Nonmarine Sedimentary Rocks of Tertiary Age in the Lake Mead Region, Southeastern Nevada and Northwestern Arizona
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s w -RN~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 24000000 I- Nonmarine Sedimentary Rocks of Tertiary Age in the Lake Mead Region, Southeastern Nevada and Northwestern Arizona GEOLOGIC AL-SURVEY PI/ F E S SIONAL PAPER 1259 Nonmarine Sedimentary Rocks of Tertiary Age in the Lake Mead Region, Southeastern Nevada and Northwestern Arizona By ROBERT G. BOHANNON GEOLOGICAL SURVEY PROFESSIONAL PAPER 1259 A study of the age, nomenclature, lithology, and tectonic history of Miocene continental deposits in the transition zone between the Great Basin and Sonoran Desert X15 UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1984 UNITED STATES DEPARTMENT OF THE INTERIOR WILLIAM P. CLARK, Secretary GEOLOGICAL SURVEY Dallas L. Peck, Director Library of Congress Cataloging in Publication Da Bohannon, Robert G. Nonmarine sedimentary rocks of Tertiary age in the Lake Mead region, southeastern Nevada and northwestern Arizona. (Geological Survey Professional Paper; 1259) BibliographW 72 p. Supt. of Does. No.: 1 19.16: 1. Rocks, Sedimentary. 2. Geology, Stratigraphlc-Miocene. 3. Petrology-Mead, Lake, region (Ariz, and Nev.) I. itle. II. Series. QE471.B682 562'.5'0979312 81407893 AACR2 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 CONTENTS POV Pg Abstract ........................................... 1 Thumb Member-Continued Introduction ........................................ 2 Depositions environments. 29 Acknow n ................................dentn 3 Facies and provenance . 31 Stratigraphic nomenclature ............................ 9 Bitter Ridge Limestone Member . 33 Historical development ............................ 3 Distribution and thickness. 33 Proposed stratigraphic nomenclature .................. 6 Lithology . 34 Age of the Tertiary rocks .............................. 9 Calcareous lithologies. 34 Description of rock unnt ............................... 14 Noncalcareous lithologies. 36 Horse Spring Foingaon ............................ 15 Sedimentary structures. 36 Rainbow Gardens Member ...................... 15 Facies and depositions environments. 37 Distribution and thickness ................... 15 Lovell Wash Member. 41 L thology ................................. 15 Distribution and thickness. 41 Clastic and siliceous ithologies ............. 15 Lithology . 42 Carbonate and sulfate lithologies ........... 18 Carbonate rocks, claystome, and chart . 42 Sedimentary structures .................. 19 Tuffaceous and elastic lithologies. 44 Facies and their distribution ............... 20 Sedimentary structures. 46 En ts of deposition .................. 22 Depositional environments. 48 Thumb Member ............................... 23 Informally named rock units . 49 Distribution and thickness ................... 23 Red sandstone unit . 49 Litho togy................................. 23 Rocks of the Grand Wash trough . 54 Clastic rock types ....................... 23 Muddy Creek Formation . 56 Carbonate and sulfate rocks ............... 28 Paleogeologic and paleotectonic evolution. 68 Tuffaceous rocks and volcanic flows. 28 References cited . 67 Sedimentary suctures .................. 29 ILLUSTRATIONS PLATE 1. Geologic and paleogeologic maps of the Lake Mead region ................................................ In pocket Page FIGURE 1. Location map of the Lake Mead region .............................................................. 3 2. Diagram showing general lithology, stratigraphy, previous nomenclature, and proposed nomenclature for Tertiary rocks of the Lake Mead region .5 3. Maps showing Tertiary geology and sample sites in the Muddy Mountains area, Virgin Mountains-Grand Wash Cliffs area, and Frenchman Mountain area .13 4. Measured stratigraphic sections of the Rainbow Gardens Member of the Horse Spring Formation .16 5. Photograph of the resistant limestone unit in the upper part of the Rainbow Gardens Member .19 6. Photograph of the magnesite-bearing strata within the Rainbow Gardens Member .20 7. Map showing distributions of the five lithofacies of the Rainbow Gardens Member .21 8. Generalized stratigraphic sections of the Thumb Member of the Horse Spring Formation .......................... 24 9. Photographs of parallel continuous bedding within sandstone of the Thumb Member .26 10. Map showing distributions of conglomerate and finegrained facies of the Thumb Member and relative abundances of rock types. 