770 



HEEZEN 



[chap. 27 



fied by coring or drilling. Worzel (1959) has correlated a prominent sub-bottom 

 echo in the east tropical Pacific with a 2-4-in. thick bed of volcanic ash. Heezen 

 et al. (1958) have correlated a prominent sub-bottom echo with the base of the 

 Recent anaerobic layer in the Cariaco Trench. Gorsline and Emery (1959) 

 have mapped turbidity-current beds in the San Pedro Trough. In areas of 

 particle by particle deposition where one or two prominent 5-20-fm deep sub- 

 bottom echoes can be traced over a wide area they typically show deepening in 

 the small depressions, thinning over gentle rises, and are totally absent on 

 steeper inclines, indicating that, owing to gentle scour, sedimentation has been 

 greater in the depressions (Fig. 22). 



D>.-: 



Pm'f 



\i^>i'< 



8 NAUTICAL MILES 



/440 



Fig. 22. Precision Depth Recorder echogram made west of Panama by R.V. Vema. The 

 strong, sub-bottom echo, 5-10 fm below the sea floor, is reflected from the Worzel Ash. 

 Note the slight, distinct variations in thickness of the sediment above the ash. These 

 variations are presumably due to the combined action of bottom scour and minor 

 gravitational movements. 



The gentle but continuous wafting siiggested by Riedel in explanation of the 

 distribution of Radiolaria would seem to be a satisfactory explanation of these 

 configurations. In other areas the simple sub-bottom echo, while still detectable, 

 became intensely disturbed, possibly suggesting that the bed is intermittent or 

 has been folded. 



The occurrence of a series of closely spaced reflectors on an otherwise smooth 

 bottom (Heezen et al, 1959, pi. 5) may indicate deep-sea dunes (2400 fm) 

 produced by strong bottom scour. Echogram profiles taken in depths of 300 fm 

 beneath the Gulf Stream were interpreted in a similar manner. 



Photographs of manganese nodules (Dietz, 1952; Elmendorf and Heezen, 

 1957) taken in great depths of the Atlantic Ocean floor, and more recently on 

 the Easter Island Ridge of the southeast Pacific (Revelle, 1958), tell us that 

 sedimentation in these areas is virtually at a standstill. Features in the sediment 

 around the nodules resemble scour marks. Thus the gentle scour and reworking 

 of the abyssal red clay is supported both by the study of cores and by examina- 

 tion of the bottom photographs. 



