Sequence Stratigraphy and Depositional Systems of the Lower Silurian Medina Group, Northern Appalachian Basin
CASTLE, JAMES W., Cabot Oil and Gas Corporation, Pittsburgh, PA
Detailed sedimentological analysis of 3500 ft of continuous core from 44 wells in Pennsylvania, Ohio, Ontario, New York, and West Virginia, combined with regional study of geophysical logs, results in new interpretations of sequence stratigraphy and depositional systems in Lower Silurian siliciclastic rocks of the northern Appalachian basin. Above a type-1 sequence boundary at the base of the Medina Group are a lowstand systems tract and a transgressive systems tract that are represented, respectively, by the Whirlpool Sandstone and by the overlying Cabot Head Shale. The thickest sandstones in the Medina Group occur in the Grimsby Sandstone, which is interpreted as a highstand systems tract with basinward-prograding parasequences. Sea level rise after Grimsby parasequence deposition i represented by marine-shelf shale in the uppermost part of the Medina Group.
Based on facies successions in the cores, four mappable depositional systems are interpreted for the Grimsby Sandstone and correlative sandstone units: (1) wave-dominated middle shelf, (2) wave- and tide-influenced inner shelf, (3) tide-dominated shoreline, and (4) fluvial. The wave-dominated middle-shelf system, which includes very fine-grained shelf-ridge sandstones encased in marine shale, is the most basinward system, occurring from Ontario through parts of eastern Ohio. Wave-formed lamination and storm-produced bedding are present in the shelf-ridge sandstones, which coarsen upward. The wave- and tide-influenced inner shelf system, recognized in cores from Ontario (eastern Lake Erie) and eastern Ohio, includes very fine- to fine-grained sandstones that contain wave-produced prima y structures and tidal bedding. The tide-dominated shoreline system occurs in the subsurface of western New York, northwestern Pennsylvania, and northeastern Ohio, and includes tidal-channel, tidal-inlet, tidal-flat, and shoreface deposits. Thick, tidally bedded fine- to medium-grained sandstone beds and intensely burrowed, interbedded sandstone and shale are present in the shoreline system. Upward coarsening in cored parasequences of the shoreline system indicates progradation. The parasequences are terminated in a landward direction by coarse-grained fluvial-channel deposits and minor, associated finer grained facies of the fluvial system, which is represented by part of the Tuscarora Formation in central Pennsylvania and West Virginia.
Shoreward, across the northern Appalachian basin, the influence of tidal processes relative to wave processes generally increased, which may have been related to distance across the shelf, water depth, and shoreline configuration. The shoreline may have been deltaic in some areas and straight in other areas.
AAPG Search and Discovery Article #91005 © 1991 Eastern Section Meeting, Pittsburgh, Pennsylvania, September 8-10, 1991 (2009)