140 



STRUCTURAL GEOLOGY OF NORTH AMERICA 



in the Upper Cretaceous ( Ewing et al., 1939 ) . Richards thinks the M zone 

 in the Upper Cretaceous is the contact between the Magothy and Mata- 

 wan or Magothy and Merchantville formations. See section 32, Fig. 10.2. 

 If so, about 700 feet of Upper Cretaceous strata, which generally underlie 

 the Magothy, and 1000 feet or more of Lower Cretaceous would be in the 

 semiconsolidated layer. In southeastern Virginia the Eocene rests on 

 the Lower Cretaceous, and the M zone is probably absent; but perhaps 

 seaward the Upper Cretaceous comes in again, and the zone is present. 



Contour of Crystalline Basement Surface 



In a paper of 1950, Ewing et al., report on profiles off Cape May, New 

 York, and Woods Hole, and concluded that the Precambrian surface does 

 not slope constantly toward the Atlantic Ocean basin floor but has a pro- 

 nounced reversal of dip at a depth of 16,000 feet before the margin of the 

 shelf is reached off Long Island and Delaware Ray. Structure contours 

 on the surface are shown on Fig. 10.6. Farther south off the Cape Fear 

 arch the slope of the crystalline floor reflects the arch nearly to the mar- 

 gin of the shelf (Richards, 1945, 1947; Rerry, 1948; Hersey et al, 1959). 

 The surface is lost seaward over the Rlake Plateau, where no seismic 

 record of it or deeper boundaries of velocity layers were obtained. See 

 Fig. 10.7. The strike of the surface veers westward in South Carolina 

 and northern Florida. Near Jackson, Florida, the surface dips steeply 

 southward and is lost at a depth of 19,000 feet. The basement contours 

 here are distinctly discordant to contours drawn on the top of the 

 Cretaceous (Fig. 10.6). 



In the shelf profiles off Long Island and Delaware Ray the unconsoli- 

 dated sediments thicken gradually outward under the shelf. In the upper 

 section of Fig. 10.7 Heezen et al. (1959) show a ridge of basement rock 

 at the shelf margin and then an abrupt fall-off apparently of fault nature. 

 Oceanward is a second basin in which the unconsolidated and consoli- 

 dated sediments attain a maximum thickness of 33,000 feet ( 10.3 kilome- 

 ters). The unconsolidated layer thins over the deep Atlantic floor to about 

 2 kilometers, but becomes much thicker again on the approaches to the 

 Rermuda Rise and Mid-Oceanic Ridge. 



The seismic profiles across the Atlantic Coastal Plain and continental 

 shelf to date have been summarized by Drake et al. (1960), and these 



writers point out that a ridge of basement rock near the edge of the shelf 

 is a common feature. It separates two sedimentary troughs, one under the 

 shelf, and another in deeper water under the shelf slope and rise. The 

 ridge and basins can be seen in the upper section of Fig. 10.7 and section 

 A-A' of Fig. 11.34. The sediments in the inner or shelf trough have been 

 drilled in several places along the Atlantic Coastal Plain and are mostly 

 shallow water sands, silts, and clays. Cores of the upper part of the sedi- 

 ments of the outer trough have revealed features attributed to slump- 

 ing, sliding, and turbidity currents, and are in part similar to graywackes. 

 Drake et al. point out that the size of the troughs and the thickness and 

 character of sediments in them are similar to the early Paleozoic troughs 

 of the Appalachians as restored by Kay (1951) and that here is a good 

 representation of the miogeosyncline (inner trough) and eugeosyncline 

 (outer trough). Compare with Figs. 11.17, 6.6, and 6.15. Evidence of past 

 volcanism in the outer trough is present in the form of partially buried 

 seamounts with large magnetic anomalies. 



The eugeosyncline, according to the above view, develops largely on 

 the oceanic crust, and represents, when uplifted, an accretion to the 

 continent. The theory appears very attractive when related to the Paleo- 

 zoic Cordilleran geosyncline. 



Submarine Canyons. Comprehensive submarine surveys of the whole 

 of the continental shelf and slope of the northeastern United States have 

 been made since 1930, using the most advanced methods, and the results 

 were published in 1939. Chart 1 of the publication, "Atlantic submarine 

 valleys of the United States and the Congo submarine valley" (Veatch 

 and Smith, 1939) is a composite of all the modern work from Cape 

 Hatteras to Georges Rank. The same results are presented in reduced 

 scale and somewhat simplified on the Tectonic Map of the United 

 States. 



The shelf is a fairly smooth plain and a continuation of the emerged 

 Coastal Plain. The most prominent feature is the Hudson channel, which 

 is entrenched 50 to 150 feet in the shelf from the mouth of the Hudson 

 River to near the edge of the shelf. South of the channel are many shallow 

 depressions and low, irregular ridges trending generally parallel with the 

 shore. They have been likened to bars and lagoons. Northeast of the chan- 

 nel, the shelf is a regular oceanward slope, perhaps rilled with many 



