28 Geologic Mapping in the West (Posters)

This poster session highlights geologic mapping in the western Cordillera and Rocky Mountains. We encourage submissions that explore bedrock and surficial geology, geophysics, hydrology, hazards, and more. Both professional and student submissions are welcome.

Tuesday, 19 May 2026
8:00 AM - 5:30 PM
Alvarado D/E

Mapping and Detailed Basin Analysis to Elucidate Central Basin and Range Miocene Extension, the 7.5' Bitter Spring Quadrangle, Southern Nevada

Paper number: 28-3
Program: Technical Sessions
Time: 8:15 AM – 8:15 AM

Authors

Abstract

The 7.5' Bitter Spring Quadrangle (BSQ), southern Nevada, is situated between the highly-extended Colorado River Extensional Corridor and the northern Basin and Range. The Lake Mead region contains world-class exposures of Phanerozoic sedimentary units, Sevier-age thrusting, and major normal, strike-slip, and transtensional Cenozoic fault systems. Cambrian through Permian sequences record Paleozoic, marine passive-margin deposition and are in thrust contact with underlying, non-marine Triassic through Cretaceous outcrops. The Miocene Rainbow Gardens and Horse Spring Formations, and informally-named red sandstone unit of Bohannon (1984), record the initiation and development of Basin and Range extension. Tertiary-Quaternary alluvial surfaces presumably record integration of the Colorado River and tributaries.

Previous work in this region advanced our understanding of continental rifting and led to new hypotheses and questions about the structural evolution of continental rifts.  Based on detailed, spatially-focused studies, Dubendorfer and Simpson (1994) and Umhoefer et al. (2010) proposed stages of deformation.  This map results from a larger study to better understand this deformation, and is based upon detailed mapping from 2004 to 2025. The main period of Miocene faulting is divided into five stages. During Stage 1, ~16-14.5 Ma, extension via mostly north-south normal faults was well-underway. The best evidence for active faulting is found within lowest member of the Horse Spring Formation, the Thumb Member, in outcrops to the east and west of the BSQ.  The BSQ best illuminates faulting in the remaining four stages. Stage 2 ranges from 14.5 to 13.86 Ma, during Bitter Ridge Limestone deposition, and likely includes initiation of the White Basin Fault and other northeast-striking, left-lateral oblique normal faults.  Stage 3, 13.86 to 13.46 Ma, was a time of major tectonic reorganization and increased faulting based on sedimentation rates within Lovell Wash Member units.  Stage 4, 13.46 to 11.5 Ma, is marked by the development of northwest-trending faults that begin to partition the White Basin, decreased sedimentation rates, activity on the Muddy Peak Fault, continued activity on northeast-trending faults, tilting of beds creating a fanning dip sequence within the red sandstone unit, and activity on the Bitter Spring Valley Fault. During Stage 5, post-11.5 Ma, northwest faults continued to offset the red sandstone unit, while evidence of activity on northeast-trending faults is found across the region. 

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410541

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