Depositional systems of the mid-Tertiary Gene Canyon and Copper Basin Formations, eastern Whipple Mountains, California

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Content DEPOSITIONAL SYSTEMS OF THE MID-TERTIARY GENE CANYON AND COPPER BASIN FORMATIONS, EASTERN WHIPPLE MOUNTAINS, CALIFORNIA by Thomas Daniel Mason 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 1985 UMI Number: EP58757 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. Dissertation Publishing UMI EP58757 Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106- 1346 UNIVERSITY OF SOUTHERN CALIFORNIA THE GRADUATE SCHOOL UNIVERSITY PARK LOS ANGELES. CALIFORNIA 90007 This thesis, w ritten by Thomas Daniel Mason under the direction of h.?:?....Thesis Com m ittee, and approved by a ll its members, has been p re sented to and accepted by the D ean of The G raduate School, in p a rtia l fu lfillm e n t of the requirements fo r the degree of ......Masten..Q£..Scienca.................. Dean THESIS COMMITTEE ACKNOWLEDGEMENTS This study was supported in part by a graduate research award from the Department of Geological Sciences, University of Southern California. Additional assistance was provided by the sedimentary petrology and sedimentology lab at the University of Southern California. My sincere thanks and appreciation go to Dr. Robert H. Osborne for introducing me to a very interesting sedimentation and tectonics problem. His continued encouragement and technical assistance were invaluable. I would also like to thank Drs. Gregory Davis and Lawford Anderson for their insightful discussions concerning the structural and petrologic evolution of the Whipple Mountains region. Finally, I wish to thank my wife, Jane. Her assistance in the field was indispensable, and her continued encouragement, patience and support greatly contributed to the completion of this project. ii ABSTRACT The Whipple Mountains occur within a tectonic terrane characterized by metamorphic core complexes and mid- Tertiary extensional tectonic features. Portions of the range are mantled by volcanic and sedimentary strata of the Gene Canyon and Copper Basin Formations, which were deposited syntectonically in intermontaine basins. These basins developed as a result of extensional Oligo-Miocene detachment faulting and associated high-angle normal faulting in the upper plate of the Whipple Mountain detachment complex. These deposits have, therefore, recorded the mid-Tertiary tectonic history of this region. Eight sedimentary and two volcanic lithofacies in the easternmost part of the range are defined in terms of their physical characteristics and principal sequence of subfacies, as determined by Markovian statistical techniques. Their depositional environments range from the apex of an alluvial fan to distal playa lake environments associated with an alluvial plain. These lithofacies are grouped into proximal, medial, and distal assemblages, whose lateral and temporal variations reflect relative tectonic changes between the source terrane and depositional basin. The sediment dispersal systems were determined from paleocurrent data, compositional and textural trends, and the distribution of lithofacies mosaics; and are related to the different tectonic models developed for the Whipple Mountains. The Gene Canyon Formation consists of a wide range of lithofacies which exhibit rapid lateral and vertical variation. The primary sediment dispersal systems were directed towards the southeast, probably along the longitudinal axis of the depositional basin. Local areas of high relief to the east and southwest were the major source of monolithic talus and debris-flow deposits. The younger Copper Basin Formation consists of rythmically-bedded, predominantly mid-alluvial fan deposits, with relatively minor vertical and lateral variations, as compared to the Gene Canyon Formation. The major sediment dispersal systems were similarly directed towards the southeast, along the longitudinal axis of the basin floor. The marked change in depositional style from variable and laterally discontinuous lithofacies to rythmically- bedded deposits first occurs in the upper part of the Gene Canyon Formation. In addition, an eastward shift in paleocurrent direction was noted at this stratigraphic position. These features suggest a change in the local tectonic setting and may reflect the initiation of detachment-related deformation. Incremental movement along the detachment fault and associated high-angle normal faulting of the upper-plate resulted in the rhythmic deposition characteristic of the Copper Basin Formation. Subsequent isostatic uplift was the final stage of detachment-related deformation. TABLE OF CONTENTS PAGE ACKNOWLEDGEMENTS i i ABSTRACT................................................ iii LIST OF FIGURES.........................................ix LIST OF TABLES.........................................xiii INTRODUCTION ............................................. 1 General Statement .................................... 1 Location of Study Area................................. 4 Purpose and Scope .................................... 4 Geologic Setting .................................... 7 Previous Work.........................................12 Methods of S t u d y ..................... 14 LITHOFACIES ANALYSIS.....................................17 Introduction.........................................17 Boulder Breccia Lithofacies..........................24 Lithofacies Description............................ 24 Environmental Interpretation......................30 Massive Conglomerate Lithofacies......................32 Lithofacies Description............................ 32 Environmental Interpretation......................43 Channeled Conglomerate and Sandstone Lithofacies. . .46 Lithofacies Description............................ 46 Environmental Interpretation......................57 Alternating Conglomerate and Sandstone Lithofacies. .59 Lithofacies Description............................ 59 Environmental Interpretation......................70 Diffuse Sandy Conglomerate Lithofacies............... 72 Lithofacies Description............................ 72 Environmental Interpretation......................78 Pebbly Sandstone Lithofacies.......................... 84 Lithofacies Description............................ 84 Environmental Interpretation......................88 Interbedded Fine Sandstone and Siltstone Lithofacies.97 Lithofacies Description............................ 97 Environmental Interpretation ................... 109 Calcareous Siltstone and Mudstone Lithofacies . . . Ill Lithofacies Description........................... Ill Environmental Interpretation ................... 116 Volcanic and Volcaniclastic Lithofacies ........... 118 Lithofacies Description........................... 118 Environmental Interpretation ................... 121 STRATIGRAPHY ........................................... 122 Introduction.......................................... 122 Gene Canyon Formation .............................. 129 Desilt Wash Section............................... 133 Parker Dam School Section.........................139 Red Mountain Section ............................ 145 Copper Basin Formation............................... 152 Parker Dam School Section.........................156 Reconnaissance Sections........................... 160 Section A ......................................160 Section B ......................................164 Section C ......................................165 Osborne Wash Formation ............................ 166 Textural Analysis .................................. 168 Introduction .................................... 168 Discussion of Results............................. 169 Petrography.......................................... 176 Introduction .................................... 176 Discussion of Results............................. 182 Diagenesis........................................189 DEPOSITIONAL MODEL.......................................198 Tectonic Setting .................................. 198 Sediment Dispersal Systems........................... 206 Gene Canyon Formation............................. 206 Copper Basin Formation .......................... 223 Depositional History..................................229 Gene Canyon Formation (32-18 m.y.b.p.) 229 Copper Basin Formation (18-16 m.y.b.p.)..........233 SUMMARY AND CONCLUSIONS..................................236 REFERENCES CITED ...................................... 240 viii LIST OF FIGURES FIGURES PAGE 1. Diagramatic cross-section across the Whipple Mountains illustrating structural and strati- graphic relationships........................ 2 2. Location map showing the Whipple and Buckskin Mountains, as well as the surrounding ranges . 5 3. Geologic map of central and eastern Whipple Mountains, San Bernardino County, southeastern California .................................... 9 4. Extreme range of clast sizes and monolithic compositions of the boulder breccia lithofacies.................................... 25 5. Adamellite/limestone unit of the boulder breccia lithofacies...................... 27 6. Lenticular nature of massive conglomerate lithofacies............................... 33 7. Massive conglomerate lithofacies showing internal fabrics and subrounded clasts .... 36 8. Massive to crudely stratified boulder conglomerate lithofacies ..................... 38 9. Typical sequence of subfacies and textures for the massive conglomerate lithofacies ......... 41 10. Internal configuration of the channeled conglomerate lithofacies ..................... 48 11. Channeled conglomerate lithofacies showing clast imbrication........................ 50 12. Transition probability matrix and spider diagram for the channeled conglomerate lithofacies............................... 53 13. Typical sequence of subfacies and textures for the channeled conglomerate lithofacies .... 55 ix Figures (cont'd) PAGE 14. Alternating conglomerate and sandstone lithofacies showing segregation of textures and the planar-erosional contacts............. 61 15. Alternating conglomerate and sandstone lithofacies showing internal features.......... 63 16. Transition probability matrix and spider diagram for the alternating conglomerate and sandstone lithofacies.......................... 66 17. Typical sequence of subfacies and textures for the alternating conglomerate and sandstone lithofacies.................................... 68 18. Internal texture, fabric and primary sedimentary structures of the diffuse sandy conglomerate lithofacies ..................... 73 19. Primary sedimentary structures of the diffuse sandy conglomerate lithofacies ............... 76 20. Transition probability matrix and spider diagram for the diffuse sandy conglomerate lithofacies................................. 79 21. Typical sequence of subfacies and textures for the diffuse sandy conglomerate lithofacies . . 81 22. Stream-washed pebbly sandstone lithofacies exhibiting planar laminae.................. 86 23. Primary sedimentary structures within the pebbly sandstone lithofacies ................. 89 24. Transition probability matrix and spider diagram for the pebbly sandstone lithofacies.. 91 25. Typical sequence of subfacies and textures for the pebbly sandstone lithofacies ............. 93 26. Internal configuration of the interbedded fine sandstone and siltstone lithofacies....... 99 27. Sedimentary structures of the interbedded fine sandstone and siltstone lithofacies....... 102 x Figures (cont'd) PAGE 28. Transition probability matrix and spider diagram for the interbedded fine sandstone and siltstone lithofacies...................... 105 29. Typical sequence of subfacies and textures for the interbedded fine sandstone and siltstone lithofacies................................. 107 30. Subaerial dessication features of the calcareous siltstone-mudstone lithofacies. . . 114 31. Volcanic and volcaniclastic lithofacies of the Gene Canyon Formation...................... 119 32. Pooled probability matrix and spider diagram for the Gene Canyon and Copper Basin Formations................................. 124 33. Index map indicating location of measured and reconnaissance sections.................... 127 34. Typical exposure of the Gene Canyon Formation................................... 130 35. Desilt Wash stratigraphic section......... 134 36. Transition probability matrix and spider diagram for the Desilt Wash stratigraphic section..................................... 136 37. Parker Dam School stratigraphic section. . . . 140 38. Transition probability matrix and spider diagram for the Parker Dam School section. . . 143 39. Red Mountain stratigraphic section ........... 146 40. Transition probability matrix and spider diagram for the Red Mountain section..... 148 41. Typical exposure of the Copper Basin Formation................................... 152 42. Parker Dam School section.................. 156 43. Reconnaissance sections of the Copper Basin Formation................................... 160 xi Figures (cont’d) PAGE 44. visher plots of the pebbly sandstone, inter bedded fine sandstone and siltstone and calcareous siltstone-mudstone lithofacies. . . 170 45. Passega diagram after Bull (1972)............. 173 46. McBride ternary diagram of grain compositions for the Gene Canyon and Copper Basin Formations.................................... 177 47. Diamond diagram after Basu et al. (1975) . . . 179 48. Histogram of quartz grain types after Basu et al. (1975).................................. 182 49. Ostrocod shells in calcareous siltstone- mudstone lithofacies.......................... 185 50. Diagenesis of the Copper Basin and Gene Canyon Formations.................................... 189 51. Diagenetic alterations of framework grains . . 191 52. Diagenetic history of sample 21 from the Copper Basin Formation ........................ 194 53. Paleogeographic reconstruction of the Whipple Mountains from late Oligocene through Miocene time (from Teal & Frost, 1982)............... 200 54. Theoretical model of relationship between detachment faulting and uplift (from Spencer, 1984)........................................... 203 55. Paleogeographic model of Gene Canyon Formation showing location of caldera complex........... 207 56. Fence diagram of Gene Canyon Formation .... 210 57. Lithofacies map of Gene Canyon Formation . . . 212 58. Diagramatic cross-section of the Gene Canyon Formation near Parker Dam School ............. 215 59. Sediment dispersal systems map for the Gene Canyon and Copper Basin Formations ........... 217 60. Fence diagram of the Copper Basin Formation. . 223 xii LIST OF TABLES PAGE 1. Lithofacies and depositional environments........ 18 2. Lithofacies and sedimentary structures of flu vial deposits (modified from Miall, 1978) .... 20 3. Summary of lithofacies characteristics ........ 22 4. Summary of petrographic data................... 176 xiii INTRODUCTION General Statement The Whipple Mountains of southeastern California occur within a belt of Cordilleran Mesozoic to Tertiary metamorphic core complexes, which extends from Sonora, Mexico to British Columbia (Davis and Coney, 1979). Some of these metamorphic core complexes are mantled by Cenozoic volcanic and sedimentary strata which were deposited syntectonically into intermontaine basins, whose character and tectonic history remain unresolved. The eastern Whipple Mountains provide an excellent opportunity to study these mid-Tertiary continental deposits. More than 3000 m of stratigraphic section have been preserved (Fig. 1), with essentially continuous exposures which enable detailed vertical sequence analysis. Previous workers (Terry, 1972? Anderson and others, 1979; Davis and others, 1980? Evans, 1979; Podruski, 1979? Lopez, 1981? Thurn, 1982b? Frost, 1984) have concentrated on structural and petrologic relationships, and relatively little attention has been directed to associated sedimentary dispersal systems. 1 Figure la. Diagramatic cross-section across the Whipple Mountains illustrating structural and strati graphic relationships. The rocks have been divided into three units: (1) lower-plate mylonitic complex (mxln)? (2) upper-plate crystalline complex (xln); and (3) upper-plate mid-Tertiary volcanic and sedimentary strata (T]_ 5 5 Gene Canyon Fm. , T2 = Copper Basin Fm.)f (modified from Davis and others, 1980). lb. Diagramatic columnar section illustrating major units and age relationships. 2 sw NE POST-DETACHMENT FAULTING 15.9 mybp 17-18 mybp Possible Initiation of Detachment faulting 26-32mybp (?) OSBORNE WASH FORMATION COPPER BASIN FORMATION (T2) GENE CANYON FORMATION CRYSTALLINE UPPER-PLAtE (b) Location of Study Area The Whipple Mountains are located in San Bernardino County, California, and lie approximately 20 km north of Parker, Arizona (Fig. 2). They are adjacent to Lake Havasu and form the eastward bend in the Colorado River, along the California-Arizona state line. Highway 172 and a network of dirt roads, which are maintained by the Metropolitan Water District of Southern California, provide central access to all stratigraphic sections in the study area. Purpose and Scope of Study The preservation of the Gene Canyon and Copper Basin Formations has provided an important record of the mid- Tertiary tectonic history of the Whipple Mountains. Our knowledge of the nature and timing of sedimentation asso ciated with detachment-fault tectonics may be increased greatly by the detailed analysis of these continental deposits. This study has two main objectives. The first is to identify the principal lithofacies of the Gene Canyon and Copper Basin Formations, and the second is to relate sediment dispersal patterns to the tectonic evolution of the Whipple Mountains. Lithofacies may be defined in 4 Figure 2. Location map showing the Whipple and Buckskin Mountains, as well as the surrounding ranges (from Podruskif 1980). 5 NEV. Needles • ARIZONA Wikleup Area of Index map MOHAVE \M T N S . STUDY WHIPPLE ( 89) MTNS Vidal Junction TU R TLE MTNS BUCKSKIN Parker Rice RIVERSIDI MTNS Wickenbun MARIA \M T N S Wenden. Blythe PLOMOSA MTNS terms of their characteristic sedimentologic properties, which reflect an interpretable sequence of depositional processes. These include thickness, bedding contacts, texture and fabric, sedimentary structures, lithologic compositions, and the vertical and lateral relationship of these features. Interpretations concerning the depositional environments and sediment dispersal systems may be made, from which a process-response model may be inferred. The second objective may be addressed by the collection and analysis of paleocurrent data, textural and compositional trends, and the spatial distribution of lithofacies. It is emphasized that this study is limited to the eastern part of the Whipple Mountains? therefore, only limited conclusions concerning the tectonic history of the region can be drawn. It is hoped that this study will provide a basis for future investigations, from which a more comprehensive depositional and tectonic model will be developed. Geologic Setting The Whipple Mountains occur within a tectonic terrane characterized by Mesozoic to Tertiary metamorphic core complexes and Oligo-Miocene extensional tectonic features (Davis and others, 1980). Cordilleran metamorphic core 7 complexes have been recognized along an irregular belt from Sonora, Mexico to southwestern Canada (Armstrong and Hansen, 1976? Anderson and others, 1979? Coney, 1980? Davis and others, 1980? Rehrig and Reynolds, 1980). Along the Colorado River trough, these metamorphic core complexes are characterized by regionally extensive, low- angle normal faults, termed detachment faults, which separate mylonitic "core" rocks from relatively unmetamorphosed upper-plate rocks (Davis and others, 1980). The rocks of the Whipple Mountains and those of the co-extensive Buckskin and Rawhide Mountains have been divided into three units based on this regional detachment surface shown in Figure 3 (Shackelford, 1976? Davis and others, 1980? Frost 1981, 1984). The lower-plate contains mylonitic orthogneisses, paragneisses, and granitic plutons, much of which possess a consistent shallow dipping northeast and southwest-trending lineation (Davis and others, 1977, 1980). These rocks are considered to be autochthonous relative to the upper plate, and are often referred to as the mylonitic "core". The upper-plate rocks are separated into a crystalline complex and the overlying mid-Tertiary continental volcanic and sedimentary strata. The crystalline complex is composed of Precambrian nonmylonitic, quartzo-feldspathic gneisses, a Precambrian 8 Figure 3. Geologic map of central and eastern Whipple Mountains, San Bernardino County, southeastern California,. The location of the Desilt Wash, Parker Dam School and Red Mountain measured stratigraphic sections are shown (modified from Davis and others, 1979a). 9 MIO-PLIO. OSBORNE WASH FM. TERT{MIO.-OLIG GENE CANYQN 8 COPPER BASIN FMS. UPPER-PLATE XA LIN E ROCKS LOWER-PLATE MYLONITIC ROCKS LOW ER-PLATE NON-M YLONITIC ROCKS \ STUDY- \yAREA f jl /li/iV * \ /Whipple Wash 7 ////V7 Mi D E SILT^ 7>WASH < a * & ‘S E C : w M M Y L O N IT IC /V y ' W h t , 4 w M w j ^^RKEft’ DAM s c h o o i^ s e c : -7fTRED MOUNTAIN X s e c t io n v y & C ; yr u t „v Arizona + *** u V * ’ > « • $ 1 Jr > , *A A r >Vi^A< ‘ f L CALIFORNIA ‘ I • ; * y * > t > : —v . / r • W HIPPLE d e t a c h m e n t f a u l t granite porphry and related intrusive rocks, and a variety of Mesozoic and Tertiary intrusive rocks (Anderson and others, 1979a? Davis and others, 1980). Oligo-Miocene sedimentary and volcanic strata of the Gene Canyon, Copper Basin, and Osborne Wash Formations unconformably overlie this crystalline complex (Fig. 2). Coarse-grained conglomerate and sandstone assemblages assigned to these formations are considered to represent alluvial fan deposits (Teel and Frost, 1982, Frost, 1984). It also has been suggested that the presence of volcanic flows and ash-flow tuffs indicate a nearby caldera complex (Frost, 1984? Teel, 1983), which may in fact, control detrital sedimentation patterns (Vessell and Davies, 1981). Both units of the upper-plate have been extensively faulted and distended to the northeast by high-angle normal faults, associated with the detachment surface (Davis and others, 1980). The tectonic history of this region has received considerable attention, which has centered primarily on the temporal and genetic relationships between (1) the mylonitic gneisses of the lower plate, (2) the low-angle detachment fault, (3) the high-angle normal faults of the upper-plate, and (4) the upwarping of the ranges into their characteristic domal configurations (Davis and others, 1980? Teel and Frost, 1982). The current concensus among workers is that a genetic relationship 11 does exist between mylonitization of the lower-plate and the spatially associated detachment faults (Davis and Coney, 1979? Rehrig and Reynolds, 1980? Lister and Davis, 1983? Davis and others, 1983). A genetic relationship between detachment faulting and the high-angle normal faults of the upper-plate has also been established by Davis and others (1980). The possible temporal and genetic relationship of these tectonic elements with the upwarping of the ranges, however, is less clear. Contrasting models have been suggested (Davis and others, 1980; Davis and Coney, 1979? Reyhrig and Reynolds, 1980; Spencer, 1982, 1984; Teel and Frost, 1982? Frost, 1984), and will be discussed in association with the proposed depositional model. There is general agreement, however, that the depositional trends of the mid-Tertiary volcanic and sedimentary deposits reflect the tectonic history of the Whipple Mountain region. Previous Work One of the earliest geologic studies in the Whipple Mountains was made by F. L. Ransome (1931). He was apparently the first geologist to recognize low-angle faults in the Colorado River trough, although he considered the faults we now recognize as the Whipple detachment fault to be a thrust fault (Davis and others, 1980). 