Abstract

Sedimentary rocks and associated fossils are the direct record of past life and its environmental context. Few methods allow direct and precise dating of this important archive, making it difficult to correlate climate, biologic, and tectonic events preserved in terrestrial records. We sampled lacustrine tufa calcite from the middle member of the Miocene Barstow Formation to further test the feasibility of uraniumlead dating of terrestrial carbonates. Five samples yield U-Pb ages (2σ) of 14.81 ± 0.39 Ma, 15.30 ± 0.25 Ma, 15.39 ± 0.15 Ma, 16.14 ± 0.40 Ma, and 16.24 ± 0.23 Ma. These precise ages are consistent with existing ages from K-Ar and 40Ar/39Ar dating of intercalated volcanic ashes. Our most precise U-Pb age on tufa has an uncertainty of <1% (150 ka), which is equivalent to uncertainties for K-Ar ages from the same time period. Our results indicate that U-Pb dating of carefully sampled and screened sedimentary carbonates provides much needed age constraints in purely terrestrial sequences and can be a powerful tool for correlation even in deposits with structural complexity and rapid facies changes.

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