FLEXURAL AND SEDIMENTARY RESPONSES OF THE APPALACHIAN FORELAND BASIN TO NEOACADIAN OROGENY

Major Mississippian clastic wedges in the Appalachian foreland basin have long been considered by most workers to reflect either the Acadian or Alleghanian orogenies, but recent structural and tectonic developments now suggest the presence of a kinematic regime involving the transpressional convergence of the Caroline terrain with eastern Laurussia that is different than those of the preceding Acadian or succeeding Alleghanian orogenies. Although this convergence probably began in latest Devonian time, the first sedimentary indication in the Appalachian basin is the localized presence of a subtle Devonian-Mississippian unconformity, representing bulge moveout. This was followed by major flexural subsidence throughout the basin and development of deep, anoxic conditions reflected in the Sunbury Shale and its equivalents. The Sunbury shows the most extensive, anoxic black shales in the basin, attesting to the intensity and widespread distribution of deformational loading that accompanied the orogeny. By Early Mississippian time, loading-related subsidence had largely abated and a phase of loading-type relaxation ensued in the orogen, resulting in the extensive, cratonward-prograding Borden-Grainger-Price-Pocono clastic wedge, a wedge so extensive that it filled the Appalachian basin and prograded into parts of the Illinois basin; parts of the basins not filled are represented by the generally clastic-deficient Ft. Payne Formation. By latest Osagean time, however, this progradation had been halted by the eastward return of the bulge, which cut off sediment supply to the basin. In combination with a eustatic lowstand and a subtropical climatic setting, this cutoff generated ideal conditions across the old delta platforms for the widespread deposition of shallow-water carbonates, apparent in the Greenbrier-Newman-Slade-Monteagle limestones. By mid-Chesterian time, carbonate deposition waned, as marginal-marine clastics, representing the final flexural phase of unloading-type relaxation, prograded across the basin in the Pennington-Mauch Chunk formations. This progradation continued until earliest Pennsylvanian time, when it was truncated by an Early Pennsylvanian unconformity representing the true inception of Alleghanian orogeny and continent-continent collision.