to the Devonian Domba Conglomerate of

Norway (Nemec and others, 1984), seems the most reasonable of the inteipretations

discussed above.

Coarser-grained deposits of the Breccia members

Dibblee (1968) interpreted the prominent breccia and megabreccia of the Mud Hills

Formation as debris flow or landslide deposits shed off rugged crystalline basement

highlands then adjacent to the present day Mud Hills. We concur with his interpretation

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and refine it by suggesting that these breccias are predominantly rock avalanche

deposits.

A key observation in our interpretation is the common presence of jigsaw and crackle

brecciation (Shreve, 1968; Yamold and Lombard, 1989) in the Middle and Upper

Breccias (Fig. 3.14a,b). This occurrence of slightly rotated and separated clasts which

can be fit back together like a jigsaw puzzle confirms the presence of matrix during

emplacement of the deposit and may indicate a lack of large scale turbulence (e.g.,

Yamold and Lombard, 1989). According to Yamold and Lombard (1989), well

developed jigsaw fabric is good evidence of rock avalanche deposition. A related fabric

which occurs in the Upper Breccia Member consists of trails of fine-grained clast

material radiating from the clast into the matrix. These features are less well documented

than jigsaw breccia, but have been observed in modem Himalayan rock avalanches

(Hewitt, 1988).

Felsite breccia lenses in the upper part of the Middle Breccia also yield evidence on

the origin of the deposits. These breccia lenses must represent either primary igneous

structures within the quartz monzonite basement or sedimentary features formed during

deposition. The former origin is more likely because no felsite clasts are found mixed in

with immediately overlying quartz monzonite breccia. Original source area stratigraphy

commonly is preserved in both rock avalanche deposits (e.g., Hsu, 1975; Krieger,

1977; Eisbacher, 1979; Hewitt, 1988; Yamold and Lombard, 1989) and rock-slumps

and slides, but the pervasive brecciation throughout the Breccia members of the Mud

Hills Formation, as well as the abundance of jigsaw/crackle breccia and the undulatory

basal contacts (Fig. 3.7) are more characteristic of rock avalanches (Yamold and

Lombard, 1989).

An interesting feature of the Middle Breccia Member is the occurrence of large, folded

but coherent pods of bedded sandstone and breccia, isolated within the middle of the

unit west of Rainbow Canyon (Fig. 3.6). Because relict stratigraphy is commonly

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Fig. 3.14. Photographs of sedimentary textures and structures in the coarse-grained breccia units of the Mud Hills Formation in Owl Canyon, a) jigsaw/crackle brecciation in felsite breccia of the Middle Breccia Member (ruler is -15 cm long); b) jigsaw/crackle brecciation in breccia of the Upper Breccia Member - angular white pebbles in upper center of photograph were derived from the leucocratic vein forming the upper part of the boulder on which the ruler rests; c) clastic dike of red tuffaceous sandstone (outlined) in the Lower Breccia Member.

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preserved in rock avalanche deposits, it is usually assumed that vertical mixing within

avalanches is not widespread (e.g., Yamold and Lombard, 1989). The presence of

these coherent pods indicates that such mixing does occur, and may argue for a flow

mechanism (e.g., Hsii, 1975) rather than a slide mechanism (e.g., Shreve, 1966,1968)

in the transport of rock avalanches.

Whereas the Middle and Upper Breccia Members of the Mud Hills Formation are

clearly rock avalanche deposits, we initially interpreted the upper 58-64 m of the Lower

Breccia as a thick series of amalgamated debris flows, based on the lack of

jigsaw/crackle breccia or megabreccia and the occurrence of cmde vertical variations in

fabric and cement on a 2-10 m scale. Correlation of some features of the Lower Breccia

with the facies model of Yamold and Lombard (1989) for rock avalanche deposits,

however, may also favor a rock avalanche origin for parts of this unit. Yamold and

Lombard (1989) document three zones within rock avalanche deposits: a basal mixed

zone overlies disturbed or undisturbed substrate, and is in tum overlain by a breccia

sheet consisting of a lower dismpted zone and upper matrix-poor zone. The matrix-

poor zone is dominated by jigsaw and crackle breccia and megabreccia, whereas the

dismpted zone contains matrix-rich breccia of generally smaller grain size. Additional

features of the dismpted zone include subhorizontal comminuted slip surfaces and

subvertical clastic dikes of mixed avalanche and substrate material. According to

Yamold and Lombard (1989), the mixed and matrix-poor zones are best developed in

distal deposits of rock avalanches.

Several features similar to those in the disturbed zones documented by Yamold and

Lombard (1989) are seen within the Lower Breccia: (1) The unit as a whole is matrix-

rich, although vertical variations are seen; (2) Clastic dikes of a red tuffaceous

sandstone, similar to that observed in the basal sandstone and breccia section in

Solomon Canyon, are found in the Lower Breccia Member in Owl Canyon (Fig. 3.14c);

(3) Thin, fine-grained horizons within the Lower Breccia may be the result of

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comminution on dip-parallel surfaces. If these features are indicative of a rock

avalanche disrupted zone origin for the Lower Breccia Member, then the slickenside

striations observed on the base of the unit in Owl Canyon may be primary depositional

structures rather than the result of interbed slip during folding of the Barstow syncline.

Depositional features of the Lower Breccia which do not support a rock avalanche origin

include the generally low relief basal contact of the breccia section of the unit, as well as

the lack of jigsaw/crackle brecciation. In addition, distal rock avalanche deposits

commonly overlie fine-grained sediments of the alluvial plain (Yamold and Lombard,

1989), rather than alluvial fan deposits such as those comprising the basal part of the

Lower Breccia.

In summary, the Lower Breccia Member exhibits features which are compatible with

either a rock avalanche or a debris flow interpretation. It is possible that this member

contains a mixture of debris flow and rock avalanche deposits, although differentiation

between individual flow units is difficult. Certainly, it is unlikely that the entire upper

56-64 m of the Lower Breccia represents the disrupted zone of a single rock avalanche,

for such zones are commonly not more than 10 m thick (Yamold and Lombard, 1989).

The repeated vertical variations in matrix content on a ~10 m scale argue for multiple

depositional events, whatever the mode of transport, as does the occurrence low in the

section at Solomon Canyon of swirly laminations defined by calcite cement, which may

reflect pedogenesis. An overall upward increase in maximum clast size is observed

through the Lower Breccia. Such a textural trend might be expected in a prograding

system dominated by debris flow deposition or within the deposits of a single rock

avalanche. The significance of this coarsening-upward trend is less easy to explain,

however, in terms of a multiple rock avalanche model.

The number of depositional events represented by each of the Breccia members is

important in determining whether the geometry of these units is a function of subsidence

or of depositional processes. The Lower Breccia obviously was deposited during

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multiple events, but the Middle and Upper Breccias could have resulted from single

events (based on thickness). The vertical progression of facies documented by Yamold

and Lombard (1989) is not clearly observed in either the Middle or Upper Member, the

mixed and disrupted zones appear to be thin or missing, possibly indicating that these

are proximal deposits. One indication that multiple events may have been involved is the

occurrence in the upper part of the Middle Breccia of a number of thin, fine-grained,

dip-parallel horizons which may be comminuted slip zones. According to Yamold and

Lombard (1989), these comminuted slip zones are largely restricted to the dismpted

zone in the lower part of rock avalanches. No such evidence is observed within the

Upper Breccia Member, however, and it is possible that this unit was deposited in a

single event, based on geometry (Fig. 3.3).

Provenance

The Pickhandle, Mud Hills, and Barstow Formations were deposited in a long,

narrow, NW-trending basin (Dibblee, 1968; Link, 1980; Dokka and Woodbume, 1986;

Dokka, 1989a; Woodbume and others, 1990). Clast lithologies, and facies and

thickness trends, indicate that rocks of the Barstow Formation were derived from

highlands to either the north or south, depending on their proximity to the respective

basin boundary (Link, 1980; Woodbume and others, 1990). Volcaniclastic rocks of the

Pickhandle Formation in the Mud Hills were probably supplied by volcanic centers to

the north and east at Lane Mountain and in the Calico Mountains (Dibblee, 1968; this

study) (Fig. 3.6). The provenance of basement-derived detrital rocks in the lower

Pickhandle Formation and the Mud Hills Formation, however, has remained obscure.

Dibblee (1968) noted the similarity between clasts of the Mud Hills Formation breccias

and crystalline basement exposed immediately north of the Mud Hills in the Williams

Plain (Fig. 3.2), but was at a loss to explain the occurrence of felsite clasts and the

paucity of diorite debris in the Mud Hills Formation given the present day distribution of

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these lithologies. Walker and others (1990) suggested that the Pickhandle and Mud

Hills Formations were derived from farther to the northwest, based on comparison of

exotic clast lithologies in the lower part of the Pickhandle Formation with basement

outcrop at Alvord Mountain. Data presented below refute this notion and demonstrate

that a large part of the Mud Hills Formation was in fact derived from a now buried or

eroded source south of the Mud Hills.

Direct indicators of sediment dispersal patterns in the Mud Hills Formation are rare

and are confined to the finer-grained units. The only potential indicator of transport

direction in the coarser facies are NNE-SSW trending grooves on the base of the Lower

Breccia Member in Owl Canyon. It is not clear, however, whether these are

sedimentary or tectonic features. Paleocuirent indicators are most abundant in the basal

sandstone and breccia sequence of the Lower Breccia Member. Measurements of cross

bedding and of "contact" and "isolate" imbricated clasts (terminology of Laming, 1966)

indicate overall northerly dispersal patterns (vector mean = 006°) within the unit.

Several measurements from the eastern Mud Hills show a secondary trend of west-

directed paleocurrents (Fig. 3.15). Limited data from the Solomon Sandstone and the

"Red T uff marker bed show patterns similar to those from the Lower Breccia Member

(Fig. 3.15). We feel confident in concluding that the reciprocal of the mean measured

paleocuirent direction indicates provenance because (1) basal strata of the Lower Breccia

were deposited in an alluvial fan environment, and (2) the Mud Hills Formation was

deposited in a narrow, NW-trending basin (Dibblee, 1968; Link, 1980; Dokka and

Woodbume, 1986; Dokka, 1989a; Woodbume and others, 1990). A southern source

terrane may also help to explain Dibblee's (1968) observation that no source for the

felsite clasts can be found on the Williams Plain. If our interpretation is correct, then we

are faced with explaining the present lack of a suitable source terrane south of the Mud

Hills. We address this issue below.

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Fig. 3.15. Paleocuirent directions measured in the Mud Hills Formation and lowermost Barstow Formation, a) Pebble/cobble imbrication in Lower Breccia Member near Coon Canyon: n=12; vector mean=334°; b) Pebble/cobble imbrication in Lower Breccia Member near Owl Canyon: n=17; vector mean=059°; c) Cobble imbrication in Owl Conglomerate in Owl Canyon: n=5; vector mean=200°; d) Minor fold axes in Upper sandstone Member in Owl Canyon: Dots, fold axes; Semicircular arrows, sense of asymmetry; Black arrow, generdized slip line; e) Cross bedding in Lower Breccia Member: n=24; vector mean=009°; f) Cobble imMcation in Owl Conglomerate in Solomon Canyon: n=5; vector mean=200°; g) Cross bedding in Solomon Sandstone Member in Solomon Canyon: n=6; vector mean=062°; hi) Cross bedding in Barstow Formation east of Copper City Road: n=4; vector mean=265°; i) Imbrication and cross bedding in Pickhandle Formation upper tuff breccia and sandstone tongue east of Solomon Canyon: n=7; vector mean=209°; j) Cross bedding in "Red Tuff' marker bed of Pickhandle Formation middle tuff breccia tongue east of Solomon Canyon: n=8; vector mean=285°; k) Pebble/cobble imbrication in Lower Breccia Member east of Solomon Canyon: n=6; vector mean=284°.

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In contrast to the lower parts of the Mud Hills Formation, the Upper Breccia Member

likely had a northern source, as did the overlying (northern) Owl Conglomerate (Fig.

3.15) (Woodbume and others, 1990). Not only is the Upper Breccia absent in its

expected stratigraphie position on the south limb of the Barstow syncline (Woodbume

and others, 1990), but geometric relations indicate that it probably pinches out not far

south of its southemmost surface exposure (Fig. 3.3). In contrast, underlying members

of the Mud Hills Formation thicken markedly to the south (Fig. 3.3). In determining the

timing of this change from southem to northern provenance, the origin of the Upper

Sandstone Member, which direcdy overlies the "Red Tuff and underlies the Upper

Breccia in Owl Canyon, assumes critical importance. Reliable primary paleocuirent

indicators are rare in the Upper Sandstone Member. Ripple forms, when measurable,

give a range of southerly to northwesterly orientations, but these small bedforms

probably reflect highly localized current directions (e.g., Allen, 1966). The sense of

shear in synsedimentary folds can, however, be used to determine paleoslope direction

(e.g., Collinson and Thompson, 1982). Because soft sediment folds within the Upper

Sandstone Member show the same sense of asymmetry on all scales from syn-

depositional, intrabed (Fig. 3.13b) to post-depositional, multiple bed (Fig. 3.13a), it is

reasonable to conclude that they all formed as a result of downslope movement. Using

the method of Hansen (1971), it is evident that folds within the Upper Sandstone reflect

southerly mass movement (Fig. 3.15).

In summary, lithologie, paleocuirent, and rock body geometric data indicate that the

lower part of the Mud Hills Formation was derived from a quartz monzonite basement

terrane to the south of the Mud Hills -19.8-18.7 Ma. A northern source of similar

basement lithology became dominant shortly following deposition and reworking of the

pyroclastic Pickhandle tuffs bisecting the formation (-18.7 Ma). This northern sediment

source, the Williams Plain, dominates drainage patterns in the area to this day.

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STRUCTURE OF THE MUD HILLS

The Mud Hills is dissected by several sets of faults. Predominantly northwest-

striking right-slip faults of late Neogene age dominate the area (Fig. 3.3). These faults

are related to regional right shear associated with Pacific-North American plate boundary

motions and form part of the recently recognized Eastern California Shear Zone (Dokka

and Travis, 1990ab). Late Neogene regional right shear also resulted in folding of the

Barstow syncline, which is spectacularly exposed in Rainbow Basin and Owl Canyon.

Although the sense of motion on strike-slip faults of the Mud Hills is dextral (Dibblee,

1968), small horsetail splays off these faults in the western Mud Hills show left

separation due to a dip slip component resulting from their orientation relative to major

fault strands (Fig. 3.3).

Less well documented are the west-northwest- to northwest-striking, north-dipping

normal faults in the Mud Hills (Fig. 3.16). Although abundant, most of these faults are

small (<10 m offset) and are not represented on Figure 3.3. The faults are difficult to

trace through the Breccia Members of the Mud Hills Formation, but they appear not to

cut the Mud Hills-Barstow Formation contact and may therefore predate Barstow

deposition.

Recognition that the Middle Breccia Member is repeated in Ross Canyon (Fig. 3.3)

led us to consider two hypotheses for the structure and stratigraphy of the east-central

Mud Hills. Our initial idea was that the contact between the tuff breccia tongue and the

upper breccia unit east of Owl Canyon is a repetition of the Pickhandle Formation-Mud

Hills Formation basal contact (cf. Ingersoll and Diamond, 1990). In this scenario, the

upper contact of the Lower Breccia Member in Solomon Canyon, along with its

eastward continuation as the lower contact of the tuff breccia tongue in Ross Canyon

(Fig. 3.3), constitute a north-dipping normal fault. The Solomon Sandstone would

actually be a correlative of the basal sandstone and breccia sequence of the Lower

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Breccia Member, rather than a separate member, and both of the breccia units exposed in

Solomon Canyon would belong to the Lower Breccia Member. Evidence in favor of

this hypothesis is that the Lower and Upper Breccia Members west of Ross Canyon are

remarkably similar in composition, fabric and thickness, and both members overlie light

greenish gray to yellowish gray lapilli tuff breccia of Pickhandle affinity. In addition,

the upper contact of the Lower Breccia Member in the Solomon Canyon area is steeply

dipping and overlying beds are slightly disrupted.

The second, preferred, hypothesis is that the stratigraphy and structure are as shown

on Figures 3.3 &3.5 and described above. We favor the second hypothesis for the

following reasons: (1) We found no direct evidence that the lower contact of the

Pickhandle tuff breccia tongue/Solomon Sandstone is a fault, or that it dips north;

indeed, the contact dips steeply southward in Solomon Canyon and clearly dips

southward both west of the Ross Canyon-Solomon Canyon drainage divide and east of

Solomon Canyon near the Mud Hills fault (Fig. 3.3); (2) Stratigraphie relationships

similar to those in Solomon Canyon are observed west of Owl Canyon, where these

units are not repeated by faulting (Fig. 3.3). In Rainbow Canyon, south-dipping,

greenish gray lapilli tuff breccia capped by red tuffaceous sandstone underlies the Upper

Breccia Member and overlies the Middle Breccia Member with a south-dipping contact

(Fig. 3.3). In Owl Canyon, the greenish gray lapilli tuff breccia is absent but the red

tuffaceous sandstone is present (Fig. 3.7), and although the Lower and Upper Breccia

Members are very similar in lithology, they are not repeated by faulting (Fig. 3.3); (3)

Just west of Ross Canyon a north-dipping, east-striking, normal fault separates the

Upper Breccia Member from the overlying Middle Breccia and is responsible for the

repetition of stratigraphy (Fig. 3.3). This fault changes strike westward to become a

northwest-striking, left-slip fault (Fig. 3.3). On the east flank of Ross Canyon and to

the east a steeply south-dipping fault separates the Upper and Middle Breccias (Fig.

3.3).

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The high-angle upper contact of the Lower Breccia Member in Solomon Canyon is an

unconformity. Disruption of beds overlying the contact is similar to that seen at the

contact between the Upper Breccia and the upper Pickhandle tuff breccia and sandstone

tongue in Solomon Canyon, which also appears to be quite steeply dipping (Fig. 3.3)

but is not faulted (Fig. 3.17). The steepening of overlying beds into both of these

contacts may be due to deformation of less competent sandstone, shale, and tuff units

against the buttresses formed by more competent breccia units during late Neogene

folding of the Barstow syncline.

DISCUSSION

Several workers previously have interpreted the Pickhandle/Mud Hills and Barstow

Formations as syn- and post-extension units, respectively, based on age and textural

trends (Dokka and Woodbume, 1986; Dokka and others, 1988; Dokka, 1989ab;

Glazner and others, 1989; Walker and others, 1990; Woodbume and others, 1990).

The evolution of the basin in which these rocks were deposited, however, remains

controversial (cf. Dokka, 1989a; Walker and others, 1990; Bartley and others, 1990).

Issues include the nature and location of the southem boundary of the basin, the location

of source areas, timing relationships, lower plate uplift history, and the original dip of

the underlying detachment. In the following discussion, we perform a basin analysis

using data presented above as well as published stratigraphie, petrologic, and

thermochronologic data. The results of this analysis provide a number of constraints on

the structural evolution of the area that may be used to evaluate previously published

tectonic models.

Paleogeography

The Tertiary sedimentary record in the central Mojave Desert begins with widespread,

thin, fluvial deposits of the Oligocene or older Jackhammer Formation (McCulloh,

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Fig. 3.16. Photograph of northwest-striking normal faults (arrowed) exposed in Owl Canyon. Key to abbreviations: Tpt, Pickhandle Formation; Tmbl, Lower Breccia Member, Tmbm, Middle Breccia Member.

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Fig. 3.17. Photograph of contact relations between Upper Breccia Member (Tmbu) and upper Pickhandle Formation tuff and sandstone tongue (Tpst).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 163

1952; Lambert, 1987). The first evidence of early Miocene deposition in the Mud Hills

area is the local accumulation of up to 200 m of fine- to coarse-grained, crystalline

basement-derived sediments, basalt, and tuff at the base of the Pickhandle Formation.

Walker and others (1990) identified the provenance of the basement-derived rocks in

this unit as Alvord Mountain, then northeast of the Mud Hills. The subsequent

accumulation of voluminous, predominantly pyroclastic volcanics attests to the

importance of volcanic activity during the early stages of development of the Mojave

Extensional Belt (Travis and Dokka, 1991, in prep.). The extent and geometry of the

basin at this time are, however, difficult to define. It is not until deposition of the Mud

Hills and Barstow Formations that we can recognize a distinct physiographic and

structural basin.

Rocks of the Mud Hills and Barstow Formations were deposited in a narrow, NW-

trending trough (Dibblee, 1968; Link, 1980; Dokka and Woodbume, 1986; Woodbume

and others, 1990). Woodbume and others (1990) estimated the width of this basin

during deposition of the Barstow Formation to be ~8 km. Sedimentation in the basin

during accumulation of the Mud Hills Formation (-19.8-18.1 Ma) appears to have been

dominated by catastrophic rock avalanche events, with alluvial fan and lacustrine

processes relegated to a more minor depositional role. Major sediment source areas

during deposition of the Mud Hills Formation included basement terranes both south

and north of the Mud Hills. A southem quartz monzonite source terrane controlled

sediment dispersal early during the accumulation of the formation, but after-18.7 Ma a

northern source of similar composition became dominant. A major change in the nature

of the southern source terrane occurred -16.5 Ma, as metamorphic clasts begin to appear

in southerly-derived deposits of the Owl Conglomerate exposed on the south limb of the

Barstow syncline at this time (M. O. Woodbume, written communication, 1991).

Although transverse depositional systems obviously were dominant in the basin,

westerly paleocurrents in the eastern part of the Mud Hills may refiect some longitudinal

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transport by a fluvial system along the basin axis. The recognition of lacustrine deposits

in the Mud Hills Formation, however, along with their preeminence in the upper part of

the Barstow Formation, suggests that the basin underwent interior drainage.

Active tilting during deposition of the Mud Hills Formation is recorded by the

divergent geometry and intraformational unconformities within the unit. In addition,

rock avalanches are commonly known or inferred to be triggered by seismic activity

(e.g, Mudge, 1965; Shreve, 1966,1968; Eisbacher, 1979), giving further credence to

previous interpretations of the unit as a syntectonic deposit associated with extensional

faulting. An angular unconformity of ~ 20° separates the Mud Hills and Barstow

Formations, suggesting that Mud Hills Formation strata formed topographic relief

during deposition of much of the Barstow Formation. This is shown by the basal onlap

of the northern Owl Conglomerate and by the occurrence of a stranded Barstow

shoreline marked by discontinuous limestone beds and large stromatolites encrusted on

granitoid boulders of the Upper Breccia Member in the western Mud Hills.

In contrast to the Mud Hills Formation, the overlying Barstow Formation appears not

to record significant syn-depositional tilting. The presence of local breccias and

intraformational unconformities recognized by Woodbume and others (1990), however,

may imply some tectonic activity. Because the basal Owl Conglomerate on the south

limb of the Barstow syncline is in equivalent stratigraphie position to the upper Mud

Hills Formation, however, the lowermost part of the Barstow Formation (on the south

limb) is probably syntectonic in origin. The Barstow Formation is dominated by thick

alluvial fan deposits in its lower parts and by finer-grained deltaic and lacustrine strata

higher in the section. The unit is a fining-upward package which has been interpreted to

record infilling of the basin after extension had ceased (Dokka and Woodbume, 1986;

Dokka and others, 1988; Dokka, 1989a; Glazner and others, 1989; Walker and others,

1990; Woodbume and others, 1990).

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The limits of the basin in which the Mud Hills Formation was deposited are obscure.

The nature and location of the southem boundary to the basin is particularly intriguing,

both for its controversy (cf. Dokka, 1989a; Walker and others, 1990) and because a

major change in the southem source terrane occurred subsequent to the Mud Hills

Formation, during deposition of the lower Barstow Formation. The position of the

southern basin margin at the start of Mud Hills deposition is constrained by upper plate

petrologic variations. Sedimentary rocks now exposed in the upper plate in the

Waterman Hills and assigned to the Pickhandle Formation by Dibblee (1967) and

Walker and others (1990) have clasts that are compositionally different from those of the

Mud Hills Formation (cf. Walker and others, 1990; this paper). Because they overlie

volcanic rocks which are equivalent to the thick Pickhandle Formation tuff breccias in

the Mud Hills (Walker and others, 1990), these sedimentary rocks occupy the same

stratigraphie position as the lower Mud Hills Formation and cannot be derived from

north of the Mud Hills as suggested by Walker and others (1990). Sedimentary rocks

now exposed in the Waterman Hills therefore accumulated in a separate basin, south of

the basin in which the Mud Hills Formation was deposited. The southem boundary of

the "Mud Hills" basin is, thus, located in the area between the Waterman Hills and the

Mud Hills.

Clues as to the nature of the southem basin margin may be found in the geometry of

the Mud Hills Formation and perhaps in its sedimentary history. The divergent

geometry and intraformational unconformities of the formation (Fig. 3.3) indicate

asymmetric subsidence and are consistent with the half graben structural setting

predicted by previous tectonic models (Dokka and Woodbume, 1986; Dokka and

others, 1988; Glazner and others, 1989; Dokka, 1989a; Walker and others, 1990).

Alluvial fan deposits at the base of the formation indicate topographic relief and a quartz

monzonite basement terrane to the south, but imply little else about the nature of the

basin margin. The presence of rock avalanche deposits in the Mud Hills Formation,

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however, indicates substantial relief at the basin edge and is, thus, consistent with a

faulted margin. Large rock avalanches commonly are derived from fault scarps and

other cliff faces (e.g., avalanches in Switzerland (Heim, 1882; Hsu, 1975), the

Himalayas (Hewitt, 1988), Montana (Mudge, 1965), and northwestern Canada

(Eisbacher, 1979)), although they may also develop on the limbs of antiforms, where

preexisting planes of weakness such as bedding exert a first order control (e.g., Mudge,

1965). Based on the data presented and discussed above, we interpret the southern

boundary to the basin as a fault which is presently buried in the region between the Mud

Hills and the Waterman Hills. This inferred fault is in equivalent position to the Fossil

Bed Road fault of Dokka and others (1988).

