URANIUM-SERIES DATING OF CARBONATE (TUFA) DEPOSITS ASSOCIATED WITH QUATERNARY FLUCTUATIONS OF PYRAMID LAKE, NEVADA

Uranium-series dating of dense tufa deposited in a small cave, at form er lake margins, and in large tufa mounds clarifies the timing of lake -level variation during the past 400,000 yr in the Pyramid Lake basin. A moderate-sized lake occasionally overflowed the Emerson Pass sill a t elevation of similar to 1207 m between ca. 400,000 and 170,000 and f rom ca, 60,000 to 20,000 yr B.P., as shown by Th-230/U-234 ages of the cave samples, Th-230-excess ages of tubular tufas, and average isochr on-plot ages of shoreline-deposited tufas. (By comparison, modern Pyra mid Lake is similar to 50 m below this sill). There is a lack of tufa record during the intervening period from ca, 170,000 to 60,000 yr B.P . After ca. 20,000 yr, Pyramid Lake underwent abrupt changes in level and, based on previous C-14 ages, reached its highest elevation (ca 13 35 m) at ca. 14,000 yr B.P. The youngest uranium-series ages are compa rable with previously reported C-14 ages. (C) 1996 University of Washi ngton.