SYNTHESIS OF THE PALYNOSTRATIGRAPHY OF THE POTOMAC FORMATION IN THE MIDDLE ATLANTIC REGION OF THE UNITED STATES AND IMPLICATIONS FOR AQUIFER CORRELATION

Confined aquifer sands of the Potomac Formation provide critical groundwater resources in many parts of the densely populated Washington, DC, to New York, NY corridor. The distribution and connectivity of these sands is complex because of the fluvial environment of deposition. We have assembled a regional palynostratigraphic synthesis for this unit that better resolves the aquifer stratigraphy in an area of interest extending from the Coastal Plain of northeast Maryland to northern Delaware to southern New Jersey.

The pollen record of the Potomac Formation records of the dawn of the angiosperms, with related floral changes contributing to recognition of palynomorph zones. The first zonation, established by Brenner (1963), created two zones and a number of subzones in Maryland outcrops and boreholes, mostly using spores, with ages based on ties to European floras. This zonation was refined by Doyle and Robbins (1977) and extended into higher strata utilizing the ranges of numerous angiosperm pollen types, largely from two Delaware boreholes. We have used these previous studies as a starting point for a palynostratigraphic synthesis that encompasses more than 20 outcrop and borehole locations examined in our own studies in Delaware as well as data from other studies in Maryland and New Jersey. The palynological assemblages identified at these sites provide independent criteria that supplement geophysical log interpretations to constrain subsurface correlations within the Potomac Formation in this area of interest.

Palynostratigraphy confirms a stratigraphic model in which southeastward-dipping strata a) onlap a basement unconformity at the bottom of the formation and b) are overlain by Cretaceous and Pleistocene unconformities that truncate successively older Potomac strata updip. These correlations suggest that three lithologic subdivisions can be recognized and correlated in much of the region. However, occurrences of stratigraphically useful pollen and spores are irregularly distributed vertically and geographically. Although palynomorph-bearing dark clay lithologies are common, much of the Potomac Formation consists of paleosols and fluvial sands that do not yield spores or pollen. The irregular nature of palynomorph occurrences limits the resolution of stratigraphic control.