The northern boundary to the basin presumably lies near the edge of the Williams

Plain, based on facies distributions in the Barstow Formation (Link, 1980; Woodbume

and others, 1990). The presence of northerly-derived, thick, alluvial fan strata of the

Owl Conglomerate and rock avalanche deposits of the Upper Breccia Member of the

Mud Hills Formation, might be taken as evidence for a steep, faulted margin. Dibblee

(1968) found no evidence for a faulted margin in this area, however, and southerly,

divergent stratal tilts in the Mud Hills Formation support the interpretation of this

boundary as a less active feature than that in the south. Continental half graben

commonly exhibit extensive alluvial fan-braid plain deposits derived from the hanging

wall (dip slope), whereas footwall-derived fans adjacent to major fault scarps tend to be

areally and volumetrically small (e.g., Denny, 1965; Crossley, 1984; Leeder and

Gawthorpe, 1987; Cavazza, 1989). In addition, less active basin margins may be

characterized by the dominance of streamflow processes over debris flow processes on

fans (e.g.. Steel, 1976). The excellent development of streamflow-dominated alluvial

fan conglomerates at the base of the Barstow Formation on the north limb of the

Barstow syncline is consistent with a dip slope interpretation for the northern margin of

the basin. In light of this interpretation, the presence of northerly-derived rock

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avalanche deposits in the upper part of the Mud Hills Formation underscores the fact that

such events may be generated on an apparently unfaulted basin boundary (although

unrecognized smaller antithetic faults could be present).

Little is known about the eastern and western boundaries of the basin. Westerly

transport directions, along with thinning of the Mud Hills Formation and thickening of

the Pickhandle Formation in the easternmost part of the Mud Hills suggest that the

eastern part of the basin was dominated by the volcanic centers of the Calico Mountains

(Dibblee, 1968) and Lead Mountain (Lambert, 1987). Barstow Formation sediments,

as well as unnamed coarse clastic rocks of presumed Tertiaiy age, are found at the edges

and in the middle of the valley separating the Mitchel Range and the Calico Mountains.

The presence of these deposits indicates that the basin may have continued as far east as

Lead Mountain. In this area, a transfer zone between the Waterman extensional terrane

and the Daggett terrane probably terminated, or at least partitioned the basin (Dokka and

Woodburne, 1986) (Fig. 3.1).

The basin margin in the western Mud Hills is not exposed, but its presence may be

inferred from the distribution of the Pickhandle, Mud HUls and Barstow Formations

strata. No Pickhandle or Mud Hills Formation rocks are seen west of Fossil Canyon

(Fig. 3.3), and the overlying Barstow Formation thins across this area (Woodbume and

others, 1990). In addition, Woodbume and others (1990) note that the upper part of the

Barstow Formation in the westem Mud Hills is predominantly derived from the west.

We consider that the basin boundary in this area is probably a buried transfer fault

between the Waterman and Edwards extensional terranes (Fig. 3.1). It is not exposed

as it is overlapped by the post-extension basin fill of the Barstow Formation. A

concealed fault of unknown significance was previously inferred to lie at the same

location by Dibblee (1967).

In summary, the Mud Hills Formation was deposited in a NW-trending half graben

-19.8-18.1 Ma. The basin was filled with coarse-grained quartz monzonite debris

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derived from both the south and the north. Similar extensional basins probably existed

to the southwest, and traces of their presence are evident in the tilted upper plate clastic

rocks in the Waterman Hills. Basement debris of the Mud Hills Formation was derived

from the southern source early during the accumulation of the unit (-19.8-18.7 Ma), but

after -18.7 Ma the northern source (Williams Plain) became dominant The appearance

of more abundant metamorphic clasts in southerly-derived deposits of the Owl

Conglomerate -16.5 Ma (M. O. Woodbume, written communication, 1991) provides

evidence of a change in the nature of the southern source teirane at his time.

Tectonic implications

Perhaps the most interesting and important feature of the Mud Hills Formation is that

its development can be linked directly to crustal-scale events in the evolution of the

Mojave Extensional Belt. Thermochronologic data indicate that shear zone rocks now

exposed in the Mitchel Range-Hinkley Hills-Waterman Hills metamorphic core complex

cooled extremely rapidly from above 320°C to below 70°C near 20 Ma, about the time at

which deposition of the Mud Hills Formation began (Fig. 3.18) (Jones, 1990; Jones

and others, 1990). Pressure constraints from metamorphic petrology indicate that rocks

of the shear zone occupied depths of 10-17 km just prior to extension (Heniy and

others, 1989; Henry and Dokka, 1990), suggesting an equilibrium geothermal gradient

of 20-30°C/km (Henry and Dokka, in press) and a final burial depth of <3.5 km at -20

Ma. Jones (1990) demonstrated that the most reasonable mechanism for such rapid

uplift of these rocks is tectonic denudation by detachment faulting. By -16.5 Ma, lower

plate rocks of the Waterman Hills were exposed at the surface, as evidenced by the

occurrence of clasts of mylonite and other metamorphic rocks in southerly-derived

sandstones of the southern Owl Conglomerate. This coincidence in timing and location

of upper and lower plate events not only reiterates the syntectonic nature of early

Miocene rocks of the

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Rapid cooling ~20 Ma 400-1 / Ar-Ar Muscovite O 300 Ar-Ar Biotite CD In. 3 "W 2 0 0 - o h H FT Zircon Q. O 100 4 — 1—I FT Apatite I— ' emergence of lower plate 0 I I I » ' ■ I " I n - n - If' f 1 1'" T n ' I : I I j i I I II I 10 20 30 40 50 A g e (M a)

Fig. 3.18. Cooling path for lower plate rocks of the Waterman Hills and Mitchel Range, based on ^^Ar/^^Ar and fission track thermochronology data from Jones (1990) and Dokka and others (1991), and geological constraints discussed in text.

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Mud Hills, but may also allow us to constrain tectonic models of rifting in the Mojave

Extensional Belt.

Several models have been advanced for the tectonic evolution of the Waterman

terrane. Dibblee (1968) cited the widespread bipolar pattern of bedding attitudes as

evidence that the Barstow Formation was deposited in a symmetrical crustal downwarp,

but thought that the Pickhandle/Mud Hills basin may have been fault-bounded (though

such faults have not been identified). Dokka and Woodbume (1986) proposed that the

Pickhandle, Mud Hills, and Barstow Formations were deposited in an upper plate

extensional fault basin overlying a low-angle, crustal-scale, normal fault (detachment)

(Fig. 3.19a). Surfacing of the lower plate in the Waterman Hills was theorized to have

occurred by a combination of tectonic denudation and isostatic uplift. Dokka and others

(1988) later suggested as a second hypothesis that the final stages of uplift of the lower

plate occurred along a normal fault now buried between the Waterman Hills and the Mud

Hills (Fossil Bed Road fault). According to the Dokka and others (1988) model, the

Pickhandle and Mud Hills Formations were deposited on the hanging wall of this fault

(Fig. 3.19b). Glazner and others (1989) and Walker and others (1990) presented a

model similar to that of Dokka and Woodbume (1986), although they did not address

the issue of lower plate exposure. The main differences between the model of Glazner

and others (1989) and Walker and others (1990) and that of Dokka and Woodbume

(1986) lie in the proposed basin dimensions, sediment sources, timing relationships,

and lower plate uplift history (Fig. 3.19c). In contrast to the low-angle detachment

models of previous workers, Bartley and others (1990) suggested that the detachment

now exposed in the Waterman Hills and Mitchel Range had a substantially greater dip at

the time of extension (Fig. 3.20). According to Bartley and others (1990), the

detachment attained its present gentle inclination by flexural compensation during

unroofing of the lower plate (i.e., "rolling hinge" model of Wemicke and Axen (1988),

Buck (1988), and Hamilton (1988)).

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Iron M ountan Waterman Hits Mud Hiite a) B3fstowFm.

Mud Hib Ftn

b) Iron Mountain Waterman H3b Mud Hilb

Hoad fault

PiddjandhFrn. c) (M . Mud Hib

Iron Mountain W aterman HiVs Mud Hilb BarsfowFm-s,

Fig. 3.19. Tectonic models for the development of the Waterman terrane. a) simplified after Dokka and Woodbume (1986); b) simplified after Dokka and others (1988); c) simplified after Glazner and others (1989). Models a) and b) depict a conceptual NE-SW cross section through the terrane at the end of Barstow Formation deposition (~13.4 Ma) (Woodbume and others, 1990). Model c) shows similar cross sections for ~19 Ma (upper figure) and 13 Ma (lower figure).

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Fig. 3.20. Rolling hinge model for the tectonic development of the Waterman terrane (based on models of Wemicke and Axen (1988), Buck (1988), and Hamilton (1988)). a) Preextension geometry. Dip of incipient detachment (50°) corresponds to that indicated by Bartley and others (1990) for the Waterman terrane (their Figure 1 ); b) Minor extension. Upper levels of the detachment have flattened out due to flexural compensation and are no longer at a favorable dip for fault movement. Dashed line shows an incipient new fault which will cut through the hanging wall of the detachment, c) Major extension. A series of tilted fault blocks containing upper plate basement and syntectonic strata are stranded on the footwall of the detachment. These fault blocks formed by continued flattening out of upper levels of the detachment and the sequential development of new faults as the active detachment passed beyond dips favorable for fault movement. Note that excisement of the hanging wdl of the detachment predicts younging of fault blocks and the sedimentary packages they carry with proximity to the hanging wall basin, as well as petrologic affinity between sedimentary rocks in the fault blocks and those in the hanging wall basin.

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a) hcpient fault 0 -1

• •»•♦• • • • 10 H / %%%%% X \

2 0 -»

indpiont faut b) ^ Hanging wall bask)

I 10 H o8-

Tiled fauti c) blocks K Incipient fault Hanging waB basin

I 10 H OI

20 - *

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In order to remain viable, tectonic models for the development of the Waterman

terrane must be compatible with data on the paleogeography and the sequence of events

outlined above. In a very general sense, all of the published low-angle detachment

models are compatible with these results, although there are obvious disagreements in

timing, provenance, and basin dimensions in the case of the Glazner and others (1989)

and Walker and others (1990) models. The new data presented here do not permit

differentiation of the models of Dokka and Woodbume (1986) and Dokka and others

(1988). The change in the nature of the southern source terrane ~16.5 Ma may be

ascribed to tectonic/erosional unroofing of the lower plate in the Waterman Hills or to

the suppression of a southern quartz monzonite source by uplift of the Waterman Hills.

Both of these hypotheses explain the present-day absence of a suitable southern source

at the surface. Similarly, a change in the dominant sediment soiu-ce from south to north

could result from various events: erosional destruction of the southern source terrane,

cessation of activity on the Fossil Bed Road fault, or the development of increased relief

in the Williams Plain by antithetic faulting.

Because the "rolling hinge" mode of extension advocated by Bartley and others

(1990) involves the sequential removal of fault-bounded slices from the hanging wall of

the detachment ("excisement" of Lister and Davis (1989)), it has at least two major

implications for the evolution of upper plate basins. Firstly, successive fault blocks and

the sedimentary packages they carry should become older with increasing distance from

the hanging wall basin (Buck, 1988). Secondly, the continued removal of pieces of the

hanging wall basin should result in a complex stratigraphy involving multiple recycling

of sediments over a short time period, perhaps similar to the cannibalization of foreland

basin sediments by advancing thrust sheets (e.g., Burbank and Raynolds, 1988). It is

not possible to resolve differences in age between upper plate syntectonic sedimentary

units at different localities within the Waterman terrane, because the major episode of

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detachment faulting in the Mojave Extensional Belt was extremely shortlived (Jones,

1990). We can, however, discount the excisement model of upper plate extension

because upper plate rocks now exposed in the Waterman Hills exhibit petrology and

sediment dispersal patterns different to correlative strata in the Mud Hills.

To summarize, paleogeographic and timing constraints favor the original models of

Dokka and Woodbume (1986) and Dokka and others (1988). Similar models proposed

by Glazner and others (1989) and Walker and others (1990) do not correctly predict the

paleogeographic development of the area. No data are presently available to test the

Dokka and Woodbume (1986) and Dokka and others (1988) models against each other,

thus, we present altemative conceptions of the basin in which the Pickhandle-Mud Hills

and Barstow Formations were deposited (Fig. 3.21). The evidence presented by

Bartley and others (1990) for both extension and contraction in lower plate rocks of the

Waterman terrane may indeed indicate flexural rotation of the detachment during

unloading, but the magnitude of that rotation remains unknown. The predictions of the

Wemicke and Axen (1988), Buck (1988), and Hamilton (1988) rolling hinge models for

upper plate stmcture and stratigraphy do not match data from this area. If these

predictions for the behavior of the upper plate of a steeply dipping, crustal-scale normal

fault are correct, it is unlikely that the Mitchel detachment was originally steeply dipping.

CONCLUSIONS

The following conclusions were reached during this study;

(1) Early Miocene crystalline basement-derived breccias exposed in the Mud Hills

north of Barstow, Califomia have previously been considered a part of the Pickhandle

Formation (Dibblee, 1967). These breccias differ markedly from volcaniclastic strata in

the type area of the Pickhandle Formation ~6 km northeast of the Mud Hills at

Pickhandle Pass. The distinctive lithologie character of these basement-derived breccias

and their substantial thickness warrants their differentiation from the predominantly

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1 7 6

WalwiMnHiM Üpfmpl0hiÊUtttoda. 1 têfgtfytrod^

Waterman Hifo Mud Hlb Wifiaim Ptaîn

Pic*h«ftdl« I ^ I Fofmaion 6#f«*ow Formetom l'T :J Upp* pWWoyAUln# bewmenl sn*«r zone rod(« ^ 3 Müd Fofmzlon

Fig. 3.21. Schematic models for the early Miocene tectonic development of the Mud Hills, a) based on the tectonic model of Dokka and Woodbume (1986); b) based on the model of Dokka and others (1988).

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volcaniclastic Pickhandle Formation and their designation as a separate unit, the Mud

Hills Formation.

(2) The Mud Hills Formation is ~300 m thick and is subdivided into five mappable

members that include three coarse-grained breccia units, and two finer-grained

sandstone and shale units. The unit both overlies and interfingers to the east with the

predominantly pyroclastic Pickhandle Formation, and is unconformably overlain by the

Barstow Formation. The Mud Hills Formation accumulated between ~19.8 Ma and

-18.1 Ma.

(3) Rocks of the Mud Hills Formation are interpreted as rock avalanche and debris

flow deposits, with subordinate alluvial-fluvial and lacustrine deposits. Basement

debris of the Mud Hills Formation was derived from a southern source early during the

accumulation of the unit (19.8-18.7 Ma), but after -18.7 Ma a northern source

(Williams Plain) became dominant. A major change in the nature of the southern source

terrane took place -16.5 Ma, based on changes in the composition of southerly-derived

sedimentary rocks. This change was likely related to tectonic/erosional unroofing of

lower plate rocks in the Waterman Hills.

(4) The Mud Hills Formation likely accumulated in a NW-trending half graben that

had both northerly and southerly crystalline basement sediment sources. Similar

extensional basins probably existed to the southwest, and traces of their presence are

evident in the tilted upper plate clastic rocks in the Waterman HiUs.

(5) Stratigraphie data from the Mud Hills and thermochronologic data from the

Waterman Hills support the published tectonic models of Dokka and Woodbume (1986)

and Dokka and others (1988) for the region. The rolling hinge model of extension by

high-angle, crustal-scale normal faulting (Wemicke and Axen, 1988; Buck, 1988;

Hamilton, 1988) is precluded because the sffatigraphy of the upper plate in the

Waterman terrane does not match the predictions of the model. Structural evidence for

both extension and contraction of shear zone rocks in the Waterman Hills may reflect

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flexural rotation of the detachment in response to unloading (the "rolling hinge")

(Bartley and others, 1990), but the magnitude of that rotation is unknown.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Stratigraphie architecture of extensional 4 basins: insights from a numerical model of sedimentation in evolving half graben

INTRODUCTION Mathematical models linking geodynamic, basin-forming processes with those of

erosion and sedimentation have potential to generate new insights into the fundamental

controls on stratigraphie architecture of evolving basins. In addition, such models

provide a testing ground for previously established conceptual models of basin fill.

Several disparate approaches have been taken to modelling the stratigraphie evolution of

geologic basins. Many workers investigating the large-scale tectonic processes of basin

development have made the simplifying assumptions that no erosion of landforms takes

place and that topographic basins fill instantaneously with sediment (or water) up to a

specified horizontal datum (e.g., McKenzie, 1978; Jarvis and McKenzie, 1980; Turcotte

and Schubert, 1982; Buck, 1988). Consequently, these geodynamic models generate

no predictions about lithology or stratigraphie architecture (Cross and Harbaugh, 1990).

At the other end of the spectrum are studies which focus mainly on understanding the

dynamics of sediment erosion, transport, and deposition (e.g., Paola, 1988,1990;

Anderson and Humphrey, 1990). Subsidence rate is a direct input to these models,

rather than being the result of simulated tectonic processes, and uplift is hot treated

directly. Such models provide little information on the influence of specific tectonic

regimes on stratigraphie architecture, although the general effects of uplift, subsidence,

and climatic variations may be examined by varying model sediment supply,

subsidence, and rates of sediment transport (e.g., Angevine and others, 1990).

Linking the basin-forming and sedimentary processes within a numerical model is

complicated by our limited understanding of the regional-scale dynamics of erosion.

179

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 180

sediment transport, and deposition. This limitation is particularly true of the subaqueous

realm, in which the stratigraphie effects of varying sea level are of substantial interest.

As a result, many integrated stratigraphie models of basin evolution take a geometric

approach to clastic deposition by maintaining an equilibrium topographic profile (e.g..

Pitman, 1978; Turcotte and Willeman, 1983) or by setting limits on topographic slope

(e.g., Strobel and others, 1990). In contrast, simulation of carbonate deposition is

accomplished using empirically-derived relationships between autocthonous

sedimentation rate and water depth, combined with a geometric approach to the dispersal

of erosional debris (e.g.. Read, 1986; Bice, 1988; Bosence and Waltham, 1990; Gildner

and Cisne, 1990). Few stratigraphie models have linked basin-forming processes with

a dynamic approach to erosion, transport, and deposition of sediment. One exception is

the work of Flemings and Jordan (1989) on synthetic stratigraphy of foreland basins.

These authors presented a model in which the generation of topography by thmsting and

consequent loading of the lithosphere is coupled with erosion and sediment transport

approximated as diffusive processes.

In this paper, we present an integrated numerical model which simulates the

tectonostratigraphic evolution of continental half graben. Erosion and sediment

transport are treated as diffusive processes and sediment grain size variations are

modelled in terms of perfect sorting during deposition. The model is completely

specified by six variables: fault geometry and slip rate, coefficients of erosion and

sediment transport, the effective elastic thickness of the lithosphere, and the grain size

distribution of sediment generated by weathering/erosion of bedrock. Model results

indicate that the diffusion and perfect sorting analogies adequately simulate the geometry

and textural characteristics of sedimentary units deposited in extensional half graben of

the southwestern U.S. In addition, the model generates several new insights into the

dépendance of topography, sediment package geometry, and textural trends on initial

fault geometry, rates of faulting and rates of erosion vj. sediment transport.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 181

DESCRIPTION OF THE MODEL

Three groups of processes control basin development in our model. Fault

movements create new topography, which is subsequently levelled by erosion and

sedimentation. Isostacy acts to compensate for the redistribution of mass caused by the

other two processes. No account is taken of the effects of thermal perturbations on

uplift/subsidence. The assumptions of the model are: (1) The lithosphere responds to

applied loads in the same way as does a thin elastic plate overlying a fluid substratum;

(2) Deformation of the hanging wall of major faults approximates vertical simple shear,

(3) Erosion rates and sediment transport rates are directly proportional to topographic

slope; (4) Depositional processes result in perfect sorting of sediment, such that all

grains of a given size class carried within a flow are deposited prior to fallout of particles

belonging to the next smaller size class.

Faulting

The model allows up to two major normal faults with an initial linear or listric form.

These faults merge downwards with a flat detachment at a user-specified depth.

Additional input parameters include fault spacing, surface dip direction and magnitude,

and horizontal slip rate. At each timestep during execution, an increment of horizontal

movement is applied to each fault block; hanging wall collapse is accomplished by

vertical simple shear. Slip rate may be varied through time, such that individuEiI faults

may undergo periods of reduced activity and subsequent reactivation or termination, and

new faults may be formed.

Isostatic compensation

Subsequent to fault movement, and again after sediment erosion and redistribution,

the lithosphere is allowed to undergo isostatic readjustment. The analogy to the flexure

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 182

of a thin elastic plate has been used successfully by several authors to model the

deformation of the lithosphere in foreland basins (e.g., Beaumont, 1981; Jordan, 1981;

Karner and Watts, 1983; Flemings and Jordan, 1989), subduction zones (Turcotte and

Schubert, 1982), island chains (Walcott, 1970; Turcotte and Schubert, 1982) and, more

recently, rifts (Buck, 1988; Weissel and Karaer, 1989). The vertical deflection of such

a plate {w(x) ) in response to a distributed load {qJix} ) may be described by:

.2 .2 £ + + q £ x ) (1) dx dx^ dx

where g is the acceleration due to gravity, pm the mantle density, P the horizontal force

on the plate, and D the flexural rigidity of the plate. Flexural rigidity is a function of the

elastic thickness of the plate (A):

(2)

(1 - V)

where E is Young's modulus, and v Poisson’s ratio. Equation (1) is arranged in an

implicit finite difference form for solution in the model (Appendix III; cf. Bodine,

1981).

Erosion and sedimentation

The basis of the model of sediment erosion and redistribution is the assumption that

sediment flux is directly proportional to topographic slope. Experimental, theoretical,

and observational data validate this assumption and the basic equations derived from it

for subaerial environments (e.g.. Culling, 1960,1965; Schumm, 1967; Ruxton and

McDougall, 1967; Nash, 1980; Begin and others, 1981). The stratigraphie fill of

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 183

foreland basins and simple half graben have been successfully modelled using this

approach (e.g.. Angevine and others, 1990; Flemings and Jordan, 1989 ). The slope-

sediment transport assumption may be expressed as:

(3)

where S is the sediment flux, h elevation, x distance, and k the coefficient of sediment

transport. The continuity equation,

dh _ 1 dS dt ' (i.^)d x (4)

where 0 is porosity, describes the change in elevation (sedimentation or erosion)

through time {t) at any point in terms of a balance between the amount of sediment

arriving at that point and the amount leaving. Substitution of (3) into (4) leads to the

sediment diffusion equation:

dh _ 1 d l

Terms accounting for subsidence and sediment supply from out-of-plane are not shown

in (5), but can easily be included (e.g.. Angevine and others, 1990). The physical effect

of applying equation (5) is that convex-up slopes are eroded, whereas concave-up areas

are sites of deposition. The equation is formulated in an implicit finite difference scheme

(Appendix III) and solved using numerical methods outlined in Press and others (1986).

Grain Size Variation

Downstream decrease in grain size in the alluvial environment is accomplished both

by abrasion and by selective deposition of larger clasts (e.g., Paola, 1988). Selective

deposition appears to be the dominant process, however, for rates of fining increase

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 184

with increasing rates of deposition (Angevine and others, 1990) and are typically several

orders of magnitude greater than abrasion rates estimated from laboratory studies (cf.

Krumbein, 1941; Kuenen, 1956; Brierly and Hickin, 1985). Mechanisms of

downstream fining by selective deposition are poorly understood (Paola, 1990;

Flemings, 1990). In this paper we follow the lead of Paola (1988,1990), by assuming

that selective sorting during deposition is perfect (i.e., all gravel is deposited from the

flow before any sand is laid down). According to Paola (1990), this greatly simplified

approach is justified by the high level of uncertainty in the grain size distribution of

sediment supplied to the system by weathering and erosion. Our model assumes that

sediment supplied to the depositional system by breakdown of bedrock falls into only

two size classes: gravel and sand. The relative proportions of these classes are specified

in the model input. Erosion of previously deposited gravels results in a grain size

distribution identical to that produced by bedrock weathering, whereas erosion of sandy

units produces only sands.

RESULTS

The output of the model includes three major manifestations of the underlying

control processes: basin geometry, sediment package geometry, and sediment grain size

variation in time and space. In this section, we examine the sensitivity of these

dependant variables to changes in each of the input parameters.

