F-4 



eugeosyncline . 



More recently, multi-channel seismic reflection surveys 

 have shown numerous prominent reflectors at great depths 

 along the continental margin, leading to estimates of up to 

 12 km of sediments lying beneath the shelf off New York 

 (Schlee et al . , 1976). Drilling of the COST B2 well off New 

 Jersey (Smith et al . , 1977; Scholle et ad., 1977) penetrated 

 4.8 km of Mesozoic and Cenozoic sands and shales interpreted 

 as having been deposited in shallow water environments. This 

 volume of sediments, along with numerous geophysical measure- 

 ments over the years, has prompted investigations seeking 

 subsidence mechanisms besides simple sediment loading of 

 geosynclines. It seems likely that much of the excessive 

 subsidence of the Baltimore Canyon trough is due to crustal 

 thinning related to the initial opening of the Atlantic some 

 200 m.y. ago. Steckler and Watts (1978) have summarized the 

 hypotheses for crustal thinning into three main groups: 1) 

 subcrustal cooling and thinning at the time of rifting; 2) 

 crustal stretching due to regional extension and marginal 

 loading; and 3) deep metamorphism increasing crustal density. 



The Georges Bank depocenter, represented here by the 

 Oceanographer and Lydonia Canyons, is located in an area of 

 lesser subsidence than Baltimore Canyon. Strata of similar 

 age are much shoaler than in Baltimore Canyon. As a result, 

 backcutting of Baltimore Canyon has exposed formations only 

 younger than 25 m.y.B.P. (Gibson et al . , 1968), whereas the 

 Oceanographer and Lydonia Canyons have excavated many geo- 

 logical formations that are typically as old as 95 m.y.B.P. 



