continuous reflector that was noted to truncate underlying 

 reflectors. This unconformity was irregular and incised with 

 depressions up to 14 m in depth (Figure 3-27) . Depressions were 

 infilled with the basal part of the overlying acoustic unit. In 

 general, this upper unit was well-laminated. Internal reflectors 

 were continuous, parallel, and flat-lying to dipping. Onlap fill 

 and cut-and-fill structures were noted within the deeper 

 depressions. The upper unit was inferred to consist of post- 

 glacial fluvial, estuarine, and marine deposits of gravel, sand, 

 silt, and clay. 



The closed depressions noted within Brenton-A and B 

 (Figures 3-23 and 3-24) are thought to be surface expressions of a 

 relict, post-glacial drainage system that directed flow to the 

 southwest along the landward side of the Charlestown-Buzzards Bay 

 recessional moraine. The unconformity observed between the two 

 seismic units discussed represents the stream-cut surface of this 

 relict system. The irregular 20 fm (120 ft) bathymetric contour in 

 the southeastern half of Brenton-A does not reflect a subsurface 

 drainage system; rather, this depression was the result of modified 

 surface relief of the glacial drift deposit. 



Narragansett Bay : The Holocene stratigraphy in 

 Narragansett Bay, as interpreted from seismic profiles, has been 

 reported in detail by McMaster (1984) . In East Passage, the trunk 

 valley was infilled by >20 m of glacial drift. These deposits were 

 cut by the post-glacial fluvial unconformity and overlain by as 

 much as 10 m of estuarine sand, silt, and clay. An 8.4 m vibra- 

 core obtained from a location approximately 0.7 km south of the 

 disposal site off Prudence Island yielded fine-grained (silt) , 

 Holocene marine sediments up to 7,000 years in age (McMaster and 

 Friedrich, 1985) . Paleomagnetic analyses indicated that the upper 

 1.5 m of core material had been highly disturbed due, presumably, 

 to human activity. 



3.2.3 Surface Textural Characteristics 



Brenton-A: Four textural bottom types were mapped at the 

 Brenton-A area based on side scan sonograph interpretations and 

 grain size data from NED (Figures 3-28 and 3-29, Tables 3-5 and 3- 

 6) . Unit 1, which predominated in this area, consisted of a 

 relatively uniform and featureless (no bedforms) silt-sand-gravel 

 surface with, occasionally, scattered boulders. This unit 

 characterized much of the northeast-southwest trending, 37 m closed 

 contour depression and extended to the southeast through the 

 central portion of the region. Unit 2 was found to either side of 

 unit 1, in slightly shallower depths. This unit was characterized 

 by sand with gravel depressions of approximately 1 m relief. 

 Dimensions of these depressions varied greatly. The average 

 feature appeared approximately 50 m by 50 m, but some were linear 

 (10-20 m wide by 40-50 m long) with no preferred orientation. 



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