SIDESCAN-SONAR IMAGERY, MULTIBEAM BATHYMETRY, AND SURFICIAL GEOLOGIC INTERPRETATIONS OF THE SEA FLOOR IN EASTERN RHODE ISLAND SOUND

Sidescan-sonar and multibeam echo sounder data collected during National Oceanic and Atmospheric Administration survey H11320 were used to interpret the surficial sea-floor geology in Rhode Island Sound off Sakonnet Point, Rhode Island. This study provides information on sediment distribution, which is important for the study of benthic habitats and provides a framework for future research. The study area consists of a central basin and two deeper bathymetric lows in the southwest surrounded by a bathymetric high to the north and a moraine in the southeast. The basins have relatively flat floors scattered with boulders, which are ubiquitous in much of the study area. Bathymetric highs are associated with the moraine, sand wave fields, and isolated tabular erosional outliers. Three sedimentary environments comprise the sea floor within the study area. Areas of erosion or nondeposition are primarily located on the flanks of ridges and on the moraine and are characterized by the presence of rocks and boulders and a rugged topography. Sedimentary environments characterized by coarse-grained bedload transport are located mostly on the northern bathymetric high where sand waves indicate active sediment transport. Environments characterized by sorting and reworking are located in the basins and on parts of the ridges where there is moderate backscatter in the sonar imagery and a lack of bedforms and gravelly sediment. Bathymetric lows are associated with a southwest-draining channel system that cut into the underlying glacial sediments during the last sea-level low stand. The moraine trends to the northeast appearing to align with the Elizabeth Islands-Buzzards Bay end moraine line, suggesting an age of 18 ka. The tabular erosional outliers are likely composed of cohesive glaciolacustrine sediments that were eroded during lake drainage and subsequent subaerial exposure of the inner shelf. Benches around the northern edges of the bathymetric lows are interpreted to be wave-cut terraces and may be a record of paleoshorelines. These features parallel the bathymetry and are present at depths similar to paleoshorelines reported elsewhere in Rhode Island and Long Island Sounds.