GEOLOGIC FRAMEWORK OF THE MASSACHUSETTS INNER CONTINENTAL SHELF: NEW RESULTS FROM HIGH-RESOLUTION SEAFLOOR MAPPING WITHIN BUZZARDS BAY, VINEYARD SOUND, AND WESTERN NANTUCKET SOUND

Between 2009 and 2011 the Buzzards Bay, Vineyard Sound, and western Nantucket Sound inner continental shelf was the focus of a collaborative seafloor-mapping project between the U.S. Geological Survey and the Massachusetts Office of Coastal Zone Management. Chirp, single-channel, and multi-channel seismic reflection, towed sidescan-sonar, swath bathymetry, and seafloor sediment sample, photograph, and video data were acquired over > 800 km² of the three coastal embayments. Synthesis and interpretation of these data and some previously collected data have yielded a series of high-resolution seafloor geologic maps that include nearly-seamless coverage of swath bathymetry, acoustic backscatter, surficial sediment texture, geologic outcrop, and structural and isopach renderings of shallow geologic framework components. The results build upon those of previous investigations that broadly defined the subsurface stratigraphy of basement and coastal plain rocks, late Pleistocene glacial sediments, and Holocene fluvial, estuarine, and marine deposits, and the corresponding geologic history related to the advance and retreat of Laurentide ice sheet lobes and subsequent Holocene sea level rise. While our interpretations mostly correlate with previous results, the higher density and spatial resolution of the new data enabled us to map the thickness and distribution of post-glacial sediments in greater detail. Additionally, we propose new interpretations regarding the recessional histories of the Buzzards Bay and Cape Cod Bay ice lobes on the basis of seafloor morphology and the spatial relations of newly differentiated seismic stratigraphic units interpreted to represent ice-marginal deposits.