27 11. Photograph of a continuous, well-defined conglomerate bed in the Thumb Member .28 12. Photograph of one of the green tuff units in sandstone of Thumb Member .28 13. Diagram of lithology and bedforms of a microsection in the Echo Wash area .29 14. Photograph of rocks measured for the Echo Wash microsection .30 15. Map showing northern basin margin of the Thumb Member .31 III IV NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN LAKE MEAD REGION Page FIGURE 16. Photographs of the northern margin of the Thumb basinn 32 17. Map showing distribution of outcrops of the Bitter Ridge Limestone Member of the Horse Spring Formation. 33 18. Photographs of bedding characteristics in the Bitter Ridge Limestone Member .... ............................ 35 19. Photomicrograph of sample from type section of Bitter Ridge Limestone Member ............................... 36 20-23. Photographs of sedimentary structures in the Bitter Ridge Limestone Member- 20. Conglomerate interfingering with limestone ................................................... 37 21. "Teepee" structures ....................................................................... 38 22. Large circular depressions on a dip slope ....................................................... 39 23. Stromatolitic structures ................................................................... 40 24. Map showing distribution of outcrops of the Lovell Wash Member of the Horse Spring Formation .................. 41 25. Photographs of three lithofacies in carbonate and claystone beds of the Lovell Wash Member ...................... 43 26. Photomicrographs of three characteristic textures of limestone in the Lovel Wash Member ...................... 44 27. Photograph of a small chert dome from Lovel Wash Member, showing original bedding .......................... 45 28. Photograph of thick, wellxposed tuff unit in Lovell Wash Member .......................................... 45 29-31. Photonicrographs of samples from the Lovell Wash Member 29. Shard-rich, matrix-supported tuffaceous rock . ........................................ 46 30. Reworked, partially altered tuffaceous rock ..................................................... 47 31. Pumice lapilli in sparry calcite matrix ......................................................... 47 32-36. Photographs of the Loveil Wash Member: 32. "Teepee" structure in thick limestone bedset ................................................... 48 33. Contorted thin bedding .................................................................. 48 34. Eggshell-like hemispheroids ....... ......................................................... 49 35. A large carbonate mound ................................................................... 50 36. Rip-up structure .......................................................................... 51 37. Map showing distribution of informal rock units ........................................................ 51 38. Generalized geologic map of red sandstone unit in White Basin .............................................. 52 39. Photograph of red sandstone, siltstone, and claystone in the red sandstone unit ................................. 53 40. Photomicrograph of sandstone from the red sandstone unit ................................................. 54 41. Photograph of Paleozoic carbonate rocks abutting conglomerate of the red sandstone unit along the Muddy Peak fault.. 54 42. Generalized geologic map of the Grand Wash trough area .................................................. 55 43. Map showing present distribution and probable original extent of Muddy Creek Formation in the Lake Mead region .... 57 44. Cross section from a point north of Muddy Peak to area southwest of Gale Hills ...... .................. ........ 65 TABLES pagp TABLE 1. Selected previously published K-Ar ages for Tertiary rock samples from the Lake Mead region .......................... 10 2. Zircon fission-track ages of samples from Tertiary rocks of the Lake Mead region ..................................... 11 3. Analytical data for zircon fission-track age determinations ....................................................... 12 s * f NONMARINE SEDIMENTARY ROCKS OF TERTIARY AGE IN THE LAKE MEAD REGION, SOUTHEASTERN NEVADA AND NORTHWESTERN ARIZONA By ROBERT G. BOHANNON ABSTRACT Twenty-two fission-track age determinations on zircon extracted from basal parts of airfall tuff beds in the Horse Spring Formation Tertiary sedimentary rocks form a broad belt north of Lake Mead and the two informally named rock units are concordant with in eastern Clark County, Nev., and western Mohave County, Ariz., previously published K-Ar age determinations. These determinations between Las Vegas and the Grand Wash Cliffs. This belt stradles the indicate that the Thumb Member ranges from 17.2 to possibly diffuse boundary between the Great Basin and Sonoran Desert sec- 13.5 my. old, the Bitter Ridge Limestone Member from 13.5 to about tions of the Basin and Range structural province and is directly adja- 13.0 my. old, and the Lovell Wash Member from 13.0 to 11.9 my. old. cent to the Colorado Plateaus province.