Woods Hole Science Center
Sidescan-Sonar Imagery and Surficial Geologic Interpretations of the Sea Floor in Central Rhode Island Sound
Sidescan-sonar imagery shows that 7 backscatter patterns exist in the study area (fig. 9). These patterns have been interpreted using bathymetric and seismic-reflection data (fig. 10). The patterns are interpreted to be: sand waves, boulders, benches, low, moderate, and high backscatter, and trawl marks. These areas are often gradational, many overlap, and the boundaries are inferred.
Areas of tiger-striped backscatter can be seen across most of the northern, western, and southeastern parts of the study area. These features occur on the bathymetric highs and are interpreted to be sand waves (fig. 11). Most sand waves in the northwestern part of the study area have straight crests that are oriented north-south with 10-20 m wavelengths. Sand waves that lie closer to the channel, in the central, eastern, and southern parts of the study area, tend to have curved and sinuous crests oriented to the east or east-northeast and wavelengths of 20 to over 100 m. Superimposed on some of the sand waves near the channel are megaripples with several-meter wavelengths and crests oriented to the northwest (fig. 12). The sediment transport direction in the central part of the study area is interpreted to be north-south between the ridge and channel, perpendicular to the sand-wave crests. Along the northern part of the study area, sediment transport is in an east-west direction, as indicated by orientations of the sand-wave crests. Sand waves along channels in the north tend to be oriented both north-south and east-west, suggesting there is a component of north-south flow through the smaller channels in addition to the east-west flow (fig. 11).
High-backscatter targets with low-backscatter shadows exist throughout the study area, with the highest concentration in the southeast (fig. 13). These targets are interpreted to be boulders up to several meters wide. They are present on both bathymetric highs and in the channel where glacial drift is exposed. The boulders are glacial erratics and ice-rafted debris deposited during the Wisconsin glacial period.
Discontinuous, curvilinear, sharply contrasting high- and low-backscatter patterns cross through the central and southeastern parts of the study area in a northeast-southwest direction (fig. 10). These features, which occur along the northern edge of the central channel and along the northern edge of the bathymetric low in the southeast in water depths of 30-36 m (figs. 11, 12, and 13), are composed of paired scarps and horizontal sea floor notched into the regional bathymetric slope. The features in the western and southeastern corners of the study area are mostly 35-36 m deep and those in the northeast are generally 32-33 m deep. The features tend to exist near the edge of stratified sediments and where fluvial sediments or till outcrop (figs. 7 and 8). Similar backscatter patterns seen in the sidescan-sonar imagery of NOAA Survey H11320 were associated with small hills and scarps up to 0.5 m in height in the multibeam bathymetry and were interpreted to be wave-cut terraces along paleoshorelines (McMullen and others, 2007). Although these features are smaller than the resolution of the bathymetric and seismic-reflection data in the study area, they are presumed to be benches with small hills and scarps, like those to the east, because they continue through both surveyed areas (McMullen and others, 2007).
Several areas of low backscatter are visible in the sidescan-sonar imagery within the bathymetric lows in the northwest, south-central, and southeast (figs. 9 and 10). Low backscatter generally indicates finer grained sediments and lower energy conditions. These areas are located in the deepest parts of the study area.
Areas of moderate backscatter that are otherwise featureless are located in much of the central trough and on parts of the bathymetric highs (fig. 10).
Four areas of high backscatter are located in the southern part of the study area, where they occur on the flanks of bathymetric highs (fig. 10). High backscatter is usually associated with coarser grained sediment and higher energy conditions.
Several areas with lines of either low backscatter or paired high and low backscatter are located in the channel in the central part of the study area and along the northern and southeastern edges of the study area (fig. 12). These are interpreted to be trawl marks from fishing boats. Driscoll (1996) found that between 1991 and 1994, trawl marks increased in distribution and density in Block Island Sound, suggesting an increasing impact of anthropogenic activity on the sea floor.