12 A few years later, Kemnitzer (1937) concluded that no low-angle faults occurred in the Whipple Mountains, and that high-angle normal faults and doming were the dominant structural features. He regarded the contacts between Tertiary strata and crystalline basement complex as non conformities. Furthermore, he described the type section for the Gene Canyon Formation at Desilt Wash and provided a general description for the type area of the Copper Basin Formation. Further interest in the Whipple Mountains was minimal until Terry (1972) mapped a low-angle fault between the upper- and lower-plate crystalline assemblages. Lower- plate mylonites and the fault were interpretated as being of thrust origin and of Mesozoic age. However, detachment-fault contacts between Tertiary strata and lower-plate crystalline basement, were considered to be nonconformities. Anderson (1971) studied low-angle faults in the Eldorado Mountains of Nevada, and presented evidence that they were in fact extensional tectonic features. This important conclusion stimulated a re-evaluation of the understanding of the geology of this region. Davis and his associates (1977, 1980) considered the Whipple fault to be a Cenozoic structure unrealted to Mesozoic thrust faulting. They first developed a model with a gravitational sliding mechanism to account for the 13 upper and lower plates of the Whipple Mountains and areas to the east. Subsequent field evidence led Davis and others (1982) to alter this model in favor of one in which the low-angle detachment surface between plates is of extensional origin. Several students from the University of Southern California have mapped various parts of the Whipple Mountains and contributed significantly to the overall geologic understanding of this region (Evans, 1979; Krass, 1980; Lingrey, 1979; Podruski, 1979; Krass, 1980; Rowley, 1981; Thurn, 1982a; Frost, 1984). The first sedimentologic study of the Whipple Mountains was provided by Teel and Frost (1982) and Teel (1983). This work concentrated on the type locality of the Copper Basin Formation, where general facies patterns were interpreted in terms of the tectonic setting. This study was limited to the Copper Basin Formation and provided useful information concerning the sediment dispersal systems in this part of the depositional basin. Methods of Study Field data for this study were obtained at two scales. Reconnaissance work and regional outcrop sampling provided generalized stratigraphic information and lithologic trends, from which a more detailed sampling 14 program was developed. This work was greatly assisted by a regional geologic map compiled by G. A. Davis and Eric Frost (unpublished). More detailed work which forms the data base for this study included vertical sequence analysis and lithofacies mapping. Three stratigraphic sections (Fig. 3) were measured and described using the Jacob staff and Brunton compass technique (Compton, 1962). These sections were selected for their excellent exposures, lack of faulting and position within the depositional basin. A standardized form developed from preliminary reconnaissance work was used to record lithologic characteristics and the vertical sequence of subfacies for statistical treatment (Miall, 1978). Lithofacies were defined (see lithofacies section) for each of the measured sections. General facies mapping of the Gene Canyon Formation was helpful in determining the configuration of each facies and the lateral facies associations. The Gene Wash and Black Peak quadrangles were enlarged to a scale of 1:12,000, and used as base maps. At each outcrop, the maximum clast size was determined by measuring the apparent maximum diameter of the largest 10 clasts, within a i m radius. A mean value was then recorded following the technique of Miall (1969). Conglomerate to sandstone ratios were estimated visually, where detailed sections were not measured. 15 Selected samples of several lithofacies were analyzed by thin section textural techniques discussed by Blatt and others (1980). The induration of these deposits precluded sieve analysis. Conventional petrographic techniques were used to analyze 23 thin sections from samples collected in the measured sections. At least 400 points on each thin section were counted using the Glagolev-Chayes method (Galehouse, 1971). The error associated with sampling 400 grains at random is +5% at a 95% confidence interval (Van der Plas and Tobi, 1965). Paleocurrent azimuths were corrected for tectonic tilt using a Wulff stereographic net and following the procedures outlined by Ragan (1973). Rose diagrams were then plotted and the vector means computed following the methods described by Potter and Pettijohn (1977). 16 LITHOFACIES ANALYSIS Introduction The shape, internal characteristics, and spatial distribution of sedimentary deposits are a function of the transport processes and environments of deposition. Several levels of organization may be observed in the field, and represent different scales of process-response models. The key to environmental interpretations, therefore, is the association of each level of organization, or sequence of sedimentary structures, to the geologic processes which control their deposition (Fisher and McGowan, 1967; Heward, 1978a; Blatt and others, 1981). Three scales of stratigraphic analysis were chosen to record the variation of sedimentary features in the Gene Canyon and Copper Basin Formations (Table 1). The primary working unit is the lithofacies, which typically ranges from 3 to 25m thick, and varies considerably in lateral continuity. It is defined by a consistent set of physical characteristics which are recognizable at the outcrop scale, and exhibits a specific sequence of grain size and sedimentary structures. It is emphasized that these units often are transitional with each other and that judgments must be made concerning contacts. Lithofacies are 1 7 Table 1. Lithofacies and Depositional Environments. Subfacies symbols modeled arte- Miall (1978) Lith o facies 1. Boulder Breccia Depositional Environment Talus ?. Massive Conglomerate Major Channels 3. Channeled Conglomerate Braid Channels fl. Alternating Braid-Sheetflood Conglomerate/Sandstone 5. Diffuse Sandy Conglomerate 6. Pebbly Sandstone 7. Interbedded Sandstone/Siltstone Calcareous SiItstone/Mudstone Sheetflood Sheetflood-Al luvial Plain Subfacies Gms, Gm Gms, Gm Gm, Gh, Gt Gh, Gt, Sh Sh, Gt, St-Sp Sh, SI, St-Sp Alluvial Plain-Playa Sh, FI, St-Sp-Sr Alluvial Plain-Playa FI, Fm, Sr Lithofacies Assemblage 18 considered to represent distinct geomorphic features, limited in time and space, which are found associated with several different depositional environments. The most distinguishing features are clast size and fabric, which suggest that the controlling factors are related to relative fluctuations of transport energy and particle size. Lithofacies may be described as a set of subfacies as defined by Miall (1977, 1978), which are observed within individual beds as primary sedimentary structures. These have been listed in Table 2 and are related to depositional flow regimes. A detailed discussion of these bedforms is presented below for each lithofacies. Large scale sequences or Lithofacies assemblages (Rust, 1978) vary from 25 to 250m thick. These sequences are analagous to vertical profile models of Miall (1977, 1978), and megasequences of Heward (1978a, 1978b). They are interpretated in terms of specific depositional environments, which are dominantly controlled by the local tectonic setting. The recognition of depositional cycles or sequences may be enhanced by the use of Markovian analysis. This technique classifies the sedimentary features into a limited number of facies "states," with the number of vertical transitions between each of these states then counted and tabulated. The computational procedure for 19 Table 2.. Lithofacies and sedimentary structures of fluvial deposits (modified from Miall, 1978). The asterisk denotes a newly defined facies. Facies Code Lithofacies Sedimentary structures Interpretation Gms massive, matrix supported gravel none debris flow deposits Gm massive or crudely • bedded gravel horizontal bedding imbrication longitudinal bars lag deposits, sieve deposits *£n gravel, stratified horizontal bedding longitudinal bars lag longitudinal bars lag deposits Gt gravel, stratified trough crossbeds minor channel fills Gp gravel, stratified planar crossbeds linguoid bars or del taic growths from old< bar remnants Sm sand, medium to v. coarse, pebbly massive planar bed flow St sand, medium to v. coarse, may pebbly solitary (theta) or grouped (pi) trough crossbeds dunes (lower flow regimel Sp sand, medium v. coarse, may be pebbly solitary (alohal or arouoed (omikronl planar crossbeds linguoid, transverse bars, sand waves (low; flow regime) Sr sand, very fine to coarse ripple marks of all types ripples Mower flow regime) *Sh sand, very fine to very coarse, may be pebbly horizontal lamination, parting or streaming 1ineation planar bed flow (1. and u. flow regime S': sand, fine low angle ( 10°) crossbeds scour fills, crevasse splays, antidunes FI sand, silt, mud fine lamination, very small ripples overbank or waning flood deposits Ft mud, silt massive, desiccation cracks overbank or drape deposits 20 evaluating Markovian processes is outlined by Harbaugh and Bonham-Carter (1970). The probability of occurrence of each transition is then calculated, and a tree diagram is produced indicating the principle vertical sequence. A probability matrix was constructed for each lithofacies which contained sufficient transitions between facies states, and was quite helpful in distinguishing lithofacies. This technique also was applied to the sequence of lithofacies to determine their overall preferred order and to establish lithofacies associations. Ten operational lithofacies were defined in terms of shape, stratification, texture, sedimentary structures, vertical sequence, lateral variability, and lithologic composition (Table 3). These lithofacies contain features consistent with alluvial fan/plain and lacustrine depositional units (Blissenbach, 1954? Bluck, 1967? Bull, 1972? Miall, 1978? Heward, 1978b? Nilson, 1982)? and include (1) disorganized breccia and conglomerate, (2) coarse-grained very poorly-sorted pebbly sandstone, (3) bidirectional to radiating paleocurrent patterns, (4) upward-fining beds, (5) lenticular and channel-shaped beds, (6) laterally persistent bed sets, (7) abundant red iron oxide cement, (8) locally distinct terrigenous compositions, and (9) lack of marine fossils. This section examines each lithofacies in detail, determines preferred vertical sequences, and provides 21 Table 3. Summary of lithofacies characteristics. Lithofacies 1. Boulder Breccia 2. Massive Cgl. 3. Channeled Cgl. 4. Alternating Cgl/Ss 5. Diffuse Sandy Cgl. 6. Pebbly Sandstone Lower Bed Configuration Bounding Surfaces Texture & Fabric Sedimentary Structures Lenticular, laterally Markedly erosional to Boulders to silt; very poor sorting; Chaotic & massive, rare discontinuous. irregularly depositional. v. angular to angular; clast- and grading; rare Sm, Sh, FI matrix-supported. Lenticular, laterally discontinuous. Erosional, convex down ward; planar erosional less Lenticular, laterally Erosional, convex discontinuous. downward. Tabular to lenticular, laterally persistent to discontinuous. Slightly erosional to Planar erosional. Boulders to clay, very poor sorting; subangular to subrounded; clast- and matrix-supported. Cobbles to coarse sand; poorly sorted; subround-subang. Clast- supported; rare imbrication. Cobbles to coarse sand; moderate sorting; subangular-subrounded; clast-supported. Massive to crudely bedded, both normal & reverse grading; weak imbrication; uncommon Sf, SI, St, Rare Sr. Massive to crudely bedded; imbricated; rapid litho- logic changes; St, Gt, Sh, Gh. Planar-bedded to irregular rapid lithologic changes; conglomerate horizons; Sh, Gh. Tabular & laterally persistent. Slightly erosional to planar erosional. Pebbles to sand; moderate sorting, subangular-subrounded; clast- supported. Massive to crudely bedded; conglomerate horizons; scour pockets; St, Sp, Sh, Gt. Tabular & laterally very persistent. Planar erosional to nonerosional. Med. to coarse-grained sand; pebbles; moderately-well sorted subangular to subrounded. Massive to planar- laminated, low-angle cross beds; Sh, Sm, SI, Sr. Climbing ripples; parting lineations; rip-up clasts. 22 Table 3 (cont'd) Lithofacies 7. Interbedded Ss/Slts 8. Calcareous Slts/Mudst 9. Volcaniclastic 10. Volcanic flow Bed Configurative Sheetlike; laterally very persistent. Lower Bounding Surfaces Planar erosional to gradational. Sheetlike to tabular; Planar nonerosional laterally very persist- to gradatical. ent to discontinuous. Texture & Fabric Medium-grained sand to silt; well- sorted; subangular to subrounded. Fine sand to clay; well-sorted; subangular to subrounded. Sedimentary Struct, Planar-laminated; sm. scale cross laminae, convolute and dewateR structures; climbing ripples; T i n i n g - u p w a R d Sp-St, Sr, fm, fl. T':T:V 1 ’ - s m . scale cross laminae; mudcracks; tracks. - See description in text. - See description in text. ^ environmental interpretations. Boulder Breccia Lithofacies Lithofacies Description The boulder breccia lithofacies form some of the most spectacular units in the Whipple Mountains, and are characterized by their highly-resistant nature, monolithic composition, and distinct colors (Fig. 4). They are found throughout the Gene Canyon sections and are notably absent from the Copper Basin Formation. They are recognized primarily by their extremely large clast size. Several of these units can be mapped laterally for hundreds of meters before abruptly pinching out, giving them a pronounced discontinuous nature and lenticular shape at the stratigraphic level (Fig. 5). Thicknesses are quite variable, ranging to as much as 200 m. Several of these units exhibited a general thickening trend towards the southeast. The lower contacts are markedly erosional and irregular, with truncation of the underlying units often exceeding 10 m. Where contacts are planar erosional, they are recognized by abrupt textural and compositional changes, usually along a highly-irregular depositional surface. The upper contacts are typically gradational with finer-grained lithofacies, and less 24 Figure 4. Extreme range of clast sizes and monolithic compositions of the boulder breccia lithofacies. (a) Adamellite. Clasts are typically leucocratic and often have large orthoclase phenocrysts. (b) Granodiorite. This distinctive unit has abundant mafic xenoliths is gray in color and displays spheroidal weathering. (c) Adamellite boulder measuring 1.7 m in length. Note the extreme range of sizes. This unit lies depositionally on top of a sandstone lithofacies. (d) Adamellite boulder breccia showing large range of clast sizes. Also note the angular clasts and their chaotic fabric. Measuring stick is 1 m in length. 25 Figure 5. Adamellite/Limestone unit of the boulder breccia lithofacies. This unit is lenticular, as indicated by dashed line, and exhibits thickening and coarsening to the southeast. 27 2 8 commonly, are planar to shallow erosional. The most distinctive feature of this lithofacies is the extreme range in size distribution (Fig. 5). Sizes range from a silty/sandy matrix to blocks exceeding 10 m. Boulders are typically from 1-3 m in diameter and often compose more than 70% of this lithofacies. The size « distribution, therefore, is highly skewed to the coarse fraction. An overall increase in the mean size distribution and maximum clast size was observed to the southeast. This trend was most apparent in the adamellite/limestone unit found in Desilt Wash. The internal character of this lithofacies is extremely disorganized and chaotic, with both clast- and matrix-supported fabrics. Clast-supported fabrics were generally show a polymodal size distribution, and display no preferred shape or spatial orientation of grains. Although some matrix-supported fabrics exhibited both normal and reverse grading, they most often occur as non graded, homogenous units. The size distribution is typically bimodal with a sandy/silty matrix supporting the larger cobbles and boulders. Clasts are very angular and show little evidence of abrasion. Primary sedimentary structures are rare, but consist of massive to planar-laminated sandstone (Sm, Sh) and conglomerate horizons (Gh). These are generally less than 3 m thick and nearly always occur near the top of this 29 lithofacies. Thinly-laminated siltstone units (FI) sometimes occur, and are associated with the sandstone near the top of these lithofacies. Environmental Interpretation The extreme chaotic nature of these deposits coupled with their monolithic composition and coarse grain size indicates little transport, with relatively little or no sorting, and a rapid loss of transport energy. Landslides may account for these features and may have produced the thick talus deposits observed in the Gene Canyon Formation. Talus deposits include falls, slumps, flows, slides, and rock avalanches (sturzstroms), which commonly are transported from inter-stream slope areas of the mountain front to an alluvial fan surface (Nilson, 1982). These deposits are quite often interbedded with debris-flow and mud-flow deposits, and are, therefore, difficult to recognize in ancient deposits. Bull (1972) has observed that they are similar to sieve deposits as described by Hooke (1967), and can be distinguished only by their downslope increase in particle size, overall increase in size distribution, and poorer sorting. These characteristics were observed in the Gene Canyon Formation and are valuable paleoslope indicators. 30 The geometry of this lithofacies is particularly diagnostic of talus deposits, with the rapid lateral terminations and interfingering with water-laid deposits (Blissenbach, 1954). Eisbacher (1979) made similar observations and noted that rock avalanches can flow down fan channels, giving them a linear shape, and resulting in the deposition of debris across both proximal and distal portions of a fan. Thick talus deposits have been recognized along seismically active mountain fronts (Bull, 1964b, 1972, 1977? Heward, 1978b, Nilson, 1982) with steep slopes and deep weathering, such that repetitive landslide deposits form mappable facies, and locally, marker beds for stratigraphic analysis. This is certainly the case in the Gene Canyon Formation, where monolithic breccias have been traced for several kilometers (Frost, 1983) and mapped for hundreds of meters in the present study. The single most impressive feature of this lithofacies, however, is the large size and angularity of the clasts. Brookfield (1980) has documented similar cases with clasts as much as 30 m in diameter, and notes that they are surely indicative of talus deposits. The lack of sorting and abrasion also strongly support a landslide mechanism. The massive and planar-laminated sandstone (Sm, Sh) probably reflect the reworking of this lithofacies by 31 high-velocity fluvial currents. As such currents waned, rare thinly-laminated siltstone (FI) was deposited. They were encountered infrequently due to their low potential for preservation. Several episodes of talus deposition account for the features observed within this lithofacies. Massive Conglomerate Lithofacies Lithofacies Description The massive conglomerate lithofacies consists predominantly of cobble to boulder conglomerate, and forms characteristic reddish-orange (10R6/6) resistant ridges. This lithofacies is found throughout the Gene Canyon, but is much more widespread in the lower portions of the Copper Basin Formation. The external configuration of this lithofacies is distinctly lenticular or channel-shaped (Fig. 6). The lower bounding surface is concave-upwards and markedly erosional, often truncating the underlying unit in excess of 15 m. Thicknesses at the channel axis range from 5 to 15 m, and occasionally exceed 50 m. This unit is laterally discontinuous and rapidly changes into finer- grained lithofacies. The upper contact commonly is gradational with the channeled conglomerate lithofacies or alternating conglomerate and sandstone lithofacies. 32 Figure 6. Lenticular nature of massive conglomerate lithofacies indicated by dashed line. (a) A major stream channel in the Gene Canyon Formation located at Desilt Wash, associated with the apex of an alluvial fan. The axial deposits are more than 50 m thick. (b) A relatively small stream channel, which truncates a sandstone lithofacies. Channel is 7 m thick and located at Red Mountain. 