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PROVENANCE STUDIES IN THE ATLANTIC COASTAL PLAIN: WHAT FISSION-TRACK AGES OF DETRITAL ZIRCONS CAN TELL US ABOUT THE EROSION HISTORY OF THE APPALACHIANS
The oldest single-grain zircon FT ages from bedrock in the Appalachian Plateaus, Valley and Ridge, and the westernmost Blue Ridge Provinces are significantly older (800 Ma to 2,960 Ma) than those from the eastern Blue Ridge and Piedmont (<800 Ma). Detrital zircon single-grain FT ages from outcrop and shallowly buried (<411 m) sands in the ACP provide a record of detritus coming from source terranes in the Appalachians during the Mesozoic and Cenozoic. In Virginia and Maryland, all zircon single-grain FT ages in ACP sands of Early Cretaceous through mid-Oligocene age are <800 Ma, suggesting source terranes in the Piedmont and eastern Blue Ridge. Old zircons (>800 Ma) comparable in age to those in the westernmost Blue Ridge, Valley and Ridge, and Appalachian Plateaus do not appear in ACP sands until early Miocene time.
Similarly, clasts of Paleozoic chert that originate in Paleozoic rocks in the Valley and Ridge, west of the Blue Ridge and Reading Prong, first appear in abundance in ACP sediments from New Jersey south to the Virginia-North Carolina border in the late Oligocene(?)-Miocene.
Our interpretation of the zircon and lithic detritus data is that major east-flowing Atlantic rivers, such as the ancestral Potomac, did not breach the Blue Ridge-Reading Prong and begin eroding rocks to the west until mid-Oligocene to early Miocene time. Prior to that, drainage from the western Appalachians was to the west-southwest, into the Gulf of Mexico. Mesozoic through mid-Oligocene ACP sediments were derived entirely from erosion of Piedmont and eastern Blue Ridge rocks, possibly with minor input from distant volcanic centers of Cretaceous to Eocene age.