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

Interpretative maps of sea-floor features and sedimentary environments were produced using multibeam bathymetric and sidescan-sonar data collected by NOAA and sediment samples and bottom photography collected by the USGS from 36 stations in a 114-km² area of Block Island Sound, southeast of Fishers Island, New York. The features and sedimentary environments are attributed to glacial history, subsequent sea-level change, and modern processes. Wisconsinan glaciation striated bedrock and deposited drumlins and glaciolacustrine sediments in the study area. Large current-scoured bathymetric depressions formed when a glacial lake in present-day Block Island Sound drained and fluvial channels eroded the underlying strata. The marine incursion and strong modern tidal currents flowing into and out of Long Island Sound maintained and continue to erode the depressions. A bathymetric depression in the western part of the study area is flanked by ridges of cobbles on its eastern and northern sides. Bottom photographs from within a 100-m deep current-scoured bathymetric depression show outcrops of glaciolacustrine sediment on its eastern wall. Glaciolacustrine rip-up clasts are also on the surrounding sea floor, indicating that currents are actively eroding and transporting these sediments. Obstacle marks are visible in the bathymetric data around many boulders and their asymmetry indicates generally westward net sediment transport. Bottom photography data show that boulders in the study area tend to be covered with sessile flora and fauna. The coarse-grained sediment on bathymetric highs and flanking a bathymetric depression along with the exposure of glaciolacustrine sediments in another depression indicates modern environments characterized by erosion or nondeposition. Areas with sand waves and megaripples are found in sedimentary environments characterized by coarse-grained bedload transport and tend to have scattered shells and shell hash, burrows, and worm or amphipod tubes. Sedimentary environments characterized by sorting and reworking have generally finer sand and contain anthropogenic features of trawl marks preserved on the sea floor. These geologic interpretations of sea-floor features and environments provide base maps for resource management and studies on benthic ecology.