33 (b) 34 Several orientations of the channel axis were obtained, and generally indicated a northwest trend. This "massive" lithofacies is very poorly sorted and ranges from a silty/sandy matrix to boulders greater than 70 cm in diameter. Cobbles and bounders typically comprise from 50 to 70% of this lithofacies, and the average maximum clast size is from 30 to 40 cm. Interbedded pebbly sandstone beds occasionally occur. These exhibit much better sorting and range from 1 to 4 mm in diameter. Both clast- and matrix-supported fabrics occur in this lithofacies (Fig. 7). Matrix-supported conglomerate is bimodal, with cobbles and boulders "floating" in a sand or silt matrix. Clast-supported conglomerate is commonly polymodal and disorganized. Clasts are subangular to subrounded and do not exhibit an obvious preferred or ientation. Internally, beds are massive to crudely stratified (Gms, Gm, Gh), with occasional normal or reverse grading (Fig. 8). Reverse grading is more often associated with matrix-supported conglomerate, whereas normal grading is common in clast-supported conglomerate. Beds ranging from 1 to 5 m thick are laterally persistent (greater than 10 m) and have planar nonerosional to gradational contacts. Thin, lenticular, pebbly sandstones which measure from 1 to 3 m thick and 5 to 10 m horizontally, 35 Figure 7. Massive Conglomerate Lithofacies showing internal fabrics and subrounded clasts. (a) Poorly-sorted, clast-supported massive conglomerate. Note the irregular basal contact cutting into an andesite flow (G.C. Formation, Desilt Wash). (b) Poorly-sorted, reverse-graded (?), matrix- supported debris flow deposit. The matrix is mostly coarse, pebbly sandstone (C.B. Formation, Gene Reservoir). 36 (a) <b) 3 7 Figure 8. Massive to crudely stratified boulder conglomerate lithofacies of the Gene Canyon Formation. Note the thick interbedded sandstone. This unit is distinctly lenticular. The location is at Red Mountain looking eastward. 38 39 are generally massive to crudely laminated (Sm, Sh). The lower contacts are gradational, whereas the upper contacts are planar to shallow erosional. Low-angle stratification (Gl, SI) and trough cross-beds (Gt) were occasionally noted as were small-scale scour and fill structures. Few measurable paleocurrents were found, due to the low angles and lack of exposed bedding surfaces. Directional data was obtained, however, by channel orientations and textural trends. Statistical vertical sequence analysis was not applied to this lithofacies, as the number of subfacies transitions were insufficient. However, several patterns were observed in the field (Fig. 9). Clast-supported, massive conglomerate (Gm) commonly grades into crudely stratified conglomerate (Gh), and this coupler is repeated numerous times. This coupler is occasionally succeeded by horizontal, low-angle and trough cross-laminated sandstone (Sh, SI, St). Matrix-supported conglomerate was observed to grade to horizontal and low-angle sandstone units (Sh, SI), which were occasionally topped by thinly-laminated to ripple cross-laminated silty sandstone (Sr, Sh, FI). It is stressed, however, that these sequences were subordinate to the overall massive nature of these deposits. 40 Figure 9. Typical sequence of subfacies and textures for the massive conglomerate lithofacies: Gm-Gh; Gm Gh-Gl—Gt. 41 42 Environmental Interpretation The external geometry, large clast-size distribution, and fabric suggest a high-energy, channelized, depositional environment; as would be associated with a proximal alluvial fan (Blissenbach, 1954? Bluck, 1967? McGowan and Groat, 1971? Bull, 1972? Steel and Wilson, 1975? Heward, 1978b? Brookfield, 1980; Schumm, 1977; Nilson, 1982). Deposits similar to these have been classified by several workers as streamflood (Steel and Aasheim, 1978), upper fan (Boothroyed and Ashley, 1975), facies B (Bluck, 1967), Gt (Miall, 1978), stream channel (Bull, 1972) and massive conglomerate lithofacies (Sailer and Dickinson, 1982). The development of the massive conglomerate lithofacies was controlled by two types of depositional processes; streamflow and debris-flow. Although these processes are clearly different in mechanism, their close spatial association has resulted in the juxaposition of their respective deposits. Hooke (1967) has noted the potential of stream flows channeling into older debris flow deposits, due to the fine-grained textures often associated with the matrix of the latter. Probably the most diagnostic feature of this lithofacies is the deep incision of the underlying unit. This is typical of alluvial fan channels or pediment 43 channels in which rainfall is concentrated from the drainage areas of interfan streams which concentrate the runoff from adjacent fans, and of intrafan streams draining old fan surfaces (Bull, 1964a; Brookfield, 1980). Blissenbach (1954) also noted that alluvial fans in the process of degradation are normally cut by streams that tend to form deep, narrow channels, near the apex of the alluvial fan. In his study of modern alluvial fan channels and laboratory models, Hooke (1967) demonstrated that channel incision occurs above an intersection point, or in the proximal region of the fan. And, quite significantly, he suggests that abnormally deep fanhead incision is related to tectonic movements, and not climatic changes. This is a refinement of earlier work by Bull (1964a, 1964b). Several of these units, however did not show evidence of erosion of the underlying lithofacies. They displayed planar, nonerosional bases typical of viscous debris flow deposits (Hooke, 1967; Harms and others, 1975). Debris flow deposits are one of the principal components of most alluvial fan successions in both semi-arid and periglacial environments (Rust, 1978). They represent sediment gravity flows in which the matrix is silt or mud, which, therefore, have sufficient strength to support large clasts (Johnson, 1970). These deposits have been recognized by numerous workers and are generally poorly 44 stratified, contain few obvious sedimentary structures, and are matrix-supported. Although generally associated with proximal regions of the alluvial fan, they have also been documented on distal parts of an alluvial fan (Sharp and Nobles, 1953). Hollywood (1982) suggests that proximal debris flows may be differentiated from distal flows on the basis of lateral extent. Flows within the entrenched portion of the fan should have channel-like shapes and are laterally discontinuous, whereas distal flows are generally laterally continuous. The deposits observed in this study display characteristics of both regions of the alluvial fan, however, proximal debris flows were more common in the Gene Canyon Formation than the Copper Basin Formation. Also characteristic of this lithofacies is the large clast size population, and the occurrence of both clast- and matrix-supported fabrics. Stream-channel deposits can be distinguished from more distal braided deposits by their coarser-grained sizes (Bull, 1972), which indicate a higher velocity profile and close proximity to the source. Stream channel deposits are typically massive to crudely stratified, clast-supported and contain relatively small amounts of clay and silt? features indicative of alluvial fan stream-channel processes (Bluck, 1967? Steel and Wilson, 1975? Miall, 1977, 1978). Primary sedimentary structures which occur clast-supported and contain 45 relatively small amounts of clay and silt. These features are indicative of alluvial fan stream-channel processes (Bluck, 1967; Steel and Wilson, 1975; Miall, 1977, 1978). Primary sedimentary structures which occur near the top of these deposits include planar and ripple cross-laminated sandstone. These features are similar to waning flow subfacies as reported by Miall (1977r 1978), and represent the final stages of storm-generated floods. Channeled Conglomerate Lithofacies Lithofacies Description The channeled conglomerate lithofacies is composed predominantly of sandy, pebble to cobble conglomerate. This lithofacies is typically pale reddish-brown (10R5/4) and moderately to well-lithified. It is found throughout the Gene Canyon and Copper Basin Formations, and comprises approximately 10% of the measured sections. The shape of this lithofacies is sheetlike or tabular. Sedimentary "packages" appear laterally persistent, extending for hundreds of meters; whereas thicknesses are usually less than 10 m. The lower bounding surface is typically shallow- to planar- erosional, usually with less than 2 m of relief. The upper contact commonly is gradational with the alternating conglomerate and sandstone lithofacies. 46 The internal character, however, is dominated by rapid lateral variations and lenticular bar and channel forms. Channels have erosional, concave-upwards, lower- bounding surfaces, and gradational to planar-nonerosional upper contacts (Fig. 10). Maximum thicknesses are from 1 to 3 m, and range from 2 to 6 m laterally. Bars have gradational to share lower-bounding surfaces and convex- upwards upper contacts (Fig. 10). They are commonly 1 to 2 m thick and range from 2 to 4 m laterally. Vertical stacking of these lenticular bed forms gives a distinctive internal homogeneity, due to lack of persistent depositional surfaces. The size distribution is polymodal, with 40 to 60% conglomerate, 25 to 30% sandstone, and the remainder siltstone. The conglomerate is commonly pebble-and cobble-size, which ranges from 2 to 10 cm. The sandstone is very coarse-grained and typically has oversized clasts entrained within the matrix. The fabric is predominantly clast-supported, with rare matrix-supported horizons. Clasts are subangular to subrounded and generally display and anisotropic fabric. Clast imbrication, however, occasionally occurs in channels (Fig. 11), from which directional information was obtained. Several internal sedimentary structures occur in these irregular-and crudely-bedded deposits. Channels commonly exhibit a fining-upwards trend from cobbles to 47 Figure 10. Internal configuration of the channeled conglomerate lithofacies (G.C. Formation, Desilt Wash). (a) Bar form with flat base and convex-upwards shape. The measuring stick is 1 m. (b) Channels cut into sandstones. Note depth of incision and vertical stacking of channels. The clipboard is 26 cm long. 48 1 — — Figure 11. Channeled conglomerate lithofacies showing clast imbrication. Note that the oriented clasts are relatively flat. The measuring stick is 1 m long (G.C. Formation, Red Mountain). 50 51 sandy siltstones at the top. Bars, on the other hand, are typically massive, and in rare instances, exhibit coarsening-upwards trends. Both of these morphologic forms, however, are topped by siltstone, which appears to drape across bars and into channels. Massive (Gm) to crudely-stratified (Gh) conglomerate are the dominant subfacies. Low-angle and trough cross bedded conglomerate and sandstone (G1,Gt,S1,St) are generally associated with broad, shallow scours. Small- scale current ripple laminae (Sr,Fl) are relatively uncommon and only observed in the upper fine fractions, which drape over bars and channels. Horizontal pebble trains are typical of areas lacking channels or bars, and are usually associated with massive to horizontally- laminated sandstone (Sm,Sh). Paleocurrent data was obtained from cross-bedded sandstone and pebbly conglomerate, channel axes, and clast imbrications. Markovian analysis indicated two major vertical sequences occur more than 73% of the time, both of which demonstrate upward-fining trends and a succession of sedimentary structures indicative of decreasing flow regime (Figs. 12,13). Channels and low-angle scours are generally massive at the base (Gm,Gms), and grade into a crudely-stratified, very sandy conglomerate (Gh-Gl). This sequence is repeated and forms the dominant feature of this lithofacies. A variation of this sequence shows a 52 Figure 12. Transition probability matrix and spider diagram for the channeled conglomerate lithofacies. 53 CO Gm -Gm : Gh-Gl Gt-Gp Sh-Sl St-Sp s ~ oo Fl-Fm Gm-Gms .73 .17 Gh-Gl .45 .36 .18 Gt-Gp .36 CO .21 Sh-Sl .19 .56 .13 .11 St-Sp .56 .22 .22 Sr .29 CO • = 3 " .29 Fl-Fm .33 .67 .45 .36 Sh-Sl 54 Figure 13. Typical sequence of subfacies and textures for the channeled conglomerate lithofacies: Gm/Gms- Gh/Gl; Gm/Gms-Gh/Gl-Sh/Sl-Sr. 55 Gh-Gl Gm-Gms Sr-Fl -Flm Sh-Sl Gh-Gl Gm-Gms St-Sp Gt-Gp Gh-Gl Sh-Sl Gh-Gl Gm-Gms Gh-Gl Gm-Gms very sandy conglomerate overlain by trough cross-bedded conglomerate and pebbly sandstone, which are, in turn, overlain by planar-laminated sandstone. Environmental Interpretation Frequent lateral and vertical variations in grain size and internal stratification indicate a depositional environment where rapid fluctuations of flow velocity, depth, and particle size are found. Also, the presence of interfingering channel and bar deposits implies a considerable variation of flow competence. Together, these features suggest a transitional depositional environmental from stream channels discussed above, to braided streams (Bluck, 1967, Miall, 1977, 1978? Brookfield, 1980). Braided stream environments are typically associated with the medial region of an alluvial fan (McGowan and Groat, 1971; Heward, 1978b; Brookfield, 1980? Nilson, 1982), below the intersection point where over-saturation causes sediment to be rapidly deposited and flow becomes wider and shallower (Brookfield, 1980). The conglomerate is deposited as longitudinal bars (Rust, 1972? Smith, 1970? Miall, 1977, 1978) and backfilling of the associated channels (Bull, 1972). Longitudinal bars originally develop as diffuse channel lags (Hein and Walker, 1977) 57 which continue to aggrade vertically, trapping finer particles as the current velocity decreases. Flow diversion then occurs causing channeling of the banks and underlying sediment (Miall, 1977). This system of braided distributary channels gives an overall impression of sheetlike deposition (Bull, 1972). The internal character, however, of shallow channels and bars, and the lack of a principle surface of accumulation (Steele, 1974) gives an appearance of laterally discontinuous and irregular bedding or a distinctly heterogeneous character. The migration of longitudinal bars, therefore, results in the deposition of massive to poorly-defined horizontal and low-angle inclined strata (Rust, 1972? Smith, 1974? Hein and Walder, 1977? Miall, 1977). In addition, channels and scour pockets are filled by finer- grained conglomerate and sandstone which display trough and planar cross-laminated bedforms, and more rarely, ripple cross laminae. These sedimentary structures are the result of waning flood currents (Miall, 1977, 1978). The vertical sequence indicates a decreasing flow regime which is attributed to waning flood currents over both bars and within channels. Each cycle, therefore, is initiated as a new bar or channel develops and is terminated as the current wanes over that particular location. The coarsening upwards cycles represent 58 migrating longitudinal bars, which places more proximal coarser-grained conglomerate over finer grained distal conglomerate. This lithofacies is transitional between the stream channel and interbedded conglomerate and sandstone lithofacies. It is considered to represent proximal braid deposits which have sufficient confinement to channelize, yet retain their rapid shifting character. These deposits, therefore, are likely to be found above the intersection point on an alluvial fan. Similar deposits have been described by several workers as massive conglomerate (Sailer and Dickenson, 1982), lower fan and pediment (Brookfield, 1980), mid-fan sandstone lobe association (Heward, 1978a), Gj alluvial fan (Rust, 1978), stream channel (Bull, 1972) and upper distal fan (McGowan and Groat, 1971). Alternating Conglomerate and Sandstone Lithofacies Lithofacies Description The alternating conglomerate and sandstone lithofacies is found extensively throughout the Gene Canyon and Copper Basin Formations, forming approximately 30% of the measured sections. These lithofacies are extremely well-lithified and exhibit a distinct horizontal 59 segregation of textures (Fig. 14). They range in color from pale red (10R6/2) to pale reddish brown (10R5/4), and are typically composed of 30 to 40% conglomerate. These lithofacies have distinctive sheet-like shapes, which are laterally persistent, extending for hundreds of meters without a significant change. The lower contacts are both gradational to very shallow erosional. The upper contacts are both gradational and erosional depending on the subsequent lithofacies. Thicknesses are quite variable, with stacked sequences exceeding 150m. Lateral truncation occurs where higher-energy lithofacies cut channels into these deposits, which were subsequently backfilled by coarse conglomerate. The outstanding feature of this lithofacies is the textural segregation of sandstone and conglomerate. The sandstone is coarse-grained to pebbly and moderately well- sorted. The conglomerate is composed of mostly pebbles and cobbles, which are from 0.5 to 1.0 cm in diameter, and is very-poorly sorted. The conglomerate is typically clast-supported, contains well-rounded clasts of which generally do not exhibit preferred orientation. Conglomerate also is quite commonly found as thin pebble horizons, 1 to 2 clasts thick, entrained with a coarse sandstone matrix (Fig. 15). The internal character of this lithofacies is dominated by massive to crudely-bedded sandstone and 60 Figure 14. Alternating conglomerate and sandstone lithofacies. Note the distinct segregation of textures and the planar-erosional contacts. The measuring stick is 1 m long (G.C. Formation, Desilt Wash). 6 1 62 Figure 15. The Alternating Conglomerate and Sandstone Lithofacies (G.C. Formation, Desilt Wash). (a) The rythmically interbedded nature of this lithofacies. Note the low relief of the conglomerate horizons. The measuring stick is 1 m long. (b) Planar cross-laminae within the sandstone portion of this lithofacies. The pencil is 14 cm long. 63 conglomerate. The sandstone is primarily composed of massive (Sm) to planar laminae (Sh), with transitions to low-angle cross-laminae (SI). Both fining- and coarsening-upward sequences occur within beds, but the overall textural trends are generally ungraded. At the top of many of these beds are thin planar (Fig. 15) and trough cross-bedded sandstones (St, Sp), which are sometimes accompanied by small-scale current ripple cross laminae (Sr) . The conglomerate includes massive (GM), planar- stratafied (Gh) and low-angle cross-stratafied (Gl), which usually are in beds from 1 to 2 m thick. Shallow scours are filled by trough cross-bedded conglomerate and sandstone (Gt, St), which are tangential to the basal surface. These are usually less than 1 m thick. The major sequence identified by Markovian analysis is a cyclic repetition of horizontally bedded conglomerate and sandstone (Figs. 16,17). A variation of this sequence exhibits the deposition of trough and planar cross stratified conglomerates on top of the flat-bedded conglomerate, which are, in turn, overlain by planar- laminated sandstones. This cycle occurs 27% of the time and is usually associated with shallow scours. 65 Figure 16. Transition probability matrix and spider diagram for the alternating conglomerate and sandstone lithofacies. 66 t / ) EE , _ Q. ( _ Q- cd 1 O | CD ■ OO I OO E s z . 1 +-> 1 _£Z 1 4-> CD CD CD OO OO Gm-Gms .30 .70 Gh-Gl .27 .60 .13 Gt-Gp .43 .43 .14 Sh-Sl .28 .44 .17 .11 St-Sp .25 .38 .25 .13 Sr .33 .67 Fl-Fm .50 .25 .25 Gm-Gms Sh-Sl ^ Gh-Gl * 2a St-Sp Sr Teo" . 27| |.43 Gt-Gp #5° Fl-Fm 67 Figure 17. Typical sequence of subfacies and textures for the alternating conglomerate and sandstone lithofacies: Gm-Sh/Sl-Gh/Gl-Gm. 68 ■pnr Gh-Gl FI Sp Sh-Sl Gt Sh-Sl Gm Sh-Sl Gh-Gl Gm Gh-Gl Sh-Sl G m Sh-Sl 69 Environmental Interpretation The alternating conglomerate and sandstone lithofacies has many characteristics similar to the channeled conglomerate lithofacies. In particular, it displays frequent lateral and vertical variations in grain size, which implies rapid shifts in depositional energy. However, the increased lateral continuity of discrete beds and the overall lack of channeling gives this lithofacies much more of a sheetlike appearance, which is markedly different. These features are consistent with the fore mentioned model and probably represent more distal braided alluvial fan deposits (Miall, 1977, 1978). In addition, this facies contains many of the features characteristic of sheet flood deposits, as described by Blissenbach (1954), Doeglas (1962), Bull (1964a, 1964b, 1972) and Steel (1974, 1978). The dominant feature of this lithofacies is the segregation of textures into discrete "packages" of rythmically interbedded conglomerate and sandstone. The horizontally-bedded pebbles and cobbles were deposited as low-relief longitudinal bars (Rust, 1972? Smith, 1970? Miall, 1977, 1978) and anastamosing channels (Bull, 1972). Deposition is caused by widening of the flow into shallow bands or sheets and concurrent decrease in depth and flow velocity (Bull, 1972). Therefore, channels were limited 70 to very shallow scours and were quickly filled with low- angle to trough cross-bedded conglomerate and sand. Several workers have shown that the lack of channeling and the development of sheetlike braided deposits are most commonly associated with the region below the intersection point on an alluvial fan (Bull, 1964a, 1964b? Hooke, 1967? Heward, 1978b). This would place these deposits distal to the channeled conglomerate lithofacies? a concept which also is supported by the better sorting and finer-grained texture. The observed sequences were caused by periodic flooding associated with storm water runoff. During peak flood conditions, rapid flow velocities produce longitudinal bars (Rust, 1978) , and less commonly, transverse bars (Smith, 1970) . As the flood currents wane, finer-grained, horizontally-bedded and cross-bedded units are produced. Their probability of occurrence, however, is limited as subsequent floods remove the upper part of the cycles, with only the upper flow regime deposits preserved. An upward-fining sequence was associated with this lithofacies with each sequence representing a discrete depositional event. 