"The Monongahela Group extends from the base of the Pittsburgh coal to the base of the Waynesburg coal. It is divided into the Pittsburgh and Uniontown Formations at the base of the Uniontown coal. The group is about 270 to 400 feet (82 to 122 m) thick in Pennsylvania, increasing in thickness irregularly from the western edge of the state to western Fayette County....It is entirely nonmarine" (Edmunds, et al., 1998, p. 156).

"…dominated by limestones and dolomitic limestones, calcareous mudstones, shales, and thin-bedded siltstones and laminites... The only sandstone of significant thickness within the formation lies directly above the Pittsburgh coal complex. A major fluvial channel system, flowing north to northwest through what is now Greene and Washington Counties, deposited an elongate sandstone body up to 80 feet (24 m) thick and several miles (kms) wide" (Edmunds, et al., 1998, pp. 156-157).

Pittsburgh Formation - The lower half of the Pittsburgh Formation contains the majority of mineable coals in the Monongahela Group. The Pittsburgh coal, which defines the base of the formation and group is unusually continuous, covering thousands of square miles (km²) and is unusually thick (5 to 10 ft; 1.5 to 3 m) for a coal of western Pennsylvania. The other major coals are the Redstone and Sewickley.

Pittsburgh Coal to Redstone Coal Interval - Pittsburgh coal overburden varies from sandstone to limestone to shale. Sandstone was deposited in the distributary channel running through the southwestern corner of Pennsylvania. An isopach map of the thickness of this channel is shown below. Limestone or shale was deposited in the lakes and on the mud flats. The Redstone limestone is "patchy", which may simply be due to variable carbonate content of the lake-deposited sediments. The intervening shales may be a lateral, less calcareous, facies of the freshwater limestone.

Blue Lick Coal to Sewickley Coal Overburden - The Pittsburgh coal often has a rider coal positioned approximately 5 to 50 ft (1.5 to 16 m) above it. This rider coal is usually thin and not economically significant. However, in Somerset County the coal at this stratigraphic horizon, named the Blue Lick, attains mineable thickness.

Pittsburgh Formation Limestones - Monongahela lacustrine limestones exhibit extensive lateral continuity. The lakes in which these laterally extensive limestones were deposited had to have been very large. For example the Benwood limestone above the Sewickley coal covers more than 4,000 square miles (10,000 km²) (Berryhill, et al.,), and Eggleston (1993) notes that the Redstone limestone extends from Somerset County, PA to as far south as Cabell County, WV, and as far west as Morgan County, OH. Eggleston (1993) concludes that the Redstone limestone was deposited in a "very shallow lake that was subject to periodic subaerial exposure during drier periods." As evidence of drying she refers to desiccation breccia, root traces, and lack of original bedding. Evidence of shallow depositional conditions include rounded intraclasts, broken and nested shells, and bioturbated limestone. Berryhill et al. (1971) arrived at similar conclusions, using similar evidence, concerning the depositional environment of other limestones in the Monongahela and Dunkard Groups. Berryhill et al. suggest that the limestones were deposited in shallow lakes, where water depth "probably never exceeded a few feet" and that "(n)either fossil evidence nor physical properties of the rocks indicates any influence of marine conditions" (Berryhill et al., 1971, p. 34). In addition to the limestones, the sandstones and shales of the Monongahela Group are also often calcareous. The environmental interpretations of Eggleston (1993) are consistent with the paleoclimatic interpretations of Cecil et al. (1985) and Donaldson et al. (1985) for this stratigraphic interval.

References

Berryhill, H.L., Jr., S.P. Schweinfurth, and B.H. Kent, 1971. Coal-bearing Upper Pennsylvanian and Lower Permian Rocks, Washington Area, Pennsylvania. USGS Prof. Paper 621.

Cecil, C.B., R.W. Stanton, S.G. Neuzil, F.T. DuLong, and B.S. Pierce, 1985. Paleoclimate controls on Late Paleozoic sedimentation and peat formation in the central Appalachian Basin. Intern. J. of Coal Geol., v. 5, pp. 195-230.

Donaldson, A.C., J.J. Renton, and M.W. Presley, 1985. Pennsylvanian deposystems and paleoclimates of the Appalachians. Intern. J. of Coal Geol., v. 5, pp. 167-193.

Edmunds, W.E., V.W. Skema, and N.K. Flint, 1998. Pennsylvanian. Part II. Stratigraphy and Sedimentary Tectonics. Geology of Pennsylvania, pp. 149-169.

Eggleston, J.R., 1993. Redstone and Fishpot limestones. In: Geology of the Southern Somerset County Region, Southwestern Pennsylvania. Guidebook, 58th Annual Field Conference of Pennsylvania Geologist, pp. 145-146.

Skema, V.W., C.H. Dodge and J.R. Shaulis, 1991. Lithologic character and correlation of marine units in the Conemaugh Group (Upper Pennsylvanian), western Pennsylvania (abstract). Geol. Soc. of America, Northeastern and Southeastern Section Meeting, v. 23, no. 1, p. 128.

Williams, E.G., 1960. Marine and fresh water fossiliferous beds in the Pottsville and Allegheny Groups of western Pennsylvania. Jour. of Paleontology, v. 34, pp. 908-922.