USING CONODONTS TO DIFFERENTIATE LITHOLOGICAL CYCLES WITHIN THE PENNSYLVANIAN MIDDLE MINNELUSA FORMATION, BLACK HILLS, SOUTH DAKOTA, USA

The Minnelusa Formation is a thick and lithologically diverse formation that extends across North Dakota, South Dakota, Montana, and Wyoming. In South Dakota, the Minnelusa Formation is informally subdivided into lower, middle, and upper units. The middle Minnelusa is diagnosed by its exceptional lithological heterogeneity, including shale, carbonate, and sandstone varieties. This focused study of the middle Minnelusa from the Black Hills reveals that the unit comprises a repetitive series of lithological cycles. Each cycle has a similar stacking pattern with a basal sandstone unit overlain by limestone and shale. For the three primary outcrops sampled, at least eight lithological cycles were observed at Boulder Canyon Road (BCR), eight at Vanocker Canyon Road (VCR), and four at Hot Brook Canyon (HBC). These cycles record environmental changes from a nearshore to an offshore marine environment, and the shale and limestone intervals at the top of several cycles contain well-preserved conodonts indicating a marine origin. Five biostratigraphically distinct Middle to Upper Pennsylvanian conodont faunas were recovered from these marine units. An upper Desmoinesian assemblage was recovered from VCR cycle 1. Two Missourian assemblages were recovered, one indicative of the Idiognathodus cancellosus Zone from VCR cycle 3 and the other indicative of the Streptognathodus gracilis Zone from VCR cycle 6. Two Virgilian assemblages were recovered, one indicative of the Heckelina simulator Zone from VCR cycle 7, VCR cycle 8, and HBC cycle 2 and the other indicative of the lower part of the “Streptognathodus” group 4 virgilicus Zone from HBC cycle 3. Two conodont biofacies were recognized within the middle Minnelusa, the low energy, offshore, Idiognathodontidae-Idioprioniodus biofacies, and the higher energy, nearshore Idiognathodontidae-Hindeodus biofacies.