71 Diffuse Sandy Conglomerate Lithofacies Lithofacies Description The diffuse sandy conglomerate lithofacies is composed of greater than 50% pebbles of which are interspersed within a very coarse-grained to granular sandstone. This lithofacies is moderately lithified, and is distinctly grayish-orange (10YR714). Exposures are poor and uncommon, and this lithofacies constitutes less than 10% of the measured sections. It generally occurs in the lower parts of the Gene Canyon and Copper Basin Formations. The overall shape is difficult to ascertain due to erosion of this lithofacies; however, the general lack of channeling, horizontal bedding surfaces, and fabric homogeneity suggest a sheetlike or tabular deposit. The lower contact is gradational to very shallow erosional, and is typically associated with coarser-grained, alternating conglomerate and sandstone lithofacies. The erosional surfaces are planar to low-angle, with very low relief. These generally occur as distinct changes in texture and fabric (Fig. 18). The upper contact varies from gradational to deeply erosional, where higher-energy channels have cut into its surface. Laterally, these units are transitional with both finer- and coarser- 72 Figure 18. Internal texture, fabric and primary sedimentary structures of the diffuse sandy conglomerate lithofacies (G.C. Formation, Parker Dam School). (a) Horizontally-laminated sandy conglomerate overlain by low-angle to trough cross laminated sandy conglomerate. Note the homogenous nature of the fabric. The pencil is 12 cm long. (b) Small-scale ripple cross-laminae interbedded with crudely laminated sandy conglomerate. The pencil is 14 cm long. 73 7 4 grained lithofacies. Maximum thicknesses are from 15 to 20 m with horizontal dimensions as much as 60 m. This lithofacies is characterized by moderate to good sorting, and a distinctly homogenous internal fabric (Fig. 18). The size distribution is polymodal, with a mean clast size from 6 to 8 mm. Maximum clast size is from 2 to 4 cm, with rare outsized clasts ranging up to 70 cm in diameter found floating in a sandy conglomerate matrix. The fabric is clast-supported, with no apparent preferred orientation of the subrounded and subspherical clasts. The internal character of this lithofacies is dominated by broad, shallow lensing, and crudely- stratified beds. These beds range from 20 to 50 cm thick, and typically grade into one another. Primary sedimentary structures consist predominantly of planar to low-angle pebbly sandstone laminae (Sh,Sl) and trough cross laminated sandy conglormerate (Gt, St) shown in Figure 19. These beds tend to fine upwards, and are bounded by planar erosional surfaces. Shallow scours which are filled with low-angle to trough cross-laminated pebbly sandstones also occur. These laminae typically intersect the base of the scour at a very low angle or parallel the basal surface. Thicknesses are generally less than 1 m. Small-scale ripple cross-laminae also occur (Fig. 18) and are usually less than 10 cm thick. 75 Figure 19- Primary sedimentary structures of the diffuse sandy conglomerate lithofacies (Gene Canyon Formation, Desilt Wash). (a) Large-scale, trough cross-bedded sandy conglomerate overlying planar laminated pebbly conglomerate. The measuring stick is 1 m long. (b) Large-scale planar cross-bedded sandy conglomerate overlain by pebbly sandstone. The pencil is 14 cm long. 76 (b) 77 The major sequence indicated by Markovian analysis is the cyclic repetition of horizontally-laminated sandy conglomerate and pebbly sandstone (Figs. 20,21). In addition, a sequence of trough cross-laminated conglomerates overlying horizontally bedded conglomerates was observed. These were commonly topped by horizontally- laminated sandstone. Environmental Interpretation In contrast to the interbedded conglomerate and sandstone lithofacies, there is no distinct segregation of sand and gravel-sized particles in the diffuse sandy conglomerate lithofacies. Pebbles and sand are thoroughly mixed into a homogenous, moderately well-sorted unit, which was deposited under relatively constant flow conditions. That is, fluctuations of flow velocity and transported clast size were significantly reduced, and resulted in more uniform sets of textures and sedimentary structures. Deposits with similar features have been described as distal alluvial fan facies (McGowan and Groat, 1971), sheetflood deposits (Bull, 1972), very sandy conglomerate lithofacies (Sailer and Dickinson, 1982). Below the intersection point on an alluvial fan, the braided channels divide into numerous, smaller braided channels which radiate outward across the toe of the fan. The low slope and lack of flow confinement result in thin 78 Figure 20. Transition probability matrix and spider diagram for the diffuse sandy conglomerate li thofacies. 79 </) EE ( _ O- , ___ Cl CD CD CD CO CO 1 £E 1 -C 1 +-> 1 +-> s- CD CD CD CO CO co E Li_ I Gm-Gms Gh-Gl Gt-Gp Sh-Sl St-Sp Sr Fl-Fm .27 .60 .13 .29 .57 .14 .67 .20 .13 .50 .13 .38 .50 Gt-Gp 80 Figure 21. Typical sequence of subfacies and textures for the diffuse sandy conglomerate lithofacies: Gh/Gl-Gt-Sh/Sl. 8 1 |o«bC poq, oop » Ooo&® OOOC? v 00 OOP oowr, * cr<» o*o. __ * * * OOa • - - . rfvf>o oSc> . • • Q ttaO .... , 0* Sh-Sl Gh-Gl Sp Sh-Sl Gt Sh-Sl Gt Sh-Sl Gt Gh-Gl Sh-Sl Gh-Gl 82 sheets of water, usually less than 30 cm (Bull, 1964b, 1972) deep, spreading across the distal fan surface and depositing sheetlike sandy conglomerate. Bull (1972) also noted that these types of deposits are generally well- sorted and may be cross-bedded, laminated, or massive. The crudely stratified sandy conglomerate probably accumulated on low-relief longitudinal and transverse bars during high flow. As they migrated downstream, the forset beds accreted to their downstream ends (Sailer and Dickinson, 1982). Graded and trough cross-bedded conglomerate and sandstone probably formed in shallow interbar channels, or scours found at the lee of bars (Miall, 1977). Planar cross-bedded pebbly sandstone is attributed to migration of transverse or linguoid bars (Smith, 1970; Miall, 1978). Miall (1977) also noted that flood conditions or a confinement of channels was required, and that these bedforms are relatively rare in braided-stream environments. However, Hein and Walker (1977) suggested that gravel bars with forsets may develop in the same channel as longitudinal bars under conditions of reduced sediment and water discharge. Eynon and Walker (1974) suggested that their origin was a product of delta like growth from an eroded bar remnant. The observed upward-fining sequences are attributed to cyclic flood conditions in which the flow regime was steadily reduced. The homogenous texture was due to 83 sorting, as these deposits occur near the distal reaches of the fan surface. Fine-grained textures are generally absent, as they were eroded by subsequent high flows. The variation of this sequence was caused by either the infilling of shallow depressions or migration of bar forsets. It is though that the former was generally the case, as longitudinal bars typically acrete and less commonly migrate downstream. Pebbly Sandstone Lithofacies Lithofacies Description The pebbly sandstone lithofacies consists of more than 70% medium- to coarse-grained sandstone with minor conglomeratic horizons and siltstone lenses. This lithofacies is well lithified and has a pale red (10R6/2) color. It is found throughout both the Gene Canyon and Copper Basin Formations and forms approximately 25% of the measured sections. These units have a tabular or sheet-like geometry, extending laterally for several hundreds of meters. The lower contact is usually gradational, and less frequently erosional, exhibiting both shallow and deep incisions by higher-energy channels. Thickness varies from 2 to 10 m, and is generally much less than the lateral dimensions. 84 Lateral facies relationships include both finer- and coarser-grained lithofacies, and are usually transitional with each other. This lithofacies is generally well-sorted as indicated by the cumulative log-normal plot (Fig. 44). The mean grain size is 0.6mm (0.750) and has a standard deviation of 0.880. Maximum size of grains with a well- sorted population is 2 mm, with outsized clasts ranging up to 10 cm. The outsized clasts are usually isolated within the sand matrix. Grains are in contact with each other and are subangular to subrounded. They show no obvious preferred orientation. However, on bedding surface exposures, parting lineations were observed, where rocks broke along preferred planes which were parallel to planar laminations within the rock. The internal character of this lithofacies is dominated by massive to planar-laminated sandstone (Sm, Sh) shown in Figure 22. The massive appearance is often caused by weathering, in which calcite coatings form on the rocks. Where washed by recent streams, these units often have well-developed planar laminae, which occur in sets as much as 3 m thick. These planar laminae are commonly transitional with low-angle, cross-laminated sandstones, with tangential bases (Si). Planar (Sp) and trough (St) cross-laminated sandstone is far less common and form in sets ranging from 10 to 30 cm thick. Ripple 85 Figure 22. Stream-washed pebbly sandstone lithofacies exhibiting planar laminae. Note the pebble horizon within the sand matrix. The measuring stick is 1 m long (C.B. Formation, Parker Dam School). 86 cross-laminae (Sr) occasionally occur, varying from 1 to 2 cm thick (Fig. 23). Climbing ripples also occur (Fig. 23), and range from 1 to 2 cm. Fluid escape structures are rare but were found in association with climbing ripples. Irregular siltstone rip-up clasts occasionally were observed floating in the sandstone matr ix. Sandstone of this facies commonly exhibits normal grading. Bedding-plane structures include ripples with heights from 2 to 3 cm, cut and fill structures, and rare grooves. The vertical sequence is dominated by horizontal and low-angle cross-laminated sandstone. Several patterns emerged, however, from Markovian analysis (Figs. 24,25). Conglomerate horizons are usually followed by planar- laminated sandstone? this sequence is repeated several times. Occasionally, a complete sequence of Gh-Gl--- Sh-Sl St-Sp Sr fl-fm was noted, which indicates a decreasing flow regime. Environmental Interpretation Laterally extensive, sharply-based sandstone in association with distal braided stream deposits suggests a sheet-flood origin (Bull, 1964a, 1972? Rahn, 1976? Heward, 1978b? Brookfield, 1980). These deposits usually are 88 Figure 23. Primary sedimentary structures within the pebbly sandstone lithofacies (C.B. Formation, Parker Dam School). (a) small-scale planar cross-laminae. The pencil is 14 cm long. (b) small-scale climbing ripples. The hammer is 26 cm long. (c) bedding surface ripples. The lense covered is 5 cm in diameter. (d) silt rip-up clasts within a sandstone matrix. The hammer is 26 cm long. 89 9 0 Figure 24. Transition probability matrix and spider diagram for the pebbly sandstone lithofacies. 91 Gm -Gm : Gh-Gl Gt- G p Sh-Sl St-Sp s- GO EE U_ i Li_ Gm-Gms Gh-Gl .80 .20 Gt-Gp Sh-Sl .18 .64 .18 St-Sp .83 .17 Sr .70 .30 Fl-Fm 1.0 ,20 .80 .64 t . St-Sp Sh-Sl Gh-Gl .83 .70 Sr - 2 0 Fl-Fm 92 Figure 25. Typical sequence of subfacies and textures for the pebbly sandstone lithofacies: Sh/Sl-St- Sr. 93 lm V X \ «60 - < > P 0 0 °~* OBO Sh-Sl Sr FI Sp Sh-Sl Gh-Gl Sh-Sl Sr St Sh-Sl Sm Sh-Sl St Sh-Sl Sm Sh-Sl 94 found in the distal region of an alluvial fan (McGowan and Groat, 1971; Steel, 1974) and interfinger with basin plain alluvial sediment. In fact, many of these units might actually represent desert-floor ephemeral stream systems, characteristic of the axial basin plain (Brookfield, 1980) . The laterally persistent sheet-like geometry of this lithofacies and the lack of channeling imply a relatively flat to very low-angle depositional surface. Sediment laden flood waters spread out in thin sheets and deposit thinly-bedded, well-sorted sediment (Bull, 1964a, 1972). Constant lateral shifting and the intermittent frequency of flood discharge inhibit entrenchment of the underlying surface, and results in the vertical accretion of planar- laminated, pebbly sandstone. The well-sorted nature of these deposits and their relatively fine-grained size distribution implies a longer transport distance than previously discussed lithofacies. The cumulative curve (Fig. 44) is similar to the sheetflood cumulative curves of Brookfield (1980) and suggest transport by saltation. This lithofacies occasionally receives coarser-grained fan sediment, which commonly occurs as out-sized clasts or thin pebble to cobble horizons. These clasts are transported from more proximal portions of the alluvial fan during extremely high flood conditions, when flow velocities are higher 95 than normal. As flow conditions return to normal, coarse sediment is entrained within pebbly sandstones. The dominance of planar-laminated sandstone and the ubiquitous parting lineations suggests that deposition occurred under transitional flow regime conditions (Harms and Fahnstock, 1965; McKee and others, 1967). Parting lineation has also been suggested to be the result of low- velocity currents (Picard and High, 1973). However, the thick, unchanging sequences and lithofacies associations do not support waning flow conditions, which would be required for low flow velocity. The planar and trough cross-laminated sandstone is caused by the migration of linguoid (lobate and transverse) bars (Collinson, 1970; Smith, 1971a, 1972; Boothroyd and Ashly, 1975; Miall, 1977). If the channels are relatively straight and the formation of bars insignificant, they may be due to dune (megaripples) or sand-wave migration (Smith, 1971a, 1972). Both processes may have been involved in the deposition of this lithofacies, with the larger-scale cross-laminae associated with migrating ripple bedforms. Small-scale ripples and ripple cross-laminae form during low-water stages across the top of low-relief bars (Smith, 1971a, 1972). They also form on reactivation surfaces during rising water conditions (Miall, 1977). The observed climbing ripples, convolute laminations, and 96 rare dewatering structures imply rapid sedimentation rates due to a sudden loss of depositional energy. This often is caused by over-bank flooding, ponding, or currents entering a playa lake. Rip-up clasts also occur and probably were due to channels entrenching into distal plain/lacustrine deposits. They also may be attributed to caving in of channel banks usually associated with the basin floor. The vertical sequences are once again associated with episodic flooding, so characteristic of semi-arid regions. Planar-laminated sheetflood deposits accumulated under high flow regimes. As the flood waters receded, dunes migrated across the bar surfaces, producing planar and trough cross-laminated bedforms. At low water conditions, small-scale ripple cross-laminae developed. The low probability of encountering cross-laminations, however, is attributed to their low preservability, as subsequent flood conditions remove the top of the preceding deposit. In addition, the sheetflood mechanism is by far the dominant process associated with this lithofacies. Interbedded Fine Sandstone and Siltstone Lithofacies Lithofacies Description The interbedded fine sandstone and siltstone 97 lithofacies is composed of discrete beds of sandstone and siltstone, with sandstone/siltstone ratios ranging from 3:1 to 1:4. Conglomeratic stringers are rare and comprise less than 5% of this lithofacies. Within the Gene Canyon Formation, these units are typically pale yellowish brown (10YR6/2) to pale brown (5YR5/2), whereas they are pale red (10R6/2) in the Copper Basin Formation. Exposures are infrequent due to the friable nature of these deposits. This lithofacies is nearly always found in association with the fine-grained calcareous siltstone-mudstone lithofacies. The external shape of this lithofacies is tabular and sheet-like. The lower and upper contacts are commonly gradational with both finer and coarser-grained lithofacies. The upper contact occasionally displays shallow channeling. The thickness of this lithofacies varies considerably and is transitional with fine-grained lithofacies, making measurements difficult. The lateral extent, however, is very continuous, measuring upwards of 200 m before being removed by erosion. Fine grain size and extreme lateral continuity are the distinguishing characteristics of this lithofacies. These units exhibits a bimodal segregation of textures into discrete fine-sandstones and siltstones (Fig. 26). The sandstone is well-sorted as shown by the cumulative log-normal plot (Fig. 44). The mean grain size 98 Figure 26. Internal configuration of the interbedded fine sandstone and siltstone lithofacies. (a) illustrates convex-upward bar, and (b) illustrates conves-downward channel (G.C. Formation, Desilt Wash). 99 100 is 0.2 mm (2.250) with a standard deviation of O.3OJ0. The siltstone to very-fine sandstone is well-sorted (Fig. 44), with a mean grain size of 0.07 mm (3.75J0). Sand grains are subrounded to rounded and clast-supported. Internally, this lithofacies predominantly consists of tabular to lenticular beds, which are distinctly defined by abrupt contacts. The sandstone units are tabular and lensoidal with upper surfaces commonly convex- up (Fig. 26). Thicknesses vary from 1 to 3 m and range laterally as much as 50 m. The lower contacts are commonly planar erosional and the upper contacts are gradational. Interbedded siltstone is usually tabular, with thicknesses as much as 2 m. These siltstone beds are laterally quite persistent. The intercalation of discrete sandstone and siltstone beds is diagnostic of this lithofacies. The sandstone subfacies are primarily massive to planar-laminated units (Sm,Sh), which vary in thickness from a few centimeters to 3 m. Planar and trough cross laminated units (Sp,St) are less common, and range from 4 to 10 cm thick (Fig. 27). Ripple cross-laminated units (Sr) rarely occur. Scour and fill structures are common, and are filled with sand and silt with low-angle cross laminae. Beds typically fine-upwards, with rare gravel lag deposits at the base. 101 Figure 27. Sedimentary structures of the interbedded sandstone and siltstone lithofacies. (a) Ripple cross-laminae. Note possible dish structures below. Lense cover is 5 cm in diameter (C.B. Formation, Gen Reservoir). (b) Bedding surface ripples. Lense cover is 5 cm in diameter (C Dam School). (c) Small-scale planar, is 14 cm long (G.C. Wash). (c) Small-scale trough, is 14 cm long (G.C. Wash). B. Formation, Parker cross-laminae. Pencil Formation, Desilt cross-laminae. Pencil Formation, Desilt 102 f p 1 0 3 Subfacies within the siltstone include planar- laminated (FI), ripple cross-laminated units (Sr), and less commonly, beds with climbing ripples. Siltstone beds usually fine-upwards and range from 10 to 15 cm thick, with abrupt, nonerosional upper and lower contacts. They also are found to top sandstone beds, usually less than 10 cm thick, and represent waning flow conditions. Bedding surface structures include scour and fill, generally less than 5 cm deep, and subaerial dessication features, such as up-turned mud partings and mudcrack polygons. Two vertical sequence scales were established for this lithofacies (Figs. 28,29). Individual sedimentation units, from 50 to 120 cm thick, are commonly fining- upwards sequences: Sm(Gm)-Sf-Sp-Sr-Fl. Convex-upward sequences (bars) usually lack well-formed planar cross beds, but displayed the following succession: Sf-Sl-Sr-Fl. Scour and fill sequences are similarly fining-upwards, but contain trough cross-laminated sandstone units (St) and gravel lag deposits (Gt), which conform to the low-angle depositional surface, and have the following sequence: (Gm-Sm)-Sh-St-Fl. At the lithofacies scale, the vertical sequence of beds were both thickening and coarsening, as well as thinning and fining upsection; however, the former is more common. 104 Figure 28. Transition probability matrix and spider diagram for the interbedded fine sandstone and siltstone lithofacies. 105 tn E j _ CL CL £ CD | CD CD oo i go Li_ 1 E _cr +-> 1 -c 1 4-> S- 1 i — CD CD CD oo CO CO Ll_ Gm— Gms Gh-Gl 1.0 Gt-Gp Sh-Sl .27 .07 .67 St-Sp .10 .71 .19 Sr .13 .82 Fl-Fm .71 .04 .29 .67 .27 .82 ♦ Fl-Fm Sh-Sl St-Sp . 7 1 .29 .71 106 Figure 29. Typical sequence of subfacies and textures for the interbedded fine sandstone and siltstone lithofacies: Sm-Sh/Sl-Fl/Fm. 107 2m f ? Q O O ^ ? f V > >ng< > Q p D b n - r \ ? > £ * > —v f T 5 • Fl-Fm Sh-Sl Sm Gh-Gl Fl-Fm Sr Sh-Sl Sm Sh-Sl FI Sr Sp Sh St Sh-Sl Fl-Fm Sh-Sl Sm Fl-Fm Sh-Sl Sm 108 This lithofacies displayed the most subfacies transitions within the Gene Canyon and Copper Basin Formations, and provided excellent paleocurrent data. Environmental Interpretation The fine-grain size, laterally continuous beds, channel and bar geometries, and abundance of sedimentary structures suggest a relatively low-energy depositional environment where traction currents and suspension processes are common. Although several depositional environments can be envisaged with these features, the vertical and lateral facies relationships suggest that these deposits represent basin-floor, alluvial plain sedimentation (Steel, 1974; Steel and Wilson, 1975; Brookfield, 1980; Nilson, 1982), where fluvial and lacustrine processes dominate. Several alluvial plain facies interfinger with alluvial fans and include braided and meandering channels, levees, and playa lakes. These systems are typically oriented parallel to the adjacent ranges and are, therefore, perpendicular or at high-angles to outbuilding fans (Nilson, 1982). A complex interrelationship of fan and alluvial plain facies make these facies difficult to distinguish in ancient deposits (Larsen and Steel, 1978) . These alluvial plain deposits are distinguished from more 109 proximal lithofacies by their better sorting, finer-grain size, lack of mud and debris flows, and extensive sheet sands. The laterally persistent beds are caused by channel widening and deposition of sand sheets during early stages of discharge. Construction occurs during later stages, when fining-upwards cycles develop as the flood wanes (Schumm and Lichty, 1963; Steel, 1974; Brookfield, 1980). The distinctive channels and bars (Fig. 26), which are laterally transitional with sheet sand, and interbedded with siltstone, represent constant high-flow velocities within a fluvial system. Their close proximity to lacustrine deposits might also suggest a deltaic or channel splay depositional mechanism. Both models explain their fining and coarsening-upwards sequences. Planar (Sh) and low-angle (SI) cross-laminated sandstone units are deposited by high-flow regime currents associated with sheetflood processes as discussed above. Trough cross-laminated sandstone may be attributed to either scour fill or the migration of linguoid bars and dunes associated with fluvial systems (Smith, 1971a, 1972; Miall, 1978). Planar cross-laminated sandstone units represent bar forsets, or possibly overbank deposits (Mckee and others, 1967) . Small-scale ripple and ripple- drift cross-laminae are associated with low-flow regimes during waning flood conditions. Reactivation surfaces 110 represent fluctuating current velocities and sediment supply. The dominant sequence of sedimentary structures indicates a decreasing flow regime, which is repeated episodically. The interpretation is that of intermittent flooding associated with semi-arids regions. A fluvial meandering model could also produce this sequence, and may, in fact, contribute to these deposits. The coarsening upward cycles may be attributed to prograding channel bars and/or small deltas building into ponded waters. To summarize, this lithofacies probably represents several depositional subenvironments which are associated with alluvial fan fringes. Paleocurrent data is insufficient to determine if these deposits were transported along the basin axis, or radiated outward from a point source* Their close association with lacustrine deposits, however, places them distal to an actively developing alluvial fan-fluvial sequence. Calcareous Siltstone - Mudstone Lithofacies Lithofacies Description Exposures of the calcareous siltstone-mudstone lithofacies is limited, because it generally has been 111 eroded and buried by more recent sediment. Where preserved, the lithofacies forms colorful light olive grey (5YR/2), greyish red-purple (5RP4/2) and pale reddish- brown (10R5/4) hills. This lithofacies consists of more than 60% siltstone and mudstone, and from 10 to 20% fine grained sandstone. Limestone and conglomerate horizons make up the remainder of this unit. These units are found predominantly in the lower sections of the Gene Canyon Formation, and are rarely found in the Copper Basin Formation. This lithofacies is quite persistent, and ranges up to 200 m laterally. The thickness varies from 10 to 30 m, giving an overall sheet-like shape. The lower contacts are nearly always gradational with the interbedded fine grained sandstone and siltstone lithofacies, and may be defined on the basis of the calcareous mudstone component. The position of the upper contact varies significantly, depending on the overlying lithofacies. In fact, at several localities boulder breccia and massive conglomerate lithofacies cut deep incisions into this unit. Laterally, this unit is commonly transitional with the interbedded fine-sandstone and siltstone lithofacies. The grain size of this lithofacies ranges from 0.01 to 0.18 mm, with a mean grain size of 0.06 mm (4.2 0) shown in Figure 44. Lenses of coarse-grained sandstone and infrequent pebble horizons also occur. 