GEOLOGIC MAPPING OF BOSTON HARBOR AND APPROACHES

The Massachusetts Office of Coastal Zone Management (CZM), the U.S. Geological Survey (USGS), and the National Oceanic and Atmospheric Administration - National Ocean Service (NOAA-NOS) began a cooperative program in 2003 designed to map the geology of the Massachusetts inner shelf. The goal of this partnership is to create a series of high-resolution geologic maps of the seafloor at a scale of 1:25,000 that depict the varied topography and surficial sediment distribution in shallow nearshore waters (i.e., inside the 3-mile state limit). Geologic seafloor mapping data supports management of ocean resources and monitoring of natural and anthropogenic environmental changes. This recently completed series of 5 maps, which encompasses Boston Harbor and approaches, is based on sidescan-sonar and bathymetric data from hydrographic surveys conducted in 2000-2001 by NOAA-NOS. Bottom photographs, video and sediment grab samples were collected in 2004 to assist in interpretation of the geophysical data. The integration of geophysical data and new sampling data provide an improved understanding of the geologic framework and insight into the geologic and anthropogenic processes that have shaped the sea floor in the region.

Six seafloor map units were defined based on bathymetric relief, depth, slope, acoustic backscatter, and analyses of sediment samples and photographs of the sea floor. Boston Harbor is a formerly glaciated landscape with numerous drumlins (harbor islands) separated by submarine channels. The seafloor morphology varies from gently sloping sub-tidal flats to areas of rugged elevated topography. The seabed of the Inner and Outer Harbor is typically fine-grained sediments and has been extensively dredged for navigation purposes. The harbor approaches are characterized by rugged topography created by exposed bedrock and boulders, and areas of smooth topography that are generally composed of sand. Many anthropogenic features (e.g. tunnels, dredge spoil dumps, shipwrecks) are resolved by the high-resolution (2 meters/pixel) data.