Figures 4.1 and 4.2 provide an introduction to the model output. Figure 4.1

illustrates two stages in the development of a pair of half graben. Relevant input

parameters are given in the caption to Figure 4.1; unless otherwise stated, these values

apply to all model output shown below. Figure 4.2 shows how sediment package

geometry and grainsize distribution in sediments may change during the evolution of a

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 185

Fig. 4.1. Stages in the tectonostratigraphic evolution of two half graben. Key to symbols: Heavy lines, faults; Dashed lines, pre-faulting marker horizons; Dash-dot lines, previous sediment surfaces (timelines); Shaded areas, gravel facies. In this model, the bounding faults are both listric, with a surface dip of 60° and a depth to detachment of 10 km. Coefficients of erosion and sediment transport have equal values {k^kf= 2.4 x 10^ m^ yr^). The model timestep is 10^ yr, and the sediment surface is plotted every 6 timesteps. Horizontal slip rate is 1 cm-yr^. The effective elastic thickness of the lithosphere is set at 5 km. These values apply to all model runs shown here, unless otherwise stated.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 186

CM E E T-- II o o 00 00 LÜ

o o

O o co CD

o o in LO

o o

o o co co

o o c\j (M

(L U > |) (a i> l) M J d e a m d 0 Q

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 187

0.0 0.5

20 30 40 50 km

0.0 0.5

20 30 40 50 km

V.E. = 4.2

Fig. 4.2. Geometric and textural trends associated with syn- and posttectonic sedimentation in a single half graben. A. Model output at the end of active faulting. B. Model output after posttectonic infilling of the basin. Weathering/erosion of basement rock is assumed to generate sediment with a grain size distribution of 40% gravel and 60% sand. All other input parameters as in Figure 4.1.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 188

single half graben. Features of interest include: (1) Syntectonic units are highly

asymmetric and underfill the basin, whereas posttectonic depositional packages are

symmetrical and overlap earlier basin boundaries; (2) During times of active fault

movement, gravel is trapped near the basin edge on the fault-bounded side of the basin

but progrades far out into the basin on the dipslope side. In posttectonic times, coarse

grained sediment from the fault-bounded side is initially able to reach farther out into the

basin, but overall retrogression is observed. The dominant control on facies patterns in

Figure 4.2 is sediment body geometry. Because most erosion takes place outside the

basin in this simple model run, the basin receives sediment of a constant grain size

distribution. Accordingly, a fixed area of each sedimentary unit consists of gravel. If

we assume equal flux from both sides of the basin, it is evident that a progression

towards more widely spread, symmetrical units results in rétrogradation of coarse

grained facies, whereas a change from asymmetric to symmetric packages (of equivalent

width) causes rétrogradation on the more gently dipping side of the basin and

progradation on the steeper side (Fig. 4.3).

Fault geometry and rate of slip

The model assumptions of a flat detachment and hanging wall deformation by

vertical simple shear yield differing basin geometries depending on the initial shape of

the bounding fault. Listric fault geometries generate asymmetric basins which appear

similar to actual half graben produced by rollover of the hanging wall of normal faults

(Fig. 4.4). In contrast, linear boundary faults simulate deforaiation of the hanging wall

by secondary, antithetic faulting and yield full graben (Fig. 4.4). Not surprisingly, full

graben exhibit symmetrical sediment body and grain size trends (Fig. 4.5). Bulk

rotation of the entire hanging wall block, such as occurs in association with domino-

style deformation, is not simulated in this model.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 189

" — T' 1 1 1 1 1

Fig. 4.3. Effect of sediment package geometry on facies architecture. A. Changing extent of unit B. Changing shape of unit Stippled regions fill 40% of each unit and represent gravel facies; unshaded areas depict sands. Dashed line indicates basin axis.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 190

E o 00

o co

o 'îl-

o CM

O LO O

(ai>|) L)}d8a Fig. 4.4. Differing basin geometries produced by varying the initial shape of the major bounding faults.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 191

0.0

^ 0.2

0.3 20 40 km30

V.E. = 34

Fig. 4.5. Symmetrical basin fill patterns resulting from a single linear fault dipping at 60° and passing downwards into a flat detachment at 10 km. All other input parameters as in Figure 4.1.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 192

Depth to detachment has only a minor effect on basin evolution. Shallow

detachments result in narrower basins, but stratigraphie patterns are unaffected (Fig.

4.6).

Traditional fault theory and observation of faults in outcrop indicate that normal

faults should initially form with dips of ~60° from horizontal (Anderson, 1951; Hubbert,

1951). The fact that most active, large, seismogenic normal faults dip between 30° and

60° lends further credence to this theory (Jackson, 1987; Jackson and White, 1989).

Not all normal faults dip steeply, however, for rotational movements during extension

may substantially decrease initial dips (e.g., Morton and Black, 1975; Proffett, 1977).

In addition, some workers have argued, based on field evidence, that crustal-scale

normal faults may form with low initial dips (5°-10°) in certain settings (e.g., Wernicke,

1981, 1985; Yin, 1989).

Initial fault dip at the surface affects both the size of the basin and the nature of

sedimentation within it. Steeply dipping listric,faults produce narrow basins and overall

aggradational-retrogradational facies patterns on both sides of the basin; however, basal

sedimentary strata exhibit initial progradation on the fault-bounded side of the basin and

rétrogradation on the dip side (Figs. 4.7a,b). In contrast, gently dipping faults generate

wider basins in which footwall-derived deposystems show strong progradation,

whereas hanging wall systems are entirely retrogradational (Figs. 4.7c,d). Figures

4.7a-d illustrate basin evolution on a short timescale (3.6 x 10^ years). When basin

development is modelled over several million years, overall facies architecture appears to

be independant of fault dip (Fig. 4.7e). Syntectonic facies patterns shown in Figure 4.7

are influenced not only by sediment body geometry, but also by contrasts in sediment

flux from opposing sides of the basin (Fig. 4.8). For a given amount of extension, the

ratio of fault dip to stratal dip (and, thus, footwall to hanging wall sediment flux) is

greater for gently dipping faults than for steeply dipping faults (Wernicke and Burchfiel,

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 193

0.0 1~li¥Tirnf— E

s z -4-^ Q. CD Q

0.2 20 30 40 50 km

0.0 E

IZ Q. CD Q

0.2 20 30 40 50 km V.E. = 80

Fig. 4.6. Variations in basin width produced by varying depth to detachment. A. Detachment at 10 km. B. Detachment at 5 km. All other input parameters as in Figure 4.1.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 194

0.0 E

0.1 CD Q

0.2 20 30 40 50 km

0.0

E £ 0.1 Q. CD Q

0.2 20 30 40 50 km

V .E .= 79

Fig. 4.7. Stratigraphie effects of varying the initial surface dip of a single, listric, basin- bounding fault. A. Dip = 80°. B. Dip = 60°. C. Dip = 30°. D. Dip = 10°. E. Dip = 30°, Model time = 3.6 x 10® yr. All other input parameters as in Figure 4.1.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 195

0.0 E

Q. .1 0) O

0.2 20 30 40 50 km D) stratigraphie layering not depicted 0.0 E

Q. .1 CD Q

0.2 20 30 40 50 km V.E. = 40

E) 0.0 E 0.4 Q. CD Q 0.8 20 40 60 80 km V.E. = 23

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 196

S 1 /S 2 = 1

S2 S I

(3 1 /8 2 > ll' E) |S 1 / S 2 < l \

Fig. 4.8. Effect of differential sediment flux on facies architecture. A. Sediment input from left hand side of basin only. B. Sediment input from right hand side of basin only. C. Equal sediment flux from both sides of basin. D. S è m e n t flux greater from left hand side of basin than from right hand side. E. Sediment flux greater from right hand side of basin than from left hand side. Stippled regions represent gravel facies; unshaded areas depict sands. Dashed line indicates basin axis.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 197

1982, their Fig. 10). Posttectonic sediment dispersal patterns in Figure 4.7 are strongly

affected by basin geometry. Progradation of footwall-derived deposystems associated

with relatively minor extension on gently dipping faults results from a progressive

rightward shift of the posttectonic basin axis with time (Figs. 4.7c,d). This

phenomenon is caused by underfilling of the structural basin during active extension,

with the result that the posttectonic basin inherits renmant structural topography from the

syntectonic stage (cf. Figs. 4.7b,c). When the period of active extension is prolonged,

however, the structural basin is entirely filled during the syntectonic stage and

posttectonic stratigraphie patterns are similar to those associated with steeply dipping

faults (Fig. 4.7e).

Estimates of extension rates in continental extensional settings vary by at least one

order of magnitude. For example, the Basin and Range province of the southwestern

U.S. is currently extending at a rate of ~1 cm yr^ (Buck, 1988), whereas calculations

based on timing relationships (Dokka, 1989; Dokka and others, 1991) and interpreted

magnitude of extension (Glazner and others, 1989) of the early Miocene Mojave

Extensional Belt in southern California yield rates of ~10 cm-yr^. Figure 4.9 illustrates

the model effects of varying extension rate on stratigraphie architecture. The main

stratigraphie manifestation of high extension rates is the development of a thicker

posttectonic section due to underfilling of the basin during times of active faulting (Fig.

4.9).

Effective elastic thickness of the lithosphere

The flexural rigidity of the lithosphere is controlled by its effective elastic thickness

(Te), which is in turn a function of the temperature structure and the curvature of the

plate (Buck, 1988; Watts and others, 1980; Kusznir and Kamer, 1985). Terrestrial

values of Te may range from 2 km to 80 km (e.g., Kamer and Watts, 1983; Watts and

others, 1980; Kusznir and Kamer, 1985; Lyon-Caen and Molnar, 1983).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 198

stratigraphie layering not depicted

E 0.0 1.0 Q. Q) 2.0 Q 10 2030 40 50 60 70 km

B) posttectonic stratigraphie layering not depicted

' e 0.0

10 3020 40 50 60 70 km

V.E. =4.5

Fig. 4.9. Stratigraphie effects of varying extension rates. A. Horizontal slip rate = 1 cm-yr^ B. Horizontal slip rate = 10 cm-yr^. AU other input parameters as in Figure 4.1.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 199

0.0

S. 0.1

0.2 20 30 40 50 km

B) 0.0

s z Q. CD Û

0.2 20 30 40 50 km

V .E .= 79

Fig. 4.10. Stratigraphie patterns associated with different values for the effective elastic thickness of the lithosphere (Te). A. Te = 0 km. B. Te = 70 km. All other input parameters as in Figure 4.1.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 200

Under a fixed load, the effect of increasing Te is to decrease the maximum deflection

of the plate and to increase the area over which this response takes place. In our model,

increasing the effective elastic thickness of the lithosphere spreads the effect of loading

by extension and sedimentation and results in a basin which becomes progressively

wider with time after extension ceases (Fig. 4.10). Consequently, more pronounced

posttectonic rétrogradation of coarse-grained facies is observed when Te is large than

when it is small (cf. Figs. 4.7, 4.10).

Coefficients of erosion and sediment transport

Sediment erosion and redistribution are controlled by the choice of transport

coefficients (k). Empirically and experimentally derived values of k may vary by several

orders of magnitude (see Flemings and Jordan (1989) for a recent compilation) and

reflect differing transport mechanisms in differing environments, as well as variations in

climate, vegetation, and source rock lithology (e.g., Anderson and Humphrey, 1990;

Nash, 1980). Transport coefficients in hillslope settings, where landforms degrade by

mass flow processes, are typically of the order lO'^-lO’^ m^ yr^ (e.g., Nash, 1980;

Colman and Warson, 1983; Hanks and others, 1984), whereas coefficients associated

with fluvial channel processes have values -lO^m^ yr^ (e.g.. Begin and others, 1981;

Kenyon and Turcotte, 1985). Use of these values in large-scale 2D models of basin

evolution results in an unsatisfactory match between model results and actual examples

of basin fill (Flemings and Jordan, 1989). The discrepancies result from

underestimating the effective erosional transport coefficients in the plane of the model.

In practice, small scale surface roughness and steep valley sides parallel to the plane of

the model have a significant effect on sediment flux in upland regions (Anderson and

Humphrey, 1990). Anderson and Humphrey (1990) suggest that a scaling factor of

10^-10^ should be applied to erosional transport coefficients in large-scale models. This

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 201

suggestion is supported by consideration of average fill rates for foreland basins

(Flemings and Jordan, 1989).

Our model addresses the variation of sediment transport coefficients with

environment by assigning different values of k at any specific location according to

whether erosion or deposition is taking place. Figures 4.11-4.13 illustrate the effects of

changing the ratio between the rates of erosion (kg) and sediment transport (k,). In order

to isolate these effects, fault movement occurs only during the first two timesteps, after

which the basin is allowed to fill for a further 30 timesteps. Lines representing

successive sediment surfaces are plotted after the first and second timesteps and

subsequently after each six timesteps. Figure 4.11 shows a model run in which kg=k,.

The output is similar to those of Figures 4.1 and 4.2; key observations are that the fault

scarp degrades by shallowing of the topographic slope and that sedimentary units lap

onto the scarp.

Figure 4.12 shows model output for which kg=100k(. In this case the system is

transport limited. Note the development of progradational sediment body geometries

both within the basin and on the footwall. These are caused by backing-up of sediment

behind an advancing depositional 'front'. Erosional/depositional modification of the

fault scarp is similar to that in Figure 4.11.

In Figure 4.13, kg=0.01k,. Features of interest include: (1) Sediment is transported

away from the fault scarp, which retreats but remains steep throughout most of its

history; (2) A steep, cliff-like erosional feature has developed on the dipslope side of the

basin. This is probably a result of removing erosional debris from the vicinity of its

source, thereby preventing sediment from lapping up against the slope and modifying

the erosional profile. Similar observations have been made by Anderson and Humphrey

(1990); (3) Although landform evolution is drastically different in Figures 4.11 and

4.13, the end results are hard to distinguish since the erosional topography may not be

preserved.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 202

O in

o m

o 2 o

o o co co o co

o CVJ o o OJ CVJ

o ou q CVJ CD CVJ C > C ) c > CD CD CD CD CD CD

(LU>1) n i d e a (LU>j) q j d e a (LU>i) q i d e a

00 ü

Fig. 4.11. Effects of changing the ratio between rates of erosion and sediment transport: I. ke=k. All other input parameters as in Figure 4.1.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 0 3

O i n o in

g o o

i l ! v o ï- co o c o o co

o CM o CM O CM

CD CM T f q CM -M; O O O

(uj>l) nidaa (uj>|) iiid ea (ui>|) nidaa

Cû ü

Fig. 4.12. Effects of changing the ratio between rates of erosion and sediment transport: II. X:e=100A:. Ail other input parameters as in Figure 4.1.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 0 4

E O CD

O m

o o lO LO

o o o \ —

o o co coo co

o o (M CM o o o

CD CM T t CD CM '( f

(IU)|) Mîdea (iu>i) MîdeQ CD ü

Fig. 4.13. Effects of changing the ratio between rates of erosion and sediment transport: III. )te=0.0U',. All other input parameters as in Figure 4.1.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 0 5

Grain size distribution

The grain size distribution of sediments generated by weathering of bedrock is

poorly understood, but is, in part, related to source rock lithology and climate (e.g.,

Pettijohn, 1975). In this paper, we assume that the weathering products of crystalline

basement contain 40% gravel size material (Pettijohn, 1975; his Fig. 3-11). The effect

on model output of varying this fraction is simply to change the areal proportions of

gravel and sand in the basin fill.

DISCUSSION

Comparison of model with observed stratigraphie relationships

Synthetic stratigraphies generated by our model compare favorably with stratigraphie

relationships observed in several extensional terranes. Seismic data from the early

Miocene Mojave Extensional Belt of southern California illustrate both the wedge-

shaped geometry of syntectonic units and the more evenly distributed nature of

posttectonic strata (Fig. 4.14a). Posttectonic amalgamation of basins shown in Figure

4.14a is duplicated in the model output of Figure 4.14b. At a much larger scale, syn-

and posttectonic stratigraphie geometries in the North Sea basin bear a striking

resemblance to the model relationships (cf. Figs. 4.15,4.2). An important implication

of the sedimentary unit geometries generated by the model is that posttectonic

stratigraphie onlap of extensional basin margins can be produced under a variety of

conditions, without recourse to temporal variations in the effective elastic thickness of

the lithosphere (e.g.. Watts and others, 1982) or to differences in the distribution of

extension between the crust and lithospheric mantle (White and McKenzie, 1988).

Model predictions for the distribution of coarse- and fine-grained facies within half

graben match theoretical and observed relationships. Several studies have demonstrated

that alluvial fan-braid plain conglomerates derived from the hanging wall dominate

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 206

W e s t E ast

1 Km

0.0

J 0.4

o 0.8 Q

48 km

Fig. 4.14. A. Seismic reflection profile from western Mojave Desert, showing an early Miocene extensional half graben and its syn- and posttectonic fill (after Dokka, 1989a). B. Model output for two half graben. Input parameters as in Figure 4.1.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 0 7

(spuooas) aujii

Fig. 4.15. Annotated line drawing of a regional deep seismic line from the North Sea (after White and McKenzie, 1988).

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 208

syntectonic sedimentation in half graben, whereas footwall-derived fans adjacent to

major fault scarps have limited areal extent (e.g., Denny, 1965; Crossley, 1984; Leeder

and Gawthorpe, 1987; Mack and Seager, 1990). Syn- to early posttectonic facies

relationships are exposed in the Mesilla basin of the Rio Grande rift (Fig. 4.16; Mack

and Seager, 1990). The observed aggradation-progradation of footwall-derived

depositional systems and aggradation-retrogradation of hanging wall systems correlates

well with model relationships (cf. Figs. 4.16,4.7b). Sedimentary rocks of syntectonic

origin exposed in the Mud Hills area of the Mojave Extensional Belt were deposited

predominantly by catastrophic rock avalanches and do not exhibit the grain size

variations predicted by the model, but overlying syn- to posttectonic strata of the

Barstow Formation fine-upwards at most locations, indicating overall posttectonic

rétrogradation of coarse-grained depositional systems (Woodbume and others, 1990;

Travis and Dokka, in review).

The true effects of fault dip on sedimentation are difficult to gauge from published

studies, but it is interesting to note that Morley (1989) has proposed, on the basis of

work in the East African Rift, that basins bounded by gently dipping faults may be

characterized by wider dispersal of coarse-grained sediments.

Several authors recently have attempted to describe extensional basin stratigraphy in

terms of systems tracts associated either with various stages of basin development or

with intrabasin changes in base level (Vail, 1987; Scholz and others, 1990; Prosser,

1991). The predicted and observed stratigraphie relationships discussed above fit well

with the scheme of Prosser (1991; Fig. 4.17), which is based on reflector

configurations observed on seismic reflection profiles. Wedge-shaped rift initiation

systems tracts (Fig. 4.17) likely correspond to the initial footwall deposystem

progradation/hanging wall rétrogradation observed in most of the model output and to

the syntectonic, rock avalanche-dominated stratigraphie packages of the Mojave

Extensional Belt. Rift climax systems tracts reflect increased ordering of the system and

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 2 0 9

(D C <

os Q

0 1n c I I I

Fig. 4.16. Cross section of the northern Mesilla basin (modified after Mack and Seager, 1990). Key to abbreviations: pC, Precambrian granite; Pz, Paleozoic carbonates; Tv, Tertiary volcanic rocks; Tf, Eocene "early rift" basin fill (first stage of rifting); QTc, Camp Rice Formation piedmont-slope facies; QTcf, Pliocene-Quaternary Camp Rice Formation axial fluvial facies. N.B., Mack and Seager (1990) consider progradational part of footwall derived QTc to be of "postorogenic" origin, despite subsequent movement on major boundary fault

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Fig. 4,17. Sequence stratigraphie model for continental extensional half graben (modified after Prosser, 1991). Key: 1, Rift inititation systems tract, characterized by wedge-shaped geometry, internal discontinuities and hummocky reflectors; 2, Rift climax systems tract, consisting of offlapping hanging wall systems and ag^ading footwall systems, overlain by aggradational systems on both sides of the basin and drape across underlying units; 3, Post-rift systems tract, characterized by subparallel, disctontinuous reflectors and strong onlap of the basin margins.

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are represented by syntectonic, aggradational to retrogradational facies patterns in the

model output and, perhaps, by conglomerates assigned to the lower Barstow Formation

in the Mud Hills of the Mojave Extensional Belt. Deposits of the post-rift systems tract

infill the topography created during extension; they correspond to posttectonic units in

both model and outcrop.

Limitations of the model

The model presented here is appealing, both for its simplicity and for its ability to

predict observed stratigraphie relationships. Nevertheless, there are several

shortcomings, which are addressed below.

Under most input conditions, model stratigraphies are dominated by aggradational-

retrogradational facies architecture. In contrast, meter to kilometer scale coarsening-

upwards cyclothems, indicative of prograding basin margin deposystems, are a common

component of some extensional basins (e.g.. Steel, 1976; Blair and Bilodeau, 1988).

Several factors may influence the ability of the model to produce prograding sequences

similar to those seen in the stratigraphie record. Small-scale progradation resulting from

incremental fault movements is below the resolution of the model output shown here,

but is reproducible by plotting facies distributions both at the beginning and at the end of

each model timestep. The lack of larger scale sequences may be related, in part, to the

failure of the sediment diffusion equation to account for the effects of slope length on

sediment flux. According to Statham (1977), sediment yield from fluvial systems in

eroding environments increases with increasing slope length. Footwall catchement areas

in active extensional settings are small in relation to hanging wall drainage systems, but

undergo expansion over time (e.g.. Hunt and Mabey, 1966; Crossley, 1984; Leeder and

Gawthorpe, 1987; Blair and Bilodeau, 1988). Consequently, sediment flux to footwall-

derived deposystems should increase with time, resulting in progradation. These

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relationships may be modelled, provided that the slope length dépendance of sediment

flux in eroding environments is understood (cf. Hirano, 1975).

Cyclicity in the continental sedimentary record may also be related to base level

changes and to climate. Modelling the effects of base level changes requires an

understanding of the dynamics of subaqueous sediment transport (see below), whereas

climate affects subaerial weathering and transport rates. Anderson and Humphrey

(1990) and Hirano (1975) discuss the effects of weathering rates on sediment supply,

whereas Angevine and others (1990) illustrate the dépendance of sediment transport

coefficients on stream discharge and investigate the stratigraphie effects of varying

sediment flux.

Further limiting factors on the utility of the model include the difficulty of assigning

appropriate coefficients of erosion and transport for subaerial sedimentary processes, the

effects of out-of-plane sediment transport, and the treatment of subaqueous deposition.

Calculated coefficients of erosion and sediment transport in the subaerial realm vary by

several orders of magnitude as a result of differences in climate, vegetation, source

lithology, and sedimentary process (e.g., Anderson and Humphrey, 1990; Flemings

and Jordan, 1989; Nash, 1980). Overall model stratigraphie relationships are preserved

under a variety of input conditions, but landscape evolution and fine-scale stratigraphy

are strongly dependent on the ratio between rates of erosion and transport (Figs. 4.11-

4.13). In addition, transport mechanisms (and, hence, transport coefficients) vary with

structural setting and/or topography. According to Steel (1976), steeper, more active

basin margins are characterized by the dominance of debris flow processes, whereas

less active boundaries exhibit mostly stream flow. In order to model these phenomena,

it is necessary to distinguish, within the model framework, regions dominated by fluvial

processes vi\ those characterized by mass flow processes.

Sediment transport perpendicular to the extension direction has a significant

influence on extensional basin stratigraphy (Steel and Gloppen, 1980; Frostick and

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Reid, 1987; Gawthorpe and others, 1990; DiGuiseppi and Bartley, 1991; Travis and

others, in review). User specification of sediment flux gradients perpendicular to the

plane of the model is easily accomplished, but satisfactory treatment of these dynamic

processes requires development of a 3-dimensional model.

Our discussion so far has centered on fluvial and subaerial mass flow processes,

since the diffusion model of sediment transport has only been validated for these

environments. Kenyon and Turcotte (1985) have applied the diffusion analogy to delta

front sedimentation, based on the assumption that creep due to wave action and

bioturbation is directly proportional to slope. This assumption may not be valid for

other subaqueous sedimentary processes, such as slope failure, turbidity currents, and

hemipelagic sedimentation.

In summary, many of the basic shortcomings of the model are related to our

simplified treatment of the dynamics of erosion and sedimentation. Improvements

currently under consideration include addition of a third spatial dimension, treatment of

the sediment flux-slope length relationship, segmentation of the model into areas

dominated by different sediment transport processes, and treatment of weathering as a

separate process.

CONCLUSIONS

The following conclusions were reached during this study:

(1) Integration of tectonic and sedimentary processes within a numerical model

allows investigation of the controls on extensional basin stratigraphy.

(2) Models based on the assumptions of a linear dépendance of sediment flux on

slope and perfect sorting of sediment during deposition yield many of the fundamental

stratigraphie relationships observed in continental half graben. In addition, these models

generate several new insights into the controls of fault geometry, rates of faulting, and

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rates of erosion v^.sediment transport on landscape evolution, sediment package

geometry, and facies distributions.

(3) Over short time periods, basins bounded by steeply dipping listric faults exhibit

overall aggradational-retrogradational facies architecture, whereas the wider basins

generated by gently dipping boundary faults are characterized by progradation of

footwall-derived deposystems and rétrogradation of hanging wall systems. Gross

stratigraphie patterns in longer-lived basins appear to be independant of initial fault dip.

(4) Depth to detachment and rates of fault slip affect basin width and the thickness of

the posttectonic sediment package, respectively. Variation in these parameters have little

influence on model facies distributions.

(5) Varying the elastic thickness of the lithosphere affects model posttectonic basin

width and facies patterns. It seems unlikely, however, that we can isolate these effects

from the stratigraphie record until the other prime controls on sediment distribution are

better understood.

(6) Absolute rates of sediment dispersal have less effect on landscape evolution and

stratigraphy than does the ratio between rates of erosion and sediment transport. In the

subaerial environment, rates of erosion are typically less than those of sediment

transport (e.g., Flemings and Jordan, 1989; Anderson and Humphrey, 1990). Under

these conditions, hillslopes evolve by parallel retreat rather than by shallowing, and

erosional debris is quickly redistributed away from its source area towards regional

topographic lows.

(7) Model limitations reflect our poor understanding of the dynamics of sediment

erosion and redistribution. Significant improvements to the model would include

addition of a third spatial dimension, treatment of the sediment flux-slope length

relationship, segmentation of the model into areas dominated by different sediment

transport processes, and treatment of weathering as a separate process. More

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sophisticated treatment of tectonic processes may also improve the ability of the model to

reproduce and to predict extensional basin stratigraphies.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The Prologue to this dissertation posed three questions:

(1) What is the regional stratigraphy in a continental extensional setting, and what

are the geometric and textural trends associated with the syn- and posttectonic basin fill?

