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Figure 13. Sedimentary environments found in the study area include those characterized by processes of erosion or nondeposition, coarse-grained bedload transport, sorting and reworking, and deposition.

On the basis of backscatter tones and features observed in the sidescan-sonar imagery, four sedimentary environments can be distinguished in the study area (fig. 13). Lighter backscatter tones tend to indicate higher energy environments and darker backscatter tones tend to indicate lower energy environments. The historic sediment samples from the study area tend to be characteristic of the sedimentary environments from which they were collected; however, boundaries of the environments were not determined using sediment-sample data, as only five samples are available from within the study area. The sedimentary environments are characterized by the processes of erosion or nondeposition, coarse-grained bedload transport, sorting and reworking, and deposition.

Erosion or nondeposition

Sedimentary environments characterized by processes of erosion or nondeposition are located on the bathymetric highs. These environments produce high backscatter on the sonar image and include areas of rocks and erosional outliers. One historic sediment sample of gravel came from this sedimentary environment located in an area of high backscatter. Processes of erosion and nondeposition are found in higher energy environments where currents tend to winnow away finer grained sediment, leaving a coarser grained lag of sand and gravel.

Coarse-grained bedload transport

Also on the bathymetric highs are sedimentary environments characterized by the process of coarse-grained bedload transport. This environment exists where sand waves are present. These are relatively high-energy environments where currents maintain sand waves. The three historic samples of sand collected in the study area were from this sedimentary environment.

Sorting and reworking

Sedimentary environments characterized by sorting and reworking processes are found primarily between bathymetric highs and lows. The sea floor in this environment typically consists of fine sands and mud (Knebel and others, 2000). This environment is characterized by moderate backscatter and tends to be found between environments characterized by the process of deposition and higher energy environments.

Deposition

Throughout much of the channel and the depression on the northeastern bathymetric high, environments are characterized by fine-grained deposition. One historic sample of mud came from this sedimentary environment, which displays low backscatter in the sidescan-sonar imagery. This low-energy environment with weak currents, if any, allows finer grained sediment, such as muds, to accumulate.