Abby Dennett David Selby Justin E. Birdwell Jeffrey T Pietras 2022 <div id="abstracts" class="Abstracts u-font-serif"><div id="ab0005" class="abstract author" lang="en"><div id="as0005"><p id="sp0080">The range of<span>&nbsp;</span>¹⁸⁷Re/¹⁸⁸<span>Os values measured from samples of five organic-rich lacustrine&nbsp;<a class="topic-link" title="Learn more about mudstones from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/mudstone" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/mudstone">mudstones</a>&nbsp;units in the&nbsp;<a class="topic-link" title="Learn more about Eocene from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/eocene" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/eocene">Eocene</a>&nbsp;Green River Formation in the easternmost Uinta Basin covaries with organic matter diversity driven by changing water column conditions. A set of samples from the Douglas Creek Member has the highest pristane/phytane ratio and lowest β-carotane/</span><i>n</i>-C₃₀<span>&nbsp;ratio compared to overlying units, indicating deposition in an oxic-anoxic environment with low&nbsp;<a class="topic-link" title="Learn more about salinity from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/salinity" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/salinity">salinity</a>&nbsp;that would have allowed for the accumulation of a diverse assemblage of aquatic organisms. These samples define the broadest&nbsp;</span>¹⁸⁷Re/¹⁸⁸Os range of 1504. In contrast, samples from the R6 and Mahogany zones possess lower pristane/phytane ratios and higher β-carotane/<i>n</i>-C₃₀<span>&nbsp;ratios indicating deposition in a more restricted&nbsp;<a class="topic-link" title="Learn more about lacustrine environment from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/lacustrine-environment" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/lacustrine-environment">lacustrine environment</a>&nbsp;with elevated salinities and alkalinities that would have limited aquatic organic matter diversity. The R6 and Mahogany zones have the narrowest range of&nbsp;</span>¹⁸⁷Re/¹⁸⁸Os values measured in this study of 254.9 and 154.6, respectively. As noted by previous workers, these results suggest that organic matter diversity plays a primary role in determining the range of<span>&nbsp;</span>¹⁸⁷Re/¹⁸⁸Os ratios in a sample set, and in turn the uncertainty of Re-Os age determinations from organic-rich sedimentary rocks.</p><p id="sp0085">The Re-Os data from the R3 zone and R6 zone yield ages of 49.7&nbsp;±&nbsp;3.4&nbsp;Ma and 42.0&nbsp;±&nbsp;18&nbsp;Ma, respectively, which are statistically indistinguishable based on 2σ uncertainty from three previously reported Re-Os age determinations and those provided by<span>&nbsp;</span>⁴⁰Ar/³⁹<span>Ar&nbsp;<a class="topic-link" title="Learn more about geochronology from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/geochronology" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/geochronology">geochronology</a>&nbsp;of interbedded volcanic ash beds. Although the age uncertainty is high, these findings further highlight the importance of Re-Os geochronology in lacustrine basins, particularly those with thick mudstone successions that lack volcanic ash layers, reliable&nbsp;<a class="topic-link" title="Learn more about biostratigraphy from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/biostratigraphy" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/biostratigraphy">biostratigraphy</a>, or magnetostratigraphic control. In these cases, even ages with large uncertainties can be useful to constrain burial history and thermal history models.</span></p><p id="sp0090">Together, the initial<span>&nbsp;</span>¹⁸⁷Os/¹⁸⁸<span>Os ratios of five sets of samples analyzed from the Uinta Basin define the largest Os isotope&nbsp;<a class="topic-link" title="Learn more about stratigraphic record from ScienceDirect's AI-generated Topic Pages" href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/geological-record" data-mce-href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/geological-record">stratigraphic record</a>&nbsp;from any lacustrine basin compiled to date and record a shift from a value of 1.40 to 1.48 between the R3 and R4 zones in the lower part of the Parachute Creek Member. This small shift may signify a change in the chemical weathering products that entered the lake preserved 20 to 50&nbsp;m above the contact between the Douglas Creek and the lower Parachute Creek members during a period when the basin transitioned from a shallow lake with mostly open hydrology to an alkaline lake with more frequent basin restrictions.</span></p></div></div></div><ul id="issue-navigation" class="issue-navigation u-margin-s-bottom u-bg-grey1"></ul> application/pdf 10.1016/j.chemgeo.2022.120939 en Elsevier The role of organic matter diversity on the Re-Os systematics of organic-rich sedimentary units: Insights into the controls of isochron age determinations from the lacustrine Green River Formation article