(2) What are the basin fill patterns associated with individual tectonic elements of a

continental extensional terrane? (3) How can stratigraphie relationships be used to constrain tectonic models of

rifting?

Intervening Chapters provide partial answers to all of these questions, based on

previous literature, on field studies of the Mojave Extensional Belt, and on quantitative modelling of tectonic and sedimentary processes. The emphasis has been on non

marine basins which are not dominated by lacustrine sedimentation; conclusions fiom

this work may, thus, differ in some respects from those studies concerned with

subaqueous processes. The following paragraphs summarize the major conclusions of

this dissertation and provide the rationale for future research. More detailed conclusions

are found at the end of each Chapter.

WHAT IS THE REGIONAL STRATIGRAPHY IN A CONTINENTAL

EXTENSIONAL SETTING?

In Chapter 2, it was found that the regional stratigraphie framework of the Mojave

Extensional Belt is tripartite, consisting of (i) pre- to early synextension volcanic

deposits, unconformably overlain by (ii) syntectonic, basement-derived breccia and

megabreccia with local finer-grained units, overlain by (iii) a posttectonic fining-

upwardsequence of gravel, sand, shale, and limestone. Important variations on this

216

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theme include the early influx of basement-derived debris (e.g., Alvord Mountain), the

locally overwhelming influence of volcanic centers (e.g.. Calico Mountains), and the

local presence of overall coarsening-upward trends (e.g., central Edwards terrane).

Widespread pre- to early syntectonic volcanic deposits similar to those of the

Mojave Extensional Belt are found in several other Cenozoic extensional terranes in the

western United States, suggesting a causal link between early volcanism and later

surface expressions of rifting. Discreet early volcanic stages are not observed,

however, in similar, detachment-dominated extensional terranes in other parts of the

world; nor are such intervals seen on a consistent basis in rifts characterized by other

structural styles.

Textural trends in syn- and posttectonic stratigraphie units of the Mojave Extensional

Belt record the creation and subsequent destruction of landforms associated with

deformation of the upper plate and unroofing of lower plate rocks during regional

extension. Syntectonic sedimentary rocks at some localities are dominated by massive

breccias and megabreccias which were emplaced by landslides and rock avalanches

(e.g., Daggett Ridge, Mud Hills). Elsewhere, alluvial fan deposits exhibit coarsening-

upward sequences tens to hundreds of meters thick which are likely related to discrete

episodes of fault movement followed by inactivity (e.g., Azucar Mine, Rodman

Mountains). Lacustrine strata seem to be poorly represented in the syntectonic section,

although tilted Tertiary limestones are exposed along the southern margin of the Hinkley

Hills in the Waterman terrane and near Boron in the Edwards terrane. In contrast,

sedimentation in some active extensional orogens may be dominated by lacustrine

processes (e.g.. East African Rift). Posttectonic deposits of the Barstow Formation and

equivalents provide evidence for widespread lacustrine conditions subsequent to active

extension. The fining-upwards nature of posttectonic rocks in the Mojave Extensional

Belt is likely a signature of internally drained basins; post-extension development of

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throughgoing drainage may result in coarser-grained rocks being deposited (e.g., Eberly

and Stanley, 1978; DiGuiseppi and Bartley, 1991).

The diffusion model of sedimentation in evolving half graben described in Chapter 4

generates geometric and textural relationships similar to those observed in the Mojave

Extensional Belt and elsewhere. Model syntectonic sediment packages are highly

asymmetric and underfill the basin, whereas posttectonic units are symmetrical and

overlap earlier basin boundaries. During times of active fault movement, gravel is

trapped near the basin edge on the fault-bounded side of the basin but progrades far out

into the basin on the dipslope side. In posttectonic times, coarse-grained sediment from

the fault-bounded side is initially able to reach farther out into the basin, although overall

retrogression is observed. One obvious weakness of the numerical model is its inability

to produce coarsening-upwards cyclothems similar to those seen in syntectonic alluvial

fan deposits. This failing may result, in part, from oversimplified treatment of the

dynamics of sediment erosion and redistribution (see Chapter 4).

What then of the contention of Blair and Bilodeau (1988) and Heller and others

(1988) that basin fill coarsens upward overall, with the widespread distribution of

gravels throughout the basin corresponding to post-orogenic times? Certainly, many

basin fills exhibit a coarsening-upward motif (e.g., Blair and Bilodeau, 1988; Mack and

Seager, 1990), and it does seem likely that the first response to uplift/subsidence is the

deposition of relatively fine-grained rocks (e.g.. Steel and Wilson, 1975; Steel, 1976;

Reward, 1978; Leeder and Gawthorpe, 1987). On the other hand, some basins exhibit

overall fining-upwards trends (e.g., Bluck, 1980; Manspeizer, 1985; Williamson and

others, 1987; Stoakes and others, 1991; this study), and at some locations the

occurrence of faulting-induced catastrophic sedimentation events, such as rock

avalanches and landslides, may obscure the syntectonic stratigraphie record (Chapters 2

& 3). One critical question may be the rate at which alluvial fans prograde once faulting

has ceased. In foreland basin settings, this process may be slow, or perhaps obscured

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by the effects of basin size. Geologic evidence from the smaller rift basins suggests,

however, that progradation in these settings may be rapid enough to have a significant

effect in the interval between successive spasms of activity on individual faults (Frostick

and Reid, 1987; Steel and others, 1977; Steel and Gloppen, 1980; Chapters 2 & 4 of

this study). The evolution of facies patterns may also be strongly affected by other

factors. Pre-extension drainage may partially control sediment supply to the basin (e.g..

Fossil Canyon transfer fault in the Mud Hills?). Changing drainage patterns and/or lake

levels may result in changes of local base level, whereas eustacy affects basins

connected to the sea (Chapter 1). Climatic variations may have a dramatic long-term

effect on sediment erosion and transport rates and, thus, sediment body geometry and

facies patterns (Chapter 4). Finally, differing tectonic styles may affect stratigraphie

relationships, as discussed in Chapter 4 and below.

Regional stratigraphie understanding of the Mojave Extensional Belt remains

incomplete. Many correlations of syntectonic units are based on lithostratigraphy (e.g.,

Dibblee, 1967), although posttectonic strata of the Barstow Formation yield abundant

vertebrate fauna and have long been the subject of detailed biostratigraphic studies (e.g.,

Merriam, 1919; Frick, 1937). Recent studies have integrated biostratigraphic,

magnetostratigraphic, and isotopic data (e.g., Woodbume and others, 1990;

Woodbume, 1991), but the distribution of control points remains sparse, as does our

understanding of the interrelationship between studied stratigraphie sections. Future

stratigraphie work requires better chronostratigraphic control; the common occurrence of

tuffaceous beds and basalts throughout both syn- and posttectonic sections makes the

area a good candidate for additional isotopic dating studies.

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WHAT ARE THE BASIN HLL PATTERNS ASSOCIATED WITH INDIVIDUAL

TECTONIC ELEMENTS OF A CONTINENTAL EXTENSIONAL TERRANE?

Distinctive stratigraphie patterns are associated with individual tectonic elements of

the Mojave Extensional Belt (Chapters 2 & 3). A complex paieogeographic history of

basin formation and dissection associated with the progressive breakup of the upper

plate during extension is recorded by rocks now exposed in the breakaway zone of the

Daggett terrane. Sediment dispersal patterns in this area were dominated by drainage

from unextended regions across the Kane Springs transfer boundary of the terrane,

although erosional debris was also derived from the fault scarp of the breakaway.

Similar dominant sediment dispersal patterns, subperpendicular to extensional axes,

characterize the extensional Homelen Basin of Devonian age in western Norway (Steel

and others, 1977; Steel and Gloppen, 1980) and the strike-slip Ridge Basin of

California (Crowell, 1982). In these basins, tectonic transport of depositional packages

away from their point of origin by continued extension resulted in shingling of

successive sedimentary units. An important implication of this geometry of deformation

and sedimentation for breakaway basins is that, like strike-slip basins, total stratigraphie

thicknesses may be related less to the magnitude of vertical subsidence than to the

amount of lateral extension.

Despite the dominance of cross-transfer zone sediment dispersal patterns near the

breakaway of the Daggett terrane, strike-slip elements elsewhere in the Mojave

Exensional Belt had a variable effect on patterns of sediment accumulation. Syn- and

posttectonic sedimentary rocks exposed near the Kane Springs transfer zone in the

eastern Newberry and Rodman Mountains were mostly derived from across the strike-

slip boundary, with additional components eroded from tilted upper plate fault blocks

within the extensional terrane. In the Waterman terrane, however, the Lane Mountain

transfer zone appears to have had no effect on sediment accumulation in the Mud Hills

until deposition of the posttectonic part of the Barstow Formation.

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In contrast to proximal areas, upper plate extensional basins farther removed from

the breakaway zones of the Mojave Extensional Belt exhibit fairly straightforward early

Miocene structural and stratigraphie relations. Sedimentation patterns in simple

continental half graben are well documented (e.g., Alexander and Leeder, 1987; Leeder

and Gawthorpe, 1987; Frostick and Reid, 1987). Model simulations described in

Chapter 4 show good agreement with these observational data.

Syntectonic sedimentary processes associated with the topographic evolution of the

Mitchel Range-Hinkley Hills-Waterman Hills metamorphic core complex in the

Waterman terrane are preserved in deposits of the Mud Hills and Barstow Formations

(Chapter 3). Syntectonic rocks of the Mud Hills Formation are interpreted as rock

avalanche and debris flow deposits, with subordinate alluvial-fluvial and lacustrine

deposits. These strata likely accumulated in a NW-trending half graben that had both

northerly and southerly crystalline basement sediment sources. Basement debris of the

Mud Hills Formation was derived from a southern source early during the accumulation

of the unit (19.8-18.7 Ma), but after ~18.7 Ma a northern source (Williams Plain)

became dominant. This change was likely related to tectonic/erosional unroofing of

lower plate rocks in the Waterman Hills. The syn- to posttectonic Barstow Formation is

composed of alluvial fan, fluvial, and lacustrine strata. Outcrops of the Barstow

Formation in the southern part of the Mud Hills contain a minor component of

metamorphic clasts derived from final unroofing of the adjacent core complex -16.5 Ma.

Sediment dispersal patterns in some parts of the Mojave Extensional Belt may reflect

tectonic elements that are less well understood. For example, little work has been

reported on sedimentation patterns in the breakoff zones of the Mojave Extensional Belt;

at Alvord Mountain in the Waterman terrane, Fillmore and Miller (1991) postulate the

presence of an early antithetic fault on the basis of alluvial fan deposits.

The Mojave Extensional Belt provides much room for detailed stratigraphic-

sedimentologic studies. The work presented here merely scratches the surface of

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understanding. Several authors recently have attempted to describe extensional basin

stratigraphy within a sequence stratigraphie framework (Vail, 1987; Scholz, 1990;

Prosser, 1991). Accurate classification of the regional and local stratigraphie framework

of the Mojave Extensional Belt in terms of sequence stratigraphie models requires more

work, but the data and model results presented in this dissertation favor the approach of

Prosser (1991) (Chapter 4).

Two immediate opportunities for further research into local stratigraphie

relationships of the Mojave Extensional Belt present themselves:

(1) The hypotheses presented here that breakaway zone evolution through basin

formation and subsequent dissection can be documented from the stratigraphie record,

and that cross-transfer zone sediment dispersal dominates sedimentation in the

breakaway zone, require further documentation.

(2) The Barstow Formation in the Mud Hills area provides an excellent opportunity

to investigate the effects of tectonics and lake level fluctuation on lacustrine basin

stratigraphy through detailed sequence stratigraphie studies. A good chronostratigraphic

framework built from biostratigraphic, magnetostratigraphic and isotopic data exists for

the area (Woodbume and others, 1990), and the occurrence of several distinctive tuff

marker beds throughout the section would facilitate mapping of the lateral variation of

sequence stratigraphie elements.

HOW CAN STRATIGRAPHIC RELATIONSHIPS BE USED TO CONSTRAIN

TECTONIC MODELS OF RIFTING?

In Chapters 2 & 3, the traditional approaches of measuring facies relationships and

paleocurrents, together with structure mapping and recording of clast compositions, are

used to infer tectonic events and styles of deformation. This is best illustrated in

Chapter 3, where the ability to correlate upper plate stratigraphie events to lower plate

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uplift history, together with paieogeographic analyses, allowed testing of a crustal-scale

tectonic model of extension.

The Mud Hills Formation is a ~300 m thick unit of syntectonic breccia units, with

subordinate sandstones and shales, which accumulated in a northwest-trending half

graben between ~19.8 Ma and ~18.1 Ma. Similar extensional basins probably existed to

the southwest, and traces of their presence are evident in the tilted upper plate clastic

rocks in the Waterman Hills. Thermochronologic data indicate that shear zone rocks

now exposed in the Mitchel Range-Hinkley Hills-Waterman Hills metamorphic core

complex cooled extremely rapidly from above 320°C to below 70°C near 20 Ma, about

the time at which deposition of the Mud Hills Formation began (Jones, 1990; Dokka and

others, 1991). Pressure constraints from metamorphic petrology indicate that rocks of

the shear zone occupied depths of 10-17 km just prior to extension (Henry and others,

1989; Henry and Dokka, 1990, in press), suggesting an equilibrium geothermal gradient

of 20-30°C/km (Jones, 1990) and a final burial depth of <3.5 km at ~20 Ma. By ~16.5

Ma, lower plate rocks of the Waterman Hills were exposed at the stuface, as evidenced

by the occurrence of clasts of mylonite and other metamoiphic rocks in southerly-

derived sandstones of the southern Owl Conglomerate. Because the "rolling hinge"

mode of extension advocated for the Waterman terrane by Bartley and others (1990)

involves the sequential removal of fault-bounded shoes from the hanging waU of the

detachment ("excisement" of Lister and Davis (1989)), such a model has at least two

major implications for the evolution of upper plate basins. First, successive fault blocks

and the sedimentary packages they carry should become older with increasing distance

from the hanging wall basin (Buck, 1988). Secondly, the continued removal of pieces

of the hanging wall basin should result in a complex stratigraphy involving multiple

recycling of sediments over a short time period, perhaps similar to the cannibalization of

foreland basin sediments by advancing thrust sheets (e.g., Burbank and Raynolds,

1988). It is not possible to resolve differences in age between upper plate syntectonic

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sedimentary units at different localities within the Waterman terrane, because the major

episode of detachment faulting in the Mojave Extensional Belt was extremely shortlived

(Jones, 1990). The excisement model of upper plate extension can, however, be

discounted, because upper plate rocks now exposed in the Waterman Hills exhibit

petrology and sediment dispersal patterns different to correlative strata in the Mud Hills.

New approaches to investigating tectonics through examination of the sedimentary

record are suggested by the quantitative model of sedimentation in evolving half graben

presented in Chapter 4. The good match of model results with observed stratigraphie

relationships suggests that it may be possible to use the model in a predictive sense and

to infer certain tectonic attributes based on the comparison of model output with

observed stratigraphy. For example, model results indicate that short-term facies

evolution is affected by fault dip, whereas the depth to detachment controls basin size

(Chapter 4). Similarly, Morley (1989) asserted, on the basis of work in the East African

Rift, that sediment thickness and facies patterns, along with volcanism, may be

controlled by fault dip and depth to detachment. In addition, Morley (1989) made the

provocative suggestion that, if levels of detachment in rifts become shallower with time

due to changing thermal structure, it may be possible to generate coarsening-upwards

sequences that are not related either to declining extension rates or to changes in

sediment supply.

If the model presented in Chapter 4 is to be used as a predictive tool, it would benefit

from a more comprehensive treatment of tectonic processes. Inclusion of lithospheric

behaviors which allow simulation of metamorphic core complex evolution (e.g., lower

crustal flow, rolling hinge) would increase the utility of the model as a predictive tool in

stratigraphie studies of detachment-dominated extensional orogens. It is essential,

however, that one is able distinguish the effects of tectonics from other controlling

factors such as climate if comparisons of this type are to made. In addition, an

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improved understanding of the dynamics of sediment erosion and transport may help the

predictive ability of the model.

In conclusion, work presented in this dissertation continues the process, essentially

begun by T. W. Dibblee and notably continued by R. K. Dokka and M. O. Woodbume,

of documenting the tectonostratigraphic evolution of Miocene rocks deposited in

response to the development of the Mojave Extensional Belt. Several new conclusions

have been arrived at, and some of these may be applicable in attempts to understand the

stratigraphie evolution of other extensional orogens. Nevertheless, much remains to be

done.

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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPENDIX A. MEASURED SECTIONS OF THE MOJAVE

EXTENSIONAL BELT

Note: These are transcribed field notes. They contain observations, speculations, and many abbreviations. Facies abbreviations follow those of Rust and Koster (1984),

except for the addition of facies designations G l, which is clast-supported breccia with

random fabric, and G2, which is very clast-rich matrix supported breccia with random

fabric.

I. MEASURED SECTION OF THE MUD HILLS FORMATION IN OWL CANYON (TYPE SECTION)

(Measured by Chris Travis on 27-28 February, and 3 March, 1989.)

Beginning at the top of the section...

Owl Conglomerate is gryish orange to yellow-reddish at base, becoming gm upwards.

This staining at top Upper Breccia mbr Mud Hills Fm/base Owl Conglom. may imply

hiatus. Some basal reworking of Upper Breccia mbr of Mud Hills fm. Consists

predom. of Sh and imbricate Gm facies. [Sample OC-89-13 of clasts from Owl

Conglom. NB. does Owl Conglom. here contain VRFs? -see gmish felsite(tuff?) in

sample]

Angular unconformity to Owl Conglomerate [multiple photos]

Top Upper Breccia member Mud Hills Formation

• 81 m of gryish orange grntd breccia. Unbedded, mostly massive. Apparent overall

246

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coarsening-up trend: Ave gmsize pebble-lge cobble in lower 27 m, sm cobble-sm bldr

27-36 m, Ige cobble-med. bldr, with higher frequency of Ige bldrs above 36 m. Texture

varies from v.clast-rich, matrix supp. to matrix-rich clast supp.(G2-Gl), mostly clast-

rich G2 except nr top of unit. Note little dispersal of gms subsequent to breakage -see

photos. Also see some jigsaw fabric in leucocratic rx nr. Photo 8.16 location. [Photo

8.13 of grntd clast which has undergone little dispersal since breakup; Photo 8.14 of

typical appearance of matrix-rich grntd breccia; Photo 8.15 of typical appearance of

clast-rich gmtd breccia; Photo 8.16 of leucocratic debris in matrix concentrated nr.

leucocratic vein in gmtd bldr, Photo 8.17 of dispersal into matrix of fractured edge of

gmtd bldr marked by dioritic inclusion. Pencil marks farthest extent of biotite

debris].[Sample OC-89-12 of clasts from upper gmtd breccia mbr]

Grain size range: Peb-lge bldr

Max. grain size: 55 cm below 27m mark; 1.6 m above 27m

Ave. grain size: peb-lge cob below 27 m; Ige cob-med bid above

Roundness: angul-subang

Shape: subeq-bricklike

Sorting: poor

Packing: matrix-supp clast-rich to clast-supp matrix-rich

Composition: granodiorite, subordinate leucocratic vein material and rounded, finer-

grained dioritic clasts and inclusions. Matrix fine-grained with granules and pebbles

of gmtd and minor visible clay.

Base Upper Breccia member

Top Upper sst member

• 16.5 m gryish-orange VFG-pebbly sst, in four facies (A-D). Upper contact to Upper

Breccia member on east side of canyon appears fairly sharp, from CG-granule sst into

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gmtd breccia with sandy matrix. On west side of canyon, sst is overlain by a few

meters of v.weathered tuff-tuff breccia containing clasts of mafic blkrk. From distance,

contact has several meters of relief.

A) Granule sst. Fairly sharp, non-erosive contacts. Lateral continuity 5-10's m.

Massive to possibly reverse graded at base with aligned biotites (in general biotites are in

thick books and are unaligned). Upper parts of beds may or may not exhibit slight

fining.

Grain size range: gm-sm pb

Max. grain size: 3 cm

Ave. grain size: gm

Roundness: angul

Shape: subeq

Sorting: poor

Packing: poor pkd

Composition: gmtd with minor purplish RF's. Matrix MG-clay

B) CG-pebble sst. 5 cm-10 cm beds, sharp contacts. Fairly continuous laterally.

Normal grading, parallel laminations, ripple cross-laminations (sets ^ cm thick),

aligned biotites.

C) CG-pebble sst. 2-5 cm bedded. Reverse graded, with aligned biotites. Texture and

composition otherwise same as for facies B.

D) Fg-VFG sst and clay. 1 mm-10 cm tabular to discontinuous bedded, sharp contacts,

interbedded with thin (<1 cm thick) muds. Laminated, wavy laminated, and ripple

cross-laminated, with biot. defining laminations. May contain granules at base of

thicker beds and exhibit normal grading. May show flaser bedding, loaded

microripples, and soft sediment folding. [Sample OC-89-9 of FG-MG sst with ripples,

load casts and burrows]

Grain size range: FG-VFG

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Max. grain size: gm

Ave. grain size: VFG

Roundness: well rad

Shape:

Sorting: well

Packing: well pkd

Composition: arkosic-gratd, much biotite visible. Non-calcareous cement.

Sequence is as shown in Fig. 3.12, Chapter 3.

• 60 era pale red, EG sst. Sharp contacts, appearing undulatory, but affected by

faulting. Appears to contain some ~3 era thick, normally graded laminae (MG-FG).

Contains a few pebbles of gmtd and tuff nr. top.

Grain size range: FG

Max. grain size: Feb

Ave. grain size: FG

Roundness:

Shape:

Sorting: mod-well

Packing:

Composition: qtz and feldsp visible. Few grntd and tuff pebbles nr. top.

• 70 cm v.pale orange to It red, CG-VFG sst. 2-5 cm beds with sharp, planar bases.

Laminated, may be normally graded. Overall fining upwards.

Grain size range: CG-VFG

Max. grain size:

Ave. grain size:

Roundness:

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Shape:

Sorting: well

Packing:

Composition: tuffaceous.

• 80 cm v.pale orange pebbly/muddy breccia. Appears to be crudely bedded on 20 cm-

40 cm scale, with gradational contacts. Possibly reverse graded at bases, normally

graded at top.

Grain size range: Sm peb

Max. grain size: 13 cm

Ave. grain size: sm peb

Roundness: angul

Shape:subeq

Sorting: v.poor

Packing: clst-supp matrix-rich

Composition: gmish and purplish VRFs in FG-clay matrix.

• -1.7 m Lt. red to v.pale orange muddy, tuffaceous pebble breccias and VFG sst. 1-

15 cm beds. Lower 1.2 m shows gradational to sharp contacts, tuff clasts aligned

parallel to bedding and flattened and irregular in shape. Upper 50 cm shows sharp

bases, gradational tops, and normal grading

Grain size range: Peb-VFG

Max. grain size: 8 cm

Ave. grain size: peb

Roundness: angul

Shape: irreg.-subeq

Sorting: v.poor

Packing: v.poor

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Composition: Wt., powdery tuff with biot, and gmish and purplish VRF's in FG-

muddy matrix

• 1.2 m v.pale orange muddy granule sst. Lower contact gradational, upper sharp, non-

erosive. Massive. Probably a debris flow. Clay prob. of volcanic origin.

Grain size range: Gm

Max. grain size: 5 cm

Ave. grain size: gm-smpb

Roundness: angul

Shape: subeq

Sorting: v.poor

Packing: poor pkd

Composition: qtz, feldsp, gmtd RF's, felsite RF's, in matrix of clay and MG sand

• 60 cm Lt. red pebbly granule sst. Mostly massive, top few cm appear finer. Upper

contact gradational. Probably Gms

Grain size range: Gm-peb

Max. grain size: 12 cm

Ave. grain size: gm

Roundness: angul

Shape: subeq-bricklike

Sorting: v.poor

Packing: poor pkd

Composition: gmtd and powdery wt tuff pebbles-cobbles in matrix of qtz, feldsp,

and tuff granules, biot, and VFG-clay

Base o f Upper sst member on east side o f canyon

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Basal part of Upper sst member on canyon floor described below (equivalent to lower

part of unit described above on east side of canyon)

• 20 cm Lt. red FG sst. Massive, well sorted. Feldsp and biot. visible

• 10 cm Pale red VFG-pebble sst. Sharp contacts. Massive. [Sample OC-89-11 of sst]

Grain size ran^e: VFG-peb

Max. grain size: 2.5 cm

Ave. grain size: VCG-gm

Roundness: angul

Shape: subeq-irreg

Sorting: v.poor

Packing: poor pkd

Composition: irregularly shaped-swirled. It colored tuffs, and gmtd constituents in

VF-fg matrix

Assorted Notes on the Upper sst Member on West side of Owl Canvon

[Photos 8.6, 8.7 of burrows in FG sst, looking down on bedding

ssurface. Note biot orientation -flat in plane of bedding, except in

burrows, where it is perpendicular to bedding and forms curved

foliations =biogenic mineral foliation!]

[Photos 8.8, 8.9 of minor isoclinal folds]

[Photo 8.10 of asymétrie to south, biot-rich bed/lamination in MG sst

beneath a 50 cm thick granule to pebble lens. Underlying this are ripple

festoons showing SW or NE orientations]

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Ripple drifts generally suggest currents to south and west (probably

wind-induced forms). Westerly component appears dominant.

