COMPOUND STRUCTURE OF THE MOST PROMINENT BAR IN THE BEATTY JUNCTION BAR COMPLEX, DEATH VALLEY, CA, AS SEEN IN A RECENT NEAR-SURFACE GEOPHYSICAL SURVEY

The Beatty Junction bar complex is composed of four bars, constructional shoreline deposits that mark lake oscillations at the northernmost extent of a stand of Lake Manly, which has intermittently occupied Death Valley. Thus, these structures provide information needed to constrain the magnitude of Lake Manly at a given time, important for understanding past climates here.

Our ground-penetrating radar (GPR) survey of the most prominent of the four bars (Spit B, from Klinger [2001, in USGS OFR 01-51, ed. by M. N. Machette et al.]) has revealed the presence of a smaller bar that has been completely buried by the larger structure now at the surface, indicating a more complicated lake level history than was previously recognized. The smaller buried bar, formed first, is imaged with four transverse GPR lines (at 6-8 m depth); its crest is located 20-40 m lakeward of the crest of the overlying bar. The larger surface bar must therefore represent a relative lake level rise/shoreward shift of deposition.

The Beatty Junction bar complex generally lies to the east of a hill that is a southern spur of the Kit Fox Hills. Spit B directly abuts the east side of the hill. Longshore transport directions from the west to the east (i.e., from the hill outwards) are supported by offlapping reflectors seen in the longitudinal GPR lines from Spit B, as well as by layers that thin to the east in the longitudinal seismic refraction line. In a similar study of paleo-shorelines of Lake Lahontan, Adams and Wesnousky (1998: GSA Bulletin, 110) listed controls on the size and the type of shoreline feature developed as: slope, amount and nature of sediment available, accommodation space, and the length of time a given lake level persists. In this case, the much larger size of Spit B, compared to neighboring bars, may primarily reflect the increased sediment supply provided when lake level rose to first intersect the hill.

Klinger (2001) summarized age estimates to date for Spit B, centering on OIS 2 or OIS 6. The Hanaupah Shoreline Deposit at Tule Spring, described by Warnke and Ibbeken (2005, in: Encyclopedia of Coastal Science, ed. by M. L. Schwartz), is broadly similar to the bar complex in elevation and may be considered equivalent in time. A 36Cl age for the Hanaupah deposit has been reported as 128-145 ka, placing it within OIS 6 (Machette et al, 2003, GSA Abstracts, 35: 96-3).