112 Grains are generally in contact with each other, but often exhibited a polymodal distribution, making it difficult to distinguish between framework grains and matrix. Abundant calcite cement also disguised the nature of the fabric. Detrital grains are typical subrounded to rounded. The internal character of this lithofacies is distinctly stratified, composed of tabular beds measuring from 30 cm to 5 m thick. Beds are bounded by gradational to planar non-erosional contacts, which are marked by changes in texture and/or color. Thin, lenticular sand horizons are present within the massive siltstone. The subfacies are primarily thinly-laminated to massive siltstone units (Fl,Fm). Ripple cross-laminae (Sr) occasionally occur, and range from 2 to 4 cm thick. Wavey to lenticular laminae are much prevalent and are generally associated with the sandier portions of this lithofacies. Subaerial dessication features are common and include mudcracks and upturned mudstone edges (Fig. 30), and small-scale sand dikes. Calcareous and cherty nodules and ribbons occur throughout this lithofacies, and aided in its recognition. Fine tracks and thin burrows (?) also are present. Statistical vertical sequence analysis was not performed, as the number of subfacies transitions were judged insufficient. However, a decreasing mechanical 113 Figure 30. Subaerial dessication features of the calcareous siItstone-mudstone lithofacies (G.C. Formation, Desilt Wash). (a) Upturned edges and polygonal mudcracks. Scale is 10 cm long. (b) Well-formed mudcracks in silty limestone. Pencil is 14 cm long. 114 energy sequence was noted within individual beds with the following sequence of subfacies: Sh-Sr-Fl. In addition, the beds tend to coarsen and thicken upwards. Environmental Interpretation The fine-grained nature of these deposits, ubiquitous carbonate (see petrography section), and lateral continuity of beds require a low-energy, quiet-water depositional setting. These features are consistent with lacustrine deposition (Picard and High, 1972); Link and Osborne, 1978? Brookfield, 1980; Fouch and Dean, 1982; Nilson, 1982), which range from small, temporary lakes associated with overbank flooding and temporary ponding, to large, permanent bodies of water. The thin, laterally continuous beds which occur as layers within the sequence of alluvial fan lithofacies were deposited during pluvial intervals, as extensive blanket-like deposits (Bull, 1972). Less continuous, more-localized units are interpreted as temporary ponding, caused by either overbank flooding or channel avulsion (Brookfield, 1980). The latter are generally much more indurated, due to the high percentage of carbonate cement, and typically display many more subaerial dessication features. The thicker and laterally continuous sequences near Red Mountain (easternmost section) were deposited in 1 1 6 deeper, quiet-water, where suspension processes dominated and deposits were interbedded with fluvial or deltaic deposits. The thinly-laminated siltstone beds are very fine grained and well-sorted, indicating suspension and low- energy traction processes. Thin-bedded, fine-grained sandstone was often observed within these sequences and are considered to represent periodic flooding, where sand is transported beyond the fringes of a fan or to within ponded areas. Mudcracks, small-scale sand dikes, upturned shale polygons, and thin laminae of gypsum all imply subaerial exposure where dehydration and clay shrinkage occur. Although relatively uncommon, ripple cross-laminae (Sr) were observed to overlie planar-laminated (Sh), thinly-bedded sandstone. These ripple cross-laminae represent ripple migration across low sand bars during waning flood conditions (Smith, 1971, 1972). They also have been described to occur on reactivation surfaces as current velocities increase (Miall, 1978). 117 Volcanic and Volcaniclastic Lithofacies Lithofacies Description The volcanic and volcaniclastic lithofacies are found in the upper part of the Gene Canyon Formation and are absent from the Copper Basin Formation within the study area. They occur most frequently to the north of the study area, where they are found throughout the Gene Canyon Formation (Aubrey Hills-Lower Whipple Wash), and also as resistant cliffs within the Copper Basin Formation. The volcanic units were not emphasized in this study, and will, therefore, only be described briefly. Several individual volcanic flows were recognized which are characteristically interbedded with the channeled conglomerate lithofacies. The flows range from 5 to 15 m thick and are quite weathered (Fig. 31). Frost (1983) has described similar volcanic flows as probably being andesites or basaltic andesites, with plagioclase phenocrysts having An contents of approximately 40 to 50%. Several volcaniclastic units also were noted, which consist of water-laid tuffs and breccias. The tuffs are 5 to 20 m thick. Volcanic breccias were noted at the base of several andesitic flows, which are very poorly-sorted and matrix-supported (Fig. 31). These breccias range from 2 to 5 m thick and are very similar to the boulder breccia 118 Figure 31. Volcanic and volcaniclatic lithofacies of Gene Canyon Formation (Desilt Wash). (a) Volcanic breccia which is very poorly- sorted and matrix-supported. Jane is 1.7 m tall. (b) Massive andesite flow overlying scoriaceous unit. The measuring stick 1 m long. (a) (b) 120 lithofacies described earlier. Ash-flow tuffs with flattened pumice fragments were not observed within the study area. Environmental Interpretation The volcanic and volcaniclastic rocks of the Gene Canyon Formation may have been associated with a caldera complex, which was located in the vicinity of the Aubrey Hills-Lower Whipple Wash area (Frost, 1983). This conclusion is based largely on the ash-flow tuffs and thick volcanic sequences observed in the nearby Abrey Hills. The volcanic flows and water-laid tuffs within the study area, therefore, would represent more distal volcanic deposits, which commonly are interbedded with valley-fill sediment (Vessell and Davies, 1981). N. Suneson, however, has suggested that these units were probably derived from local rhyolitic domes, as the tuffs are not welded (N. Suneson, personal communication, 1983). The thick volcanic flows within the Copper Basin Formation, on the other hand, are thought to have been derived from several different localities where abundant hypabyssal rocks (intrusive andesites) have been observed (Frost, 1983). 121 STRATIGRAPHY Introduction The mid-Tertiary volcanic and sedimentary strata which overlie the upper plate crystalline complex are sub divided into three units: the Gene Canyon Formation, the Copper Basin Formation, and the Osborne Wash Formation (Ransome, 1931). These formations are generally separated by unconformities in the eastern Whipple Mountains, and have been distinguished primarily by their angular relationships to the underlying crystalline basement complex and to each other. Details of their specific lithologies and the distribution of their lithofacies have not been defined. This section presents the vertical and lateral sequence of lithofacies within the Gene Canyon and Copper Basin Formations. A brief description of the Osborne Wash Formation is included to demonstrate the changes in depositional style, which occurred subsequent to detachment-related deformation. Lithofacies are commonly grouped into assemblages by their vertical and lateral associations (Miall, 1978? Rust, 1978? Heward, 1978b? McGowan and Groat, 1972). These lithofacies assemblages are considered to represent 122 specific depositional environments forming the basis from which the depositional setting can be reconstructed. The sequence of lithofacies observed in this study is similar to those identified by McGowan and Groat (1972), and have, therefore, been classified as proximal, medial, and distal alluvial fan deposits. In addition, features consistent with alluvial plain (Steel and Wilson, 1975) and lacustrine deposition (Picard and High, 1972) have been recognized and are classified accordingly. These associations are shown in Table 1. Transition probability matrices were constructed for each section to determine the most probable sequence of lithofacies. These have been correlated across sections to determine both the areal and vertical distribution of lithofacies. Caution should be exercised when using this statistical model for single sections, however, as the number of lithofacies transitions are limited. Pooling of lithofacies from all of the sections provides significant results concerning probable sequences, and also assists in recognizing the lithofacies assemblages (Fig. 32). Three detailed stratigraphic sections were measured along the eastern portion of the Whipple Mountains in order to establish the vertical and lateral distributions of lithofacies (Fig. 33). In addition, several reconnaissance sections were studied to determine general 123 Figure 32. Pooled probability transition matrix and spider diagram for the Gene Canyon and Copper Basin Formations. S1.-S10 represent lithofacies 1-10 (Table 3) . 124 si S2 S3 S4 S5 S6 S7 S8 S9 S10 SI .44 .22 .11 .11 .11 S2 .22 .39 .05 .05 .11 .17 S3 .11 .26 .32 LO o .11 .05 S4 .05 .36 .18 .23 .05 .05 .05 S5 CO o .15 .54 .15 .08 S6 .08 .08 .25 .33 .17 o 0 0 S7 .17 o 0 0 .17 .08 .25 o 00 , .08 .08 S8 .17 .50 .33 S9 .75 .25 S10 .14 .57 .29 .50 125 trends within the Copper Basin Formation, where lithologic variation was more limited (Fig. 33). These sections provide excellent exposures of both the Gene Canyon and Copper Basin Formations, and allowed for sampling across a portion of the depositional basin. They also were selected to minimize structural complications due to faulting. The sections were measured from the basal crystalline complex to the top of the mid-Tertiary strata, which were usually in fault contact with crystalline basement complex. Sections were measured along stream cuts, and as far as possible, along a linear transect normal to structural strike. The Desilt Wash section is located between Parker Dam School and Gene Reservoir (Fig. 33). This section is the westernmost section studied in detail, and has excellent exposures along intermittent stream valleys. In fact, this locality originally was described by Kemnitzer (1937) as the type section for the Gene Canyon Formation. Approximately 580 m of continuous Gene Canyon section and 160 m of Copper Basin Formation are exposed. A well- defined angular unconformity separates these two formations, which is enhanced by a distinctive color change from reddish brown to reddish orange. The central measured stratigraphic section is located near Parker Dam School, for which it is named (Fig. 33) . 126 Figure 33. Index map (Gene Wash and Black Peak quadrangles) indicating location of measured sections and reconnaissance sections (DW = Desilt Wash; PDS (GC) = Parker Dam School, Gene Canyon Formation; PDS (CB) = Parker Dam School, Copper Basin Formation; RM = Red Mountain; Reconnaissance Sections = A,B,C) . 127 Black Meadow . ' Landing : - i i & F uxr* N/ A T I 0 s p s 1 8 \___ Bluegill ^ cni - Island c r c c l T 12 N U G E Hi Isle f 'TntaR? a’ v i grounds > / ■ X- 1 /w \ j- Lamed Landing f\ n v ^ 4 , n X A MAXIMUM MAXIMUM EL EVA T tON elay S la P ' . \ : r n l L PARKFR DAM Wa r * “ y^Hav(t$u^ Spring * A{ Parker. Dairf . (BK'jlu l Ji-M y / U M PIN G S T A ^ , A . />£• * ^ g a & ^ s~ ' x ? 5 x u NNEL basin Eureka el ay ,jSta 1079 Moo Keys —-- + ~ L ^r-—j h j B risto l l ’ , i c . \ ' : ' 4K£? S p ring | X " U\' P ro s p e c t Xnnel Head AOC 6M 40 a 0 . Wash 172 i g k 128 In addition to its central location, this section was selected for the excellent exposures of ridge-forming, boulder breccia and massive conglomerate lithofacies. The lower contact is covered by recent alluvium, which accounts for the limited 380 m of exposed Gene Canyon Formation. The Copper Basin Formation is 480 m thick and forms possibly the best preserved section outside of the type locality as described by Teel (1983). The two formations are separated by a high-angle normal fault (Frost, 1984). The southern section was measured at Red Mountain, across the Colorado River (Fig. 33). This locality provided the thickest combined section, measuring 860 m. The contact between the Gene Canyon and Copper Basin Formations, however, was difficult to establish due to the lack of a clear angular unconformity and the presence of similar lithologies. This section also provided data from the Copper Basin Block, or a more southerly position within the depositional basin. Gene Canyon Formation The Gene Canyon Formation (Fig. 34) is the oldest of the three mid-Tertiary formations and nonconformably overlies the upper plate crystalline complex. The beds 129 Figure 34. Typical exposure of the Gene Canyon Formation outlined by dashed line. View looking toward the northwest from Red Mountain. 130 131 are parallel to subparallel with the contact of the underlying crystalline rocks (Frost, 1983? Teel and Frost, 1982), and nearly always exhibit a steeper dip than the overlying Copper Basin Formation (Davis and others, 1979a). The angular unconformity between the two formations, however, grades to a conformable relationship at Red Mountain and other localities to the south and is often difficult to define (E. Frost, personal communication, 1982) . The type section has been described as a buff- colored, coarse-grained sandstone, fanglomerate, and interbedded breccia, composed of metamorphic and volcanic fragments (Kemnitzer, 1937). In a regional study, Frost (1983) described and interpreted the lithologies as interbedded lakebeds, debris flows, fanglomerate, volcanic and tuffaceous flows, limestone and pebbly sandstone. He observed a vertical profile for the Colorado River synform as follows: (1) basal units are nearly all lakebeds, pebbly siltstones, or arkosic (Gruss) sandstones, (2) mid- and upper-portions are predominantly coarse debris flows, landslide deposits, and coarse fanglomerates, and (3) the upper portions are primarily volcanic flows. Details of lateral associations and the vertical sequence of lithofacies, however, were not addressed. 132 The age of the Gene Canyon has not been precisely determined. However, bracketing has indicated an age between 32 and 18 m.y.b.p. (Davis and others, 1982). An artiodactyl track was discovered by Kemnitzer (1937), which suggested an age of late Oligocene to early Miocene. Isotopic studies of basal volcanic flows have given dates which range from 25.7 + 1.7 m.y.b.p (K/Ar, biotite) to 31.8 + 3.2 m.y.b.p. (K/Ar, biotite) or late Oligocene (Martin and others, 1980; Davis and others, 1982). Basal volcanic flows of the overlying Copper Basin Formation yield dates ranging from 17.1 + 0.8 m.y.b.p. (K/Ar, whole rock) to 18.7 + 0.6 m.y.b.p. (K/Ar, whole rock) or mid- Miocene (Martin and others, 1980; Davis and others, 1980b). The ages of the correlative Artillery Formation in the adjacent Rawhide and Buckskin Mountains are compatible with these ages (Lasky and Webber, 1949; Ebberly and Stay, 1978; Luchitta and Sunneson, 1980). Desilt Wash Section The Desilt Wash section (Fig. 35) is composed of several cycles of lithofacies, which are grouped into assemblages, representing a set of closely-related depositional environments. The sequence of lithofacies and assemblage groupings is confirmed by the transition 133 Figure 35. Desilt Wash stratigraphic section. 134 desilt wash gure 36. Transition probability matrix and spider diagram for the Desilt Wash stratigraphic section. S^-S^q represent lithofacies 1- (Table 3). SI S2 S3 S4 S5 S6 S7 S8 S9 SI 0 SI .33 .33 .33 S2 .67 .33 S3 .13 .50 .13 .13 .13 S4 .44 .22 .11 .22 .22 S5 .25 .50 .25 S6 .33 .33 .33 S7 .50 .17 .17 .16 S8 .67 .33 S9 .50 .50 S10 1.0 S6 * -^ n -— S7 137 probability matrix (Fig. 36). Each of the previously described lithofacies were observed in this section, which suggests rapidly changing depositional environments ranging from the proximal regions of an alluvial fan to distal alluvial fan and lacustrine deposits. The lower part of this section, which nonconformably overlies the crystalline upper plate, is composed predominantly of distal to medial fan assemblages, with an interbedded proximal landslide deposit. Several cycles of fining-upward sequences were observed, where conglomerate is overlain by finer-grained lithofacies. These cycles probably represent the early stages of fan development, where distal lobes began to prograde onto older alluvial plain deposits. Overlying these deposits is a thick sequence (94 m) of very coarse-grained proximal deposits, which include landslide, stream-channel and debris flow lithofacies. Several fining-upward sequences were observed, where channeled conglomerate lithofacies grade into braided deposits. The mid-portion of this section is composed of distal lithofacies, which are predominantly alluvial plain and lacustrine deposits. Several fining-upward cycles were noted in which sandy conglomerate was topped by thick sequences of fine-grained sandstone and siltstone. 138 Interestingly enough, a very thick boulder breccia deposit was interbedded with these distal lithofacies, which may indicate a relatively long transport distance of this proximal lithofacies, or abrupt local tectonic activity. The first sign of volcanic influence is a thin tuffaceous bed immediately overlying these fine-grained lithofacies. This isolated volcaniclastic lithofacies is overlain by a thick sequence of sandstone and conglomeratic sandstone in which both coarsening- and fining-upward sequences occur. Overlying the sandstone "package" is the dominant lithofacies association of the upper part of this section. More than 125 m of braided-stream deposits which represent mid-fan deposition are present. They are characterized by a rhythmic alteration of thin conglomerate horizons and sandstone. An overall coarsening-upward sequence was observed, where the frequency of channels, percent conglomerate and maximum clast size increases. This sequence is interpretated as prograding alluvial fan lobes in which more proximal deposits were deposited on more distal deposits. This section is topped by a volcanic sequence which includes andesite flows, tuffs, and stream-channel conglomerate. Three separate volcanic flows were noted which are interbedded with air-fall and water laid tuffs. 139 Figure 37. Parker Dam School Stratigraphic Section. S C H O O L SE CTI ON STRUCTURES LITHOLOGY and < ■ WENT WORTH MAX. PARTICLE CCNGL. RALE OCURRENT o c3°o^ qO ^ c o ° O CD-O X-S1Q - w SECTION COVERED WITH ALLUVIUM METER Reworking of these deposits resulted in the deposition of coarse-grained conglomerate which usually exhibit channeling. Parker Dam School Section The Parker Dam School section (Fig. 37) is dominated by proximal to medial alluvial fan lithofacies, and a thick capping sequence of volcanic and volcaniclastic deposits. From the probability transition matrix (Fig. 38), it is apparent that many transitions occur between the boulder breccia and massive conglomerate lithofacies (1-2,2-1). Also, lithofacies 3,4, and 5 are typically found in association, which agrees with the lithofacies assemblage model, indicating proximal and medial braided stream deposits of an alluvial fan. The lower contact and basal portion of this section are covered by recent alluvium. The lowermost exposed lithofacies are proximal to medial fan associations which coarsen-upwards. The distinctive diffuse sandy conglomerate is typically overlain by the interbedded sandstone and conglomerate lithofacies (5-4). This sequence has gradational contacts and represents the progradation of medial braided stream deposits over more distal braided to sheet-flood deposits. 142 Figure 38. Transition probability matrix and spider diagram for the Parker Dam School section. S^-Sio represent lithofacies 1-10 (Table 3). 143 SI S2 S3 S4 S5 S6 S7 S8 S9 S10 SI .75 .25 S2 .67 .33 S3 .50 .50 S4 .33 .67 S5 o LO .50 S6 1 .0 S7 S8 S9 1.0 SIO .15 .42 .28 .15 S5 ■ 50 > S6 144 A major change occurs above these lower deposits, with the appearance of a thick sequence of volcanic flows and the occurrence of a very distinctive interbedded massive conglomerate unit. The lowermost andesite is truncated by a major feeder-channel composed of boulders and cobbles, with massive to crude stratification. The axis of the channel is more than 30 m thick and trends northwest. The lower contact is convex-downward, and abruptly pinches out laterally into the fine-grained lithofacies. This unit is overlain by volcanic flows and is interpreted as a proximal channel, perhaps very near the apex of an alluvial fan. The upper part of this section is dominated by thick sequences of monolithic boulder breccias, which, in turn, are overlain by a sequence of volcanic and volcaniclastic lithofacies. The boulder breccias lithofacies consist of very poorly-sorted, clast- and matrix-supported units with boulders exceeding 10 m in diameter. A change in composition from leucocratic adamellite to adamellite with abundant mafic zenoliths was observed, and used to separate two major phases of deposition. The chaotic nature of these deposits did not permit further division. Several volcanic flows cap this section, which are interbedded with channeled-conglomerate lithofacies and one volcaniclastic lithofacies. The upper contact is a 145 Figure 39. Red Mountain Stratigraphic Section. 