Large-Scale, Tectonic Controls on the Origin of Paleozoic, Black-Shale, Source-Rock Basins in the Appalachian Foreland-Basin Region

Black shales are common lithologies in the Appalachian Basin, ranging from Middle Ordovician to Pennsylvanian in age. Dark coloration indicates greater-than-normal amounts of organic matter, making many of these shales hydrocarbon source beds, or reservoir rocks when fractured. Organic matter in these shales may comprise up to 20 % by weight, and 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 are thought to be flexural, foreland-basin manifestations of distinct episodes of tectonism, called tectophases, during an 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. 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 characteristic of the foreland basins, and today, these shales are major hydrocarbon source and reservoir rocks in the Appalachian Basin and adjacent parts of the craton in central and eastern United States. During the final Alleghanian collisional orogeny, however, two final tectophases are represented by shallow-water, terrestrial or marginal-marine dark shales, which are not good source rocks and reflect a change in tectonic style and the presence of secondary, glacio-eustatic influences.

As hydrocarbon source and reservoir rocks, foreland-basin black shales are clearly the product of distinctive tectonic frameworks and histories. The models applied here have also proven useful in understanding black shales from other U.S. foreland basins and from foreland basins in Europe, South America, and Asia. Aside from the potential economic value of these shales, they may provide additional controls on the timing and location of related tectonic events.