MANTLE DYNAMIC IMPACT ON PASSIVE MARGIN EVOLUTION: IMPLICATIONS FOR THEIR ARCHITECTURE AND DERIVED SEA LEVEL HISTORIES

The eastern U.S. coastal plain is characterized by Cenozoic strandlines and adjacent shallow marine shelf sequences that define a series of warped surfaces that vary in altitude along the length of the coastal plain by as much as 100+ m. We model Pliocene and younger depositional sequences as these are very well preserved and uncertainties in calculations of various potential contributors to their current elevations are small. The late Early Pliocene depositional history of coastal plain is characterized by flooding events within the Yorktown Fm in Virginia and northern North Carolina, and correlative Duplin, Charlton, Reysor sequences in southern North Carolina, South Carolina, and Georgia and unnamed and Cypresshead units in Florida. These units are bounded to their west by the Orangeburg scarp from Georgia to southern North Carolina, and the correlative Chippenham and Thornburg scarps farther north. The Orangeburg scarp represents a wave cut step in the topography, the age of which is determined by the presence of marine late Early Pliocene (~3.25 Ma) sediments at its base. The base of these Pliocene marine sediments and adjacent scarp define an undulating surface that varies by more than 60 m in elevation from Georgia to Virginia. At the time of deposition the Orangeburg scarp was a horizontal feature that has subsequently been significantly distorted. Both younger and older sequences and associated scarps are warped to varying degrees along the coastal plain. Remaining glacial isostatic adjustment might be responsible for up to 15 m of shoreline uplift, but with virtually no variation along strike. The 3D flexural response to off-shore sediment loading is between +10 and -10 m of elevation change. Dynamic topography is the largest contributor to current elevation variations of these paleo-shorelines with a total amplitude >60 m since 3 Ma centered on the Norfolk Arch. Retrodicted paleogeographies that account for all these perturbations accord well with independently drawn paleogeographic reconstructions, a quite rigorous test of our calculations. Dynamic topography appears to have played a significant role in the evolution of the coastal plain that is unaccounted for by any model of passive margin evolution and may significantly contaminate signals related to sea level variations from these and other settings.