SURFICIAL GEOLOGIC MAP OF THE BLACKWATER NWR, MD INTERPRETED FROM LIDAR AND COREHOLE DATA

A LiDAR digital elevation model (DEM) with 30 cm contour intervals was produced for the Blackwater NWR (on the Blackwater, Golden Hill, and Taylors Island 7.5” Quadrangles) in 2002. The DEM was originally produced to forecast the impact of rising sea level on the wetlands and low lying terrain during the next century, but its high resolution has facilitated a new interpretation of the geomorphology of the area and a new surficial geologic map. The geomorphic history has been interpreted from cross-cutting relationships of deposited and eroded landforms. We combine the geomorphology with data derived from numerous shallow (<30 m deep) cores to produce a process-based map of the near-surface geologic framework.

Surficial and shallow subsurface deposits in the study area include Upper Miocene to late Holocene sediments from fluvial, estuarine, eolian, and littoral environments. The dramatic impact that Pleistocene to Holocene climate variability has had on the Blackwater area is clearly evident in mapping the surficial geology of this region. The sub-surface Pleistocene map units represent estuarine platforms with inset cut-fill river deposits that result from rising and falling sea levels. This dissected estuarine platform is the local substrate for a complex allostratigraphic unit consisting of large dune fields, sand sheets, fluvial deposits, and raised-rim ephemeral ponds. This assemblage of surficial deposits is an artifact of a former landscape that was constructed, eroded and modified by cold climate processes during the Last Glacial Maximum (LGM). When climate began to warm after the LGM, the drainage system was deeply incised. Holocene sediment filled the incised valleys as sea level rose, and sedimentation continues in the topographic lows today. Late Holocene deposition predominantly consists of organic-rich silty peat and underlies most of Blackwater Lake and the surficial wetlands.

The surficial map and subsurface data help to define the antecedent landscape and ~5,000 year history of wetlands fill in the Blackwater River valley. Climate change, sea level rise, subsidence, and land use have all contributed to the complex geologic history of this landscape.