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STRUCTURAL GEOLOGY OF NORTH AMERICA 



Cretaceous); of a glauconitic greensand, Navarro (equivalent of Mon- 

 mouth); of an indurated green silt not older than Miocene; and of an 

 impure glauconitic sandstone, late Tertiary in age, were broken from 

 the walls of newly charted canyons cutting the southern margin of 

 Georges Bank (Stetson, 1936). The thickness of the Tertiary sediments 

 cannot exceed 1500 feet, and the top of the Upper Cretaceous ranges 

 between 1450 and 1800 feet below sea level. Glacial drift and recent 

 material mantle the gentler slopes, but in several places the older forma- 

 tions crop out on the steeper slopes. It is clear, therefore, that the Atlantic 

 Coastal Plain, made up of Tertiary and Cretaceous sediments, continues 

 eastward from the New York region and forms Georges Bank. 



George H. Chadwick has prepared sections across the Gulf of Maine 

 and Georges Bank showing the composition and evolution of the sub- 

 merged coastal plain, and he has given permission to reproduce them, 

 although they have not been published. See Fig. 11.33. These sections 

 integrate the erosional surfaces, the sediments of Georges Bank as recog- 

 nized by Stetson, an extension of the Nova Scotian Triassic trough, 

 eustatic changes in sea level, and two stages of glaciation. The Lower 

 Cretaceous Potomac is a projection from the New Jersey coastal plain 

 and has not been sampled by the dredge. 



Seismic Profiles. Refraction profiles have been run by Drake et al. 

 (1954) and Officer and Ewing (1954) of the Gulf of Maine and con- 

 tinental shelf off Nova Scotia. These support the conclusions of Chad- 

 wick and Stetson as far as the unconsolidated and semiconsolidated sedi- 

 ments go (Cenozoic and Cretaceous). Compare Figs. 11.32 and 11.34, 

 B-B'. It will be seen that the unconsolidated sediments are very thin 



over the Gulf of Maine and only thicken under the shelf slope. 



The Triassic trough sediments are believed to exist, as Chadwick 

 pictured them, on the basis of the layer that yielded the 3.7-4.02-km/sec 

 velocities (Drake et al., 1954). 



The crystalline basement appears complicated by layers with lower 

 than normal velocities. The 4.6-km/sec velocity layer south of Yarmouth 

 (section C-C), the 4.52-5. 13-km/sec layer under the shelp slope and 

 rise off Nova Scotia (sectionA-A'), and the 5.11-4.78-km/sec layer in 

 the same place off Georges Bank (section B-B') are the cases in point. 

 They have been interpreted by Drake et al. to be part of the crystalline 

 basement on the grounds that Katz et al. (1953) found two layers in 

 Maine, recording at Falmouth, with a velocity of 5.34 km/sec for the 

 upper and 6.24 km/sec for the lower. These are both somewhat higher 

 than the presumed equivalents under the Gulf of Maine. In a study of 

 the Outer Ridge and Blake-Bahama basin (reviewed in Chapter 10) a 

 5.2-km/sec velocity layer on a =•= 6.5-km/sec velocity layer was theorized 

 to be a mass of extruded basalt on the typical "oceanic basalt" layer. The 

 Gulf of Maine "anomalous layer" has velocities somewhat slower than the 

 'Volcanic" layer under the Outer Ridge, and also lies on the crystalline 

 basement—not on the ocean basalt layer. It would appear, therefore, that 

 the anomalous layer is part of the Paleozoic complex of New England. It 

 could be a mildly metamorphosed Carboniferous basin type of deposit, 

 or conceivably a Mississippian (?) volcanic accumulation. 



The floor of the continental shelf and shelf slope sediments off Nova 

 Scotia and Georges Bank show a depression or trench similar to that off 

 New Jersey. Refer to Fig. 10.6. 



