Figure 27 shows that the bedrock surface in the vicinity of Long Sand 

 Shoal is undulatory and has been deeply scoured by Pleistocene glaciers. The 

 irregular bedrock surface has been filled and covered by flat-lying sedimen- 

 tary strata which in turn are overlain by the almost horizontal strata com- 

 prising the shoal. Apparently, bedrock is about 11 meters beneath the shoal 

 at fix 214; cores 66 and 67A show that the strata on top (which make up the 

 shoal) are fairly uniform, fine and medium grain-size sands with thicknesses 

 of 3.2 and 4.7 meters, respectively. Core 68, taken on the southern flank in 

 31 meters of water, shows that the sand pinches out and overlies older fine- 

 grained sediments which probably are the horizontal strata filling the bedrock 

 depressions shown in Figure 27. Cores 64 and 65 to the north of the shoal 

 contain silty, very fine and fine-grained sediments which would be unsuitable 

 for fill material. 



Horizontal Scale 



Navigation Fixes 



CERC LINE I2i a 



204 





Water 



(m) 



Figure 27. Interpreted profile of seismic line 121a over Long Sand Shoal. 

 The Bedrock surface closely underlies the shoal which is charac- 

 terized by highly stratified sandy sediments covering both bedrock 

 and channel-filling fine-grained sediments. 



Based on the limited seismic and core data available, Long Sand Shoal 

 probably contains enough suitable sand to be designated a high potential 

 deposit. Most of the largest grain-size sand is apparently concentrated along 

 the main axis of the shoal or on the southern flank; the sediments on the 

 northern flank are finer grained and have higher percentages of silt and clay. 

 Using a thickness of 2.7 meters over the area shoaler than the -6-meter con- 

 tour yields a total sand volume of 6 million cubic meters. Although Long Sand 

 Shoal offers potential as a deposit, more densely spaced sediment samples are 

 needed to reliably define its potential. 



IV. SUMMARY 



A detailed geological study was made of Long Island Sound using high- 

 resolution seismic reflection profiles and long sedimentary cores to decipher 

 the Quaternary geologic stratigraphy of the Sound and, in particular, to 

 assess the sand and gravel resources. 



Results from the study show that the deepest acoustic reflector is the 

 bedrock surface which crops out on the Connecticut mainland and slopes south 

 to depths of -250 meters at the north shore of Long Island. Bedrock is over- 

 lain in isolated areas by Cretaceous strata which have been deeply scoured by 

 multiple glacial advances and long episodes of fluvial erosion. Pleistocene 

 sediments are ubiquitous as thick accumulations of firm lacustrine silt and 



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