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U.S. Geological Survey Open-File Report 2013–1060

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


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The data and interpretations in this report are based on NOAA survey H12012 completed offshore in eastern Long Island Sound during 2009 and USGS verification cruises completed during 2010. These data yield new geologic perspectives of Long Island Sound's dynamic sea floor that show the terrain of the seabed, allow interpretations of composition and processes, and provide information on sediment transport and benthic habitat. Together, these data provide 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 15 m at mean lower low water to more than 74 m. Shallowest areas occur on top of a marine delta in the northwestern part of the study area. Deepest depths are found in a scour depression near the entrance to The Race.

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 present multibeam bathymetric data include exposed bedrock outcrops, lag deposits of boulders from winnowed Pleistocene drift, deep scour depressions, and a gravelly 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. Transverse sand waves, which in places exceed 4 m in relief, prevail on top of the marine delta where sand is abundant; barchanoid sand waves prevail in isolated fields on the gravelly pavement where sand is more limited. Regardless of sand-wave morphology, orientation and asymmetry predominantly indicate net west-southwestward sediment transport, and the presence of ripples and megaripples on the stoss slopes of the bedforms suggests that transport is active. Dredge spoils of the Cornfield Shoals Disposal Site are the only anthropogenic artifacts visible in the bathymetric data.

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