POST-DEPOSITIONAL DISPLACEMENT OF PLIOCENE SEA LEVEL IMPRINTS ALONG THE SE COAST OF THE USA

Proxy data suggest that during the Mid-Pliocene Warm Period (MPWP) atmospheric CO2 levels were roughly similar to today (between 350 and 450 ppmv) and that global average temperature was elevated by as much as 3°C with respect to pre-Industrial values. Estimates of sea level (SL) during the MPWP range from +10 m to more than +40 m relative to present (Raymo et al., 2011, Nature Geoscience), reflecting uncertainties in our knowledge of the sensitivity of the East Antarctic, West Antarctic and Greenland Ice Sheets to modest climate warming.

The southeast US coastal plain is of historic importance in MPWP SL studies (Dowsett and Cronin, 1990, Geology). Since the late 1960s, the Pliocene shorelines in this area have been the subject of numerous studies aimed at defining shoreline traces, fossil deposits and elevation. In North and South Carolina, the Orangeburg Scarp can be regarded as the shoreline of the Duplin formation. Further south, in Georgia and north Florida, the Trail Ridge shoreline is associated with the Wicomico Formation, and in South Florida, it is known as the Haines City Ridge (Winker and Howard, 1977).

In this study we present the results of a field expedition aimed at documenting the elevation of MPWP shoreline traces from North Carolina to Florida. High-accuracy GPS vertical measurements were acquired across the topographic-geomorphic margin of the Pliocene and younger shorelines. The shoreline margin was identified and mapped on Digital Elevation Models at 33 locations. Six of these field surveys showed a particularly well-defined shoreline morphology with excellent horizontal continuity.

Field data and analysis of DEMs provide evidence that the Pliocene shoreline margin is complex and is the result of numerous, high-frequency oscillations that were imprinted during a general regressive phase of the Pliocene associated with the buildup of continental ice at the onset of the Quaternary. Analyzed in light of glacial isostatic adjustment, the shoreline is inclined rising to the north as much as 50 m across the study area, mostly as a result of post-depositional deformation due to processes related to dynamic topography.