33 YEARS OF USGS CONTINUOUS CORE DRILLING IN THE MID-ATLANTIC COASTAL PLAIN: KEY TO UNDERSTANDING LITHO-, BIO-, CHRONO- AND SEQUENCE-STRATIGRAPHIC RELATIONSHIPS

33 years of USGS core drilling in the mid-Atlantic Coastal Plain has dramatically improved our understanding of its geologic and hydrogeologic stratigraphic framework. Since the drilling of the Oak Grove Core in 1976 the USGS has obtained several dozen cores from both up- and down-dip that vary from ~15-500 m. These cores provide essential data for the accurate dating and characterization of lithic units. Geophysical logs of the coreholes provide guidance for interpretation and correlation with other boreholes. Some cores have provided samples for water quality (core squeezing) and extremophile (bacterial) investigations while others have provided the most accurate characterization of aquifers and confining units and guided monitoring wells screen emplacement. Lithio-, bio-, chrono-, and sequence stratigraphic analyses of the cores reveal and allow mapping of unconformity-bounded allostratigraphic units with variable facies, distribution and preservation. The resultant detailed chronostratigraphy enables accurate estimates of sedimentation rates. These cores often provide the most complete section of units and may become their allo-stratotype sections. The cores have led to the discovery of subsurface Oligocene stratigraphic units that have no outcrop in the mid-Atlantic. Cores have provided the accurate stratigraphic detail needed for mapping Coastal Plain structures such as the Stafford, Brandywine, and James River fault zones. These cores also provide ground truth for seismic interpretation. It was the drilling of the Exmore core that led to the discovery of mixed-age lithologies, shocked quartz and ultimately the Chesapeake Bay impact crater.