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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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## Bathymetry

| Title PageList of FiguresConversion FactorsAbstractIntroductionSettingMethods**Bathymetry**Sidescan SonarSedimentsBottom PhotographyGIS Data CatalogSummaryAcknowledgmentsReferencesContactsIndex | Click on figures for larger images**Figure 11.**Hill-shaded multibeam bathymetry of National Oceanic  and Atmospheric Administration survey H12296 in Block Island Sound.**Figure 12.**Map showing interpretations of sea-floor features in  the National Oceanic and Atmospheric Administration survey H12296 study area.**Figure 13.**Detailed multibeam bathymetric image of sand waves, megaripples,  boulders, scour depressions, and erosional outliers in the east-central part of  the study area.**Figure 14.**Detailed multibeam bathymetric image of barchanoid megaripples,  boulders, and trawl marks in the southwestern part of the study area.**Figure 15.**Detailed multibeam bathymetric image showing modern  marine sediments surrounding scour depressions that contain erosional outliers  and locations of stations NBIS47 and NBIS50.
Water depths in the study area range from about 2 m close  to shore to about 33 m along the southern side of the area (fig. 11). Several  sea-floor features are visible in the MBES data including boulders, sand waves,  scour depressions, modern marine sediments, and trawl marks (fig. 12). These  features often overlap, and boundaries are gradational.

 Boulders occur in patches, mostly within shallow depths  less than 20 m (figs. 13, 14, and 15). They are generally several meters wide  and a couple of meters high, but they can reach over 10 m in width. The  boulders are lag deposits of winnowed, ice-proximal glacial drift, deposited  during the northward retreat of the Laurentide Ice Sheet, some time before the  Harbor Hill-Roanoke Point-Charlestown-Buzzards Bay end moraine was constructed,  about 18,000 to 23,000 years ago (Uchupi and others, 2001; Balco, 2011). Exposed  boulders indicate modern high-energy environments where processes of erosion  and nondeposition are prevalent.

In general, sand  waves within the study area tend to have crests that are oriented perpendicular  to shore, and wavelengths of several hundred meters. Megaripples occur with crests  that are oriented either parallel or perpendicular to shore, have much smaller  wavelengths (from about 20 to 50 m), and are observed either alone or  concurrently with the larger sand waves. Two areas of megaripples in the  western part of the study area, have east-west oriented crests with wavelengths  from about 20 to 40 m and wave heights from 20 to 30 cm. The southern-most of  these two areas also has sand waves with northwest-southeast oriented crests, 500-m  wavelengths, and 1- to 2-m heights. In the western part of the study area there  is a section of barchanoid megaripples that have horns pointing to the east, which  indicates eastward sediment transport (fig. 14; McKee, 1966). A large area of  sand waves and megaripples in the east-central part of the study area consists  of megaripples with north-south oriented crests and about 20- to 90-m wavelengths,  superimposed on northwest-southeast oriented sand waves with about 500-m  wavelengths and 2-m heights (fig. 13). Further to the east are megaripples with  northwest-southeast oriented crests, several-hundred-meter wavelengths, and  0.5- to 1-m heights. The northwest-southeast oriented sand waves and  megaripples in the study area reflect bedload transport associated with shore-parallel,  tidally driven, and wind-driven currents in this area (White and White, 2012). It  is likely that megaripples with crests that are oriented parallel to shore, reflect  sediment reworking and transport that is associated with wind-driven waves, and,  in deep water, with storm waves. Sand waves and megaripples commonly grade into  nearby scour depressions and boulders. 

 Scour depressions occur throughout much of the study area,  and are characterized by 20- to 50-cm deep indentations with steep sides, and  coarser grained sediment on the floor of the depression than on the surrounding  sea floor (fig. 15). These are common features within Block Island and Rhode  Island Sounds (McMullen and others, 2007, 2011, 2012, 2013; Poppe and others,  2011, 2012) and are also called cross-shore swaths (Oakley and others, 2009;  Boothroyd, 2009). Oakley and others (2009) describe cross-shore swaths near the  Rhode Island shoreline as areas formed from the down welling of seaward flowing  water during storms. Morang and McMaster (1980) believed these features were  formed from rip currents, waves, and (or) storm-generated flows. Scour  depressions are thought to be maintained because their coarse-grained floors  create turbulence in bottom water, which scours finer material and prevents it  from being deposited in these areas (Murray and Thieler, 2004). Thus, scour  depressions indicate high-energy, sedimentary environments of erosion and  nondeposition. Scour depressions in the study area are surrounded by modern  marine sediments, and often contain erosional outliers of these undisturbed,  featureless, sandy sediments within the depressions.           In addition to geologic features, anthropogenic features such  as trawl marks can also be seen on the sea floor (fig. 14). Trawl marks are  seen in the deeper areas along the southern edge of the study area. They appear  as straight to curvilinear troughs caused by fishing gear dragging along the  sea floor. |
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U.S. Department of the Interior |            U.S. Geological Survey                                                URL: http://pubs.usgs.gov/of/2014/1018/html/bathy.html            Page Contact Information: GS Pubs Web Contact                        Page Last Modified: Friday, February 21, 2014, 12:18:46 PM