Sequence stratigraphy of a carbonate-clastic ramp: The Cincinnatian Series (Upper Ordovician) in its type area

摘要

The Cincinnatian Series consists of five depositional sequences; each is composed of a thin transgressive-systems tract and a much thicker, shallowing-upward, highstand-systems tract. Whereas the highstand-systems tracts are relatively poor in micrite but rich in terrigenous shale, the transgressive-systems tracts are composed of cleaner carbonate facies, ranging from coarse-grained calcarenites to micrite-rich wackestones and micstones. This alternation of carbonate- and terrigenousfacies types resulted from the trapping of clastic sediment near its presumed source in the Appalachians during a relative sea-level rise, and it may be a feature common to sequences in other mixed carbonate-clastic settings. These depositional sequences were produced by eustatic fluctuations, as shown by the limited extent to which the sequences can be traced into more proximal portions of the Appalachian Basin. The recognition of depositional sequences clarifies the regional paleogeography and simplifies regional lithostratigraphic nomenclature. Within current biostratigraphic resolution, sequences in the Cincinnatian Series appear to have chronostratigraphic significance and thereby offer an additional means of intrabasinal correlation in the Upper Ordovician.