UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY Geology of a part of the southern Monte Cristo Range, Esmeralda County, Nevada By Steven W. Moore Open-File Report 81-710 1981 This report has not been edited for conformity with U.S, Geological Survey editorial standards or stratigraphic nomenclature. CONTENTS Page Abstract. ............................ 1 Introduction .......................... 3 Background, purpose, and scope ............... 3 Location and geographic setting .............. 5 Methods used in the investigation ............. 7 previous investigations .................. 9 Geologic time scale used .................. 11 Acknowledgments. ...................... 14 Stratigraphy .......................... 15 Ordovician System ..................... 15 Palmetto Formation .................. 15 Distribution, contact relations, and thickness ................... 15 Lithology .................... 16 Age and correlation ............... 16 Depositional environment ............. 18 Tertiary System ...................... 18 Oligocene to Miocene Series .............. 18 Castle Peak tuff ................. 18 Lithology of the unwelded tuff member .... 20 Lithology of the white welded tuff member ................ 25 Lithology of the red welded tuff member ................ 28 Age and correlation ............. 31 Mode of deposition ............. 33 Older Andesite .................. 35 Lithology .................. 35 Age and correlation ............. 37 Miocene Series .................... 37 Miocene andesite flows .............. 37 Lithology of the fine-grained andesite. ................. 38 Lithology of the coarse-grained andesite. ................. 40 Age and correlation ............. 42 Esmeralda Formation Blair Junction sequence ... 44 Distribution and thickness. ......... 46 Lithology of member 1............ 51 Stratigraphy Page Tertiary System Oligocene to Miocene Series Esmeralda Formation Blair Junction sequence continued Lithology of member 2 ............ 58 Lithology of member 3 ............ 62 Lithology of member 4 ............ 65 Fossils ................... 70 Age and correlation ............. 75 Depositional environment .......... 80 Diagenesis. ................. 86 Miocene or Pliocene(?) series ............. 91 Post-Esmeralda Formation volcanic rocks. ..... 91 Lithology .................. 94 Age and correlation ............. 94 Pliocene(?) Series .................. 96 Basaltic andesite ................ 96 Lithology .................... 97 Age and correlation ............... 100 Quaternary System ..................... 100 Older alluvium .................... 100 Younger alluvium ................... 101 Structural Geology ....................... 102 Regional tectonic setting ................. 102 Pre-Cenozoic structure ................... 105 Cenozoic structure ..................... 106 Unconformities ....................... 106 Faults ........................... 107 Folds. ........................... 113 Discussion ......................... 117 Geologic History. ........................ 120 Conclusions ........................... 123 References. ........................... 126 Appendix A: Table of selected radiometric ages for Cenozoic rocks in southwestern Nevada ....... 134 Appendix B: Measured stratigraphic sections .......... 140 Measured section SS I ................... 140 Measured section SS II ................... 150 Measured section SS III. .................. 154 Measured section SS IV ................... 157 ii ILLUSTRATIONS Page PLATE 1. Geologic map and cross sections of a part of the southern Monte Cristo Range, Esmeralda County, Nevada ................ In pocket FIGURE 1. Location map showing the mapped area within Nevada Coal Withdrawal No. 1 ............ 4 2. Index map of part of southwestern Nevada ....... 6 3. Late Cenozoic geologic time scale .......... 13 4. Photograph of chert of the Ordovician Palmetto Formation ................. 17 5. Regional correlation chart of Cenozoic rocks in selected areas of southwestern Nevada ....... 19 6-12. Photographs showing: 6. Members of the Castle Peak tuff at Castle Peak ................. 21 7. Unwelded member of the Castle Peak tuff .... 23 8. White welded member of the Castle Peak tuff . 27 9. Red welded tuff member of the Castle Peak tuff. 30 10. Older andesite unit at Dragonback Hills .... 36 11. Fine-grained andesite overlying the Castle Peak tuff .................. 39 12. Basal breccia of the coarse-grained andesite unit ................ 41 13. Stratigraphic columns showing correlation of measured stratigraphic sections ............... 48 14. Map showing locations of measured sections and fossil samples ................... 49 iii Page FIGURE 15. Diagram showing correlation of the Blair Junction and Coaldale sequences of the Esmeralda Formation ................ 