Using U-Pb ages of Miocene tufa for correlation in a terrestrial succession, Barstow Formation, California

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 diffi cult 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 uranium-lead 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 40 Ar/ 39 Ar 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.

Synconvergent surface-breaking normal faults of Late Cretaceous age within the Sevier hinterland, east-central Nevada

“…A sample from the base of Member B in the type section was analyzed by TIMS at Stony Brook University following the method of Cole et al (2005). The resulting errorchron age of 66.1 ± 5.4 Ma (MSWD = 34) corroborates earlier Maastrichtian to Paleocene fossil age assignments (Fouch, 1979;Good, 1987).…”

“…Recent studies of lacustrine carbonates have shown that calcite may contain elevated levels of uranium due to complexation with organic matter from sources such as microbial mats, and that U-Pb ages representing depositional ages may be determined by TIMS (Cole et al, 2005).…”

Directly dating geologic events: U‐Pb dating of carbonates

“…For example, in the youngest carbonate dated with U‐Pb, the age without considering U series disequilibrium was ∼25% younger than the corrected age [ Richards et al , 1998]. Disequilibrium expected in typical waters because of an absence of initial 230 Th would result in a change in the 238 U‐ 206 Pb age of −109 ka, and an initial 234 U/ 238 U activity ratio of 2 would result in a change of +354 ka [ Cole et al , 2005]. Thus, in older (Paleozoic and Mesozoic) deposits, any reasonable disequilibrium is likely accommodated within age uncertainties.…”

Uranium-Lead, Diagenetic Processes

“…Some examples of carbonates that are formed by meteoric fluids that have been dated by U-Pb are cave deposits such as flowstones and stalagmites (Richards et al 1998;Polyak et al 2008;de Ruiter et al 2009;Cliff et al 2010;Pickering et al 2011;Woodhead et al 2012;Woodhead and Pickering 2012;Bajo et al 2012;Pickering et al 2013) and paleosol (fossil soil) carbonates such as nodules and rhizoliths (Hoff et al 1995;Winter and Johnson 1995;Rasbury et al 1997;Wang et al 1998;Leeder et al 2008;Ludvigson et al 2010). Lacustrine (lake) carbonates have also been shown to have good potential for U-Pb dating (Cole et al 2005;Druschke et al 2009). …”