INTRACONTINENTAL RIFT BASIN STRATIGRAPHY RECORDS MIDDLE MIOCENE EXTENSION, UPPER HORSE SPRING FORMATION IN THE WHITE BASIN, LAKE MEAD REGION, NV

The Lake Mead area is a natural laboratory to study extensional processes. The Horse Spring Formation (HSF) records the main phase of Basin and Range extension from 17-13 Ma. As part of a larger study to better understand extensional processes and Cenozoic paleogeography, we examined the upper HSF in detail along the south side of the Muddy Mountains. The ~200 m thick Bitter Ridge Limestone Member (BRL) of the HSF is exposed continuously over many kilometers. The conformably overlying Lovell Wash Member (LWM) is widely exposed in the White Basin and contains a mix of siliciclastic, evaporitic and carbonate units. Using measured sections and many tuffs found in both members, we create a detailed chronostratigraphic framework for the upper HSF and correlate marker beds across numerous faults. With the detailed stratigraphy, 1:10,000 mapping and structural analyses, we document depositional, faulting style and paleogeographic changes during extension, from 14.5 to 13 Ma. This includes a change from a single, large basin with persistent, uniform shallow lacustrine conditions of the BRL to a mix of depositional environments that vary laterally and vertically in the LWM. We divide the LWM into eight stratigraphic packages that represent basin-wide changes in depositional environments. Our data suggest that during upper HSF deposition, the Lake Mead Fault System and White Basin Fault controlled the overall basin geometry but during latest LWM deposition, the Muddy Peak Fault became active. Facies variation suggests additional syndepositional faulting occurred within the LWM basin and created local topographic variability. Stromatolitic facies vary rapidly laterally and abruptly across some northwest-trending structures: these are likely small growth faults that created varying depths in small lakes. Faults may have also controlled localized hot springs, which in turn created unusually diverse microbialite facies. Global climate changes may have also contributed to facies variability but, overall, we interpret that the stratigraphy records a shift in the style and location of faulting. A change in the sedimentation rate from ~435-450 m/My for the BRL and much of the LWM to ~250 m/My in the uppermost LWM may record the start of waning of the main phase of extension. Structural analyses confirm EW to ENE extension directions.