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Geology and structural evolution of the southern Shadow Mountains, San Bernardino County, California

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Geology and structural evolution of the southern Shadow Mountains, San Bernardino County, California

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Content INFORMATION TO USERS This manuscript has been reproduced from the microfilm master. UMI films the text directly from the original or copy submitted. Thus, some thesis and dissertation copies are in typewriter face, while others may be from any type of computer printer. The quality of this reproduction is dependent upon the quality of the copy submitted. Broken or indistinct print, colored or poor quality illustrations and photographs, print bleedthrough, substandard margins, and improper alignment can adversely afreet reproduction. In the unlikely event that the author did not send UMI a complete manuscript and there are missing pages, these will be noted. Also, if unauthorized copyright material had to be removed, a note will indicate the deletion. Oversize materials (e.g., maps, drawings, charts) are reproduced by sectioning the original, beginning at the upper left-hand comer and continuing from left to right in equal sections with small overlaps. 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GEOLOGY AND STRUCTURAL EVOLUTION OF THE SOUTHERN SHADOW MOUNTAINS, SAN BERNARDINO COUNTY, CALIFORNIA by Mary Alice Parke A Thesis Presented to the FACULTY OF THE GRADUATE SCHOOL UNIVERSITY OF SOUTHERN CALIFORNIA In Partial Fulfillment of the Requirements for the Degree MASTER OF SCIENCE (Geological Sciences) December, 1997 1997 Mary Alice Parke Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UMI Number: 13 89978 UMI Microform 1389978 Copyright 1998, by UMI Company. All rights reserved. This microform edition is protected against unauthorized copying under Title 17, United States Code. UMI 300 North Zeeb Road Ann Arbor, MI 48103 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. UNIVERSITY OF SOUTHERN CALIFORNIA THE GRADUATE SCHOOL UNIVERSITY PARK LOS ANGELES, CALIFORNIA 90007 ‘U This thesis, written by Mary Alice Parke under the direction of h.^T.....Thesis Committee, and approved by all its members, has been presented to and accepted by the Dean of The Graduate School, in partial fulfillment of the requirements for the degree of ACKNOWLEDGEMENTS Many are deserving of thanks and gratitude for their patience, instruction and support during the preparation of this thesis. Thanks go to my parents, Bill and Molly, for encouraging me to do what I wanted to do. This study would not have been possible without the financial support of the Terry Shackleford Fund of the Department of Geological Sciences at the University of Southern California, as well as the financial support of Sigma Xi. To my eastern Mojave peers; Kim Bishop, Todd Battey, Tom Brudos, Yu Hao, and Ken Fowler, many thanks, guys, for the enlightening discourses and wonderful evenings around the campf ire. Thanks to Chris Carlson, Elizabeth Martin, Kathy Campbell, Kate Whidden, Pete Bentham and Eric Bender for moral support. For their guidance through the political and bureaucratic morass of the University, thanks are owed to Rene, Virginia, Denise, Sue, Cindy, Desser, John, and Tom Henyey. Many thanks to Scott Patterson and Doug Burbank for their patience and instruction. Infinite patience, careful instruction, field savvy and wise counsel - for all of these I owe a debt of gratitude to Gregory A. Davis. ii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. For Glenda, and all the other "good witches" of the world - they show us that our strength comes from within. iii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE OF CONTENTS iv ABSTRACT xiii INTRODUCTION 1 Location and Accessibility 5 Methods 6 Geologic Setting 8 Previous Work 15 SHADOW VALLEY BASIN SEDIMENTS 16 General Statement 16 Age Constraints 16 Paleoclimate 21 Basement for the Shadow Valley Basin 22 Alluvial Units 24 Allochthonous Sheets 24 Landslide Deposits (Carbonate Megabreccias) 27 Lacustrine Sediments 27 Alluvial Unit 1 (T^J 28 Alluvial Unit 2 (T^) 31 Clast Composition 35 Paleocurrent Indicators 38 Carbonate Megabreccias (Tctr) 38 Alluvial Unit 3 (T^-) 43 Clast Composition 46 Paleocurrent Indicators 47 Lacustrine Sediments (TH1) 49 Provenance of Miocene Ciastic Sediments 52 Alluvial Unit #1 52 Alluvial Units #2 and #3 54 Lacustrine Sediments 55 Depositional Environment of Shadow Valley Basin 55 Younger Alluvial Sediments 59 Older Alluvium 59 Colluvium 59 Alluvium 60 Landslides 60 MIOCENE ALLOCHTHONOUS SHEETS 61 General Statement 61 Lower Allochthon (LA) 69 Occurrences 69 Stratigraphy 74 Nooah Formation (0N) Sultan Formation (Ds) Monte Cristo Formation (MIIC) Middle Allochthon 1 (MAX) 77 Occurrences 77 Geology 82 Bonanza Kina Formation iv Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Intrusive Igneous Rocks Middle Allochthon 2 (MA^) 86 Upper Allochthon (UA) 90 Occurrences 90 Geology 92 Basement Gneisses Crystal Soring Formation Diabase Noonday Dolomite Pre-emplacement Deformation of Allochthons 108 Lower Allochthon 108 Middle Allochthon 1 109 Middle Allochthon 2 116 Upper Allochthon 116 Pre-Cenozoic Deformation Mesozoic Deformation Syn-emplacement Deformation of Allochthons 132 Lower Allochthon 132 Middle Allochthons 133 Upper Allochthon 138 Provenance of Allochthons 143 Lower Allochthon 143 Middle Allochthons 146 Upper Allochthon 148 Analogous Deposits 151 Emplacement of Allochthonous Sheets 154 Initial Breakaway of Allochthons 156 Formation of Basal Friction Carpet 165 Role of Fluids 169 STRUCTURAL GEOLOGY 171 General Statement 171 Accidental Structures 171 Syn- and/or Post-basinal Rotation 172 Post-basinal Oblique-slip Faulting 175 Post-basinal Strike-slip Faulting 179 The Halloran Hills Detachment Fault 181 EVOLUTION OF THE SHADOW VALLEY BASIN 185 REFERENCES 191 LIST OF FIGURES vi LIST OF TABLES xii LIST OF PLATES xii v Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. LIST OF FIGURES Figure 1. Schematic representation of the Garlock fault as a boundary between the highly extended Basin and Range Province to the north, and a relatively unextended Mojave Block to the south. SJV=San Joaquin Valley, SN=Sierra Nevada, PV=Panamint Valley, DV=Death Valley, NR=Nopah Range, KR=Kingston Range (Figure 4 of Davis and Burchfiel, 1973). 2 Figure 2, Generalized location map of the Shadow Mountains study area showing location of various physiographic and structural features discussed in text, KRD=Kingston Range detachment fault, HHD=Halloran Hills detachment fault, RT=Riggs thrust fault, KR=Kingston Range, MM-Mesquite Mountains, CM=Clark Mountains, SVB=Shadow Valley Basin. Ruled box is location of study area. 4 Figure 3. Location of Shadow Mountains study area with respect to adjoining 7.5' quadrangles. Modified from 1984 USGS Provisional Map Series, Kingston Spring Quadrangle. 7 Figure 4. Generalized stratigraphy of Precambrian units found within the Shadow Mountains study area (after Wright and Troxel, 1974). 9 Figure 5. Generalized and schematic reconstruction of Shadow Valley basin in the southern Shadow Mountains showing the relative positions of the various units found in the study area. Horizontal scale is approximately 1:12,000. The diagram is exaggerated vertically to show d e t a i l . 18 Figure 6. Distribution of main crystalline basement types in the Shadow Valley basin area. KPP=Kingston Peak pluton, TQM=Teutonia quartz monzonite, pv=Pliocene volcanics (4.3 Ma) . Other abbreviations after Figure 2. 20 Figure 7. Diagrammatic representation of similarities between carbonate megabreccias (lower unit) and basal friction carpets (upper deposit). Part (a) shows a megabreccia exposed beneath intact lithologies. Part (b) shows these same units exposed at the level of the dashed line in (a). In the absence of the upper part of the allochthon, the megabreccia appears identical to the basal friction carpet. 26 Figure 8. Diagram keyed to various topographic features referenced in text. The diagram is a schematic of Plate I. Arrows indicate area of Plate I being discussed in text. 29 Figure 9. Alluvial sediments of unit T ^ . View is southeast on western flank of hill 1149T. 30 vi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 10. Location of Plate I being discussed in text. 33 Figure 11. Sediments of T^, exposed in through canyon of Evening Star wash. View is northwest. 34 Figure 12. Clasts of Tapeats sandstone within in through canyon of Evening Star wash. 37 Figure 13. (a) Cobble imbrications from one horizon in TMa,, Evening Star wash. (b) Composite plot of paleocurrent data from exposures of T,^. in Evening Star wash. Plus sign = channel axes, square = groove set on base of one debris flow, small dot = one set of climbing ripples, large dot = one set of ripple foresets. 3 9 Figure 14. (a) View of megabreccia intercalated with sediments of below the base of middle allochthon 2 west of hill 1099T. View is toward the north. (b) Interdigitation of dolomitic (light color) and limestone (darker) lithologies within megabreccia of (a). 41-42 Figure 15. Arrows showing approximate area of outcrop of in Plate I . 44 Figure 16. (a) Sediments of TMa3 exposed along Evening Star wash. View is to the north. (b) Clasts within TMa,. 45 Figure 17. (a) Plot of ripples and ripple foresets in TVa3. (b) Bidirectional current data from TMaJ. plus sign = grooves on bases of beds, small dots = channel axes. 48 Figure 18. Fine-grained lacustrine sediments of TH1. 50 Figure 19. Location of area being discussed in text. 51 Figure 20. (a) Relationship of T,^ to probable source in Halloran Hills area. (b) Schematic cross section of (a) . No scale. 53 Figure 21. Present spatial relationship between Clark Mountains (CM) and the Shadow Mountains (SMS) showing probable direction of transport of and TMa3. 56 Figure 22. Perspective sketch of Shadow Valley basin sediments and source terranes of TMal_3. 57 Figure 23. Generalized and schematic reconstruction of Shadow Valley basin in the southern Shadow Mountains showing the relative positions of the various units found in the study area. Horizontal scale is approximately 1:12,000. The diagram is exaggerated vertically to show detail. 63 vii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 24. Generalized cross-sectional views of an allochthon showing basin morphology for (a) section transverse to the transport direction and (b) section parallel to the transport direction. Qualitative velocity profiles of an allochthon for (c) intact cap moving more rapidly than basal friction carpet and (d) velocity distributed through the entire allochthon. 66 Figure 25. Location map showing approximate outcrop area of the lower allochthon in Plate I. 70 Figure 26. Intact Paleozoic carbonates of the lower allochthon. Small knob in the right of the photo is approximately 80 m high. View is to the southwest toward hill 1137T and the powerline road. 71 Figure 27 . Contact between the lower allochthon and TMal exposed on the east side of hill 1137T. Angular, brecciated clasts form the La are mixed with more rounded clasts from TMal in a zone approximately 1 m thick. View is toward the east. 73 Figure 28. Generalized stratigraphic column of the Paleozoic carbonates that comprise the LA (after Hazzard, 1937, and Hewett, 1956). 75 Figure 29. Map showing the general location of MA, in Plate I- ‘ 78 Figure 30. (a) Basal friction carpet of MA, showing large block of intact carbonates surrounded by finer breccias. (b) Zone of MA: friction carpet showing uniformity of size of some breccias. 80 Figure 31. Eastern view of M A X near Francis Peak. Rugged topography at crest is intact cap, which rests upon lowrelief basal breccias, which rest upon alluvial sediments. 81 Figure 32. Generalized stratigraphic column of the Cambrian Bonanza King Formation (after Gans, 1974). 83 Figure 33. Location of igneous stocks in M A X. 85 Figure 34. Location of MA^ on Plate I. 87 Figure 35. (a) Striae along the base of MA;, in Evening Star wash. Orientation of Striae is 347/6. (b) Contact between MA2 and TMa2 in Evening Star wash. View is toward the northwest. 89 Figure 36. Distribution of the upper allochthon and the location of its western (UAw) and eastern (UAe) exposures. 91 viii Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 37. Generalized stratigraphic column of the units that comprise the UA (after Wright and Troxel, 1974) . 93 Figure 38. Spotted quartzite from the Precambrian Crystal Spring Formation. Reduction spots formed around feldspar grains in a predominantly quartzose rock. 97 Figure 39. Occurrences of the blue quartzite from the Crystal Spring Formation. 98 Figure 40. Generalized relationship between the Noonday Dolomite and older Precambrian crystalline and sedimentary units (after Wernicke et a l ., 1988). 100 Figure 41. Location of well-exposed, intact outcrops of the basal Noonday conglomerate (see Plate I). 105 Figure 42. Location of area being discussed in text. 107 Figure 43. Location of the only fault mapped within the LA. 110 Figure 44. Area of MA, mapped in detail. This map is the inset in Plate I. Ill Figure 45. (a) NNE-striking faults within MA,. (b) NWstriking faults and folds within MA,. ’ 114-115 Figure 46. (a) Brittle, high-angle fault truncating the northwest limb of folded Crystal Spring lithologies. (b) Line drawing of (a). 121 Figure 47. Location of well-exposed, stacked sequence of thrusts within the UA. 123 Figure 48. Diagrammatic sketch of the relationships and relative positions of stacked, low-angle faults within the UA. 124 Figure 49 . Locations of well-exposed Noonday-over-Crystal Spring fault. 126 Figure 50. (a) Fault contact between phyllitic beds of the Crystal Spring Formation below silty dolomite of the Noonday Dolomite. (b) Line drawing of (a). 127 Figure 51. Diagrammatic sketch of the relationship between transport direction of allochthons and the predicted orientation of clastic dikes. 135 ix Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 52. Location on Plate I of major faults within MA, that separate the intact cap of the allochthon (in the hanging wall) from basal breccias (footwall). 137 Figure 53. Location of area of Plate I being discussed in text. 140 Figure 54. (a) View east of well-exposed tear fault within the UA. (b) View north of fault in (a). Note dilational fractures oriented perpendicular to the fault plane. 141 Figure 55. Location on Plate I of low-angle normal fault within the UA. 144 Figure 56. Distribution of Paleozoic carbonates (shaded) autochthonous to the Shadow Mountains study area. R=Resting Springs Range, N=Nopah Range, MM=Mesquite Mountains, CM=Clark Mountains, MR=Mescal Range, B=Baker, Calif. Box shows approximate location of study area. Unshaded enclosed areas are allochthonous carbonates within the Shadow Valley basin. 147 Figure 57. Spatial relationship between the Shadow Mountains and the probable source terrane of the UA at Shadow Mountain. Arrow shows inferred transport direction. 150 Figure 58. Schematic representation of extension in the breakaway zone of a detachment fault. Breakaway zone fills with sediment derived from the unextended headwall. Sedimentary unloading allows allochthons to spall off of the headwall and slide into the basinal sediments. 158 Figure 59. (a) View north of Shadow Mountain showing the sequence of granitic gneisses, diabase and the Crystal Spring Formation that are also found within the UA in the Shadow Mountains. (b) Line drawing of (a). 160 Figure 60. Reconstruction of possible relationships between the UA, bedding in the Crystal Spring Formation, and a possible rotated Winters Pass (WP) thrust at Shadow Mountain. By rotating the WP thrust 3 0° to the west, Crystal Spring strata within the UA can be matched to similar units at Shadow Mountain, thereby accommodating a shallow initial westward dip at the base of the UA. 162 Figure 61. Diagram showing the progressive deformation of an allochthon and formation of the basal friction carpet as it slides across a fixed substrate. (a) As the allochthon moves, its entire base becomes brecciated, thereby allowing further ease in moving downslope under the influence of gravity. (b) As the allochthon continues to move, the basal friction carpet is left behind, and new breccias are formed. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (c) At some point during downslope motion, the intact cap of the allochthon may slide off the front end of the friction carpet, thereby becoming a new source for fractured basal breccias. The contact between the breccias and the cap need not be a sharp one; evidence was seen in the field for both a gradual breccia development over a wide zone of deformation and for sharp fault contacts between the cap and the breccias. 167 Figure 62. View toward the south of the "tail" of the UA (in the right of the photo) exposed at higher structural levels than the playa sediments into which it settled (to the left and beneath the alluvial veneer) . 173 Figure 63. (a) View north of the large basin-repeating normal fault on the west side of the study area. Dark gray Bonanza King carbonates of M A t in the footwall to the east are juxtaposed against light-colored playa sediments in the hanging wall to the west. (b) View of same fault as in (a) approximately 100 m to the north. 176 Figure 64. Location on Plate I of large normal faults with associated anticlinal folds. 178 Figure 65. (a) View toward the north of normal faults offsetting the base of MA, in the through canyon of Evening Star wash. (b) Line drawing of (a) . 180 Figure 66. Diagram showing location of early extension along the KRD (hatchured) and probably north-directed influx of sediment Halloran Hills area. Large arrow shows extension direction, smaller arrow shows direction of sediment and LA transport. 186 Figure 67. Intrusion of the Kingston Peak pluton (KPP) into the KRD forces extension to the south (and possibly west; McMackin, 1988) of the pluton. Ruled areas indicate active extension. Extension along the HHD creates a topographic low, allowing large alluvial fan systems to source out of the Clark Mountains and Mescal Range a r e a s . Large arrow shows extension direction; small arrows show sediment transport direction. 188 xi Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Table Table Plate Plate Plate LIST OF TABLES 1. Characteristic Features of the Shadow Mountains Allochthons 64 2 . Deposits Analogous to the Shadow Mountains Allochthons 153 LIST OF PLATES I . Geologic Map of the Southern Shadow Mountains II. Geologic Cross Sections Through the Southern Shadow Mountains III. Pen and ink drawing of the T^, in through canyon of Evening Star wash back pocket back pocket back pocket Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ABSTRACT Extensional tectonism related to detachment faulting has disrupted a unique sequence of Miocene rock units in the southern Shadow Mountains of southeastern California. Fanglomerate and playa sediments, intercalated with four areally extensive (0.25-60 km2) allochthonous sheets, have been rotated to the northeast and repeated by steeply west-dipping normal faults. This basinal sequence comprises part of the allochthonous upper plate of a southwest-rooting Halloran Hills detachment fault system, a recently recognized, regionally extensive detachment system active during the Middle to Late Miocene. The allochthonous sheets within the stratigraphic sequence record gravity gliding at the earth's surface on an impressive scale. These structurally complex allochthons contain crystalline and sedimentary units ranging from 1.7 Ga to Mississippian in age and preserve Mesozoic, and possibly, Precambrian deformational events. Pre-Tertiary structures have been overprinted by brittle Tertiary structures related to allochthon emplacement. The lowest of the four allochthons within this part of the Shadow Valley basin appears to be locally derived. The structural style and stratigraphic relationships within the higher three sheets, however, are suggestive of source terranes 6-15 km from their present locations. Kinematic data within these sheets, deformation of sediments beneath xi i i Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. the sheets, paleocurrent data from the Tertiary alluvium, and provenance of Tertiary clastic sediments and allochthons are all indicative of a west- to northwestdipping paleoslope during their Miocene time of emplacement. The proposed source terranes for the fartravelled allochthons thus lay to the east and southeast of the southern Shadow Mountains. The source of the uppermost, composite allochthon is Shadow Mountain, currently located 6 km ENE of the eastern exposed edge of this sheet. The Precambrian crystalline and sedimentary units that comprise this upper allochthon have marked similarities in structural style and intrusive relationships to the same units at Shadow Mountain. These relationships strongly suggest that the allochthon was displaced from its Shadow Mountain source terrane along a shallowly dipping fault located very near the earth's surface. In light of regional tectonic considerations, it is proposed that this surface may have been related to faulting in the breakaway zone of the Halloran Hills detachment fault or fault system. By analogy, emplacement of the other allochthons within the basin is also proposed to be genetically linked to detachment faulting in the region. No compelling evidence for a Halloran Hills detachment fault is evident in the Shadow Mountains, but structural relationships in parts of the basin adjacent to xi v Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. this area strongly support the presence of a detachment fault at depth. The gently to moderately northeastdipping Shadow Mountains sequence may be deposited unconformably on an older, steeply dipping basinal section to the east at Shadow Mountain. North and south of the Shadow Mountains, the Tertiary section has been rotated along numerous normal faults; parts of the basin dip as steeply as 60°. The regional trend of these west-dipping faults is NNW and the dips of bedding are to the NE. The lack of evidence for a detachment fault in the southern Shadow Mountains is attributed to its rather central location within the allochthonous upper plate of the Halloran Hills detachment fault. The Shadow Valley basinal sequence may have been detached and translated above the inferred detachment fault without appreciable upper-plate normal faulting and associated rotation. xv Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. INTRODUCTION Cenozoic crustal extension in the Basin and Range province of southeastern California was at one time postulated to end abruptly southward at the left-lateral Garlock fault (Davis and Burchfiel, 1973). These authors proposed that the Garlock fault separated the highly extended Basin and Range province to the north from the "stable" Mojave block to the south, which, until recently, was believed to have been little affected by extension in this eastern California region (Figure 1). The recognition of a detachment fault system within the Mojave block by Dokka (1986) showed that the Garlock fault did not delineate the two provinces as previously thought. However, Dokka's Waterman Hills detachment system is located west of the eastern exposed terminus of the Garlock. The discovery of the southwest-rooting Kingston Range detachment fault (KRD) northeast of the projected trace of the Garlock fault (Burchfiel and others, 1983) led these authors to suggest that the Garlock fault, along its eastern trace, had acted as a lateral ramp along the southern margin of the Kingston Range allochthon. Hence the Garlock fault apparently separated the highly extended Basin and Range to the north from regions to the south where crustal extension had not been recognized. The projected southern extension of the KRD and the projected eastern extension of the Garlock strike-slip 1 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. SN NR PV SJV DV KR SAN ' GARLOCK FAULT ANDREAS FAULT — MOJAVE DESERT Figure 1. Schematic representation of the Garlock fault as a boundary between the highly extended Basin and Range Province to the north, and a relatively unextended Mojave Block to the south. SJV=San Joaquin Valley, SN=Sierra Nevada, PV=Panamint Valley, DV=Death Valley, NR=Nopah Range, KR=Kingston Range (Figure 4 of Davis and Burchfiel, 1973) . Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. fault lie beneath the modern alluvium of Kingston Wash. The Shadow Mountains study area for this thesis are a part of the Tertiary Shadow Valley basin of Hewett (1956), one of the largest Tertiary basins in this eastern Mojave region with an areal extent in excess of 600 km2 (Figure 2) . The northern exposures of the Shadow Valley basin sediments, including the Shadow Mountains, are situated in an area that, at the onset of this project, were expected to record the effects of the interaction between the Garlock fault and the KRD. In an attempt to document the extent of these faults, field studies by Davis and Burchfiel (Davis and others, 1990; Davis, in progress) suggested the existence of a detachment fault system south of the projected Garlock trace. The postulated occurrence of a Halloran Hills detachment fault system (HHD), whether it be a southern continuation of the KRD or a separate fault system, cast doubt upon the likelihood of a Kingston Wash-located Garlock fault as a possible terrane-bounding structure. Hence, an additional purpose of this study came to be the analysis of the possible presence of detachment faulting in the region. A second goal of this study was to map and describe in detail the sediments that comprise this part of the Shadow Valley basin. Volcanic rocks constitute lower portions of the Shadow Valley basin sequence in areas to the north and south, but are not exposed in this study 3 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. NV XRD KR CA SM SMS CM RT dV > HHD •SH MR Squaw Min HH Approximate extent of SVB sediments (Hewett. !956) 15 km Baker Figure 2. Generalized location nap of Shadow Mountains study area showing location of various physiographic and structural features discussed in text. KRD=Kingstcn Range detachment fault, HHD=Halloran Hills detachment fault, RT= Riggs thrust fault, KR=Kingston Range, MM=Mesquite Mountains, CM=Clark Mountains, MR=Mescal Range, HH=Halloran Hills, SH=Silurian Hills, SM=Shadow Mountain, SMS=Shadow Mountains, SVB=Shadow Valley basin. Ruled box is location of study area. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. area. The exposed Tertiary section here consists of alluvial and lacustrine sediments intercalated with large gravity-driven allochthonous sheets whose origins and modes of emplacement were poorly understood. The size of these allochthons (as large as 60 km2) , good exposure and the intact nature of the sheets make the Shadow Mountains a world-class locality for the study of gravity gliding. Location and Accessibility The Shadow Mountains are located in eastern San Bernardino county, California, approximately 3 0 km northeast of the town of Baker (Figure 2). From Baker, the study area can most easily be reached by driving approximately 40 km north on U. S. Interstate 15 to the Cima Road offramp. A Los Angeles Department of Water and Power powerline maintenance road crosses the Cima Road 14.6 km to the north of the offramp. A jeep trail (mining access road) leading into the study area extends north from the powerline road 15.1 kilometers west of the Cima Road. The powerline road forms the southern boundary of the area mapped. The study area can also be reached by driving 15 kilometers north on U. S. Route 127 from Baker to the powerline road. The western limit of the study area is 20.3 km east of Route 127 along the powerline road. 5 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Methods This study is primarily based on observations made in the field. Fieldwork included 53 days of detailed field mapping at a scale of 1:12,000 between March, 1989 and January, 1991. The base map used in this study was a composite of 1984 USGS provisional 1:24,000 7.5 minute topographic map sheets (Figure 3). Individual sheets were spliced together and then enlarged to a scale of 1:12,000. In instances where an even larger scale was needed, the base map was enlarged on a xerox copier, and data were then transferred to the smaller 1:12,000 base. Stratigraphic sections were measured using a Jacob's staff and Abney level. All measurements were made in meters. Structural measurements were made with a Brunton hand-held compass. Mapping was in part augmented by Landsat Thematic Mapper images provided by John Ford of the Jet Propulsion Laboratory, Pasadena, California. Stereo plots were made using the program STEREONET, v. 4.1, by Richard Allmendinger, for the Apple Macintosh computer. Interpretations and conclusions herein are primarily based on the compiled geologic map of the study area (Plate I). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. CA QUADRANGLE LOCATION Study area 116 ° Adjoining 7 . 5 ’ q u a d r a n g l e n a m e s Figure 3. Location of Shadow Mountains study area with respect to adjacent 7.5' quadrangles. Modified from 1984 USGS Provisional Map Series, Kingston Spring Quadrangle. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Geologic Setting Precambrian Framework The middle to late Precambrian history of the greater Death Valley region was dominated by the deposition of a thick (ca 2500 m) sequence of clastic and carbonate rocks known as the Pahrump Group. Three formations comprise the Pahrump Group. From stratigraphically lowest to highest they are the Crystal Spring Formation (rests nonconformably on granitic gneisses), the Beck Spring Dolomite and the Kingston Peak Formation (Figure 4). A significant unconformity is present in the upper part of the Kingston Peak Formation, and it has been argued that this upper member of the Kingston Peak should be given formational status. Deposited unconformably on all members of the Pahrump Group, as well as on crystalline basement, is the late Precambrian Noonday Dolomite, the basal unit of the Cordilleran miogeocline. Subsequent sedimentation within the miogeocline continued with few interruptions until the Sevier Orogeny (see Mesozoic framework). Rifting along the proto-Pacific margin of North America is the purported cause for the unconformity between the Noonday Dolomite and the units upon which it rests. This rifting event has long been believed to be the only deformational event to affect the Death Valley region during the Late Precambrian. The relatively recent 8 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 4. Generalized stratigraphy of Precambrian units found within the Shadow Mountains study area (after Wright and Troxel, 1974). 9 recognition of metamorphosed clasts of lower parts of the Kingston Peak Formation above the unconformity near the top of the formation led Walker et al. (1986) to conclude that a regional deformational event had occurred prior to deposition of this upper part of the Kingston Peak. Their observation that this event must have been Precambrian in age was further supported by the fact that the Noonday Dolomite was unaffected by this deformation. This conclusion is somewhat controversial (Wright and Troxel, 1987; Walker and others, 1987), and to date, the controversy has yet to be resolved. Mesozoic Framework In the region surrounding the study area, Late Mesozoic compressional structures are overprinted by Cenozoic extensional features. The southern extension of the foreland fold and thrust belt in southern California is represented by the Clark Mountains thrust complex, which generally lies east of the study area and includes from north to south, the Mesquite Mountains (MM), the Clark Mountains (CM) and the Mescal Range (MR; Figure 2) . Three major thrust plates comprise the thrust complex. From east to west, and structurally lowest to highest, they are the Keaney/Mollusk Mine, Mesquite Pass, and Winters Pass thrust plates (Burchfiel and Davis, 1977, 1988) . 10 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Of particular interest to this study are the Mesquite Pass and Winters Pass plates. The Mesquite Pass plate is a potential provenance for Tertiary deposits of the Cambrian Bonanza King Formation in the Shadow Mountains. The Mesquite Pass plate consists of numerous individual thrust sheets, most of which demonstrate considerable crystal-plastic flow as well as complex folding and thrusting relationships (Burchfiel and Davis, 1988). Thrusting in the Mesquite Pass plate involves Precambrian granitic gneisses as well as Precambrian and Cambrian clastic and carbonate units of the Cordilleran miogeocline (ibid.). The Winters Pass plate at Shadow Mountain (G. A. Davis, pers. comm., 1989) is a potential source for the upper allochthonous sheet of Precambrian units found in the Shadow Mountains. The Winters Pass thrust fault places Precambrian crystalline and sedimentary units on top of Precambrian and Cambrian miogeoclinal sedimentary rocks in the underlying Mesquite Pass plate. Sedimentary units above granitic gneiss and diabase in the Winters Pass plate include the Precambrian Pahrump group and younger Precambrian and Cambrian sedimentary units of the miogeocline (Burchfiel and Davis, 1971). The time of thrusting of the Winters Pass plate is probably early Cretaceous. The plate contains a Mesozoic granitic pluton (ca 155 Ma) west of Pachalka Spring in the 11 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Clark Mountains and the thrust contact here is intruded by a late synkinematic diorite with an age of ca 127 Ma (G. A. Davis, pers. comm., 1990). Cenozoic Framework No known rock units of Lower Tertiary age occur in the eastern Mojave Desert area (Hewett, 1956). The Cenozoic stratigraphic record of this region is limited to Miocene and younger events. During the Miocene, crustal extension north and east of the Kingston Range (Figure 2) occurred along and above the southwest-rooting Kingston Range detachment fault (KRD; Burchfiel and Davis, 1988) . North- to northeast-tilted Cenozoic, Paleozoic and Precambrian strata in the hanging wall are juxtaposed against Late Precambrian and Cambrian rocks of the lower plate across the gently southwest-dipping KRD (Burchfiel and Davis, 1988). The Cenozoic sediments were initially deposited on Cambrian sedimentary rocks and serve to constrain the timing of movement of the KRD. Age dates of 12.1 and 12.5 Ma have been reported for volcanic units within the tilted Cenozoic upper plate rocks (ibid.) which provide a lower limit for extension along the KRD. An upper age limit for motion on the KRD is not clearly established. McMackin (1988) has suggested that the Kingston Peak pluton, which has cooling ages of 12.8 and 12.2 Ma (Calzia and others, 1987) cuts the detachment 12 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. fault and caused a cessation of motion along it northeast of the Kingston Range. However, the presence of 12.1 Ma volcanics within the tilted upper plate sediments suggests that either southwest-directed extension on the KRD continued after the pluton cooled, or that the 12.1 Ma age of these volcanics is too young. A second series of Tertiary sediments locally overlaps upper plate rocks of the KRD. This sequence includes large boulders of the Kingston Peak pluton, as well as volcanic clasts that have been dated at 11.6 Ma (Calzia, pers. comm., 1990). Although their relationship to KRD extension is not clear, they may provide an upper limit for deformation along this fault. Recent reconnaissance studies by Burchfiel and Davis have revealed a possible southern extension of the KRD (Burchfiel, Davis and Parke, 1990). The Halloran Hills detachment fault (HHD; Figure 2) may extend as far south as the central Mescal Range, and appears to underlie the Shadow Mountains (G. A. Davis, pers. comm., 1990). Tertiary sediments have been repeated and tilted eastward along west-dipping normal faults in a region extending from Kingston Wash in the north, to south of Interstate I15, where Davis (G. A. Davis, pers. comm., 1991) has identified several klippen of steeply tilted Tertiary strata above a subhorizontal fault (the HHD ?) . A possible western exposure of the HHD occurs in the 13 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Silurian Hills (Figure 2) where Kupfer (1960) has described the Riggs thrust, a younger-over-older fault that places hanging wall Riggs carbonates and Tertiary sediments on lower plate Pahrump Group sedimentary units. If the Riggs thrust, or any part of it, since it appears to be a composite surface, is indeed the HHD, then the Shadow Mountains were affected by extension on this heretofore unrecognized detachment fault. Previous Work Although the Shadow Mountains are mentioned in a 1950 abstract on chaotic breccias (Jahns and Engel, 1950), the earliest comprehensive study of the Shadow Mountains area was included in a report on the Ivanpah quadrangle by D. F. Hewett, published in 1956. Hewett's mapping was on a scale of 1:125,000 (Hewett, 1956, his Plate 1). He interpreted a sheetlike, stratiform assemblage of Precambrian crystalline and sedimentary rocks resting on Tertiary strata to be a consequence of Tertiary thrusting (his "Playground thrust"). Hewett's Playground thrust plate is equivalent to the upper allochthon of this study. A lower allochthonous sheet of Paleozoic carbonate rocks embedded in the Tertiary section was not considered by Hewett as a thrust-faulted assemblage. In 1966, a Master's thesis on the Shadow Mountains area was completed by R.C. Wilson, Jr., at Rice University 14 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (Wilson, 1966) . His study entailed geologic mapping on a scale of 1:20,750 and included a proposed emplacement mechanism for what he interpreted as gravity slide sheets within the area. Burchfiel and Davis (1971) mention the sheets in the Shadow Mountains as being a part of the Tertiary stratigraphic section in that area. Reynolds and Nance (1988) published a study of the age and history of a part of the Shadow Valley Basin. Their data primarily include paleontological data that help constrain the age of sediments within the basin (Reynolds and Nance, 1988). Although their report was based on a Halloran Hills sedimentary section located southeast of the area covered by this study, their stratigraphic section is believed to be a part of the same basin (note areal extent of Shadow Valley basin; Figure 2) . Abstracts by Parke and Davis (1990) and Davis, Burchfiel and Parke (1990) presented early results of recent mapping in the area by the authors. Ongoing studies include detailed mapping in the region by G. A. Davis (north and southeast of the study area), Kim M. Bishop (Halloran Hills), Ken Fowler (Shadow Mountain area) and Tom Brudos (western Mescal Range), from the Department of Geological Sciences at the University of Southern California. 15 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. SHADOW VALLEY BASIN SEDIMENTS General Statement Although the clastic sediments that comprise the Tertiary section in the Shadow Mountains are Miocene in age, allochthonous sheets within this sequence contain rocks that range in age from Proterozoic to Mississippian. The allochthonous sheets within the Shadow Valley section are unique, and as such, will be treated separately from the sediments within the basin. This entire stratigraphic sequence has been repeated by a major north-striking normal fault that has dropped the base of the uppermost allochthon in the study area down to the west approximately 600 m with respect to the footwall (see section A-A', Plate II). None of the Tertiary units in the Shadow Mountains have been given formational names, and they are herein referenced on the basis of relative positions within the stratigraphic sequence (Figure 5). TMa1 is the oldest alluvial unit in the study area sequence and TMa3 the youngest of the alluvial deposits mapped in the area. Age Constraints On the basis of unconformities at the base of the stratigraphic section, Hewett (1956) placed the Shadow Valley basin in the Middle Tertiary. Structural relationships led him to the conclusion that the basin 16 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 5. Generalized and schematic reconstruction of Shadow Valley basin in the southern Shadow Mountains showing the relative positions of the various units found in the study area. Horizontal scale is approximately 1:12,000. The diagram is exaggerated vertically to show detail. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 00 must be post-Laramide and pre-Pliocene. The sediments of the Shadow Valley basin were further constrained by Hewett (ibid.) to be of Miocene in age because of general depositional and lithologic similarities with paleontologically dated basins in the southern California region. The age of the Tertiary section present in the study area has been further constrained by paleontological studies conducted by Reynolds and Nance (1988) . As mentioned above, the section studied by these authors (ibid.) is located in the Halloran Hills. Fossil rodents recovered by Reynolds and Nance have a broad age range from the Hemmingfordian (18 Ma) to recent. Based on the fossil assemblage present in this section, these authors have placed the average age of these sediments at around 11.5 Ma. Isotopic ages for sediments in and around the Shadow Valley basin constrain the sedimentary section to the Middle and Late Miocene. In the Silurian Hills, a K-Ar date on a volcanic flow near the base of the section gave an age of 13.3 Ma (J. Calzia, pers. comm., 1990). Dates from volcanic deposits north of the study area fall between 11.1-11.6 Ma (ibid.). Further limits on the age of the Shadow Valley basin are based on radiometric dates from basaltic flows in the Halloran Hills north of Interstate 15 (pv, Figure 6). 19 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. NV 4 x m m k « k i ( at x x >KPpc x xl CA M M SM SMS CM SH MR ::tqm HH Baker Figure 6. Distribution of main crystalline basement types in the Shadow Valley basin area. KPP=Kingston Peak pluton, TQM=Teutonia quartz monzonite, pv=Pliocene volcanics (4.3 M a ) . Other abbreviations after Figure 2. 20 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. These flows, dated at 4.3 Ma (J.Calzia, pers. comm., 1990), dip very gently to the northeast. In contrast, the Tertiary section in the Shadow Mountains generally dips approximately 25° northeast. Although the basalts do not tightly constrain the age of the basin, they do provide a possible upper limit for the age of deformation in this area. In light of the data presented above, the best estimate for the age of the Shadow Valley sediments mapped in the Shadow Mountains is Middle to Late Miocene. In the absence of further age constraints, this sequence is herein assumed to have been deposited after 13.3-11.6 Ma. Paleoclimate The paleoclimate in the Shadow Mountains area during the Middle Miocene was less arid than what is seen today in the eastern Mojave desert. Savannas and low scrub were apparently common in this region until approximately 3-4 million years ago, when xeric plant types first appeared in the paleobotanical record of the southwest Cordillera (J. A. Wolfe, 1985). According to Wolfe (ibid.) deserts as they are known today did not develop until after the Miocene. However, sedimentary depositional environments in the region during the Middle Miocene were apparently similar to the subaerial alluvial fans and playa deposits seen in the modern desert southwest. The majority of the Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Tertiary basin fill in the Shadow Mountains area is comprised of fan deposits and playa sediments, deposits which according to Blissenbach (1954) are characteristic of semiarid to arid conditions. Basement for Shadow Valley Basin Within the area mapped, the Tertiary stratigraphic section is never seen in depositional contact with basement. However, two very distinct crystalline basement types do crop out within several kilometers of the study area (Figure 6). Precambrian crystalline basement, composed of granitic gneisses dated at 1.7 Ga (J. L. Anderson, pers. comm., 1990), is exposed at Shadow Mountain, approximately 6 km ENE of the study area. Burchfiel and Davis (1971) assign these crystalline rocks to the Winters Pass thrust plate. Proterozoic diabase sills and dikes (ca 1069-1087 Ga; Heamon and Grotzinger, 1992 ms) intrude both the gneisses and sediments of the Proterozoic Crystal Spring Formation. The relationship between this basement assemblage and Tertiary sediments at Shadow Mountain appears to be tectonic; the Precambrian and Tertiary units are juxtaposed across a high-angle fault and no depositional relationships have been recognized (G.A. Davis, unpub. mapping). Recent mapping by Fowler (K.A. Fowler, pers. comm., 1990) show that the Tertiary sediments at Shadow Mountain may actually be 22 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. older than the Tertiary section found in the Shadow Mountains area of this study. His mapping suggests that the sediments in the Shadow Mountains have been deposited unconformably on an older, steeply northeast-tilted Tertiary section. However, evidence for this relationship is not apparent within the Shadow Mountains study area. The basement in the Halloran Hills (Figure 6), located southwest of the study area, consists of Proterozoic crystalline rocks equivalent to those at Shadow Mountain, a sequence of miogeoclinal rocks that range in age from Cambrian to Mississippian, and Cretaceous quartz monzonites belonging to the Teutonia intrusive complex (TQM; DeWitt, 1980). Mapping by Bishop 1 km south of the thesis study area reveals that the Tertiary section mapped in this study rests unconformably on an older Tertiary sequence containing volcanic and carbonate units, which is in turn deposited unconformably on Teutonia and its miogeoclinal wall rocks. Hewett (1956) states that the only locality where the basal units of the Shadow Valley basin can be seen resting on basement is in Riggs Wash, approximately 8 km west of the study area. Here, the Tertiary sediments are deposited directly on TQM. Hence, the most likely candidate for the crystalline basement underlying the Tertiary sedimentary sequence mapped in the study area is the TQM. 23 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Alluvial Units Three alluvial units have been mapped in the study area and are referred to as TNa1-THa3, from oldest to youngest as they occur in the stratigraphic sequence (Figure 5). The designations used here have no implication for the basin as a whole. All four units are alluvial fan deposits laid down subaerially by ephemeral streams. Differentiation of these units was based on both clast composition and grain size. In the case of TMa2 and TMa3 the distinction between the two is based on their relative positions below and above the middle carbonate allochthons, respectively. Alio chthonous Sheets Four allochthons have been identified in the Shadow Mountains area (for detailed descriptions of the allochthons, see "MIOCENE ALLOCHTHONOUS SHEETS"). Based on their relative positions in the stratigraphic sequence (Figure 4), they have been denoted the lower allochthon (LA) , middle allochthon 1 (MA.,) , middle allochthon 2 (MA2) and the upper allochthon (UA). The first three allochthons are composed of Paleozoic carbonates, whereas the upper allochthon is composed of Precambrian crystalline and sedimentary rocks. In this study, allochthonous sheets are differentiated from megabreccia landslide deposits that 24 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. also occur within the Tertiary section. The most significant criterion in distinguishing between the two types of deposits is the degree to which the deposit has been internally brecciated. Allochthonous sheets generally retain a significant degree of internal coherence, such that structural and sedimentological features can be mapped (c.f. landslides below). In cases where pervasive shattering has occurred, Tertiary deposits have been mapped as allochthons based on the extent and thickness of the deposits and the ability to map individual formations (stratigraphy established) within them. The basal zones of the allochthons are marked by brecciated equivalents of the lithologies that comprise the sheets. The basal breccias or "friction carpets" are assumed to "feather", or thin, laterally, as one moves away from the center of an allochthon (c. f. Blackhawk landslide, Shreve, 1968). This characteristic of the breccias may lead to some confusion in distinguishing an allochthon-related friction carpet from a carbonate megabreccia (Tcbr) of landslide origin (see below) . Figure 7 shows schematically how the two deposits could be mapped as the same unit: In the absence of the intact, upper portions of an allochthonous sheet, the friction carpet is similar in appearance to the megabreccia in outcrop. Although the carbonate breccias in the study area are 25 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. generally interpreted as rapid landslide deposits, the possibility exists that some are the feathered distal margins of allochthons not exposed within the study area. V' % > \ ■ Figure 7. Diagrammatic representation of similarities between carbonate megabreccias (lower unit) and lasal friction carpets (upper deposit). Part (a) shows a megabreccia exposed below an allochthon with a basal breccia beneath intact lithologies. Part (b) shows these same units exposed at the level of the dashed line in (a) . In the absence of the upper part of the allochthon, the megabreccia appears identical to the basal friction carpet. 