Subsurface correlation and paleogeography of a mixed siliciclastic-carbonate unit using distinctive faunal horizons: Toward a new methodology

摘要

Regional correlation of mudrock-siliciclastic units is challenging, largely owing to the apparent featurelessness of fine-grained intervals. This difficulty is multiplied when subsurface correlation is necessary for paleogeographic reconstruction. In this study, faunal-marker tracing and limestone-pattern matching have permitted subsurface correlation of the Alexandria submember of the Kope Formation (Edenian Stage, Upper Ordovician) over a 193-km transect in southwest Ohio. The faunal markers are thin (<10 cm), widespread deposits of skeletal debris exhibiting faunal associations, degrees of preservation, or both, that distinguish them from other fossil deposits in host mudrocks. Subsurface correlations corroborate interpretations of southwest Ohio paleogeography and demonstrate the usefulness of techniques presented here. Geographic trends in the data indicate that the average seafloor slope over much of the Cincinnati region was near zero. Evidence also indicates a northwest-dipping paleoslope approximately normal to the study transect; this is likely a transition from the Kope environment into the Sebree Trough, a narrow basin with poorly understood morphology. A change from limestone-rich to limestone-poor facies, accompanied by replacement of oxic by dysoxic fauna, takes place over a maximum distance of 40 km between two localities along the transect. This represents improved constraint on the Kope-Sebree Trough boundary.