LATE MIOCENE FOSSIL LAKES OF THE LAKE MEAD REGION: IMPLICATIONS FOR PALEOGEOGRAPHY AND TRANSTENSIONAL FAULTING DURING BASIN AND RANGE EXTENSION

The Oligo-Miocene Horse Spring Formation (HSF), found throughout the Lake Mead region, records the onset of and main phase of extension and reflects a response to tectonic activity on contractional, extensional, and strike-slip features. The Lake Mead Fault System (LMFS) and Las Vegas Valley Shear Zone (LVVSZ) are the two major strike-slip systems. The Bitter Ridge Limestone (BRL) member of the HSF records microbial-dominated carbonate deposition in a shallow lacustrine setting from 14.5 to 13.8 Ma. The BRL can serve as an offset marker that is crucial for testing tectonic reconstructions and the driving mechanisms for extension. In order to accomplish this, however, it must first be determined if the BRL was deposited in one or more lake basins, and if in more than one, which field locations go with each basin. Once this is determined, high-resolution stratigraphic correlation within the basin(s) will allow for more refined tectonic reconstructions and a better understanding of the evolution of faulting. Previous workers have assumed that the BRL represents the deposits of one lake but outcrops span both sides of the LVVSZ and the Muddy Mountains, and may represent deposition in two lakes. We set out to test this assumption and refine the stratigraphy using a range of tools, including detailed measured sections, isotope and XRF geochemistry, and tephrochronology. Initial geochemical results from an Innov-X handheld XRF suggest (a) possible deposition within two separate basins; (b) BRL outcrops north of the LVVSZ and west of the Muddy Mountains are correlative; and (c) outcrops on the east side of the Muddy Mountains are geochemically dissimilar from those on the west. Alternatively these geochemical differences could be caused by lateral variation in local sources. Ongoing geochemical work will help test and refine these initial hypotheses. Distinguishing between these alternative explanations is crucial to reconstructing the Miocene paleogeography of the region and, hence, its tectonic evolution.