50 16-31. Photographs showing: 16. Basal conglomerate of member 1 of the Blair Junction sequence .......... 52 17. Tuffs and claystones in the lower part of member 1 of the Blair Junction sequence. 54 18. Shale in member 1 of the Blair Junction sequence. ................. 56 19. Sandstone at the base of member 2 of the Blair Junction sequence .......... 59 20. Siltstone overlying sandstone in member 3 of the Blair Junction sequence. ...... 61 21. Pebbly sandstone and tuff in member 3 of the Blair Junction sequence ........ 64 22. Upper lahar in member 4 of the Blair Junction sequence showing lenses of sandstone ................ 66 23. Lahar in Member 4 of the Blair Junction sequence. ................. 67 24. Sandstone and volcanic breccia in a lahar in member 4 of the Blair Junction sequence. ................. 69 25. Pharyngeal arch of Mylopharodon sp. ..... 74 26. Silicified wood in member 1 of the Blair Junction sequence .......... 76 27. Erosion-resistant dikes of intrusive andesite. ................. 92 28. Hornblende andesite dike that intrudes the Castle Peak tuff. ............. 93 IV Page FIGURE 29. Photograph showing intrusive plug of rhyolite .... 95 30-31. Photographs showing: 30. Basaltic andesite flow. ........... 98 31. Columnar-jointed basaltic andesite. ..... 99 32. Generalized map of major fault traces and major regional structural features in part of southwestern Nevada .............. 103 33. Photograph of upturned beds of member 3 of the Blair Junction sequence, along a fault. ...... 108 34. Frequency diagram of fault trends in the southern Monte Cristo Range ............ 109 35. Photograph of slickensides along a fault in older andesite flows. ............... Ill 36. Equal-area stereographic plot of slickensides .... 112 37-38. Photographs showing: 37. Zone of intense folding in member 1 of the Blair Junction sequence ........ 114 38. Thickening of lignitic shale in the hinge of a fold in member 1, Blair Junction sequence ............. 116 TABLES Page TABLE 1. Samples analyzed by X-ray diffraction ......... 24 2. Checklist of fossils from the Blair Junction sequence of the Esmeralda Formation ......... 71 vi Geology of a part of the southern Monte Cristo Range, Esmeralda County, Nevada By Steven W. Moore ABSTRACT The Blair Junction sequence of the Esmeralda Formation in the southern Monte Cristo Range, Esmeralda County, Nevada, is informally defined as a discrete local stratigraphic assemblage of middle to upper Miocene, dominantly lacustrine sedimentary rocks. The Blair Junction sequence, which is divided into four informal members, is interpreted as being deposited in shallow, fresh, alkaline water of a marginal lacustrine environment with local influences of intermit­ tent paludal and fluvial depositional conditions and contemporaneous volcanic activity. Older stratigraphic units include middle Miocene andesite units and upper Oligocene to lower Miocene ash-flow tuff. The Cenozoic volcanic and sedimentary rocks unconformably overlie the Ordovician Palmetto Formation. Post-Esmeralda Formation units include upper Miocene to Pliocene andesite, rhyolite, and Quaternary alluvial deposits. Member 1 of the Blair Junction sequence of the Esmeralda Forma­ tion is correlative with the coal-bearing unit at the north base of the Silver Peak Range. However, the amount and the grade of coal (lig­ nite or carbonaceous shale) generally decreases northward. In fine­ grained lacustrine rocks, authigenic minerals include montmorillonite, opal-CT, authigenic potassium feldspar, and minor zeolites. The southern flank of the Monte Cristo Range is traversed by three major sets of faults, the trends of which average N. 57 E., N. 77 E., and N. 42 E., respectively. These trends are anomalous with respect to the more northerly trends typical of the Basin and Range Province. These faults, some with large vertical throw, offset upper Oligocene to Pliocene rocks. Slickensided fault planes provide some evidence for left-oblique slip on these northeast-trending faults, which is analogous in part to the relative sense of movement along the fault system in the nearby Candelaria Hills. A zone of lo­ cally intense folding, suggestive of compressional stresses acting subparallel to an adjacent fault, also suggests a lateral or oblique component of movement along this fault. INTRODUCTION Background, Purpose, and Scope Coal was discovered north of the Silver Peak Range in Esmeralda County, Nevada in 1893. This discovery spurred great local interest in the early 1900's because of scarcity of fuel in the region and the energy demand of mining operations in the nearby gold-boom towns of Goldfield, Candelaria, and Tonopah. In 1911, lands in four townships in this area, including much of the Monte Cristo Range,