Pennsylvanian System
Allegheny Group
The Section below shows the general stratigraphy for the Pennsylvanian Allegheny Group of Southwestern Pennsylvania. The boxes to the right represent the section exposed at each outcrop site. Click on the box to visit the page for that site. - NO SITES IN ALLEGHENY GROUP
KEY: Gray is shale, blue is limestone, black is coal, and yellow is sandstone.
Allegheny Group (the following is paraphrased from http://www.dep.state.pa.us/dep/deputate/minres/districts/cmdp/Chap08-1.html)
The Allegheny Group is one of two groups within the Pennsylvanian that contains the majority of economically mineable coals. Marine units occur only below the upper Kittanning underclay and, with minor exceptions, nonmarine limestones occur only at or above that unit" (Edmunds, et al., 1998, p. 154). Williams (1960) interpreted the rocks in the Allegheny Group above the upper Kittanning coal (and including the Johnstown limestone below the coal) to be freshwater. This interpretation was based on fossil Esterids (now referred to as conchostracans or "clam shrimps"). Conchostracans are found above the upper Kittanning, lower Freeport, and upper Freeport coals (e.g., Williams, 1960; Edmunds, 1968). New analysis and interpretations of the ecology of conchostracans during the Pennsylvanian Period support a marginally-brackish environment.
M
arine units occur over large areas and at several stratigraphic horizons within the lower Allegheny Group. The paleoenvironment of the marine zones can vary regionally and vertically. A good example of this is the Vanport horizon which occurs above the Clarion coal. The marine limestone facies covers an area of at least hundreds of square miles (km2) and is over 20 ft (6 m) thick in the center of the basin. A maximum thickness of over 40 ft (12 m) has been recorded for a small area in northeastern Lawrence County. Core logs in the files of the PaGS were examined to determine the extent of the Vanport zone south of the area mapped by Williams and Keith (1963). The presence of the Vanport marine horizon cannot be confirmed for the southwestern corner of the state because sandstone typically occupies this horizon. This sandstone may indicate non-deposition of limestone and the presence of distributary channels that emptied into the Vanport sea. Alternatively, but less likely, the sandstone may occupy channels that cut through and removed the limestone.According to Edmunds et al. (1998) the Allegheny Group:
"was specifically defined to include all of the economically significant coals present in that part of the Pennsylvanian sequence. The thickness of the formation is between 270 (82 m) and 330 feet (100 m) in Pennsylvania, and there is no obvious regional trend. The Allegheny Formation is a complex, repeating succession of coal, limestone, and clastics, ranging from claystone or underclay to coarse sandstone....No individual bed or lithosome is universally persistent, but some coals, marine shales, and limestones seem to be fairly continuous over thousands of square miles (thousands of square kilometers). The group is fairly uniform in its lithologic diversity…The Allegheny Formation contains six major coal zones. The coal in each zone may exist as a single, more-or-less continuous sheet, as a group of closely related individual lenses, or as a multiple-bed complex in which the various beds can be separated by tens of feet or merge into a single thick coal" (pp. 153-154).