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U.S. Geological Survey Open-File Report 2010–1150

Sea-Floor Geology and Topography Offshore in Eastern Long Island Sound


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The data and interpretations in this report are based on NOAA survey H11997 completed offshore in eastern Long Island Sound during 2008 and a USGS verification cruise completed during 2010. These data yield new geologic perspectives of Long Island Sound's dynamic sea floor and show the composition and terrain of the seabed, providing information on sediment transport and benthic habitat. Together, this information provides a fundamental framework for research and management activities in this part of Long Island Sound.

Surveyed depths within the study area range from less than 16 m at mean lower low water to more than 108 m. The shallowest areas occur north of Plum Island on the submerged flanks of the Harbor Hill-Roanoke Point-Fishers Island moraine. The deepest depths are found in a scour depression near the northern entrance to Plum Gut.

As noted in previous work, funnel-shaped geometry of the eastern sound constricts tidal flow, producing currents of greater strength, coarser grained sediments, and sedimentary environments characterized by processes associated with erosion or nondeposition. Evidence for these high-energy conditions revealed by the multibeam bathymetry data include exposed bedrock outcrops, lag deposits of boulders on the moraine surface, deep scour depressions, and a gravel pavement that armors the sea floor throughout much of the eastern part of the study area.

Sedimentary environments characterized by processes associated with coarse bedload transport and sandy sediments become more prevalent in the study area as the sound widens westward and tidal-current speed decreases. A large field of transverse sand waves, which in places contains individual waves that exceed 20 m in relief, stretches across the western part of the study area, and scattered smaller fields of transverse and barchanoid waves are present. Regardless of sand-wave morphology, orientation and asymmetry predominantly indicate net westward sediment transport, and the presence of ripples and megaripples on the stoss slopes of the bedforms suggests that transport is active.

Anthropogenic artifacts visible in the bathymetry data include dredge spoils at the Cornfield Shoals Disposal Site and two shipwrecks.

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