SEA-FLOOR GEOLOGY IN NORTHEASTERN BLOCK ISLAND SOUND ADJACENT TO POINT JUDITH, RHODE ISLAND

Multibeam-echosounder and sidescan-sonar data collected by the National Oceanic and Atmospheric Administration in northeastern Block Island Sound as part of survey H12023 are combined with sediment and bottom-photography data collected by the U.S. Geological Survey to interpret sea-floor features and sedimentary environments in a 52-square-kilometer area offshore of Rhode Island from Green Hill to Point Judith. Gravel and gravelly sediment are predominant in water depths less than 20 m, reflecting high-energy sedimentary environments. Boulders, which are often several meters in diameter and overgrown with sessile flora and fauna, are probably part of the southern flank of the Harbor Hill-Roanoke Point-Charlestown-Buzzards Bay moraine, deposited about 18,000-21,000 years ago. Sand prevails in the deeper offshore areas and in isolated patches along the shore, indicating areas with relatively lower energy sedimentary environments. Bottom photography shows that sandy areas are generally rippled, with shells and coarser sediment collecting in the troughs. Scour depressions—areas of the sea floor with a coarser grained, rippled surface lying about 0.5 meter below the surrounding modern marine sea floor—and erosional outliers of modern marine sediments contained within the depressions are located both in a band near shore and also offshore in deeper parts of the study area. Textural and bathymetric differences between scour depressions and modern marine sediments stand out in the sidescan-sonar imagery as sharp tonal contrasts. We believe these features are formed from, and maintained by, bottom stresses associated with storm waves. Shore-perpendicular features with no discernible relief and alternating areas of coarser and finer grained material tens of meters wide are visible in the sidescan-sonar imagery in the north-central part of the study area. These features are not visible in the bathymetric data and their origin is unknown.