NEW SHORELINE AGES AND OIS-6 LAKE-LEVEL HISTORY OF LAKE MANLY, DEATH VALLEY

Despite a long history of Lake Manly research, a well-dated core record of the last two major lake cycles [186-120 ka (OIS-6) and 35-10 ka (OIS-2)], and numerous attempts to date shoreline deposits, the pluvial history as recorded in the ages and elevations of Lake Manly shorelines has remained controversial and unresolved. To this end, we initiated a basin-wide survey of high-level shorelines in areas of relative tectonic stability. We see remarkable consistency in elevations of two distinct high-level shorelines spanning northern, central (west side), and southern Death Valley. The highest formed at ~60 m above sea level (masl). Slightly lower, and notably prominent shorelines mark lake levels at ~46 masl. Similarities in degree of preservation suggest shorelines at both levels formed during the same pluvial cycle. New U-series tufa ages from 46-m shorelines on Johnson Canyon fan and a basalt hill (BH) on the north side of Blackwater fan (BF) are best bracketed between 129.5 ± 1.6 ka and 122.4 ± 1.1 ka, and indicate a high lake level late in the OIS-6 pluvial. Further details of OIS-6 lake fluctuations are recorded on BF where a high beach ridge (57 m) built onto the south flank of the BH during (what we now take as) the OIS-6 high stand. Following lake regression from the high stand, the 57-m beach ridge was deeply incised on the flank of the BH and inset by a younger fan. The timing of lake regression isn't known. Remarkably, however, the fan that was inset during lake regression records a later transgression of Lake Manly in the way of shorelines and overlying beach deposits that can be traced on the fan to a maximum elevation of 46 m. Importantly, the relations show that the late OIS-6 lake at 46 masl was a late-transgressional phase, not a recessional lake level and thus implies an earlier OIS-6 age for the high stand. Our new observations and age constraints agree well with the lacustrine record from CORE DV93-1. We interpret that: 1) the 60 m shorelines mark the original, quasi-stable, highstand of Lake Manly reached in the earlier part of the OIS-6 pluvial (186-166 ka interval); 2) lake regression (recorded at BF) likely occurred during a time of higher lake salinities (166-144 ka interval); and 3) late OIS-6 transgression to 46 masl matches a deep lake interval at 122 ka, but perhaps more likely correlates to the deep lake interval ending at 128 ka.