26 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Landslide Deposits (Carbonate Megabreccias) Landslide deposits in the Tertiary sequence are mapped separately and are denoted "Tcbr", for Tertiary carbonate breccia. They are distinct, lenticular bodies of highly brecciated and shattered, dark grey carbonates, usually from the Cambrian Bonanza King Formation, that resemble rock avalanche deposits (sturzstroms of Hsu, 1975). Internal structures such as foliation, folds and original bedding are not preserved, and the carbonates have a highly chaotic appearance in outcrop. They are similar to the chaotic "matrix-rich breccia" facies of Yarnold and Lombard (1989), except that they show no evidence of mixing with the alluvium upon which they were deposited. Rare lenses of younger carbonates (possibly Upper Paleozoic, based on fossil assemblages; J. K. Schubert, pers. comm., 1990) also occur as landslide deposits within the mapped area. Lacustrine Sediments Fine-grained sandstones, siltstones and shales with interbedded tuffaceous units have been mapped as playa deposits or lacustrine sediments. These deposits are labeled TML and appear to be the youngest Tertiary sediments within the study area. These poorly consolidated rocks are buff, tan and cream colored and very clay rich. Occasional tuffaceous beds are found 27 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. within this sequence, and where possible, have been mapped separately. Davis (pers. comm., 1991) reports that alluvial units THa2 and TMa3 (see below) become progressively finer textured 1-2 km along strike to the north of the map area, and acquire an increasingly lacustrine appearance as siltstones and shales become predominant. This transition represents a facies change from a fan environment grading northward into a lacustrine depositional setting. Alluvial Unit 1 (TMa1) The oldest Tertiary sedimentary unit in the study area outcrops in the southwestern part of the area mapped and is best exposed on the southwest flank of hill 1149T. This hill is 1 km north of the powerline road, and 0.8 km NNE of hill 1137T (Figure 8; Plate I). This unit, TMa1, consists of well-indurated, fluviatile gravels and conglomerates and alluvial debris flows and conglomerates (Figure 9). It is entirely composed of fine-grained, micritic, subangular to subrounded lithic carbonate fragments that resemble late Paleozoic to early Mesozoic formations exposed in the eastern Mojave region. The carbonate clasts are pale grey, blue-grey and blue-black in color. The beds range from several cm to 3/4 m in thickness. 28 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1099T 1153T 1181T 1159T A 1077.0AT 1128T 1182 T 1174T 1121T 1167T 1106T 1146T ^ 1059T 1149T 2-118A 1176T 1137T 1 km Figure 8. Diagram keyed to various topographic features referenced in text. The diagram is a schematic of Plate I. Arrows indicate area of Plate I being discussed in text. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 9. Alluvial sediments of unit T„a1. View is southeast on western flank of hill 1149T. 30 Well-sorted to moderately well-sorted beds tend to be clast supported, whereas poorly sorted to moderately sorted beds are generally matrix supported. The brick red matrix consists of silt-sized to very fine sand-sized mud and sand-sized clasts of these same units. Individual clasts range from sand-sized to boulders up to l m in length. All beds within this unit are polylithologic from hand specimen to outcrop scale. The various carbonate clasts do not appear to be strongly metamorphosed or foliated, although they exhibit moderate recrystallization. No channel deposits or other paleocurrent indicators were observed within this unit. The unit TMa1 has an areal extent of slightly more than 0.5 km2, and an exposed thickness of approximately 80+ m. The base of TMa1 is not seen in the study area. Its uppermost underlie the lowest allochthonous sheet in the southwest corner of the map area. The top of this unit has been observed northwest and southeast of hill 1149T, as well as on the southeast flank of hill 1137T. Alluvial Unit 2 (TMa2) Deposited on the lower allochthon is an alluvial unit labeled TMa2. The base of this unit is exposed on the east side of hill 1149T (Figure 8) and consists of a reddishcolored, matrix supported conglomerate. The clasts are composed of lithic fragments of fine-grained, micritic 31 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. late Paleozoic to early Mesozoic carbonates. Clasts derived from crystalline and clastic sedimentary rocks were not recognized near the base of the section. The clast-to-matrix ratio in this unit is lower than in TMa1, and large boulders (up to 1.6 m diameter) are common. Outcrops of this unit are sparse. A continuous section is inferred to lie immediately beneath slope wash for almost 1.5 km northeast of hill 1149T and has an apparent thickness of 600 m (Fig. 8). The upward transition from Paleozoic carbonate clasts to basement crystalline units suggests a continuous unroofing within the source terrane of these sediments. In the northwest corner of the study area, the uppermost 80 m of TMa2 are exceptionally well exposed in the wash north of hill 1099T (through canyon of Evening Star wash; Fig. 10) . At this locality the unit consists of well indurated, dark red fanglomerates (Fig. 11 and Plate III). Individual beds are generally poorly sorted, matrix supported, cobble to boulder debris flows. Some beds within this unit display inverse grading. The clasts have a maximum size of 1.5 m and a minimum size of 1/2 cm. The matrix varies from coarse to fine sand. Moderatelysorted to well-sorted beds in this unit are rare and consist of several cobble conglomerates and gravel beds. Thin (2-10 cm) discontinuous lenses of fine sand, and less commonly silt, are found sporadically interspersed 32 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1099T 1153T 1181T 1159T A 1077. OAT 1128T Francis Peak 1132T A 1174T 1121T 1167T 1106T 1146T A 1059T 2-118A 1176T 1137T 1 km Figure 10. Location of Plate I being discussed in text. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 11. Sediments of TMa2 exposed in through canyon of Evening Star wash. View is northwest. 34 throughout the unit at this locality. The average bed thickness of coarse-grained sediments is approximately 1 m. The top of THa2 has been arbitrarily placed at the base of the upper carbonate gravity glide sheet, middle allochthon 2 (to be discussed later) , which crops out on both sides of the wash. Note that in section C-C', Plate II, that the contact between these two units is marked as an apparent unconformity. Clast Composition The clasts found within TMa2 appear to record the unroofing of a part of the Clark Mountains. As mentioned earlier, the base of this unit consists of debris flows containing Upper Paleozoic to Mesozoic carbonates. An across-strike traverse northeast from hill 1149T (see Fig. 6) permitted the identification of several distinctive lithologies weathering out of this unit and occurring as float. These same lithologies also occur as clasts in the well-exposed outcrops of TNa2 in the northwest corner of the study area. Among the most distinctive of these lithologies are carbonate rocks from the Mississippian Monte Cristo Fm. and the Devonian Sultan Fm., phyllites of the Cambrian Carrera Formation and from the Eocambrian Wood Canyon Formation, quartzites from either the Zabriskie Quartzite (Cambrian) or Johnnie Formation (Precambrian), and basement units characteristic of the 35 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Clark Mountains area. The basement lithologies include coarsely crystalline red granites, medium- to fine-grained amphibolite, quartzofeldspathic gneisses, garnetiferous gneisses and granitic augen gneisses. The presence of the Cambrian Tapeats sandstone (Figure 12) and Cretaceous Delfonte volcanics within this alluvial unit provides an excellent tie to the Clark Mountains area as a provenance for TMa2, since the only exposures of these units in this region lie in the autochthon below and east of the Clark Mountains thrust complex. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 12. Clast of Tapeats sandstone within TMa2 in through canyon of Evening Star wash. 37 Paleocurrent Indicators Few paleocurrent indicators were identified within TMa2. Data sets from the canyon of Evening Star wash are limited to two zones of imbricate cobbles, one boulder pile-up, one set of climbing ripples, one set of asymmetric ripples, one set of ripple foresets, several channel axes, and basal grooves on several beds. These data are shown in Figure 13. Although the data set is limited, the apparent dominant flow direction in this interval is approximately 290. This is in good agreement with the probable eastern source of clasts within TMa2. Carbonate Megabreccias (Tcbr) Four or possibly five landslide deposits (Tcbr) are intercalated with the alluvial sediments of TMa2 at the southern end of the study area (Figure 5; Plate I). Two of these breccias are exposed just north of the powerline road, north and northeast of hill 1157T (see section C-C', Plate II). These deposits consist of highly chaotic, thoroughly disrupted limestones and dolomites of the Cambrian Bonanza King Formation. They are interpreted to be a single deposit offset on a west-dipping normal fault, and might be distal, brecciated equivalents of the lower allochthon, although rocks in exposed parts of this allochthon appear to be younger than Bonanza King. The along-strike length of the western deposit is nearly one 38 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Equal Area a. / Figure 13. (a) Cobble imbrications from one horizon in THa,, Evening Star wash. (b) Composite plot of paleocurrent data from exposures of TUa2 in Evening Star wash. Plus sign=channel axes, square=grcove set on base of one debris flow, small dot=one set of climbing ripples, large dot=one set of ripple foresets. 39 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. km, whereas the eastern exposure is approximately 1/2 km long. Their exposed width is approximately 2 0-30 m, and the maximum observed thickness is 3-4 m. These landslide(s) dip approximately 20° northeast. East of these two deposits, a third brecciated exposure of Bonanza King carbonates crops out along the powerline road. This landslide is thought to lie above playa sediments, but it may also be an offset equivalent of the units just described above. North along strike from these megabreccias, four additional lenses of Tcbr consisting of Bonanza King carbonates are intercalated with the alluvial sediments of TMa2 (Plate I) . West of hill 1099T (Figure 10), a megabreccia composed of Paleozoic carbonates is intercalated with TMa2, and crops out approximately 60 m from the top of this alluvial unit (Figure 14a). The age of the carbonates that comprise this landslide is approximate; rare brachiopods are the only distinctive fossils within the deposit (J. K. Schubert, pers. comm., 1990). This landslide is highly shattered, and locally consists of a fine rock flour. In other parts of the landslide, the carbonates appear to have behaved like a fluid; finegrained breccias consisting of predominantly dolomite are interfingered with more limestone-rich breccias (Figure 14b). The textures are similar to igneous textures 40 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. extent of 0.2 km2 and a maximum thickness of 2 0 m. The basal contact is not well exposed, but seems to parallel bedding in the underlying alluvium. Figure 14. (a) View of megabreccia intercalated with sediments of TMa2 below the base of middle allochthon 2 west of hill 1099T. View is toward the north. (b) Interdigitation of dolomitic (light color) and limestone (darker) lithologies within megabreccia of (a). 41 Alluvial Unit 3 (TMa3) A third alluvial deposit, TMa3, rests unconformably on top of the middle allochthons. This alluvial unit crops out sporadically in the valley of Evening Star wash (Fig. 15). It is moderately well-indurated and forms low, rounded hills. The best exposures of the TMa3 are adjacent to modern stream channels where undercutting and active scouring of the rocks have exposed fresh surfaces (Fig. 16a) . THa3 is approximately 750 thick in the northern end of the study area and tapers to the south, where it is approximately 230 m thick (see Plate I). In its southern extent, TMa3 has a gradational contact with the unit TMa2, as the middle allochthon(s) taper out to the south, thereby juxtaposing the arbitrary "bottom” and "top", respectively, of the two alluvial deposits. THa3 is mapped separately from THa2 because of its finer grain size and better sorting, but it may, in fact be a finer equivalent of TMa2. The clast composition of the two units is similar. This unit demonstrates the only potential example of ponding above or behind one of the large allochthons: medium- to coarse-grained sandstones are locally found within 10 to 30 m of the top of the middle allochthons, which may be explained as a result of the allochthon below this unit acting as a barrier to downslope flow, thereby allowing finer-grained sediments to "pond" behind it. True ponding of sediments in the 43 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1099T 1181T 1159T A 1077. OAT 1128T Francis Peak 1182T A 1174T 1121T 1167T 1106T 1146T A 1059T 1149T 2-118A 1176T 1137T 1 km Figure 15. Arrows showing approximate area of outcrop of THa3 in plate x - Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 16. Star wash. (a) Sediments of TMa3 exposed along Evening View is to the north. Hammer is 18 cm long. (b) Clasts within TMa3. 45 form of playa or lacustrine deposits was not evident. Near its base, this deposit is composed of coarse to verycoarse, cobble to gravel debris flows with occasional boulder horizons. These coarse sediments grade upward into medium-grained sands and gravels with sparse lenses of conglomerates. The alluvium of TMa3 shows an overall fining upsection, and the upper contact of this deposit locally interfingers with lacustrine sediments, but generally shows a sharp change to finer-grained, silty to clay-rich playa deposits. The top of the unit is mapped where lacustrine sediments become the dominant lithology, and is dashed in many localities due to poor exposure. In general, the unit is well bedded, with individual beds ranging from several centimeters to 1/2 m in thickness. TMa3 is dark red to reddish-orange in color. Clast Composition The composition of clasts within TMa3 is similar to that found in TMa2 (Figure 16b) . Clastic sedimentary rocks occurring in this unit include the Cambrian Carrera Formation (green phyllites) and maroon phyllites of either the Carrera Formation or the Eocambrian Wood Canyon Formation. A variety of carbonate clasts are also present within TMa3, but individual formations were not identifiable. Crystalline lithologies found within this unit include garnet gneisses, quartzo-feldspathic 46 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. gneisses, amphibolite, and coarsely crystalline granites. The Tapeats sandstone is present within TMa3 in the study area as cobbles within coarse-grained sediments. Approximately 2 km north of the northern end of the study area, Davis (G. A. Davis, pers. comm., 1991) has identified a large (700 m x 1200 m in cross section) fan lobe within TMa3 that contains abundant clasts of cobbles and boulders (up to 2 m) of Tapeats sandstone. This fan lobe may be the reason for the apparent northward thickening of THa3. No clasts of Delfonte volcanics were recognized in this unit. It is worth noting that none of the Shadow Valley basin clastic sediments contain clasts of the lithologies found at Shadow Mountain, which is much closer to the study area than the probable Clark Mountains source of these sediments. Paleocurrent Indicators Paleocurrent data from TMa3 include asymmetric ripples, ripple foresets, channel axis trends and basal groove trends. These data are shown in Figure 17. The unidirectional data suggest that flow was toward the west or northwest (Figure 17a). Bidirectional data (channel axes, basal grooves; Figure 17b) are oriented in a northeast-southwest direction. It is also worth noting that several opposing sets of cross beds were found one on top of the other in a herringbone pattern. In light of 47 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Equal Area CL. Circle = 40 % bEqual Area Figure 17. (a) Plot of ripples and ripple foresets in TMa3* Bidirectional current data from TMaJ. Plus sign=grooves on bases of beds, small dots=channel axes. 48 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. these contradictory data, no confident estimate of a dominant flow direction for THa3 can be made. Lacustrine Sediments (TMl) A sequence of light colored, fine-grained siltstones, shales and sporadic tuffs is deposited above the unit THa3. This sequence has been mapped as lacustrine (playa) sediments, and is labeled Tm on Plates I and II. These sediments are exposed discontinuously along the base of the west flank of the hills labeled Shadow Mountains on Plate I and east of these hills as well (Figure 18) . Correlative lake sediments also crop out approximately 2 1/2 km southwest of the "Shadow Mountains" hills, where a normal fault has repeated the entire Tertiary sequence. The playa sediments weather as low rounded mounds and are poorly indurated. They are best preserved beneath resistant capping units such as tuffs, younger gravels, or allochthonous sheets (Fig. 19). An intact section of this unit has not been recognized within the study area, but mapping by Davis (G. A. Davis, in progress) indicates that these lacustrine sediments become gypsiferous and thicken to the north and northwest. The absence of a complete section is probably due to the fact that these sediments are almost exclusively found beneath the upper allochthon, where the uppermost horizons are highly complicated by folding. A second explanation for an incomplete section 49 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 18. Fine-grained lacustrine sediments of TMl. 50 1099T 1153T 1181T 1159T A 1077.0AT 1128T Francis Peak 1182T A 1174T 1121T 1167T 1106T 1146T A 1059T 1149T 2-118A 1176T 1137T 1 km Figure 19. Location of area being discussed in text. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. of lacustrine sediments may be that these sediments were tilted prior to the emplacement of the upper allochthon (see section entitled "STRUCTURAL GEOLOGY"). An estimated 10-2 0 m of these deposits may occur beneath the upper allochthon. Where possible, individual tuffs within this sequence have been mapped separately. Provenance of Miocene Clastic Sediments Alluvial Unit #1 Paleocurrent indicators are absent in the lowest alluvial deposit in the study area, but TMa1 is believed to be derived from the Halloran Hills area (Figure 20). The carbonate clasts that comprise THa1 have apparently experienced contact metamorphism, because they are bleached and recrystallized, but show no evidence of fabrics attributable to regional metamorphism. G. A. Davis (pers. comm., 1990) has suggested that the lithologies present within this deposit resemble Lower Mesozoic (Triassic Moenkopi?) and possibly Upper Paleozoic carbonates in the region. A lack of fossils in this unit makes identification of individual formations nearly impossible, however. The mountain ranges east of the study area (Clark Mountains, Mescal Range and Mesquite 52 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A //■/IT 1137 r A /-f if (<9> Figure 20. (a) Relationship of T„a, to probable source in Halloran Hills area. (b) Schematic cross-section of (a). No scale. 53 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Mountains) are all possible provenances for this unit, although most of the carbonates of this general age range in these eastern ranges have escaped metamorphism and recrystallization. A more reasonable, local source for TMa1 is found south of the study area in the Halloran Hills, where a nearly complete Paleozoic stratigraphy is preserved and crops out within 1 km of the southwestern corner of the study area. Paleozoic strata in the Halloran Hills have been intruded by the Cretaceous Teutonia plutonic complex (DeWitt, 1980), and are characterized by recrystallization and bleaching due to contact metamorphism. Since the clasts within TMa1 display metamorphic features similar to those of the units in the Halloran Hills, it is likely that these sediments were derived from this southerly provenance. Alluvial Units #2 and #3 An across-strike traverse of the unit TMa2 from hill 1149T (Fig. 8, p. 29) to approximately 200 m northwest of hill 1188T suggests that the unit records a continuous unroofing sequence from Upper Paleozoic carbonates through Precambrian basement. This observation is based on lithic clasts that occur as float in colluvium that obscures the outcrop. G. A. Davis (pers. comm., 1990) has suggested that these lithologies, and particularly those of the 54 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. crystalline units, are similar to those found in the Clark Mountains, 15 km ESE of the area being discussed (Fig. 21) . The presence of the Cambrian Tapeats sandstone and Cretaceous Delfonte volcanics is strong evidence for an eastern source for these units in the Clark Mountains. Paleocurrent data recorded in TMa2 are indicative of flow toward the west to NNW. Paleocurrent indicators from TMa3 are inconclusive, but clast compositions within this unit also support a Clark Mountains provenance. Lacustrine Sediments Playa deposits in the Shadow Mountains area are considered to be local in extent and there is nothing in the fine silts, clays and occasional fine sands of these deposits to suggest sediment provenance. Depositional Environment of Shadow Valley Basin Based on the nature of the clastic sediments described above, the Shadow Valley sequence is assumed to have been deposited in an intermontane basin (Fig. 22). The sediments record deposition in a fan/playa system upon which large allochthonous sheets were emplaced. Little evidence exists within the study area to suggest that these sheets disrupted sedimentation and/or drainage patterns within this fan/playa system. In general, the clastic sediments in this part of the Shadow Valley basin 55 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. IS k-a* Figure 21. Present spatial relationship between Clark Mountains (CM) and the Shadow Mountains (SMS) showing probable direction of transport of TMa2 and TMa3. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 22. Perspective sketch of Shadow Valley basin sediments and source terranes for Tu . .. Ha 1 • j (J) ^1 appear little disturbed by the episodic emplacement of large volumes of basin fill in a series of geologically instantaneous events. THa1 was dominated by sheet flow, and debris flows early in its development (unit THa1) . This unit may have been fluvially reworked, or represent the distal reaches of an isolated drainage or small fan located southwest of the study area. The presence of carbonate megabreccia deposits within TMa2 suggests that elevated topography or active fault scarps or both were present along the margins of the basin during deposition of this unit (c.f. Tin Mountain landslide; Burchfiel, 1966). Coarse sediments, debris flows and relatively immature sediments that comprise TMa2 suggest deposition in a medial fan environment with its source in the Clark Mountains. If so, then this fan system was enormous. Unit THa3 marks a shift to a more distal fan environment, either as the fan system matured or the alluvial plain migrated toward the head of the fan. Playa sediments suggest that the basin was closed, and the lack of interfingering within the study area between alluvium and playa deposits suggests a stable depocenter and fairly stable fan system. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Younger Alluvial Sediments Younger alluvial sediments are defined as those sedimentary deposits that post-date Miocene deformational and sedimentation events. They are assumed to have been deposited on top of the basin described above, after its basin was deformation. These sediments are thought to reflect a paleotopography similar to that seen today. Older Alluvium Older alluvium includes Plio-Pleistocene lacustrine sediments, alluvial fans and any other such deposits that are obviously not Miocene or Recent. They are nondeformed, with bedding either being horizontal or reflecting the paleotopography at the time of their deposition. Older alluvium is denoted "Oa" in Plate I, and is generally limited to lower elevations adjacent to modern stream channels. Colluvium The term "colluvium'* is applied to modern-day sediments that cover older rocks in places other than stream beds and channels. These deposits include slope wash, covered intervals and other forms of nonlithified surficial sediments. This term does not include soils, which are rare in the study area. These deposits are mapped as "Qc" in Plate I. Where the older lithologies 59 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. beneath Qc are known or strongly inferred, the cover has been omitted. Alluvium The term "alluvium" is used to designate those sediments which occupy modern stream channels and washes. Labeled "Qal", these sediments consist of fluvial gravels and sands deposited during infrequent flash floods. No attempt has been made to differentiate incised alluvium from the most recent channels within modern drainages. As such, this sedimentary unit is mapped in a fairly general sense, with most contacts between alluvium and older rock units shown as dashed lines to give the reader the general idea of where modern washes are located. Landslides Quaternary landslides are modern mass wasting features that have displaced bodies of older rock by slump, rock slide, block glide or other surficial mechanical processes. They are marked by arrows showing the relative sense of motion of the mass wasting feature and are labeled "Qls" on Plate I. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. MIOCENE ALLOCHTHONOUS SHEETS General Statement Within the Shadow Mountains study area, four intact, allochthonous sheets of older rock units are intercalated with the alluvial sediments of the Miocene Shadow Valley basin sequence (see Fig. 23) . As mentioned earlier, these large sheets are mapped separately from carbonate megabreccias (Tcbr) based on their internal coherence and, to a lesser degree, their size (Table l lists the more salient features of these deposits). From the stratigraphically lowest to the highest position within the exposed sedimentary section they are the lower allochthon (LA), middle allochthon 1 (MA^ , middle allochthon 2 (MA2) and the upper allochthon (UA) . The age and composition of the rock units found within the allochthons varies from sheet to sheet. Ordovician to Mississippian carbonates comprise the LA, Cambrian carbonates make up MA1 and MAj, and Precambrian crystalline and sedimentary units comprise the UA. The gross morphology of the allochthons is that of large, lobate to lensoidal bodies. Figure 24 shows some of the characteristic morphological features of the allochthons as well as qualitative velocity profiles for these deposits. In general, a vertical profile through an allochthon passes from the substrate (sedimentary unit 61 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 23. Generalized and schematic reconstruction of Shadow Valley basin in the southern Shadow Mountains showing the relative positions of the various units found in the study area. Horizontal scale is approximately 1:12,000. The diagram is exaggerated vertically to show detail. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. U) Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Allochthon Lower Allochthon Middle Allochthon 1 Middle Allochthon 2 Upper Allochthon TABLE #1. Characteristic Features of the Shadow Mountains Allochthons Composition OrdivicianMississippian carbonates Basal Friction Carpet NO Areal Extent (km2) 0.25 Volume (m3) Source Area Minimum Distance and Direction Traveled 1.75x106 Halloran Hills 1 km NE Cambrian Bonanza King carbonates Cambrian Bonanza King carbonates Precambrian crystalline and sedimentary units YES NO YES 60-70 >25 4.5X107 1.8x10® 8x10® Claik Mtns? 15km W NW Clark Mins ? 15km W NW Shadow Mountain 6 km WNW 0\ Figure 24. Generalized cross-sectional views of an allochthon showing basic morphology for (a) section transverse to the transport direction and (b) section parallel to the transport direction. Qualitative velocity profiles of an allochthon for (c) intact cap moving more rapidly than basal friction carpet and (d) velocity distributed through the entire allochthon (modified from Melosn, 1986; and from Shreve, 1568). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. s W *0 a X <3 \ > I liun ui uoqisod leoiyeA jiun ui UOIJISOd IBOIU0A 66 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. velocity beneath the sheet) into a basal friction carpet. Very little mixing has occurred between the allochthons and the substrate: the basal contacts of the allochthons are very sharp, and, with the exception of the upper allochthon, appear to parallel bedding planes in the substrate. The friction carpets consist of shattered and brecciated equivalents of the rock units that comprise the allochthons. These brecciated rocks are formed as the allochthons move across the substrate (see section on allochthon emplacement). The presence of a basal breccia appears to be related to the overall thickness of the allochthon; thinner sheets (LA and MA2) are completely shattered, whereas thicker allochthons (MA, and UA) have a distinct basal brecciated zone. Thickness variations within the friction carpet appear to depend on several factors, including the size of the sheet, the composition of both the substrate and the formations comprising the sheet and, very likely, the morphology of the surfaces upon which the sheets traveled. Above the friction carpet, intact rock units cap the basal breccias. The transition between friction carpet and intact lithologies is gradational in the distal, or marginal parts of the allochthons. In the central parts of the allochthons the boundary between shattered and intact lithologies is very sharp and can be mapped as a fault or series of faults. The intact caps of the 67 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. allochthons can be mapped: Individual rock units, sedimentary features and structures related to older deformational events are well-preserved. Where units in an individual allochthon are not mappable (e.g. a transitional brecciated zone), they are labeled undifferentiated. Deformation within the allochthons can be divided into two separate phases. Pre-emplacement deformation includes Mesozoic contact metamorphism of LA, Mesozoic thrust faulting, foliation development and contact metamorphism of the middle allochthons, and Mesozoic thrust faulting and foliation development within the UA, as well as possible Precambrian deformation of the UA. Overprinting these older structures are brittle features related to allochthon emplacement. Included in this category are the shattering of the allochthons (especially LA and MA2) , friction carpets, and brittle faults (most notably tear faults in the UA) . The structures and features formed during the two phases are treated below in sections entitled "Pre-emplacement Deformation", and "Synemplacement Deformation", respectively. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Lower Allochthon (LA) Occurrences An allochthonous sheet of Paleozoic carbonates designated the lower allochthon (LA; Fig. 25) crops out in the southwest corner of the study area. It lies atop the lowest stratigraphic unit, TMa1, within the study area (see Section C-C', Plate II). The sheet consists of a brecciated, but stratigraphically intact section of Paleozoic carbonates to the west and chaotic equivalents of the same units to the east (Fig. 26) . The formations that comprise the LA are the Ordovician Nopah Formation (0N) , the Devonian Sultan Formation (Ds) , and the Mississippian Monte Cristo Formation (M^). Based on the intact nature of the western exposures of this carbonate sheet, this unit has been mapped as an allochthon, rather than a landslide deposit of megabreccia type; the eastern exposures appear to be a distal or feathered lateral margin of the LA. Western exposures of the LA can be seen immediately north of the powerline road, striking northwest and southeast from hill 1137T (Figure 25) . In this area, the LA has been folded into a broad, doubly-plunging anticline whose main axis trends NNW-SSE. Eastern, highly brecciated chaotic equivalents of the LA are located along the powerline road, approximately 400 m east of the area 69 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. MA 1099' 1153T 1181T A 1077.0AT 1128T Francis Peak 1182T UA A 1 17 4T UA 1121T 1106T 1146T A 1059T UA 2-118^ LA 1176T 1 km Figure 25. Location nap showing approximate outcrop area of the lower allochthon in Plate I. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 26. Intact Paleozoic carbonates of the lower allochthon. Small knob in the right of the photo is approximately 80 m high. View is to the southwest toward hill 1137T and the powerline road. 71 described above. These eastern exposures of the LA strike northwest and NNW of the road, and despite numerous small offsets, can be traced along strike for nearly 2 km. This part of the lower allochthon is fairly thin, not exceeding 4 m. These breccias dip between 20-40° northeast. The correlation of carbonate rocks in these two areas of exposure is based on the presence of THa1 immediately beneath each of the deposits. Also, the degree of recrystallization, metamorphism and bleaching is very similar in both deposits. The LA ranges between 1 and 25 m in thickness and has an areal extent of 1/4 km2. The basal contact of this sheet, which appears to parallel the bedding in the underlying alluvial unit, is best exposed approximately 80 m east of the crest of hill 1137T, just north of the powerline road, and dips 37° NE at this location. A narrow (1 m) zone of intermixed clasts from the underlying alluvium and angular, brecciated clasts from the allochthon marks the contact between the two units (Fig. 27). The top of the LA is best exposed just east of hill 1149T, where debris flows of TMa2 have been deposited on chaotic breccias of the marginal portion of the LA. 72 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 27. Contact between the lower allochthon and IMal exposed on the east side of hill 1137T. Angular, brecciated clasts from the LA are mixed with more rounded clasts from TMal in a zone approximately 1 m thick. View is toward the east. 73 Strat igraphy Nopah Formation (0N) The Ordovician Nopah Formation is the oldest identifiable unit within the intact exposures of the lower allochthon (G. A. Davis, pers. comm., 1990). It is a grey, somewhat cherty, limestone with sparse fossil crinoid columnars (Hazzard, 1937; see figure 28). Nopah strata crop out along the southwestern and west sides of hill 1137T. Bedding could not be identified within this unit. Sultan Formation (Ds) On the east side of hill 1137T, the Devonian Sultan Formation forms the east-dipping flank of the folded LA. The Sultan Formation, which occurs in a normal stratigraphic sequence above the Nopah, is here separated from the older unit by a fault. The oldest part of the Sultan present on hill 1137T is either the Ironside or Valentine member (lower and middle members, respectively) . The lower portions of the Sultan are identified by distinctive stromatoporoids, which can be seen just south of the crest of hill 1137T. Bedding within this part of the Sultan Formation strikes north and dips 85° east. The eastern flank of hill 1137T is capped by the Crystal Pass member of the Sultan Formation. This upper member of the Sultan contains crinoids and possible rugose corals 74 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. c o c5 siI f cn 2 o o ."2 to ^ w o is 2 c o +i c o c to — - 5 E o c ' o > CD Q c 2 3 CO CD +1 Yellow Pine Limestone Bullion Member Dawn Anchor Member Crystal Pass Member Ironside Dolomite Valentine Member Missing Section c ca — .2 E o o t o Figure 28. Generalized stratigraphic column of the Paleozoic carbonates that comprise the LA. (After Hazzard, 1937, and Hewett, 1956). Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. (Hazzard, 193 7), and is in fault contact with the Nopah Formation at the north end of hill 1137T. Monte Cristo Formation (M,,,.) North of hill 1137T, the Mississippian Monte Cristo Formation (Mmc; Figure 28) crops out on the north side of a small window through the lower allochthon. The basal member, the Dawn Anchor Limestone, has an apparent strike and dip of approximately N65W 90 at this location. The recrystallized and bleached unit is identified by elongate chert nodules that roughly approximate bedding in the otherwise massive grey limestone (see figure 28; G. A. Davis, pers. comm., 1990). The chert nodules make up approximately 5-10% of the unit and range in length from 15-60 cm. The contact between the Dawn Anchor and the overlying Bullion member strikes N66W across the small knob 500 m north of hill 1137T. No bedding is evident in the Bullion, a massive, sugary dolomite. However, the depositional contact between these two members is vertical. The upper member of the Monte Cristo Formation, the Yellow Pine Limestone, occurs in a normal stratigraphic sequence above the Bullion about 600 m north of hill 1137T. This unit crops out on the northwest side of the small knob 500 m north of hill 1137T, and becomes buried beneath alluvium just north and west of this point. 76 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Middle Allochthon 1 (MA^ Occurrences Above the unit TMa2 throughout most of the study area is an allochthonous sheet of Cambrian Bonanza King carbonates; metamorphosed and bleached carbonate in the southern part of this allochthon are probably of Middle Paleozoic age, although no diagnostic fossils were found to support this conclusion (Figure 29; see section on geology of this allochthon). This sheet, labeled MA1, crops out for 4.1 km along strike, and measures 720 m at its maximum map width. The structural thickness of this deposit ranges from a minimum (0 m) at its southeasternmost exposure to a maximum of 145 m, approximately 400 m northwest of Francis Peak. In spite of a markedly brittle overprint, older Mesozoic structures and stratigraphic continuity have been preserved within intact caps of Bonanza King carbonates located above the friction carpet that marks the base of the allochthon. The basal friction carpet consists of shattered Bonanza King carbonates. This disrupted zone is slightly less than 25 m thick at the northwest end of the allochthon, where the breccias appear to be thickest. The basal breccias have a minimum thickness of approximately 3 m at the southeastern end of the allochthon. The size of fragments that comprise the breccias varies greatly. Locally, blocks as large as 30 m in maximum dimension are 77 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 10SS 1153T ‘ 23T UA LA 1149T UA 1176T i1137T 1 km Figure 29. Map shewing the general location of MA in Plate I . 1 78 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. readily identified (Fig. 30a). These large blocks are surrounded by zones of smaller fragments that range from several centimeters to as much as a meter in maximum dimension. In other localities, the breccias are more homogeneous; all fragments are roughly the same size (Fig. 30b) . There is no apparent order to the occurrence of different size zonations within the friction carpet. The allochthon breccias differ from the landslides in that neither rock flour nor fluidization was observed. To this point, the friction carpet has been described as a persistent feature at the base of MA1. While this may be the general rule, local variations have been recognized in the field. In fact, southwest of Francis Peak, no breccias were observed at the base of the allochthon. This "breccia-free" zone continues northwest along the base of the allochthon along strike for approximately 800 m. At this point, breccias mark the base of the allochthon for another 250 m north, where they disappear and intact Bonanza King again crops out along the base of MA1 for nearly 700 m. Resistant caps of structurally complex, intact Bonanza King carbonates (see below) are found above the friction carpet in several parts of MA1. The boundary between the intact caps and the friction carpets can in places be mapped as a fault or series of faults; in other localities within MAV intact lithologies grade into 79 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 30. (a) Basal friction carpet of MA, showing large block of intact carbonates surrounded by finer breccias, (b) Zone of MA, friction carpet showing uniformity of size of some of the breccias. 80 friction carpets with no discernible discrete boundary (Fig. 31). Within the intact caps of MA,, only one area, where a distinctive Cambrian stratigraphy permitted elucidation of the complex structure, was mapped in detail (see inset, Plate I). Figure 31. Eastern view of MA, near Francis Peak. Rugged topography at crest is intact cap, which rests upon lowrelief basal breccias , which rest upon alluvial sediments. 81 Geology Bonanza Kina Formation Limestones and dolomites within MA1 have been assigned to the Middle Cambrian Bonanza King Formation (Cbk) , a regionally extensive unit in the eastern Mojave Desert region that was originally defined by Hazzard and Crickmay (1933). This unit marks the onset of predominantly carbonate sedimentation in the Cordilleran miogeocline (see Figure 32). The Bonanza King Formation, composed of dolomites and silty dolomites interbedded with minor limestone and cherty limestone (Hazzard and Mason, 1936) was initially subdivided into five members by these authors. Three members are commonly mapped in the field: a lower Papoose Lake member, a middle silty dolomite member (unit 7a of Gans), and an upper Banded Mountain member (Gans, 1974; Figure 32) . A moderate to strong metamorphic foliation is present throughout most of the Bonanza King Formation within MA,. The carbonates have been strongly recrystallized and folded so that foliation to bedding angles are quite variable. In other instances the foliation was measured to be nearly parallel to original bedforms within the formation. In the field, a stratigraphic sequence is difficult to establish if a distinctive silty dolomite unit is not present. This is a 10 to 15 m thick, orangecolored marker horizon within the formation, whereas the 82 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. meters 30 Ironsiae Memoer 69 Sultan i.mestone ’01 S-uran ' 24Q Ev Scrags Ooiomiie 37 Eureta Quartzite 462 Poccnto Group 369 Nooan Formation ’2 Ounaeroerq Shaie Memoer 893 Coroneio Springs Formation 8anceo Mountain Memoer or Bonanza King Formanoni 41 Umi ?A 455 Bonanza King Formation !Papoose Lake Memoer) CaOiz Formation Figure 32. Generalized stratigraphic column of the Cambrian Bonanza King Formation (after Gans, 1974). 83 remainder of the formation is dark orangish-grey to black in outcrop and of roughly the same dolomitic composition. Cross beds within the silty dolomite assist in determining stratigraphic tops, as clear bedforms and sedimentary structures are generally absent in the rest of the formation. Through the course of this study, identification of members within the Bonanza King was possible only in the presence of the orange dolomite; otherwise, the formation was not differentiated. A stratigraphic sense of top and bottom was also difficult to establish in the absence of this member. Intrusive Igneous Rocks Small bodies of medium- to fine-grained intrusive rocks occur sporadically throughout the Bonanza King Formation within the allochthon (Fig. 33) . Although no age constraints are available, they are thought to be Mesozoic (probably Cretaceous) in age (G. A. Davis, pers. comm., 1990); similar small intrusive bodies are widespread throughout the Clark and Mescal Ranges. Medium-grained intrusive rocks are granodioritic in composition. These rocks crop out at two localities within the allochthon as small stocks 25 m wide in maximum dimension. In hand sample they are greenish to greyish white in color, with amphibole laths (5 mm) surrounded by a matrix of plagioclase. In thin section, the amphiboles 84 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1099' 1153T 1181T A 1077. OAT 1128T Francis Peak 1182T U A A 1174T UA 1121T 1167T 1106T 1146T A 1059T UA LA 1149T 1176T .1137T 1 km Figure 33. Location of igneous stocks in MA,. 85 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. have been altered to chlorite, and the textures preserved in the rock appear to be magmatic. No foliation or magmatic alignment of mineral grains is evident, suggesting that these rocks were either intruded after deformation of the Bonanza King host rocks or were unaffected at the time of their deformation. Fine-grained intrusives are thought to be equivalents of the coarser-grained rocks described above. The rocks are a pistachio green color in hand sample, and crop out as thin (maximum 1 m), sporadic, discontinuous sill or dike-like bodies. In thin section the rocks appear to have been completely altered to chlorite and other minerals. Both fine and coarse grained lithologies weather recessively. These igneous rocks have not been recognized elsewhere in the study area. Middle Allochthon 2 (MA2) At the northwestern end of the study area, a second carbonate allochthon has been identified (MA2; Fig. 34) . This sheet of Bonanza King and younger carbonates is approximately 1.6 km long where exposed in the study area, and has a maximum exposed width of 400 m. This allochthon extends approximately 2 km beyond the northern boundary of the area mapped where it tapers out completely. The thickness of this allochthon is at a minimum at its 86 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1153T 1181T \1159T 1077.0AT 1128T Francis Peak 182T UA £.11747 UA 1121T 167T 1106T 1146T 1059T UA LA 1149T 2-118^ 1176T [1137T 1 km Figure 34. Location of MAj on Plate I. 87 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. southernmost exposure, approximately 1.2 km NNW of Francis Peak, where the sheet is less than 2 m thick and appears to rest directly on MA1. The maximum thickness of MA2 within the study area occurs on the small hill 500 m NNW of hill 1099T where it measures approximately 45 m thick. Within the study area, the southern two-thirds of MA2 is composed of brecciated Bonanza King carbonates, whereas the northern 1/3 of the unit is composed of younger, brecciated unidentified carbonates. This allochthon has been mapped as a single sheet, but may, in fact be two; an older, southern sheet of Bonanza King and a younger, northern sheet of possibly upper Paleozoic units. Because field relationships between the two different lithologies are unclear— the possible contact between the "two" sheets lies in the through canyon of Evening Star wash, that cuts across the carbonates just north of hill 1099T— this unit has been mapped as a single sheet. In Evening Star wash the well-exposed contact with TMa2 is marked by a thin (2.5 cm) layer of rock flour/fault gouge (Figure 35a). This spectacular contact dips 19° northeast. Just west of hill 1099T, the contact is also well-exposed, and also dips 19° northeast. No gouge was observed at this location. The base of the sheet appears to lie parallel to bedding in underlying TMa2 (see Figure 35b) . 