UNLV Retrospective Theses & Dissertations 1-1-1988 Sedimentological constraints on Middle Miocene extensional tectonism of the southern Las Vegas Range, southern Nevada Jack Edward Deibert University of Nevada, Las Vegas Follow this and additional works at: https://digitalscholarship.unlv.edu/rtds Repository Citation Deibert, Jack Edward, "Sedimentological constraints on Middle Miocene extensional tectonism of the southern Las Vegas Range, southern Nevada" (1988). UNLV Retrospective Theses & Dissertations. 49. http://dx.doi.org/10.25669/xzfv-h1j7 This Thesis is protected by copyright and/or related rights. It has been brought to you by Digital Scholarship@UNLV with permission from the rights-holder(s). You are free to use this Thesis in any way that is permitted by the copyright and related rights legislation that applies to your use. 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University Microfilms International A Bell & Howell Information Company 300 North Zeeb Road, Ann Arbor, Ml 48106-1346 USA 313/761-4700 800/521-0600 Order Number 1388257 Sedimentological constraints on Middle Miocene extensional tectonism of the southern Las Vegas Range, southern Nevada Deibert, Jack Edward, M.S. University of Nevada, Las Vegas, 1989 UMI 300 N. Zeeb Rd. Ann Arbor, MI 48106 SEDIMENTOLOGICAL CONSTRAINTS ON MIDDLE MIOCENE EXTENSIONAL TECTONISM OF THE SOUTHERN LAS VEGAS RANGE, SOUTHERN NEVADA by Jack Edward Deibert A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology Department of Geoscience University of Nevada, Las Vegas August, 1989 The thesis of Jack Edward Deibert for the degree of Master of Science in Geology is approved. Examining Committee Member, Peter L. Guth Ph.D. Examining Committee Member, David L. Weide Ph.D. (5 w w ^ — ■— _______________________ Graduate Faculty Representative, Peter Starkweather Ph.D. Graduate Dean, Ronald Smith Ph.D. University of Nevada, Las Vegas August, 1989 ABSTRACT An integrated depositional-tectonic model is proposed for the reconstruction of a Miocene lacustrine basin in the southern Las Vegas Range, Nevada, that was fragmented during a period of extensional tectonism. The model suggests that the sediments were deposited in an isolated basin during initiation of normal faulting in the Sheep Range extensional allochthon 12 to 16 Ma. After 12 Ma, deposition in this basin was terminated by high-angle block faulting followed by oroflexural rotation along the Las Vegas Valley shear zone. The Middle Miocene sequence is exposed as isolated outcrops within grabens between the Sheep Range, Fossil Ridge, and the Las Vegas Range. Collectively, the recognition and interpretation of sedimentary facies, analysis of the provenance of terrigenous material, correlation of volcanic ash units, and mapping of structural features indicate that the isolated exposures represent only fragments of a once laterally continuous sequence. This sequence is informally defined as the Gass Peak formation and does not correlate lithostratigraphically with the Ho se Spring Formation nor with any nearby Tertiary deposits. The lower, Fossil Ridge member is 75 m thick and consists of bioclastic limestone with minor interbeds of conglomerate. The upper, 260-m-thick Castle Rock member consists of limestone, dolomite, siltstone, sandstone, conglomerate, and iii megabreccia with minor tuffaceous and gypsiferous beds. Both of these members represent deposition in a shallow, tectonically active, permanent lake into which episodic, high-intensity stream discharge deposited coarse clastic sediment 12 to 16 Ma. Post 12 Ma, north-striking, high- angle normal faults displaced the Gass Peak formation on both sides of what is presently Fossil Ridge. Subsequently, oroflexural bending, which was the result of drag along the Las Vegas Valley shear zone, reactivated the north-striking faults as strike-slip faults and rotated them into their present northeast-striking orientation. The age and unique depositional basin of the Gass Peak formation support published models that the Sheep Range extensional allochthon was an extensional terrane separate from the Lake Mead area. Additionally, the proposed model suggests that oroflexural deformation along the Las Vegas Valley shear zone is younger than 12 Ma and is younger than normal faulting. TABLE OF CONTENTS Abstract............................................. iii List of Figures...................................... vii List of Tables...................................... viii Acknowledgements...................................... ix Introduction.......................................... 1 Regional Geology...................................... 4 Problems of Vertical Rotation..................... 10 Tertiary Stratigraphy.................................. 11 Gass Peak Formation............................... 11 Fossil Ridge Member.......................... 14 Castle Rock Member........................... 17 Quaternary Deposits............................... 26 Age of Gass Peak Formation........................ 27 Regional Correlation.............................. 28 Lithofacies and Depositional Processes................. 32 Bioclastic Limestone Lithofacies.................. 32 Carbonate-Clast-Dominated Conglomerate Lithofacies...................................... 3 6 Thin-Bedded Limestone, Sandstone, and Siltstone Lithofacies............................. 43 Quartz Arenite-Clast-Dominated Conglomerate Lithofacies...................................... 47 Megabreccia Lithofacies........................... 51 Depositional Model.................................... 54 Early-Stage of Deposition......................... 55 Late-Stage of Deposition.......................... 57 v Tertiary Structure.................................... 59 Normal Faults.................................... 59 Timing of Normal Faults and Oroflexural Movement.... 66 Summary of Tertiary Geologic History................... 67 Discussion of Regional Implications.................... 73 References............................................ 76 Appendix A ............................................ 82 Appendix B ............................................ 83 vi LIST OF FIGURES Figure Page 1 Regional location map of study area............. 2 2 Stratigraphic column for the Sheep and Las Vegas ranges............................... 5 3 Generalized geologic map of the study area....... 6 4 Stratigraphic column for the Gass Peak formation................................. 12 5 Features of the Gass Peak formation............. 16 6 Features of the Gass Peak formation............. 20 7 Location map for volcanic ash samples and correlative ash unit....................... 23 8 Rare earth element graph of volcanic ash units......................................... 25 9 Lithofacies map of the Gass Peak formation...... 3 3 10 Stratigraphic occurrence of the lithofacies of the Gass Peak formation..................... 34 11 Lithofacies of the Gass Peak formation.......... 37 12 Composite rose diagram for conglomerate clast imbrications for the Gass Peak formation........ 40 13 Stereonet diagram for sedimentary structures 46 14 Schematic diagram for the early-stage and late-stage of deposition....................... 56 15 Stereonet of tectonic folds and faults.......... 62 16 Geologic north-south cross-section.............. 64 17 Schematic block diagram for the deposition and subsequent deformation of the Gass Peak formation..................................... 69 vii LIST OF TABLES Table Page 1 Chemical analysis of selected tuff samples....... 18 2 Summary of tuff components of selected samples.... 22 3 Conglomerate