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Surficial Geology in Central Narragansett Bay, Rhode Island: Interpretations of Sidescan Sonar and Multibeam Bathymetry


Sedimentary Environments

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Sedimentary Environments
Figure 10. Map showing distribution of sedimentary environments.
Figure 10. Map showing distribution of sedimentary environments. Click on figure for larger image.

Interpretations of the study areas' sedimentary environments are based on bathymetry, acoustic backscatter and sediment samples. We identified three different sedimentary environments in the study areas: fine-grained deposition, sorting and reworking, and erosion or nondeposition (fig. 10).

Sedimentary environments characterized by processes associated with deposition generally have low backscatter, gently sloping or deep bathymetry, and fine-grained muddy sediments. Much of the flat regions in the study areas, including the northern parts of West Passage and western parts of East Passage, are depositional environments. Deposition is also occurring in the trough to the south of Prudence Island and in a bathymetric low east of Plum Point. The U.S. Army Corps of Engineers studied the tidal flow in Narragansett Bay and found that the area southwest of Prudence Island experiences about 3 hours of slack water every tidal cycle, which possibly influence the surficial sediment (McMaster, 1960). Areas of fine-grained deposition tend to occur in non-constricted areas of the Bay, where current influences are weak.

Sedimentary environments dominated by processes associated with sorting and reworking have moderate backscatter and contain fine sands. In East Passage, environments of sorting and reworking are located in the trough and along its flanks in the southern part of the study area and to the east of Dyer Island in intermediate depths. In West Passage, sorting and reworking of sediments is taking place throughout much of the trough, surrounding the depositional environment in the north and in the far western side of the study area. Sorting and reworking is generally occurring in areas that transition from depositional to erosional environments where currents are moderately strong.

Erosional or nondepositional environments produce high backscatter, are located in shallow water (and in some deeper troughs), and are composed of coarse-grained sediments. They are located near the shores of islands and the mainland, and in parts of the trough in West Passage. These areas are susceptible to waves and currents, which winnow away the finer-grained sediments.

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