are also characterized as gas-bearing sediments (McMaster, 1984) . 

 Valley basins, such as the one in East Passage, are favorable sites 

 for the trapping of interstitial gas bubbles that are produced by 

 the decomposition of organic matter. 



Sediments within areas Brenton-A and Brenton-B are 

 composed of sand, gravel, and boulders derived from the reworking 

 of the extensive glacial drift deposits. A relict channel system, 

 trending northeast-southwest through the two areas, is buried by up 

 to 14 m of inferred fluvial estuarine and marine silt, sand, and 

 gravel. This system was rendered inactive after submergence by the 

 last marine transgression. An offshore surface expression of the 

 system persists due to incomplete infilling prior to the marine 

 submergence. Bathymetric contour configurations elsewhere 

 throughout the survey areas reflect the antecedent relief of the 

 glacial terrain. 



4.2.2 Side Scan Sonar Observations 



Side scan sonar observations revealed the poorly sorted, 

 coarse material associated with the glacial deposits. Sand, 

 gravel, and scattered boulders characterized most of the textural 

 units (Table 3-5) . Side scan data also indicated that recent sand 

 deposits, winnowed from the glacial and post-glacial sediments, 

 occurred in a thin (< 3 m) patchy veneer over the coarser-grained 

 material. Underlying topographic relief, bed roughness, 

 turbulence, and bottom stress contributed to the shape and 

 distribution of these sand deposits. Units 1, 2, and 5 (Brenton-B) 

 represented this potentially depositional sand veneer surface. 

 Units 3 and 4 represented highly reworked drift surfaces from 

 which, perhaps, the sand was derived. 



Active bedload erosion, transport, and deposition were 

 indicated, most notably in the Brenton-B area, by the megaripple 

 and sandwave forms. These forms are produced by current speeds in 

 excess of 50 cm/ sec (1 kt) . Given that average fairweather current 

 speeds are reported to be less than 0.6 kt in this area (Spaulding, 

 1979) , it is presumed that these forms occurred episodically in 

 response to storm-driven currents. When active, megaripple 

 migration produces a reworking of approximately the upper 3 cm of 

 sediment. Sandwaves are capable of reworking the upper 1 m of 

 sediment cover. During fairweather periods when the forms would be 

 inactive, sandwaves would be expected to persist while megaripples 

 would be expected to degrade in form. Swift et al. (1979) report 

 that this is the case in similar water depths off Long Island, NY. 

 It is interesting to note that the preferred orientation of both 

 forms, in Brenton-A and B, indicated that net sediment transport 

 was to the south-southwest. Predominant northeasterly storm winds 

 are capable of producing a south-southwesterly current due to the 

 effects of geostrophic flow. The paucity of bedforms in the 

 Brenton-A area, in comparison to the widespread distribution in 



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