SECT. 2] ABYSSAL PLAINS 351 



physiographic provinces. Tlie distribution of sands and silts correlates very 

 well with the pattern of i)hysiograi)hic provinces. In the abyssal plain areas, 

 all the cores contained sand or silt, whereas on the Bermuda Rise, sand or silt 

 was definitely lacking, except on the archipelagic plain around the Bermuda 

 Pedestal, where the sediments are obviously derived from Bermuda. On 

 the continental rise, both sand-silt and purely clay-ooze cores are found, 

 indicating that although canyons and channels running down the slope distri- 

 bute the sands and silts over wide areas, broad, slightly elevated areas of the 

 continental rise receive for a relatively long period of time only clays and 

 pelagic oozes; whereas cores obtained from canyon floors, canyon banks, 

 abyssal cones and deep-sea fans receive sands and silts intermittently and 

 fairly regularly. A few of the sands and silts shown on Fig. 28 may be related 

 to scour b}" bottom currents. Probably not all of them are due entirely to 

 turl)idity-current sorting (Heezen, 1959). The surface characteristics of the 

 sediments on the abyssal plains, as revealed by preliminary studies of bottom 

 photographs, do not seem to show any striking contrast with the sediments of 

 the continental rise or of the oceanic rises. Both are covered with the small 

 mounds, holes and tracks of benthic life (see Chapter 18). 



7. Geophysical Characteristics 



A. Seismic Reflection 



Seismic-reflection measurements were the earliest geophysical investigations 

 made in the abyssal-plain areas (Hersey and Ewing, 1949). It M^as early noted 

 that seismic reflections taken from the abyssal plains were markedly diff"erent 

 from those observed on the adjacent oceanic rises. The records obtained from 

 the Bermuda Rise and the continental rise generally Avere relatively simple, 

 showing one, two or three sub-bottom reflections, which, in general, could be 

 correlated over relatively long distances. Records obtained on the abyssal plain 

 in contrast showed a strong bottom reflection followed by a series of reflections 

 so closely spaced that they formed a continuous reverberation (Fig. 29). At first 

 the difference between the two types of records was not understood. The records 

 from the rises, which showed a relatively few strong and persistent reflectors, 

 were interpreted as evidence of persistent layering in the relatively thick sedi- 

 ments covering the rises. The lack of discrete correlatable sub-bottom reflections 

 in the abyssal plain areas was interpreted as evidence of very thin sediment. 

 Seismic-reflection records on the abyssal plains have been satisfactorily re- 

 interpreted as multiple reflections from the thinly bedded sands and pelagic 1 

 sediments underlying the abyssal plain. As examples, typical records from the | 

 Bermuda Rise and from the adjacent Sohm Abyssal Plain are illustrated in 

 Fig. 29. 



In the first seismic -reflection exploration of the deep sea, measurements 

 were spaced every 2 to 10 miles, corresponding to a shot interval of 15 min to 

 1 hr. Reflecting horizons which would not correlate at this interval were con- 

 sidered either not significant or too complex to be investigated on a regional 

 reconnaissance basis. 



