SEA-FLOOR GEOLOGY IN CENTRAL RHODE ISLAND SOUND SOUTHEAST OF POINT JUDITH, RHODE ISLAND

The U.S. Geological Survey (USGS) and the National Oceanic and Atmospheric Administration (NOAA) collaboratively study sea-floor environments off the northeast coast of the United States. During 2008, NOAA survey H11996 collected multibeam-echosounder data in a 65-square-kilometer area in central Rhode Island Sound, southeast of Point Judith, Rhode Island. During 2010, the USGS collected bottom photographs and sediment samples from 25 stations in this study area and used these data to interpret distributions of sea-floor features including scour depressions, sand waves, trawl marks, and dredge spoils. Scour depressions, which have steep sides, relatively flat floors, and a variety of shapes spanning 10s to 100s of meters (m) wide and about 0.5 m deep, cover the bathymetric highs in much of the study area. Sediments on the floors of the scour depressions tend to be coarser grained than those on the surrounding sea floor. Sand waves, having east-west oriented crests and wavelengths up to about 40 m, are located mostly in the southwest. Trawl marks are concentrated on the flanks of bathymetric highs across the northern part of the study area. Dredge spoils from the Providence River and Harbor are located at disposal site 69b in a bathymetric low in the western end of the study area. Sediments at most stations are sand or silty sand, but sediments at eight of the stations are boulders, gravel, or gravelly sediment. Coarser sediments are generally located on top of bathymetric highs where higher energy sedimentary environments are prevalent and bottom photographs show boulders and cobbles are commonly overgrown with hydrozoans and anemones. The bathymetric high in the northwestern part of the study area has a bouldery, hummocky appearance suggesting it is a submerged part of the Point Judith moraine, a segment of the 18,000-year-old Harbor Hill-Roanoke Point-Charlestown-Buzzards Bay moraine line. Finer grained sediments dominate in bathymetric lows and on the flanks of bathymetric highs where lower energy sedimentary environments prevail; burrows, accumulations of shells and shell debris, amphipod communities, and worm tubes are common in these areas.