Load casts v.abundant. Other synsed defm appears abndt also

Larger cross-beds suggest southward flow

[Photo 8.11 of upright synsedimentary folds in biot-rich laminae in FG

sst]

Note This all has a rather turbiditish look about it!

[Photo 8.12 of large-scale minor fold hinge, asymétrie to south]

Note Granule sst may exhibit basal reverse grading and lamination,

followed by massive interval, followed by laminated, normally graded

interval. See also normal and reverse graded laminations. These=good

Lowe-type sequences?

As a whole, rx are v.biotite-rich. Biot. mostly aligned parallel to

bedding

First impressions of rocks on east side and in canvon floor

1) Tend to form fining upwards sequences

2) Burrowing, with upright biotites fairly common

3) In granule-pebble sst, biot non-aligned, still in thick books. In more

laminated and/or finer grained sst (eg. at top of granule beds and in beds

above), biot aligned parallel to bedding and in individual flakes/smaller

books. I think this reflects sediment gravity flows vg. stream flow.

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4) As a whole, I would say that these seds. were deposited as lacustrine

turbidites

5) Note soft sediment defm v.common here also -microfolding and

Red Tuff at higher level is FG-CG, poorly sorted sst of arkose and tuff

Lowest occurrence of Red Tuff is as a pebbly-muddy breccia of VRFs

and felsite, overlain by pinkish, CG, laminated tuffaceous sst (SH and

Gms)

Possible base of Upper sst member on east side of canyon. Note that on west side

canyon, sst is unconformable on Red Tuff, which drapes (conformably?) the Middle

Brecia member

Top of Tuff Breccia tongue o f Pickhandle Formation

15 cm Mod reddish orange FG-VFG sst. =Woodbume's Red Tuff. Sharp contacts.

Massive. [Sample OC-89-10 of sst (Woodbume's Red Tuff)

Grain size range: FG-VFG

Max. grain size: CG

Ave. grain size: FG-VFG

Roundness: angul

Shape:

Sorting: med

Packing: well pkd

Composition: qtz, feldsp, biot, in VFG matrix

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• 50 cm Mod reddish orange VFG-pebble sst. Bottom contact not seen (presumed sharp

as elsewhere -eg. west side canyon -see photos). Top contact sharp. Normal grading

Grain size range: VFG-Peb

Max. grain size: 5 cm

Ave. grain size: FG-CG

Roundness: angul

Shape:

Sorting: v.poor

Packing: poor pkd

Composition: gmtd, feldsp, qtz, biot, felsite at base, minor tuff at top. Matrix

reddish, VFG, tuffaceous(?)

Base of T i^ Breccia tongue of Pickhandle Formation

Top Middle Breccia member of Mud Hills Formation

• 19 m of gryish-orange to dk yellowish orange boulder breccia (A) with felsite breccia

lenses(B)

A) Gryish orange -dk yellowish orange boulder breccia (Gl). Massive, clast supported,

matrix poor. [Sample OC-89-7 of granitoid clast from this interval]

Grain size range: Bldr

Max. grain size: 1.5 m

Ave. grain size: sm bldr

Roundness: angul

Shape: subeq-slt. elongate

Sorting: v.poor

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Packing: clast-supp, matrix poor

Composition: granitoid. Sparse matrix of MG-pebble size.

B) Lt. gry felsite cobble breccia (Gl). 5 cm to 2 m thick lenses, spaced every 1 m-12

m; lateral extent lO's m; irregular boundaries. Lenses appear to parallel general attitude

of unit. Lenses are monolithologic. Felsite clasts generally confined to these lenses,

except near top of unit where some slight mixing of granitoid and felsite is seen.

[Sample OC-89-8 of felsite breccia]

Grain size range: Cob

Max. grain size: 25 cm

Ave. grain size: pb-medcb

Roundness: angul-v.angul

Shape:

Sorting: v.poor

Packing: clast-supp

Composition: Lt. gry felsite. Matrix of felsite granules, wt.-bm calcite. In upper

part of unit on east side canyon is some matrix of gmtd sand.

• 15 m of Gryish orange to mod. reddish orange cobble-boulder breccia. Becomes more

gryish orange upwards. Seems to be bedded; 1.5 m-4 m thick beds defined by v.clay-

rich zones which appear to be sheared (contain broken clasts). Contacts irregular, ~40

cm relief. Upper 5 m appears unbedded. Suggest these are debris flows. Sheared

contacts represent flow bases or shear within flow? Note: - may instead be part of rock

avalanche dismpted zone [Photo 8.5 of three apparent debris flows. Note felsite breccia

lens at base].

• 1 m Lt. gry felsite monolithologic breccia lens.

Grain size range: Cob

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Max. grain size: 25 cm

Ave. grain size: pb-medcb

Roundness: angul-v.angul

Shape: Sorting: v.poor

Packing: clast-supp

Composition: Lt. gry felsite. Matrix of felsite granules, wt.-bm calcite.

• 46 m Gryish orange to mod reddish bm weathering, It gry gmtd brecca (Gl).

Massive, unbedded. Seems to have suffered little relative displacement of clasts.

Aplite/graphic granite lens exhibits jigsaw fabric. This is the house-sized breccia

interval. Interpret this as a/some rock avalanches. [Sample OC-89-5 of graphic granite

from mono(bi)lithologic lens of leucocratic graphic granite and milky qtz breccia

containing milky qtz clasts up to 5 cm across; Sample OC-89-6 of granitoid clast from

this interval] [Photo 8.3 of possible jigsaw fabric in felsite breccia; Photo 8.4 of jigsaw

fabric in aplite-graphic granite breccia].

Grain size range: Cob-bldr

Max. grain size: at least 30 m

Ave. grain size: Igcb-lgbldr

Roundness: angul-v.angul

Shape:subeq

Sorting: v.poor

Packing: clast-supp

Composition: Coarse-grained granodiorite containing aplitic veins and finer-grained,

more mafic inclusions, minor leucocratic graphic granite-milky qtz in breccia lenses

near base of unit. Matrix MG-granules of gmtd; apparently little clay.

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Assorted Notes on the Middle Member Granitoid Breccia

1) Felsite breccia lenses have pronounced bimodal grainsize: matrix is

much finer and may consist of gmtd material. This =good for scree

2) Mixing of felsite and gmtd clasts tends to occur near top of unit

3) If grntd breccia is scree, why is gmsize distribution less markedly

bimodal? A. weathering characs?

4) If gmtd breccia is scree, why doesn't felsite filter down into presumed

pore space? A. pores filled before felsite deposition or pores never empty

5) Could gmtd and felsite have different sources? A. No? no analog

source of pure felsite known

6) Why is felsite in lenses? What controls their geometry? A. This is

preservation of original intrusive stratigraphy in a rock avalanche.

7) Rock avalanche may lower gradient, cause damming and fine-grained

sedimentation (i.e. Upper sst mbr)

Base of Middle Breccia member

Top of Sandstone and Breccia Lentil between Lower and Middle Granitoid Breccia

members', possibly equivalent to Middle Sandstone and Tuff member of Solomon

Canyon? Probably equivalent to entrained sandstones in Middle breccia member in

Rainbow Canyon and to west. Note see sketch in notebook for geometry of unit and

contact relations.

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Eastern Part (east side of canyon)

5 m of Gryish orange granule-FG sst. 1 cm-10 cm beds with sharp planar contacts.

Lower and upper contacts of unit sharp. Coarser beds may be normally graded. Beds

arranged in 15 cm-20 cm fining upwards sequences. Unit overlain by, and intruding

apparent white boulders (bouldery weathering?) of fiactured gmtd and/or gmtd breccia.

"Boulders" contain non-calcareous yellow silty laminae subparallel to bedding. [Sample

OC-89-34 of sst] [Photo 8.2 of three fining upwards sequences in this interval]

Grain size range: FG-gm

Max. grain size: 5 cm

Ave. grain size: CG-pb,CG-gr,MG,FG

Roundness: angul

Shape: subeq

Sorting: poor(CG), well(FG)

Packing:

Composition: gmtd and its constituents, minor purplish VRF's

•3 m Gryish orange cobble-boulder breccia (Gms). Sharp contacts. Slight fabric of

south-dipping cobbles. Matrix contains some small (~40 cm in size) folded pods of FG-

MG sst.

Grain size range: Cob-bldr

Max. grain size: 1.5 m

Ave. grain size: Igcb-smbldr

Roundness: angul-subangul

Shape:

Sorting: v.poor

Packing: matrix-supp, v.matrix-rich

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Composition: granodiorite, minor puiplish VRFs. Matrix of CG arkose, pebbles

and clay

• 60 cm by 2 m exposure of mod orange pink MG arkose = local patch of outcrop.

• 1 m-30 cm thick It gry weathering. It gry-bm stromatolitic/algal limestone with

gastropods. [Sample OC-89-3 of limestone]

Western Part feast side of canvon’i

• ~4 m thick by few m wide lens of matrix-rich granitoid breccia. Smaller grainsize than

that described below. Contains reddish (more clay-rich?) stringers

• 30 cm of v.pale gry, laminated limestone, with ^ cm thick lenses of sst. Lower

contact sharp, irregular, upper sharp, slightly wavy

Top of Lower Breccia member

* 2 cm-30 cm of mod orange pink MG-CG sst. Gradational lower contact. Upper

contact sharp, irregular

• ~7 m of Gryish orange granitoid breccia. Fines and becomes more matrix-rich

upward: basal parts v.clast-rich, Ige cobble-small boulder size; upper parts matrix-rich,

Ige pebble-lge cobble size.[Photo 7.31 of Clast-supported appearance of basal part of

this bed; Photos 7.32,7.35,7.36 taken near upper contact of lower member in this area:

Photo 7.32 of "bedding" in grntd breccia. These are slightly finer grained intervals,

possibly debris flow shear planes? - note see no alteration, gouge or vein fill; Photo

7.35 of thin "bedding" in 2 m-3 m thick gmtd breccia unit, defined by areas of slightly

more matrix; Photo 7.36 of a 2 m-3 m debris flow(?) showing thin "bedding"(shear?)

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near top and bottom. The rock is a matrix supp gmtd breccia (30 cm, ave pebble-med

cobble, CG-mud matrix)-corresponds to 3 m thick gmtd breccia described in eastem

part above]

• 57 m of Gryish orange granitoid cobble-boulder breccia [mostly Gms, G2]. Sharp

lower contact marked by fault plane grooves. Locally see a basal, 7 cm thick layer of

mod reddish orange, FG-MG sst {Granule, (FG-MG), angul where visible, subeq,

poor, well pkd; qtz, feldsp, biot, gmtd, no tuff visible}. This sst may be injected into

lower few metres of unit, subparallel to known normal faults. Bedding in unit not

obvious, but lower 40 m contains greyer, 5 cm-20 cm thick, more resistant,

discontinuous layers subparallel to bedding. These appear to be calcite cemented, and

while not flow tops may indicate pedogenic processes. Note these are same layers

recognized as being dismpted on Spring 1988 trip (see that notebook). Clast abundance

is very high (though predom. matrix supp) in 39 m-45 m interval, as in section

described above [cf. Photo 7.31]. Above 47 m, average grainsize increases from Ige

pebble-lge cobble to med cobble-small boulder. [Sample OC-89-2 of granitoid breccia

and typical clasts] [Photo 7.27 of typical lower mbr matrix supp gmtd breccia, with

gryer more resistant clast(?) or bed; Photo 7.28 of red sst (red 'tuff) injected into base

of unit; Photo 7.29 of apparent bedding in gmtd breccia represented by greyer more

resistant bands -short axis field of view ~8 m; Photo 7.33,7.34 of gmtd breccia units

on west side of Owl Canyon, taken from the east side near the top of the lower member] Lower 47 m

Grain size range: Cb-bldr

Max. grain size: 2.5 m

Ave. grain size: Igpb-lgpb

Roundness: angul-subangul

Shape: subeq-bricklike

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Sorting: v.poor

Packing: matrix-supp, clasat-poor to rich

Composition: CG granodiorite, aplite (tends to be bricklike in shape). It. gry felsite,

minor VCG leucocratic feldspathic rock, rare purplish VRF's and pink-red CG

granodiorite. Matrix MG-pebble plus clay

Above 47 m

Grain size range: Cb-bldr

Max. grain size: 1.5 m

Ave. grain size: mcb-sbldr

Roundness: angul-subangul

Shape: subeq-bricklike

Sorting: v.poor

Packing: matrix-supp, clast-poor to rich

Composition: CG granodiorite, aplite (tends to be bricklike in shape). It. gry felsite,

minor VCG leucocratic feldspathic rock, rare purplish VRF's and pink-red CG

granodiorite. Matrix MG-pebble plus clay

Base of Mud Hills Formation -contact is a slip plane marked by grooves

Top of Pickhandle Formation

Gryish pink-lt. gmish gry tuff breccia and tuff.[Sample OC-89-1 of tuff from ~25 m

below contact in base of a bed of "flattened hole tuff (eutaxitic?)

Grain size range: Pebbly

Max. grain size: 1 cm

Ave. grain size: 5-10 mm

Roundness: angul

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Shape:

Sorting:

Packing: matrix-supp

Composition: Purplish, gmish, braish VRF's and mafic blkrk. Matrix of FG-CG

qtz, feldsp, biot., clay.

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n. MEASURED SECTION OF THE MUD HILLS FORMATION IN RAINBOW

CANYON

(Measured by Chris Travis on 25 February, 1989)

Beginning at the top of the section...

Top Upper Breccia Member of Mud Hills Formation

•~130 m Gryish orange cobble-boulder breccia. Massive, unbedded. Unit appears

identical in lithology to lower member in Rainbow Canyon and Upper member in

Solomon Canyon. [Samples RC-89-4 of gmtd clasts from this member]

Grain size range: Cb-bldr

Max. grain size: 6 m

Ave. grain size: (Igcb-sm bldr)

Roundness: angul

Shape: subeq

Sorting: v.poor

Packing: csupp-msupp

Composition: CG friable granodiorite with rare finer grained biot rich inclusions and

leucocratic vein rocks. Matrix of CG gmtd and clay

Base of Upper Breccia member

Top of Tuff breccia tongue of Pickhandle Formation

In one location see a i m thick by 2 m long pod of fairly well sorted VFG-MG red tuff.

This must be Owl Canyon "red tu ff equivalent, and is definitely Solomon Canyon

upper "red tu ff equivalent.

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•~10 m Lt. gmish gry tuff-pebbly tuff breccia. Upper 3 m is less resistant, and tends to

be thinly bedded (reworked). Unit is similar to the lower (sampled) flow in Ross

Canyon, although no gas segregation pipes are seen.

Grain size range: CG-peb

Max. grain size: 2.5 cm

Ave. grain size: (CG-1 cm)

Roundness: angul

Shape:subeq

Sorting: med-poor

Packing: matrix supp

Composition: wt. tuff, blkrk, purplish and pinkish VRFs., and more rare pumice

remains. Matrix of CG tuff and VFG-clay.

Base of Tuff breccia tongue of Pickhandle Fm

Top of Middle Breccia member o f Mud Hills Fm

•~60 m pinkish and whitish granitoid breccia. Massive, unbedded. Occurs as 5 m-15 m

diameter patches of more and less leucocratic granodiorite/granite. Pinker lithologies are

more biotite-rich. These accumulations are now brecciated, but may once have been

giant boulders. Base of unit exhibits 1 m-5 m diameter lenses of gryish orange pebbly

CG sst and a larger lens of bedded CG leucocratic gmtd sst and fesite breccia. These

lentils are entrained, similar to those observed farther to the west (cf. Photos). Near top

of unit see a ~4 m thick by 10 m long lens of pinkish red clast supported cobble-boulder

breccia with matrix of CG sst-clay, which may or may not be in place. At one location

at top of interval find a single 3.5 m diameter boulder of felsite breccia (similar to that

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observed in Owl Canyon) at base of gm tuff member. [Samples RC-89-3 of gmtd clast

types, including a clast from the felsite boulder at top of unit).

Base of Middle Breccia member

Top of Lower Breccia member

•~60 m Gryish orange cobble-boulder breccia. Lower contact obscured. Generally

massive, but do see dip parallel crude fabric on a 2 m-5 m scale which may be related to

presence of calcareous laminae through rock. These may be related to ancient caliche

(cf. Solomon Canyon lower member). [Sample RC-89-2 of gmtd breccia] [Photo 7.22,

7.23,7.24 of approximate bedding surface views of gmtd breccia of lower member]

•~10 m of poorly exposed CG-VCG sst and clay-rich CG-granule sst and Shale(?).

CG-VCG sst is horizontally laminated, ~10 cm bedded with sharp contacts; CG-Gm sst

is massive, friable, gryish orange, clay-rich, composed of gmrtd constituents. 2 m-6 m

interval is covered by slope wash of CG-gm sst, and may contain shales. This interval

may correspond to Facies C. described below

•6 m of Gryish orange FG sst, CG-granule sst, pebble and boulder breccia (Sh, Gm,

Gl/Gms, FI). IN sequence: Sh with FI (11 transitions), G2(Sm), Sh with FI (20

transitions), Sh, Gms/G2, Sh, Gm, Sh, Sm, Sh, Gm, G2, Gm, Gms, Sh. [Sample

RC-89-1 of gmtd clasts]

A. FG sst (FI). ~2 cm thick, planar beds, horizontally laminated, well sorted,

interbedded with 4 cm thick beds of MG-CG sst (Sh), massive to horizontally

laminated, sharp contacts, with clay laminae.

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B. VCG-sm pebble sst/breccia. 2 cm-10 cm thick beds with sharp, erosive basal

contacts with ~5 cm relief scour and fill structures and sharp upper contacts. Horizontal

laminations.

Grain size range: CG-peb

Max. grain size: 4 cm

Ave. grain size: (VCG-1.5 cm)

Roundness: angul

Shape: subeq-

Sorting: poor

Packing: mod well pkd

Composition: gmtd RF's, feldsp, qtz, biot., minor felsite

C. MG-granule sst, med-poor sorted, granule laminae, -10 cm thick beds; interbedded

with Facies A above.

D. MG sst-bouldder breccia (Gl/Gm). ~50 cm thick beds. Imbricate, coarsetail

graded.

Grain size range: MG-bldr

Max. grain size: 30 cm

Ave. grain size: (cb-gm)

Roundness: angul

Shape: subeq-elongate

Sorting: v.poor

Packing: poorly pkd

Composition: gmtd RFs, minor felsite. Matrix sand-clay

E. CG-boulder breccia and pebble breccia (G2/Gms). -30 cm-70 cm thick beds.

Massive.

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Grain size range: CG-bldr

Max, grain size: 60 cm

Ave, prain size: (cb-gm)

Roundness: angul

Shape:

Sorting: poor

Packing: matrix supp

Composition: Matrix muddy-granule

•2.5 m of Gryish orange FG-pebble sst/breccia and yellowish gry mudstone (Fm, Sh,

minor Sp, Gl). Sequence: Fm(l), FI, Sh, G l, Sh, Sh, G l, Sp, Sh, G l, Sh, Sp, Sh,

Sp, Sh. [Photo 7.20 looking south at lower member sst on east side Rainbow Canyon,

showing facies Gm and Sh. Cobble imbrication points west, scour trough above

hammer head points NNE; Photo 7.21 showing general view of lower member sst on

east side of Rainbow Canyon]

A. ~50 cm thickness of yellowish gry mudstoneFm(l). Gradational lower contact to

tuff, gradational upper contact to FG sst. Massive to slightly fissile.

B. Gryish orange FG-pebble sst. 3 cm-10 cm thick beds, laterally continuous on 1 m to

IQ’s m scale. Contacts sharp, planar to gradational. Planar- to cross-laminated.

Grain size range: FG-peb

Max. grain size: 2.5 cm

Ave. grain size: (FG,CG-VCG,VCG-gm)

Roundness: angul

Shape:subeq

Sorting: well(fine)-poor

Packing: well pkd

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Composition: feldsp, qtz, biot, gmtd RF's, v.rare VRF's

C. Gryish orange small pebble breccia (Gl). 70 cm-15 cm thick beds. Gradational to

sharp contacts. Massive.

Grain size range: sm peb

Max. grain size: 2 cm

Ave. grain size: (5 mm)

Roundness: angul

Shape: subeq

Sorting: poor

Packing: clast supp; poor pkd

Composition: Gmtd RFs. Matrix muddy-CG

Base of Lower Breccia member of Mud Hills Formation

Top ofPickhandle Formation

Yellowish gry tuff to tuff breccia. Massive. Gradational upper contact to Mud Hills

Formation.

Grain size range: CG-sm cob

Max. grain size: 5 cm

Ave. grain size: (CG-smpeb)

Roundness: angul

Shape: subeq

Sorting:

Packing: matrix supp

Composition: wt. tuff(qtz+feldsp+biot), minor VRF's, blkrk, CG qtz, feldsp, MG

biot. Matrix VFG-clay size tuff.

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m. MEASURED SECTION OF THE MUD HILLS FORMATION IN SOLOMON

CANYON

(Measured by Chris Travis on 23-24 February, 1989).

Beginning at the top of the section...

Owl Conglom = CG gmtd sst to cobble/boulder conglom. Lwr contact sharp, planar.

Predom gmtd RFs and breakdown products, abndt purplish andésite porphyry, other

VRFs, felsite?, rare schistose MRFs, poss VFG tuff in matrix.

Bas Barstow Fm

Top ofTuffaceous sandstone and tuff breccia tongue ofPickhandle Fm

•8 m Mod orange-pink to It. gmish-gry (predom. o-p) weathering, gryish-pink tuff

w/sparse, scattered RF pebbles. Unbedded, massive. Lwr contact sharp. Pclastic or

lahar. [Sample SO-89-11 of tuff]

Grain size range: Tuff

Max. grain size: 5 cm

Ave. grain size: (1 cm)

Roundness: angul

Shape: subeq

Sorting: poor

Packing: matrix supp

Composition: pebbles-granules purplish VRFs, bmish & gmish VRFs, blkrk, wt

tuff, rare pumice frgs, bm tuff, bm MG-CG sst, gmtd.

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•4 m Yellowish-gry to gryish-orange weathering, yellowish-gry to It gry, CG-FG

tuffaceous sst (Sh, minor FI). 1 cm-20 cm beds; sharp, planar contacts; ungraded to

normally graded at base, may be parallel laminated; no overall trends, though pebbles

are restricted to lag at base of unit. Lateral extent unknown, appears to thin to south in

west canyon wall. Suggest this is a Channel fill sequence.

Grain size range: Peb-FG

Max. grain size: 2 cm

Ave. grain size: (MG)

Roundness: ang-subang

Shape: subeq

Sorting: med-well

Packing: well pkd

Composition: purplish, bm, pink VRF pebbles, qtz, feldsp, biot.

•8.5 m Yellowish-gry to gryish orange pink, VFG-VCG sst (Fl-Sh). 15 cm-2.5 m beds;

sharp planar to scoured contacts (5 cm relief); coarser and thicker beds normally graded

to massive, finer and thinner beds parallel laminated to massive; slight fining upwards

overall. Upper contact sharp wavy erosional (15 cm scoured relief). Facies Sh, FI, Fm,

and Sm(?).

Grain size range: VFG-VCG

Max. grain size: VCG

Ave. grain size: (F,C,&MCG)

Roundness: coarser ang

Shape: equant

Sorting: med-well

Packing: well pkd

Composition: CG=predom. wt tuff; FG=tuff + ?

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•1.5 m Pale gmish-yellow tuff and tuff breccia. Pebble-rich layers; 2 cm-5 cm laminae;

30 cm bed at top. Lower contact gradational, upper planar, sharp. Reworked lahar top?

Grain size range: Pebble

Max. grain size: 3 cm

Ave. grain size: (5-15 mm)

Roundness: angul

Shape: subeq

Sorting:

Packing: clast supp

Composition: pale gmish-yellow tuff, chloritic(?) schist, purplish, pink, and bm

VRF's

•3 m Lt gry to gryish orange tuff breccia. Matrix supported; massive, except at top

where see a layer of scattered bm tuff bouders ^ 0 cm in diameter. Lower contact

sharp, planar to wavy (30 cm relief) to apparently gradational; upper contact gradational.

A lahar? [Photo 7.18 of better cemented nodules in this tuff]

Grain size range: MG-bldr

Max. grain size: 5mm40cm

Ave. grain size: (2.5 mm)

Roundness: angul

Shape: subeq

Sorting: poor

Packing: matrix supp

Composition: Blkrk, red ,gm and bm VRF's, chert, gmtd, schist clasts, in MG-clay

size matrix

•~4 m missing section -lost in move from west to east side of canyon to avoid

numerous, small down-to-north normal faults

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•7 m Lt bm weathering, gryish orange pink tuff. Matrix supported, massive. Lower

contact gradational, upper sharp. A lahar? [Photo 7.17 of gmtd breccia lens in tuffs on

west face Solomon Canyon. Normal fault on right, normal conatct on left Estimated

min. thickness 8 m, max. clast size ~3 m]

Grain size range: VFG-peb

Max. grain size: 5 cm

Ave. grain size: (granule)

Roundness: angul

Shape: subeq

Sorting: poor

Packing: matrix supp

Composition: wt tuff, blkrk, biot, purplish, bm and gm VRF's, gmtd, in VFG-clay

size matrix

•10.5 m Lt bm (5YR5/6) VFG-VCG tuffaceous sst and bentonite (Sh, Fm, FI). Similar

to middle sst and middle red tuff. 1 cm-25 cm tabular beds; sharp contacts; local minor

(1 cm) scour and fill structures with pebble lags; parallel laminated; normally graded.

Bentonite more abundant above 7 m level, beds <3 m thick. Upper contact gradational.

