GOLDEN SPIKES IN THE DELAWARE COASTAL PLAIN: ADVANCES IN SUBSURFACE STRATIGRAPHY THROUGH WIRELINE CORING

The use of wireline coring in Delaware over the last 20 years has caused a step change in understanding the subsurface geology of the Delaware Coastal Plain. Wireline cores reveal details of sedimentology, micropaleontology, sequence stratigraphy, and age of significant stratigraphic events in important aquifer intervals and confining beds, as well as the nature of the crystalline basement rocks that lie beneath the Coastal Plain. The value of wireline cores was highlighted in a Delaware Geological Survey (DGS) project in 1999-2000 to investigate faulting and earthquake risk potential in the northern part of the Coastal Plain (New Castle). Two coreholes drilled by Gene Cobbs II and Gene Cobbs III of the United States Geological Survey (USGS) resulted in improved understanding of the stratigraphy of the non-marine Cretaceous Potomac Formation and structure of the underlying basement rocks. Later in 2000 the USGS team drilled a deep (1,470 ft) core hole along the Delaware Coast (Bethany Beach), part of International Ocean Discovery Program (IODP) Leg 174AX, that yielded an outstanding record of coastal plain sedimentation and sea-level change from the Oligocene to the Quaternary. The DGS began its own wireline coring program in 2002 using the DGS drill rig with technical assistance by the USGS drillers. Between 2004 and 2017, DGS driller Steve McCreary cored 16 sites, reaching depths of as much as 720 ft. USGS drilling by Gene Cobbs III and Jeff Grey in 2012 obtained deeper core records from two core sites in central Delaware that sampled Quaternary to Upper Cretaceous section. These coring projects have greatly improved understanding of the stratigraphy and geological characteristics of the Cretaceous (Potomac) aquifers of northern Delaware, the Miocene (Cheswold, Frederica, and others) aquifers of southern Delaware, and the Paleogene (Rancocas) aquifer of central Delaware. The cores have also provided a subsurface context for surficial geological mapping and permit the basement rocks to be tied to rock units that outcrop in Delaware’s Piedmont. The results of two decades of DGS and USGS coring in Delaware provide invaluable geological “golden spikes” that allow details of subsurface geology to be calibrated to geophysical logs and extrapolated broadly around the Delaware Coastal Plain.