88 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 35. (a) Striae along the base of MA2 in Evening Star wash. Orientation of striae is 347/6. (b) Contact between MA2 and TMa2 in Evening Star wash. View is toward the northwest. 89 Upper Allochthon (UA) Occurrences The uppermost allochthon within the Shadow Mountains study area is an intact sheet of Precambrian crystalline and sedimentary rocks that crops out over an area of approximately 60 km2 within the study area (Figure 36). This allochthon is present in other areas to the north and west of the study area. The upper allochthon (UA), with few exceptions, has been emplaced upon fine-grained silts and clays of the lacustrine unit Tm and is generally overlain by young (Pleistocene to Recent), nondeformed alluvial deposits. The UA has been offset along a normal fault that places hanging wall exposures down to the west. As such, exposures of the UA are described as "eastern" (eastern footwall exposures) and "western" (western hanging wall exposures), based on their relationship to this fault (see Figure 36) . Outcrops of the UA occur as elongate ridges or series of hills punctuated by small valleys underlain by either modern alluvium or TMl. The ridges and hills generally strike northwest, which is the overall trend of the Shadow Mountains section. Unlike the other three allochthons in the study area, the basal friction carpet of the UA is very thin, with a maximum thickness of 3-5 m in the study area. Many exposures of the base of the UA show only a thin layer of breccias. The paucity of basal breccias beneath the UA 90 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. MA 1099 1153T / / 1181T v1159T A 1077.OATFrancis Peak 1146TA UAe 2-118^ LA 1149T 117ST ,1137T 1 km Figure 36. Distribution of the upper allochthon and the location of its western (UAw) and eastern (UAe) exposures. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. can be attributed to at least two factors: emplacement of the UA occurred across predominantly water-saturated playa sediments, and rock units within the UA (e.g. gneiss and diabase) are less prone to brecciation than the brittle limestones and dolomites that comprise the other allochthons. The lack of a distinctive friction carpet at the base of the UA means that the bulk of this allochthon consists of an intact cap of rock in which older features are well preserved. The characteristic disposition of units within the UA is that of thin, fault-bounded slices of crystalline and sedimentary units arranged in a crude, pseudostratigraphic order. Most of the faults separating individual units are low angle; both older-over-younger and younger-over-older juxtapositions have been identified. Younger-over-older fault relationships are very common in the UA, which is a structural style not commonly recognized in this region (see section on preemplacement deformation of the UA) . Geology Basement Gneisses The upper allochthonous sheet in the Shadow Mountains (UA) consists of Proterozoic crystalline and sedimentary units. A generalized stratigraphic column of these units in the greater Death Valley area is shown in Figure 37. 92 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Quartzite. siltstone and dokxnita Quartzite, gsneratty feidspethic Quartzite Siltstone Quartzite siltstone. dokxnrte. Quartzite. abundantly Sandstono MQ SitStOna. § Dokxnita, siliceous Mudstone Mostly granitic gnetss Dolomite, wed Diabase Chert Limestone, strometoiitic Clastic dolomite and Quartzite abundantty crossbedded Mixed sedimentary rocks; quartzite, clastic dolomite and quartzrtic dokxnrta. percent meorubtea Clastic quertzitic dolomite and sntstone Arkosic conglomerate sandstone and siltstone §s Dokxnita, siltstone and than one percent maoiubtes Clasbc. limestone, thinly Arkosic sandstone and siltstone uitstono and vmm; graoad beds and ^gantic darts common; may be as much as 1800m. thick Mixed sedimentary rocks; sandstone. siltstone. she*. 1 § S i E x 21 £ 3 i8 a Figure 37. Generalized stratigraphic column of the units that comprise the UA. (After Wright and Troxel, 1974). 93 The granitic gneisses that serve as basement for the overlying sedimentary sequence have been dated at 1.7 Ga in the Death Valley region (Wasserberg and others, 1959). The gneisses in the Shadow Mountains are moderately to strongly foliated, and based on their deformational histories, have been subdivided into two separate units. Gneisses with a moderate foliation are labeled Xg1, and strongly deformed and mylonitized gneisses are labeled Xg2. The relationship between the two is discussed in the section on pre-emplacement deformation of the UA. Crystal Spring Formation The basal Precambrian sedimentary unit in the Death Valley region is the Crystal Spring Formation (Ycs) . This formation is the oldest unit in the Pahrump Group, which consists of the Crystal Spring, Beck Spring, and Kingston Peak Formations (Fig. 37). The Crystal Spring Formation has been divided into seven members (Roberts, 1974) . The divisions of Roberts are too detailed for the purpose of this study. As such, no attempt has been made during the course of this study to identify various lithologies with individual members, except where relative positions within the formation have been established. Basal pebbly quartzites have been mapped separately, and a distinctive bluish quartzite in the western exposures of the UA has been separately mapped in order to demonstrate structural 94 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. continuity. A simplified stratigraphy of the Crystal Spring Formation is shown in Figure 37. The seven members of Roberts (1974) can be described in terms of a three-fold stratigraphic sequence: a basal conglomeratic and arkosic clastic sequence; a middle carbonate sequence; and an upper mixed sequence of carbonates, arkosic sands, siltstones and shale with subordinate conglomerate (Roberts, 1974; Wright and others, 1974) . The majority of the Crystal Spring lithologies in the Shadow Mountains study area appear to belong to the basal clastic sequence, which warrants a more detailed description of these sediments. In the greater Death Valley area, the base of the Crystal Spring Formation is typically marked by a distinctive quartz cobble/pebble conglomerate derived from the underlying granitic gneisses upon which the unit lies with profound nonconformity. The base of the Crystal Spring Formation is not exposed within the Shadow Mountains study area. Regionally, the basal conglomerate is overlain by a thick (ca. 350 m) sequence of arkosic sands with subordinate pebbly beds and conglomeratic layers. The uppermost 50-75 m of this sequence is characterized by interbedded brown- to black-weathering silty carbonates and cream- to white-colored quartzites. Individual carbonate beds are 10-50 cm thick, and the quartzites range from approximately 35-70 cm in thickness. 95 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. The carbonates tend to form small ridges, whereas the quartz ites (as well as most of the clastic sediments) tend to form rubbly outcrops that often weather recessively. One of these quartzites, a medium-grained, commonly cross-bedded arkosic quartzite, is very distinctive: It is believed to belong to the upper part of this clastic sequence (G. A. Davis, pers. comm., 1989; Fig. 38) based on its position within the formation on the eastern side of Shadow Mountain. When deformed, this quartzite acquires distinctive black reduction spots which, when ellipsoidal, tend to define a foliation within the unit. These spots are composed of opaque oxides accompanied by minor amounts of biotite. They appear to form around what may have been isolated feldspar clasts in a predominantly quartzose rock. The top of the clastic sequence is regionally marked by a massive purple mudstone (Stewart, ibid.). A purple mudstone has not been recognized within in the study area, but a possible equivalent to this mudstone may be present in the southwestern part of the study area, where a very fine-grained quartzite that weathers to a bluish color has been mapped separately (Fig. 39) . Above the mudstone is a sequence of dolomites, algal limestones, silty dolomites and subordinate clastic interbeds. These carbonates reach as much as 190 m in total thickness in the Alexander Hills (Stewart, 1974). 96 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 38. Spotted quartzite from the Precambrian Crystal Spring Formation. Reduction spots formed around feldspar grains in a predominantly quartzose rock. 97 1099' 1153T 1181T 1S9T 1077.0AT 1128T Francis Peak ■1182T UA MA, &1174T UA 1121T 167T 1106T 1059T UA LA 1149T 2-11 1176T l1137T 1 km Figure 39. Occurrences of the blue quartzite from the Crystal Spring Formation. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Brown- to black-weathering silty algal carbonate units are widespread within the Crystal Spring in the study area, but they are thought to belong to the basal clastic sequence described above. The upper member of the Crystal Spring Formation is composed of mixed lithologies. The actual stratigraphy of this member is poorly documented (ibid.). The only lithologies in the Shadow Mountains study area that may belong to this upper, mixed sequence are a series of reddish to purplish phyllites (Fig. 40). These finegrained, thinly bedded units are not described in the literature pertaining to lower parts of the Crystal Springs Formation, hence they might possibly belong to the upper part of the formation. The age of the Crystal Spring Formation is generally considered to be 1.2-1.1 Ga, based on its association with diabase sills and dikes that intrude it (see below). These diabases have been correlated with 1.2-1.1 Ga sills and dikes in northwestern Arizona (Hammond, 1982), on the basis of chemical and mineralogical similarities. The age of the Crystal Spring Formation, which Hammond (ibid.) believes was nonlithified upon intrusion of the diabases, is thereby taken to be only slightly older than these intrusive rocks. 99 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. ARGUS S INYO RANGE NO I’AH RANGE EASTERN Sl’HING M O U N TA IN S FRENCHMAN MOUNTAIN 5iJ.lV , , , Lijfmvha. -'.r,? ,1*' - 4 Q u aitzito a Clastic ^ ^ W edge AZ ILC UASAl M7 SANOSIONf UHCON St’HINl INANZA 14 1 7 Ga UASEMENT Pahrump KAUIAU IIM E S IO N E Figure 40. Generalized relationship between the Noonday Dolomite and older Precambrian crystalline and sedimentary units (From Wernicke et al., 1988). o o Diabase In the Death Valley area, the Crystal Spring Formation and its associated gneissic basement have been intruded by diabase sills and dikes (YD; see Figure 37). The diabase is a distinctive, coarse- to medium-grained, dark green intrusive rock that commonly develops a black desert varnish. At Saratoga Springs (southern Death Valley) and at Shadow Mountain the diabases are seen intruding basement gneisses, but their most common regional occurrence is as thick sills within the upper 1/2 of the Crystal Spring Formation (Wright, 1968). Alteration of impure dolomites above the sill has produced economic deposits of "talc" (tremolite; ibid.). Multiple sills and dikes have been identified at Shadow Mountain, which gets its "Shadow" from the largest of these bodies. The intrusions cut across the depositional contact between granitic gneisses and the overlying Crystal Spring Formation (Fowler, in progress). The granitic gneisses at Shadow Mountain have been extensively intruded by diabase, and the contact between gneisses and Crystal Spring lithologies has also been a common localized zone of diabase intrusion. Occurrences of the diabase sills and dikes in the Shadow Mountains study area are limited to exposures of the upper allochthon. Within the UA, the diabase forms approximately 60% of the exposed crystalline rock, and 101 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. occurs extensively along the contact between the Crystal Spring Formation and the underlying basement gneisses (see Plate I). None of the diabase bodies in the Shadow Mountains area are found intruding the Crystal Spring Formation except along its contact with Precambrian gneisses. The maximum exposed thickness of the major sill is approximately 500 m. The smaller diabase bodies within the basement gneisses range in size from 10 m x 25 m, to 25 m x 120 m. Where well exposed, the diabase is commonly foliated within several centimeters of its contact with both the gneisses and the Crystal Spring Formation, and appears to have generally been intruded parallel to both the foliation in the basement and to the bedding in the Crystal Spring Formation. The contact between the diabase and Crystal Spring rocks is now characteristically steep, dipping between 60° and 80°. With the exception of the foliated contacts mentioned above, no other fabrics are readily evident in the diabase. Discontinuous quartz veins commonly occur at the contact between gneisses and diabase. Even though several mining prospects are found within the UA in the Shadow Mountains, extensive metamorphism and/or alteration of country rocks related to intrusion of the diabase has not been recognized in the study area. 102 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Noonday Dolomite The Noonday Dolomite (ZN) , generally regarded as the basal unit of the Cordilleran miogeocline, lies with profound unconformity on older Pahrump Group sediments and crystalline basement throughout the greater Death Valley region (Wright and others, 1974; Figure 40). [An exception to this stratigraphic relationship has been described by Kupfer (1960) west of the study area in the Silurian Hills, where a concordant (conformable?) contact separates the Noonday Dolomite from the upper Kingston Peak Formation]. The Noonday Dolomite has been assigned a Late Precambrian age, because the oldest Cambrian trilobites appear in the stratigraphic record in the Wood Canyon Formation, well above the top of the Noonday (see Figure 37). The carbonate rocks of the Noonday are conformably overlain by the Johnnie Formation. Two very distinct facies have been identified within the Noonday Dolomite in the greater Death Valley area. A platform facies is composed of two dolomitic members that contain algal mats and algal mounds (Williams and others, 1974). This facies occupies the northern 2/3 of the exposed Noonday basin. A basin facies consists of clastic limestones, breccias, siliceous sandstone and shale. This facies crops out primarily north of the Silurian Hills and west of the Kingston Range (Williams and others, 1974) . 103 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. In the Shadow Mountains, only the platform facies has been recognized. A distinctive quartz pebble conglomerate marks the base of the Noonday Dolomite at one Noonday-on-gneiss locality in the southwestern Mesquite Mountains (G. A. Davis, pers. comm., 1990). In the Shadow Mountains, the base of the Noonday Dolomite is locally marked by a very similar conglomerate to that described above. Here, however, the Noonday lies unconformably atop quartzites and phyllitic quartzites of the Crystal Spring Formation; this basal conglomerate has been observed at five localities (Plate I). This conglomerate consists of large quartz pebbles (3-5 cm) in a matrix of coarse quartz sand to gravel. The conglomerate is best exposed 400 m northwest of hill 1121T (Figure 41) and is 2.5 m thick at this locality. To the author's knowledge, no other known occurrences of the conglomerate have been reported throughout the Death Valley region. Hence the basal conglomerate is probably an isolated facies, but a potentially important one in linking the Shadow Mountains Noonday section to a provenance proximal to the Mesquite Mountains. In western exposures of the UA, the conglomerate is locally preserved beneath dolomites of the shelf facies. The basal conglomerate is this area is highly brecciated, and the contacts between Noonday and units below it are 104 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1099' 1153T 1181T J159T A 1077. OAT 1128T Francis Peak 1174T UA 1121T 167T 1106T 1146T A 1059T UA LA 1149T 1176T i1137T 1 km Figure 41. Location of well-exposed, intact outcrops of the basal Noonday conglomerate (see Plate I ) . 105 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. apparently faults. The largest mapped exposure of Noonday in this part of the UA occurs on hill 1106T, where silty dolomites cap most of the hill (Fig. 42). On the southeastern flank of the hill, the Crystal Spring quartzites are in contact with granitic gneisses (Xg1) along a near-vertical, curviplanar fault. The Noonday Dolomite appears to have been emplaced or deposited across this fault, because fine-grained dolomites and brecciated quartzites lie above gneisses to the north of the fault and Crystal Spring quartzites to the south. Brecciated exposures of the conglomerate are also found above basement gneisses (Xg1) approximately 500 m south of hill 1106T. On the south side of hill 1106T, brecciated basal Noonday conglomerates are mixed with what appear to be brecciated quartzites from the Crystal Spring Formation. At this locality, silty Noonday carbonates are apparently in fault contact with the Crystal Spring below it, and the Noonday lithologies are also in fault contact with mylonitized gneisses of Xg2 (see Plate II) . The gneisses are brecciated, and this fault is probably an equivalent of the low-angle normal fault that has been mapped on the northeast side of hill 1106T (Plate I; see below). 106 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1099' 1153T 1181T & 1077. OAT 1 1 2 8 T Francis Peak MAi A 1 1 7 4 T UA 1121T 1146T A 1059T UA LA 1149T 2 -1 18A" 1176T i1137T 1 km Figure 42. Location of area being discussed in text. 107 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Pre-emplacement Deformation of Allochthons The deformational history of the allochthons within the Shadow Mountains area is complicated by the successive overprinting of older events by younger deformations (Table 1). Rocks within each of the allochthons has experienced several phases of deformation. In order to clarify the structural evolution of these complex allochthons, each phase of their deformation has been addressed separately. In identifying individual events, a distinction has been made between structures related to deformations that occurred prior to emplacement of the allochthons, and structures directly related to allochthon emplacement. Deformation of the entire Shadow Valley basin sequence is treated in a subsequent section, since these younger structures are not unique to the allochthons. Lower Allochthon Pre-emplacement deformation of the Paleozoic carbonates that comprise the lower allochthon (LA) is limited to recrystallization and possible stratigraphic attenuation. No evidence of foliation or a penetrative cleavage was noted in the strata of the LA, although the units in the LA seem to have experienced some degree of contact metamorphism. It is possible that the units in the LA have been attenuated; G. A. Davis (pers. comm., 108 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1990) believes that the various members of the Devonian Sultan and Mississippian Monte Cristo Formations are thinner in the Shadow Mountains than at Clark Mountain localities. One fault is present in the LA that may predate its emplacement. The Ordovician Nopah Formation is juxtaposed against the upper, Crystal Pass member of the Devonian Sultan Formation across a north-striking, vertical fault (N4°E 90°) that is located 150 m NNE of hill 1137T (Figure 43; section C-C', Plate II). A sense of displacement on this fault was not evident in the field. The entire contact between the Nopah and Sultan Formations could be marked by this fault; the Ironside/Valentine members of the Sultan Formation appear to become thinner along this contact as the fault is traced north from hill 1137T. Middle Allochthon 1 The older of the two middle allochthons, MA1, records a complex structural history. A penetrative foliation is present in the Bonanza King carbonates throughout the allochthon. Because of the internal complexity of this allochthon only about one square kilometer was mapped in detail (see inset of Plate I; Figure 44) . This area extends north from just east of Francis Peak to the northwestern end of this unit. 109 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1099 1153T . V, 1181T 1077.0AT 1123T Francis Peak £ 1174T UA 1121T 1106T 1146T A 1059T UA 2-118^ LA 1149T 1176T i1137T 1 km Figure 43. Location of the only fault mapped within the LA. 110 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TN A rvlti a.^a. * / /') > 1 1 1 1 0; km Figure 44. Area of MA1 mapped in detail. This map is the inset in Plate I. 11 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. As mentioned above, establishing stratigraphic sequence or location within the formation is difficult in the absence of the distinctive yellow-weathering silty dolomite (middle member of the Cambrian Bonanza King Formation). Where a sequence/position can be established, the relative sense of motion and offset on faults has been appropriately designated. Where a sequence could not be established, faults were mapped with no indicated sense of offset unless kinematic relations were ascertained in the field. Two dominant orientations of faults are present in MA2. One set, mostly thrust and high-angle faults, trends in a NNE-SSW direction (Figure 45a) . Two of the high angle faults have left-lateral strike-slip displacement. The second set strikes between NNW and NW (Figure 45b) . Approximately 1/2 of these faults dip southwest at angles between 25° and 75°. One northeast-dipping thrust occurs in this group. Normal faults within this set have a downto-the-southwest sense of offset. The northwest-striking faults appear to be the older set, as most of these faults are cut by northeast-striking faults (see inset, Plate I). None of the faults mapped in this allochthon have a ductile fabric along them. The general assumption about the ages of the different faults in MA2 is that the thrust faults are Mesozoic in age. Certainly, not all brittle 112 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 45. (a) NNE-striking faults within MA1. (b) NWstriking faults and folds within MA1. 113 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 114 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 115 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. faults within the allochthon are necessarily Tertiary in age. Several northwest-trending folds within MAj are possibly related to Mesozoic deformation (Figure 45b). One large, NW-plunging syncline-anticline pair is located one km NW of Francis Peak. The limbs of these folds are undulose and have several minor, open folds within them. A second, smaller syncline-anticline pair was mapped approximately 800 m NNW of Francis Peak. These are also northwest-plunging, open folds. A steeply plunging anticline was mapped 200 m NW of Francis Peak. The fold is fault-bounded on its western limb and plunges to the northwest. No brittle structural features related to folding have been recognized within the allochthon. Middle Allochthon 2 The allochthon labeled MA2 is thoroughly shattered. Sedimentary features and map-scale structures are not preserved in this allochthon. Although the carbonates are foliated and recrystallized, identification of structures related to pre-emplacement deformation of this allochthon was not possible. Upper Allochthon Within the UA, pre-emplacement stratigraphic, structural and intrusive relationships are well-preserved. 