Facies Sh, Fm, FI. [Sample SO-89-10 of tuffaceous sst]

Grain size range: VFG-spb

Max. grain size: 5 mm

Ave. grain size: (VF-CG,gn)

Roundness: angul-subang

Shape:subeq

Sorting: med-poor

Packing: well pkd

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Composition: VCG-pebbles=qtz, feldsp,gmtd, wt tuff, rare purplish VRF's; Finer-

grained incl. tuff, qtz, feldsp, biot. Non-calc cernent

Base ofTuffaceous sandstone and tuff breccia tongue ofPickhandle Fm

Top o f Upper Breccia unitofMud Hills Fm

[Photo 7.15 of upper tuff unit, looking east] [Photo 7.16 of Upper Granitoid Breccia to Upper Tuff (sst) contact in Solomon Canyon]

•70 m Lt gryish orange to gryish orange cobble-boulder breccia (Gms, G1/G2).

Massive -no bedding, but see apparent grain size variations on 7-10 m scale, and

horizons with more/less matrix and/or clay component; mean grain size in horizons:

med-lge cobble, Ige cobble-sm boulder, Ige-med boulders. Lower contact fairly

gradational to volcanic clay; some clay mixed into lower part of breccia unit. Upper

contact fairly sharp, non-erosive (see Photo 7.16). [Photos 7.13,7.14 of typical Upper

Granitoid Breccia unit].[Sample SO-89-9 of granitoid clasts].

Grain size range: Cob-Bldr

Max. grain size: 2.5 m

Ave. grain size: (m-lc,lc-sb,l-mb)

Roundness: angul-subang

Shape: subeq-slight elongate

Sorting: v.poor

Packing: matrix supp, clast-med to clast-rich

Composition: Qtz monzonite/granodiorite, minor feldspathic and biotite-rich

components. Matrix MG-pebbles of same. Clay content variable, med-low. Non-

calc cement.

Base of Upper Granitoid Breccia unit

Top tuff breccia tongue ofPickhandle Fm

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•80 cm Red volcanic clay (Fm). Apparently fairly gradational/mixed into overlying

Granitoid Breccia

•1 m Moderate reddish orange to It gry FG-VFG sst (Sh). 1 cm-25 cm tabular beds;

sharp planar contacts; normally graded from MG-VFG in 1 cm laminae; parallel

laminated (note: see TXB to east cf. Photo ?). Lower contact sharp, upper gradational.

This is Red Tuff of Woodbume. [Sample SO-89-8]

Grain size range: FG-VFG

Max. grain size: MG

Ave. grain size: (FG)

Roundness:

Shape:

Sorting:

Packing: well

Composition: Larger grains appear to be qtz and feldsp

•14.2 m Lt gry to yellowish gry lapilli tuff breccia. Matrix supported; massive. Grades

up into 50 cm of popcorn-weathering tuff and clay. [Sample SO-89-7 of clasts from

lapilli tuff breccia].

Grain size range: Peb-sc

Max. grain size: 10 cm

Ave. grain size: (206 cm)

Roundness: angul

Shape: subeq

Sorting: v.poor

Packing: matrix supp

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Composition: Purplish andesitic volcanics, wt-gm felsitic clasts, highly weathered

pumices (possibly predominant as qtz+feldsp+clay), blkrk, tuff breccias, qtz

monzonite, biotite schist, schist. Matrix of clay and MG qtz and feldsp

•4 m of volcanic clay, MG-VCG tuffaceous sst, and muddy tuffaceous breccia (FI, Sh,

Gms). Epiclastic

A) Lt gry to gmish giy, popcorn-weathering clay. 1 cm to 1 m beds

B) Gryish orange pink MG-VCG sst. 1 cm-20 cm thick beds; upper and lower contacts

sharp; lateral continuity -100 m; parallel stratification.

Grain size range: MG-Peb

Max. grain size: 15 mm

Ave. grain size: (MCG,CVG)

Roundness: angul

Shape: subeq

Sorting: med

Packing: poor pkd

Composition: Qtz, feldsp, tuff, biot, purplish VRF's. Matrix clayey

C) Muddy tuffaceous pebble-boulder breccia. One 50 cm bed; poor lateral continuity;

matrix supported; massive.

Grain size range: Peb-bld

Max. grain size: 40 cm

Ave. grain size: (1 cm)

Roundness: angul

Shape: subeq

Sorting: v.poor

Packing: matrix supp

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Composition: tuff breccia, puiplish VRF's. Matrix of tuff fragments and volcanic

clay.

•8 m Lt red to It gry weathering yellowish gry MG-pebble tuff/lapilli tuff breccia. May

weather to a popcorn appearance. Larger clasts coarsen upward. Upper and lower

contacts gradational. Pyroclastic?

Grain size range: MG-Peb

Max. grain size: 3 cm

Ave. grain size: MG base; MG-sp top

Roundness: angul

Shape: subeq

Sorting: v.poor

Packing: matrix supp

Composition: Wt tuff and/or pumice weathered to clay, blkrk, bm, purplish and

gmish andesitic volcanics, and minor pinkish gry dacitic tuff of feldsp, qtz, biot and

glass.

Base tuff breccia tongue ofPickhandle Fm

Top Solomon Sst member of Mud Hills Fm

•15.5 m (6 m added to account for small fault on west side canyon) Lt bra (5YR6/4)

Bentonite, FG sst, and MG-VCG sst. Sst beds 2 cm-25 cm thick, sharp based, rapidly

grading up into bentonite at top. Sst beds separated by 0 cm to 3 m of bentonite with

stringers of VFG sst. Sst bed continuity varies between 1-10 m and lO's of meters.

Coarser sst more common in upper 2/3 of sequence; may be normally graded. FG and

CG sst exhibit parallel laminations, ripple TXB (5 cm sets), low angle PXB, and

festoon TXB. Sst more abundant on east side of canyon: accumulations of 10 cm-20 cm

bedded sandstone <1 m thick are separated by ~1 m bentonite. Note: bentonite may

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contain volcanic lithics. [Photo 7.2 of festoon TXB; Photo 7.8 of ripple TXB in FG sst-

note 5 cm sets, discontinuous flaser-like bedding; Photo 7.11 of middle tuff and part of

middle sst on east side Solomon Canyon, showing more and less well bedded parts of

tuff; Photo 7.12 of middle tuff and part of middle sst on west side Solomon Canyon,

closeup view] [Sample SO-89-5 of FG sst; Sample SO-89-6 of CG sst].

Following description is of the MG-VCG sst:

Grain size range: MG-Peb

Max. grain size: 16 mm

Ave. grain size: (CG-VCG)

Roundness: angul

Shape: subeq

Sorting: poor

Packing: poor(muddy matrix)

Composition: qtz, feldsp, qtz monzonite, leucocratic vein rx, purplish andésite

porphyry, felsite, blkrk. Calcareous cement.

Base Solomon Sst membert

Top Lower Breccia Member

•34.5 m Gryish orange cobble breccia and boulder breccia (G1,G2). No bedding

apparent. In lower 8 m may see muddier horizons on 5-10 m scale intervals. In lower 8

m, ave grainsize is sm-med cobble. From 8 m, grainsize increases upwards, culminating

in a 3 m thick horizon at 24 m. From 24 m to 34.5 m, ave grainsize is Ige cobble to

small boulder in matrix of clay-pebble size clasts (G2). At 8 m see accumulation of

angular, brnish gry, fine-grained dacite pebbles-cobbles. Upper contact abrupt.

Grain size range: Sm cb-bdr

Max. grain size: >2 m

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Ave, grain size: (sm-med cb base, sm bldr top)

Roundness: angul

Shape: subeq to brick-like

Sorting: poor-v.poor

Packing: clast-rich, matrix supp to clast supp.

Composition: qtz monzonite/granodiorite, minor felsitic vein rx, v. minor diorite,

local dacite.

•15.7 m Gryish orange cobble-boulder breccia (G1,G2,P). Lower 4 m contains red,

muddier horizons; 4-10 m section is v. clast-rich G l facies; unit massive otherwise.

Lower 2 m has "swirly" laminated appearance caused by occurrrence of calcite and/or

dolomite (HCL reaction only on fresh or powdered surface) = probable ancient

pedogenic carbonate. [Photo 7.6 of base of Granitoid breccia with "swirls" of carbonate

cement; Photo 7.7 of "swirls" in granitoid breccia] [Sample SO-89-4 of granitoid clasts].

Grain size range: Peb-bldr

Max. grain size: 50 cm

Ave. grain size: (sm-m cb)

Roundness: angul

Shape: subeq

Sorting: poor

Packing: clast-rich, matrix supp to clast supp

Composition: qtz monzonite/granodiorite. Matrix consists of FG-granule size clasts

of same lithology; minor clay constituent.

•1.4 m reddish bm muddy-sandy cobble breccia (Gms). No bedding; becoming more

matrix-rich upwards. Lower contact gradational. Calcareous cement near upper contact.

Grain size range: Peb-bldr

Max. grain size: 50 cm

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Ave, grain size: (Ige peb-sm cob)

Roundness: angul-md

Shape: subeq

Sorting: v.poor

Packing: matrix supp, clast poor

Composition: granitoid, felsite. Reddish bm matrix of same lithology-clay to granule

size.

•6.3 m It gry cobble-boulder breccia (Gl). Unbedded; generally massive; clast

supported matrix-rich to matrix-poor. VFG sst described below forms "injected"

stringers (probably mobilized in flow). Lower contact fairly shaip, upper gradational.

Think this is a v.clast-rich debris flow. [Sample SO-89-3 of felsite breccia].

Grain size range: VFG-bldr

Max. grain size: 30 cm

Ave. grain size: (5-15 cm)

Roundness: angul

Shape: subeq

Sorting:

Packing: clast supp, matrix-rich to matrix-poor

Composition: Flow banded, v.lt gry felsite, qtz monzonite, minor biot-poor graphic

granitoid. Felsite predominant, tending to form monolithologic layers. Matrix,

where seen is FG-CG w/minor clay. Non-calcareous cement.

•2 m VCG-pebbly sst, VFG tuffaceous sst, sandy volcanic clay, and pebble-boulder

breccia (facies in sequence: Sh,Fm,Fl,Sh/Gm,Sp or low angle St,Gms,Gms).

A) Gry popcorn weathering MG sst. Massive. Gradational contacts.

feldsp, qtz, biot, clay.

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B)Brick-red, VFG tuffaceous sst. Parallel laminated. Shaip contacts. Looks like

Woodbume's Red Tuff. Labelled Bed #30 BA 88. [Sample SO-89-2 of tuffaceous sst].

C) Lt gry CG-pebble breccia. 10 cm-50 cm beds, sharp bases, gradational tops;

horizontal stratified to low angle PXB or TXB.

Grain size range: CG-peb

Max. grain size: 3 cm

Ave. grain size: (granule)

Roundness: angul

Shape: mostsubeq

Sorting: med-poor

Packing: well pkd

Composition: qtz monzonite, feldsp, qtz, biot, felsite, minor purplish volcanic rock

fragments and schist.

D) Gryish orange pebble-cobble breccia (Gms). 90 cm bed, lower contact gradational,

upper sharp; fairly massive, but upper 30 cm more matrix supported and rich in red

mud.

Grain size range: CG-bldr

Max. grain size: 70 cm

Ave. grain size: (4-15 cm)

Roundness: angul

Shape:subeq

Sorting: v.poor

Packing: matrix supp

Composition: qtz monzonite, felsite. Matrix of clay and CG arkosic material.

•7 m poorly exposed volcanic clay and felsite breccia as described above.

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•3.5 m Gryish orange CG-large cobble breccia (G2). Massive to possibly v.crudely

bedded. Lateral! continuity ~50 m. Matrix supported. Probable debris flow. [Sample

SO-89-1 of granitoid clasts from this unit).

Grain size range: MG-lg cob

Max. grain size: 20 cm

Ave. grain size: (CG-smcb)

Roundness: angul

Shape: eq-subeq

Sorting: v.poor

Packing: matrix supp

Composition: granite/qtz monzonite, qtz, feldsp, biot. Felsite appears only at top of

unit. Matrix MG-CG sst and minor clay. Non-calcareous cement.

•3 m Red-orange weathering VCG sst-pebble breccia (Sh,Gm,Gp).

•60 cm Lt gry pebble breccia (Gm-Sh). Clast supported; normal grading. Lower scoured

contact (30 cm relief) with cobble to boulder lag. [Photo 7.3?]

Grain size range: VCG-bldr

Max. grain size: 40 cm

Ave. grain size: (5-15 mm)

Roundness: angul

Shape: subeq

Sorting: poor-v.poor

Packing: clast supp

Composition: qtz monzonite, felsite. It purple andésite porphyry.

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•30 cm-60 cm Red-lt gry FG-VCG sst (Sh,St-one 6 cm set). 3 cm to 10 cm beds, shaip

basal contacts; maybe normally graded with basal pebble lag, also horizontal, wavy

(ripple drift), and asymptotic TX laminations.

Grain size range: FG-peb

Max. grain size: 3 cm

Ave. grain size: (CG)

Roundness: angul

Shape:

Sorting: med-poor

Packing:

Composition: qtz, feldsp, biot, pebbles of qtz monzonite and felsite.

•60 cm Granule-pebble breccia (Gm). Max thickness 60 cm, grades laterally to facies

Sh, overlies ~5 cm thick FG sst; basal erosive contact (10 cm relief); lateral extent at

least 5 m. Grades up over to cm into sst described above. Elongate clasts may be

bedding parallel [Photo 7.4 of this and underlying sequence. See sketch in notebook].

Grain size range: VCG-peb

Max. grain size: 5 cm

Ave. grain size: (1 cm)

Roundness: angul

Shape: eq-elong

Sorting:

Packing: clast supp, well pkd

Composition: qtz monzonite, felsite, purplish andésite porphyry, minor bmish

VRF’s and schistose RF's.

•2.5 m Orange weathering, It gry VCG sst-pebble breccia (Sh-Gm). 5-10 cm laterally

discontinuous beds of pebble breccia and VCG sst having typical "wadi-sst" appearance;

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may show normal grading; scour and fill structures (normal graded) quite common. In

middle of unit see a single bed of red, well sorted, FG sst; 10 cm thick, ~10 m lateral

extent; sharp wavy base and top. Base of bed overlying FG sst contains scattered

boulders (<40 cm) of purple andésite porphyry, felsite and siliceous rock. Note clasts

angul except for one well rounded boulder, well rounded clasts of Jackhammer Fm in

float. [Photo 7.4 of this and overlying sequence. See sketch in notebook].

Grain size range: CG-bld

Max. grain size: 40 cm

Ave. grain size: (VCG-pb)

Roundness: angul

Shape:

Sorting: poor-v.poor

Packing:

Composition: qtz monzonite, feldsp, biot, gmish-gry felsite, purplish andésite,

blkrk, bm aphanitic VRF's, minor chert, milky qtz, schistose RF's. Minor VFG

matrix. Non-calcareous cement.

•2.1 m Purplish, friable (weathering to clay) tuff breccia. Lower contact gradational,

upper sharp and fairly planar. Massive.

Grain size range: Gm-pb

Max. grain size: 20 mm

Ave. grain size: (3 mm)

Roundness: angul

Shape: equant

Sorting: poor

Packing: matrix supp.

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Composition: wt tuff, purplish andésite, minor blkrk. Matrix of puiplish volcanic

clay.

•1 m Granule sst (Sh). Lower 5 cm more pebbly. Upper contact gradational, fining

rapidly.

Grain size range: CG-pb

Max. grain size: 5 cm

Ave. grain size: (gm)

Roundness: angul

Shape: eq-tabular

Sorting: poor

Packing: well pkd

Composition: feldsp, felsite, reddish VRFs, qtz monzonite, minor blkrk and biotite

schist of "Pickhandle-type."

•90 cm pebble to small cobble breccia (Gms). Lower contact not seen, upper sharp and

irregular with -10 cm relief. Massive.

Grain size range: Pb-bldr

Max. grain size: 30 cm

Ave. grain size: (4-10 cm)

Roundness: angul

Shape: subeq-tab

Sorting: v.poor

Packing: clast-rich, matrix supp.

Composition: pink-weathering qtz monzonite. It gry felsite with small feldsp

phenocrysts, and aplite, in clay to pebble matrix of same composition.

Base of Mud Hills Formation.

Top ofPickhandle Formation

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Contact slightly obscured. Contact best seen between Ross Canyon and Solomon

Canyon. Red coloration of granitoid sst may be due to presence of small faults (which I

have avoided). Note All qtz monzonite is med- to coarse-grained and biotite-rich unless

otherwise stated.

Pickhandle Fm consists of Gmish-gry tuff breccia. Flattened fabric defined by

weathered out pumice holes. Pyroclastic. Note on "fresh" surfaces pumices appear as

biotite + clay.

Grain size range: ash-lap

Max. grain size: 5 cm

Ave. grain size: (1 cm)

Roundness: angul

Shape:

Sorting: v.poor

Packing: matrix supp

Composition: blk aphanitic RF's (blkrk), asstd. bm, pink, purplish, and gry VRF's,

and (ex-) pumice. Matrix of feldsp, biot, qtz xtals, and clay.

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IV. MEASURED SECTION OF THE UPPERMOST PICKHANDLE FORMATION

EAST OF COPPER CITY ROAD

(Measured by Chris Travis on 7-8 March 1989).

Beginning at the top of the section...

Top ofTuffaceous Sst and tuff breccia tongue ofPickhandle Fm

•~145 m of gryish pink andésite and andésite breccia. Lithology similar to underlying

unit described below. Lower ~75 m clast supported, with average grainsize in small

boulder range and maximum grainsize -1.5 m. Good flow banding and some vesicular

texture seen in clasts. At -27 m level see apparent 10 m diameter, flow banded block of

andésite. At 75 m level brecciation appears to be in situ, consisting of rectangular

blocks of andésite separated by "mortar" of breccia. By -95 m level the rock appears to

be unbrecciated or else brecciated in v.large blocks. By -105 m level breciation is again

aparent, and the upper part of the unit is well brecciated (cobble-small boulder size), and

does not exhibit flow banding. [Sample CC-89-10 of crystal-rich andésite from 90 m

level -has been sunbaked]

•70 m of gryish pink andésite tuff breccia. Unbedded, massive, clast supported. Same

lithology as overlying unit. [Sample CC-89-9 of tuff breccia]

Grain size range: Peb-bldr

Max. grain size: 1.5 m

Ave. grain size: (pebb-sm bldr)

Roundness: angul

Shape: subeq-irreg

Sorting: v.poor

Packing: c-supp

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Composition: Porphyritic-aphanitic tuff of fldsp, biot, hornblende.

—100 m of interbedded tuffaceous breccias and CG ssL Sequence: 2.5 m A(Gms), 4 m

B(Sm), 1 m C(Sh), 7 m B(Sm), 1.5 m C(Sh), 1.5 m A(Gms) containing sudmd

boulders of tuff which weather nodular and exhibit calcitic interiors, 1.7 m C(Sh), 20 m

B(Sm)(poorly exposed), 6 m A(Gms), 20 m predominantly of Gm(cf. A,B), with

minor crudely bedded (~20 cm) Sh, and some Gms in beds up to 2 m thick and

increasing in importance upwards, 3 m A(Gms), 17 m Gms with minor gmtd pebbles-

cobbles, 14 m Gm in 5 cm-25 cm thick beds, with more minor A(Gms)(max grainsize 5

cm, ave pebble) and C(Sh)in 15 cm-1 m thick beds -see fairly abundant subordinate

gmtd clasts. [Sample CC-89-7 of A(Gms); Sample CC-89-8 of gmtd cobble]

A. Mod reddish orange muddy, pebble-cobble tuff breccia (Gms). Massive, apparently

sharp contacts. These are probably lahars: Why not pyroclastic?

• no pumice, no flattened fabric

• lithics distributed throughout grainsize range and

throughout rock

• same color as other epiclastics. Pyroclastics in this area

tend to be greenish to yellow grey, unless they are welded

(cf. W.Cadys) which these are not

• I don't know

Grain size range: Peb-cob

Max. grain size: 40 cm

Ave. grain size: (peb & cob)

Roundness: angul-submd

Shape: subeq-elong

Sorting:

Packing: matrix supp

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Composition: It pinkish weathering tuff, purplish poiphyritic VRF's, subordinate

gmtd RF's, blkrk, felsite? Matrix muddy-CG

B. Granule-CG sst of same lithology and texture as A (Sm). Very muddy.

C. CG-VFG sst of same lithology and texture as B (Sh). 2 cm-25 cm beds with sharp

planar basal contacts. Normally graded with occasional basal lag of granules-pebbles.

May form ~1 m thick fining and thinning upwards sequences. Contain less It colored

tuff than A & B -are composed predominantly of VRFs and minor gmtd. Note may be

some shaler also in this facies.

•85 m of pale red to gryish red sst and breccia, and gryish yellow gm tuff breccia and

tuff. Four basic lithologies Seems to be little difference between red and green

varieties -see samples.seen. Gradational lower contact.

A l. Pale red-gryish red FG-granule sst (St, Sh,). 5 cm -1 m beds with sharp contacts.

Parallel stratification to low angle trough cross strat in 30 cm-1 m thick sets with trough

widths up to 10 m. Local layers of tuff and VRF pebbles may define laminae and cross

laminae, and may be reverse graded on -1.5 cm scale. Possible HCS also seen(?).

Facies A occurs in <3 m-4 m thick packages, interbedded with Facies B and/or C.

These packages are laterally continuous on lO's m scale. [Photo 8.26 of trough cross

bedding in this unit; paleoflow to the right-northward] [Sample CC-89-2 of red sst;

Sample CC-89-6 of a coarser grained, more orange colored sst from higher in the

section]

A2 Pale red-gryish red MG-CG pebbly-muddy sst (Sm). 1 m-2 m thick massive beds,

interbedd with VCG PXB sst (Sp)

B. Pale red-gryish red pebble breccia (Gm, Gt, Gms). Gm/Gt variety in 5 cm-20 cm

thick beds with sharpo contacts. Parallel laminated to trough cross bedded similar to

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Facies A. Gms variety in 1 m-1.5 m thick beds with sharp contacts. Massive. [Sample

CC-89-3 of Gm variety of red pebble breccia]

Grain size range: Peb

Max. grain size: 2 cm

Ave. grain size: (gm-peb)

Roundness: subangul

Shape: subeq

Sorting: v.poor

Packing: matrix supp & clast supp

Composition: tuffs, grntd RF's, VRF's. Matrix MG-CG in clast supp beds, Clay-

MG in matrix supp beds.

C. Gryish yellow gm pebble tuff breccia (Gm, Gt). Similar to Gm variety of Facies B,

except for color. Also contains tuff clasts up to 10 cm in diameter, and is dominated

compositionally by tuff. Gmtd clasts are more rare. Beds up to 1 m thick. [Sample

CC-89-4 of pebble tuff breccia]

D. Gm tuff (Tp). ~1 m-1.5 m thick intervals, invariably overlain by Facies C. May

weather to clayey appearance. Massive. [Sample CC-89-5 of gm tuff]

Sequence/Facies count: 2 m mixed Al/B (red Gt & red St); 3.7 m A l (red St); 4 m C

(grn Gm), D (gm Tp), C (gm Gm), B/C (redgrn Gm), C (gm Gm); 2.6 m Al (red St),

B (red Gms), Al (red St); 4 m C (gm Gt), A l (red Sh), C (gm Gm), B (red Gm), C

(gm Gm), B (red Gm); 5.3 m C (gm Gm), D (gm Tp), C (gm Gm); 2 m CG A l (red

Sh); 12.6 m A2 (red Sm, Sp); 2 m A2 (red Sm, Sp); 1 m red Sh; 1.5 m B/C (gmred

Gm), A l (gmred Sh); 70 cm B/C (gmred Gms); 1 m B/C (gmred Gm), A l (red Sh),

minor B/C (gmred Gms); 3 m Al (gmred Sh); 1 m B (red Gms)(max bed thickness 20

cm); 3 m pebbly A, B (gmred Sh, gmred Sh) with gmish tuff content decreasing

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upward; 35.6 m predominantly CG (max 15 cm) red Sh and pebbly red Shin 5 cm-25

cm parallel sided sharp bounded beds, with more minor red Gm (mostly as lamination),

and red Gms in 25 cm-30 cm thick beds. Puiplish VRF's, blkrk and felsite(?) are more

common at this level

• 12.8 m Pale red-pale reddish bmtuffaceous breccia (Gms). Appears massive. Matrix

supported. Very similar to tuffs lower in the section, but exhibits no flattened fabric.

Gradational contact to underlying and overlying units.

Grain size range: Peb-cob

Max. grain size: 20 cm

Ave. grain size: (pb-smcob)

Roundness: angul

Shape: subeq-elong

Sorting: v.poor

Packing: matrix supp

Composition: yellowish tuffs similar to those described below, dk purple VRF's,

grntd, and minor other RFs (dacite fairly abundant near top). FG-CG matrix

•32 m more and less well indurated pale red-yellowish gry lapilli tuff breccia.

Grain size range: Peb

Max. grain size: 15 cm

Ave. grain size: (sm peb)

Roundness: angul

Shape: subeq; tuff clasts elong

Sorting: v.poor

Packing: matrix supp

Composition: yellowish-wt FG tuff with biot, grntd, dk colored VRF's. Matrix

FG-VCG

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Upper 3.6 m of lapilli tuff breccia as described above. Unbedded. Grainsize and

induration increases upwards. Upper contact gradational.

Underlain by 30 cm of well bedded (5 cm-15 cm thick beds) of pale yellowish bm

MG-granule, very poorly sorted arkose containing VRF's.

