RESOLVING THE SEQUENCE STRATIGRAPHY OF SUPERPOSED CHANNEL SANDSTONES WITHIN THE ALLEGHENY GROUP OF THE APPALACHIAN BASIN

Pennsylvanian strata commonly contain thick, multistory sandstones that have been interpreted as fluvial channel, deltaic distributary channel, or incised valley (IVF) deposits. The Lower Freeport Sandstone member of the Allegheny Group (Middle Pennsylvanian) in the Appalachian basin was earlier interpreted as a deltaic distributary deposit. More recently, similar sandstones higher in the section have been interpreted as IVF deposits. Each interpretation has different sequence stratigraphic implications and, while the location of these deposits within the distal Appalachian basin makes the IVF interpretation more attractive, data from southeastern Ohio show that unit is an amalgamation of both models. Detailed sections measured show that the Lower Freeport Sandstone in southeastern Ohio is composed of two discrete lithologies. A lower medium-grained sandstone, which lies abruptly on marine mudstones and siltstones, occurs as discontinuous ribbons up to 1 km wide with paleoflow toward the northwest. A coarse- to very coarse-grained sandstone sheet, which varies from 1- 3 km wide, unconformably overlies the lower unit and locally incises through the underlying marine sediments and into the Middle Kittanning coal below. This upper sandstone contains a basal pebble lag and shows no vertical changes in grain size. Paleoflow was to the north-northeast. These data are interpreted suggest that the Lower Freeport Sandstone is composed of a basal distributary channel sandstone of a delta system that records early highstand progradation and an upper, possibly braided fluvial, IVF deposit within a valley system formed during the late highstand and lowstand. The sequence boundary for this cycle therefore is not located at the base of the Lower Freeport sandstone but, instead, at the base of the upper sandstone unit.