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Are the Benches at Mormon Point, Death Valley, California, USA, Scarps or Strandlines?

Published online by Cambridge University Press:  20 January 2017

Jeffrey R. Knott*
Affiliation:
Department of Geological Sciences, California State University, Fullerton, P.O. Box 6850, Fullerton, California, 92834-6850
John C. Tinsley III
Affiliation:
United States Geological Survey, 345 Middlefield Road MS-975, Menlo Park, California, 94025
Stephen G. Wells
Affiliation:
Desert Research Institute, 2215 Raggio Parkway, Reno, Nevada, 89512
*
1To whom correspondence should be addressed. E-mail: Jknott@exchange.fullerton.edu.

Abstract

The benches and risers at Mormon Point, Death Valley, USA, have long been interpreted as strandlines cut by still-stands of pluvial lakes correlative with oxygen isotope stage (OIS) 5e/6 (120,000–186,000 yr B.P.) and OIS-2 (10,000–35,000 yr B.P.). This study presents geologic mapping and geomorphic analyses (Gilbert's criteria, longitudinal profiles), which indicate that only the highest bench at Mormon Point (∼90 m above mean sea level (msl)) is a lake strandline. The other prominent benches on the north-descending slope immediately below this strandline are interpreted as fault scarps offsetting a lacustrine abrasion platform. The faults offsetting the abrasion platform most likely join downward into and slip sympathetically with the Mormon Point turtleback fault, implying late Quaternary slip on this low-angle normal fault. Our geomorphic reinterpretation implies that the OIS-5e/6 lake receded rapidly enough not to cut strandlines and was ∼90 m deep. Consistent with independent core studies of the salt pan, no evidence of OIS-2 lake strandlines was found at Mormon Point, which indicates that the maximum elevation of the OIS-2 lake surface was −30 m msl. Thus, as measured by pluvial lake depth, the OIS-2 effective precipitation was significantly less than during OIS-5e/6, a finding that is more consistent with other studies in the region. The changed geomorphic context indicates that previous surface exposure dates on fault scarps and benches at Mormon Point are uninterpretable with respect to lake history.

Type
Research Article
Copyright
University of Washington

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