SURFICIAL GEOLOGY OF CENTRAL NARRAGANSETT BAY, RHODE ISLAND: INTERPRETATIONS OF SIDESCAN SONAR AND MULTIBEAM BATHYMETRY

The U.S. Geological Survey is working cooperatively with the National Oceanic and Atmospheric Association (NOAA) to map surficial geology in estuaries along the coast of the northeastern United States. In this study of NOAA Survey H11310, high-resolution bathymetric, sidescan-sonar, and textural data were used to map surficial sediment distribution and sedimentary environments in central Narragansett Bay. Three sedimentary environments were found in the study area: fine-grained deposition, sediment sorting and reworking, and erosion or nondeposition. Areas exhibiting low backscatter in the sidescan-sonar imagery usually have fine-grained sediments and were interpreted to be depositional environments. These areas have a smooth seafloor and are located in about 7-m to over 20-m deep water where the depth and bathymetry protect these environments from wind- and tide-driven currents. Moderate backscatter and fine sands are typical in environments of sediment sorting and reworking. These areas transition from environments of deposition to erosion or nondeposition and are located where the seafloor varies in depth. High backscatter and coarse-grained gravel and gravelly sediment characterize erosional or nondepositional environments, which are located in shallow or constricted areas where currents winnow the sediments. Sedimentary environments of erosion or nondeposition tend to be located near-shore, while depositional environments are generally further from shore and sorting and reworking environments are between the two. The sidescan-sonar mosaic shows an area of striped backscatter, interpreted as erosional pathways, oriented down-slope and funneling into a bathymetric low. The mosaic also shows anthropogenic features including pipelines or cables, shipwrecks, piers, and pilings. Circular depressions, about 40 m in width, interpreted to be pockmarks from sediment outgassing are also apparent in the sidescan-sonar mosaic and bathymetry imagery.