LAKE LEVELS AND SEDIMENTARY ENVIRONMENTS DURING DEPOSITION OF THE WONO AND TREGO HOT SPRINGS TEPHRAS IN THE LAKE LAHONTAN BASIN, NEVADA AND CALIFORNIA, USA

In the 1970s and 1980s, Jonathon O. Davis painstakingly constructed a tephrochronologic framework for the Lahontan Basin by documenting the occurrences, environmental settings, and ages of multiple tephra units spanning the late Quaternary (Davis, 1978). This framework has proved invaluable to subsequent researchers working to reconstruct the history of paleoenvironmental fluctuations in the Lahontan Basin and other lake basins in the western U.S. The ages, depositional settings, and elevations of various tephra layers provide direct evidence of lake levels through time. As an example, Davis (1983) defined lake level to be between 1256 and 1260 m during deposition of the Trego Hot Springs bed (THS; 23,200 +/- 300 14C yr B.P.), based on exposures of the tephra in a deltaic complex along Squaw Creek. A lake at this elevation in the western subbasins of Lahontan would cover approximately 6500 km2 and have a volume of about 360 km3. Subsequently, Benson et al. (1997) reinterpreted the Squaw Creek exposures and presented isotopic evidence that Lake Lahontan was much smaller during the deposition of the THS bed, probably close to the size of modern Pyramid Lake. New and existing evidence from outcrops of the THS bed in deltaic, beach, and other lacustrine deposits confirms Davis' (1983) original interpretation that Lake Lahontan was near 1260 m when the tephra was deposited.

Benson et al. (1997) also inferred that lake level during deposition of the Wono bed (27,300 +/- 300 14C yr B.P.) was at or below 1207 m, the spill point from Pyramid Lake to the Smoke Creek-Black Rock subbasin of Lake Lahontan. Outcrops of the Wono bed in deltaic deposits and lacustrine sands, however, indicate that Lahontan was at an elevation of at least 1215 m at that time in the western subbasins, indicating a much larger lake than inferred by Benson et al. (1997). In the Carson Sink subbasin, the Wono bed is contained within coarse beach gravels at an elevation of about 1204 m, indicating a lake surface at about this elevation. Accurate delineation of the family of lake-level curves for the Lahontan Basin is important not only for paleoclimatic reconstructions but also for specifying spatio-temporal variations in the water load for isostatic rebound modeling.