2. Bottom Deposits of Regions with Special Submarine Configuration 



A. Bottom Deposits of Ridges and Banks 



There are ridges and banks believed to be a product of tectonic activit\;, 

 and, leaving aside one case of foundation rocks, the others are built of fossil- 

 bearing Tertiary sediments whose summits are covered with coarse- 

 grained sand formation. Fine sand is distributed around the ridges to 

 considerable depths. There are many such ridges and banks arranged 

 parallel to the coast of the Japan Sea, while on the Pacific coast they 

 have been recognized only at the mouths of the Suruga and Tosa bays. 



B. Botto7n Deposits of Submarine Valleys 



The submarine valleys of the Japanese Seas are imbricate in pattern, 

 with steep sides which expose fossiliferous Tertiary formations, and the 

 valley wall of the submarine valley of Shikoku exposes Pleistocene rocks. 

 Gravel and fine sand is distributed on the walls of the submarine valleys, 

 while in the central parts are found muds. Judging from the grade 

 distribution of the sand and mud in the vicinities of submarine valleys 

 it appears that at present the valleys are being gradually filled by the 

 action of oceanic currents. 



TRANSPORTATION AND DEPOSITION BY TURBIDITY 



CURRENTS 



By Ph. H. Kuenen, Groningen, Holland 



[A bstract] 



Experiments by the author confirmed that water containing a load 

 of sediment will flow under the influence of gravity along the bottom 

 of a body of standing water. The maximum density is 2 ; with higher 

 concentration the turbid current changes to a slide. 



Turbidity currents of high density might be set up on the sea floor 

 by : (a) slumping, the material becoming more diluted ; (5) by a mud- 

 flow on land reaching the coast and continuing along the bottom ; (c) 

 the waters of a heavily silt-laden river, if heavier than sea-water, might 

 flow along the bottom beyond the mouth ; {d) by churned up mud 

 initiating a flow, which picked up more sediment and thus increased its 

 density. 



A remarkable property of currents of high density is the fantastic 

 power of transportation. With a destiny of 2 individual blocks can be 

 transported 14,000 times as heavy as by a clear current of equal velocity 

 and volume. 



It appears likely that the currents might transport great volumes of 

 silt, sand, and even coarser fractions over large distances of submarine 

 slopes. They should continue over basin floors and mount the opposite 

 slope owing to their great momentum, then depositing the coarser 

 fractions. The fine silt and mud would be carried back again to the deeper 

 parts of the basin. 



Experiments showed that the deposits tend to be graded from coarse 

 to fine both in a horizontal and vertical direction. It is believed that 

 this mechanism may account for some cases of graded bedding in which 

 coarse fractions are involved, and that are remarkable for the wide 

 expanse over which each bed occurs, resting on undisturbed fine fractions 

 of the preceding bed. Also the emplacement of deep-sea sands far from 

 the original source might be accounted for in this manner. 



216 



