Geophysical surveys of a pluvial lake barrier deposit, Beatty Junction, Death Valley, California, USA

Abstract

We used ground penetrating radar (GPR) and seismic refraction to image the internal stratigraphy of a beach barrier deposit at Beatty Junction, Northern Death Valley, to better understand its depositional environment in the context of variations in the level of the former Lake Manly. The deposit is a gravelly bar ~500m long with 4m of surface relief that formed at a shoreline of Lake Manly during the end of the last Pleistocene ice age. The GPR profiles provide subsurface images that we interpret as progradational foreset beachface strata in the uppermost 2m and the surface of an earlier bar at depths of 2 to 6m. We conclude that the crest of the bar migrated in a landward direction during the construction of the uppermost 4m of the bar as lake level rose. The seismic survey indicates a sharp velocity increase from 760m/s to 1510m/s at the base of the bar, which we interpret as the boundary between well-sorted gravelly beach deposits, and underlying older fan deposits. The depth of the base of the bar varies between 5m and 10m. The elevation of the bar is comparable to that of other shoreline features in Death Valley that formed during the MIS 6/5e (186–120ka) highstand. Measurements of fault slip on the nearby Northern Death Valley fault have documented only strike–slip motion. In absence of any evidence for significant vertical uplift in the area during the late Pleistocene and Holocene, we conclude that the bar probably formed during MIS 6/5e. This conclusion is subject to uncertainty due to discrepancies in age dates reported for the deposit.