SEDIMENT DEPOSITION IN LAS VEGAS BAY, LAKE MEAD, NV: IMPLICATIONS FOR MARINE SANDSTONE DISTRIBUTIONS

Lake Mead is the largest anthropogenic reservoir on the Colorado River. It provides a favorable environment to relate coastal and deep water sedimentary processes because the basin’s history and evolution are highly constrained. Eleven sediment cores, two bathymetry surveys and multiple GPR transects have been used to understand the distribution of sediment within Las Vegas Bay of Lake Mead, NV.

A 20 m draw down of the lake over the last three years has subaerially exposed portions of the Las Vegas Wash delta that formed during a period of high lake level. A GPR survey conducted on an exposed portion of the delta suggests that this technique, in conjunction with photo-mosaic mapping, can yield favorable results as to the morphologies, internal structures and development of successive lobes within this portion of the delta. These data provide an outcrop record of the relationships between the deposition and erosion of near shore deltaic facies that may be related to the deposition of sediment and generation of hiatuses in the deepwater facies observed in core.

The bathymetry surveys show a 1.4 km progradation of the Las Vegas Wash delta into the lake during the last three years with minimal sedimentation occurring beyond the current delta front. Sand is present throughout many of the cores collected near the mouth of the Las Vegas Wash. In more distal cores sand layers have primarily been observed in the upper portions of the cores. These distal sand deposits are interpreted to represent relatively young episodic turbidites, traveling up to 10 km from their assumed Las Vegas Wash point source. The lower portions of these cores consist primarily of clay deposited onto a gravelly pre-impoundment surface. Several horizons within the cores collected in Las Vegas Bay have been dated and can be correlated using chemostratigraphic techniques.

An overall increase in the grain size of sediment deposited outboard of Las Vegas Wash suggests that early flows associated with this ephemeral stream were not sufficient to generate sandy turbidites that extended significant distances beyond the delta front. However, as discharge down the wash and sediment influx into the reservoir have increased with the growth of Las Vegas, so too have the occurrence and magnitude of sandy turbidites within Las Vegas Bay.