GEOLOGIC MAPPING OF THE MASSACHUSETTS INNER CONTINENTAL SHELF FOR SCIENCE-BASED MANAGEMENT OF OCEAN RESOURCES

The U.S. Geological Survey, in cooperation with the Massachusetts Office of Coastal Zone Management, is mapping the shallow seafloor (depth < 100 m) in the vicinity of Cape Ann, Massachusetts. The need for geologic and bathymetric maps is driven by ongoing management concerns about declining fisheries and the impacts of offshore construction projects (e.g., pipelines, wind farms, and liquified natural gas terminals). Maps depicting sediment type, distribution, and thickness support new ecosystem-based approaches to managing ocean resources. Accurate depictions of surficial geology and associated biota on the seafloor are important first steps toward protecting essential fish habitat, delineating marine reserves, and assessing changes in habitat due to natural or human impacts. Approximately 450 km² of the inner shelf were mapped at 1:25,000 scale, using data from interferometric and multibeam sonars (swath bathymetry), sidescan sonar (acoustic backscatter), chirp seismic-reflection profiling (stratigraphy and structure), direct sampling and bottom photography/video. The glaciated, bedrock-framed seafloor of the region is characterized by extreme changes in bathymetric relief and covered with a wide variety of surficial materials. The bathymetry and surficial geology are complex, due to widespread bedrock outcrops, as well as the influence of deglaciation and multiple transgressive/regressive events over the last 18,000 years.

The challenge is how to represent accurately the great heterogeneity of geologic environments in a format that is useful to managers and policy makers. For regional mapping purposes, we define six physiographic zones: Nearshore Basin, Rocky Zone, Shelf Valley, Nearshore Ramp, Bay-Mouth Shoal, and Outer Basin. The zones are delineated based on seafloor morphology (depth, slope, relief), and the dominant characteristics of seafloor materials (bedrock, sediment grain size, thickness of sediment deposits). Although considerable variation exists within each zone, this method allows efficient mapping of large areas and presents geologic information in a readily useable format.