Southeastern Section - 63rd Annual Meeting (10–11 April 2014)

Paper No. 3
Presentation Time: 8:40 AM


DEJONG, Benjamin D., The Johnson Company, 100 State Street, suite 600, Montpelier, VT 05602,

The Blackwater National Wildlife Refuge (BNWR) on the eastern shore of Maryland resides in a low-relief, low-elevation landscape that is presently threatened by rising sea level. Tide gauge data indicate that the sea level rose approximately 3.48 mm/yr relative to the land surface over the period from 1943-2006, almost twice that of the eustatic (global) rate of sea level rise (~1.8 mm/yr). So the land surface itself is subsiding at the same rate that sea levels are rising, which has been attributed to a range of potential natural and anthropogenic sources. Refuge managers are actively responding to these high rates of sea level rise by identifying suitable corridors for the landward migration of salt marsh habitat as well as mitigation options such as decreasing groundwater withdrawal in the region to reduce land subsidence rates.

This study uses lithology and geochronology derived from coring and augering to fully characterize the surficial geology of the BNWR landscape and explore the past behavior of sea level in this region. The focus of this investigation is the Kent Island Formation, which was derived from the most recent major depositional episode in the region. This unit was penetrated and analyzed in over 70 locations, from which 11 locations were selected to produce a chronology for deposition using OSL geochronologic methods.

Results suggest fluvio-estuarine deposition during marine isotope stages (MIS) 5a and 3, when eustatic sea levels were >20 m and >80 m lower than present, respectively. These results corroborate findings from sites in Virginia down to North Carolina that suggest that the regional sea level history of the mid-Atlantic is more complicated than patterns seen in eustatic sea level records. These data may support the interpretation that a proglacial forebulge affected regional land surface elevations during previous glacial cycles. If estuarine deposition during MIS 5a and 3 was made possible by a continuously collapsing forebulge that was uplifted during MIS 6, then the lag times associated with these processes (approaching 80 ky) suggest the potential for many more millennia of adjustment from a Last Glacial Maximum (~22 ka) forebulge and render prospects for reversing current subsidence trends via mitigation unlikely.