Stratigraphic effects of the Acadian Orogeny in the autochthonous Appalachian Basin

The autochthonous Appalachian basin is located in western West Virginia, southern Ohio, eastern Tennessee, and eastern Kentucky. It is bounded on the east and the south by detachment fronts formed in the Alleghenian orogeny and to the west by the uplifts of the Cincinnati arch and Waverly arch. The Wallbridge unconformity of Early Devonian age corresponds to the boundary between the Tippecanoe and Kaskaskia sequences. It resulted primarily from a major eustatic drawdown but also marks the initial effects of the Acadian orogeny. Subsequent development of a foreland basin included deposits of the Needmore, Marcellus, and Mahantango shales. In the Middle Devonian, growth of this initial foreland basin was terminated by the regional Taghanic unconformity. The Taghanic unconformity was developed during a period of eustatic sea-level rise and is tectonic in origin, resulting from peripheral bulge reactivation during the Acadian orogeny. Development of a successor foreland basin followed, including deposition of the Genesee through the Ohio shales. Maximum Catskill delta progradation in the latest Devonian was followed by a period of postorogenic delta destruction, accompanied by glacio-eustatic sea-level drawdown. Recognition of two distinct foreland basins suggests that the Acadian orogeny outboard of the autochthonous Appalachian basin resulted from two distinct impacts on the continental margin. The first occurred in the Early Devonian and was relatively small. The second occurred in the Middle Devonian and was large. This interpretation requires a three-plate tectonic model. One possible interpretation would attribute the first impact to obduction of an Avalonian microcontinent and the second to collision with Gondwanaland.