MIOCENE EXTENSIONAL TECTONICS AS RECORDED BY THE HORSE SPRING FORMATION, LAKE MEAD AREA, NEVADA: GEOCHEMICAL ASH CORRELATION WITHIN THE THUMB MEMBER

Previous studies from the Lake Mead Area undertaken to understand Central Basin and Range extension consist primarily of mapping and structural analyses. These results have been crucial in understanding certain aspects of uplift and extension, however, many questions cannot be addressed until sedimentary and volcanic units, as well as the basins into which they were deposited, are better understood. The Miocene Horse Spring Formation (HSF), found throughout the Lake Mead Area, contains four members that span much of the time during which extension occurred, and is often used in tectonic reconstructions but has not been studied in detail within many key fault blocks. The Thumb Member, deposited during the early phase of extension (from ~16-14 Ma), is especially important to these reconstructions. It, however, displays rapid lateral and vertical facies changes, often over 10's to 100's of meters, making it difficult to use lithostratigraphy alone for correlation. Fortunately, the HSF contains numerous well-exposed ashes that can be used as marker beds. The Thumb Member alone contains 12-15 separate ash layers. By using a geochemical ash fingerprinting technique, combined with ⁴⁰Ar/³⁹Ar geochronology, detailed mapping and measured sections, we are able to identify and delineate informal “lower, middle and upper” Thumb sub-members for the first time. To identify key ash marker beds, we performed XRF analyses of over 80 whole-rock ash samples. Trace element concentrations from 20 samples within 3 mapped continuous layers were analyzed statistically to establish which elements are most reliable for determination of a geochemical fingerprint. Six elements, Rb, Ce, Cr, Ga, Nb and Ni, are the most reliable. These elements were then used to correlate unknown ashes within and between fault blocks. This technique allows for the construction of an accurate stratigraphic framework as well as correlation across important extension-related faults. This in turn contributes to a better understanding of uplift, basin development, and dismemberment in this structurally complex area.