SECT. 3] 



ESTUARIES, DELTAS, SHELF, SLOPE 



647 



that the sedimentation on continental shelves results largely from Pleistocene 

 processes, which were much more efficient than those acting now, except, per- 

 haps, for the Arctic Ocean and Antarctica, which may provide a picture of the 

 conditions existing formerly on other shelves, and for some restricted areas 

 where large overloaded rivers flow into the sea (e.g. the Hwang Ho and the 

 Yangtze in the western parts of the Gulf of Pohai and the East China Sea: see 

 Niino and Emery, 1961). Even in coastal regions which were not directly 

 affected by glaciations, the terrestrial supply is likely to have been much larger 

 during the cold periods of the Pleistocene, when the shelf, under periglacial or 

 "pluvial" climates, was widely exposed as dry land and the sedimentary dis- 

 charge was higher. The deposition proceeds now at a much lower rate, 

 especially on the outer parts of the shelves which are no longer within the reach 



165" 



160" 



155° 



150 145 



140° 135° 



130" 125" 



72" 



70' 



68' 



Inset 



72° 



See 



inset 



'^"oo-X^" 



/ 



-^-Ji Pt. Barrow 

 £iii _J 



Median diameter 

 in microns: 



• <4 



o 4-8 



A 8-62 



+ 62-1000 yij 



X >1000 Banks 

 ils 



Cape 

 Lisburne 



A 



rnfff 



Barrow <^ 



165° 



160° 



155° 



rP • 200--** • 



^Mackenzie 



76° 



150° 



145 



140° 



135° 



130° 125° 



Fig. 22. Median diameter, expressed in microns, of sediments on shelf and slope in Beaufort 

 and north-east Chukchi Seas. Contours in fathoms. (From Carsola, 1954, fig. 4.) 



of big sedimentary supplies owing to the retreat of shorelines after the post- 

 glacial transgression. The reduction in grain size from outer to inner shelves 

 may also be due to a general reduction in river competence, and thus have a 

 climatic significance. 



The low rates in terrigenous sedimentation may have prevailed for a much 

 longer time off arid or semi-arid regions, where river discharge, although it was 

 increased during pluvial Pleistocene periods, was not able to compete with more 

 humid climates. This is probably the case for the continental shelf of Western 

 Austraha (Carrigy and Fairbridge, 1954), and the Persian Gulf (Emery, 1956; 

 Houbolt, 1957), where the hot dry chmate results in slow accumulation, 

 relative abundance of organogenic calcareous deposition, and dominance of 

 silt and clay in the non-calcareous fraction, partly contributed by wind supply. 



