SEA-FLOOR GEOLOGY IN CENTRAL RHODE ISLAND SOUND, SOUTH OF SAKONNET POINT, RHODE ISLAND

The U.S. Geological Survey and National Oceanic and Atmospheric Administration are working together to study sea-floor geology and sedimentary environments along the northeastern coast of the United States. As part of this effort, 63 square km of multibeam-echosounder data, collected during hydrographic survey H11995 in central Rhode Island Sound, were verified with sediment samples, bottom video, and still photographs. Together these data are used to map distributions of boulders, scour depressions, megaripples, and relatively undisturbed modern marine sediments. Much of the eastern part of the study area is bouldery owing to lag deposits on a submerged segment of the Charlestown-Buzzards Bay moraine. Bottom photography shows boulders are generally overgrown with sessile fauna and flora including hydrozoans, algae, and anemone. Scour depressions, presumably formed by long-period storm waves, and erosional outliers of Holocene sediments within the scour depressions dominate the western part of the study area as well as several large areas in the east. The sea floor in the scour depressions tends to have coarser grained sediment than that of the undisturbed modern marine sediments comprising the intervening erosional outliers and surrounding sea floor. The coarseness of sediments on the moraine, along isolated bathymetric highs, and in scour depressions likely creates turbulence in bottom-water currents, limiting redeposition of finer grained sediment and helping to maintain these features. Where present, sandy sediments are typically rippled and have well-developed infaunal communities as evidenced by the abundance of burrows and worm tubes. Several small fields of megaripples in the west have bedforms with 20-m wavelengths, 20- to 50-cm heights, and east-west oriented crest lines. Mud content of sand increases within bathymetric lows, such as in the southeast, suggesting lower energy environments and processes associated with deposition.