256-5 Paleosols through time: implications for the paleobiological record of the Lower Cretaceous Cloverly Formation, Bighorn Basin, Wyoming

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

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Abstract:

Integrating paleopedology with nonmarine sequence stratigraphy is crucial for understanding basin dynamics, landscape evolution, and soil formation through time. The Himes Member of the Cloverly Formation of Wyoming, a unit recognized for its paleontological significance and geochronological importance, has yet to have its paleosols interpreted from a pedological standpoint. The Himes is of particular significance because it has been hypothesized to contain a substantial unconformity and to be the landward record of the Thermopolis transgression. Here, we correlate a succession of nonmarine to marine strata from the upper Himes, using 5 stratigraphic columns from a 74 km transect along the northeast flank of the Bighorn Basin of Wyoming. This correlation, along with geochemical data from X-ray diffraction, X-ray fluorescence, and scanning electron microscopy, was used to evaluate the characteristics of paleosols as possible evidence of unconformities and a potential transgressive succession. Our results indicate that the lower part of the Himes Member is a low-aggradation systems tract (LAST) containing amalgamated fluvial channel sandstones followed by a succession of spodosols, with one particularly well-developed paleosol representing a significant unconformity in the lower part of the upper Himes. The upper part of the upper Himes Member preserves a transition to a high-aggradation systems tract (HAST) dominated by a succession of gleysols that pass upward into transgressive systems tract (TST) paralic deposits in the uppermost Himes, which in turn pass upwards into the marine portion of the TST in the Sykes Mountain Formation and Thermopolis Shale. Our geochemical data reveal that paleosol development and exposure can be tracked through changes in clay composition, as well as the transgression of the Western Interior Seaway, particularly through the appearance of jarosite, formed by the oxidation of pyrite and other iron sulfides. Our characterization of Himes paleosols suggests a re-evaluation of paleobiological patterns through the Himes Member to reflect the presence of a substantial hiatus and an upsection shift from inland, higher-elevation settings to coastal, lower-elevation settings. Interpreting paleosols with horizon-by-horizon paleopedology may be more broadly applicable in the recognition of unconformities and transgressive records in non-marine settings. 

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

doi: 10.1130/abs/2025AM-9197

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