WHEN IS A LIMESTONE NOT A LIMESTONE?: CALCITE REPLACEMENT OF GYPSUM OF THE PERMIAN MINNEKAHTA LIMESTONE

The Permian Minnekahta Limestone of the Black Hills of South Dakota, overlain and underlain by continental redbeds, has long been a marker bed within the stratigraphic column. This unit has previously been interpreted as a tidal flat deposit based upon the presence of stromatolites in carbonate rock. However, our preliminary observations question the original composition at time of deposition. Petrography indicates that the Minnekahta is dominated by pseudomorphs after bottom-growth gypsum crystals, suggesting calcite replacement of bedded gypsum. Sedimentary structures include alternating lamina of millimeter-scale bottom growth gypsum and mud drapes, as well as mudcracks, raindrop imprints, and stromatolites. This assemblage and its close association with continental redbeds indicates that this was likely a shallow saline-lake deposit. Other “carbonates” in Permo-Triassic continental redbed sequences throughout the midcontinent of North America, such as those in the Goose Egg, Chugwater, Nippewalla, and Spearfish, may have similar continental origin.