302-6 Late Eocene Initiation of Loess and Its Significance in Western Wyoming, USA

Session: Aeolian Systems in Time and Space (Posters)

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Abstract:

The late Paleogene loess in the western USA is an important archive of terrestrial paleoclimatic and paleoenvironmental changes. In this study, we integrate field sedimentology and bulk sediment grain-size analyses from a section in western Wyoming to evaluate a prior interpretation of an abrupt shift from fluvial to loess deposition. Our field observations show an upward transition from interbedded lenticular sandstones and siltstones in the lower Wagon Bed Formation, to a transitional interval of interbedded massive, well-sorted, very fine-grained sandstones and lenticular conglomerates and sandstones in the upper Wagon Bed Formation and lower White River Formation, and finally to continuous, massive, very fine-grained sandstones within the rest of the White River Formation.  Grain-size data reflects this lithologic shift, with broad, multimodal distributions in the lower unit, bimodal, loess-like distribution in the upper unit, and variations between the two in the middle unit. Cluster analysis further categorizes the samples into three groups: Cluster 1 is characterized by a multimodal distribution, with a predominant mode at ~20-50 μm and notable amounts of clay and very fine sand; Cluster 2 is marked by a broad and coarsely skewed distribution, with modes at ~5 μm, ~60 μm and ~110 μm; and Cluster 3 shows bimodal distributions, with a dominant mode at ~80 μm and a subordinate mode at ~10 μm. Clusters 1 and 2 dominate in the lower half, whereas Cluster 3 first appears in the upper Wagon Bed Formation and becomes increasingly dominant upward. These changes suggest a shift in depositional environment from fluvial to a mix of fluvial and eolian, ultimately transitioning to purely eolian conditions. Therefore, the transition from fluvial to eolian deposition in this section is gradual. Maximum Depositional Ages (MDAs) from detrital zircon U-Pb geochronology, which align with North American Land Mammal Ages and show a decreasing trend upward, constrain loess initiation to ~39 Ma, ~3 Myr earlier than previously recognized in this region. This finding supports a previous interpretation of the eastward diachronous onset of late Paleogene loess in the western USA. The initiation of loess in the western USA coincides with the onset of loess in the Xining Basin, China, suggesting a global increase in atmospheric dust around 40 Ma.

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

doi: 10.1130/abs/2025AM-6691

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