Open-File Report 2014–1018
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.