MAPPING AND GEOCHEMICALLY ANALYZING HORSE SPRING FORMATION THUMB MEMBER MIOCENE TUFFS TO RECONSTRUCT EXTENSIONAL DEFORMATION IN THE BITTER SPRING VALLEY, LAKE MEAD REGION, NEVADA

Active mapping and research of central Basin and Range extension in the Lake Mead region aims to reconstruct complex geologic events using structural features and stratigraphy. A key part of this research is determining a detailed stratigraphic framework of the Miocene Horse Spring Formation. Past work by St. Thomas, USGS, and Northern Arizona students and researchers documents much of this stratigraphy and the faulting across the Lake Mead region. Their recent hypothesis is that deformation in this region happened in stages, on different fault sets at different times, from 17 to ~11 Ma and younger (Lamb et al., 2022). A few areas have yet to be mapped and analyzed stratigraphically. We set out to determine the upper Thumb Member volcanic stratigraphy in one of these unstudied areas, the Bitter Spring Valley, in order to 1) identify and describe distinctive tuffs that can be mapped across fault blocks, 2) use these marker beds to map previously unrecognized faults, and 3) determine the origin of the tuffs.

In the field, we observed numerous tuffs, measured two detailed sections, and collected samples. We are now conducting petrographic and scanning electron microscope (SEM) geochemical analyses of these samples to test field hypotheses. Field and petrographic observations confirm that the two measured sections, from the western and central Bitter Spring Valley, are the same two-meter sequence of altered, glass-shard-rich and phenocryst-poor, ash-fall and reworked tuffaceous layers. A third tuff in the eastern Bitter Spring Valley is finer-grained but also glass-rich and phenocryst-poor. Initial geochemical results from major element SEM analyses suggest the three tuffs are the same. This is the youngest thick tuff in the Thumb Member. Stratigraphically below this tuff, is another two-meter-thick tuff with a distinctive pattern: a gray, glassy unaltered 20-40 cm thick base and altered, green upper part. We used the “grey to green” tuff as a marker bed to identify offsets and map faults. Ongoing SEM and electron microprobe analyses of additional tuffs will help us identify possible sources, including eruptions from the Southern Nevada Volcanic Field, Snake River Plain Volcanic Field, and Colorado River Extensional Corridor south of Lake Mead.