SECT. 2] 



ABYSSAL PLAINS 



353 



largely limited to intermontane basins and perliaps averaging one-tenth of the 

 thickness found below the abyssal floor. 



The sediment blanket beneath the abyssal plains covers a sub -bottom relief 

 similar to the abyssal hills, strongly suggesting that the abyssal plain was 

 created by the burial of pre-existing relief. The contrast in average sediment 

 thickness between the abyssal hills and the mid-oceanic ridge on the one hand 

 and the abyssal plain on the other is easy to understand in the above terms, but 

 the similarity in sediment thickness between the abyssal plains and the abyssal 

 rises presents a difficulty. The smoothing, it would seem, was accomplished in 

 the course of the deposition of a small fraction of the total sediment thickness. 

 Otherwise the sediment thickness beneath the plains should be measurably 

 larger than beneath the oceanic rises if we assume that sedimentation began in 

 both areas at the same time and has been of similar type in the two provinces. 

 It is beyond the scope of the present paper to consider the validity of these 

 assumptions. Suffice it to say that it may not be legitimate to assume that the 





Prominent Scarp in Abyssal Plain 





2900 rif^qfy. 



-i|iiiil^:i#7;'H=' 





East 



Seaward Edge. Madeira Abyssal Plain 

 Note prominenl sub-bottom echo. 



West 



10 Nautical Miles 



Fig. 30. Precision Depth Recorder records from the Madeira Abyssal Plain. Depth in 

 fathoms. (After Heezen et al., 1959.) 



sedimentation began at the same time on the rises as on the plains. This 

 problem is involved in the general problem of the geology of the ocean basins in 

 relation to continental drift. 



Sub-bottom reflections have been observed on Precision Depth Recorder 

 records in the continental rise, on oceanic rises and many other environments 

 of the deep-sea floor. However, the character of the sub-bottom reflections 

 observed in the abyssal plains is generally quite different from those observed 

 on the rises. Over most of the abyssal plains, sub-bottom reflections are either 

 absent or, if present, persist as correlatable continuous horizons for relatively 

 short distances. This is in sharp contrast to the oceanic rises, where sub-bottom 

 horizons can be traced for dozens or hundreds of miles. However, a marked 

 exception to this rule often occurs near the seaward edges of the abyssal plains, 

 where sub-bottom reflections become very strong. Some of the deepest sub- 

 bottom reflections traced by ordinary echo-sounders in deep water have 

 been observed near the seaward edges of the Madeira and Nares Abyssal 



