TIMING OF PLUVIAL LAKE-FILLING EVENTS IN THE WESTERN GREAT BASIN

Internally drained valleys of the Great Basin, western U.S., were repeatedly filled with deep lakes and desiccated during the Pleistocene. The most prominent and best-preserved geomorphic features of these lakes formed during highstands between 17 and 14 ka (toward the end of marine oxygen isotope stage 2, or OIS 2). Previous studies of older deep-lake cycles relied mainly on cores and a few stratigraphic exposures beneath the youngest deposits and were dated by tephrochronology and paleomagnetic techniques. According to the ruling paradigm for Lake Lahontan, OIS 2 lakes were about as large as the older lakes; higher shorelines of previous lake cycles, if recognized, were ascribed to effects of tectonics or stream capture. However, work since 1995 has documented shoreline features of older lakes (based on weathering characteristics) that reached higher levels due to climatic effects in at least seven basins in the western Great Basin, located throughout Nevada and extending into adjacent northern California and southern Oregon. Analysis of topographic maps suggests that nearly all basins in this area contain remnants of older shorelines at higher elevations than those formed in the latest Pleistocene.

Combined field and geochronological studies in three basins demonstrate the presence of shoreline deposits associated with at least two or three lakes that were older and larger than the OIS 2 lakes. We dated shoreline sequences using cosmogenic 36Cl depth profiles on barrier beaches deposited at or near a highstand and TIMS U-series dating on lacustrine tufa, which records the presence of a lake but not necessarily a highstand. Geochronological results indicate that all three basins contain shorelines of lakes present between about 125 and 200 ka (correlative with OIS 6) that were significantly higher (+17 to +35 m) than the younger OIS 2 shorelines. At least two of the basins also contain shoreline remnants at intermediate heights (-4? to +15 m higher than OIS 2) that were apparently deposited during OIS 4, a relatively brief glacial stade in the region. These OIS 4 deposits indicate that the lakes can respond very rapidly to relatively short climate perturbations. Even higher lakes in several basins are represented by pre-OIS 6 shorelines that reflect effectively wetter conditions than those present during OIS 2.