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Sidescan-Sonar Imagery, Multibeam Bathymetry, and Surficial Geologic Interpretations of the Sea Floor in Rhode Island Sound, off Sakonnet Point, Rhode Island


List of Figures

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List of Figures
Figure 1. Location map of NOAA Survey H11320 in Rhode Island Sound.

Figure 1. Location map of NOAA Survey H11320 (red polygon) in Rhode Island Sound. Location of stratigraphy profile A-A' from figure 3 is also shown.

Figure 2. Location of figures, bathymetric profiles, historic seismic-reflection profiles, and sand and rocky sediment samples. Figure 2. Location of figures (red polygons), bathymetric profiles (green lines), historic seismic-reflection profiles (blue lines, O'Hara and Oldale, 1980), and sand (yellow circles) and rocky (red circle) sediment samples (Poppe and others, 2003) within the study area.
Figure 3. Interpreted stratigraphy of Rhode Island Sound from O'Hara and Oldale (1980). Figure 3. Interpreted stratigraphy of Rhode Island Sound from O'Hara and Oldale (1980). Acoustic units include: PzZ - Proterozoic to Paleozoic bedrock, Ku - Cretaceous coastal plain sediments, Qdo - Wisconsin glacial drift deposits, Qdm - Buzzards Bay moraine deposits, Qfe - Holocene fluvial and estuarine deposits, and Qm - Holocene quiet-water marine deposits. Boundaries are dashed where inferred. Vertical exaggeration: 40X. Location of stratigraphy profile shown as A-A' in figure 1.
Figure 4. Map showing the location of end moraines (solid black polygons) in southern New England and New York.

Figure 4. Map showing the location of end moraines (solid black polygons) in southern New England and New York. The Ronkonkoma-Nantucket moraine shows the maximum extent of the Laurentide ice sheet 20 ka, while the Harbor Hill-Roanoke Point-Charlestown-Buzzards Bay moraine shows a retreated position of the ice sheet around 18 ka (Uchupi and others, 1996). Submarine ridges (dotted lines) mark the underwater extensions of the moraines. H11320 study area shown as a gray polygon. Modified from Gustavson and Boothroyd (1987).

Figure 5.  Photo of NOAA Ship RUDE (Courtesy of NOAA).

Figure 5. Photo of NOAA Ship RUDE (Courtesy of NOAA).

Figure 6. Hill-shaded bathymetry of the study area.

Figure 6. Hill-shaded bathymetry of the study area. Four areas outlined in black show multibeam bathymetry from H11320, while the rest of the bathymetry is from the NGDC Coastal Relief Model. Multibeam bathymetric data have been gridded at 4 m and depths represent mean lower low water level in multibeam data and either mean low water or mean lower low water in Coastal Relief Model data. (Divins and Metzer, n.d.)

Figure 7. Sidescan-sonar imagery of NOAA Survey H11320.

Figure 7. Sidescan-sonar imagery of NOAA Survey H11320. Darker regions represent low backscatter and generally finer grained sediments; lighter regions represent high backscatter and generally coarser grained sediments.

Figure 8. Interpretation of sidescan-sonar and multibeam bathymetric imagery.

Figure 8. Interpretation of sidescan-sonar imagery and multibeam bathymetry showing areas of rocky and gravelly sediment, sand waves and crest orientations, sediment transport direction, hummocky sea floor, tabular erosional outliers, small hills and scarps, and trawl marks.

Figure 9. Detailed sidescan-sonar imagery and bathymetry illustrating the relatively smooth central basin floor scattered with rocks and boulders in the south and adjacent ridge to the north.

Figure 9. Detailed sidescan-sonar imagery and bathymetry illustrating the relatively smooth central basin floor scattered with rocks and boulders in the south and adjacent ridge to the north. Sand waves are located on the ridge and a scarp crosses the ridge in an east-west direction. Sand waves have curved crests oriented east-west and wavelengths of 40 to over 100 m in the west and about 50 m in the east. High-resolution multibeam bathymetry has been overlaid on bathymetry from the NGDC Coastal Relief Model. Figure location is shown in figure 2.

Figure 10. Bathymetric profiles of sand waves showing slip faces oriented northward.

Figure 10. Bathymetric profiles of sand waves showing slip faces oriented northward. Profile C-C' is of sand waves in the eastern field of east-west oriented crests, which have wavelengths of about 50 m. Profile D-D' is of sand waves in the northwestern corner of the study area where wavelengths are also about 50 m. Location of profiles shown in figure 2.

Figure 11. Detailed view of sidescan-sonar imagery showing mottled backscatter and corresponding bathymetry.

