PLIOCENE MARINE TRANSGRESSION OF THE COASTAL PLAIN, EASTERN UNITED STATES, THE CASE FOR DYNAMIC TOPOGRAPHIC UPLIFT

The geology of the coastal plain of the Eastern U.S., with its complex record of onlapping and offlapping shorelines and associated facies, together with significant along strike variability in these patterns provides a natural laboratory for investigating the interplay of processes that impact the stratigraphic and topographic evolution of this region. Considerable recent interest has focused on the potential role of dynamic topography arising from buoyancy driven flows in the mantle and their potential effects on interpretations of long-term sea level. We focus on the Pliocene, specifically the various transgressive units that onlap unconformably across the coastal plain in VA, NC, SC, GA, and northern FL. In northern GA, across SC, and southern NC marine facies of the Duplin Fm extend westwards up to at least the Orangeburg Scarp where these strata attain elevations in excess of about 100 m, increasing towards the N. The northward continuation of the Orangeburg Scarp and the Duplin are erosionally truncated, making it impossible to trace the maximum heights farther north across northern NC and southern VA. Farther north in VA the Yorktown onlaps over the Fall Line and attains elevations in excess of 80 m at this southward facing erosional edge. Farther north the Yorktown zero edge swings east around MD and only younger Pliocene Beaverdam estuarine sediments are preserved in DE at low elevations (<30 m), and farther north in NJ marine Pliocene sediments are preserved only at very low elevations (<10 m) near Cape May. In all areas, this surface slopes toward the modern coast. The complex topography along the Pliocene flooding surface requires significant along strike and across strike differential vertical motions. Vertical motions arising from dynamic topography, flexural loading offshore, erosion-related flexural unloading onshore, together with glacial isostatic responses are incorporated in our analysis. We find considerable support for our recent dynamic topography models in which density contrasts in the upper mantle give rise to significant vertical motions, both uplift and subsidence along the eastern seaboard of the U.S. since the Pliocene, and directly contrary to other models that show only dynamic topographic subsidence.