249 



The phosphorite is associated with glauconite. Both materials are authigenic 

 deposits and are typical of shallow sea-floor environments that have a very 

 low rate of accumulation of detrital or biogenic sediments. The area of their 

 maximum concentration is just such an environment. However, there is a 

 strong probability that the phosphorite is residual from weathering and ero.sion 

 of outcrops of phosphatic strata. On the adjacent land .several Miocene and 

 Pliocene formations contain phosphorite that is exploited chiefly for fertilizer. 

 The seaward regional dip should permit these formations to crop out on the 

 slope, if the slope is a fault partly mantled with sediments, as Sheridan and 

 others (1905) indicate is true of the East Florida Escarpment. Topographic 

 irregularities on the slopes also suggest that narrow structural terraces were 

 developed by outcrops of resistant strata (Ileezen and others, 1959). The 

 question of quantity and quality of the phosphorite can be determined only 

 l)y detailed sampling and laboratory analyses. During 1963 the total value of 

 phosphorite mined on land in the United States was only about $140 million 

 (D'Amico, 1964) ; thus mining of it from the sea floor may be uneconomical. 



MANGANESE NODULES 



Many samples and bottom photographs (Pratt, 1963) have shown an abun- 

 dance of nodules and crusts of manganese oxide on the Blake Plateau (fig. 1) 

 between depths of 750 and 1,050 m. The nodules occur in a thin blanket of 

 globigerina and pteropod ooze lying on Pliocene and Miocene limestones. They 

 range in size from sand grains to flat masses nearly a meter in length, but 

 the majority are subspherical nodules 10 to 20 cm in diameter. The encrusta- 

 tions are solid4ooking coatings on the bottom, covering continuous areas of 

 at least 10 sq m, the space depicted by underwater photographs that show them ; 

 they are rarely if ever removed by sampling devices. 



Analyses reported by Mero (1960, 1962) suggest that the nodules from the 

 Blake Plateau are of low quality for metallurgical purposes as compared with 

 those that have been dredged from the deep-sea floor of the Pacific Ocean. 

 However, it is possible that higher quality nodules occur at the seaward side 

 of the Blake Plateau, which is more distant from diluting detrital sediments. 

 The nearness of the Blake Plateau to ports and its relatively shallow depth 

 may permit this area of the sea floor to be one of the first to be economically 

 exploited for manganese. The total value of manganese of comparable quality 

 mined in the United States during 1961 was only about $1.5 million (D'Amico, 

 1964), but the manganese nodules from the sea floor contain trace elements 

 (mainly cobalt, nickel, and copper) perhaps more valuable than their manga- 

 nese matrix. 



PETROLEUM 



A sizable fraction of the oil and gas production of the world already comes 

 from the shallow sea floor off the coasts of the United States and Europe, and 

 in the Persian Gulf. The Atlantic Coastal Plain sediments are not very petrolif- 

 erous, but farther seaward these sediments become thicker, more marine, 

 and possibly better source beds of petroleum. Suitable petroleum-bearing struc- 

 tures may be associated with large tectonic structures that exist in three main 

 areas shown by figure 1. The southern area is along a seaward extension of the 

 Cape Fear arch, which is well known on the adjacent land. A second area 

 just southeast of New York City follows a major strike-slip fault. The north- 

 ern area underlies the continental shelf between two tectonic troughs mapped 

 by Drake and others (1959). The basement rock of tiie ridge between the troughs 

 is overlain by 1.5 to 2.5 km of Cretaceous to Pliocene strata. Farther south- 

 west the ridge underlies deep water of the continental slope, but farther 

 northeast it becomes shallower in the vicinity of Sable Island (beyond the 

 limits of fig. 1) and it probably extends to the northwestern part of the 

 Grand Banks of Ne\vfoundland. Detailed geophysical exploration may reveal 

 the presence of isolated high portions of the ridge worthy of testing by drilling. 



At the base of the continental slope is a broad gently sloping feature, the 

 continental rise. Continuous seismic profiling indicates that it consists of 

 numerous interbedded strata of alternating high and low acoustic impedence. 

 These layers probably consist of sandy turbidites separated by clayey hemi- 

 pelagic beds — a flyschlike deposit. Future testing by drilling may show that 

 the clays are petroleum source beds and that the sands are reservoir beds 

 (Emery, 1963). 



