40 Aeolian Systems in Time and Space
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.
Sunday, 19 October 2025
1:30 PM - 5:30 PM
HBGCC, 212AB
Identifying eolian sediments from the late Eocene-Oligocene Renova Formation in southwestern Montana
Paper number: 40-8
Program: Technical Sessions
Time: 3:20 PM – 3:35 PM
Authors
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Majie FanUniversity of Texas at Arlington, Arlington, TX
Abstract
Eolian deposits are important archives of continental climate changes and surface processes. Latest Eocene-Oligocene loess has been documented at several sites in the western U.S., suggesting that its presence may be widespread. During this time, the fluvial Renova Formation was deposited in southwestern Montana, and it exhibits sedimentary characteristics that indicate an eolian component. This study tests the hypothesis that the Renova Formation in the Sage Creek Basin was deposited in a mixed fluvial-eolian environment. We conducted bulk sediment grain-size analyses on 111 siltstone and fine-grained sandstone samples collected from low-energy channel and floodplain deposits, and quartz surface microtexture analyses for ten selected samples based on field sedimentology and grain-size data interpretation. The grain-size distributions are characterized by a non-uniform multimodal distribution, with a common peak at ~30 um for about half of the samples. Non-parametric end-member analyses show that three end-members are sufficient to explain the distributions. EM1 exhibits a negatively skewed multimodal distribution, featuring a dominant mode at ~30 µm, a subordinate mode at ~5 µm, and a minor mode near 1 µm. EM2 is characterized by a slightly negatively skewed distribution, with a mode at a ~110 µm and a minor fine tail. EM3 displays a bimodal distribution, with a dominant mode at ~400 µm and an indistinct minor mode at ~5 µm. EM3 occurs mostly in the Eocene deposits characterized by frequent channel avulsion and erosion, whereas EM1 and EM2 are predominant in the Oligocene low-energy fluvial deposits, which have few channels. A comparison of these characteristics with the late Paleogene loess in Wyoming shows that EM1 mostly represents eolian transport by long- and intermediate-term suspension. EM2 mainly reflects local eolian saltation during dust storms, while EM3 is indicative of fluvial bedload transport and deposition in proximal floodplains. Quartz microtexture analyses further show that the Eocene EM3–rich deposits exhibit predominantly fluvial features such as conchoidal fractures and linear steps, while the Oligocene EM1- and EM2–rich deposits contain abundant eolian features such as upturned plates and dish-shaped depressions. These findings show enhanced eolian activity that most likely recycled local fluvial floodplain deposits in southwestern Montana following the drastic cooling at the Eocene-Oligocene transition.
Geological Society of America Abstracts with Programs. Vol. 57, No. 6
doi: 10.1130/abs/2025AM-413030
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