Zybala, Jonathan G.¹, Andrew D. Hanson¹, David C. Twichell², Mark J. Rudin¹, Brenda J. Buck¹ 
(1) University of Nevada Las Vegas, Las Vegas, NV 
(2) U.S. Geological Survey, Woods Hole, MA

ABSTRACT: Sequence Stratigraphic Evolution of Las Vegas Bay in Lake Mead, NV

Lake Mead is the largest reservoir on the Colorado River. The lake’s history and evolution are well constrained by historic records and recent field studies. These data permit the establishment of cause and effect relationships between the reservoir’s history and observed sedimentary deposits within Lake Mead. 
A 20 m drawdown of the lake over the last three years has subaerially exposed deltaic deposits that formed during periods of high lake level at the head of Las Vegas Bay. The current lowstand provides an outcrop record of the delta’s stratigraphy that can be related to deepwater facies and hiatuses observed in cores offshore of the delta. 
Photomosaic mapping and eleven sediment cores have been used to understand the distribution and temporal relationships of the sediment within Las Vegas Bay. Several horizons within the cores have been correlated using chemostratigraphic techniques. These data in conjunction with historic lake level fluctuations and discharge records from Las Vegas Wash have been used to constrain the sequence stratigraphy of Las Vegas Bay. 
The early history of the basin is characterized by low sediment influx and seasonal fluctuations in lake level. The latter history of the lake is characterized by large sediment influx, coarser material, and longer period changes in lake level. Our preliminary results have identified and dated two regional unconformities, a prolonged transgression, and headward erosion caused by the migration of a knick point in Las Vegas Wash. These events can be tied to historic changes in both lake level and fluvial discharge.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.