The Spatial and Temporal Scales of Tectonic Accommodation In the Back-Bulge Zone of the Appalachian Foreland Basin during Alleghenian Deformation

Exposures of the Middle to Late Pennsylvanian Allegheny, Conemaugh, and Monongahela Groups in the back-bulge zone of the Alleghenian foreland basin in southeastern Ohio show that tectonics affected the rate of formation of accommodation over a wide range of temporal and spatial scales. Backstripping of the entire section using realistic initial porosity values and recently updated age estimates for the boundaries of the three groups shows that there was a continuous increase in the long-term rate of formation of accommodation from 30 m/m.y. to >100 m/m.y. over the span of 9 m.y. Graphic correlation of closely spaced, detailed measured sections shows that the interval between the Upper Brush Creek and Ames limestones in the Glenshaw Formation (lower Conemaugh Group, circa 1 m.y. duration) varies in thickness by up to 50%. Isopach maps of intervals between marine bands reveal the presence of trends that parallel the leading edge of the Appalachian fold-and-thrust belt and are interpreted to be the result of low-amplitude folding affecting accommodation on a time scale of 0.1-0.5 m.y. Confirmation of the presence of far-field effects of thrusting exists in the form of two sets of clastic dikes that bracket the Ames Limestones (302 Ma). These fine-grained sandstone dikes are 1-10 m long and 0.2-0.3 m thick, and intrude both highstand and transgressive mudstones in discontinuous outcrops spanning a distance of 5 km. Contoured poles to planes of 61 dike orientations reveal two perpendicular sets. The dominant set is perpendicular to the Allegheny Front, which when combined with compaction of the dikes and host rock, is further evidence of repeated structural deformation on a scale of < 100 k.y. related to the regional stress field of the Late Pennsylvanian.