PROVENANCE STUDIES IN THE ATLANTIC COASTAL PLAIN: WHAT FISSION-TRACK AGES OF DETRITAL ZIRCONS CAN TELL US ABOUT THE SOURCE OF SEDIMENTS

Fission-track (FT) analysis of zircon is helping to define the Paleozoic through Cenozoic history of the Allegheny Plateau, Valley and Ridge, Blue Ridge, Piedmont, and Coastal Plain provinces of the Eastern United States. The oldest single-grain zircon FT ages from the Allegheny Plateau, Valley and Ridge, and the west side of the Blue Ridge are significantly older (800 Ma to 2,960 Ma) than those from the eastern Blue Ridge and Piedmont (<800 Ma). Zircon single-grain FT ages from outcrop and shallowly buried (<411 m) sediments in the Coastal Plain provide a record of detritus coming from source terrains during the Mesozoic and Cenozoic. In Virginia and Maryland, all zircon single-grain ages from Lower Cretaceous through uppermost Oligocene Coastal Plain sands are <800 Ma, suggesting source terrains in the Piedmont and eastern Blue Ridge. Old zircons (>800 Ma) comparable in age to those in the western Blue Ridge, Valley and Ridge, and Allegheny Plateau do not appear in Coastal Plain sediments until the early Miocene.

The fluvial and marginal marine Miocene and Pliocene deposits shown on the 1989 Coastal Plain Map of Virginia by Mixon and others include abundant Paleozoic chert and other rock types that are found only west of the Blue Ridge. Congruent with the old zircon ages (>800 Ma) these distinctive lithologies occur in the Miocene and younger deposits; they do not appear in older units.

The preliminary interpretation is that major drainage from the western Blue Ridge, Valley and Ridge, and Allegheny Plateau was to the west prior to the Miocene, and major east-flowing Mid-Atlantic rivers did not breach the Blue Ridge until early Miocene time.