Summary

Sediment samples and bottom photography were obtained by the U.S. Geological Survey from the sea floor in Block Island Sound, northeast of Gardiners Island, New York, in order to aid in delineating sedimentary environments and interpretations of sea-floor features visible in bathymetric data collected in this area by the National Oceanic and Atmospheric Administration during hydrographic survey H12299. Features on the sea floor include bedforms, such as sand waves and megaripples; boulders; and scour depressions. 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 made of small sand waves and megaripples with wavelengths of several tens of meters. Bedforms indicate sedimentary environments of coarse-grained bedload transport. 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. Boulders are in fields on many bathymetric highs, most of which likely mark exposures of recessional ice-front positions, the northern flank of the Ronkonkoma-Block Island-Nantucket terminal moraine, and (or) basal tills on top of coastal-plain deposits. The boulders, which are commonly overgrown with sessile fauna and flora, indicate sedimentary environments of erosion or nondeposition. A large scour depression is along the northwestern edge of the study area adjacent to The Race, where strong tidal currents flowing through a constricted area have scoured the sea floor. Gravel armors the eastern wall of the depression where finer sediments have been winnowed away. Generally flat and featureless areas of the sea floor, such as those along the southern and northeastern parts of the study area and southwest of the large scour depression, are composed of modern marine sediments that are being sorted and reworked, and these sedimentary environments likely have the lowest energy in the study area.