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U.S. Geological Survey Open-File Report 2013–1003

Sea-Floor Geology in Northeastern Block Island Sound, Rhode Island


Summary

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Multibeam-bathymetric and sidescan-sonar data collected as part of National Oceanic and Atmospheric Administration survey H12023 are used in combination with sediment textural data and photography obtained during U.S. Geological Survey cruise 2012-002-FA to describe sea-floor features and sedimentary environments in an area of northern Block Island Sound. The sea floor in the study area is sandy nearshore and along the southern parts and gravelly and bouldery in the central areas. Sand is often rippled, and shells and coarser sediment tend to collect in the troughs, while finer sediment is on the crests. Boulders, making up the southern flank of the Point Judith moraine, are often overgrown with sessile fauna and flora. Scour depressions, which lie between the exposed moraine and areas of mostly sandy Holocene sediments, have rippled floors about 0.5 meter deeper and coarser grained than adjacent modern sediments. They are formed from and maintained by bottom stresses from storm waves. Scoured areas nearshore tend to be oriented parallel to the coast and have northward oriented tapering extensions, whereas scour depressions in the south tend to be large and east-west oriented or, where small, north-south oriented. An area of low, broad bedforms in the north-central part of the study area is visible in the sidescan-sonar imagery as high- and low-backscatter stripes from coarser grained troughs and finer grained crests, respectively.


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