Mid-Pliocene shorelines of the US Atlantic Coastal Plain

For nearly a century, the Atlantic Coastal Plain (ACP) of the United States has been the focus of studies investigating Pliocene and Pleistocene shorelines, however, the mapping of paleoshorelines was primarily done by using elevation contours on topographic maps. Here we review published geologic maps and compare them to paleoshoreline locations obtained through geomorphometric classification and satellite data. We furthermore present the results of an extensive field campaign that measured the mid-Pliocene (~ 3.3-2.9 Ma) shorelines of the Atlantic Coastal Plain using high-accuracy GPS and digital elevation models. We compare our new dataset to positions and elevations extracted from published maps and find that the extracted site information from earlier studies is prone to significant error, both in the location and, more severely, in the elevation of the paleoshoreline. We also investigate, using geophysical modeling, the origin of post-depositional displacement of the shoreline from Georgia to Virginia. In particular, we correct the elevation of our shoreline for glacial isostatic adjustment (GIA) and then compare the corrected elevation to predictions of mantle flow-induced dynamic topography (DT). While a subset of these models does reconcile the general trends in the observed elevation of the mid-Pliocene shoreline, local discrepancies persist. These discrepancies suggests that either (i) the DT and GIA models presented here do not capture the full range of uncertainty in the input parameters; and/or (ii) other influences, such as sediment loading and unloading or local fault-driven tectonics, may have contributed to post-depositional deformation of the mid-Pliocene shoreline that are not captured in the above models. In this context, our field measurements represent an important observational dataset with which to compare future generations of geodynamic models. Improvements in models for DT, GIA and other relevant processes, together with an expanded, geographically distributed set of shoreline records, will ultimately be the key to obtaining more accurate estimates of eustatic sea level not only in the mid-Pliocene but also earlier in the Cenozoic