Regional sedimentology and stratal surfaces of a Lower Silurian clastic wedge in the Appalachian foreland basin

Abstract

ABSTRACT Detailed investigation of continuous cores from 44 wells and borehole geophysical logs from 200 wells extends sedimentological interpretations and sequence stratigraphic correlations of Lower Silurian siliciclastic rocks across a major part of the Appalachian foreland basin. Stratal surfaces in the Medina Group and Tuscarora Sandstone are correlated regionally across depositional strike, made possible by the large subsurface database. Above a sequence-bounding unconformity at the base of the Medina Group are an incised-valley fill and a transgressive systems tract represented by the Whirlpool Sandstone and the overlying Cabot Head Shale. The thickest sandstones, and most important hydrocarbon reservoirs, are in the Grimsby Sandstone, which is interpreted as a highstand systems tract with basinward-prograding deposits formed during a normal regression. Three major depositional systems are recognized for the Grimsby Sandstone: (1) wave-dominated shelf, (2) wave- and tide-influenced inner shelf, and (3) tide-dominated shoreline. In the landward direction, the Grimsby Sandstone shoreline deposits are correlative with coarse-grained fluvial and estuarine deposits of the Tuscarora Sandstone. Marine-flooding surfaces in the Medina Group are correlated regionally with flooding surfaces interpreted in the Tuscarora Sandstone. A marine-flooding surface in the upper part of the Grimsby Sandstone, as identified in cores from Ohio, Ontario, and western Pennsylvania, is correlated with the contact between fluvial/estuarine facies of the Tuscarora Sandstone and the overlying, much finer-grained Castanea Member in central Pennsylvania. This flooding event marks the first landward shift in facies of the Lower Silurian highstand accumulation, and is therefore recognized as an important, regionally correlative surface. A second regionally extensive marine-flooding surface occurs between uppermost Lower Silurian argillaceous sandstones and overlying carbonate rock and shale of the Clinton Group.