116 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Very little brecciation associated with the emplacement of this allochthon has occurred, leaving the Precambrian units that comprise the UA remarkably nondisrupted. Several older deformational events are recorded in the crystalline and sedimentary units of the UA. All of the pre-emplacement events are pre-Cenozoic, and two may be as old as Proterozoic. Pre-Cenozoic Deformation Two pre-Cenozoic deformational events have been identified within the western exposures of the UA in the vicinity of hill 1059T and hill 1106T (Figure 42; Plate I). The older of these two events involved ductile deformation of crystalline basement and overlying quartzites of the Crystal Spring Formation and may have included thrusting. This high temperature event produced a well-defined foliation, lineations, and locally, mylonitic fabrics within the Precambrian granitic gneisses labeled Xg2. Although the gneisses now lie below the Crystal Spring quartzites, the Crystal Spring section is overturned on the basis of preserved cross-bedding. The mylonitic fabrics decrease in intensity away from the contact between the gneisses and the overlying quartzites, which strongly suggests that this contact is a fault. Basal coarse clastic units of the Crystal Spring Formation are omitted along this poorly exposed contact, which 117 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. further suggests a fault relationship between the gneisses and Crystal Spring lithologies. Stretching lineations in the mylonitic basement (310/50) closely approximate lineations in the overlying Crystal Spring spotted quartzite (305/47). Crystal Spring rocks above this fault have been folded into a large overturned, northeast-trending anticline with both limbs of the fold dipping to the northwest (Plate I; Plate II, section E-E'). This structure crops out on hill 1059T and can be traced across the wash to the northeast. A hinge for this fold was not observed. It may be that a fault recognized 200 m NE of hill 1059T has omitted this part of the fold, or that structural closure once lay above present-day topography. Locally, intrafolial isoclinal folds are present within individual carbonate beds of the Crystal Spring Formation, but their orientations do not elucidate the orientation of the larger structure. Whereas the foliation and mylonitic fabric within the gneisses, as well as the bedding in the Crystal Spring Formation appear to parallel the inferred trace of the fault, the lineations and axial traces of folds within both units trend at high angles to its trace. These relationships rule out possible strike-slip deformation, because lineations along a strike-slip fault would be expected to form parallel to the trace of the fault. The 118 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. fault geometry most easily reconciled with the structural data is that of a thrust placing gneisses over Crystal Spring lithologies. These units were subsequently folded and overturned, yielding the present-day geometry described above (see section E-E', Plate II). Post-dating this ductile event and predating the tectonic emplacement (or deposition?) of the Noonday Dolomite (see Mesozoic Deformation) is a fault that truncates the northwest limb of the fold described above. This brittle fault, which places moderately foliated granitic gneisses (Xg1) against Crystal Spring quartz ites, can be traced discontinuously from 350 m southeast of hill 1106T for nearly a kilometer to the southwest (Figure 46). A probable offset equivalent of this fault crops out on the small knob 400 m northeast of hill 1106T. The juxtaposition of granitic gneisses with significantly different deformational histories strongly implies a significant amount of offset along this fault. The primary constraint on the age of the deformational events described above is the presence of the Noonday Dolomite above both the ductiley deformed Crystal Spring Formation and the moderately foliated gneisses of Xg1. East of hill 1106T, silty dolomites of the Noonday are superposed above these gneisses and Crystal Spring quartzites along what appears to be a brittle fault; the contact beneath the Noonday is marked 119 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. by brecciated quartzites (Figure 46). These breccias do not appear to belong to the basal conglomerate of the Noonday, but they may be derived from quartzit.es of the Crystal Spring Formation. West of this locality, approximately 100 m south of hill 1106T, brecciated basal Noonday conglomerate is mixed with brecciated Crystal Spring quartzites above a brittle, low-angle normal fault that places Noonday and Crystal Spring over mylonitic gneisses (Xg2) . This fault is discussed in the next section. At another locality, 500 m south of hill 1106T, brecciated quartzites that may be derived from the basal Noonday conglomerate mark the contact between silty dolomites from the Noonday and gneisses of Xg1. The juxtaposition of the Noonday Dolomite above these Precambrian units may be related to the younger-over-older structural stacking within the UA. This structural style seems to be the rule rather than the exception at this structural level within the fold-and-thrust belt during Mesozoic (?) deformation (see below). The inferred Mesozoic age of the Noonday faulting therefore provides an upper limit to the age of the structures found beneath it. Hence, the deformational events described above are not tightly constrained by the Noonday faulting. 120 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 46. (a) Brittle high-angle fault truncating the northwest limb of folded Crystal Spring lithologies. (b) Line drawing of (a) . 121 Mesozoic Deformation The best evidence for Mesozoic deformation within the UA is found in the exposures of this sheet in the eastern one-third of the study area (Figure 47). Four low-angle faults involving both older-over-younger and younger-overolder superpositions have been mapped (closed and open barbs, respectively, Plate I; see section B-B', Plate II). These faults usually involve the Crystal Spring Formation and the Noonday Dolomite. Fault-bounded slices of crystalline basement (Xg1) and diabase are also present, but are not as common as the sedimentary units. A ductile foliation is present within lithologies of the Crystal Spring Formation along faults separating Noonday Dolomite from the Crystal Spring. From lowest to highest within the stacked sequence, the four fault-bounded packages consist of gneiss + diabase, lithologies of the Crystal Spring Formation, parts of the Noonday Dolomite, and a second sheet of Crystal Spring units (Figure 48). Younger over older low-angle faults are here interpreted as thrusts, since they occur in a structural sequence in which the older-over-younger juxtaposition of the Crystal Spring Formation on the Noonday Dolomite is definitely a thrust fault, and there is no evidence within this stacked sequence of low-angle faults to suggest that more than one event produced these structures. The most 122 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1099 1153T 1181T V1159T 1077.0AT 1128T Prancis Peak 182T UA A 1174T UA 1121T 167T 1106T 1146T A 1059T UA LA 1149T 2 -118^ 1176T i1137T 1 km Figure 47. Location of well-exposed stacked sequence of thrusts within the UA. 123 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 48. Diagrammatic sketch of the relationships and relative positions of stacked low-angle faults within the UA. 124 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. common younger-over-older fault relationship within the UA places the Noonday Dolomite in the hanging wall and the Crystal Spring Formation in the footwall of a fault that crops out throughout the eastern exposures of the UA. This juxtaposition of units that normally occur in stratigraphic succession is mapped as a fault for two reasons: (a) the Crystal Spring lithologies along the fault have a well-developed foliation that trends parallel to the fault, and (b), the conglomerate that marks the base of the Noonday Dolomite is often omitted along this contact. This fault is well-exposed at several localities within the UA (Figure 49). On the southwest flank of hill 1167T, the Noonday has been emplaced over phyllites of the upper member of the Crystal Spring Formation. This relationship also is preserved northeast of hill 1167T, where silty dolomites rest above a folded phyllitic sequence of Crystal Spring rocks (Figure 50). The fold within the Crystal Spring units is here defined by the basal Noonday conglomerate, found in depositional contact with these same phyllites (see Plate I). However, the fault between Noonday and Crystal Spring at this locality has omitted this distinctive conglomerate. Other exposures of this fault are found east of hill 1182T (Figure 49). In this area, the Noonday on Crystal Spring fault occurs within a stacked, imbricate sequence 125 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1099' 1153T 1181T J159T A 1077. OAT 1128T Francis Peak •1182T UA A 1 17 4T UA 1121T 1106T 1146T £ 1059T UA LA 1149T 2-1 ISA* 1176T i1137T 1 km Figure 49. Locations of well-exposed Noonday-over-Crystal Spring fault. 126 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 50. (a) Fault contact between phyllitic beds of the Crystal Spring Formation below silty dolomites of the Noonday Dolomite. (b) Line drawing of (a). 127 of folded low-angle faults. The Crystal Spring lithologies below the fault are silty carbonates and quartzites of the lower, clastic member of the formation. The fault is best exposed 100 m northeast of hill 1182T. The basal Noonday conglomerate in locally preserved along this fault on the west flank of hill 1128T. Other less common younger-over-older fault relationships within the UA involve the Noonday Dolomite above granitic gneisses (Xg1) , and the Crystal Spring Formation above Xg1 and diabase (Y0) . The Noonday-ongneiss relationship is preserved at two localities, 3 00 m east, and 400 m southeast of hill 1174T. The basal Noonday conglomerate is not present at either of these exposures. Southeast of hill 1174T, the Noonday Dolomite is also found above the Crystal Spring Formation, which suggests that the Noonday-on-gneiss fault is the same as the fault that places Noonday on the Crystal Spring Formation (see section B-B'; Plate II). A third Noondayon-gneiss fault is located in the western exposures of the UA. This fault has been described in the section on Precambrian deformation (see above). Exposures of the Crystal Spring Formation in fault contact with gneisses and diabase in a younger-over-older relationship are limited to two windows through the stacked fault sequence. One of these windows is located on the southwest flank of hill 1128T. At this locality, 128 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. folded quartzites and silty carbonates of the Crystal Spring are juxtaposed against a package of gneiss and diabase. The coarse basal pebble/cobble conglomeratic units of the Crystal Spring Formation are not present here, which is highly suggestive of a fault relationship between the crystalline units and the Crystal Spring sediments above them. A small, doubly-plunging anticline forms a small dome through which the lower-plate crystalline rocks are exposed. This same relationship is preserved in a second window exposed 600 m NNW of hill 1128T. Two occurrences of older-over-younger thrusting have been identified within the UA. The juxtaposition of the Crystal Spring Formation atop Noonday Dolomite occurs within the stacked thrust sequence located east of hill 1182T (Figure 49). Here, folded and foliated silty carbonates and quartzites of the Crystal Spring are thrust over a thin (maximum thickness = 30 m) sliver of Noonday Dolomite. This thrust sheet of Crystal Spring rocks is the highest sheet within the imbricate sequence, occurring above the Noonday-over-Crystal Spring fault (which may be equivalent to the Noonday-over-gneiss fault elsewhere) and above the Crystal Spring-over-gneiss + diabase juxtaposition (see Figure 48). Along this uppermost thrust, the thin sliver of Noonday Dolomite is locally omitted. Hence, in some places Crystal Spring lithologies 129 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. are juxtaposed against Crystal Spring lithologies along this fault. Field relationships and the presence of a ductile foliation within the Crystal Spring Formation both above and below the fault-bounded sliver of Noonday Dolomite suggest that the younger-over-older, Noonday on Crystal Spring thrust and the older-over-younger, Crystal Spring on Noonday thrust are coeval. The fault-bounded sliver of Noonday Dolomite appears to have been caught up as an imbricate slice beneath the highest Crystal Spring thrust sheet. The Noonday slice has locally been omitted along this fault, and the local preservation of parts of the basal Noonday conglomerate suggest that the base of the Noonday Dolomite has acted as a locus of discrete slip (decollement). This interpretation is supported by the presence of Noonday Dolomite in depositional contact with the Crystal Spring Formation below it, 200 m northwest of hill 1182T. The basal Noonday conglomerate is resting on the Crystal Spring Formation at this locality. As one follows this depositional contact to the east, the Noonday is omitted by the higher Crystal Spring thrust, and then reappears as a fault-bounded imbricate slice, sandwiched between the two Crystal Spring thrust sheets further to the east. The tectonic interleaving of imbricate thrust sheets in a younger-over-older sense, while not common, has been 130 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. documented within the foreland fold-and-thrust belt in this southern California region. G. A. Davis (pers. comm., 1991) has stated that imbricate slices of both younger-over-older and older-over-younger thrusts locally occur beneath the Winter's Pass thrust, the structurally highest thrust in this region. The fault-bounded slices below the Winters Pass plate include slivers of Sterling Quartzite, Johnnie Formation and the Noonday Dolomite (see Figure 37, for the relative stratigraphic relationship between these units). At Shadow Mountain, the inferred source terrane for the UA (see section on allochthon provenance), the Winters Pass thrust is located well below the structural level at which the UA was located prior to its emplacement. Hence the tectonic stacking seen in the UA cannot be attributed to complexities beneath the Winters Pass plate. This implies that there may have potentially been a fourth thrust sheet, located structurally above the Winters Pass plate in this part of the thrust complex, or that the Winter's Pass plate had developed internal imbricates of both older-over-younger and younger-over-older types. To the author's knowledge, structural complexities attributable to Mesozoic thrusting, such as those seen in the UA, have not been reported within the upper plate of the Winters Pass thrust, except at Shadow Mountain, where mapping by Burchfiel and Davis (1971) and Fowler (1992) has 131 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. identified an east-directed thrust fault within the Winter's Pass plate [Fowler has also mapped Precambrian gneisses over Precambrian Crystal Spring units in the northwest Shadow Mountain block]. Older-over-younger thrusting occurs locally within the Crystal Spring Formation. In the northeast corner of Plate I, basal pebble conglomerates of the Crystal Spring Formation have been thrust over younger Crystal Spring strata. The age of this thrust is not clear, but it is probably Mesozoic. Syn-emplacement Deformation of Allochthons Lower Allochthon Deformation associated with the emplacement of the lower allochthon seems to be limited to intense shattering and brecciation of the Paleozoic carbonates that comprise this sheet. No mixing of lithologies within the sheet has occurred, however, and the shattering is not so intense that individual formations cannot be identified. Similarly, negligible mixing of alluvial sediments beneath the allochthon and the brecciated allochthonous carbonates has occurred. One high-angle fault within the LA described previously may be related to its emplacement, but its age is equivocal. 132 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Middle Allochthons As in the case of the LA, intense brecciation of the middle allochthons is thought to be related to the emplacement of these sheets. In particular, the allochthon MA2 has been shattered to such a degree that large-scale features such as bedding, folds and faults are not distinguishable in the field. The basal exposures of the allochthon MA1 have also been thoroughly brecciated, and this basal friction carpet appears to be directly related to the Miocene emplacement of the allochthon (see section on emplacement mechanisms). Structural and kinematic data from the middle allochthons that can be directly related to their emplacement are sparse. Basal contacts between the allochthons and underlying alluvium are knife-sharp, and with the important exception of occasional clastic dikes of alluvium within the allochthons, no mixing of allochthonous Bonanza King carbonates and the underlying TMa2 has been recognized. Clastic dikes, composed of very fine grained alluvial material, cut the base of the allochthons and have been measured at several localities (Evening Star Wash and just west of Francis Peak). These dikes are thought to be the result of overpressurization of fluid-rich sediments beneath the allochthons, such that the sediments are injected along dilatant zones at the bases of the sheets during transport. Dilatancy is 133 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. thought to occur at high angles to the transport direction of the sheets (i.e. the dikes would strike roughly perpendicular to the transport direction; Figure 51). The clastic dikes measured in Evening Star Wash strike between N65E and N73E and have vertical "walls". Their thicknesses range from 5 to 50 cm. Near Francis Peak, the one clastic dike observed strikes N70E and has vertical sides. This dike is 1.5 m wide at its base, and tapers to 2 cm wide near the top. This dike is approximately 6 m long. Fault gouge marks the bases of the middle allochthons in several localities. In Evening Star wash, a 2.5 cmthick layer of rock flour occurs between the alluvial unit TMa2 and MA,. Slickenlines on the bottom of this layer trend between N8W and N13W, with a negligible plunge. South of Evening Star wash, the base of the allochthon is marked by a distinct set of corrugations. These grooves occur on a wavelength of 5 cm, and trend between N80W and N90W. Fault gouge was also recognized along the base of MA2 approximately 350 m north of Francis Peak. No kinematic data were present at this locality. The structural data listed above strongly suggests that the transport direction of the middle allochthons was toward the NNW or SSE. The provenance of the Bonanza King carbonates that comprise these allochthons supports a transport direction toward the NNW (see section on 134 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. A \t •cL-H^4 Figure 51. Diagrammatic sketch of the relationship between transport direction of allochthons and the predicted orientation of clastic dikes. 135 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. allochthon provenance). That the kinematic data from the bases of the allochthons are related to their emplacement is clear; these features probably represent the very final stages of motion of the allochthons, since the slickenlines would not survive transport over any great distance. The vertical orientation of clastic dikes at the base of the allochthons also suggests that these features formed at the latest stages of allochthon motion. Folding or further deformation of the dikes would be expected if they had formed at a time during allochthon emplacement that would require their transport within the breccias. Faults within MA, have been described previously, and the relative ages of the majority of the faults is highly equivocal. However, several of these faults delineate a sharp boundary between brecciated basal deposits and intact Bonanza King carbonates higher in the sheet. One such fault has been mapped from 80 m east of Francis Peak, NNW for 800 m along its strike (Figure 52). The fault dips southwest between 25-40°. A second, similar fault is located 700 m northwest of Francis Peak. This fault also dips southwest and separates intact Bonanza King carbonates above the fault from breccias below it to the northeast. These faults may be related to allochthon emplacement, and the timing of their formation probably coincides with the formation of the basal breccias. 136 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1099' 1153T ~ Hr 1181T 1077. OAT 1128T Francis Peak 182T UA A 1174T UA 1121T -o 167T 1106T 1146T A 1Q59T UA LA 1149T 2-1 1 8 a 1176T .1137T 1 km Figure 52. Location on Plate I of major faults within MA1 that separate the intact cap of the allochthon (in the hanging wall) from basal breccias (footwall). 137 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Upper Allochthon Faulting and brecciation related to allochthon emplacement is generally easier to differentiate in the UA than in lower sheets for several reasons. Pre-emplacement deformation in the UA seems to be confined to ductile events and the lithologies of the UA are less prone to brecciation than the dolomites and limestones present in the other allochthons. It should be noted that the normal faults within the UA that were described earlier are confined to the allochthon and do not extend into the underlying Tertiary sediments. These faults therefore predate emplacement of the sheet, and postdate older ductile deformation, but further constraints on their age cannot be made. Breccias within the base of the UA are much less extensive than beneath lower, carbonate allochthons, and are probably related to the initial separation of the sheet in its source terrane and subsequent motion over alluvial fans prior to reaching the playa sediments upon which it now rests. The UA appears to have moved extensively across fine silts and clays of the unit Tm , and friction on this interface would probably produce negligible brecciation of the Precambrian units in the allochthon. Brittle faults related to the emplacement of the UA are limited to strike-slip faults and normal faults. Two 138 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. large strike-slip faults have been mapped within the UA that do not cut the sediments below it. These faults are thought to be tear faults within the allochthon, and as such are good indicators of the relative transport direction of the sheet (see section B-B', Plate II). The more prominent of these faults is located along the drainage at elevational mark 1077.OAT, 2.5 km NNW of where the jeep trail leaves the powerline road and enters the study area (Figure 53). This fault, which strikes N68E, has displaced the northern exposures of the allochthon to the east in a right-lateral sense relative to the offset equivalent of this block to the south. It has approximately 1 km of displacement along its exposed length, based on the offset intrusive contact between Precambrian diabase and the Crystal Spring Formation. On the northwest side of the drainage the fault surface is well exposed, and dips 64° to the northwest (Figure 54a) . Slickenlines on the surface of the fault have an orientation of 068/12, and also suggest right-lateral motion. Vertical clastic dikes injected into the allochthon north of the fault strike between N15W-N3 5W (Figure 54b); similar features south of the fault strike N10E. These structures are also suggestive of relative northeast-southwest transport. Deformed playa sediments are exposed at a structurally higher level than the base of the allochthon 139 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1099' 1153T 1181T J159T 1077.0AT 1128T Francis Peak 1182T UA £ 1174T UA 1121T 167T 1106T 1146T 1059T UA LA 1149T 2-118/* 1176T ,1137T 1 km Figure 53. Location of area of Plate I being discussed in text. 140 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 54. (a) View east of well-exposed tear fault within the UA. (b) View north of fault in (a). Note dilational fractures oriented perpendicular to the fault plane. 