302 Aeolian Systems in Time and Space (Posters)
This session explores research focused on all aspects of aeolian processes and landforms, including dunes, dust, and loess in modern and ancient aeolian systems on Earth and other planets.
Wednesday, 22 October 2025
8:00 AM - 5:30 PM
HBGCC, Hall 1
Deciphering Orbital-Scale Climate Variability from Late Eocene–Early Oligocene Loess in the western USA
Paper number: 302-6
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM
Authors
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Majie FanThe University of Texas at Arlington, Arlington, TX
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Yiquan MaChengdu University of Technology, Chengdu
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Steven FormanBaylor University Dept of Geosciences, Waco, TX
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Dennis TerryTemple University, Philadelphia, Pennsylvania
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Ran FengUniversity of Connecticut, Storrs, Connecticut
Abstract
The late Eocene–early Oligocene loess deposits in the western USA provide critical insights into terrestrial climatic and environmental responses to the rapid global cooling at this transition. Here, we present high-resolution bulk sediment and quartz grain-size data from the Flagstaff Rim (Wyoming) and Toadstool Geologic Park (Nebraska) sections. By conducting sedimentology study and end-member modeling analysis (EMMA), we identified four end members with different sediment transport mechanisms at each site. At Flagstaff Rim, the loess primarily comprises well-sorted unimodal EM2 and EM3, characterized by grain-size modes at ~ 35 µm and 70 µm, respectively, indicating short-distance suspension and near-surface saltation by local winds. The fluvial deposits are dominated by EM1, a multimodal distribution with a primary mode at ~5 µm and a minor fine-sand component, and EM4, a poorly sorted unimodal distribution peaking at ~300 µm, representing suspension and near-bed saltation in fluvial environments, respectively. At Toadstool, the loess also predominantly contains EM2 and EM3, though these are slightly finer than those at Flagstaff Rim. The fluvial sediments are primarily composed of EM1 and EM0 that has a bimodal distribution of clay to very fine silt, both indicative of riverine suspension. Spectral analysis of grain-size data of the loess reveals cycles at various intervals. Based on the sedimentation rates constrained by absolute ages of volcanic ash, the cycles identified include ~ 100-kyr eccentricity, ~40-kyr obliquity, and ~21-kyr precession cycles during the late Eocene at Flagstaff Rim, and ~100-kyr eccentricity cycle during the early Oligocene at Toadstool. The low sedimentation rate at Toadstool has suppressed higher-frequency orbital signals. Astronomical tuning to the 100 kyr eccentricity cycle establishes high-resolution timescales for both sections. Loess accumulation initiated at Toadstool around 33 Ma, roughly 3 Myr later than at Flagstaff Rim, supporting an eastward shift in loess deposition across the western interior. Further spectral analysis of EM2 + EM3 at Flagstaff Rim shows dominant obliquity and precession signals, suggesting the influence of seasonality on near-surface wind dynamics.
Geological Society of America Abstracts with Programs. Vol. 57, No. 6
doi: 10.1130/abs/2025AM-414656
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