NEOACADIAN TECTONISM AND THE ORIGIN OF THE SUNBURY BLACK SHALE, EAST-CENTRAL UNITED STATES

The Sunbury Shale is a widespread, Lower Mississippian, organic-rich, black-shale unit, which occurs throughout eastern Ohio, eastern Kentucky and western West Virginia. The Sunbury is separated from the underlying Upper Devonian Bedford-Berea sequence by an unconformity at the Devonian-Mississippian boundary. The Sunbury is thickest at 26 m near the intersection of Kentucky, Virginia and West Virginia and thins toward the Cincinnati Arch, where only a few centimeters of the unit remain. Sunbury equivalents also cross the Cincinnati Arch into the Illinois and Michigan basins. To the east, in northern West Virginia, western Maryland and western Pennsylvania, the Sunbury grades into the much thicker (>70 m) and more clastic-rich, black Riddlesburg Shale.

Devonian-Mississippian black-shale units formed due to deformational loading and accompanying foreland-basin subsidence, associated with Acadian and Neoacadian orogenies, representing a north-to-south, transpressive collision between the southeastern margin of Laurussia and several Avalonian terranes, allowing Devonian black-shale basins to migrate southwestward in time. However, by earliest Mississippian time, Sunbury-Riddlesburg basin migration shifted abruptly to the east with the inception of the Neoacadian Orogeny, representing initial collision of the exotic Carolina terrane with the New York promontory and the subsequent, southwestward, transpressional migration of the terrane and orogeny. In response, the initial Sunbury-Riddlesburg basin formed just cratonward of the orogen at areas behind the New York promontory and expanded southwardly accompanying dextral transpression in the orogen. The overlying Price-Pocono-Borden-Grainger clastic units represent an extensive subaerial and subaqueous delta complex that developed during erosion of Neoacadian highlands. This delta complex and overlying parts of the Appalachian Mississippian System probably reflect a single, loading-related, tectophase response to Neoacadian tectonism. Although the entire Neoacadian tectophase apparently persisted for ~37 Myr., the Sunbury Shale represents only the initial 3 Myr. of deposition in the subsiding Neoacadian foreland basin; hence, it records the duration of the most intense period of Early Mississippian, Neoacadian deformation.