U.S. Geological Survey Open-File Report 2007-1373
High-Resolution Geologic Mapping of the Inner Continental Shelf: Cape Ann to Salisbury Beach, Massachusetts
The inner continental shelf between Cape Ann and Salisbury Beach, Massachusetts, exhibits a complex assemblage of sediment textures related to the inherited geologic framework and reworking during postglacial sea-level changes. Rapidly changing environments associated with glaciation and deglaciation of the region have exposed the inner shelf to a variety of marine and terrestrial processes (fig. 4.12). High-relief bedrock is locally exposed on the seafloor (Rocky Zone, Shelf Valley) but otherwise is buried beneath relatively thick deposits of glacial and non-glacial sediment. The glacial deposits consist of till and glacial-marine sediment, which are overlain by deposits of fluvial, estuarine, deltaic, and marine sediment. The Merrimack River, one of the largest rivers in New England, delivered abundant sediment to construct a large sandy delta graded to a lowstand of sea level at a depth of about –50 m. Ongoing transgression across the paleodelta's upper surface (Nearshore Ramp) has removed significant material from shallow areas above the lowstand depth and eroded a distinct unconformity on top of the Pleistocene sedimentary sequence. Surficial deposits of Holocene sediment, which overlie the regional transgressive unconformity, are relatively thin and unevenly distributed. The mobile sandy sediments range in thickness from less than 0.5 m in the northern and central parts of the study area to 9 m in the south near Cape Ann. This distribution indicates long-term net transport of sediment in a southerly direction. These deposits contain approximately 121 million m³ of fine sandy sediment in the nearshore area alone, where we have sufficient seismic-reflection data to accurately map its thickness. Along with sediment derived from reworking of older deposits along the coast and inner shelf, sandy sediment supplied by the river has constructed a large barrier system. Modern processes interact with bedrock and glacial sediment to create the sandy beaches, tidal-inlet complexes (Ebb-Tidal Deltas), and other present-day landforms along the present shoreline. In depths below about –50 m, the seafloor was not exposed during the postglacial sea-level lowstand. A complete stratigraphic section is preserved in these deeper areas (Outer Basins) where fine-grained sediment is accumulating, presumably derived from erosion of shallow, formerly emergent areas of the shelf. These different sedimentary environments provide habitat for a variety of benthic-dwelling marine organisms. Geologic mapping at this scale is the first step toward determining the distribution, type, and quality of subtidal marine habitats in the Massachusetts coastal ocean.