OROGENIC FORELAND STRATIGRAPHY CONTAINS KEYS TO THE INTERNAL TECTONIC HISTORY OF OROGENS—IF YOU CAN RESOLVE IT (Invited Presentation)

Frank R. Ettensohn has had an outstanding career as a scholar, mentor of students, and sedimentary geologist. He has broken new ground with his reconstructions of parts of the sedimentary section in the southern-central Appalachian foreland. These sections reflect the evolution from stable margin/carbonate shelfs to anoxic foredeeps that received voluminous clastic sediments from the three orogenies that formed the Appalachian orogen. Mountain chains commonly form via the Wilson cycle as large or small oceans close with subduction of intervening ocean crust and arc or continental collision closes the remaining ocean. Continental crust is thickened as the previous margin is deformed, metamorphosed, and intruded. New continental crust forms increasing the mass of a continent, or island arcs, which are accreted to existing arcs or to a continental landmass. Progradation of clastic wedges onto a continent reflects both the spatial source of the wedge and its composition; detrital zircons and character of sediments help characterize the source terrane. Frank Ettensohn recognized the effects produced by oblique collisions that produce diachronous foreland sedimentation. He even subdivided Late Devonian basin sedimentation into “tectophases,” which may be further divided into stages reflecting the deepening history of a foreland basin, its filling history, and the oblique nature of collision of [now known to be Neoproterozoic-Cambrian] Avalon-Carolina that produced the Acadian-Neoacadian orogeny. Applying these principles to foreland basins in other orogens (e.g., North American Cordillera, British and Scandinavian Caledonides), we see that they display similar characteristics.