292-10 Stratigraphic Context and Preliminary Taphonomy of Mammalian Fossil Assemblages from Stewart Valley, Nevada

Session: Life and Environments Through Time and Space: Multi-Record Approaches to Stratigraphic Paleobiology, Part II

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The Stewart Valley in eastern Nevada is one of the numerous basins of the Basin and Range province that formed through extensional tectonics during the Miocene. The Gilbert Andesite and Stewart Valley Group (~16–11 Ma) comprise volcanic, volcaniclastic, lacustrine, and fluvial deposits that represent the evolution of a localized lake system in a volcanically active region. These units preserve abundant plant, insect, and vertebrate fossils for which Stewart Valley is well known. Fossil localities in the basin formed within and at the margins of a lake system that expanded and contracted over time, with different fossil assemblages forming in relation to lake transgressions and regressions. Based on results from stratigraphic simulations of extensional-basin fossil records, preservation quality and taphonomic mode are predicted to vary cyclically with lake-level fluctuations.

Plant, insect, and fish assemblages are preserved in thin-bedded tuffaceous or cherty shales and siltstones representing deposits in a relatively deep-water lake that was intermittently stratified. Mammalian and small-vertebrate fossils occur at localities representing fluvial and lake-margin deposits, and taphonomic features of the assemblages vary taxonomically and with facies. The majority of bones are disarticulated and isolated; material from small vertebrates have lower amounts of weathering and abrasion than material from large mammals. Preliminary taphonomic assessment of mammalian material from Stewart Valley indicates that assemblages are time averaged and accumulated in a low-aggradation marginal lacustrine setting. Preservation quality of terrestrial vertebrate assemblages may vary in relation to changes in lake-level: increasing in localities forming during early lake-level rise and decreasing through slowing lake-level rise in association with increased indications of reworking and subaerial exposure.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8792

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