This study focuses on the structures and Miocene sedimentary rocks of the Horse Spring Formation within the western-central Lake Mead domain in southern Nevada. The goals of this study are to better understand the ages, local facies distributions, and relation of the detailed stratigraphy to the structural setting of the Thumb, Bitter Ridge Limestone, and Lovell Wash Members of the Horse Spring Formation in the eastern Gale Hills. Other goals of this study are to better understand the timing and style of folding, faulting, and vertical axis rotations near the intersection and terminus of two regional strike-slip fault systems with opposing sense of slip, the right-lateral Las Vegas Valley shear zone and the left-lateral Lake Mead fault system. To achieve these goals, I created a detailed geologic map (1:5,000 scale) of the eastern Gale Hills, collected samples that were analyzed for tephrachronology and geochronology, collected samples for paleomagnetic analysis, and collected kinematic and other structural data from primary and secondary structures.
Two new paleomagenetic results are presented and discussed here along with the analysis of previous paleomagnetic results. Tephrachronology of ash-fall tuffs provides new age control throughout the Thumb Member within the eastern Gale Hills. Map and facies relationships provide new insights into the time-transgressive nature of the base of the Bitter Ridge Limestone Member as well as basin margin facies of the Bitter Ridge Limestone Member. Map and facies relationships also provide new insights on the nomenclature and stratigraphic position of the top of the Bitter Ridge Limestone Member and the base of the Lovell Wash Member.
Active normal faulting was present in the study area during deposition of the lower Thumb Member beginning ca. 16.0 Ma. Normal faulting continued throughout deposition of the Thumb Member. It is proposed here that the Las Vegas Valley shear zone propagated into the Gale Hills area ca. 15.0 Ma. The Bitter Ridge Limestone in the study area was deposited in an algal lake, which was located north of a north-down normal fault that shed coarse-grained sediments, derived from the uplifted footwall, into the algal lake. The relationships between the sedimentary rocks and the faults and folds suggest that the majority of the deformation in the eastern Gale Hills took place during and after the deposition of the Lovell Wash Member from 13.7 to ∼12 Ma. Deformation in the northern part of the study area is dominated by north-northeast-striking normal and left-oblique-slip faults that accommodated ∼10° - 20° amounts of clockwise rotations. The southern part of the study area is dominated by east-west-trending contractional structures: plunging folds accommodated vertical axis rotations. Strain in the Highway block accommodated the deformation pattern at the interaction of the left-lateral western Bitter Spring Valley fault and the right-lateral Las Vegas Valley shear zone. Basin inversion of the eastern Gale Hills took place after deposition of the Lovell Wash Member.
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|Advisor:||Umhoefer, Paul J.|
|Commitee:||Beard, L. Sue, Duebendorfer, Ernie M.|
|School:||Northern Arizona University|
|Department:||School of Earth Sciences and Environmental Sustainability:|
|School Location:||United States -- Arizona|
|Source:||MAI 51/02M(E), Masters Abstracts International|
|Keywords:||Basin analysis, Gale Hills, Lake Mead, Miocene, Stratigraphy, Structure|
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