ESTIMATING PALEOWIND CONDITIONS FROM LATE PLEISTOCENE BEACH DEPOSITS IN THE WESTERN GREAT BASIN

The geologic record of past wind conditions is well expressed in the coarse gravel, cobble, and boulder shorelines and beach deposits of pluvial lakes in the Great Basin and elsewhere. The goal of this research is to develop a technique, using the particle size distribution of beach deposits and mapped indicators of the direction of net shore drift, to reconstruct paleowind conditions when the lakes were present. The technique is presented in the context of testing the hypothesis that wind conditions were more severe than at present during the last highstand of Lake Lahontan (~13 ka), which only lasted years to a decade or two.

The largest 50 beach clasts were measured at six sites located around the Carson Sink, where instrumental hourly wind records are available from the period 1992-1999. At these sites, the mean largest clast size ranges from 12 to 38 cm and the fetch lengths range from 10 to 84 km. Nearshore wave height is calculated by assuming that the critical threshold velocity required to move the largest clasts represents a minimum estimate of the breaking wave velocity, which is controlled by wave height. Shoaling transformations are removed to estimate deepwater wave heights and, ultimately, wind velocity and duration. Paleowind estimates for the six beach sites are 11, 15, 15, 17, 22, and 38 m/s (25 to 86 mph), respectively. The strongest winds were calculated for a cobble/boulder beach with a fetch of 25 km. Instrumental wind records indicate that wind events of 9 to 11 m/s (30 to 35 mph) are common and that the strongest winds for the 1992-1999 period was an eight hour event with average wind speeds of about 13 m/s (30 mph). Based on this preliminary comparison, it appears that the late Pleistocene western Great Basin was a windier place than at present, at least for a brief time.