SECT. 2] 



ABYSSAL PLAINS 



361 



to receive turbidity -current sediments, one might envisage a rough sea floor j £/'/> -/c 

 and a marginal trench bounding the continental slope. Turbidity currents 

 would first fill up the trench and the rate of this filling would depend not only 

 on the amount delivered to the submarine canyons and hence the frequency of 

 turbidity currents, but it would also depend on the rate of subsidence of the 

 trench. However, given a sufficiently long time, the trench would eventually 

 be filled and the turbidity-current sediments would overflow the outer wall of 

 the trench and flow out on the deep-sea floor. As long as slopes exceeded 

 1 : 1000, the growing smoothed area would resemble the surface of the con- 

 tinental rise with many deep-sea channels, submarine canyon systems, abyssal 

 cones and deep-sea fans. However, as the basin floor begins to fill and the wedge 

 of sediment building out from the shore reaches the greatest depths of the 





iff'' 





^i:i^tiili 





Fig. 33. Two Precision Depth Recorder records across the Biscay Abyssal Plain. (After 

 Heezen et al., 1959.) 



ocean basin, a distinct change in the topography will begin to take place. As 

 turbidity currents begin to build up against the seaward side of the oceanic 

 basin, the depositional topography will change character from that resembling 

 the topography of a continental rise to that resembling the topography of the 

 modern abyssal plains. Gradually, as more sediment is supplied, the abyssal 

 plain will widen and fill. If, after some time, tectonic warping uplifts the edge 

 of the plain or a decrease in turbidity-current activity or a change in the type 

 of turbidity-current sediment takes place, certain portions of the plain will 

 become relic and no longer be receiving turbidity-current sediment. 



These relic jDlains may be relic because they have been uplifted. Some will 

 have been cut off from land by the subsidence of a new trench, as in the case 

 of the northern "Aleutian Abyssal Plain". There are areas of the sea floor of 

 very gentle relief which are distinctly elevated above the abyssal plains and 

 no longer connected to them. Such an area lies in the center of the Argentine 

 Basin. The very smooth topography in this area of nearly abyssal-plain flatness 

 may mean that this area was at one time an abyssal plain which has later been 

 uplifted and is now cut off from a supply of turbidity currents. The sediment 

 (nearly 2 km) lying beneath this rise may have been partially contributed by 

 turbidity currents before the elevation of the rise. 



13— s. Ill 



