Black-Shale Source Rocks as Indicators of Paleozoic Tectonic History in the Appalachian Foreland Basin
Frank R. Ettensohn
Department of Earth & Environmental Sciences, University of Kentucky, Lexington, KY 40506-0053
In the Appalachian Basin, black shales are common lithologies, ranging in age from Middle Ordovician to Pennsylvanian. Dark coloration suggests greater-than-normal abundances of organic matter, and many of these shales are hydrocarbon source beds, or reservoir rocks when fractured; at various times, moreover, they have also been considered as oil and gas shales or as sources for various radioactive and heavy metals. Organic matter in these shales may comprise as much as 20 % by weight, and the distinctive lithology makes mapping their distribution in time and space relatively easy.
These shales are parts of third- and fourth-order, unconformity-bound cycles that have been interpreted to be flexural, foreland-basin manifestations of distinct episodes of tectonism, called tectophases, during each orogeny. The black shales reflect deposition during rapid, loading-related subsidence in early parts of each tectophase, and hence, track the progress of orogeny in time and space. Included fossils and bentonites, moreover, provide ages for the shales and the related timing of orogenic events. Thirteen black-shale units, representing 13 tectophase cycles during four major orogenies are recorded in the Appalachian Basin. During the Taconian, Salinic, and Acadian, subduction-type orogenies, deeper water, open-marine, black shales were typical, but during the final Alleghanian collisional orogeny, two tectophases are represented by shallow-water, terrestrial or marginal-marine dark shales, reflecting a change in tectonic style and secondary, glacio-eustatic influences. As hydrocarbon source and reservoir rocks, these shales are clearly the product of distinctive tectonic frameworks and histories, and aside from any economic value, they may provide additional controls on the timing and location of those tectonic events.
AAPG Search and Discovery Article #90059©2006 AAPG Eastern Section Meeting, Buffalo, New York