SEA-FLOOR CHARACTER AND SEDIMENTARY ENVIRONMENTS OFFSHORE OF EDGARTOWN HARBOR, MARTHA'S VINEYARD, MASSACHUSETTS

Gridded bathymetry and sidescan-sonar imagery together cover approximately 37.3 square kilometers of sea floor in the vicinity of Edgartown Harbor, Massachusetts. Although originally collected for charting purposes during NOAA hydrographic survey H11346, these data, and the seismic-reflection lines and sampling and photographic stations occupied to verify them, show seabed terrain and composition, provide information on sediment transport and benthic habitat, and are part of an expanding series of studies that provide a fundamental framework for research and management activities (e.g., windfarms, cables, and dredging) along the Massachusetts inner continental shelf.

Other than adjacent to shorelines, the shallowest areas occur on an elongate bathymetric high that extends northeastward from Middle Flats. Most gradients are relatively gentle except in constricted areas where tidal currents maintain steeper slopes. Distinctive tonal patterns revealed by sidescan-sonar include: 1) complex patches of high and low backscatter targets that delineate bouldery areas on the seaward extension of Middle flats, suggesting sedimentary environments characterized by processes of erosion or non-deposition; 2) alternating bands of high and low backscatter caused by sand waves southeast of Oak Bluffs and off Cape Poge, indicative of processes associated with coarse bedload transport; 3) high backscatter caused by coarser-grained sediments and shell beds, composed predominantly of slippersnail (Crepidula) shells; and 4) low backscatter from finer-grained sediments in low-energy depositional environments of inner Edgartown Harbor. Interpretations of seismic profiles reveal a stratigraphic section composed of Late Cretaceous to Tertiary age coastal-plain deposits unconformably overlain by Pleistocene deposits of till and stratified outwash. This glacial drift was subsequently cut by streams flowing generally toward the northeast. Channels created by these streams, now mostly filled by estuarine and transitional deposits, coalesce into dendritic patterns. An unconformity cut during the last transgression separates these older deposits from overlying modern marine deposits. The marine sediments are thickest in areas more protected from waves and currents and thin or absent in more exposed areas.