SEA-FLOOR GEOLOGY AND SEDIMENTARY ENVIRONMENTS OF BLOCK ISLAND SOUND, OFFSHORE RHODE ISLAND

Multibeam bathymetry, collected during 7 NOAA hydrographic surveys in Block Island Sound, was coupled with USGS data from sampling and photographic stations to map seabed morphology and composition, provide information on sediment transport and benthic habitat, and extend a series of studies providing a fundamental framework for research and management activities along the southern New England inner continental shelf. Bouldery lag deposits, such as on the submerged segment of the Point Judith moraine, and scour depressions, such as near the passage between Block Island and the mainland, reflect high-energy conditions characterized by sedimentary processes associated with erosion. Environmental energy levels fall as depth increases and tidal flow becomes less constricted. Fields of sand waves and megaripples, such as those near the southwestern entrance to the Sound, reflect sedimentary environments characterized by processes associated with coarse bedload transport. Sand-wave and obstacle-mark asymmetry reveal directions of net sediment transport. Elsewhere, most deeper parts of the study area are characterized by a faintly rippled to undulating, bioturbated seabed composed of Holocene fine-grained, slightly muddy sand, reflecting sedimentary environments dominated by processes associated with sorting and reworking. The sea floor is flatter, muddier, and more heavily bioturbated in the protected area west-northwest of Block Island where sedimentary environments are characterized by processes associated with deposition.

Shallow scour depressions, formed and maintained primarily by storm-waves, occur in a variety of shapes and configurations near crests of bathymetric highs and on seaward-facing slopes. Some end abruptly at their deeper extents; a few connect with channel systems that continue downslope. In places, the entire Holocene section has been removed within these depressions, exposing boulders and cobbles on the winnowed relict Pleistocene surface. In other places the depressions are floored by rippled sand and gravelly sediment, but sediments in the depressions are coarser than those on the adjacent sea floor. Our bottom photography shows that the resultant close association of muddy sand- and gravel-dependent communities promotes regional faunal complexity.