UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY GEOLOGY AND MAMMALIAN BIOSTRATIGRAPHY OF A PART OF THE NORTHERN CADY MOUNTAINS, MOJAVE DESERT, CALIFORNIA BY Susan T. Miller OPEN-FILE REPORT 80-878 This report is preliminary and has not been edited or reviewed for conformity with Geological Survey standards and nomenclature Menlo Park, California 1980 CONTENTS Page Abstract——————————————————————————————————————————————————— 1 Introduction—————————————————————————————————————————————————— 2 Previous investigations————————————————————————————————————— 4 Methods and terminology————————————————————————————————————— 6 I Acknowledgements——————————————————————————————————————————— 7 Regional geologic setting———————————————————————————————————————— 8 Descriptive geology———————————————————————————————————————————— 13 Hector formation——————————————————————————————————————————— 13 Stratigraphic nomenclature—————————————————————————————— 13 General features—————————————————————————————————————— 14 Physical stratigraphy of the sedimentary rocks————————————— 18 Tephra-bearing feldspathic and lithic sandstone————————— 18 Mudrock——————————————————————————————————————————— 27 Lenticular conglomerate and sand-bearing conglomerate———— 28 Limestone and calcareous nodules————————————————————— 30 Air-fall tuff—————————————————————————————————————— 31 Petrography and stratigraphy of the basalt unit ———————————— 31 Petrography and stratigraphy of the rhyolite ash-flow tuff unit—————————————————————————————————————————:——— 32 Paleogeographic setting and depositional history———————————— 33 Paleogeographic setting———————————————————————————— 33 Depositional History——————————————————————————————— 36 Age and correlation———————————————————————————————————— 39 Paleogeographic relationship between the northern Cady Mountains and other Miocene sequences in the central Mojave Desert region——————————————————————————————— 46 Younger alluvial units—————————————————————————————————————— 51 Fanglomerate and gravel————————————————————————————————— 51 Older gravel—————————————————————————————————————————— 52 Older alluvium———————————————————————————————————————— 52 Younger alluvium—————————————————————————————————————— 52 Structure———————————————————————————————————————————————— 54 Biostratigraphy of the Hector Formation———————————————————————————— 56 Mammal ages and local faunas————————————————————————————————— 56 Lower Cady Mountains local fauna————————————————————————————— 58 Upper Cady Mountains local fauna————————————————————————————— 61 Summary—————————————————————————————————————————————————————— 64 Systematic paleontology————————————————————————————————————————— 66 Proheteromys sulculus—————————————————————————————————————— 67 Cf. Anchitheriomys? sp.————————————————————————————————————— 71 Tomarctus cf. T. hippophagus————————————————————————————————— 76 Merychippus carrizoensis———————————————————•———————————————— 81 Cf. Diceratherium sp.—————————————————————————————————————— 89 Merychyus (Merychyus) cf. M. (M.) calaminthus——————————————————— 91 Merycodus sp.-————————————————————————————————————————————— 99 Cf. Aepycamelus sp.———————————————————————————————————————— 101 "Miolabis" cf. "M." tenuis——————————————————————————————————— 105 References cited——————————————————————————————————————————————— 116 II ILLUSTRATIONS Page Figure 1 • Index map——————————————————————————•—————————————— 3 2. Diagram summarizing stratigraphic ranges of ~** key Miocene stratal sequences, Mojave Desert———————————— 5 3. Diagram summarizing areal distribution of major Miocene and(?) latest Oligocene outcrops, Mojave Desert——•——————— 9 4. Generalized columnar section, northern Cady Mountains——————— 15 5. Block diagram of paleogeographic setting, Hector Formation, northern Cady Mountains—————————————————————————————— 16 6-7. Outcrop photographs of Hector Formation showing 6. Sandstone and siltstone, middle unit———————————————— 17 7. Lenticular conglomerate, upper unit————————————————— 17 8. Diagram of major sedimentary rock types, Hector Formation——— 19 9-20. Outcrop photographs of Hector Formation showing 9. Sandstone, siltstone, and mudrock, middle unit————————— 20 10. Poorly indurated and poorly bedded sandstone, upper unit———————————————————————————————————————————— 20 11. Sandstone, upper unit————————————————————————————— 21 12. Sandstone and conglomerate, middle unit——————————————— 21 13. Conglomerate and sandstone, middle unit—————————————— 23 14. Mudrock and siltstone, middle unit—————————————————— 24 15. Conglomerate and sandstone, middle unit——————————————— 25 16. Texturally massive sandstone, middle unit, and basalt unit—————————————————————————————————————————— 25 17. Basalt unit and middle sedimentary unit—————————————— 26 18. Distal facies, middle