Underlain by 2.7 m of crudely bedded (15 cm beds) pebble tuff breccia and lapilli tuff

breccia as described above. Gradational down into

•7.2 m of lapilli tuff breccia as described above (max clast size med pebble), unbedded,

flattened clast fabric. Lateral continuity of these tuffs is suspect as induration varies.

• Underlain by 8 m poorly exposed earthy, friable tuff of same lithology

• Lower 10 5 m lapilli tuff breccia as described above, passing from less well up into

well indurated rocks. Unbedded, sharp upper and lower contacts, tuff clasts flattened

and aligned parallel to bedding. [Sample CC-89-1 of lapilli tuff breccia]

•5.5 m pale red-yellowish gry pebble tuff breccia (Gm, Gms reworked tuff). Crudely

bedded on 5 cm-1 m scale, with horizons of coarser/fmer grainsize, more/less matrix.

Sharp contacts. Internally massive to laminated. Lateral continuity difficult to assess

because of changes in induration and small faults

Grain size range: Peb

Max. grain size: 15 cm

Ave. grain size: (peb)

Roundness: subangul

Shape: subeq

Sorting: v.poor

Packing: clast supp-matrix supp

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Composition: dk purple dacite (& andésite?), gm stained tuff, grntd, rare siliceous

RP's. Matrix FG-CG qtz, feldsp, VRF's. Note may be a few MRF's throughout

this part of section.

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V. MEASURED SECTION OF THE FORMATIONS OF DAGGET RIDGE AND

SLASH X RANCH AT CENTRAL DAGGET RIDGE

(Measured by Chris Travis on 2-4 April, 1990.)

Beginning at the top of the section...

Top formation of Slash X Ranch

•170-230 m of dk. yellowish bm to black (some orange weathering) Porphyry cobble-

boulder breccia (Dibblee's Tpb). Bedding not apparent, outcrop forms resistant ledges

5-10 m thick, spaced 10-15 m apart. Intervening areas are perhaps more matrix-rich

and less resistant. Shattered and jigsaw fabrics common. At 86-100 m and 120-145 m

are intervals of basalt breccia and abundant basalt float, respectively. Near 145 m

basalt is calichified (parallel to bedding) to resemble breccia. Unit contains more fines

than granitic breccia. These are rock avalanche deposits. [Photos: 5.6 of offset clasts

with vein as marker(note point of pencil lies above part of original clast); 5.7 standing

on Tgb-Tpb (Dibblee) contact looking south and east at lensoid nature of resistant Tpb

outcrop; 5.8 of crackle breccia nr. top of unit] [Sample DR-90-11 of newly observed

type of rhyolite poiphyry which resembles granite from a distance].

Grain size range: Pb-bldr

Max. grain size: 50 cm

Ave. grain size: (3-10 cm)

Roundness: v.angul

Shape: equant

Sorting: v.poor

Packing: clast-supp, matrix-rich

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Composition: Mesozoic andésite porphyry (Ap), varying from banded porphyry to

It. gry apanitic andesite(?), to gryish-orange rhyolite porphyry which superficially

resembles granite. Matrix of fine-sand + clay + pebbles.

•60 m of gryish orange cobble-boulder granitoid breccia. No bedding apparent.

Appears to coarsen upward slightly. More matrix-rich horizons locally. Crackle and

jigsaw fabric common. Lower contact not seen, upper v.abrupt, irregular, marked

beneath by fine-grained, v.matrix-rich material. No mixing of clast types across

contact. To northeast along main ridge are monolithologic lenses of granitoid and

porphyryy breccia. In one outcrop see remains of granite to gnessic country rock

contact disturbed by brecciation. These are rock avalanche deposits. [Photos: 1.0-1.4

of crackle and jigsaw fabrics; 1.5 of quartz vein intact on left and right but brecciated

and disrupted in middle; 1.6 of slightly brecciated material; 1.7-1.8 of jigsaw fabric in

porphyry breccia; 1.9-1.11 of gneiss to granite contact and brecciation; 5.5 of

brecciated qtz vein in granitic breccia] [Samples: DR-3 of gneiss/schist to granite

contact; DR-90-7 of brecciated qtz; DR-90-8 of felsite nr qtz breccia; DR-90-9 of

grantoid breccia with gneiss].

Grain size range: Pb-bldr

Max. grain size: 12 cm Iwr, 70 cm at 36 m level

Ave. grain size: (1-5 cm Iwr, 2-10 cm +abdt larger at 36 m)

Roundness: angul

Shape: equant-brick-like

Sorting: v.poor

Packing: clast-supp, matrix-rich to poor

Composition: Coarse-grained qtz monzonite, vein qtz fairly abdnt in Iwr parts, rare

gneiss (country rock), rare felsite or qtz breccia, mostly unmixed. Matrix granules

with fine-sand to pebbles.

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•9.5 m of basalt. [Sample DR-90-6 of basalt, oriented for paleomag.]

•99.5 m of olive gry cobble-boulder porphyry breccia (Ap-andesite porphyry of

Dibblee). Possible crude bedding defined by ledges on 2-10 m scale, but no coherent

lithologie variation recognized except in lowest 3-4 m of unit (mixed and

monolithologic, 5-50 cm thick, sharp bounded beds of granitoid and Ap). Upper and

lower contacts not seen, but lower contact has several meters of relief. Generally

massive fabric with minor local shattering in lower 42.5 m (matrix-rich elsewhere).

Upper part of unit (above 76 m) more clast-supported. Float between 42.5 and 76 m

(on top of Daggett Ridge) includes basalt boulders. [Photos: 5.2, 5.3 of clast-supported

breccia (note alignment of banding in 5.3); 5.4 bedding plane photo of clast-supported

breccia nr top of unit] [Sample DR-90-5 of It. gry dacite/andesite breccia[.

Grain size range: Cobble

Max. grain size: 10 cm Iwr, 20 cm mid-upper

Ave. grain size: (2-8 cm)

Roundness: v.angul.

Shape: equant

Sorting: poor

Packing: Iwr matrix-supp, mid-upr clast-supp, matrix-rich to poor.

Composition: Almost entirely Mesozoic andésite porphyry. Some granitoid nr base.

In upper parts see local pods of It grydacit/andesite breccia similar to that at Azucar

Mine. Lower matrix of clay and fine-sand to granules.

•77 m of strongly foliated, fresh, non-vesicular basalt. Partially flow brecciated near 40

m. [Photo 5.1 of sample location] [Sample DR-90-4 of basalt from nr. brecciated zone].

•9.5 m unexposed

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•23.25 m of orange (aplite) to gryish pink (granitoid) pebble-boulder breccia. No

bedding apparent. At one location see ~4.5 m thick lens of aplite with sharp, steeply

dipping contacts (no mixing) = fault or (more likely) original intrusive relationship.

Upper and lower ontacts of unit not seen. Laterally persistent unit. Base of unit finer-

grained, matrix-rich, becoming coarser-grained, more clast-supported upward over

lower 14 m (to 50 cm max. grainsize). [Photos: 4.24 of lower, matrix-supported/rich

breccia; 4.25 of upper, clast-rich breccia].

Grain size range: Pb-bldr

Max. grain size: 15 cm Iwr, 70 cm upr

Ave. grain size: (5-10 cm Iwr, 5-25 cm upr)

Roundness: v.angul (aplite)-subangul.

Shape: equant

Sorting: v.poor

Packing: matrix-supp. Iwr clast-supp matrix-med. upr

Composition: Coarse-grained qtz monzonite & products. Also monolithologic

lenses of fine-grained aplite. Matrix medium- to fine-sand with granules and clay.

•-230 m unexposed. Float of granitoid material, granitic breccia, basalt, and diorite.

All probably in place as there is no higher source.

•-110 m of olive gry (pale orange at base due to abndt matrix), pebble to boulder breccia

of Mesozoic andésite porphyry (Ap). No bedding. Poorly exposed lower contact to

earthy material underlain by granitoid breccia. Highly shattered fabric (crackle breccia)

with little if any displacement of blocks (original foliation & jointing traceable across

clasts -see photos). High angle planes might be faults. Basal part of unit more matrix-

rich, redder in color, and crudely layered on -40 cm scale (comminution zone?).

Uppermost 15 m of unit may exhibit repeat of matrix-rich to clast-rich sequence. This

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is a landslide, or maybe a rock avalanche. [Photos: 4.13,4.14,4.19-4.23 of fabrics]

[Sample DR-90-3 of andésite with schlieren].

Grain size range: Pb-bldr

Max. grain size: 2 m blks

Ave. grain size: (2-8 cm blks)

Roundness: v.angul

Shape:

Sorting:

Packing: clast-supp matrix-rich nr base, matrix-poor, tight fitted upr

Composition: Mesozoic andésite porphyry with/without banding, also with schlieren

(banding & schlieren prob. eutaxitic). Matrix clayey at base, fine-sand elsewhere.

•~70 m unexposed. See orange-stained muddy, clast-supported, matrix-rich breccia of

Ap at top of section, assumed to correlate with base of unit described above (which is

250 m west of section measured below).

•44.25 m of granitoid breccia similar to the sequence described below, but less well

exposed. [Photos: 4.16,4.17 of upper part of interval below v.large boulders; 4.18 of

lower part of breccia (though not the base, which is muddier)].

•38 m of gryish orange pebble to boulder breccia of granotoid material. No bedding

seen. Lower contact not seen, but appears to be associated with an earthy zone.

Progressive upward increase in average and maximum grainsize, with concurrent

decrease in matrix content. Top of unit marked by a layer of ~8 m diameter boulders.

This is a rock avalanche or a very clast-rich debris flow.

Grain size range: Cl-lge bldr

Max. grain size: 15 cm base, 5-70 cm upr, 8 m top

Ave. grain size: (1-3 cm base, 5-25 cm upr)

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Roundness: angul

Shape: equant

Sorting: v.poor

Packing: clast-supp, rich in muddy matrix

Composition: Qtz monzonite and products thereof. Matrix muddy to sandy.

•19 m of yellowish gry tuffaceous sandstone and pebble-cobble breccia of volcanic

debris. Upper and lower contacts not seen but appear to be fairly abrupt. 5-30 cm

thick beds of sandstone with sharp, planar contacts, parallel and low angle cross

laminations, and occasional cobble lags. 50 cm beds of breccia, some with gradational

contacts and inverse-normal grading. Predominantly facies Sh/St, less common Gm.

[Photos: 4.9 of this sandy interval; 4.10 of coarse-grained facies; 4.11 of fine-grained

facies with cut & fill and cross laminations); 4.12 of cobble lag in sst (parallel to staff)]

Grain size range: sst, pb-cb

Max. grain size: 70 cm (brecc)

Ave. grain size: (F-C, & pebbly; pb-cb)

Roundness: subangul

Shape:

Sorting:

Packing:

Composition: sandstone tuffaceous; breccia clasts of andesiote and basalt.

•49 m of granitoid breccia as described in 55 m thick unit below, except that it is

generally slightly finer-grained and fabric (where exposed) is clast-supported, matrix-

rich. Contacts not seen.

•27 m of dk. purplish, vesicular, brecciated basalt. Contacts not seen.

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•4 m of gryish pink to mod. pink v.coarxse-grained sst and pebble breccia of volcanic

origin. Lower contact abrupt, upper irregular. 3 m and 7-80 cm bedded. Crude

horizontal stratification in finer-grained units.

Grain size range: sst & breccia

Max. grain size: 15 cm

Ave. grain size: VC sa, Pb

Roundness: subangul-angul.

Shape:

Sorting:

Packing: breccia clast-supp matrix-rich & matrix-supp.

Composition: Predominantly pinkish basalt. Also tuff, fine-grained poipohyritic

banded andésite, and rare med-fine grained granitoid frags. Matrix clay to coarse-

sand.l

•55 m v.pale orange to yellowish orange weathering pebble-boulder breccia of granitoid

debris. Lower and upper contacts abrupt. No bedding or fabric apparent through most

of unit, but clast size dramatically coarser nr upper contact. Well exposed, 6 m thick

section in upper part of unit is rubbly weathering, matrix-supported clast-rich to clast-

supported matrix-rich pebble-cobble (max 1.5 m) breccia of friable granitoid debris

with a matrix of clay to fine-sand, and pebbles.

Grain size range: clay-bldr

Max. grain size: 80 cm basal,-15 m at top

Ave. grain size: (sm pb-15 cm)

Roundness: angul

Shape: equant

Sorting: v.poor

Packing: resistant band is matrix-supp clast-rich

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Composition: Coarse-grained, somewhat friable qtz monzonite/granite (some qtz but

not enormous amounts). Matrix clay-vf sand in resistant band, mud and pebbles in

wash (similar to upper section at Azucar Mine).

Base formation of Slash X Ranch

Top formation of Dagget Ridge

—214 m of It greenish gry tuffaceous sst and tuff breccia. Lower contact not seen,

upper abrupt. Coarse-grained tuff breccia in 7.5 m thick beds with gradational

contascts, may coarsen upward. Finer-grained breccia clast-supported in 50 cm thick

sharp based, gradational top beds, massive to crudely stratified with sandier lenses.

Sst in 7-30 cm thick, laterally discontinuous beds with sharp contacts and horizontal

laminations and cut and fill structures. These are facies Gms, Gm, and Sh,

respectively. The unit consists of 19 m of poorly exposed Gms, overlain by 16 m of

well exposed Gms, overlain by 4.75 m of Gm and Sh, overlain by 174 m of well to

poorly exposed Gms. [Photos: 4.7 of facies Gm and Sh; 4.8 of coarser-grained facies

-note coarsening upward (bedding dips steeply to right)].

Grain size range: FSa-Bldr

Max. grain size: Gms: 2.5 m, Gm: 50 cm, Sh: pebble

Ave. grain size: (Gms: 3-15 cm, Gm: 1-5 cm, Sh; MG-CG)

Roundness: subangul-submd

Shape: equant

Sorting: well-poor

Packing: Breccia matrix-suppclast-rich to clast-supp.

Composition: Lt. bm weathering dk. gry, flow banded andésite. Minor porous,

crystal-rich andésite (not as much as at Azucar Mine), reddish andésite, and basalt.

Matrix fine-sand to pebble.

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VI. MEASURED SECTION OF THE FORMATIONS OF DAGGETT RIDGE AND

SLASH X RANCH NEAR AZUCAR MINE

(Measured by Chris Travis on 26-29 March 1990.)

Beginning at the top of the section...

Top formation of Slash X Ranch

•~810 m poorly exposed pebble to boulder breccia of granitoid debris. Poorly exposed

areas consist of pebble-cobble average (1 m max), rounded to angular clasts of granite

and qtz monzonite with subordinate fine-grained diorite and basalt. Crops out as surface

wash on low hills of earthy (clay rich) ground with grus and predominantly pebbles to

cobbles. Two areas of exposure found within unit -described below:

1) ~5 m of pebble-boulder breccia of debris flow origin (facies Gms). Upper and lower

contacts not seen. Unbedded to 1-30 cm bedded with gradational contacts. Massive to

basal inverse graded in thick beds, crude normal grading in thinner beds. High and

low-angle imbrication. Veiy muddy debris flow unit, probably typical of the entire

poorly sequence. [Photos: 3.12, 3.13 of this sequence; 3.14 of bedded rocks with

possible imbrication (each side of hammer handle)].

Grain size range: Pb-bldr

Max. grain size: 20 cm

Ave. grain size: pb

Roundness: angul

Shape:

Sorting: v.poor

Packing: matrix-supp, clast-rich

Composition: Granitoid frags, subordinate FG diorite and basalt. Matrix v.muddy

with coarse-sand.

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2) ~25 m of pebble to cobble breccia with minor sandstone. 7 cm to 5 m thick beds.

Thicker beds have gradational contacts and basal inverse grading. Thinner beds exhibit

imbrication (paleocurrents to northwest). Predominantly facies Gms, with subordinate

Gm (see original sketch). In upper part of section see abndt clasts of dk gry banded

porphyry and volcanic breccia v.similar to Dibblee's Tfp/Tpb of Daggett Ridge (these

clasts located nr. 1 of section 13). [Photos: 3.15, 3.16 of this sequence (looking north,

dipping west)].

Grain size range: Pb-cb

Max. grain size: 15 cm

Ave. grain size: (pb-cb)

Roundness: angul

Shape:

Sorting:

Packing: matrix-supp, matrix-rich

Composition: Granitoid and basalt, with Mesozoic volcanics higher in section.

Muddy matrix.

•12.5 m of small cobble breccia. Crudely bedded in upper few meters and lower 80 cm.

Contains an ~3 m thick, less resistant horizon in middle of unit. Mostly facies Gms.

Grain size range: Sm cb

Max. grain size: 30 cm

Ave. grain size: (sm cb)

Roundness: angul

Shape:

Sorting:

Packing: matrix-supp.

Composition: Polylithologic, mostly granitoid.

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•12.5 m poorly exposed granule-pebble breccia similar to 40 m thick unit described

below. Polylithologic composition, basalt more common than in underlying unit.

Central part of sequence includes pinchout of a laterally equivalent resistanty unit similar

to the 4.75 m thick sequence, described below. Probably facies Gms.

•4.75 m of bedded pebble breccia. Lower contact sharp, upper gradational. Lateral

continuity of this and other resistant units at this level ~30 m. Consists of ~1 m thick,

parallel sided beds, some massive but mostly showing crude horizontal stratification.

Facies Gm with a little Gms nr base. [Photo 3.11 of lower part of this section showing

sst layers].

Grain size range: Pebble

Max. grain size: 10 cm

Ave. grain size: (<1-3 cm)

Roundness: v.angul

Shape:

Sorting:

Packing: clast-supp.

Composition: Polylithologic, granitoid dominant

•~40 m of poorly exposed pebble breccia. No bedding visible. Massive where visible.

Probably predominantly Gms. One horizon of ~10 cm diameter cobbles seen within

unit.

Grain size range: Pebble

Max. grain size: 5 cm

Ave. grain size: (gm-2 cm)

Roundness: submd(bas)-angul

Shape: equant

Sorting: v.poor

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Packing: matrix-supp, matrix-rich

Composition: Mixed lithology: granitoid and dacite/andesite with rare reddish,

vesicular basalt. Matrix of clay and sand.

•15.5 m of pebble to cobble breccia. Diffuse, wavy lower contact, gradational upper

contact. Massive to crudely stratified. Better bedded above 5.5 m, with 30-100 cm,

parallel sided beds of smaller grainsize. Lower 40 cm coarsens upwards. Sequence

horizontally stratified to strongly parallel laminated. Cmde imbrication with

northeasterly orientation. Facies Gm.

Grain size range: Pb-cb

Max. grain size: 20 cm

Ave. grain size: (pb-cb Iwr pb upper)

Roundness: v.angul

Shape: equant-tabular

Sorting: v.poor

Packing: clast-supp.

Composition: Monolithologic, of granitoid debris. Matrix medium-sand to pebble.

♦2 m of pebble breccia. Lower contact not seen, upper diffuse and wavy. Crudely

stratified matrix of granules with imbricate pebbles. Facies Gm.

Grain size range: Gm-pb

Max. grain size: 5 cm

Ave. grain size: (gm-pb)

Roundness: angul

Shape:

Sorting:

Packing: larger clasts supp in matrix

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 0 6

Composition: Mixed lithology: abndnt It-med gry dacite/andesite, granitoid debris,

and rare VRF's. Sandy matrix.

•22 m poorly exposed granule-pebble breccia similar to the 14 m thick unit described

below. Differs, however, in that it exhibits crude cut and fill type bedding. A fine

grained version of facies Gm.

•~5 m of granule to pebble breccia. Crude discontinuous bedding -10 cm thick with

fairly planar contacts. Horizontally stratified. No grading apparent, but unit may

consist of ~2 m thick fming upward sequences. Fine-grained facies Gm (nearly a

pebbly sandstone). Similar to poorly exposed units below and above, but better bedded

and more resistant.

Grain size range: CGSa-pb

Max. grain size: 5 cm

Ave. grain size: (0.5-2 cm)

Roundness: angul

Shape: equant

Sorting: v.poor

Packing: clast-supp, matrix-rich

Composition: Monolithologic, of granitoid debris

•14 m of poorly exposed granule-pebble breccia. No bedding apparent except for one

70 cm thick horizon of with max clast size of 5 cm. Massive. Is this fine-grained facies Gms?

Grain size range: Gm-Pb

Max. grain size: 1-2 cm

Ave. grain size: (0.5-2 cm)

Roundness: angul

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Shape: equant

Sorting: v.poor

Packing: matrix-rich

Composition: Monolithologic, of granitoid debris. F-M sand matrix with some clay.

•2.2 m of pebble to cobble breccia similar to 5.5 m thick unit described below, but

having no bedded interval. Massive.

•1.5 m of granule-pebble breccia. Lower contact not seen, upper fairly sharp with ~1 m

of relief. Poorly bedded with ~10 cm thick beds. Fines slightly in upper 50 cm. May

be a fine-grained facies Gms?

Grain size range: Fsa-pb

Max. grain size: 3 cm

Ave. grain size: (gm-1 cm)

Roundness:

Shape:

Sorting: v.poor

Packing: clast-supp.

Composition: Granitoid rock fragments. Few clasts of It gry dacite/andesite nr

upper contact.

•8.4 m unexposed

•5.5. m pebble-cobble breccia. Lower contact fairly diffuse with ~1 m relief, upper not

exposed. Lower 3 m unbedded. Upper 2 m crudely bedded at 20-4- cm scale. Mostly

massive, although lower 40 cm coarsens up slightly and bedded part may show faint

imbricate fabric. Upper part is facies Gm, lower part facies G1 (looks like problematic

unit below). [Photos: 3.9,3.10 of upper part of this sequence, showing crude bedding

and possible imbricate fabric].

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 308

Grain size range: Pb-cb

Max, grain size: 20 cm

Ave, grain size: (5-10 cm)

Roundness: angul

Shape:

Sorting: v.poor

Packing: larger clasts supp. in matrix

Composition: Monolithologic, of granitoid debris. Sandy matrix.

•3 m of corse-sand to large pebble breccia. Lower contact sharp, wavy, upper fairly

diffuse. Lower 2 m unbedded, upper 1 m crudely bedded. No structures. Upper part

facies Gm, lower G1 (debris flow?).

Grain size range: CSa-cb

Max. grain size: 10 cm

Ave. grain size: (gm-5 cm)

Roundness: angul

Shape:

Sorting: v.poor

Packing: larger clasts supp. in matrix

Composition:

'1.6 m of small pebble breccia. Lower contact fairly sharp., upper sharp, wavy. Crude

5-8 cm thick, discontinuous bedding. Crude horizontal stratification. Facies Gms

Grain size range: MSa-lg pb

Max. grain size: 15 cm

Ave. grain size: (CSa-sm pb)

Roundness: angul

Shape: equant-tabular

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Sorting: v.poor

Packing: clast-supp

Composition: Monolithologic, of granitoid debris. Sandy matrix.

•~70 cm of gryish orange to blk weathered to It gry pebble-cobble breccia. Lower

contact not seen, upper contact sharp, cuts upsection by 7 m. Crude bedding in uper

part of section, otherwise massive. Appearance varies from fairly coherent granitoid to

highly shattered and matrix-rich. Has an odd, arcuate outcrop pattern. Problematic

interpretation: could be cataclasite of detachment, but is v.fresh and not red/weathered;

could be a rock avalanche, but has an odd outcrop pattern and cuts upsection. My guess

is that it is a rock avalanche/landslide with a high relief surface or else upper plate chaos.

[Photos: 3.4-S.7 of shattered (cataclastic) appearance of granitoid; 3.8 of upper contact

and overlying unit].

Grain size range: CG-bldr

Max. grain size: >few m

Ave. grain size: (pb-cb)

Roundness: v.angul

Shape: equant

Sorting: v.poor

Packing: clast- to matrix-supp

Composition: Monolithologic, of granitoid. Matrix of coarse-sand and

granules/pebbles.

•71 m of poorly exposed cobble breccia as rubble on dipslope. Contacts not visible, but

appear planar. 1-2 m thick beds, some crudely stratified, some internally massive.

Facies Gm and (massive) Gl. Is G1 v.clast-rich debris flows?

Grain size range: Pb-bldr

Max. grain size: 50 cm

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 1 0

Ave, grain size: (cb)

Roundness: angul

Shape: equant

Sorting: v.poor

Packing: clast-supp

Composition: Predominantly granitoid debris. Matrix of sand, clay, and pebbles.

•3.4 m of large pebble breccia. Lower contact appears gradational, coarsening upward,

upper contact not seen. Crudely bedded, internally massive. Probably facies Gm.

[Sample AZ-90-5 good sample of dacite/andesite with abndnt hornblende].

Grain size range: Pb-cb

Max. grain size: 10 cm

Ave. grain size: (Ige pb)

Roundness: v. angul

Shape: equant

Sorting: v.poor

Packing: clast-supp matrix-rich

Composition: Lt gry dacite with subordinate granitoid debris. Matrix of clay and

coarse sand (no fine sand).

•3 m of poorly exposed pebbly, matrix-rich breccia of It gry dacite/andesite with

subordinate granitoid debris and rare VRF's (including pumice). Clayey matrix.

•20 m of medium- to coarse-grained sst and pebble-cobble breccia. 15-30 cm thick beds

of horizontally stratified sandstone (facies Sh) of >6 m lateral extent. 20-40 cm thick

beds of horizontally stratified pebble breccia (facies Gm), and 1-2 m thick beds of

massive cobble breccia of >50 m lateral extent. Contacts mostly sharp, planar. Minor

facies Gms seen also. [Photos: 3.1 of poorly stratified, internally massive granitoid

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 311

breccia; 3.2 from top of hill looking west at outcrop on other side of truck (above the

diplsope); 3.3 looking south at 20 m unit overlying this one].

