ANATOMY OF A CAMBRIAN SHEET SAND: DEPOSITIONAL ENVIRONMENTS IN THE MOUNT SIMON SANDSTONE

The Mount Simon Sandstone is a quartz arenite dominated basal Cambrian unit present throughout the mid-continental United States. It has been variously interpreted as having been deposited in a tidally dominated nearshore, tidal flat, tidal channel, barrier bar, alluvial fan, and braided fluvial depositional environments. This study merges the results of well core analysis and petrophysical subsurface studies within the Illinois Basin and the surrounding area with previous outcrop studies in Wisconsin and Missouri to create a basin-wide interpretation of Late Cambrian depositional environments in the Mount Simon Sandstone. These data suggest environments dominated by braided fluvial channels with localized alluvial fans adjacent to widespread Precambrian highlands, that merged into a tidally dominated nearshore environment with increasing depth as the Illinois Basin subsided and the Sauk transgressive sequence advanced over the region. Coarser arkosic sediments appear to be locally derived and related to basement terrains. Trace fossils and transported assemblages of inarticulate brachiopods Obolus and Micromitra observed in core samples were used to delineate marginal marine environments. Marginal marine environments also contain zones of fine sand injectites within shale horizons interspersed with undisturbed sand beds, interpreted as seismites. The Mount Simon Sandstone is one of the major targets for ongoing CO₂ sequestration projects in the Midwest United States which requires knowledge of depositional heterogeneity. This study describes large scale primary depositional variability across the Midwest region of the United States and highlights the important differences in sedimentation dynamics that are present in the Precambrian/Earliest Paleozoic clastic sedimentary systems.