sedimentary unit——————————————— 26 19. Conglomerate and sandstone, proximal facies, upper unit—— 29 20. Rhyolitic welded ash-flow tuff unit——————————————————— 29 21. Diagram showing correlation of various Miocene sequences, Mojave Desert—————————————————————————————————————— 40 22. Photograph showing relationship of younger alluvial units to older sedimentary units—————————————————————————— 53 23. Diagram showing correlation of various Miocene faunas—————— 57 24. Cf. Anchitheriomys? sp. ——————————————————————————————— 72 25. Cf. Anchitheriomys? sp. ——————————————————————————————— 72 26. Cf. Anchitheriomys? sp. ——————————————————————————————— 73 27. Cf. Anchitheriomys? sp. ——————————————————————————————— 73 28. Tomarctus cf. T. hippophagus———————————————————————————— 77 29. Tomarctus cf. T. hippophagus———————————————————————————— 77 30. Merychippus carrizoensis——————————————————————————————— 82 31. Merychippus carrizoensis——————————————————————————————— 82 32. Merychippus carrizoensis—————————————————————————————— 83 33. Merychippus carrizoensis———————————————-————————————— 83 34. Cf. Diceratherium sp. ————————————————————————————————— 90 35. Cf. Diceratherium sp. ————————————————————————————————— 90 36. Merychyus (Merychyus) cf. MM (M.) calaminthus—————————————— 93 37. Merychyus (Merychyus) cf. M. (M.) calaminthus—————————————— 93 38. Merychyus (Merychyus) cf. M. (M.) calaminthus—————————————— 94 39. Merychyus (Merychyus) cf. M. (M.) calaminthus—————————————— 94 III Illustrations (continued) Page Figure 40. Merychyus (Merychyus) cf. M. (M.) calaminthus————————————— 95 41. Merychyus (Merychyus) cf. M. (M.) calaminthus————————————— 95 * 42. Merycodus sp. ——————————————————————————————————————— 100 43. Cf. Aepycamelus sp. —————————————————————————————————— 102 44. Cf. Aepycamelus sp. —————————————————————————————————— 102 45. Cf. Aepycamelus sp. ———————————————————————————————— 103 46. Cf. Aepycamelus sp. —————————————————————————————————— 103 47. "Miolabis" cf. "M." tenuis ——————————————————————————— 106 48. "Miolabis" cf. "M." tenuis ———————————————————————————— 106 49. "Miolabis" cf. "M." tenuis ——————————————————————————— 107 50 ' "Miolabis" cf. "M." tenuis ——————————————————————————— 107 51. "Miolabis" cf. "M." tenuis ——————————————————————————— 108 52. "Miolabis" cf. "M." tenuis ——————————————————————————— 108 53. "Miolabis" cf. "M." tenuis ————————————————————————————— 109 54. "Miolabis" cf. "M." tenuis ———————————————————————————— 109 55. "Miolabis" cf. "M." tenuis ————————————————————————————— 110 56. "Miolabis" cf. "M." tenuis ——————————————————————————— 110 Plate 1. Geologic map —————————————————————————————————————— 14 2. Columnar section, CADY I —————————————————————————————— 31 3. Columnar sections, CADY I la, lib, lie, III ——————————————— 58 4. Columnar sections, CADY IVa, IVb ———————————————————————— 61 5. Locality map -——————————————————————————————————————— 59 6. Fault map, central Mojave Desert——————————————————————— 48 7. Palinspastic Reconstruction, Miocene, central Mojave Desert—— 46 1. All plates are in pocket. Page number is first reference to that plate in the text. IV TABLES Page Table 1. Measurements of Proheteromys sulculus———————————————————— 70 --»* 2. Measurements of Anchitheriomys? sp.—————————————————————— 74 3. Measurements of Tomarctus cf. T. hippophagus——————————————— 79 4* Measurements of Merychippus carrizoensis—————————————————— 86 5. Measurements of Merychyus (Merychyus) calaminthus——————————— 96 6* Measurements of cf. Aepycamelus sp.——————————————————————— 104 7. Measurements of "Miolabis" cf. "M." tenuis————————————————— 112 GEOLOGY AND MAMMALIAN BIOSTRATIGRAPHY OF A PART OF THE NORTHERN CADY MOUNTAINS, MOJAVE DESERT, CALIFORNIA By Susan T. Miller ABSTRACT Approximately 1250 ft (381 m) of interbedded basalt, tuff, and volcani- clastic sedimentary rock crop out in the northern Cady Mountains, Mojave Desert, California. These rocks are assigned to the Hector Formation of Woodburne, Tedford, Stevens, and Taylor (1974). Sediments in the Hector Formation accum­ ulated in alluvial and lacustrine depositional environments. These environments occurred in a paleogeographic setting that incorporated a basin-plain complex of lakes, ponds, and marshes; a gently-inclined alluvial plain complex; and a distal alluvial-fan complex. Sediments deposited in these environments lapped onto an erosional surface that was developed on an older volcanic and sedimentary terrain. This terrain provided epiclastic volcanic and granitic constituents to the Hector Formation. Intermittent volcanism of basaltic and rhyolitic composition accompanied deposition of the alluvial/lacustrine sediments. The volcanic events are represented by a basalt flow and a rhyolitic ash-flow tuff, and by beds of vitric air-fall