STRATIGRAPHIC CORRELATION OF THE HETEROGENEOUS CRETACEOUS AGED SEDIMENTS OF THE POTOMAC FORMATION IN NEW CASTLE COUNTY, DELAWARE THROUGH EXAMINATION OF CORE, WELL LOGS, AND PALYNOLOGY

The Potomac Formation in the Middle Atlantic region is an important aquifer to one of the most populated areas in the country. This study integrated palynology with stratigraphic analysis of wireline cores and geophysical logs to understand the distribution of lithologies in the Potomac Formation in New Castle County, Delaware. The aquifer quality sands in this formation were deposited in fluvial channels between muddier interfluvial paleosols and floodplain deposits. As a result, the formation is extremely heterogenous, making it difficult to understand and predict where the aquifer sands are located.

In total, data from nine boreholes was used for this study; six older boreholes and three more recent continuously cored borehole examined for the first time for this research. Due to the heterogeneity of the Potomac Formation, geophysical logs and sediment cores alone do not allow for accurate enough correlation. The palynological analysis provided essential stratigraphic constraints. Five palynological zones/subzones have previously been defined in the Potomac Formation, with accompanying age interpretations, based on the appearance and disappearance of fossil pollen and spores, mostly from angiosperms. The first definitive appearance of the angiosperms is in the Cretaceous Period. Their evolution can be traced through the fossil record in the Potomac Formation, thus making them stratigraphically very useful. The biostratigraphic zones recognized in these cores, combined with the core and well-log lithostratigraphy, can be used to more accurately understand the distribution of sediments. Specifically, the new biostratigraphic data indicate revised stratigraphic correlations within the Potomac Formation in the western part of the northern Delaware Coastal Plain, including the interpreted stratigraphic position of aquifer sands.

The biostratigraphic and lithostratigraphic studies allow the distribution of sediments in the Potomac Formation to be more accurately understood. The lithologic interpretations give insight into aquifer connectivity and can be used to better understand groundwater flow. The new biostratigraphic interpretations increase the extent of subsurface correlations and validate the zone interpretations identified in previous studies.