GLACIAL FEATURES AND SEDIMENT TRANSPORT PROCESSES IN EASTERN LONG ISLAND AND WESTERN BLOCK ISLAND SOUNDS

The U.S. Geological Survey, in cooperation with the National Oceanic and Atmospheric Administration (NOAA) and the Connecticut Department of Environmental Protection, is producing detailed geologic interpretations of sections of the sea floor in Long Island Sound (LIS). The current phase of this cooperative research uses multibeam bathymetry and CHIRP subbottom data to study the geomorphology and processes controlling the distributions of surficial sediments and sedimentary environments. The geologic interpretations presented here, which are based on a digital terrain model produced from acoustic data collected during survey H11250G by the NOAA Ship Thomas Jefferson, cover approximately 91 km² of the sea floor in eastern LIS and western Block Island Sound.

The dominant glacial features in the study area are the northeast-trending Harbor Hill-Orient Point-Fishers Island moraine and another smaller, previously unknown, recessional moraine to the southeast. South-trending sinuous ridges and isolated bathymetric highs associated with the larger moraine are interpreted to be eskers and kames, respectively. Elevated lineations observed on exposed bedrock in the northwestern part of the study area, which trend 154^o to 162^o and parallel the striations and streamlined hills of onshore formations, show that the effects of glaciation on the bedrock continue offshore under LIS. Also, several irregular elevated lineations on the bedrock surface trend roughly 50^o to 62^o. These lineations parallel structural trends in the onshore Avalonian terrane and, therefore, may represent strike ridges.

Strong tidal currents have scoured and redistributed much of the glacial and younger sediment in easternmost LIS, and produced large bathymetric depressions adjacent to the Race that exceed 100 m in depth. Degradational processes that cause the scour depressions include seabed erosion and mass-wasting, and effects of both processes are visible in the imagery as erosional outliers and talus piles. Asymmetry of transverse sand waves, barchanoid dune orientation, and scour around isolated boulders and a shipwreck indicate that net transport is primarily toward the west and into LIS in the northern part of the study area, but primarily toward the east and out of the Sound in the southern part.