PALEOGEOGRAPHY OF THE HORSE SPRING FORMATION IN THE LAKE MEAD DOMAIN (NEVADA) FROM 18 TO 12 MA: DID A RIVER FLOW INTO OR THROUGH IT?

The Oligocene – Miocene Horse Spring Formation is well exposed across the Lake Mead domain and gives us a detailed record of basins and faulting and paleogeography from ~25 to 12 Ma. Here we present a series of 7 paleogeographic maps at million year increments from 18 to 12 Ma based on well constrained fault reconstructions, detailed mapping, stratigraphic analysis, and geochronology and tephrachronology of abundant tuffs. In the broader picture of the Southwest, we update the interpretations of Blakey, Chapin, Cather, and Karlstrom and others on the 18, 15, and 12 Ma maps. At 18 Ma a shallow carbonate lake dominated a single basin that lay in what is now eastern Lake Mead domain. At 17 Ma major faulting began with smaller carbonate lakes surrounded by streams and alluvial fans near the uplifting blocks. At 16 Ma a large evaporate lake formed. The lake changed upward to local playas and streams and volcanism increased at 15.8 Ma. By 15.5 Ma a major alluvial fan prograded into the basin from the NW basin margin and local normal faulting was breaking the basin into subbasins. At 15 Ma faulting was peaking in the eastern part of the domain and locally common to the west; the environments were clastic dominated streams and alluvial fans punctuated by major landslides from the Gold Butte fault block. At 14.5 – 13.9 Ma three large carbonate lakes (Bitter Ridge limestone) formed along new faults in the NW part of the area (Las Vegas Valley shear zone), while faulting decreased and major alluvial fans prograded across the eastern area. At 13.8 – 13 Ma the large lakes ended when more widespread faulting changed the landscape; streams and smaller lakes formed in more isolated basins during voluminous volcanism. At 12 Ma faulting rearranged the landscape again as deposition of the Horse Spring Formation ended, a few subbasins inverted, and localized basins formed dominated by the White and Boulder basins along the Las Vegas Valley shear zone and Bitter Spring Valley and Hamblin Bay faults in the west and the Hualapai Limestone carbonate lake in the east . No part of the well-exposed Horse Spring Formation records a large river. However, the thickest Miocene basins in the Lake Mead domain and adjacent areas, the Virgin depression, north Grand Wash basin, and Las Vegas valley, all remain mainly buried and may have been the terminal sink for regional rivers.