SEA-FLOOR GEOLOGY IN NORTHWESTERN BLOCK ISLAND SOUND, RHODE ISLAND

Multibeam-echosounder and sidescan-sonar data, collected by the National Oceanic and Atmospheric Administration in a 69-square-kilometer area of northwestern Block Island Sound, are used with sediment samples, video, and still photography of the sea floor, collected by the U.S. Geological Survey at 43 stations within this area, to interpret the sea-floor features and sedimentary environments. Sea-floor features include boulders, scour depressions, sand waves, accumulations of modern marine sediments, and trawl marks. Boulders, which are often several meters wide, are found in patches in shallow water depths and tend to be overgrown with sessile flora and fauna. They are lag deposits of winnowed glacial drift, and reflect high-energy environments characterized by processes associated with erosion and nondeposition. Scour depressions, which are about 0.5 meter (m) lower than the surrounding sea floor, have floors of gravel and coarser sand than bounding modern marine sediments. These scour depressions, which are conspicuous in the sidescan-sonar data because of their more highly reflective, coarser grained floors, are likely formed from erosion by storm-generated, seaward-flowing currents and then maintained by bottom-current turbulence caused by their coarse sediments. Sand waves and megaripples, which occur in environments of coarse-grained bedload transport, tend to have crests that trend either parallel to shore with 20- to 50-m wavelengths or perpendicular to shore with several-hundred-meter wavelengths. The orientation of sand-wave crests reflects sediment transport directions perpendicular to shore by wave-driven currents and parallel to shore by tidal and wind-driven currents, respectively. Areas of the sea floor with modern marine sediments tend to be relatively flat to current-rippled and sandy. Within these otherwise featureless areas, anthropogenic traces such as trawl marks can be discerned in the bathymetric data.