Summary

The U.S. Geological Survey (USGS), in cooperation with the National Oceanic and Atmospheric Administration (NOAA), is producing detailed maps of the sea floor off the coast of southern New England. The current phase of this cooperative research program is directed toward studies of sea-floor topography and its effects on the distributions of sedimentary environments and benthic communities. The data and interpretations in this report are based on the combined NOAA surveys H12009, H12010, H12011, H12015, H12033, H12137, and H12139 completed during 2009 in Block Island Sound and a USGS verification cruise completed during 2011. Together these data provide a fundamental framework for research and management activities along this part of the coastline, show the composition and terrain of the seabed, and provide information on sediment transport and benthic habitat.

The bathymetry, surface-sediment distributions, and geomorphology of Block Island Sound are products of Quaternary history and modern environmental conditions. The shallowest and deepest parts of the study area are high-energy areas characterized by sedimentary processes associated with erosion and nondeposition. Boulders and gravel cap bathymetric highs where lag deposits of Pleistocene drift are exposed, such as on submerged segments of the Point Judith and Ronkonkoma-Nantucket moraines and the interlobate deposits around Block Island. The deepest parts of the study area are associated with tidal scour and occur along its western edge and near the passage between Block Island and the mainland.

Environmental energy levels fall as depth increases and where tidal flow becomes less constricted, and sand replaces gravel as the dominant surface sediment. The sand is typically medium to coarse grained and current rippled, and fields of sand waves and megaripples are present in places such as at the entrances to the passage between Block Island and the mainland and near the southwestern entrance to the sound. These bedforms and obstacle marks reflect sedimentary environments characterized by processes associated with coarse bedload transport and can be used to interpret net sediment transport directions.

Elsewhere, most deeper parts of the study area are characterized by a faintly rippled to undulating, bioturbated seabed composed of Holocene fine-grained sand and slightly muddy sand (less than 20 percent mud), reflecting sedimentary environments dominated by processes associated with sorting and reworking. Shell beds are common in these areas, but the beds are thin and ephemeral. The sea floor is flatter, muddier, and more heavily bioturbated in the protected area west-northwest of Block Island. We interpret the sedimentary environments within this area to be characterized by processes associated with deposition.

Scour depressions, formed and maintained primarily by high-energy storm-related events, control much of the benthic diversity in the eastern part of the study area. These depressions occur in a variety of shapes and configurations on the crests and flanks 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 on the floors of the depressions are almost always coarser than those on the surrounding sea floor. We contend that the resultant close association of muddy sand- and gravel-dependent communities promotes regional faunal complexity.