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Open-File Report 2014–1018

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### Sea-Floor Geology in Northwestern Block Island Sound, Rhode Island 

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## Abstract

| Title PageList of FiguresConversion Factors**Abstract**Introduction SettingMethodsBathymetrySidescan SonarSedimentsBottom PhotographyGIS Data CatalogSummaryAcknowledgmentsReferencesContactsIndex | Multibeam-echosounder and sidescan-sonar data, collected  by the National Oceanic and Atmospheric Administration in a 69-square-kilometer  area of northwestern Block Island Sound, are used with sediment samples, and still  and video photography of the sea floor, collected by the U.S. Geological Survey  at 43 stations within this area, to interpret the sea-floor features and  sedimentary environments. Features on the sea floor include boulders, sand  waves, scour depressions, modern marine sediments, and trawl marks. Boulders, which  are often several meters wide, are found in patches in the shallower depths and  tend to be overgrown with sessile flora and fauna. They are lag deposits of winnowed  glacial drift, and reflect high-energy environments characterized by processes  associated with erosion and nondeposition. Sand waves and megaripples tend to  have crests that either trend parallel to shore with 20- to 50-meter (m) wavelengths  or trend perpendicular to shore with several-hundred-meter wavelengths. The  sand waves reflect sediment transport directions perpendicular to shore by  waves, and parallel to shore by tidal or wind-driven currents, respectively. Scour  depressions, which are about 0.5 m lower than the surrounding sea floor, have  floors of gravel and coarser sand than bounding modern marine sediments. These  scour depressions, which are conspicuous in the sidescan-sonar data because of their  more highly reflective coarser sediment floors, are likely formed by  storm-generated, seaward-flowing currents and maintained by the turbulence in  bottom currents caused by their coarse sediments. Areas of the sea floor with  modern marine sediments tend to be relatively flat to current-rippled and  sandy.**** |
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U.S. Department of the Interior |            U.S. Geological Survey                                                URL: http://pubs.usgs.gov/of/2014/1018/html/abstract.html            Page Contact Information: GS Pubs Web Contact                        Page Last Modified: Friday, February 21, 2014, 12:18:46 PM