SIDESCAN-SONAR IMAGERY AND SURFICIAL GEOLOGIC INTERPRETATIONS OF THE SEAFLOOR IN CENTRAL RHODE ISLAND SOUND

In 2004, the National Oceanic and Atmospheric Administration (NOAA) conducted Survey H11321 in central Rhode Island Sound. Sidescan-sonar and bathymetric data were acquired within a 93 km² area of seafloor located 12 km southeast of Brenton Point, Rhode Island. These data were used in conjunction with historic seismic-reflection data (Needell and others, 1983) to interpret the seafloor geology and sedimentary environments, and provide a geologic framework for additional marine research and coastal resource management studies. Four sedimentary environments are observed in the study area, including those characterized by processes of: 1) erosion or nondeposition, 2) coarse-grained bedload transport, 3) sorting and reworking, and 4) fine-grained deposition. Sidescan-sonar and bathymetric data reveal two northeast-southwest oriented, broad, moderate- to high-backscatter, low-relief ridges in the northern and southeastern parts of the study area. The ridges are topped with fields of sand waves that reflect a sedimentary environment characterized by coarse-grained bedload transport. Sand-wave crestlines in the northwest are oriented north-south and have 10- to 20-m wavelengths, whereas those in the east and south are oriented east-west and have 20- to over 100-m wavelengths. The southern side of the northern ridge contains a series of curvilinear, northeast-southwest oriented benches that are interpreted here as indicators of paleoshorelines. Historic seismic-reflection profiles show that the northern ridge is underlain by glaciolacustrine sediments, assumed to be Wisconsin in age. A central channel, which is 8 to 9 m deep and floored by low-backscatter Holocene marine and transitional sediments, stretches southwestward across the study area. Trawl marks within the channel are clearly visible within sidescan-sonar imagery as lines of low backscatter or paired high and low backscatter. Sedimentary environments characterized by sorting and reworking and deposition are found in the channel.