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

The U.S. Geological Survey (USGS), the Connecticut Department of Environmental Protection, and the National Oceanic and Atmospheric Administration (NOAA) have been working cooperatively to interpret surficial sea-floor geology along the coast of the Northeastern United States. NOAA survey H11445 covers an area of about 12 square kilometers in eastern Long Island Sound offshore of Plum Island, New York. Multibeam bathymetry and sidescan-sonar imagery from the 2008 survey, as well as sediment and photographic data from 13 stations occupied during a 2009 USGS verification cruise, are used to delineate sea-floor features, characterize sedimentary environments, and describe floral and faunal populations. Bathymetry deepens gradually offshore, but steeply into depressions at the entrance to (>100 m) and in Plum Gut (>60 m), a channel between Plum Island and Orient Point. These depressions were formed by intense tidal scour and reflect benthic environments dominated by erosion. Many small, individual, rounded bathymetric highs that coincide with complex patches of high and low backscatter targets in the sidescan-sonar imagery are concentrated adjacent to Plum Island, delineating bouldery lag deposits produced from the Harbor Hill-Orient Point-Fishers Island moraine. Boulders are typically overgrown with sponges, hydrozoans, anemone, algae, and seaweed that add to the overall benthic complexity. Sand waves, which are present on a shoal north of Plum Island and in several smaller patches around the study area, reveal areas characterized by coarse-grained sediment transport. Sand-wave asymmetry indicates that counter-clockwise net sediment transport maintains the shoal. Sand is also prevalent where the sea floor is relatively flat and backscatter in the sidescan-sonar imagery is low. Photographs of these areas generally show a current-rippled sea floor with shell debris concentrated in the ripple troughs.