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U.S. Geological Survey Open-File Report 2012–1004

Sea-Floor Geology in Central Rhode Island Sound South of Sakonnet Point, Rhode Island


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National Oceanic and Atmospheric Administration survey H11995 covers a 63-square-kilometer area in central Rhode Island Sound, south of Sakonnet Point, Rhode Island, where multibeam-echosounder data were collected. The U.S. Geological Survey collected sediment samples, bottom video, and still photography from the sea floor at 27 stations within this study area to verify a geologic sea-floor interpretation based on bathymetric data. Together, these data are used to identify sea-floor features and sediment texture, and interpret processes occurring on the sea floor. The eastern part of the study area contains bathymetric highs composed of boulders and gravel associated with the Harbor Hill-Roanoke Point-Charlestown-Buzzards Bay moraine, which was deposited about 18,000 years ago. Much of the western part of the study area and several regions in the east contain scour depressions, probably formed by currents from long-period storm-induced waves, and erosional outliers composed of undisturbed modern marine sediments. The sea floor in the scour depressions has a coarser mean grain size than that of the marine sediments. The coarser grained scour depressions likely increase turbulence in the near-bottom water, which prevents finer grained sediment from being deposited. This process probably helped to form, and continues to maintain, these features. Several small areas of megaripples with 20-meter wavelengths are visible in the bathymetry in the northwestern part of the study area. Photography of the sea floor shows that boulders tend to be encrusted with sessile fauna (for example, hydrozoans and anemones), sandy areas of the sea floor tend to be undulating to faintly rippled, and burrows and worm tubes are common.

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