--> Abstract: U-Pb Ages Of Miocene Tufa And Lacustrine Carbonates Of The Barstow Formation, Barstow Basin, Mojave Desert, Ca, by E. T. Rasbury, I. P. Montanez, G. N. Hanson, and G. Price; #90928 (1999).

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RASBURY, E. T.1, I. P. MONTANEZ2, G. N. HANSON3, and G. PRICE4
1Queens College, Flushing, NY
2Univ. California-Davis, Davis, CA
3SUNY Stony Brook, Stony Brook, NY
4Univ. California-Riverside, Riverside, CA

Abstract: U-Pb Ages of Miocene Tufa and Lacustrine Carbonates of the Barstow Formation, Barstow Basin, Mojave Desert, CA

Non-marine carbonates are widely recognized in the rock record and may have important terrestrial faunal assemblages because in such environments fossils can easily be preserved. We report here our initial results on U-Pb dating of Miocene non-marine carbonates from southern California. Six aliquots of a sample from a tufa deposit from the Owl Conglomerate Member of the Barstow Formation yield a 238U-206Pb age of 14.76+/-0.43 Ma (235U-207Pb age is 14.62+/-1.5 Ma). Uranium concentrations range from 79 to 169 ppm, Pb from 1 to 5 ppm, and 206Pb/204Pb ranges from 24 to 34.

Six aliquots from a lacustrine carbonate sample from 200 m above the tufa yield a 238U-206Pb age of 14+/-3 Ma. Uranium concentrations range from 11 to 13 ppm, Pb from 0.2 to 0.4 ppm and 206Pb/204Pb ranges from 23.8 to 25.4. Given the small range in the Pb isotopic composition of the lacustrine carbonates, it is remarkable that the sample yields a reasonable age. We hope to increase the U/Pb spread (and thus the precision) by careful sampling using petrography and fission track mapping.

Published 40Ar-39Ar ages for biotite from ashes intercalated in the Barstow Formation range from 19.3 to 13.4 Ma. The age for biotite from an ash in an adjacent basin which is 250 meters above the U-Pb dated tufa (based on correlation) is 14.8+/-0.2 Ma. The age for biotite from an ash in the adjacent basin that is projected to be stratigraphically just below the dated tufa is 16.3+/-0.3 Ma (Rak Tuff). The apparent discrepancy between the U-Pb carbonate ages and the 40Ar-39Ar biotite ages needs to be clarified.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas