PLEISTOCENE CHANNEL SEQUENCES ENTRENCHED IN THE MIOCENE CHESAPEAKE GROUP SUBSTRATE, TALBOT AND DORCHESTER COUNTIES, MD

Preliminary data from sediment cores in Talbot and Dorchester Counties, MD present previously unrecognized units and facies relationships in the Miocene and overlying Pleistocene stratigraphy. Cores discussed here include two 30+ m monitoring wells (~.5 mile centers off Egypt Road south of Cambridge, MD), a ~215 m continuous core at the Easton Wastewater Treatment Plant (~ 4 miles southeast of Easton, MD), and a ~290 m core near the Cambridge Dorchester Municipal Airport (~3.3 miles SE of Cambridge, MD). Additional shallow auger holes and vibracores supplement these deeper cores to provide a more complete picture of the subsurface.

The Miocene lower Chesapeake Group in this region has been deeply and complexly cut and filled by Pleistocene channels of the early Susquehanna and Choptank River drainage systems. The common substrate for Pleistocene channels is a reddish and greenish clay of the upper Choptank Formation that has not been described in up-dip locations. Lateral transitions in and out of channel fill deposits are abrupt. For example, the Miocene-Pleistocene contact ranges in depth from ~5.0-20.0 m below present land surface between cores spaced just 650 m apart. This undulatory contact indicates high relief on the Miocene surface during Pleistocene sea level lowstands. This antecedent topography was filled with laterally variable transgressive sequences during subsequent interglacials. Closely spaced cores of the Pleistocene Kent Island Formation reveal multiple fill packages of low stand (fluvial), transitional rising stage (estuarine), high stand (open bay-marine), and falling stage systems tracts that grade into and interfinger with one another in complex relationships.

The lateral and vertical variability of the west-central Delmarva stratigraphy indicates that this low-elevation region has long been sensitive to changes in sea level; tectonic effects can be postulated but are unproven. Emerging data from this area presents a great opportunity for studying the complex effects of changing climate and tectonic processes on facies relations beneath the Atlantic Coastal Plain. The acquisition of subsurface data from high-density drilling, geophysics, and new dating tools proves vital for accurate geologic mapping of these units.