The Origin and Significance of the Lower Mississippian Sunbury Shale in East-Central United States
The Sunbury Shale is a relatively thin, Lower Mississippian, organic-rich, black-shale unit that is prominent throughout eastern Ohio, eastern Kentucky and western West Virginia. The Sunbury is the most widespread of the Devonian-Mississippian black shales. Where it occurs in the western Appalachian Basin, it is commonly separated from the black, Upper Devonian Ohio Shale by the gray shales, silts and sands of the Bedford-Berea sequence. However, in parts of eastern and central Kentucky, where the Bedford becomes a black shale, the Sunbury merges with black Bedford and Ohio shale equivalents to become the uppermost part of the New Albany and Chattanooga shales. The unit even crosses the Cincinnati Arch into the Illinois Basin and is present in the Michigan Basin, where it attains thicknesses greater than 45 m. To the northeast in northern West Virginia and western Pennsylvania, the Sunbury grades into the much thicker (> 70 m) and more clastic-rich Riddlesburg Shale, which reflects a marginal-marine embayment. The black-shales of the Devonian-Mississippian sequence all represent subsidence and early deep-water infilling of foreland basins associated with orogeny in the east. Early to Late Devonian black shales all represent deformational loading and foreland-basin infilling associated with the Acadian Orogeny, and these shales occur in basins that migrate southwesterly in time and space. The Sunbury-Riddlesburg, however, is different in that it migrated eastward in time and space, and recent work shows that it represents foreland-basin subsidence accompanying inception of the Early Mississippian Neoacadian Orogeny in New England. The Neoacadian Orogeny represents the initial collision of the exotic Carolina terrane at New England and the terrane’s subsequent southwestward transpressional convergence with the eastern margin of Laurussia (North America). In response, the initial Sunbury-Riddlesburg basin formed just cratonward of the orogen in western Pennsylvania and northern West Virginia and subsequently expanded southwestwardly along with dextral transpression in the orogen. While the Sunbury and its equivalents represent a deeper, sediment-starved, foreland-basin infilling, the overlying Price-Pocono-Borden-Grainger clastic units represent relaxational, post-orogenic, deltaic complexes that developed from erosion of Neoacadian Mountains formed in the orogen. Economically, it is thought that the Sunbury may have sourced conventional reservoirs in the underlying Bedford-Berea sequence as well as in the overlying Lower Mississippian clastic units. However, in places throughout the western Appalachian Basin where it has sufficient thickness and TOC greater than 20%, the unit should probably be examined as a future unconventional resource.
AAPG Datapages/Search and Discovery Article #90373 © 2019 AAPG Eastern Section Meeting, Energy from the Heartland, Columbus, Ohio, October 12-16, 2019