354 HEEZEN AM) ]>AL'(;llTOX [ClIAP. 14 



Plains (Fig. 30). In such areas the snb-bottoms are prominent and reach to 

 de])ths of 20 fni beneath the sea floor. The ex]>lanation of this typical distribn- 

 tion is similar to that offered for the more deei)ly penetrating seismic-reflection 

 measurements; that is to say, in the portions of the plain close to the continental 

 rise, turbidity currents have deposited thicker and coarser beds, which contrast 

 markedly A\ith the products of normal pelagic sedimentation. Therefore, many 

 sharp reflecting horizons are present in these areas. However, due to the 

 relatively limited area covered by the single turbidity-current deposit and the 

 marked lateral variations in thickness in such a deposit, the resulting sub- 

 bottom echos do not persist for great distances. 



However, at the seaward edges of the abyssal plains, the sediment carried in 

 by turbidity currents and that resulting from normal pelagic sedimentation are 

 ])robably quite similar, both consisting of extremely fine clays. Sedimentation 

 of clays by ponded turbidity currents may greatly increase the rate of sedi- 

 mentation in these areas. In this region, certain notable events in pelagic 

 sedimentation, such as vast ash falls or changes in the carbonate content of the 

 sediments caused by relatively sudden changes in the circulation of the deep- 

 sea water-masses, could form preservable major interfaces between sediments 

 of different com])osition or different compaction and thus provide an explana- 

 tion for the very prominent sub-bottom echos found at the seaward edges of 

 abyssal plains. 



B. Seismic Refraction 



Many seismic-refraction measurements have been made in the "ocean-basin 

 floor" of the western North Atlantic (Ewing and Ewing, 1959). These results 

 can be divided into two categories, depending on \^hether the measurements 

 were made (1) on the abyssal floor or (2) on an oceanic rise. Measurements on 

 the abyssal floor of the western Atlantic revealed a simple pattern. Beneath 

 4-5 km of water lies 0.5 to 1 km of sediment and sedimentary rock with a 

 compressional wave velocity of about 2 km/sec overlying a 4.5 km/sec layer 1 

 or 2 km thick. Beneath this lies 3-4 km of oceanic crustal rocks (approximately 

 6.5 km/sec) and the sub-M mantle rocks with a velocity of about 8.1 km/sec. 

 This pattern has been observed by most workers in the abyssal floors of other 

 oceans. In contrast, seismic-refraction measurements on the oceanic rises have 

 revealed distinctly different and variable crustal layering. In the vicinity of 

 Bermuda, Officer, Ewing and Wuenschel (1952) and Katz and Ewing (1950) 

 failed to observe either the typical oceanic crustal velocities or the ty|:)ical 

 mantle velocities found in the abyssal floor. Instead, a thickened 4.5 km/sec 

 layer overlay a 7.5 km/sec deepest layer. AlthougJi the oceanic rises have 

 a distinctly different crustal structure from the abyssal ])lain no difference 

 has been noted between observations made on the abyssal plain and those made 

 in the adjacent abyssal hills. The sediment thickness in the abyssal hills is, 

 however, markedly less than on the abyssal ])lain. 



