U.S. Geological Survey Open-File Report 2011–1004
Sea-Floor Geology and Character of Eastern Rhode Island Sound West of Gay Head, Massachusetts
The USGS, in cooperation with NOAA and the Massachusetts Office of Coastal Zone Management, is producing detailed maps of the sea floor off the coast of southeastern 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 NOAA survey H11922 completed during 2008 in eastern Rhode Island Sound and two USGS verification cruises completed during 2010. 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.
Surveyed depths within the study area range from less than 24 meters to almost 43 meters. The deepest part of the study area occurs in its southwestern corner, from which depths progressively shallow both shoreward to the north and northeastward to where the shallowest depths occur. One exception is a large isolated bathymetric high located in the south-central part of the study area that rises 6 to 13 meters above the surrounding sea floor.
The surficial-sediment distribution is a product of the Quaternary history and modern environmental conditions. Gravel is prevalent on the floors of scour depressions and on top of bathymetric highs where lag deposits of Pleistocene drift are exposed. These high-energy areas are characterized by sedimentary processes associated with erosion and nondeposition. Fine-grained sand is the dominant sediment elsewhere in the shallower parts of the study area. The sand is typically current rippled and, in places, small fields of megaripples are present. These bedforms reflect sedimentary environments characterized by processes associated with coarse bedload transport. Most of the deeper parts of the study area are characterized by an undulating to faintly rippled seabed composed of Holocene muddy sand, which reflects environments dominated by processes associated with sorting and reworking.
Scour depressions, formed and apparently maintained by high-energy storm-related events, control much of the benthic diversity on the seaward-facing slopes adjacent to bathymetric highs in the study area. The entire Holocene section has been removed within these depressions, exposing the winnowed relict Pleistocene surface. We contend that the resultant close association of sand- and gravel-dependent communities, promotes regional faunal complexity.