GEOLOGIC MAP OF OFFSHORE DELAWARE: RECONCILING A TIME-TRANSGRESSIVE, AGGREGATE CORE DATASET WITH RECENT HYDROGRAPHIC AND SUBSURFACE SEISMIC REFLECTION DATA

The Delaware Geological Survey (DGS) has published a 1:40,000-scale map of the state’s offshore geology, entitled ‘Geologic Map of Offshore Delaware’. Geologic Map 25 displays seafloor-surficial geologic units of the inner continental shelf along the 40 km-long Atlantic coastline from Cape Henlopen to Fenwick Island. It covers State and Federal waters to an offshore distance of around 15 km, where water depths are on the order of 25 m. The map is based on an assessment of over 450 vibracore records and around 500 km of high-resolution subsurface ‘chirper’ seismic reflection data.

Information archived with the DGS, which includes lithologic logs, core photographs, texture information, AAR age estimates, and radiocarbon ages of select materials, was evaluated spatially in context of modern seafloor topography, as captured by 2007 USGS hydrographic survey datasets. A stratigraphic framework model based on subsurface geophysical information, collected in 2013-2015, established boundaries between map units at the seafloor. This helped reconcile the time-transgressive nature of the core dataset, which encompassed data from 1970 to 2017. Detailed descriptions of depositional units, distinguished based on texture, color, structure, and age, are included and discussed in an evolutionary context. Geologic cross sections reveal how distribution and thickness of sediment facies relate to antecedent topography, particularly paleovalley occurrence.

The geologic information contained within this map is useful for offshore sand-resource assessment. It has additional implications for offshore infrastructure projects such as windfarm and submarine cable siting, and enhances our understanding of Holocene coastal evolution and modern shelf sedimentary dynamics in Delaware.