COMPARATIVE ORGANIC PETROGRAPHY OF PENNSYLVANIAN MARINE SHALE BEDS FROM THE CENTRAL APPALACHIAN AND EASTERN INTERIOR BASINS

Laterally persistent marine shale beds occur in the Central Appalachian and Eastern Interior Basins. These beds are useful for age dating and correlation of strata, on both an intra- and inter-basinal scale. This report presents a comparison of the organic composition of these beds, as determined from reflected light petrology.

Major marine beds in the eastern Kentucky part of the Central Appalachian Basin include, in stratigraphically ascending order, the Dark Ridge, Hensley, Betsie, Kendrick, Magoffin, and Stoney Fork marine members. Lithologically, each consists of coarsening-upward silty shale units. Marine fauna, if present, is typically restricted to the marine flooding surface in the lower parts of the units. Total organic carbon contents range from <1 to 4 % by weight. Petrographically, they are dominated by terrestrial organic material, mainly in the form of vitrodetrinite, inertodetrinite and sporinite.

Major marine beds in the western Kentucky portion of the Eastern Interior (Illinois) Basin overlie coal beds in the Carbondale Formation, and represent the transgressive portions of classic cyclothem deposition. Lithologically, the marine flooding surface, and lower parts of these shales, are less silty then their Appalachian counterparts. They tend to be high in total organic content, commonly >10 % by weight. In addition, they are usually high in total sulfur content, and often contain abundant marine fauna. Petrographically, they contain increased amounts of marine organic material, principally in the form of bituminite, and alginite. In addition, solid bitumen is more common in western Kentucky shales, than it is in eastern Kentucky shale beds.

In summary, eastern Kentucky marine shale beds are dominated by terrestrial organic matter, which may be a function of proximity to source, and deposition in a rapidly-subsiding foreland basin, with estuarine circulation patterns. In contrast, western Kentucky marine shales were deposited in a more slowly subsiding basin, which was open to the Midcontinent seas. This factor contributed to increased amounts of marine organic matter.