Figure 11. Detailed view of sidescan-sonar imagery showing mottled backscatter and corresponding bathymetry. Mottled backscatter in the sidescan-sonar data is caused by the hummocky surface of moraines and glacial till inconsistently reflecting backscatter due to the variable topography and sediment distribution. Note: black line outlines high-resolution multibeam bathymetry which overlies bathymetry from the Coastal Relief Model. Figure location is shown in figure 2.

Figure 12. Sidescan-sonar imagery and Boomer seismic-reflection profile (O'Hara and Oldale, 1980) and interpretation through an area of mottled backscatter.

Figure 12. Sidescan-sonar imagery and Boomer seismic-reflection profile (O'Hara and Oldale, 1980) and interpretation through an area of mottled backscatter. Right side of sidescan-sonar image shows mottled backscatter (top image). Seismic profile shows Cretaceous coastal plain sediments overlain by Wisconsin glacial drift and Holocene fluvial and estuarine sediments. Mottling tends to occur in areas where glacial moraine sediments are either exposed or near the surface. Vertical exaggeration: 13X. Depths are based on the assumed compressional wave velocity of 1500 m/s. Location of seismic line shown in figure 2.

Figure 13. Detailed area of sidescan-sonar image showing low-backscatter targets.

Figure 13. Detailed area of sidescan-sonar image showing low-backscatter targets. These features show no relief in the bathymetry data and are thought to be patches of fine-grained sediment within larger areas of coarser grained sediment. Figure location shown in figure 2.

Figure 14. Detailed view of sidescan-sonar imagery and corresponding hill-shaded bathymetry showing tabular erosional outliers.

Figure 14. Detailed view of sidescan-sonar imagery and corresponding hill-shaded bathymetry showing tabular erosional outliers. Curvilinear shadows in the sidescan-sonar imagery correspond to the outline of small plateaus in the bathymetry. Boulders are visible in both images, on the sidescan-sonar imagery as light-toned targets with dark shadows and within the bathymetry as dark targets. The area of boulders in the lower right corner of both images shows an exposure of till. Lighter backscatter between the erosional outliers is from coarse-grained, bouldery sediment. Images are in UTM. Location of figure shown in figure 2.

Figure 15. Sidescan-sonar imagery, seismic-reflection profile, and interpretation through an area of tabular erosional outliers.

Figure 15. Sidescan-sonar imagery, Boomer seismic-reflection profile (O'Hara and Oldale, 1980), and interpretation through an area of tabular erosional outliers. Top image shows location of the seismic profile on the sidescan-sonar data and erosional outliers in the center. Lower images of seismic profile and interpretation show stratified sediments, possibly from a small glacial lake or pond, below the erosional outliers. Note irregular sea floor and estuarine sediments above laminations, likely due to prior erosion of lacustrine sediments. Boulders can be seen rising above the sea floor. Vertical exaggeration: 16X. Depths are based on the assumed compressional wave velocity of 1500 m/s. Figure location is shown in figure 2.

Figure 16. Bathymetric profiles across study area from north to south.

Figure 16. Bathymetric profiles across study area from north to south. Location of profiles shown in figure 2. Profile A-A', located in the southwestern part of the study area, crosses a bathymetric high with deeper basins to the north and south. Profile B-B', located in the eastern part of the study area, crosses a bathymetric high in the north, central basin, and moraine in the south. Arrows point to changes in slope interpreted to represent paleoshorelines.

Figure 17. Sidescan-sonar imagery, Boomer seismic-reflection profile (O'Hara and Oldale, 1980), and interpretation of an area with small scarps of outcropping strata.

Figure 17. Sidescan-sonar imagery, Boomer seismic-reflection profile (O'Hara and Oldale, 1980), and interpretation of an area with small scarps of outcropping strata. Top image shows scarp and location of seismic line on sidescan-sonar mosaic. Bottom images of seismic line and interpretation show Pleistocene glacial drift overlain by Holocene fluvial and estuarine sediments. The small scarp displayed within the sidescan-sonar mosaic is presumed to correspond to the prominent reflector between the fluvial sediments and glacial drift as it nears the surface. Vertical exaggeration: 15X. Depths are based on the assumed compressional wave velocity of 1500 m/s. Location of seismic line shown in figure 2.

Figure 18. Detailed view of sidescan-sonar image showing long lines of low backscatter interpreted to be trawl marks from fishing boats.

Figure 18. Detailed view of sidescan-sonar imagery showing long lines of low backscatter interpreted to be trawl marks from fishing boats. Figure location is shown in figure 2.

Figure 19. Map showing distribution of sedimentary environments.

Figure 19. Map showing sedimentary environments in study area characterized by erosion or nondeposition, coarse-grained bedload transport, and sorting and reworking.

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