141 within this drainage. This relationship is probably related to strike-slip motion along the fault, with the two blocks moving separately and trapping the sediments between them (see section B-B', Plate II, and section entitled "STRUCTURAL GEOLOGY"). A second possible tear fault is located approximately 1 km northwest of the first fault, and strikes variably east, northeast and ENE. The fault dips 40° to the southeast. Offset along this fault appears to be rightlateral, but simple strike-slip displacement does not account for the outcrop patterns of gneiss and diabase on either side of the fault. Thus, normal dip-slip may be the dominant displacement. Several small normal faults appear to have developed in conjunction with this particular fault. Approximately 500 m east of hill 1153T, several small half-graben have developed along normal faults that appear to be confined to the UA. These normal faults strike northwest, and intersect the larger northeast-striking fault at their northwestern termini. Lacustrine sediments appear to have been deposited on top of granite gneisses within the halfgraben, and both the gneisses and playa deposits are variously juxtaposed against Crystal Spring quartzites along the length of these faults, the faults seemingly acting as bounding limits to lacustrine sedimentation. 142 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Hence, the lacustrine sediments appear to postdate formation of these faults. Another extensional feature within the UA has been mapped in its western exposures. Northeast of hill 1106T, a shallow, west-dipping low-angle normal fault offsets older units and structures to the southwest (Figure 55) . The steep normal fault that juxtaposes granite gneisses (Xg1) and Crystal Spring rocks is repeated to the southwest in the hanging wall of this younger structure, which must have a displacement of about 300 m. This fault does not appear to affect the Tertiary sediments beneath the UA, but the base of the sheet is not exposed at this locality. It is not clear whether this normal fault is related to emplacement of the allochthon, but its shallow westward dip is suggestive of this. Provenance of Allochthons Lower Allochthon A potential source for the lowest allochthon in the study area (LA) lies 4 km to the southeast in the Halloran Hills area. The Paleozoic stratigraphy in this area may include the Ordovician through Mississippian carbonates found in the deposit on hill 1137T. The Halloran Hills Paleozoic carbonate section is a particularly attractive source, because the allochthonous carbonate units have experienced contact metamorphism and display the same 143 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1099 1153T 1181T 1077.0AT 1128T Francis Peak H82T UA &1174T UA 1121T 167T 1106T 1146T A 1059T UA LA 1149T 2-118A' 1176T i1137T 1 km F i g u r e 55. Location on Plate I of low-angle normal fault w i t h i n the UA. 144 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. general recrystallization and bleaching seen in the Halloran Hills section. The Halloran Hills metamorphism is attributable to intrusion by the composite Cretaceous Teutonia batholith (DeWitt, 1980). Other potential sources for this deposit include the country rocks of the Kingston Peak pluton and the Clark Mountains-Mescal Range area. However, the wall rocks around the Kingston Peak pluton at present consist of Precambrian and Cambrian units, and units younger than Ordovician are not now found in that area. It is conceivable that an intact Paleozoic stratigraphic section lay above the pluton subsequent to its emplacement, and then slid southward into the Shadow Valley basin, but such a scenario is unsupported by field evidence. Paleozoic carbonates in the Clark Mountains present a less likely source for this deposit. Mesozoic thrusting in this region has produced a sequence of imbricate slices of Paleozoic units, but with the exception of localized metamorphism around isolated stocks, the units in the Clark Mountains do not exhibit the bleached appearance seen in the LA at hill 1137T. The possibility also exists that the initial source terrane for the LA now lies buried beneath alluvium in an adjacent area. The only kinematic data found that could indicate a possible transport direction is a clastic dike present at the base of the allochthon. This dike has an 145 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. orientation of N48W 50SW and has a maximum width of 35 cm. Rotating the folded glide sheet back to horizontal gives the dike a northwest strike and gentle southwest dip. This suggests that transport was from either the northeast or southwest, but this clastic dike may have suffered some deformation during emplacement of the LA. Hence, a possible source for this deposit is the Paleozoic section in the Halloran Hills, but further constraints on the origin of the lower allochthon are not present. Middle Allochthons Figure 56 shows the distribution of Paleozoic carbonates autochthonous to the Shadow Valley Basin. The Bonanza King Formation crops out in all these localities (Burchfiel and Davis, 1988; Hewett, 1956; and Hazzard, 1937) . All of these exposures of the Bonanza King have been involved in Mesozoic thrusting. In the Clark Mountains and the Mescal Range, Mesozoic intrusive rocks are present in both the Mesquite Pass and Keaney/Mollusk Mine thrust plates. The deformational style and metamorphic fabrics in the Bonanza King Formation in the Clark Mountains area are similar to that seen in the allochthons. Considering the proposed west- to northwestdipping paleoslope and easterly source for the alluvial units above and below the allochthons, as well as the 146 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. N MM o CM MR 15 km Figure 56. Distribution of Paleozoic carbonates (shaded) autochthonous to the Shadow Mountains study area. R=Resting Springs Range, N=Nopah Range, MM=Mesquite M t n s ., CM=Clark Mountains, MR=Mescal Range, B=Baker, Calif. Box shows approximate location of study area. Unshaded enclosed areas are allochthonous carbonates within the Shadow Valley basin. 147 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. similarities mentioned above, these allochthons probably originated in the Clark Mountains/Mescal Range area. Shadow Mountain, located 6 km east of the eastern edge of the study area may have been the source of these middle allochthons. However, the Winters Pass thrust plate comprises the majority of Shadow Mountain (Davis, pers. comm.), and the deformation of Bonanza King units above this thrust at Winters Pass does not involve the type of interleaving and complex structural relationships seen in the Mesquite Pass thrust plate. Upper Allochthon Ductile deformation within the Precambrian sedimentary rocks, as well as ductile thrust fault contacts between Crystal Spring and Noonday units, are all suggestive of a source terrane for the UA within the Mesozoic fold and thrust belt. One and possibly two northeast-striking tear faults have been mapped within the upper allochthon. In the playa sediments, and locally in the unit THa3, northwest-trending folds have been mapped (Plate I; see structural geology). These folds are upright and asymmetric. The structural data mentioned above are highly indicative of transport of the UA toward the southwest, from a northeastern source within the fold and thrust belt. The most likely source for this allochthon in the 148 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. study area is Shadow Mountain, located approximately 8 km northeast of the Shadow Mountains (Figure 57). At Shadow Mountain, all Precambrian units found within the UA are present with the important exception of the Noonday Dolomite, but Noonday strata were probably present prior to erosion and denudation by gravity-driven slide sheets. Most of the Shadow Mountain assemblage has been mapped, and is believed to lie in the upper plate of the Winters Pass thrust (K. A. Fowler, in progress; Davis and Burchfiel, 1971; and G. A. Davis, unpub. mapping). Hence, Shadow Mountain is the most viable source for the UA in the southern Shadow Mountains. Noonday rocks in the UA were not derived from as far east as the Clark Mountains thrust complex. Here, Noonday carbonates either rest unconformably on Precambrian basement rocks (the general case), or, locally (near the northern Cima Road), on the Kingston Peak Formation, the highest unit in the Pahrump Group (G. A. Davis, pers. comm., 1990). If Shadow Mountain is indeed the source terrane for the UA, then it must have been exposed over a much larger area at the time of emplacement of the UA. Current exposures at Shadow Mountain cannot account for the initial areal extent of the UA (>60 km2 prior to subsequent deformation) . Based on the presence of the Noonday Dolomite and the Crystal Springs Formation in the upper allochthon, a southerly source for this allochthon is not likely. 149 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. NV KR CA MM SM CM SH MR Squaw Mtn HH Baker Figure 57. Spatial relationship between the Shadow Mountains and the probable source terrane for the UA at Shadow Mountain. Arrow shows inferred transport direction. 150 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Although the original depositional environment of the Pahrump Group sediments extends well south of the Silurian Hills (Wright and others, 1974; G. A. Davis, pers. comm., 1990), the southernmost extent of Pahrump Group sediments associated with Noonday outcrops would appear to be the allochthonous deposits found in the Shadow Mountains (Stewart and others, 1974). The southernmost outcrops of the platform facies of the Noonday Dolomite occur at Valley Wells within a small allochthonous sheet of Noonday and granitic gneiss (north of the interstate and west of the Cima Road, G. A. Davis, pers. comm., 1990), but the absence of Pahrump Group sediments at Valley Wells suggests that a source terrane for the UA north of the present location of the Halloran Hills seems likely. These sedimentary units are not known to occur together anywhere south of the study area. Analogous Deposits Deposits of megabreccia and glide blocks are common in the Tertiary basins of the western United States. Allochthonous sheets of older rock units have been reported in the Tertiary sediments in the Snake Range (Miller and Gans, 1983), the Daggett terrane around Barstow, California (Dokka, 1986), the Sacramento Mountains (Miller and John, 1988), the Black Mountains (Longwell, 1951; Yarnold and Lombard, 1989), and various 151 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. other Tertiary basins. These basins have several features in common. They all formed in semiarid to arid, intermontane settings, and most importantly, they all reside in the upper plate of a detachment fault or fault system. Although the morphology of these deposits has been described in some detail (e. g. Yarnold and Lombard, 1989), little attention has been given to either their genetic relationship to detachment faulting or their emplacement mechanisms. On the other hand, landslides and rock avalanches of historic and even prehistoric age are well documented, because of their potential threat to human populations and property. These rock avalanches could be analogous to the carbonate megabreccias found in the Shadow Mountains Tertiary section. Some examples of large rock avalanches are the Tin Mountain landslide (Burchfiel, 1966), the Blackhawk landslide (Shreve, 1968), the Saidmarreh landslide (Watson and Wright, 1969), the Elm landslide (Hsu, 1975), the Frank landslide (Daly and others, 1912)) and the Sherman landslide (Shreve, 1966). Huge nonterrestrial rock avalanches have been documented on Mars and the moon by Melosh (1986). Table 2 summarizes the features of some of the larger landslides mentioned above. 152 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. TABLE 2. Deposits Analogous Allochthons. LANDSLIDE Elm Sherman Frank Silver Reef Blackhawk Saidmarreh SOURCE undermined cliff undercut dip slope undercut thrust block undercut thrust block undercut thrust block undercut dip slope to the Shadow Mountains VOLUME (cu ft) LiTHOLOGY 0.4xl09 slate 1x10 9 1.3xl09 8xl09 lOxlO9 150xl09 graywacke limestone limestone limestone limestone (from Shreve. 1968) 15 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Emplacement of Allochthonous Sheets Various mechanisms have been proposed to explain the emplacement of large rock, avalanches over long horizontal distances, since a characteristic feature of these rock avalanches seems to be an extremely low coefficient of friction. Many of the rock avalanches traveled a remarkable "runout" distance over gentle slopes from their original source. Among the proposed explanations for the apparent low coefficient of friction are air-layer lubrication beneath the deposits (Shreve, 1968) , highly lubricating lithologies beneath the deposit (e.g. gypsum in the case of Saidmarreh; Watson and Wright, 1969), frictional heating resulting in vaporization of fluids (Goguel, 1978) or even localized melting of rock units (Koefels, 1979) and acoustic fluidization (explains large runout in the absence of water or an atmosphere; Melosh, 1986, 1987) . The role of water-saturated sediments beneath terrestrial deposits is also important in accommodating transport, but water seems to have been widely neglected in the description of transport mechanisms, possibly because it provides an overly simplistic, albeit feasible, explanation. The emplacement mechanisms discussed herein are in part based on the literature pertaining to large rock avalanches. Although rock avalanches could be analogous to the carbonate megabreccias in the Shadow Mountains, 154 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. several important distinctions need to be made between these deposits and the Shadow Mountains allochthons. Most descriptions of rock avalanches report initial failure of an oversteepened slope as the primary causal mechanism of emplacement (e.g. Tin Mountain landslide; Burchfiel, 1966) . Following slope failure, many of these deposits experience a period of "free fall", in which the deposit is airborne prior to impacting slopes at lower elevations. This free fall period allows for the conversion of massive amounts of potential energy into kinetic energy, as the rock debris moves unimpeded under the influence of gravity. Two important consequences arise from the free fall of the rock debris. The large amount of kinetic energy provides extra impetus for downslope motion of a rock avalanche, and may explain how these deposits appear to "climb" topography during their emplacement (c.f. Elm landslide; Hsu, 1975). The second consequence is important in distinguishing rock avalanches from allochthonous sheets. A period of free fall during which excess kinetic energy is amassed would result in a significant amount of deformation of the rock material upon its initial impact with lower slopes. Even though a rock avalanche may initiate as an intact body of rock, the landing on lower slopes would result in shattering and overall destruction of the intact body. This effect is 155 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. well documented in the literature on rock avalanches, and most of the mechanisms listed above have been formulated to explain this "fluidization" of the rock material. On the other hand, the thicker (>50 m) allochthons in the Shadow Mountains maintain a high degree of internal coherence, with shattering and comminution of rock fragments generally limited to the base of the sheets. Allochthons appear to slide, rather than flow down slope. The basal occurrences of these brecciated rocks, the large volume of intact lithologies, and the impressive areal extent of these allochthons strongly suggests that free fall dynamics were not an integral part of their emplacement history. In developing a model for the origin and emplacement of the Shadow Mountains allochthons, emphasis has been placed on the upper allochthon, since its provenance is better constrained, and structural and stratigraphic control are more evident than within the other three allochthons in this area. Initial Breakaway of Allochthons The widespread occurrence of allochthonous sheets within detachment-related terranes suggests a genetic relationship between these deposits and detachment faults. The occurrence of the Shadow Mountains Tertiary section within the upper plate of an inferred Halloran Hills or 156 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Kingston Range detachment fault (see section on structural geology) is a case in point. The current state of knowledge on the evolution of detachment faults suggests that the "daylighting" of a detachment fault or fault system occurs within a breakaway zone characterized by nonextended footwall rocks in the headwall of this zone. Hanging wall rocks become progressively extended and rotated along listric normal faults (Figure 58; Wernicke, 1988). Isostatic uplift of the extending breakaway zone results in the rotation of fault-bounded slices of units as the extending upper plate migrates away from this zone of increasing structural and topographic relief. It is upon these slices and probably the detachment fault surface that upper plate sedimentary basins are deposited. The initial breakaway of allochthonous sheets within their source terranes is postulated to occur as a result of tectonic slivers spalling off of the nonextended headwall in response to rapid sedimentary unloading. These slivers are emplaced into the sedimentary basin that has been deposited within the extending breakaway zone of the detachment system. Invoking a breakaway-zone origin for the allochthons requires that several structural criteria be met. The most important of these are: (a) that only a slight amount of rotation of the allochthons has occurred, 157 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. a BEFORE EXTENSION -supracru.stal rocks ‘ mid-crustal datum ; b BEFORE ISOSTATIC REBOUND Moho—. --------- ----- *———---------------------------------- ----------------- j Figure 58. Schematic representation of extension in the breakaway zone of a detachment fault. Breakaway zone fills with sediment derived from the unextended headwall. Sedimentary unloading allows allochthons to spall off of the headwall and slide into the basinal sediments. 158 (b) that the allochthons originated at a high structural level in the source terrane, and (c) that brittle faulting was responsible for their formation. Evidence that meets these criteria is present within the upper allochthon in the Shadow Mountains. Matching the UA to its proposed source terrane at Shadow Mountain reveals that although some rotation about a vertical axis probably occurred, negligible rotation about a horizontal axis has affected this allochthon. A "best fit" reconstruction would place the eastern exposures of the UA above exposures of the granitic gneisses, diabase and Crystal Spring Formation exposed just north and east of the peak labeled "Shadow Mountain" (Figure 59). The basis for this conclusion comes from a comparison of dips within the Crystal Spring Formation at both localities. At Shadow Mountain the average strike and dip of beds within the Crystal Spring Formation is approximately N5W 71°NE (with dips ranging from 50-95°) , whereas the Crystal Spring Formation within the upper allochthon has an average strike and dip of N70W 44°NE (ranging from 2 0-71°) . Since the base of the UA in its Shadow Mountain source terrane is assumed to have dipped gently toward the west, several different structural geometries can be invoked to alleviate the disparity in the orientation of the Crystal Spring strata between 159 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 59. (a) View north of Shadow Mtn. showing the sequence of granitic gneisses, diabase and Crystal Spring that are also found in the UA in the Shadow Mountains. (b) Line drawing of (a). 160 Shadow Mountain and the upper allochthon in the Shadow Mountains. Possibly the simplest explanation for the shallowing of dips within the Crystal Spring from Shadow Mountain to the UA is that structural complexities were present at the structural level at which the UA originated (above the present topographic surface at Shadow Mountain). This scenario would allow the base of the UA in its Shadow Mountains source terrane to originate at a shallow westdipping orientation. Assuming that structural complications were present at Shadow Mountain, a range of initial dips for the base of the UA can thus be envisioned. A second possible geometric relationship can be constructed by rotating the Winters Pass thrust at Shadow Mountain (Figure 60). Where this thrust has been mapped at Shadow Mountain, it has an average dip of 15° east (G. A. Davis, pers. comm., 1990). At Winters Pass in the western Mesquite Mountains, the Winters Pass thrust dips 3° west where it occurs in the footwall of the Kingston Range detachment, and at Shadow Mountain, the same fault dips 15° west (Burchfiel and Davis, 1988). It is possible that the Winters Pass thrust at Shadow Mountain was rotated 3 0° toward the east, subsequent to the emplacement of the upper allochthon into the Shadow Valley basin to the west (see Figure 60). By rotating Shadow Mountain 161 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 60. Reconstruction of possible relationships between the UA, bedding in the Crystal Spring Formation, and a possible rotated winters Pass (WP) thrust at Shadow Mountain. By rotating the WP thrust 30° to the west. Crystal Spring strata within the UA can be matched to similar units at Shadow Mountain, thereby accomodating a shallow initial westward dip at the base of the UA. 162 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. such that the Winters Pass thrust dips 15° west, it is possible to reconcile the dips of the Crystal Spring Formation at Shadow Mountain and within the UA to within 5-10° of each other. This geometry allows the base of the UA in its Shadow Mountain source terrane to dip gently toward the west with little subsequent horizontal rotation of the allochthon during its emplacement. At the present time, sufficient structural control is not available to determine which of these scenarios may be more accurate. Evidence that the UA in the Shadow Mountains must be derived from a relatively high level within the fold-andthrust belt has been presented in the section on Mesozoic deformation of this sheet. The presence of the Noonday Dolomite within the upper allochthon in the Shadow Mountains and the absence of this same formation at Shadow Mountain also suggests that the allochthon originated at higher structural levels than are currently exposed at Shadow Mountain. The inclusion of the structural data discussed above regarding rotation of the sheet into its current spatial arrangement also implies that the source of the UA lay well above the current level of exposure seen at Shadow Mountain. Evidence for a brittle origin for the upper allochthon is inherent in its presence within a Tertiary clastic sequence. The presence of exclusively brittle structures within the UA that can be related to its 163 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. emplacement is further support for an emplacement history characterized by brittle deformation. The detachment of a large sheet of intact crystalline and sedimentary rocks (the upper allochthon) from a source terrane composed of similar lithologies (Shadow Mountain) is thereby explained by the presence of nearly horizontal or shallowly dipping brittle Tertiary surfaces at Shadow Mountain. These surfaces were tectonically uplifted and possibly warped during isostatic rebound of a possible breakaway zone of the Halloran Hills detachment fault on the west side of Shadow Mountain. Since these surfaces occurred at high structural levels within the breakaway zone, the motion of the allochthon out of its source terrane and onto the clastic sediments of an evolving basin in the hanging wall of the HHD can possibly be explained by gravitational and body forces. Since a brittle surface appears to have formed the initial basal contact of the UA, it seems likely that a zone of fractured, sheared and brecciated rock probably existed along its trace. Hence a preexisting plane of weakness, possibly an inverted, reactivated Mesozoic thrust fault, accompanied by a lubricating layer of fragmented rock allowed the allochthon to move across Shadow Mountain into the evolving Shadow Valley basin. One such normalfault/reactivated thrust is preserved today above the 164 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Winter's Pass thrust at Shadow Mountain (Burchfiel and Davis, 1971; their figure 7; Fowler, 1992). Formation of the Basal Friction Carpet The presence of a zone of thoroughly shattered and brecciated rock along the basal exposures of the Shadow Mountains allochthons has been documented in an earlier section. The occurrences of these friction carpets seem to be related to the thickness of the allochthons, the composition of the units that comprise the allochthons, and the composition of the substrate. Thin allochthonous sheets tend to be thoroughly shattered (LA and MA2) , allochthons composed of carbonates are more shattered than the UA (composed of crystalline and well-indurated clastic sedimentary units), and allochthons that have traveled extensively across alluvial fans are more disrupted than the UA, which has moved extensively across playa sediments, but presumably had to move down a fan before reaching the playa. Integrating all of these variables into a model for the formation of a basal friction carpet would be a difficult task beyond the scope of this report. Hence the model proposed herein for the development of a basal friction carpet is for the general case of an intact sheet of unspecified composition moving down the slope of a typical alluvial fan (5-10° dip) . 