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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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## Summary

| Title PageList of FiguresConversion FactorsAbstract Introduction SettingMethodsBathymetrySidescan SonarSedimentsBottom PhotographyGIS Data Catalog**Summary**AcknowledgmentsReferencesContactsIndex
Multibeam-echosounder, sidescan-sonar, bottom-photography,  and sediment data are used to map features on the sea floor of northwestern  Block Island Sound and describe the sedimentary environments. Sea-floor  features include boulders, scour depressions, sand waves and megaripples,  modern marine sediments, and trawl marks. Boulders are lag deposits of winnowed  Pleistocene ice-proximal drift, and are often overgrown with algae and sponges  and (or) tunicates. Scour depressions, containing coarse sand and gravel, are  about 0.5 m below the surrounding sea floor of modern marine sediments, which  are composed of finer sand. Owing to the coarser grained sediment in scour  depressions than the surrounding sea floor, bottom currents are likely more  turbulent in these areas than the surrounding sea floor. This helps to maintain  these features which are probably caused by storm-generated, offshore flows. Sand  waves are typically oriented with crests perpendicular to the shore with several-hundred-meter  wavelengths; megaripples are variably oriented parallel and perpendicular to  shore with 20- to 50-m wavelengths.  The  larger sand waves reflect the shore-parallel sediment transport owing to tidal and  wind-driven currents, and the megaripples reflect sediment transport by waves  in a shore-perpendicular direction. Bottom photography shows that sandy areas  are typically rippled with coarser sediment and shells collecting in the ripple  troughs and finer sediment on the ripple crests, often snails and crabs are  present. Modern marine sediments tend to be relatively featureless, sandy areas  that often surround scour depressions and form erosional outliers within them. 

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U.S. Department of the Interior |            U.S. Geological Survey                                                URL: http://pubs.usgs.gov/of/2014/1018/html/summary.html            Page Contact Information: GS Pubs Web Contact                        Page Last Modified: Friday, February 21, 2014, 12:29:03 PM |
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