Deposition and Structural Features of the Basal Morgantown Sandstone of the Casselman Formation (Pennsylvanian) of the Greater Pittsburgh Region

In southwestern Pennsylvania, exposures of the Morgantown Sandstone (Pennsylvanian) of the Casselman Formation are comprised of generally massive to cross-bedded sandstone that varies between 3 to 20 meters thick. The base of the unit is commonly marked by pebble or cobble conglomerate containing clasts of diverse composition and rounding in a sandy matrix. Underlying units include gray to black shale and unfossiliferous limestone; these may be deformed by folding, loading structures, and/or bedding truncations. Pebble-filled fractures resembling sedimentary dikes rarely cut the underlying carbonate rocks. A thin horizon of coal 2 to 10 centimeters thick commonly crops out within the lowest three meters of sandstone. The coal is generally strongly disrupted by faults that may transect and offset the layer or be coincident, in which case the sheared coal may accommodate sliding and related detachment. Locally the basal conglomerate, which resembles a debris flow, overlies cobbly debris containing platy shale clasts, some of which may be folded. The floating clasts may rarely make rootless, isoclinal folds. Occasionally, folding and other soft sediment deformation is recorded by underlying beds, which may be truncated at the contact. In exposures lacking conglomerate and coal, the base of the sandstone rests directly upon undeformed shale. Wherever exposure is sufficient, the base of the unit is clearly undulating. \ud The unusual character of the basal Morgantown is documented throughout an area of at least 600 square kilometers. Nearly all of the 24 outcrops visited display structural and sedimentary features that suggest disruption at the basal contact with the underlying strata. The structural and stratigraphic features at the base of the Morgantown are interpreted as having formed contemporaneously with the deposition of the sandstone as a regional, abruptly emplaced sand flow. Although commonly attributed to accumulation as a channel sandstone, no channel banks or well-sorted traction deposits are known. Furthermore, the thickness of the section between the base of the Morgantown Sandstone and the laterally persistent Ames Limestone of the Glenshaw Formation varies between 10 and 50 meters throughout the Pittsburgh region. This further leads to the conclusion that the base of the Morgantown records scour.\u