SURFICIAL GEOLOGY OF THE SEA FLOOR IN LONG ISLAND SOUND OFFSHORE OF ORIENT POINT, NEW YORK

The U.S. Geological Survey (USGS), the National Oceanic and Atmospheric Administration (NOAA), and the Connecticut Department of Environmental Protection are working cooperatively to map and interpret the sea-floor geology along the northeastern coast of the United States. Continuous-coverage multibeam bathymetry and sidescan-sonar data were obtained in 2008 during NOAA survey H11446, which was conducted in a 12-square kilometer area in Long Island Sound offshore of Orient Point, New York. In addition, sediment and photographic data from 26 stations were obtained during a USGS verification cruise in 2010. Together, the bathymetric, sidescan-sonar, sediment, and photographic data are used to delineate sea-floor features and improve our understanding of processes controlling the distribution and movement of sediment. Overall, the seabed slopes gently seaward from 2 m water depth near shore to over 100 m in Plum Gut, a tidally eroded scour depression between Orient Point and Plum Island. However, topography is more complex in areas where sand waves and boulders are prevalent. Sandy areas of the sea floor are typically current rippled. Sand waves and megaripples generally have north-south-oriented crests with 10- to 20-meter wavelengths. Sand-wave asymmetry and scour around boulders indicate eastward net sediment transport in the east near Plum Gut and westward transport in the northern and western parts of the study area. Areas with boulders on the sea floor are typically hummocky and are part of a submerged flank of the Harbor Hill-Roanoke Point-Orient Point-Fishers Island moraine. Boulders are typically encrusted with sessile fauna and flora (e.g., seaweed, hydrozoans, sponges, and anemones), which dramatically increase the benthic roughness and complexity. Sand, gravel, and gravelly sediment were the only sediment textures identified in the study area; mud is winnowed and prevented from being deposited by strong (over 3 knots) currents. These data are important for benthic-habitat studies and serve as a framework for future monitoring and resource management activities.