SECT. 3] 



TURBIDITY CURRENTS 



753 



top of a turbidity-current bed is due to churning by sediment-burrowing 

 organisms. Apparently most burrowers confine themselves to the upper 10 or 

 20 cm. Thus, the 1x3 wer parts of graded beds, greater than 10 or 20 cm in 

 thickness, are rarely disturbed by burrowing. 



The shallower- water origin of at least a part of the coarser material making 

 up turbidity-current layers is frequently proven by the presence of a certain 

 species of Foraminifera and particles of calcareous algae or other plant debris. 



3000- 



4000- 



5000- 



FM 



W' 



■n 





NAUTICAL MILES 



1^ 

 



!iw iilill 



l'i'liii|!:ii|:li|liii!Fi; 



; . . . v:'i:r- .itjil, II ,.: 





;■!•■ |ii 

 -.1 Mt 



i ;l!, ,1 



' «.l', ' 'te';"'''!"' 'yt'' ■'"' ' 'rli' '■.■'■■'?, 



'.'.:.,:.; ■:-":: mt^ '''J'-'^: 



,r.fi I'- »f 





■i: ■■ ■, I 



■ .1'-, 



-5 



N 



Lio 



KM 



Fig. 9. Seismic reflection profile across the Puerto Rico Trench. Tliis profile, obtained by 

 John Ewing indicates over 1 km. of sediments below the trench abyssal plain. Beds 

 of calcium carbonate -rich graded sediment have been found intercalated in abyssal 

 clays beneath the trench abyssal plain. These sediments containing fragments of 

 calcareous algae were transported by turbidity currents from south to north into the 

 trench. 



In some layers there occurs a concentration of coarse vegetal debris several centi- 

 meters thick. These concentrations are usually associated with silts or fine 

 sands, and individual particles may be several millimeters in diameter. In other 

 layers, organic material is present but only as dark gray or nearly black pigment 

 in the upper lutite fraction. 



Frequently turbidity-current beds deposited in the greater depths of the 

 ocean have a much higher lime content than the normal abyssal lutite. This is 



