■r' 



' ■ / 



754 HEEZEN [chap. 27 



particularly true of the sharply defined layers of graded lutite often found in 

 cores of the abyssal plains of the North Atlantic. In these, the coarse fraction is 

 either absent or is represented by a mere film of silt at the base. The lime con- 

 tent is from two to three times as great as the interbedded abyssal red clay. 



A peculiarity common to all these turbidity-current layers, whether composed i 



of sand or very fine sediment, is their discordance in physical, chemical and 

 biological make-up from the environment in which they occur. They are truly | 



displaced sediments. Usually some of the material, and sometimes most of it, 

 is of shallow-water origin. However, in most cases, good sorting, thin bed- '^'''Y'",^ 

 ding and grading set deep-sea turbitites distinctly apart from typical shallow- 

 water marine deposits. 



Another highly important characteristic of these sediments is their distri- 

 bution with respect to ocean-bottom topography. Graded layers, and layers of 

 gray calcareous lutite, have been found in submarine canyons, on the continental 

 rise and on the gently sloping abyssal plains and in the abyssal plains of the j 



oceanic trenches. They have never been found on isolated rises. Some of the 

 most dramatic evidence comes from two cores taken from the narrow trench 

 abyssal plain which lies at about 4580 fm at the bottom of the Puerto Rico | 



Trench (Fig. 9). These cores consisted of abyssal red clay at their base; but ' 



each contained a graded bed of calcareous sand, silt and clay which included 

 fragments of the alga Halimeda. Since living Halimeda requires sunlight this I 



detritus must have originated near the shore . The graded layers of calcareous sand i 



also contained shells of pteropods and shallow-water species of Foraminifera, 



However, cores taken on the ridge north of the trench, in depths between , 



5000 and 6000 m contain only red clay of abyssal facies. Elsewhere, depth | 



differences of no more than 100 m have been found to be enough to separate an ' 



almost continuous succession of graded beds from uniform foraminiferal lutite. ! 



Over 200 cores have been obtained from the abyssal plains of the North I 



Atlantic. Each core taken in an abyssal plain contained either sands or silts, j 



many with shallow-water Foraminifera (Figs. 10 and 11). Most cores from the 

 flanks of the Mid-Atlantic Ridge, the abyssal hills and the oceanic rises contained '^ Jo.^S'ff . 

 only pelagic deposits, although a few at the bases of islands or from the crest { 



zone of the ridge contained coarser material (Ericson et al., 1961 ; Heezen, Tharp i 



and Ewing, 1959). Deep-sea sands and even gravels have been found associated 

 with fans or cones at the mouths of submarine canyon systems (Fig. 12). 



A concept which might be described by the term "elevator tectonics" (or 

 perhaps "lift faulting") has often been employed in the explanation of dis- 

 placed sediments interbedded in pelagic sequences (Cushman, 1941; Landes, ' 

 1952). The latest proponents of this concept are Greenman and LeBlanc (1956) 

 who invoke a 10,000-ft subsidence of the entire central portion of the Gulf of i 

 Mexico at the close of the Wisconsin (11,000 years ago) to account for the | 

 emplacement of gray silts now found on the floor of the Sigsbee Abyssal Plain. ' 

 These gray silts have been explained by Ewing, Ericson and Heezen (1958) as ] 

 the results of turbidity-current deposition of sediment contributed by the 

 Pleistocene Mississippi River. Cores from low knolls rising above the abyssal 



