CONSTRAINTS ON ORIGINAL ELEVATIONS OF HIGH-LEVEL LAKE MANLY SHORELINES DURING RECENT PLUVIAL CYCLES, DEATH VALLEY, CA

We initiated a survey of Lake Manly shorelines in the Death Valley basin to better understand lake-level histories of the most recent Lake Manly pluvial cycles. Our focus has been on high-level shorelines in areas away from the Black Mountains fault zone (BMFZ) in central Death Valley (CDV) where Oxygen Isotope Stage-6 (OIS-6) shoreline tufa (~186 ka) occurs at up to 90 m in the uplifted footwall block. Field observations at a number of sites and limited radiocarbon ages indicate the two most recent Lake Manly cycles, presumably OIS-6 and OIS-2, produced high-stand elevations of about 60 m and 46 m, respectively. For example: (1) At the Beatty Bar complex in northern Death Valley (NDV), the highest beach ridge lies at 46 m. Algal tufa from this site yielded 14C ages of 30-27 ka. No higher shorelines have been identified in this area. (2) At Park Village Ridge (NDV), shoreline gravels are found up to 60 m and are also well exposed at 35-37 m. (3) West of Furnace Creek (NDV) at the base of the basalt hill north of Blackwater Fan (BF), BF-derived beach pebbles can be traced up to 56 m. The most prominent shoreline is expressed by thick, laminated tufa deposits at 47 m. The uppermost BF surfaces in this area are clearly modified by Lake Manly, but these fan deposits do not appear to be high enough to provide the source for the 56-m gravels. Hence, the 56-m gravels were likely reworked from older, now eroded BF fan deposits during an earlier lake phase. (4) On the west side of CDV a prominent strandline occurs at 60 m on Johnson Fan. At 36-46 m, tufa-cemented beach gravels are inset into a deep channel that is in turn, incised into the fan surface exhibiting the 60 m strandline. Tufa from the inset deposits yielded a 14C age of 27.7 ka. (5) On Warm Springs Fan (CDV), shorelines extend as high as 60 m. (6) In southern Death Valley (SDV) shoreline gravels and young, fine-grained lake sediments are extensively found at ≤46 m. In contrast, the only shoreline found at >46 m is a single exposure of backset shoreline gravels at 60 m in the northern Noble Hills. The observation that the highest Lake Manly shorelines in NDV, CDV, and SDV all lie at ~60 m suggest that shoreline elevations on the west side of CDV are little-affected by the BMFZ. Further, if 60 m approximates the original OIS-6 high-stand elevation, then ≥30 m of absolute, post ~186 ka uplift has occurred in the footwall of the BMFZ.