Sequence stratigraphy and long-term paleoceanographic change in the Middle and Upper Ordovician of the eastern United States

Fourteen depositional sequences are recognized in the Middle and Upper Ordovician of the central and southern Appalachians and the Cincinnati Arch. Two of these began in the Whiterockian (W-a and W-b), six began in the Mowhawkian (M1 through M6), and six began in the Cincinnatian (C1 through C6). These sequences are typically characterized by an absence of lowstand systems tracts, relatively thin transgressive systems tracts that may contain distinctive facies in their lower part, and somewhat thicker shallowing-upward highstand systems tracts. We present a preliminary correlation chart of lithostratigraphic units of this region based on sequence stratigraphic principles. These correlations provide a framework for analyzing long-term lithologic change within the region. Across the unconformity separating the M4 and M5 sequences in the mid-Mohawkian, carbonates shift from tropical-type to temperate-type lithologies, phosphatization becomes much more widespread, and a major pulse of siliciclastic sediments is introduced. These major lithologic changes correspond closely to the Taconic orogeny in the northern Appalachians and a major flooding event within the M5 sequence, and we postulate that a combination of upwelling of cool water, nutrient influx, and increased water turbidity was responsible for this lithologic shift.