146 RED MOUNTAIN SECTION Figure 40. Transition probability matrix and spider diagram for the Red Mountain section. Si-S;lo represent lithofacies 1-10 (Table 3). 148 SI S2 S3 S4 S5 S6 S7 S8 S9 S10 SI 1.0 S2 .43 .14 .14 .29 S3 .14 .14 .29 .14 .29 S4 .40 .20 .20 .20 S5 .50 .50 S6 .22 .33 .33 S7 00 00 .67 S8 .50 .50 S9 1 .0 SIO 1 .0 SIO S9 .20 .14 .21 ♦ S4 ♦ S5 .50 .40 .14 .43 .33 .29 .50 .67 S6 S8 3^ .50 149 high-angle normal fault. Red Mountain Stratigraphic Section The Red Mountain section (Fig. 39) consists of a wide range of lithofacies which include depositional environments from the apex of an alluvial fan to lacustrine deposits. In fact, the thick sequence of calcareous siltstone and mudstone represents the distinguishing feature of this section. The transition probability matrix (Fig. 40) clearly establishes several cycles of lithofacies. The lower contact is a nonconformity in which the diffuse sandy conglomerate lithofacies overlies the upper plate crystalline complex. These have been referred to as "gruss" deposits (Frost, 1983), but probably represent well-sorted, distal braided-stream deposits. These strata are overlain by several fining-upward sequences of more proximal lithofacies which signal the development of an alluvial fan complex. A thick (75 m) sequence of very coarse-grained monolithic breccias and massive conglomerate overlies these deposits. The massive conglomerate lithofacies has an irregular basal surface and is probably part of a feeder channel system. Overlying this lithofacies is a 150 thick adamellite unit (45 m), which is extremely chaotic and represents landslide and associated debris flow deposits. This unit is capped by a matrix-supported debris flow lithofacies, which is considerably more heterogeneous in composition. A sequence of interbedded fine-grained sandstones and siltstones overly these coarse-grained proximal deposits. These are characterized by their lateral continuity and fining-upward cycles, and probably represent the outer fringes of an alluvial fan, or possibly an alluvial plain environment. These lithofacies are transitional with a thick sequence (125 m) of calcareous siltstone and mudstone lithofacies, which form the multi-colored hills of this locality. The thickness and lateral continuity of these fine-grained deposits suggests lacustrine deposition as opposed to temporary ponding or overbank deposition. Petrographic analysis indicates the presence of a number of fossils (ostrocods), which supports this interpretation. The upper-middle portion of the Red Mountain section is composed of several fining- and coarsening-upward cycles of proximal and medial alluvial fan lithofacies. These cycles represent the continued growth of the alluvial fan, in which a series of lobes prograde over one another. 151 Volcanic and volcaniclastic lithofacies form the upper part of this section, and as with the previous sections, include several volcanic flows, tuff beds, and reworked channeled conglomerate. The Gene Canyon and Copper Basin contact appears gradational and was not clearly established. Copper Basin Formation The Copper Basin Formation (Fig. 41) directly overlies both the Gene Canyon Formation and the upper- plate crystalline complex. Its relationship to the underlying rocks varies from a pronounced angular unconformity to a gradational contact (E. Frost, personal communication 1982). Locally, the Copper Basin Formation tectonically overlies the upper-plate crystalline complex. Although not observed in this study, its relationship with the overlying Osborne Wash Formation is an angular unconformity (Davis, 1980). The Osborne Wash Formation is flat-lying and postdates detachment-related tectonic activity in the region. The type locality of the Copper Basin Formation is in Copper Basin and was described by Kemnitzer (1937) as "a series of red and reddish-brown sandstones and conglomerates, with hard shales, lava flow and volcanic 152 Figure 41. Typical exposure of the Copper Basin Formation, looking southwest from Parker Dam School (Fig. 33). 153 154 breccias, presumably Tertiary in age." Teel (1983) has recently studied this formation at several localities near Copper Basin and was able to establish lithologies and general depositional cycles. The type locality was described as being composed exclusively of coarse- to fine-grained clastic material suggesting a nearby, tectonically-active, source area (Teel, 1983). A vertical sequence of lithofacies, however, was not defined. The age of the Copper Basin has been bracketed between 15 and 18 m.y.b.p. Carnivore tracks discovered by Kemnitzer (1937) suggest deposition during Miocene time. Isotopic studies of the basal volcanic flows have provided dates of 18 + 0.6 m.y.b.p., 18.0 + 0.8 m.y.b.p., and 17 + 0.8 m.y.b.p. (Martin and others, 1980). In addition, a locally intrusive "turkey-track" andesite was dated at 17.1 + 0.5 m.y.b.p. and 17.9 + 0.7 m.y.b.p. (Martin and others, 1980). Overlying this formation are volcanic flows of the Osborne Wash Formation which have been assigned dates of 13.5 + 1.0 m.y.b.p., 12.9 + 0.6 m.y.b.p., and 15.5 + 2.8 m.y.b.p. (Marvin and Dobson, 1979? Dicky and others, 1980; Martin and others, 1980). 156 Parker Dam School Section The Parker Dam School section (Fig. 42) provides a thick, relatively undeformed sequence of the Copper Basin Formation. This section was selected for the excellent exposures and relative variety of lithofacies compared with other localities. The repetitious nature of the Copper Basin Formation does not not lend itself to statistical treatment at the lithofacies level. That is, the number of lithofacies transitions are limited, nearly always gradational and difficult to distinguish. This discussion, therefore, is directed towards lithofacies assemblages that were observed in this section. It should be noted that variation within each assemblage does occur and that detailed sampling study might resolve the internal sequences. The lower contact of this section is a distinct angular unconformity in which massive boulder conglomerate irregularly overlies the Gene Canyon volcanic lithofacies. Both clast- and matrix-supported fabrics compose this chaotic to crudely-stratified lithofacies, which suggests a proximal feeder stream-channel environment with both traction and gravity-flow processes. Overlying these proximal deposits are massive to planar-laminated, coarse-grained to pebbly sandstone, with 156 Figure 42. Parker Dam School Section. 157 500m 450m 400m 350m 300m 250m 200m 150m 100m 50m 0m o & OO &0O O & C > p^s OOOO &OQO 'SSf7 c & x z E L S ' cPoOfZiS *2 * 1 . l l i < O > £ =3 > -! 2 < LU z L L I CC CC 3 o o L li _l < a MEDIAL DISTAL TO MEDIAL DISTAL N=18 X=528*E L=73% MEDIAL PROXIMAL TO MEDIAL PROXIMAL MEDIAL TO DISTAL PROXIMAL 158 minor conglomeratic horizons. This unit coarsens-upward and is characterized by laterally continuous beds. These are interpreted as a distal alluvial fan lithofacies association, in which sheetflood processes dominate. A thick sequence of massive to alternating conglomerate and sandstone lithofacies compose the middle portions of this section. The overwhelming characteristic is the cyclic repetition of lithologies, giving an overall homogenous appearance. Overall, the unit fines-upward, in which proximal massive conglomerate is overlain by channeled conglomerate and alternating conglomerate and sandstone lithofacies. Internally, however, coarsening- upward cycles are observed. These patterns would suggest a retrogressive sequence of alluvial fan deposition. An overlying sequence of interbedded fine-grained sandstone and siltstone maintains the overall fining- upward trend. Although generally massive, this unit also contains abundant small-scale cross-laminae, which provide valuable paleocurrent information for the Copper Basin Formation. Several fining-upward cycles were observed within this unit, which suggest distal alluvial fan to alluvial plain deposition. Features indicative of rapid deposition include climbing ripples, convolute bedding and rare dewatering structures. This might be attributed to sediment debouching into a standing body of water, which 159 usually occurs beyond the fringes of an alluvial fan. On the other hand, temporary ponding caused by channel avulsion or overbank deposition might produce similar sedimentary structures. The overall thickness (40 m) of this unit, however, suggests that the former is the more likely case. The upper part of this section is once again dominated by prograding mid-alluvial fan lithofacies in which more proximal conglomerate and sandstone overlie more distal diffuse sandy conglomerate. The upper contact of this section is a high-angle normal fault, which has probably removed much of the section. Reconnaissance Sections Three sections (Fig. 43) were studied at a reconnaissance level in which general lithofacies assemblages were described and their sequences noted. Textural and compositional data were recorded to assist in defining depositional trends. Section A. This is the westernmost section examined, and is located adjacent to Gene Reservoir. A number of lithofacies were observed, which were grouped into 4 lithofacies assemblages. 160 Figure 43. Reconnaissance Sections of the Copper Basin Formation. 1 6 1 162 400mr 350m- 300m- 250m - 200m- 150m- 100m- 50m - OmL O O P o V OQbOCO 0 0 0 0 0 0 0 0 * 0 0 ° S B m % s * u . u j jS > o d £ _j > -! z < U J MEDIAL TO DISTAL MEDIAL 6C>°a PROXIMAL TO MEDIAL PROXIMAL SECTION A SECTION B L L I S UL u j sl il < U J MEDIAL DISTAL PROXIMAL 4-5 soOO o MgPga poO ooo <xOo ppg POP P. OCC — k DISTAL pOOO^O oGOO0 0 * D»oO poc*’ Boo® opo*, - C S ? 8 » ■ " p o o " ' < » < * ' 6 i , v * < ? P o P 0 0 v000 COOOO Op £ * * * > £> ocooooe peso* oooo ocoo OO0 Ooe> PROXIMAL MEDIAL SECTION C The basal unit is composed of massive boulder conglomerate, which includes both matrix- and clast- supported fabrics. Although the composition of the boulders was heterogeneous, more than 60% were of volcanic origin (andesite). Several cycles of normal and reverse grading were noted, but overall, this lithofacies assemblage finer-upward. These characteristics suggest proximal alluvial fan deposition, which includes both stream channel and debris flow deposition. The thickness and repeated cycles would indicate several depositional episodes. These deposits grade into the channeled conglomerate lithofacies with occasional interbedded pebbly sandstone. This sequence has an average clast size from 4 to 6 cm and is composed of from 30 to 40% conglomerate. As above, volcanic clasts dominate this lithofacies, forming upwards of 50% of the conglomerate. This sequence is interpreted as proximal to medial alluvial fan deposits, above the intersection point, where entrenchment dominates (Hooke, 1967). A thick sequence of "classic" Copper Basin alternating conglomerate and sandstone lithofacies comprise the middle portion of this section. Shallow channels and bars interfinger and coalesce with each other, giving an overall homogenous appearance. As 163 discussed above, these are composed of numberous fining- upward cycles, which are stacked in coarsening- and thickening-upward sequences. These deposits represent the out-building of the medial portion of an alluvial fan. The lack of an overall fining or coarsening upward trend suggests that steady-state conditions prevailed during this time. That is, uplift and erosion were in equilibrium. The upper portion of this section consists of finer- grained intercalated conglomerate and sandstone units and pebbly sandstone lithofacies. Thinly-bedded, fine-grained sandstones and siltstones, with features suggesting rapid deposition also were noted. This lithofacies association is considered to represent a distal alluvial fan environment, which includes shallow braided stream and sheet flood deposits. The finer-grained sandstone and siltstone units probably represent temporary ponding or overbank deposition. Section B. This section is a thin (200 m) sequence of Copper Basin deposits which unconformably overlies the Gene Canyon Formation at Desilt Wash. This section was studied to continue the depositional sequence at this particular locality within the depositional basin. The basal part of the section is composed of a proximal boulder breccia lithofacies which is less than 164 4 m thick. The boulders average 48 cm in diameter and are supported by a matrix of coarse-grained sandstone. They are composed of almost exclusively volcanic and scoriaceous clasts. This lithofacies is capped by a massive sandstone, which is laterally quite persistent. This sequence is considered to represent debris flow deposits, which are overlain by reworked distal sheetflood deposits. The sequence which follows is very similar to the mid-fan deposits described in Section A. The lower portion is composed of alternating conglomerate and sandstone and sandy conglomerate lithofacies. Compositions are heterogeneous and clasts average from 2 to 4 cm in diameter. These are thought to represent shallow braided stream deposits of the mid-fan region. These lithofacies grade into coarser-grained assemblages, with associated increases in the frequency of channels. These channeled conglomerate and alternating conglomerate and sandstone lithofacies represent the progradation of more proximal alluvial fan deposits over mid-fan deposits. The upper portion of this section is covered with recent alluvium Section C. This is the easternmost section examined and overlies the Red Mountain Gene Canyon Section. As stated above, the conformable relationship of the two 165 formations made it impossible to establish a clear contact. Therefore, an estimated contact within the thick conglomerate sequence was used. The lower and mid-portions of this section predominantly consist of mid-fan lithofacies associations, including alternating conglomerate and sandstone, channeled conglomerate and sandy conglomerate lithofacies. This thick sequence is rythmically interbedded and clearly demonstrates the continuity of depositional processes associated with the Copper Basin Formation. No overall coarsening or fining trends were observed. The internal character consists of thin, fining-upward sequences which represent braided stream deposits of an alluvial fan. The upper part of this section is composed of proximal deposits which consist of both matrix- and clast- supported conglomerate. These represent debris flow and incised stream channel deposits associated with the area above the intersection point of an alluvial fan. A massive, coarse-grained sandstone caps these deposits and signals the end of this depositional cycle. Osborne Wash Formation The Osborne Wash Formation unconformably overlies the Gene Canyon and Copper Basin Formation, and both the 166 upper- and lower-plate crystalline complex (Davis and others, 1980). This formation is relatively undeformed and dips slightly to the southwest as a result of normal faulting associated with Pliocene basin and range type extension (Davis and others, 1980; Davis and Anderson, 1982) . This formation consists predominantly of volcanic flows, tuffs, agglomerate and debris-flow deposits. These reflect the transition from detachment-related sedimentation to a terrain dominated by basaltic volcanism (Davis and others, 1980). Isotopic data have indicated the age of the base of the Osborne Wash Formation to be between 15.9 + 2.8 m.y.b.p (K/Ar, plagioclase) and 13.5 + 1.0 m.y.b.p. (K/Ar, whole rock), or mid- to late-Miocene (Kuniyoshi and Freeman, 1974; Martin and others, 1980). These dates have been substantiated by ages taken from correlative basaltic flows in the Rawhide Mountains (Eberly and Stanley, 1978; Sunneson and Lucchitta, 1979). 167 TEXTURAL ANALYSIS Introduction The textural analysis of the Gene Canyon and Copper Basin Formations provides information about their depositional environments and sediment dispersal systems. The significance of particle size distribution in alluvial fan deposits has long been recognized by various workers (Blissenbach, 1954; Bull, 1962; Passega, 1964). However, the statistical techniques developed for sand-size populations (Passega, 1957; Visher, 1969; Friedman, 1967) are not generally applicable to coarse-grained, conglomeratic deposits; which are typically well-cemented, and, which would require unreasonably large samples for statistically valid results. Several different sampling techniques have, therefore, been devised to provide meaningful particle-size data. The most widely used sampling technique and that followed in this study, is to record the apparent maximum diameter on the largest clasts within a i m radius, from which a mean value is derived. The proportion of sediment finer than -300 (8mm) is then visually estimated to determine the percent conglomerate. In addition, representative samples from the pebbly sandstone, 168 interbedded fine-sandstone and siltstone, and calcareous siltstone and mudstone lithofacies were analyzed by thin section textural techniques to determine general grain- size distributions for the nonconglomeratic lithofacies. These data were then compared to the grain-size distribution plots of Visher (1969), and Passega (1957). Results The general distribution of maximum clast sizes are shown in Figure 59. Within the Gene Canyon Formation, a general fining trend is observed to the south from the Gene Reservoir block to the Copper Basin block at Red Mountain. The maximum clast sizes within the conglomerate lithofacies are generally smaller, the number of boulder breccia lithofacies are fewer, and the percent of nonconglomeratic sediment is significantly greater. The vertical distribution of maximum clast sizes indicates several coarsening and fining-upward cycles, with the largest clasts found in the stratigraphically lower to middle portions of the measured sections (Figs. 57,61). Maximum clast sizes which exceed 20 cm are nearly always associated with the boulder breccia lithofacies, and represent local landslide and debris flow deposits. The maximum clast size trends within the Copper Basin Formation are not readily apparent, and require a 169 statistical sampling program to determine specific clast- size trends. The measurements obtained in this study and those documented by Teel (1983), however, suggest a general coarsening to the east. The largest sizes also appear to be in the stratigraphically lowest parts of the sections, usually associated with the boulder conglomerate 1ithofacies. The representative grain-size distributions of the nonconglomeratic lithofacies are shown in Figure 44. The pebbly sandstone lithofacies has a mean grain size of 0.75 phi and a standard deviation of 0,88 phi. This lithofacies is moderately well-sorted. The interbedded sandstone and siltstone lithofacies are finer-grained and exhibit better sorting than the pebbly sandstone lithofacies. The mean grain size is 2.25 phi and the standard deviation is 0.30 phi. The finest-grained lithofacies is the calcareous siltstone and mudstone lithofacies which has a mean grain size of 4.2 phi and a standard deviation of 0.90 phi. When the data are plotted on a Visher diagram (Fig. 44) , it is apparent that the grain-size distribution for each of the nonconglomeratic lithofacies is unique. The mean grain sizes show a marked increase from the calcareous siltstone and mudstone lithofacies to the pebbly sandstone lithofacies. The pebbly sandstone and interbedded sandstone and siltstone lithofacies were 170 Figure 44. Visher plots of the pebbly sandstone, interbedded sandstone and siltstone, and calcareous siltstone and mudstone lithofacies. 171 GRAIN SIZE 0 MIDPOINTS z02&. .375. SUSPENTION SALTATION BEDLOAD PEBBLY SS CALCAREOUS SLTS & MUDS INTERBEDDED SS & SILTS o 4.0 5.0 1 . 0 2.0 30 0.0 - 2.0 - 1.0 GRAIN SIZE 0 INTERVALS 172 CUMULATIVE WEIGHT predominantly transported by traction currents and deposited as bedload material? whereas the calcareous siltstone and mudstone lithofacies were deposited from suspension in a lower-energy environment. Another common method used to analyze grain size data for environmental recognition is from a Passega diagram (Passega, 1964). The Passega plot used by Bull (1972) for recent alluvial fans is used in this study (Fig. 45) . Although the data plot outside of the fluvial ”S"-shaped portion, it is clear that each lithofacies has been deposited by a different mechanism which can be recognized in this diagram. The same "S" shaped trend is observed and the shift is probably due to the dominance of alluvial fan processes and coarse-grained textures. The pebbly sandstone lithofacies was deposited as bedload. The interbedded fine sandstone and siltstone appears to have been deposited as both bedload and graded suspension, whereas the calcareous siltstone and mudstone lithofacies was deposited from suspension. The comparison of these grain-size data with the grain-size distributions of Visher (1969) and Bull (1972) confirm the previously inferred depositional environments. Future studies of this region should, therefore, include textural analyses to assist in environmental recognition. 173 Figure 45. Passega diagram after Bull (1972) with mean data from this study plotted. 174 BEDLOAD GRADED SUSPENSION SUSPENSION M (microns) LITHOFACIES □PEBBLY SANDSTONE oINTERBEDDED FINE SANDSTONE AND SILTSTONE a CALCAREOUS SILTSTONE AND MUDSTONE PETROGRAPHY % Introduction The Gene Canyon and Copper Basin Formations have compositions which are derived from several source terranes, including the mylonitic lower-plate crystalline complex, the upper-plate crystalline complex, and from the mid-Tertiary continental deposits (Davis, 1980). The purpose of this petrographic analysis is to characterize the compositions of the Gene Canyon and Copper Basin Formations, and to determine if a significant variation exists between the two formations. In addition, petrographic techniques were used to examine the diagenetic aspects of these deposits. Ten typical samples were selected from the sand-sized populations of each formation. Additional samples were obtained from the calcareous siltstone-and-mudstone lithofacies. The results are summarized in Table 4. Framework grains were grouped into (1) total quartz, (2) total feldspar, (3) lithic fragments, and (4) accessories. Matrix constituents include clay, carbonate, chert and hematite. The data are plotted on the McBride (1963) ternary diagram (Fig. 46) and the diamond diagram of (Fig. 47), Basu et al. (1975). In addition, the Basu et al. (1975) 176 TABLE 4 SUMMARY OF PETROGRAPHIC DATA Gene Canyon Copper Basin Minerals Observed Range % Mean % Range % Mean % Total Quartz 17-43 27 12-39 21 polycrystalline >3 4-14 7 1-6 3 polycrystalline 2-3 1-4 2 1-3 2 monocrystalline non-undulose 1-6 3 2-8 3 monocrystalline undulose 11-19 15 8-22 13 Total Feldspar 20-42 29 17-35 25 Plagioclase 5-19 9 3-9 6 Potassium Feldspar 15-23 20 14-26 19 Rock Fragments 3-26 10 7-37 18 Plutonic 2-15 6 4-14 7 Volcanic Tr-2 Tr 1-12 6 Sedimentary Tr-1 Tr 1-5 1 Metamorphic Tr-9 4 1-6 4 Accessories 6-23 10 5-29 12 Muscovite Tr-3 1 Tr-1 Tr Biotite 2-5 3 2-8 4 Amphibole 1-3 2 1-6 3 Pyroxene 1-3 1 Tr-6 2 Opaques Tr-3 1 1-2 1 Epidote 1-2 1 1-4 2 Chlor ite 1-3 1 Tr-2 Tr Calcite Tr-1 Tr Tr Tr Cement 4-40 20 6-38 20 Carbonate 1-16 9 2-23 12 Hematite 2-12 6 4-11 6 Clay 1-10 4 Tr-2 1 Chert Tr-2 1 Tr-2 1 177 Figure 46. Ternary Diagram of grain compositions (after McBride, 1963) from Gene Canyon and Copper Basin Formations. 