165 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. In the previous section, an attempt was made to argue for the presence of brecciated rock along the basal surfaces of the allochthons prior to any motion of these sheets away from their source terranes. Once the allochthons started to move down into the basin under the influence of gravity, these brecciated lithologies would become further fragmented in order to accommodate shear stress along the surface between the allochthons and their bedrock source. Hence most of the shear stress is concentrated within the breccia zone (Figure 61). A consequence of concentrating the shear stress within the breccias is that the leading edge, or toe, of the intact allochthon might eventually slide off the top of the brecciated zone and rest on a surface of either solid bedrock or alluvial substrate (depending on how far the allochthon had moved when this occurred). At this point, the lubricating layer of breccia is no longer present, and the shear stress now becomes concentrated in the base of the allochthon. It is believed that stress is concentrated in the allochthon rather than in the substrate because, in a manner similar to a fluid, the velocity and momentum are concentrated in the moving body, not the stationary base over which it moves. Normal forces and momentum of the sliding block are such that motion does not cease at this point, but new breccias are created along the basal contact beneath the 166 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure cl. Diagram shewing she progressive deformation of an allcchsncn and formation of she hasal fricsion caroes as is slides across a fixed subssrase. a; As she ailochshon stoves, iss ensire base becomes brecc laced, thereby allowing fursher ease in sieving downslope end er she influence cf gravisy. '.a; As she al lochshon cons m u e s so move, she basal fricsion carpes is lefs o e h m d , anc r.sw cr eccias are formed. (c) At some p e m s she ins=cs cap os ~ n s s . ^ lochshon may slide off she fror.s end of she breccia car P. (11 LI <D nd become a new source for comminuted fragmenss cf sne cic "■* ■_<- a W • * * « ^ ^ ^ w c ■ ^ ^ between she friction sarpes and she m s a c s cap need net be a sharp one; evidence was seen in she s ieic for bosh a gradual breccia development eve r a wide zone cf de formation and for sharp fault contacts be tween she cap and she b r e c c i a s . 167 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. I toe of the allochthon. As the sheet slides over these new breccias, a trail of older breccias will be left behind. This process continues until the shear forces no longer exceed the frictional forces at the base of the allochthon, and it eventually comes to rest in the basin. The downslope momentum of the sliding block depends in part on the velocity of the block. It appears that the allochthons traveled rapidly downslope, since debris shed from the allochthons is not present in the sediments overriden by them. However, accurate estimates of actual rates cannot be made at this time. Evidence that this type of mechanism does occur in the formation of friction carpets is recorded in the allochthons of the Shadow Mountains. Beneath the well exposed base of MA2 in Evening Star wash, motion has obviously occurred at the interface beneath the allochthon and the alluvial unit TMa2 beneath it. The base of the allochthon is planar at this locality, and a thin (2 cm) layer of fault gouge marked by slickenlines occurs at the interface. Although deformation of the sediments of TMa2 has not been documented, it seems likely that the upper uppermost few meters of alluvial sediment have been rotated, mixed and generally disrupted. Mixing of the two deposits has not taken place, but the brecciated carbonates that comprise this allochthon have obviously moved along this surface. 168 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Along the basal exposures of the allochthon MA,, the basal friction carpet is highly discontinuous. The friction carpet is absent along this surface for distances as great as several hundred meters. The current spatial arrangement of intact lithologies and friction carpet may be recording the final increments of motion within this allochthon. Role of Fluids Fluids in the form of water-saturated sediments were present during the emplacement of each of the allochthons in the Shadow Mountains. The presence of clastic dikes at the base of the various allochthons, composed primarily of fine-grained muds (silts) strongly suggests that water was present within the sedimentary substrates, and played an important role in facilitating downslope motion of the allochthons. In the case of the upper allochthon, it is possible that water was present in the form of a lake, and this allochthon actually "hydroplaned" across the surface of the lake. Further analysis of the playa sediments beneath the UA is necessary before it can be determined whether standing water was present at the time of its emplacement. The fact that brecciation of the allochthons occurred in spite of the presence of water suggests that the clastic dikes may record the final stages of motion of these sheets, when excessive loading of the underlying 169 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. sediments would force the fine-grained material into the overlying allochthons. Similarly, it is possible that the load applied to the sedimentary substrates forced the fluids to migrate to the interfaces between the allochthons and sediments, thereby providing an essentially frictionless surface upon which sliding could occur. The presence of water would have contributed to decreasing friction (and a lowering of effective normal stresses) at the sediment-allochthon interface, thereby allowing the large sheets to move farther downslope than would be possible in the absence of water-rich sediments. 170 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. STRUCTURAL GEOLOGY General Statement The discussion within this section is confined to structures that affect the Shadow Valley basin sequence, thereby omitting any discussion of the structures within the allochthons. As such, the pertinent features include deformation of sediments below the upper allochthon (accidental structures) , syn- and/or post-basinal structures, and local and regional extension vis a vis the Halloran Hills detachment fault. Accidental Structures Within the study area, the mapped exposures of the upper allochthon rest almost exclusively on lacustrine clays and silts (TMl) . These sediments have been highly deformed beneath the allochthon and are best exposed southeast of hill 1137T and northeast along the drainage 1077. OAT. The deformation is such that these units were, in all likelihood, nonlithified and water-saturated at the time of emplacement of the allochthon. This hypothesis is supported by the relationship between the eastern edge of the UA (tail) and the fine-grained sediments of TMl. North and south of the drainage marked 1077. OAT, the UA and fine lacustrine sediments are juxtaposed along a contact that resembles a buttress unconformity. Closer inspection of this contact has revealed a minor amount of shearing 171 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. within the playa sediments at the interface between the two. The apparent relationship is that UA literally sank into the unconsolidated lacustrine sediments when it finally stopped moving across the playa. The deformation along this contact is basically soft sediment deformation, and is a direct result of the UA "wallowing in the mud" (Figure 62). Deformed bedding within Tml generally strikes northwest and the deformation has formed asymmetric nearly upright synclines and anticlines, which are important kinematic indicators for transport direction of the UA across these sediments; folds are asymmetric with steep limbs formed in the direction of transport, and shallow limbs dipping toward the source of the allochthon. It is assumed that where complete folds are not mappable due to the deep weathering of these deposits, the sediments are similarly deformed. The playa deposits locally have formed clastic dikes in the base of the UA. Seven dikes measured have an average strike of N20W and a strike range of N10E to N38W. They are generally close to vertical in dip. Syn- and/or Post-basinal Rotation Within the Shadow Mountains sequence, some evidence exists to suggest that a progressive rotation of the basin was occurring during the deposition of the units that 172 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 62. View toward the south of the "tail" of the UA (to the right of the photo) exposed at higher structural levels than the playa sediments into which it has settled (to the left and beneath the alluvial veneer). 173 comprise this part of the Shadow Valley basin (see section A-A', Plate II). In a very general sense, the basin fill in the Shadow Mountains shows a progressive shallowing of dips from west to east and from the lowest to highest exposed deposits. Dips in the alluvial unit TMa1 dip an average of 40° NE. Sediments of TMa2 exposed in Evening Star wash dip 19° NE, and the base of MA1 dips an average of 20° NE. Within the unit TMa3 the progressive shallowing of dips upsection is fairly evident. Basal exposures of this unit dip toward the northeast between 23-40° NE, whereas the highest exposures of the unit dip a modest five degrees. Although the shallowing of dips within this sequence is not systematic, there is a suggestion of syndepositional rotation of the these units. Whether this rotation is a result of extension during basin formation or simply due to local depositional inconsistencies or to subsequent deformation is not clear. However, it appears as though the bulk of this part of the basin had been rotated prior to the emplacement of the upper allochthon. Where exposed, the base of the allochthon is essentially horizontal, whereas the sediments below it dip as much as 4 3° to the northeast. Faults responsible for the northeast-dipping section have not been identified within the study area; most of the normal faults affecting the section have very steep dips, 174 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. and rotation of strata cannot be attributed to offset along them. Post-basinal Oblique-slip Faulting As mentioned earlier, the entire Shadow Mountains stratigraphic sequence has been repeated to the southwest by a large northwest-striking normal fault. Where exposed at one locality, this fault is steep, dipping 75° SW (Figure 63). Crystalline units of the UA and playa sediments in the hanging wall block are juxtaposed against the unit TMa2 and MA, along the length of this fault. The fault is offset 400 m to the west by a dextral strike-slip fault just north of the northern end of the mapped exposures of the hanging wall UA. The southern trace of this fault becomes buried beneath older alluvium approximately 1.5 km NW of hill 1137T. The throw on this fault is approximately 600 m based on the apparent offset of the base of the upper allochthon. Mapping by Bishop and Davis to the south and north of the study area, respectively, indicate that the length of this fault is in excess of 3 0 km. Offset along this fault appears to have a right-slip component, because the eastern exposures of the UA occur one km farther to the south than the apparent offset equivalents of the UA to the west of the fault. This fault may have occurred very late in the history of this 175 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. sr-r 176 area, and could be related to regional right-slip displacement along the southern Death Valley fault zone. A second episode of normal faulting, possibly related to the large normal fault described above, has locally affected parts of the Shadow Mountains sequence. These faults are most evident in the southwest part of the study area, where down-to-the-west repetitions of breccias and the LA have occurred along steeply west-dipping faults. The offset on these faults varies, but does not exceed 100 m. The largest of these normal faults has offset the LA to the west. This fault, which is proposed to be buried in the wash just east of hill 1137T, strikes northwest and has a length of approximately 2 km. It strikes approximately parallel to the large normal fault. Other, smaller faults are found north and east of this fault. The reason these faults are thought to involve some component of strike-slip displacement is their en echelon nature and the presence of en echelon folds located adjacent to the fault traces (Figure 64). Folding in this part of the study area is thought to reflect strike-slip offset along the various normal faults in this area. The LA is warped into a broad anticline that could have formed in response to the large, basin-repeating normal fault that is projected through the alluvium to the west of the LA (Plate I). Another small anticline has been mapped just south of the offset of the big fault. This 177 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. 1099T 11597 rrancis PeaK 1121T -O ' 167T 1106T 1059T i i46t .i 2-119A 1176T 1 km Figure 64. Location on Plate I of large normal faults with associated anticlinal folds. 178 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. northwest-trending fold, defined by warping of the base of various breccias, appears to be related to offset on the basin-repeating normal fault. As all of the folds involving Tertiary sediments are oblique to the strike of major normal faults, it is proposed that they formed as a result of these faults. Other small normal fault offsets have been mapped in the northwestern part of the study area. North of Evening Star wash, and hill 1099T, several small normal faults have progressively dropped the base of the allochthon MA2 down to the west (Figure 65) . These small faults are very close to the inferred trace of the big normal fault, and apparently are related to its development. Two small offsets in the base of the allochthon MA, have been mapped approximately 900 m northwest of Francis Peak. These faults are at a high angle to most other extensional faults within the study area, and their relative ages cannot be constrained except to say that they post-date emplacement of the allochthon. Post-Basinal Strike-slip Faulting One strike-slip fault that post-dates formation of the Shadow Mountains sequence has been mapped within the study area. This fault, which is located at the southeastern end of the area, strikes northwest and offsets the UA, lacustrine sediments, a large landslide 179 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. Figure 65. (a) View toward the north of normal faults offsetting the base of MA2 in the through canyon of Evening Star wash. (b) Line drawing of (a). 180 breccia and the oblique-slip, basin-repeating normal fault in a right-lateral sense (Plate I). The total offset along this fault is 400 m. It can be traced discontinuously to the west, where it becomes buried in the wash one km north of hill 1106T. Its eastern trace is also buried beneath older alluvium 400 m ESE of hill 1146T. The along strike length of this fault is approximately 5 km. The relationship of this fault to the rest of the study area is unclear, in that it has formed at a high angle to other structures related to the evolution of the area. The Halloran Hills Detachment Fault Within the Shadow Mountains study area, steeply westdipping normal faults and moderately to gently northeastdipping Tertiary strata do not indicate that this area lies in the upper plate of a regional, southwest- to westdipping detachment fault. However, when this area is placed within the context of a regional framework that includes the greater Shadow Valley basin, structural relationships clearly point to the presence of a Halloran Hills detachment fault beneath the southern Shadow Mountains. The relative nonrotation of the Shadow Mountains section can be attributed to a rather central location within the upper plate of this fault and, 181 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. possibly, to its late development in the evolution of the Halloran Hills detachment fault system. Southwest of the study area, Bishop (K.M. Bishop, in progress) reports that the lower exposures of the Shadow Mountains section can be traced for approximately 2 Jon south of the southern boundary of the area mapped in this study. Similarly, the section can be traced north of the study area along strike for another 6 km (G. A. Davis, pers. comm., 1990). Dips within this relatively nondisrupted section show a progressive shallowing from south in the Halloran Hills, to several km north of the area mapped in this study. Farther north, in the vicinity of Kingston Wash, G. A. Davis (in progress) reports that Tertiary strata become highly folded and disrupted and locally dip as steeply as 90°. Davis (pers. comm., 1991) has suggested that the contractional deformation in this area largely post-dates the structures mapped in the Shadow Mountains study area. In the area east of the Shadow Mountains and west of Shadow Mountain, Ken Fowler has mapped a normal faulted and steeply rotated Tertiary section within the Kingston Range allochthon that he believes largely predates the Shadow Mountains Tertiary section (Fowler, pers. comm., 1991). His impression from working in this area is that the Shadow Mountains section was deposited unconformably on this older section, subsequent to (at least part of) 182 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. its deformation. In the are around Valley Wells, just northwest of where the Cima Road intersects Interstate 15, G. A. Davis (pers. comm., 1991) reports the presence of steeply east-dipping Tertiary strata (as great as 80-85°) unconformably overlain by a gently-dipping (10-3 0°) Tertiary section. These sedimentary relationships suggest that the Shadow Valley basin has had a protracted history of sedimentation, rotation of strata, erosion, sedimentation and further rotation. It is noteworthy that the majority of these sediments dip to the east; they have presumably been rotated along west-dipping normal faults. Other regional considerations favoring a Halloran Hills detachment fault include preserved breakaway zones for the HHD at Mesquite Pass, north of the Clark Mountains, and in the Mescal Range to the south (G.A. Davis, pers. comm., 1992; T. Brudos, in progress), and steeply dipping (vertical?) Tertiary strata at Halloran Summit (Davis, p.c., 1991) whose basal exposures are apparently truncated by a subhorizontal fault surface delineated by exotic quartzite breccias. In light of the information presented above, it is proposed that the Shadow Mountains Tertiary section lies in the upper plate of the HHD. These sediments record the development of a second phase of basin evolution superposed on an older, highly disrupted Tertiary section (i.e. a section deposited on Halloran Hills-type basement 183 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. consisting of TQM + miogeoclinal strata and exposed north and south of this study area in areas currently being mapped by Davis and Bishop, respectively). The apparent lack of rotation of these sediments can be explained by their late deposition. The Shadow Mountains section may also be far removed from the "edges" of the HHD; steeplydipping Tertiary strata at Halloran Summit are located in the upper plate of the HHD, very close to the proposed southern boundary of this detachment fault. It is also possible that the HHD beneath the Shadow Mountains section is several km below the current level of exposure. 184 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. EVOLUTION OF THE SHADOW VALLEY BASIN The greater Shadow Valley basin area has been affected by Cenozoic crustal extension dominated by the development of a regional, west- to southwest-rooting detachment fault system. Northeast of the study area, extension occurred above the Kingston Range detachment fault. In regions south of the Kingston Range, the Halloran Hills detachment fault has dominated the Cenozoic evolution of the basin. Hence a discussion of basinal evolution must be placed in the framework of the Halloran Hills detachment fault. Work in progress strongly suggests that these two faults are part of the same detachment system. Cenozoic extension in this region occurred along a north-to-south progression, beginning with the development of the KRD. The breakaway zone for this fault is located in the Mesquite Mountains, east of the Kingston Range (Figure 66). Attenuation of the extending headwall of this fault produced a localized area of negative topography, thereby lowering the local sedimentary base level. In the Shadow Mountains study area, evidence for sediments with a southerly provenance (indicative of a northerly transport direction) have been identified in the lowest Tertiary unit exposed, THa1. These coarse alluvial 185 Reproduced with permission of the copyright owner. Further reproduction prohibited without permission. NV CA MM SM SMS CM Ste//n*e* t i L A i o

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Creator Parke, Mary Alice (author)

Core Title Geology and structural evolution of the southern Shadow Mountains, San Bernardino County, California

School Graduate School

Degree Master of Science

Degree Program Geological Sciences

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag Geology,OAI-PMH Harvest

Language English

Contributor Digitized by ProQuest (provenance)

Advisor Davis, Gregory (committee chair), Burbank, Douglas (committee member), Paterson, Scott (committee member)

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c16-18206

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Rights Parke, Mary Alice

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Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

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Repository Location USC Digital Library, University of Southern California, University Park Campus, Los Angeles, California 90089, USA