129-24 A Stratigraphic Reevaluation of the Mt. Simon Formation at Its Type Locality: Implications for the Elk Mound Group

Abstract:

We present new, high-resolution stratigraphic columns and facies characterization of the Cambrian Mt. Simon Formation at three localities in west-central Wisconsin, including at its type section in Eau Claire and two nearby outcrops used for correlation. Logged bed by bed from top to bottom, the columns document sand dominated lithologies, grain size trends, bedding, sedimentary structure, and trace fossils supported by field descriptions and systematic digital imagery.

The Mt. Simon Formation is the basal unit of the Elk Mound Group, resting nonconformably on the weathered Mesoproterozoic basement. It is overlain by the Eau Claire Formation and then the Wonewoc Formation, which together underlie the Tunnel City Group. Deposition occurred when a shallow sea flooded the upper Midwest, including central Wisconsin, Minnesota, and Illinois during the Cambrian. The formation is economically important: it is a regional aquifer, and a leading target for carbon-dioxide sequestration in the Illinois basin.

Despite its significance, the type section lacks a modern study that integrates a detailed measured column, advanced analytical characterization, and digital documentation; the most recent published description is more than half a century old. Re-describing the type section is timely because recent work has proposed an open-coast, wave-dominated tidal-flat model for the Elk Mound Group (MacNaughton et al., 2019). In this model, sandy, mud-poor flats preserve co-occurring storm-generated parallel lamination and fair-weather wave ripples alongside indicators of shallow water and repeated subaerial exposure, marking a sharp departure from classic mud-rich tidal interpretations. Our columns provide the physical-stratigraphic framework needed to test this model at the type locality and reevaluate the group. Planned analytical work includes thin-section petrography, SEM-imaging, and bulk XRD of gradational contacts to refine facies and provenance interpretations.

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