POST-GLACIAL FORMATION OF THE MODERN CHESAPEAKE BAY—PALEOSHORELINES FROM LGM TO PRESENT

We document the formation and growth of Chesapeake Bay using a series of paleoshorelines based on: 1) the depth to the Cape Charles seismic reflector as mapped by Coleman and Halka (1989) and Coleman and Hobbs (1989); 2) the ca. 10-0 ka relative sea level rise history of Cronin et al. (2007); and 3) with some adjustments, the late Wisconsinan (ca 20-10 ka) sea level rise history predicted by the models of Peltier (2002). We postulate that the mouth of the LGM Susquehanna was located at the shelf-edge site where NOAA bathymetric mapping and later investigators discovered linear pockmarks associated with underlying gassy deltaic deposits.

Examples of high-resolution chirp profiler data from the Chesapeake Bay mainstem are presented to illustrate the seismic character of the Cape Charles reflector and its partial to total masking by sand in some parts of the Bay, and, particularly over the Wisconsinan paleochannel system, by biogenic methane gas. We use results from deep sediment coring that penetrated the Cape Charles Formation at several sites undertaken on the R/V Marion-Dufresne in 1999 and 2003 to calibrate and interpret the paleoshoreline maps. Error sources are evaluated, including: 1) the Chesapeake post-LGM RSLR history, which affects only the ages of paleoshorelines; 2) possible regional differences in Chesapeake RSLR history; and 3) the depth of the Cape Charles reflector at time of formation, below paleo-sea level. Modern shoreline erosion processes show the magnitude of the latter error. Unconsolidated Quaternary sediments may be eroded to ca. 2 m below sea level in the littoral zone along the exposed Eastern Shore, but perhaps only a few decimeters where the Bay is advancing into older sediments (e.g., along the Calvert Cliffs on Maryland’s western shore). However, once formed, this erosional surface may be covered by some thickness of coarser sediments (e.g., beach sands); the top of these coarse sediments created the Cape Charles reflector.