1 7 8 179 QUARTZ&CHERT QUARTZARENITE • GENE CANYON * COPPER BASIN 25, /LITHIC SUB ARKOSI FELDSPATHIC LITHARENITE LITHIC ARKOSE 50 Figure 47. Diamond diagram with quartz-grain data (after Basu and others, 1975) from the Gene Canyon and Copper Basin Formations. 180 POLYCRYSTALLINE QUARTZ (2-3 crystal units per grain. > 7 5 % of total polycrystalline v quartz) NON- UNDULATORY QUARTZ / (UNDULATORY \ QUARTZ / POLYCRYSTALLINE QUARTZ t>3 crystal units per gram > 2 5 % of total polycrystalline quartz) 181 histogram (Fig. 48b) was used to display the relative percentages of nonundulatory, undulatory, polycrystalline 2-3, and polycrystalline > 3 quartz. Discussion of Results The high percentages of feldspar and lithic fragments indicate that these formations are compositionally immature and that their source terrains are local. A significant compositional difference between the Gene Canyon and Copper Basin Formations may be observed in Figure 46, which reflects increased lithic components in the Copper Basin Formation. The Gene Canyon Formation is a lithic arkose (Fig. 46) and plots within the magmatic arc province of Dickinson and Suczeck (1979). Lithic fragments comprise 10% of this formation, which were derived from a combination of plutonic and metamorphic source terrains. Sedimentary and volcanic rock fragments are relatively rare and were only observed in trace amounts. Nearly all of the monocrystalline quartz is undulose (83%) which is probably related to past tectonic stresses in the region. When the data are plotted on the Basu et al. diamond diagram (Fig. 47), therefore, they fall into the low-rank metamorphic field due to the high percentage of monocrystalline undulose quartz. The effects of tectonic stress on monocrystalline quartz can be removed 182 Figure 48. Quartz grain types from the Gene Canyon and Copper Basin Formation. (a) Basue et al. histogram (1975) indicating relative abundance of monocrystalline and polycrystalline quartz types from Holocene sand of known parentage. (b) Relative abundance of monocrystalline and polycrystalline quartz types from the Gene Canyon and Copper Basin Formations. 183 P0LYCRYS.>3 POLYCRYS.2-3 UNDULATORY NON UNDULATORY POLY CRYS.>3 POLYCRYS. 2 -3 CRYSTALS NON UNDULATORY POLYCRYS. >3 CRYSTALS POLYCRYS. 2 -3 CRYSTALS NON UNDULATORY p m ,to mr H,GH" RANK LOW-RANK r LU UIMIU METAM0R METAMOR PERCENT 100 75- 50 - 25 - POi-YCRYS. > 3 14% POLYCRYS. >3 POLYCRYS.2-3 26% 10% POLYCRYS. 2 -3 UNDULATORY 62% UNDULATORY 55% NON UNDULATORY 14% NON UNDULATORY 11% COPPER GENE BASIN CANYON (B) by combining both undulose and nonundulose quartz into one category, and then comparing the percentages of monocrystalline, polycrystalline (2-3) and polycrystalline > 3 quartz to the Basu et al. histogram (Fig. 48b). This comparison indicates that the Gene Canyon is predominantly derived from a plutonic source terrane, with mixing from both low- and high-rank metamorphic terranes. Biotite and amphiboles are the most common accessory minerals. Within the calcareous siltstone and mudstone lithofacies, numerous fragments of fossils were observed. Ostracods are the only fossil constituent which could be identified with certainty (Fig. 49). Cements compose 20% of the Gene Canyon samples and include carbonate, hematite, and clay. Carbonate cement is most abundant (9%), and occurs as intergranular cement and as a replacement of quartz and feldspar grains. Hematite also is common (6%), and generally coats grains, or more rarely, fills interstitial pores. Clays were observed infrequently (4%) and probably originated from diagenetic alteration of feldspar grains and volcanic lithic fragments. The Copper Basin samples contain more lithic fragments (18%) than the Gene Canyon samples (10%), and plot within the lithic arkose and feldspathic litharenite fields (Fig. 46). Lithic fragments are predominantly 185 Figure 49. Ostracod shells in calcareous siltstone and mudstone lithofacies (G.C. Formation, Red Mountain). 186 volcanic, which occur only in trace amounts in the Gene Canyon Formation. This shift indicates an increased input of detrital volcanic clasts as compared to the underlying Gene Canyon Formation. The percentage of quartz grains decreases to 21%, which reflects the added lithic component. It also may signal decreased sorting or decreased textural maturity. Comparison of the percentages of monocrystalline, polycrystalline (2-3), and polycrystalline > 3 quartz to the Basu et al. histogram (Fig. 48b), indicates that the dominant source terrane is again plutonic. A substantial decrease in the percentage of polycrystalline^ 3 quartz, however, may suggest a decreased contribution from low- rank metamorphic source terranes and possibly an increase from high-rank metamorphic source terranes. As with the Gene Canyon Formation, it is clear that substantial mixing has occurred. Total feldspar also decreases, which reflects a reduction in the amount of plagioclase. The observed decrease in plagioclase may have resulted from more intense weathering associated with the Copper Basin Formation. Accessory minerals show a slight increase and and consist of similar minerals as found in the Gene Canyon Formation. Although the overall matrix remained in about the same proportions, an increase in carbonate cement and a decrease in clay content was observed. Surprisingly, 188 hematite was observed in the same percentages in both formations, although the Copper Basin Formation had a distinctively red color in outcrop compared to that of the Gene Canyon Formation. Diagenesis Authigenic calcite, hematite, silica, feldspar, clay and zeolite fill the interstitial voids; and replace several types of framework grains. These constituents typically compose from 15 to 25% of the sand-sized fraction, and are responsible for the ubiquitous red color and the high degree of induration of these deposits. Calcite is the most abundant authigenic mineral and generally fills the interstitial voids as well as the voids formed by the dissolution of framework silicate minerals. It is observed as both coarsely crystalline sparite (Fig. 50c), and amorphous micrite (Fig. 50d). In several cases, more than one generation of calcite cement was noted. Calcite also replaced many of the feldspar grains, and less commonly quartz grains. Authigenic feldspar (Fig. 51b) and quartz (Fig. 51a) commonly occur as syntaxial overgrowths on isolated grains, which are distinguished by their euhedral terminations extending beyond the drusy outline of the original grains. Several workers have documented similar 189 Figure 50. Authigenic minerals of the Copper Basin and Gene Canyon Formations. (a) Authigenic clay filling an interstitial void. (b) Hematite coating detrital grains. (c) Sparry calcite filling an interstitial void. (d) Microcrystalline calcite (micrite) replac ing framework grains. 190 Figure 51. Diagenetic alternations of framework grains. (a) Silica (chalcedony) coating framework grains (biotite and quartz). (b) Potassium feldspar overgrowth on framework grain. Note the irregular boundaries of the original grain marked by the drusy coating, and the eurhedral faces of the feldspar overgrowths. 192 193 findings in their studies of the southwestern United States (Hay, 1966? Walker, 1976? Walker and others, 1978). Chalcedony or chert also were observed filling voids formed by the dissolution of framework grains. Clay minerals are common and are probably composed of mixed-layer illite-montmorillonite, similar to those described by Walker and others (1978). They typically fill the voids between framework grains or replace feldspar. Zeolites are rare and usually are associated with volcaniclastic sediment. Ubiquitous bright red to reddish-brown hematite occurs in two forms: (1) as drusy linings of voids formed by the dissolution of framework grains (Fig. 51b), and (2) as an amorphous stain on both the matrix and framework grains (Fig. 52). The precipitation of iron oxides have given these deposits their characteristic red color. Figure 52 illustrates some of the major features discussed above and their diagenetic sequence. The original quartz grain has been abraded and later coated with hematite. Subsequently, syntaxial quartz overgrowth occurred, displaying the well-formed euhedral termination. Feldspar grains have altered to clay minerals and exhibit an extremely irregular outline, which is associated with dissolution of their borders. The final stages include ubiquitous iron oxide staining and the filling of the interstitial void spaces with calcite cement. In fact, 194 Figure 52. Diagenetic history of sample 21 of the Copper Basin Formation. Q - quartz; F - feldspar, C - calcite? H - hematite, Cl - clay. 195 WtSuV.r 1 * 1 / H ( S / / / ' / / 7// i joi'iyd, ! r 1 9 6 more than one cycle of calcite cementation was often observed. It is apparent that the original compositions which were rich in quartz, feldspars and ferromagnesium silicate minerals have been substantially altered. At least two diagenetic depositional cycles were noted in some samples, and several stages of diagenetic alteration could be determined. 197 DEPOSITIONAL MODEL Tectonic Setting The depositional setting of the Whipple Mountains during mid-Tertiary time was controlled by extensional tectonic processes. The configuration of the depositional basin and the associated depositional systems of the Gene Canyon and Copper Basin Formations were the direct result of either detachment faulting and related upper-plate normal faulting (Davis, 1980), and/or the coeval development of northeast-trending antiforms and attendant synforms (Teel and Frost, 1982; Frost, 1984; Teel, 1983). The relative influence of each of these tectonic mechanisms and their temporal and kinematic relationships, however, has not been clearly resolved. The following discussion focuses on three different extensional tectonic models, which address these structural relationships. One of the earliest tectonic models was developed by Davis and others (1980), who proposed that detachment faulting in the Colorado River trough significantly predated upwarping or "doming" of the Whipple Mountains. Field relations indicated that post-tectonic uplift warped the earlier-formed regional detachment surface and associated high-angle normal faults of the upper-plate (Fig. 1). The range-forming process, therefore, was "kinematically independent of either the earlier mylonitization of crustal rocks or the younger development of regional detachment faults across them" (Davis and others, 1980). The deposition of the mid-Tertiary strata was considered to be largely controlled by detachment faulting and associated upper-plate normal faulting, and therefore, would not have been affected by post- depositional upwarping of the range. The recognition of northeast-trending synforms and attendant antiforms and the stratigraphic relationships of the Gene Canyon and Copper Basin Formations to these structural features has led Frost (1984) to develop a significantly different tectonic model. The Gene Canyon Formation has been preserved in three northeast-trending synclines which have been isolated by parallel northeast- trending anticlines (Fig. 3). Conversely, the Copper Basin Formation has been preserved both in the synclines where it unconformably overlies the Gene Canyon Formation, and on the intervening anticlines where it nonconformably overlies the upper-plate crystalline complex (Fig. 3). Frost (1984) suggests that these synforms and attendant antiforms formed near the end of Gene Canyon time and prior to the deposition of the Copper Basin Formation. This deduction is based largely on the consistent parallel relationship of the Gene Canyon strata with the underlying depositional surface, and the truncation of both of these 199 units by strata assigned to the Copper Basin Formation (Frost, 1984). These relationships suggest that uplift and extensive erosion removed the Gene Canyon deposits from the rising antiforms prior to the deposition of the Copper Basin Formation. Continued uplift and the coeval development of detachment-related growth faults resulted in the syntectonic deposition of the Copper Basin strata (Teel and Frost, 1982; Frost, 1984; Teel, 1983). According to this model, the depositional basin initially reflected the remnant topography from Mesozoic compressional tectonism, and was later folded into broad, northeast-trending antiforms and synforms, which dramatically altered the sediment dispersal systems of the region. Associated with this crustal folding, was the coeval development of a regional detachment fault and related upper-plate normal faults (Fig. 53; Teel and Frost, 1982). This tectonic model is similar to earlier models developed by Davis (1980) and Rehrig and Reynolds (1980), and proposes that crustal folding and detachment faulting are temporally and kinematically related, and that their initiation in the Whipple Mountains, therefore, was near the end of Gene Canyon time. The theoretical relationship between detachment faulting and crustal folding recently has been addressed by Spencer (1984). His model suggests that large 200 Figure 53. Paleogeographic model of the Whipple Mountains region from late Oligocene through Miocene time. Modified from Teel & Frost (1982). 201 202 PLAYA LAKE DEPOSITS AND FANGLOMERATE DEPOSITS, S-H FLO TUFTS displacement along detachment faults results in the isostatic uplift of the lower plate in response to this tectonic denudation (Figure 54). This model predicts the development of broad antiform and synform pairs oriented perpendicular to the direction of extension, and supports a temporal and kinematic relationship between detachment faulting and crustal folding. This would indicate that crustal folding occurs in response to detachment faulting, and therefore, necessarily overlaps or postdates detachment faulting. This model is, therefore, kinematically different than the tectonic model proposed by Frost (1984) and does not clearly explain the parallel relationship of the antiforms and synforms of the Whipple Mountains to the northeast direction of extension. However, it does suggest a temporal association between detachment faulting and crustal folding in contrast to the model developed by Davis and others (1980), which therefore suggests that the mid-Tertiary sediment dispersal systems may have been influenced not only by detachment-related faulting, but also by the development of subsequent antiforms and synforms. The following sections will address these different tectonic models in terms of the depositional lithofacies and sediment dispersal systems of the Gene Canyon and Copper Basin Formations. 203 Figure 54, Theoretical model of relationship between detachment faulting and uplift (from Spencer, 1984). 204 f BREAKAWAY I A 1 0 BREAKAWAY B 10 INACTIVE SEDIMENT DISPERSAL SYSTEMS The sediment dispersal systems identified in this study are based on (1) the temporal and spatial distribution of lithofacies, (2) paleocurrent dataf and (3) textural and compositional trends. In combination, these data provide important insight concerning the nature of the depositional basin and associated source terrains. Correlations have been extended across fault blocks and should be considered tentative, due to the uncertainty of the degree of extension and rotation that these individual blocks have undergone. With more areally extensive work, the sediment dispersal systems for the entire Whipple Mountains region may be resolved, and the original positions of these fault blocks determined. Gene Canyon Formation The Gene Canyon Formation consists of interbedded alluvial fan, alluvial plain and lacustrine depositional lithofacies. In addition, thick sequences of volcanic flows and volcaniclastic rocks interfinger with the sedimentary strata. The sediment dispersal systems, therefore, are controlled not only by the paleogeography, but also by the relative contribution from the source for the volcanic and volcaniclastic lithofacies. 206 Vessell and Davies (1981) have studied modern volcanic regions and have established the following lithofacies relationships: (1) volcanic core; (2) proximal volcaniclastic? (3) medial volcaniclastic? and (4) distal volcaniclastic. The criteria used to recognize these lithofacies associations are essentially the same as those for the alluvial fan-alluvial plain model, and are attributed to reworking by a similar set of depositional processes. The comparative depositional scales, however, may vary considerably, resulting in a complex interrelationship of alluvial fan and caldera-related lithofacies. Frost (1984) has developed a regional depositional model for the Whipple mountains, which postulates the presence of a caldera complex located somewhere near the Aubrey Hills-Lower Whipple Wash area (Fig. 55). This conclusion is based on the presence of thick volcanic and volcaniclastic sequences in this area, which interfinger with alluvial fan and lacustrine deposits further to the south. Suneson (personal communication, 1983) has suggested that the source for these volcanic units was more likely individual feeder dikes, based on the lack of significant welded tuffs. An overall southerly transport direction is, indicated in either case, however, with proximal volcanic and volcaniclastic lithofacies grading to more distal valley-fill sediment. 207 Figure 55. Paleogeographic model for the Gene Canyon Formation showing the concentration of volcanic rocks and marginal sediments to the south (modified from Frost, 1984). 208 N 1 R hyo litic D om es or Caldera-Related (?) V o lcanic R ocks / / V ^ A ubrey PARKER DAM Hills S C H O O L SECTIO N W h ip p le W ash STUDY A R E A ✓ P a r k e r Dam V alley - Fill ^ S edim ent DESILT WASH SECTIO N / —RED M OUNTAIN / SECTION 209 Within the study area several lateral and vertical sedimentary lithofacies trends occur (Fig. 56). The most noteworthy is the overall higher percentage of proximal lithofacies found at Desilt Wash and Parker Dam School as compared with Red Mountain. These include talus deposits, and proximal and medial alluvial fan deposits. Similarly, a thick sequence of lacustrine deposits is found at Red Mountain which has no counterpart to the north, and is apparently the distal equivalent to the alluvial fan lithofacies of Desilt Wash and Parker Dam School. These lithofacies relationships indicate an overall southerly transport direction, similar to that postulated by Frost (1984) . The talus and debris flow deposits which occur at Desilt Wash and Parker Dam School indicate very local source terrains (Fig. 57). The shape and lateral facies relationships of these units provide valuable information regarding local transport directions. The most prominent is the adamellite talus deposit at Desilt Wash, which is laterally discontinuous and interfingers with finer- grained lithofacies towards the northwest and southeast. This would suggest that the transport direction was southwest or northeast; probably from an area of high relief flanking the basin floor. Several other talus and debris flow deposits also are discontinuous to the northwest and southeast and exhibit no consistent 210 Figure 56. Fence diagram of measured stratigraphic sections for the Gene Canyon Formation showing lateral and vertical lithofacies relation ships . 211 PARKER DAM SCHOOL PARKER DAM 8CH00L DESILT WASH RED MOUNTAIN DESILT WASH EFfAr . GENE WASH BLOCK COPPER BASIN BLOCK ADAUELLITE breccia / AL^VIAL/U CUSTR(NE U c uSTR<N£ ADAMELLITE breccia ETEROQENEOU8 PHYLLITE BRECCIA / R°*'*<AL AUuv|. ' Pan AH/*lluV ial RED MOUNTAIN QUART 2,t£ Figure 57. Generalized lithofacies map of the Gene Canyon Formation between Parker Dam School and Gene Reservoir. 213 DESILT WASH PARKER D A ^ ^ C H O O L V SECTION^ B B B 133 m m m RYTHM ICALLY -B E D D E D LITHOFACIES VARIABLE LITHOFACIES VOLCANIC & V O L C A N IC LA STIC LITHOFACIES BOULDER BRECCIA LITH O FA C IES ADAME L LIT E PHYLLITE BIOTITE ADAMELLITE HETEROGENEOUS 214 thickening or thinning trends in these directions, also which supports a local southwest or northeast transport direction. An additional important lithofacies relationship was observed at Parker Dam School, where a major feeder channel (?) filled with boulder conglomerate truncates alluvial fan lobe deposits to the northwest and southeast (Fig. 58). This would also suggest a local transport direction either to the southwest or northeast. The thickest accumulation of volcanic flows and volcaniclastic deposits within the study area occur at Parker Dam School. Similar deposits were observed at both of the other sections, but overall they were thinner and had fewer individual flows. No welded ash flow tuffs were observed, which may suggest that the volcanic and volcaniclastic deposits may have actually been derived from local feeder dikes. The flow direction of these units was difficult to establish based on the geometry alone, and other directional features were not observed. Paleocurrent data were obtained from large-scale trough and planar cross-beds, small-scale ripple cross laminae, pebble imbrication and channel axes and are shown in Figure 59. The number of actual measurements were limited due to the low number of preserved directional sedimentary structures associated with alluvial fan depositional environments. Also, the structures were 215 Figure 58. Diagramatic cross-section of the Gene Canyon Formation near Parker Dam School. Note the major feeder (?) channels which truncate alluvial fan lithofacies. 216 NE RADIO RELAY STATION O Q O O \O 6 3 Co o - V A ' - ' VOLCANIC 5 Q SANDY CONGLOMERATE |g gjB O U L D E R CONGLOMERATE INTERBEDDED SANDST & CONGLOMERATE |5fS| CHANNELED CONGLOMERATE J21 ADAMELITE BOULDER BRECCIA 5 P I PHYLLITE/ADAMELLITE BOULDER BRECCIA Figure 59. Sediment dispersal systems map of the Gene Canyon and Copper Basin Formations. General lithofacies relationships, textural and compositional trends, and paleocurrent data are shown. 