Paper No. 96-19
Presentation Time: 9:00 AM-1:00 PM
INTRACONTINENTAL RIFT BASIN STRATIGRAPHY RECORDS MIDDLE MIOCENE EXTENSION, FAULTING EVOLUTION AND CLIMATIC INFLUENCES ON DEPOSITION, UPPER HORSE SPRING FORMATION AND RED SANDSTONE UNIT, LAKE MEAD REGION, NEVADA
East of Las Vegas, Miocene strata record extension and the evolving paleogeography of the Lake Mead region. To better understand tectonic, climatic and autogenic processes that produced basin fill and to test and refine tectonic models of the region, we completed detailed chronostratigraphic and structural analyses of the upper Horse Spring Formation (HSF) and Red Sandstone unit, formed during the second half of Basin and Range extension from 14.5-11.5 Ma. We suggest that faulting caused large changes in basin paleogeography and sedimentation while climate played a secondary but important role in creating stratigraphic complexity in the White Basin and Lovell Wash areas. The ~225 meter thick, 14.5 to 13.86 Ma, Bitter Ridge Limestone Member of the HSF formed in a shallow, uniform lake controlled by a few faults. The persistent lake was broken up by fault reorganization followed by deposition of the highly variable, fluvial-lacustrine facies of the HSF Lovell Wash Member from 13.86 to 12.7 Ma. From 13.86 to 13.46 Ma, sedimentation rates increase, suggesting an increase in faulting on the northeast-trending, oblique normal left-lateral White Basin fault. After 13.46 Ma, faulting shifts to the northwest-trending, normal Muddy Peak fault and other smaller northwest-trending growth faults. The Red Sandstone unit, 12.7 to 11.5 Ma, records a slowing of both sedimentation rates and faulting as well as a return to widespread, lacustrine conditions, later followed by fluvial sedimentation. Significant shifts in stratigraphic packages align with major global and regional climate changes, including the Middle Miocene climatic optimum, subsequent cooling, and swings in solar insolation due to eccentricity and obliquity cycles. This study supports earlier work that suggests faulting proceeded in discrete westward steps across the Lake Mead area, demonstrates the likely influence of global climate change on deposition, and helps constrain the paleogeographic and tectonic evolution of the region.