Paleogeographic reconstructions and stratigraphy of the Upper Thumb Member of the Horse Spring Formation, western Lake Mead, NV

This study focuses on the upper Thumb Member stratigraphy of the western Lake Mead Domain within the central Basin and Range region. The central Basin and Range is an east-west trending corridor extending from the southwestern edge of the Colorado Plateau west to the Sierra Nevada. During the Miocene epoch, the Horse Spring Formation was deposited into a single or multiple basins before and during Basin and Range extension. The Thumb Member of the Horse Spring Formation (∼17/16.5 Ma -- 14.5 Ma in the western Lake Mead area) records much of the early syndepositional deformation in the Lake Mead region and will be the focus of this study. The upper Thumb Member (∼15.5 -- 14.5 Ma) is a laterally extensive, dominantly sandstone unit containing several interbedded tuff units that have been dated and/or correlated throughout the basin.;This study concentrates on the development of, and sedimentation in, basins and sub-basins that were formed by the normal and strike-slip faulting that accommodated large-magnitude crustal extension within the central Basin and Range. Basin stratigraphy and paleogeography was assessed by studying facies changes, both laterally and vertically. The sedimentologic and stratigraphic data obtained as part of this study aided in better understanding the controls on sedimentation and the effects of faulting within the basin. The effects of paleoclimate and paleotopography were also evaluated with respect to sedimentation rates and deposition of the upper Thumb Member into these basins.;Due to the numerous tuff units present throughout the strata of the upper Thumb Member, geochronologic correlations were made along with correlations based on lithostratigraphy. These correlations were then used to create paleogeographic reconstructions of the basin at progressive time intervals.;Six paleogeographic maps were made to represent depositional environments and facies distributions over significant time intervals or at a specific time-slice focused on one well-dated and extensive tuff unit. The upper Thumb Member section was divided into intervals based on important tuffs that were correlated throughout the basin or marker beds such as a distinct megabreccia and associated debris flow deposit. The paleogeographic maps presented are as follows: 1) 15.5 -- 15.4 Ma, which spans from the upper contact of the middle Thumb Member conglomerate to just below the crystal rich tuff, 2) 15.35 Ma time-slice, which depicts the basin at the time of the crystal rich tuff deposition, 3) 15.35 -- 15.09 Ma, which spans from the crystal rich tuff to the crystal poor tuff, 4) 15.09 -- 14.8 Ma, which spans from the crystal poor tuff to the Christmas tree tuff, 5) ∼ 14.7 Ma time-slice, which depicts the basin at the time of megabreccia 4 and debris flow 2 deposition, and 6) 14.8 -- 14.5 Ma, which spans from the Christmas tree tuff to the contact of the Thumb Member with the Bitter Ridge Limestone Member.;The upper Thumb Member records the onset of faulting in the Gale Hills along the Government, West Fortress, and Southern Gale Hills normal-oblique faults between 15.5 and 15.35 Ma. The right-lateral Las Vegas Valley shear zone propagated into the study area between 15.35 Ma and 15.0 Ma. The Bitter Spring Valley fault became active after ∼ 14.8 Ma. The White Basin fault is active starting at ∼14.5 Ma. These faults were responsible for the initial creation of sub-basins within the Thumb Member basin and significantly influenced deposition both during and after Thumb Member deposition.