64-15 End-Member Modeling of Grain Size Distributions in the Blackwater Draw Formation: Evidence for Sediment Provenance Shifts Across the Mid-Pleistocene Transition

Session: Emerging Voices in Soil and Paleosol Science (Posters)

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Abstract:

Eolian sediment transport is an effective sorting mechanism and thus eolian deposits are typified by well sorted material. The Blackwater Draw Formation of West Texas and southeastern New Mexico records eolian deposition but grain-size distributions reveal generally poor sorting seemingly at odds with eolian deposition. To resolve this discrepancy, we hypothesize that the unit contains multiple end-members of grain-size distributions from different sources that ultimately mixed during deposition and subsequent pedogenesis. Grain-size distributions from two measured sections bracketing the Mid-Pleistocene Transition (MPT) were analyzed. These focus areas included a 10-meter section near Lubbock, Texas, and a 14-meter core from Bushland, Texas. Using end-member modeling (EMMAgeo) in R Studio, three to four distinct grain-size populations were identified: a sand mode (including both fine and course populations), a silt mode, and a clay mode. These are interpreted to reflect eolian traction transport, eolian suspension, and pedogenic clay formation in situ, respectively.

    The presence of all end-members and generally poor sorting suggest input from multiple sediment sources. We infer that the sand modes reflect recycling of sand, likely ultimately derived from exposures of the underlying Ogallala Formation, across the SHP as sand sheets. The silt mode represents eolian suspension deposition while the clay mode likely represents a mixture of suspension and in situ pedogenic creation of clay. Moreover, pedogenesis likely worked to homogenize the grain-size populations through illuviation. Comparisons between pre-MPT and syn- to post-MPT intervals show an increase in clay and fine sand, and a decrease in medium to coarse sand in the syn- to post-MPT deposits. An increase in the coarse silt mode input during and after the MPT, potentially was driven by deglaciation and loess deposition across the midcontinent. If so, these findings would extend the southern boundary of midcontinent loess deposition into west Texas. Ongoing work aims to integrate geochemical data with grain size trends to refine provenance interpretations.

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

doi: 10.1130/abs/2025AM-7264

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