Grain size range: M-CSa, pb, cb

Max. grain size: 20 cm (in msv brec)

Ave. grain size: (M-CSa,pb, cb)

Roundness: angul-v.angul

Shape:

Sorting:

Packing: clast-supp, matrix-poor to rich

Composition: Basal part of unit dominated by granitoid debris. Lt gry

dacite/andesite dominates above first few meters, though granitoid still abundant.

•7.2 m of pebble breccia as described in 14.5 m thick unit below. Facies Gm, volcanic

rock fragment composition.

•80 cm of gryish orange to black weathering pebble breccia. Single parallel sided bed

with lateral extent of a few meters. Sharp contacts, internally massive fabric. Black is

manganese cement crust(?). Probably facies Gm or a v.clast-rich Gms (Gl?). Not a

sieve deposit (why not?). [Photo 2.20 of this and top of underlying unit] [Sample AZ-

90-4 of granitoid clast].

Grain size range: CSa-cb

Max. grain size: 25 cm

Ave. grain size: (pb)

Roundness: v.angul

Shape:

Sorting: poor

Packing: clast-supp.

Composition: Monolithologic, of granitoid debris.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 1 2

•9 m of gryish orange granule-v.small pebble breccia. 10-40 cm thick, tabular beds

with sharp, planar contacts. Horizontally stratified to internally massive. Facies Gm.

Note upper contact of basalt unit reaches as high as this level laterally.

Grain size range: MSa-pb

Max. grain size: Pb

Ave. grain size: (CSa-v. sm pb)

Roundness: angul

Shape:

Sorting: poor

Packing: med well

Composition: dominantly granitoid debris with minor VRFs

•7 m of large pebble breccia. Horizontally stratified, but no strong bedding. Facies

Gm. Note that there are discrete lenses of more granitoid-rich and more It gry VRF-rich

debris.

•-14.5 m of pebble breccia. Crudely bedded on ~80 cm scale, decreasing to ~40 cm in

upper part of sequence. Horizontally stratified. Predominantly facies Gm, with a

laterally discontinuous, 1.8 m thick lens of facies Gms granitoid-rich cobble breccia

within the sequence. Base of sequence VRF-rich. Granitoid content increases upwrds,

but VRF-rich lenses up to 2 m thick occur throughout unit.

Grain size range: Pb-cb

Max. grain size: 20 cm

Ave. grain size: (pb)

Roundness: angul

Shape: subeq

Sorting: v.poor

Packing: clast-supp, matrix-rich

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 1 3

Composition: Predominantly It gry VRF's with subordinate basalt, granitoid, and

other VRF's. Granitoid content increases upwards (commonly as cobbles).

*11 m of pebble-boulder breccia. Lower contact scoured 2 m down into underlying

sequence. Consists of three internally massive beds of facies Gms breccia as described

in 7.2 m thick sequence below. Contacts sharp, planar. Maximum clast size 30 cm in

lowest bed, 60 cm in upper two beds. Lower bed composed of basalt clasts (section

measured close to onlapping contact to basalt). It. gry VRF's and subordinate granitoid

and pumice debris. Upper beds have more common granitoids, fewer, but large, basalt

clasts.

•4.5 m of pebble breccia. Lower contact fairly abrupt, upper irregular, scoured.

Diffuse, 20-60 cm thick, planar beds. Horizontal stratification, poor to non-existent

imbrication. Facies Gm -typical wadi deposits. [Photos: 2.16-2.17 of this unit].

Grain size range: Gm-cb

Max. grain size: 5 cm

Ave. grain size: (gm-pb)

Roundness: angul

Shape: equant-tabular

Sorting: v.poor

Packing:

Composition: dominated by It gry VRF's plus assorted VRF's fro underlying

volcanic unit (Tag of Dibblee), plus basalt. Subordinate granitoid debris. Sandy

matrix with caly and pebbles.

•7.2 m of pebble-cobble breccia. Basal contact sharp, irregular (clasts from below

protrude upwards), other contacts appear gradational. Crude bedding on ~2.5 m scale

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 1 4

defined by thin zones of finer-grained material. Internally massive beds. Facies Gms.

[Sample AZ-90-2 of It gry VRF]

Grain size range: pb-bldr

Max. grain size: 60 cm

Ave. grain size: (pb-cb)(pb-cb)(pb-cb)

Roundness: angul-subangul

Shape: equant

Sorting: v.poorv.poor

Packing: matrix-supp.matrix-supp.matrix-supp.

Composition: Vesicular red basalt. It gry VRFs, subordinate granitoid debris and

pumice. Sandy matrix with abndnt coarser lithics.Vesicular red basalt. It gry

VRF’s, subordinate granitoid debris and pumice. Sandy matrix with abndnt coarser

lithics.

•2.6 m of v.pale orange (matrix color) cobble-boulder breccia. Lower contact of unit

unconformable (deposited in a valley on basalt unit), upper irregular with protruding

clasts. Massive fabric. Lateral extent of several meters, grades into less clast-rich

breccia of same composition). Facies Gms basal lag of basalt debris. [Photo 2.15 of

basal breccia unit] [Sample AZ-90-3 of matrix].

Grain size range: Cb-bldr

Max. grain size: 60 cm

Ave. grain size: (20-50cm)

Roundness: subangul

Shape: equant

Sorting: v.poor

Packing: matrix-supp to clast-supp matrix-rich

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 1 5

Composition: Vesicular red basalt cobbles and boulders in sandy matrix with

pebbles of It gry aphanitic VRFs.

•42 m of mostly red, vesicular to amygdaloidal basalt. Lower contact not seen, upper

fairly sharp with ~47 m of relief. At least two flows, possibly more. Common well

brecciated horizons, phenocyst poor except for "oxy-horablende", some phenocryst

rich. [Sample AZ-90-lA,B of basalt and phenocryst rich basalt].

•73 m of v.lt gry tuff breccia. Lower and upper contacts not seen. Generally unbedded,

with occasional <10 cm beds of sand-granule debris. Bedding parallel variations in clast

size/density noticed but I can't make sense of them (except for obvious thin, reworked

beds). Reworked beds described below. These are lahars and fluvially reworked

volcanics. Note subsidiary presence of crystal-rich tuff similar to that in Dibblee's unit

Tab at base of section. Unit more clast-rich /clast-supp. above 19 m level. [Photos: 2.9,

2.10 general view of unit (bedding dips steeply to right in 2.10 - note clast-rich,

reworked layer; 2.11 of bedding in this unit; 2.12 showing polylithologic (volcanic)

nature of unit - note pumice clast on left of photo; 2.13 of 5 m thick, clast-rich boulder

lahar (max 1 m, ave 15 cm) at top of unit. Dominated by clasts of andésite tuff with

mafic clots, feldspars and pumices].

Grain size range: Pb-bldr

Max. grain size: 1.3 m

Ave. grain size: (cob)

Roundness: subangul

Shape: equant

Sorting: v.poor

Packing: matrix-supp matrix-rich

Composition: Lt-med gry andésite & banded andésite, dk gry (on fresh surface)

crystal-rich tuff/pumice with large aggregates of feldspar phenocrysts, reddish-gry

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 316

crystal-rich andésite tuff and weathered version of same, pale gry andésite tuff with

abndt aggregates of mafic phenocrysts and individual hornblendes. Rare layered

basalt and pink tuff. Matrix fine-grained, tuffaceous, with abndt feldspar and tiny

mafics. Above ~19 m level see more common pumice and It colored, fnable tuff,

along with older volcanic clasts similar to those in Dibbblee's unit Tf described

below.

1) 3 m thick sequence of pebbly sandstone and pebble breccia at 19 m level in 73 m

thick unit. No bedding visible, lateral extent limited to several meters. Some normal

grading from pebble breccia to medium-grained sst observed. Unit may be folded.

Grain size range: MSa-pb

Max. grain size: 20 cm

Ave. grain size: (MSa-pb)

Roundness: angul-submd

Shape: equant

Sorting: v.poor

Packing: matrix-rich sandstone

Composition: Older volcanic and plutonic debris similar to that of Dibblee's unit Tf

described below (300 m thick unit)

•~300 m of fanglomerate of Mesozoic volcanic detritus. Generally poorly exposed as a

slope wash of boulder rubble. Two well exposed sections are described below. Less

well exposed upwards. Higher exposures show fewer sedimentary structures. Slope

wash coarser-grained than outcrop = possible winnowing effect, but max clast size

definitely increases towards the top of the unit at the ridge. Overall this may be a

prograding fan unit, coarsening upward from streamflow to debris flow dominated

deposits. Clast comosition remains about the same all the way through, with older

VRF's dominant, followed by diorite.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 31 7

1) 6 m thick section of conglomerate and breccia with minor sandstone lenses at 98 m

level. Lower and upper contacts not seen. Beds 2-100 cm thick depending on grainsize

and process (coarser-grained and facies Gm thicker), with planar to wavy (50 cm relief)

contacts. Finer-grained beds have lateral extent of a few meters. Strongly imbricate

fabric, cut and fill structures common, no visible grading, horizontal stratification

common in finer-grained units. Predominantly facies Gm, with subordinate Gms and

rare Sh. Abndt imbrication measurements from this section. [Photos 1.16-1.19 of

imbrication and cut and fill structures].

Grain size range: CSa-bldr

Max. grain size: 45 cm

Ave. grain size: (pb-cb)

Roundness: subangul-submd

Shape: eq-tab,most elong

Sorting: v.poor

Packing: clast-supp to matrix-supp clast-rich

Composition: Fine- and coarse-grained diorite, basalt, aphanitic VRF's, porphyry,

chert/qtzite(?). Matrix mostly coarse sand, some clay in facies Gms

2) 52 m thick section of v.pale orange (matrix color) conglomerate somewhere above 98

m level and below top of ridge. 20-150 cm thick beds with with fairly diffuse contacts,

internally massive. Max grain size increases upwards from 40 cm at base to 1 m by 36

m level. Above this unit slope wash of pebbles-boulders commonly contains clasts >1

m in diameter. Facies Gms dominates above 24 m level (some Gm below?).

Grain size range: Gm-bldr

Max. grain size: 40 cm, 1 m

Ave. grain size: (gm-pb) (5-7 cm)

Roundness: submd

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 3 1 8

Shape: Sorting:

Packing: clast-supp

Composition: Composition at top of ridge: Porphyryr, med gry biot/homblende

andésite, porphyiitic (feldspar) andesite/basalt, Tertiary-type basalt, fine- and

coarse-grained diorite, other VRF's

Base formation of Slash X Ranch

Top formation of Daggett Ridge

•150 m of blackish red, weathered vesicular-amygdaloidal basalt flows..

•~90 m of giyish pink tuff breccia. Lower contact not seen, upper purplish and

weathered, to weathered basalt. No bedding recognized. No fabric apparent. Lateral

variation in clast size/density undecipherable. Some boulders show possible cooling

cracks. Lahar or pyroclastic? [Sample AZ-90-0 of crystal tuffs similar to those in the

western Cadys]

Grain size range: Pb-bldr

Max. grain size: 4-5 m

Ave. grain size: (2-10 cm, >1 m)

Roundness: subangul

Shape: equant

Sorting: v.poor

Packing: matrix-supp clast-poor

Composition: Pale tuff, gryish red (5R 4/2) phenocryst-rich andésite (may be flow

banded), gryish pink poorly indurated tuff, yellowish gry to It bmish gry weathered

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 319

andésite (greenish feldspar phenocrysts), crystal tuff with ~5mm phenocrysts, rare

dk gry andésite.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. APPPENDIX B. GRAVITY SURVEY ALONG FORT iRWIN ROAD, NORTH OF BARSTOW, CALIFORNIA

Equipment tested 3 April, 1988 by C. Travis, D. Henry, L. Serpa, and T. Pavlis.

Survey earned out 8-9 April, 1988 by C. Travis, using a Worden gravimeter and pole-

type magnetometer.

The following pages contain the raw data collected dining the survey and show its

progressive reduction to the full Bouger anomaly. No inteipretations are included here,

but 2D gravity models of the survey area based on surface geology and data shown

below are compatible with geologic models presented in Chapters 2 and 3 of this

dissertation. Model densities were based on values published in:

Mabey, D. R., 1960, Gravity Survey of the Wesert Mojave Desert, California:

United States Geological Survey Professional Paper 316-D, 73p.

320

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■D CD

C/) C/) A B 0 D EF 1 Date SLocation Station name Station no. Odometer mi UTM latitude (m) Kilometers north ol PBB10 2 latitude 34°57.34' 3 3-Apr-88 Barstow Central High School 1 3861.930 -6.040 8 4 Ft. Irwin road PBB10 2 3867.970 0.000 ■D 5 Barstow Central High School 1 3861.930 -6.040 6 PBB10 2 3867.970 0.000 7 PBB10 2 3867.970 0.000 8 3 3869.570 1.600

CD 9 3 3869.570 1.600 1 0 Barstow Central High School 1 3861.930 -6.040 1 1 4 3865.230 -2.740 3. 3" 1 2 PBB10 2 3867.970 0.000 CD 1 3 5 3868.745 0.775 CD 1 4 3 390.60 3869.570 1.600 ■D O 1 5 6 391.00 3870.150 2.180 Q. 1 6 7 391.65 3870.840 2.870 C a 1 7 8 392.15 3871.275 3.305 3O 1 a Copper City Road 9 392.75 3871.615 3.645 ■D 1 9 10 393.25 3872.045 4.075 O 20 Copper City Road 9 400.20 3871.615 3.645 21 10 400.70 3872.045 4.075

CD 22 11 401.20 3872.175 4.205 Q. 23 12 401.70 3872.325 4.355 24 13 402.25 3872.540 4.570 2 5 14 402.75 3872.630 4.660 ■D 26 15 403.25 3872.785 4.815 CD 27 16 403.65 3872.940 4.970 28 17 404.15 3873.085 5.115 C/) C/) 29 18 404.75 3873.310 5.340 30 19 405.70 3874.330 6.360 31 20 3873.145 5.175 32 Copper City Road 9 3871.615 3.645 7D ■DCD O Q. C g Q.

■ D CD

C/) C/) A B 0 D E F 3 3 PBB10 2 3867.970 0.000 3 4 Barstow Centrai High School 1 3861.930 -6.040 3 5 8 3 6 8-Apr-88 PBB10 2 705.50 3867.970 0.000 ■D 3 7 Ft. Irwin Road 5 706.00 3868.745 0.775 3 8 3 706.45 3869.570 1.600 3 9 6 706.95 3870.150 2.180 4 0 7 707.65 3870.840 2.870 41 8 708.15 3871.275 3.305 4 2 Copper City Road 9 708.80 3871.615 3.845 4 3 10 709.30 3872.045 4.075 3. 4 4 11 709.80 3872.175 4.205 3" CD 4 5 PBB10 2 714.10 3867.970 0.000 4 6 3 3869.570 1.600 CD ■ D 4 7 7 3870.840 2.870 O 4 8 Copper City Road 9 Q. 717.45 3871.615 3.645 C 4 9 12 718.95 3872.325 4.355 aO 5 0 13 719.50 3872.540 4.570 3 51 14 720.00 3872.630 4.660 "O O 5 2 15 720.50 3872.785 4.815 5 3 16 720.85 3872.940 4.970 5 4 20 721.60 3873.145 5.175 CD Q. 5 5 PBB10 2 726.10 3867.970 0.000 5 6 7 3870.840 2.870 5 7 20 736.50 3873.145 5.175 5 8 21 737.00 3873.345 5.375

■ D CD 5 9 22 737.50 3873.985 6.015 6 0 23 738.00 3874.620 6.650

C/) 61 24 738.50 3875.270 7.300 C/) 62 25 739.00 3876.030 8.060 6 3 26 739.60 3876.815 8.845 6 4 27 3878.415 10.445 323

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C/) C/) G H 1 J K 1 Benchmark ft Benchmark m Altimeter 1 m Altimeter 2 m Altl normalized to Base 2 Station PBB10 at start of day 3 2131.1 649.6 1880.0 651.5 8 4 2655.0 809.2 2060.0 804.0 'C■D 5 2131.1 649.6 1882.0 649.5 6 2655.0 809.2 2062.0 805.0 7 2655.0 809.2 2061.0 804.5 8 2773.0 845.2 2105.5 838.8 9 2773.0 845.2 2105.0 838.5 1 0 2131.1 649.6 1903.5 660.5 11 2274.0 693.1 1951.0 704.0 3. 1 2 2655.0 809.2 2082.5 813.5 3" CD 1 3 2124.0 842.0 1 4 2773.0 845.2 2135.0 851.0 ■DCD O 1 5 2111.0 830.0 Q. 1 6 2672.0 814.4 2103.0 823.5 C a 1 7 2102.0 821.0 3O 1 8 2102.0 821.8 "O 1 9 2107.0 825.8 O 2 0 2102.5 823.5 21 2101.0 830.0

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M %

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■D CD L M N 0 C/) C/) 1 AII2 normalized to Base Altl corrected for All 2 corrected for Altl corrected for daily 2 Station PBB10 at start of day daily drift daily drift drift and elevation (m) 3 4 5 8 6 ■D 7 8 9 1 0 11 1 2 1 3 3. 1 4 3 " CD 1 5 1 6 ■DCD O 1 7 Q. 1 8 C a 1 9 O3 2 0 "O 21 O 2 2 2 3

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"D CD P QR ST C/) C/) 1 Alt 2 corrected for daily Alt 1 error (m) Alt 2 error (m) ' Gravity reading 2 drift and elevation (m) (machine units) 3 743.8 4 563.8 CD 5 743.6 ■D8 6 563.5 7 (O' 8 430.5 9 1 0 740.9 11 825.2 1 2 561.6 3. 1 3 488.8 3" 1 4 429.2 CD 1 5 460.0 ■DCD 1 6 452.3 O 1 7 446.2 Q. C 1 8 429.5 Oa 1 9 395.2 3 2 0 "O 21 O 2 2 386.8 2 3 382.7 CD Q. 2 4 375.3 2 5 379.6 2 6 374.9 2 7 377.2 ■D 2 8 333.9 CD 2 9 269.6 (/) 3 0 172.9 (/) 31 308.3 3 2 331

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C/) C/) U V W X Y 1 Calibrated Gravity reading Gravity corrected Latitude correction Free Air correction Free Air Anomaly 2 (0.084 mgai/machine unit) for daily drift (mgal) (mgal) to PBB10 datum (mgal) (mgal) 3 -3.5 4 0.0 ■D8 5 -3.5 6 0.0 7 0.0 8 0.9 9 0.9 1 0 -3.5 1 1 -1.6 3. 1 2 0.0 3 " 1 3 0.4 CD 1 4 0.9 ■DCD 1 5 1.3 O 1 6 1.7 Q. C 1 7 1.9 Oa 1 a 2.1 3 1 9 2.4 "O O 20 2.1 21 2.4 22 2.4 CD Q. 23 2.5 24 2.7 25 2.7 26 2.8 ■D 27 2.9 CD 28 3.0

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WC/) AE AF AG AH Al AJ 3o" 1 Relative Full Souquer Absolute Free Air Absolute Full Souquer Time Decimal time Internal temp F O 2 Anomaly (mgal) Anomaly (mgal) Anomaly (mgal) 3 9:15 69 4 9:36 ■D8 5 10:00 6 10:13 7 10:29 8 10:40 9 10:44 CD 1 0 12:52 11 13:06 1 2 13:15 3. 3 " 1 3 13:26 74 CD 1 4 13:37 CD 13:49 ■D 1 5 O 1 6 13:57 Q. C 1 7 14:06 1 8 14:15 Oa 3 1 9 14:24 76 ■D 2 0 14:53 O 21 15:05 2 2 15:12 CD 2 3 15:24 Û. 2 4 15:33 78 2 5 15:41 2 6 15:51 ■D 2 7 15:59 CD 2 8 16:09 2 9 16:19 78 C/) C/) 3 0 16:29 31 16:51 78 3 2 17:03

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C/) C/î AS 1 Comments & weather 2 CD 3 "O8 4 5 CO" 6 7 8 9 1 0 3. 11 Hiçjh cirrus clouds came In 3 " 1 2 Many telephone poles CD 1 3 0.5 miles on left side "OCD 1 4 O 1 5 At road intersection Q. C 1 6 Fossil Bed Road Oa 1 7 Netx) Quad 3 1 a Copper City Rd.. rt. side; missed BM by 0.1 mi; noisy mag. "O 1 9 Noisy mag. O 20 21 CD Q. 22 2 3 24 2 5 By old house frame on right "O 2 6 Near double post CD 2 7 Irwin Road intersection

C/) 28 C/) 2 9 At bend in road 30 By section road 31 Windy gravity reading? 3 2

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C/) C/) AS 3 3 3 4 3 5 8 3 6 Hazy but fair, no clouds; Mag. rdng -2 m Iwr elev. than grav. "O 3 7 Between power lines (40-50 m each side) 3 8 Uncapped benchmart< = replacement? 3 9 No sign of track marked on map(is south of Sta. 6 on map) 4 0 41 Mag. rdng ~2 m Iwr elev. than grav. 4 2 15 m N of new BM at Copper City Rd; pwr lines = no mag. 4 3 3. 4 4 3 " CD 4 5 Mag. rdng -2 m Iwr elev. than grav; Alts prob. not stuck 4 6 Altimeter check CD ■D 4 7 Altimeter check O Q. 4 8 Altimeter check C 4 9 a 5 0 Had to back up, odometerl 3-12 distance 13-12 3O ■D 51 Cirrus clouds in from west some time in last hour O 5 2 5 3 Irwin Road intersection 5 4 By blue "Telephone 1 mile" sign CD Q. 5 5 Clear skies; Mag. rdng -2 m Iwr elev. than grav. 5 6 Altimeter check 5 7 Altimeter check 5 8 Windy, but alts not fluctuating: Mag on LHS rd, some m higher ■D CD 5 9 6 0 Some stratus clouds to N & NW, otherwise clear C/) C/) 61 6 2 Mag. poss. dodgy -nr. road and 30 m from small pwr lines 6 3 Metal pipe in concrete on track on curve -presume is SBM 2775.2; Clouds spreading 8 6 4 ■DCD O Q. C g Q.

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AS 6 5 Cloudy skies as far 8 as here, high cirrus and nimbostratus (or dusk!) 6 6 6 7 Fair weather, less hazy than 8-Apr-88, no clouds 6 8 3. 3 " 6 9 NB. magdecrease 0.5 mi S, gravdecrease here CD 7 0 RHS road, uncapped BM 71 Too many pwr lines/local relief for mag. ■DCD O 7 2 Mileage check Q. 7 3 Between 2 & 5; too many pwr lines for mag. C a 7 4 Between 5 & 3; too many pwr lines for mag. 3O 7 5 Altimeter check ■D 7 6 Between 3 & 6; there is a decent-sized fly (live) in altimeter 2 O 7 7 Altimeter check 7 8 Between 7 & 8 CD 7 9 Between 8 & 9 Q. 8 0 Between 9 & 10 81 Altimeter check 8 2 ■D CD

C/) C/)

U) OLA APPENDIX C. DERIVATION OF FINITE DIFFERENCE

A pproximations f o r m o d e l o f h a l f G r a b e n

S edimentation

Sediment diffusion equation

The sediment diffusion equation,

dh . d h d k dh (1) à

where h is elevation, t time,

distance, is formulated in an implicit finite difference scheme. The relevant finite

difference approximations are:

a/, (c'-*;) (2) à Ât

, n + 1 dh y+1 - c l (3) d x Ax

A (4) Ax^

dk ^j) (5) Ac

which, when substituted into equation (1) yield:

351

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(6) where the left hand side contains unknown elevation values at the time step n+1 and the

right hand side contains known values of the elevation at time step n. Equation (6) is in appropriate form for solution as a tridiagonal set of linear equations. An efficient

algorithm for solving such systems is given by Press and others (1986).

Flexure equation

.2 d^w 1> (7) dx + P ^ + (Pm^gw= QaM dx dx

where Qc^x) is the applied load, w the vertical displacement of the plate, Pm the mantle

density, g the acceleration due to gravity, and D the flexural rigidity of the lithosphere.

The resulting arrangement:

P\ w j+ 2 - Wj+i + 4 4 ' 2 4 2 ' ’’ ' ^ Ax Ax ^ Æc

fP>i A Wj.i + 4 2 4 ^j-2 = ^0j (8) Ax Ax

is solved as a five-band diagonal set of linear equations using a technique similar to that

for tridiagonal matrices.

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Christopher John Travis was bom in London, England on April 5th, 1960 to

Elizabeth and Harry Travis. He attended William Ellis School in Highgate, where he

completed G.C.E. A-levels in Math, Physics, and Geography. After studying

Computer Science, Materials Science, and Geology in his first year at University

College Swansea, he went on to attain a Bachelor of Science degree with joint honors in

Geology and Oceanography from the University of Wales (U.C. Swansea). Chris

subsequently enrolled at the University of South Carolina in Columbia, where he gained

a Master of Science in Geology with a thesis entitled Upper Cretaceous Sttatigraphy of

Eastern South Galala, Egypt. He then moved to Baton Rouge, where he worked on a

Doctor of Philosophy in Geology. Chris now lives in Houston, Texas with his wife, Denise McGaha (a psychotherapist), two dogs, and two cats. He is employed by BP Exploration, Inc.

353

Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. DOCTORAL EXAMINATION AND DISSERTATION REPORT

Candidate: Christopher John Travis

Major Field: Geology

Title of Dissertation: Stratigraphie Architecture of an Extensional Orogen: The Mojave Extensional Belt, Southern California

Approved:

jor Professor and Chairman

Dean of the Graduate" School

EXAMINING COMMITTEE:

Date of Examination:

March 6, 1992

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