TRANGRESSION AND REGRESSION OF PLUVIAL LAKE DIAMOND AT RAILROAD PASS, NEVADA; EVIDENCE FOR MID-LATE PLEISTOCENE OVERFLOWS AND PLUVIAL STILLSTANDS FROM STRATIGRAPHY, SOILS, AND OSTRACODES

Railroad Pass is an east-west trending topographic pass with an ephemeral channel across the Diamond Mountains located within the Basin and Range Province approximately 32 km north of Eureka, Nevada. Previous work suggests high stands of Pleistocene pluvial lakes in Diamond Valley overflowed eastward through Railroad Pass and into the fluvial drainage network of Huntington Creek. Railroad Pass is incised at least 36 m and connects Diamond Valley with Huntington Valley to the east and then becomes a tributary to Lake Lahontan through the Humboldt River. A 15m section of unconsolidated interbedded lacustrine, fluvial, and eolian derived sediments overlying highly weathered bedrock was exposed in a slope trench just west of the maximum pass elevation. Beach gravels and cobbles with a soil developed into them rest on bedrock and are overlain by a meter and a half of laminated lacustrine sands. Ostracode assemblages obtained from the lacustrine unit suggest a decrease in depth and alkalinity during deposition. Amino acid racemization results obtained from ostracodes in a clay unit directly underlying the lacustrine unit indicate a pre-late Pleistocene age. Overflow to the east resulted in a westward facies migration and five meters of beach sands and gravels correlated with continued regression are deposited atop the laminated sands. These sands and gravels are blanketed by silt, overlain by interbedded lenses of sands and gravels and another silt unit. These units probably coincide with drier climate fluctuations of either OIS 7 or OIS 5a-e. Approximately 2.5 m of well rounded gravels and sands overlie the silty sediments. A ~1 m compound carbonate soil developed on the gravels and sands caps the section (~1831-1832 m). This soil is interpreted to be an exhumed and eroded soil that has been reworked before and after at least two episodes of pedogenic carbonate deposition. Based on the elevation of the deposits, the degree of reworking, and carbonate development stages, data suggest that these uppermost sands and gravels are OIS 4 or older in age and pre-date the previously reported most recent overflow event (~22. ka) that was controlled by a high stand at ~1824 m. The incision produced during this overflow reduced the base level of Lake Diamond to ~1775 m by 13.50 ka– the beginning of closed basin conditions.