218 219 AVERAGE MAX. CLAST SIZE O >20 Cm O 10-20 Cm O 5-10Cm 0 <5 Cm □ COPPER BASIN FM □ GENE CANYON FM □ UPPER-PLATE CRYSTALLINE COMPLEX G - GRANODIORITE A - ADAMELLITE P - PHYLLITE COPPER BASIN SIZE DATA (Teel, 1983) < 9 ( 9 & N=11 X=S10"E L=69% n=-|i X=S31 E L=79% N=3 5(=S10oW L=90% PARKER DAM SCHOOL N=18 X=S28°E L=73% X=S18°E N=9 X=S12° W L=58% RED MOUNTAIN often concealed by a carbonate coating on many of the outcrops. Twenty-two measurements were obtained at Desilt Wash which indicate an average transport direction of S23°E and have a standard deviation of 34°. These measurements were obtained predominantly from distal alluvial fan and alluvial plain depositional lithofacies and indicate that the major source terrains are probably to the north- northwest. These results correspond with the regional stratigraphic relationships discussed above. A relatively small but consistent eastward shift in transport direction was noted in the upper part of the section. The average transport direction in the lower part of the section was S10°E (s=14°), which changed to S33°E (s=ll°) near the top of the section. The consistent nature of this eastward shift in transport direction may indicate an increased sediment contribution from the west. This may also be explained by the more proximal lithofacies from which the paleocurrent measurements were obtained, which represent mid- to upper-regions of alluvial fans. These fans were probably prograding in an eastward direction out into the alluvial plain, which would account for the shift in paleocurrent direction from those obtained from more distal, southerly trending alluvial plain measurements. Nine paleocurrent measurements were obtained at Red Mountain, which indicate an average transport direction of 220 S12°E and a standard deviation of 40°. These data were similar to those obtained at Desilt Wash and also suggest an overall southerly transport direction. The high degree of variation is within the + 25-50° dispersion typically associated with the braided-stream deposits (McGowen and Garner, 1970? High and Picard, 1974). This variation may also reflect the small number of paleocurrent measurements. Only three transport direction measurements were found at Parker Dam School, which indicate a S10°W transport direction. Although the number of observations are very limited, these data are in general agreement with the other sections. The textural trends indicate an overall coarsening to the north-northwest across fault blocks from Red Mountain to Desilt Wash. The significance of particle size distribution in alluvial fan deposits have been documented by several workers (Blissenbach, 1952? Hooke, 1967; Passega, 1964) , who have demonstrated a rapid fining-trend down slope from the apex of the alluvial fan. The size trends observed in this study, therefore, suggest a southerly transport direction. Reverse trends within the boulder breccia lithofacies were also observed, which probably represent landslide deposits in which the larger boulders are deposited distal to the finer-grained particles. Similar observations have been made from 221 modern talus deposits (Bull, 1964a). The compositions of the boulder breccia lithofacies provide additional transport direction information, as they are typically monolithic and can, therefore, be traced to their respective source areas. Both adamellite and granodiorite compositions were abundant at Desilt Wash and Parker Dam School. Frost (1984) suggests that the biotite adamellite unit may have come from the southern Whipples, where similar rocks are exposed below the Whipple detachment surface. The granodiorite clasts, conversely, were probably derived from the Northern Whipples (Frost, 1984). Also noted in this area were phyllite and Mesozoic breccias, which apparently were derived from the Buckskin Mountains east of the study area (Frost, 1984). The phyllite boulder breccia also occurs at the Red Mountain stratigraphic section. A thick quartzite boulder breccia lithofacies occurs at Red Mountain and was presumably derived from the western Buckskin Mountains where it is exposed at the Carnation, Billy Mack, and Rio Vista areas (Frost, 1984). Copper Basin Formation The Copper Basin Formation consists predominantly of proximal, medial, and distal alluvial fan sequences. Interbedded volcanic flows occur, usually near the base of 222 the sequence. These flows are restricted to the east- central Whipple Mountains, located northwest of the study area (Frost, 1984). Regional stratigraphic relationships indicate that thick sandstone and siltstone sequences are concentrated in the southeastern Whipple Mountains and western-most Buckskin Mountains of the Colorado River synform, which are flanked along the margins to the northwest and southeast by coarse fanglomerate facies as shown in Figure 53 (Teel and Frost, 1982? Frost, 1984). The sedimentary and volcanic sequences in the Aubrey Hills-lower Whipple Wash synform are considered to either have been deposited in a separate basin west of the Colorado River synform (Frost, 1984) or may simply represent proximal deposition which grade to more distal deposits in the Colorado River synform (Teel, 1983). The distribution of lithofacies observed within the study area, however, indicate that coarse-grained proximal deposits are not confined to the margin of the Colorado synform or within the Aubrey Hills - lower Whipple Wash synform (Fig. 60). Thick boulder conglomerate lithofacies are observed at the base of reconnaissance Section A and the Parker Dam School section, and grade laterally into more distal associations towards the "margin" of the basin. These basal conglomerates probably represent alluvial fan feeder channel deposits, which appear to 223 Figure 60. Fence diagram for the illustrating vertical relationships. Copper Basin Formation and lateral lithofacies 224 225 MEDIAL TO DISTAL *>: v . . MEDIAL DISTAL MEDIAL MEDIAL GENE RESERVOIR b l o c k s \ MEDIAL — tz\ OISTAL 0.5j5f s o XIM A L / COPPER BASIN , \ BLOCK PROXIMAL JMEDIAL TO DI3TAL n’p OO* O0N °oy\c^™°A ^ W ^ ^ i PR0X'^AL * n rP.I _ ( X PARKER DAM SCHOOL D oO PARKER OAM SCHOOL c DISTAL PROXIMAL MEDIAL truncate the underlying Gene Canyon Formation in a northeast-southwest direction. These relationships do not support a depositional margin to the northwest as proposed by Teel & Frost (1982), and appear more likely to indicate northeast prograding alluvial fans, possibly associated with fault scarps. Another noteworthy lithofacies trend is the thick sequence of sandstone, fine-grained sandstone and siltstone found near the top of the Parker Dam School and which extends laterally towards Section B. These deposits probably accumulated beyond the margins of the alluvial fans, and therefore, indicate a more central portion of the depositional basin. These deposits may, alternatively, represent interfan deposition, between major prograding alluvial fan systems. Similar deposits also were observed by Teel (1983) near Copper Basin Reservoir. The repetitious nature of this formation and the relatively minor variation of lithofacies makes it difficult to establish clear lithofacies trends within this part of the depositional basin. The most distinct trend is the general fining between Parker Dam School and Section B. The fact that most of the section is covered by recent alluvium in this area reflects the preferential erosion of finer-grained, more friable lithofacies. Thick sandstone sequences with relatively minor conglomerate 226 also were observed south and southwest of the study area. These observations support an overall fining trend to the south. Paleocurrent data were limited within the Copper Basin Formation due to the overall coarse-grained textures and ubiquitous planar-laminated sandstones. A total of 23 measurements were obtained, principally from the fine grained sandstone lithofacies, and are shown in Figure 63. An overall southerly to southeast (S25°E) transport direction was observed. The most numerous paleocurrent indicators were found at the Parker Dam School section within the sandstone and interbedded sandstone and siltstone lithofacies. Eight measurements from small-scale planar and trough cross laminae indicate a S13°E transport direction. Measurements obtained from large-scale trough and planar cross-laminated pebbly sandstones indicate an average transport direction of S43°E. Relatively few paleocurrent indicators were observed at the other sections, and were usually measured from low- angle trough-cross bedded pebbly sandstones whose resultant vectors exhibit large variations. Several measurements, however, indicate a southeasterly to southwesterly transport direction. Teel (1983) also found an overall southward (S23°E) transport direction from 22 measurements at Copper Basin Reservoir. He suggested that 227 the observed radial patterns were related to alluvial fan deposition below the intersection point. An attempt to measure clast size distributions and clast types proved unsuccessful due to the repetitious character and irregular distribution of clasts. A detailed statistical sampling plan must be implemented to provide meaningful counts within the Copper Basin Formation. A general fining to the south, however, was noted from measurements at selected localities, which reflects the facies relationships previously discussed (Fig. 59). 228 DEPOSITIONAL HISTORY The depositional history of the mid-Tertiary volcanic and sedimentary strata in the Whipple Mountains directly reflects the local tectonic setting. Numerous workers (Bluck, 1967; Bull, 1967b, 1977; Crowell, 1974b; Steel and Wilson, 1975; Steel and Gloppen, 1980; Nilson, 1982) have studied the intimate relationships between alluvial fan sedimentation and the relative changes between the source terrane and depositional basin. These changes reflect the style and rate of tectonic deformation, and commonly produce distinctive depositional signatures. The geometry of the sedimentary deposits, sediment dispersal patterns, and the vertical sequence of lithofacies are particularly diagnostic, and provide a means for reconstructing the depositional and tectonic history. Gene Canyon Formation (32-18 m.y.b.p.) The initial deposition of the Gene Canyon Formation consists of a coarsening-upward sequence of distal to medial alluvial fan deposits, and occasional interbeds of fine-grained, calcareous interfan and lacustrine deposits. These were deposited directly on top of the crystalline 229 complex of the upper plate, and represent the initial development of prograding alluvial fan systems, which extended onto the basin floor or alluvial plain. Paleocurrent data indicate that these sediment dispersal systems were directed towards the southeast. The parallel relationship between these strata and the underlying topographic surface suggests that deposition occurred prior to major uplift or folding (Frost, 1984). These initial deposits were followed by a period of relatively low-energy deposition, in which distal alluvial fan, alluvial plain and lacustrine lithofacies dominated. However, it should be noted that boulder breccia, boulder conglomerate and occasional stream channel lithofacies, which represent intermittent landslide and debris flow deposition are interstratified with these more distal deposits. Their abrupt and irregular contacts with the finer-grained distal deposits, lateral discontinuity, monolithic compositions, and extremely large clast size distributions indicate that they were derived from local areas of high relief and represent brief episodes of deposition. This would suggest a time of relatively stable conditions which was periodically interrupted by either (1) changes in relief associated with active fault scarps, or possibly (2) deeper tectonic activity which might be related either to a developing caldera complex or 230 initial movement along the detachment fault. The second interpretation appears reasonable based on the abrupt contacts observed between the boulder breccia and conglomerate lithofacies and the finer-grained lithofacies. If fault scarps were involved, gradational contacts with a sequence of lithofacies indicative of prograding fans would be developed. The interbedded boulder conglomerate and breccia lithofacies, therefore, probably were derived from local areas of high relief, and represent landslide deposits. The source for these monolithic breccias was presumably the southern Whipple and western Buckskin Mountains, which would indicate a local northeast and westward transport direction, respectively. Paleocurrent indicators within the finer- grained alluvial plain and lacustrine lithofacies, however, indicate a consistent southeast transport direction. These relationships, therefore, suggest that a northwest trending basin was present during Gene Canyon time, which was flanked to the east and southwest by areas of high relief. The dominant transport direction of the finer-grained alluvial plain lithofacies was along the basin axis to the southeast, whereas, local transport of coarser-grained deposits was normal to the basin axis. The upper part of the Gene Canyon Formation is dominated by rythmically-bedded braided-stream deposits, 231 typically associated with the mid-alluvial fan region. This part of the section is very similar to the Copper Basin Formation, and probably represents a time when relative changes in relief and erosion were essentially in equilibrium. Paleocurrent data indicate an eastward shift in transport direction associated with these coarser- grained deposits, which suggests that alluvial fan systems are beginning to prograde onto the longitudinal alluvial plain, and may be associated with developing fault scarps. This change in sedimentation style and paleocurrent direction may signal the onset of detachment-related faulting. The top of the preserved sections are predominantly volcanic flows, tuffs and stream-channel conglomerate; which indicate an increased influence from the volcanic source area to the north. It is unclear how much of the upper section was removed prior to the deposition of the Copper Basin Formation. However, the angular unconformity which separates the Gene Canyon and Copper Basin Formations suggests substantial relief and associated erosion, particularly near Gene Wash Reservoir. This angular relationship is much less pronounced towards the south and southeast across fault blocks, where the contact becomes concordant at Red Mountain. 232 The regional facies relationships and sediment dispersal systems, therefore, indicate that the Gene Canyon Formation was deposited in a northwest trending basin, with the dominant sediment dispersal systems directed towards the southeast. Areas of high relief to the southwest and east also contributed substantial coarse sediment along the margins of the depositional basin. The marked change in depositional style noted in the upper part of the Gene Canyon Formation may represent the initiation of detachment-related deformation and the associated development of prograding alluvial fan systems onto the basin floor. Alternatively, the initiation of detachment faulting may have coincided with the deposition of the boulder breccia lithofacies early in Gene Canyon time, as these deposits may be associated with fault scarps. The change in sedimentation style observed near the end of Gene Canyon time might represent the isostatic upwarping of the Whipple Mountains (Spencer Model). Copper Basin Formation (18-16 m.y.b.p.) The lowermost Copper Basin deposits vary considerably across the study area. Near Gene Reservoir, Parker Dam School and Red Mountain, these deposits consist of coarse grained, proximal alluvial fan lithofacies, which are 233 dominantly debris-flow deposits. However, at several other localities they are rythmically-bedded, medial alluvial fan lithofacies. These variations are interpreted in terms of several developing alluvial fan systems across the area. The boulder conglomerate lithofacies was associated with the proximal regions of the alluvial fan, which grades laterally and down fan to medial alluvial fan lithofacies. Paleocurrent data indicates a general southeast transport direction for these evolving fan systems. As noted for the Gene Canyon Formation, the paleocurrent measurements also were obtained from finer-grained lithofacies which may have been transported along the longitudinal axis of the basin. The remainder of the preserved sections consist of a series of stacked alluvial fan sequences. The lack of an overall progradational or retrogradational trend suggests that relative changes in relief and erosion were in equilibrium. Available paleocurrent data also indicate a consistent southeast transport direction. It is apparent that a different depositional style was active during Copper Basin time than that observed through most of Gene Canyon time. The repetitious nature of the deposits clearly reflects a relative state of equilibrium between changes in relief and erosion. The growth fault model, as proposed by Teel and Frost (1982), 234 suggests that upwarping or folding of the range occurs coevally with detachment faulting and related upper-plate normal faulting. Upwarping would keep pace with erosion such that the depo-center would remain constant and similar facies would stack on top of each other. A similar result also would occur, however, with incremental movement along a high-angle normal fault which would establish a state of equilibrium between changes in relief and erosion, such that similar alluvial fan facies would be deposited on top of each other. The lack of any clear evidence which indicates that northeast-trending anticlines contributed to the deposits of the Copper Basin Formation suggests that the latter tectonic mechanism may have prevailed during Copper Basin time. The sediment dispersal systems of the Copper Basin Formation, therefore, were probably controlled by movement along the Whipple Mountain detachment fault and associated high- angle normal faults, which resulted in continuous deposition into a northwest-trending basin. 235 SUMMARY AND CONCLUSIONS The primary objective of this study was to identify and define the major lithofacies of the Gene Canyon and Copper Basin Formations, and to determine their environments of deposition. Through the use of detailed vertical sequence analysis of several selected sections and Markovian statistical techniques, eight sedimentary and two volcanic lithofacies were identified. Their respective depositional environments range from the apex of an alluvial fan complex to distal playa lake environments. Interstratified volcanic flows and ash- flow tuffs presumably were derived either from a caldera complex in the Aubrey Hills - Lower Whipple Wash area, or from individual feeder dikes located north of the study area. The most diagnostic and environmentally sensitive characteristic of these deposits is the internal organization and sequence of clasts. These fabrics can be related directly to known processes associated with specific regions of an alluvial fan and/or alluvial plain, and was, therefore, the most useful criteria for discriminating depositional environments. 236 The second objective was to develop a depositional model and relate the sediment dispersal systems to the tectonic history of the Whipple Mountains. This was accomplished by grouping lithofacies into several assemblages which could be directly attributed to changes in the local tectonic setting. The distribution of these lithofacies assemblages combined with paleocurrent data, compositional and textural trends, were then compared to the tectonic models developed from previous structural studies (Davis & others, 1980; Frost, 1984? Spencer, 1984). Most of the Gene Canyon Formation consists of a wide range of lithofacies which exhibit rapid lateral and vertical variations. They predominantly represent distal alluvial fan and alluvial plain depositional environments with thick local accumulations of talus and debris-flow deposits. The major volcanic source lies to the north and possibly includes several rhyolite domes. Local areas of high relief to the east and southwest were the source for the talus and debris-flow deposits, and also probably contributed to the finer-grained lithofacies associated with the southeast-directed sediment dispersal systems along the axis of the basin floor. The uppermost Gene Canyon Formation and the Copper Basin Formation, alternatively, consists of repetitious, 237 rhythmically-bedded alluvial fan deposits. These deposits are dominated by mid-fan lithofacies, which exhibit relatively minor vertical and lateral variation. This style of deposition combined with a general transport direction to the southeast indicates that the relative relief of the source terrane was more or less in equilibrium with erosion. This would support a growth fault model in which displacement along northeast-facing high-angle normal faults was associated with incremental movement along the Whipple detachment fault, resulting in continuous deposition onto the basin floor. The initiation of detachment faulting probably occurred late in Gene Canyon time, where repetitious alluvial fan deposits were first observed. In fact, the monolithic breccias found throughout the Gene Canyon Formation were probably early signals of detachment- related tectonic activity. Incremental movement along the detachment fault resulted in the deposition of the Copper Basin Formation and the subsequent isostatic uplift of the Whipple Mountains. An alternative interpretation for the observed sequence of Gene Canyon and Copper Basin deposits, is that the initiation of detachment faulting coincided with the deposition of the boulder breccia lithofacies in the lower Gene Canyon Formation. Continued movement along 238 the detachment fault then resulted in the isostatic uplift of the range. The change in sedimentation style near the top of the Gene Canyon Formation, which continued throughout Copper Basin time, therefore, would be related to continuous upwarping. 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Asset Metadata

Creator Mason, Thomas Daniel (author)

Core Title Depositional systems of the mid-Tertiary Gene Canyon and Copper Basin Formations, eastern Whipple Mountains, California

Contributor Digitized by ProQuest (provenance)

Degree Master of Science

Degree Program Geological Sciences

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

Tag OAI-PMH Harvest,Sedimentary Geology

Language English

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c30-124439

Unique identifier UC11225387

Identifier usctheses-c30-124439 (legacy record id)

Legacy Identifier EP58757.pdf

Dmrecord 124439

Document Type Thesis

Rights Mason, Thomas Daniel

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

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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