SEA-FLOOR FEATURES AND SEDIMENTARY ENVIRONMENTS IN WESTERN BLOCK ISLAND SOUND, NORTHEAST OF GARDINERS ISLAND, NEW YORK

Multibeam-echosounder data, collected by NOAA in a 162 km² area of Block Island Sound, northeast of Gardiners Island, New York, are used with sediment samples and bottom photography, collected by the USGS to interpret sea-floor geology. These data and interpretations provide important base maps for future ecologic and resource-management studies and show that the sea-floor features and sedimentary environments are products of the glacial history and modern tidal regime. Features include boulders, large current-scoured depressions, exposed glaciolacustrine sediments, and bedforms such as sand waves and megaripples. Boulders and gravelly lag deposits, which indicate environments of erosion or nondeposition, are found off the coast of Gardiners Island and on bathymetric highs, likely marking areas where deposits associated with either recessional ice-front positions, the northern flank of the terminal moraine, or coastal-plain sediments covered with basal till are exposed. Bottom photographs and video show that boulders are commonly covered with sessile fauna and flora. Strong tidal currents have produced a deep scour depression along the northwestern edge of the study area and maintained channels that record paleo-drainage patterns. The sides of the depression are covered with a gravel lag. Elongate hummocks in the northwestern corner of the study area are exposed glaciolacustrine sediments. Sand waves cover much of the study area and generally have north-south-oriented crests and 100- to 200-m wavelengths. The flanks of large sand waves are commonly covered by megaripples, with wavelengths of several tens of meters, and current ripples. These bedforms indicate sedimentary environments of coarse-grained bedload transport; asymmetry of the bedforms shows that net sediment transport is to the west. Sand is the dominant sediment type and, in general, sand is finer in bathymetric lows and unconstricted areas than where high-energy sedimentary environments prevail. Sea-floor areas characterized by modern marine sediments appear featureless at the 2-meter resolution of the bathymetry and flat to current rippled in the photography. These modern environments are indicative of sediment sorting and reworking and are the lowest energy environments in the study area.