GEOLOGIC INTERPRETATIONS OF SIDESCAN-SONAR AND BATHYMETRIC DATA FROM WESTERN RHODE ISLAND SOUND

The U.S. Geological Survey (USGS) and National Oceanic and Atmospheric Administration (NOAA) are collaboratively interpreting seafloor geology along the northeastern coast of the United States. In 2004, the NOAA Ship Rude completed survey H11322, a sidescan-sonar and bathymetric survey covering about 60 square kilometers of seafloor in western Rhode Island Sound. Interpretations of NOAA sidescan-sonar and bathymetric data and USGS sub-bottom seismic-reflection profiles are used to delineate the geologic framework, seafloor features, and sedimentary environments in the study area. The seafloor in Rhode Island Sound is composed of Wisconsinan glacial drift and Holocene marine and transitional (fluvial and estuarine) sediments, which are underlain by Late Cretaceous and Early Tertiary Coastal-Plain sediments and Paleozoic bedrock. Vertical-beam echo-sounder data reveal three bathymetric highs that are as shallow as 25 m and are separated by a channel system that reaches 40 m in depth. Features and patterns in the sidescan-sonar imagery include low, moderate, and high backscatter; sand waves; scarps; fine-grained erosional outliers; boulders; trawl marks; and dredge spoils. The interpretation of backscatter and bathymetric data reveal four sedimentary environments in the study area. Environments characterized by erosion or nondeposition are located on bathymetric highs where high backscatter is produced from coarse sediment. Also in shallower areas are environments characterized by coarse-grained bedload transport, indicated by sand waves. Environments characterized by deposition are located in relatively deep areas where lower backscatter results from fine-grained sediment. Environments characterized by sorting and reworking processes prevail between bathymetric highs and lows where moderate backscatter suggests the seafloor consists of muddy sand.