INTERPRETING THE ORIGIN AND NATURE OF UNCONFORMITIES IN THE APPALACHIAN FORELAND BASIN

At least 15 regional and interregional unconformities are present in the Appalachian foreland basin. Because basin origin largely reflects tectonic loading during nearly continuous Ordovician-through-Permian orogeny on the eastern Laurentian/Laurussian margin, most of these unconformities probably have major tectonic components. Eustasy related to glaciation, however, at times may have also contributed an important signal. Discerning the relative importance of tectonic vs. eustatic signals in unconformity formation is difficult, but attention to stratigraphic association, unconformity distribution, and timing may suggest the predominance of a single component. Tectonically related unconformities typically reflect flexural bulge uplift and bound distinct flexural stratigraphic sequences related to tectophases in each orogeny. Like the bulges that form them, unconformity distribution commonly parallels the orogen or is restricted to areas behind continental promontories where convergence, and resulting flexural uplift, were especially intense. Hence, association with flexural stratigraphic sequences, coincidence of unconformity development with defined tectonic events, and local distributions relative to probable loci of tectonism support the predominance of tectonic components. In contrast, unconformities with major eustatic components are typically widespread, except for bull's-eye-like, conformable areas in the center of the basin, and commonly formed during times of maximum ice volume. Using the above criteria, all Appalachian unconformities show some tectonic signal, but only one at the Ordovician-Silurian transition reflects predominantly eustatic control. Although Appalachian unconformities at the Early-Middle Ordovician and Mississippian-Pennsylvanian transitions are nearly continent wide, their origin is more likely related to initiation of Laurentian subduction and collision with Gondwana, respectively, during which the continental response was one of impedance, resulting in widespread uplift and emergence. In short, in view of the causal, spatial and temporal proximity of the Appalachian foreland basin to nearly continuous, Paleozoic orgeny, it is probably unrealistic, except during times of major glaciation, to relate most unconformity formation there to major causes other than tectonism.