Ch. 1] BASINS 21 



Basins 



Deposits in basins, either in the ocean far from land or near shore, 

 are of many types. The depth of the sill in these basins influences 

 very materially the conditions in the basin and thus in turn the de- 

 posits. The sill may be defined as the outlet of the basin if sea level 

 were lowered to the extent that the water in the basin would be entirely 

 enclosed by dry land. If the sill is deep, the water below sill depth is 

 more likely to escape than if the sill is at shallow depth. The water 

 in basins with deep sills, therefore, is less apt to become stagnant. 

 The oxygen content of the water in any basin, however, ordinarily 

 will be less than in the overlying water, and the sediments are likely 

 to be in a higher state of reduction than similar deposits laid down 

 in more open places on the sea floor. The organic content is apt to be 

 relatively high, as the lowered oxygen content inhibits the decomposi- 

 tion of organic matter. 



Basins favor chemical precipitation, because the lack of circulation 

 or the poor circulation favors the increase in concentration of dissolved 

 substances to such an extent that a state of saturation develops. The 

 addition of chemical substances such as silica, iron, or manganese from 

 submarine springs is more likely to result in the formation of chemical 

 deposits in basins than in other places. 



If the sill is relatively shallow and the basin relatively deep, verti- 

 cal circulation, or ventilation of the water as it is called, is much in- 

 hibited, and the water becomes stagnant (Str0m, 1939). Hydrogen 

 sulphide is formed, and highly reduced sediments rich in organic 

 deposits are laid down. If the rate of influx of terrigenous debris is 

 small, deposits largely of a chemical nature form in basins. Extensive 

 deposits of salt, gypsum, potash, phosphate, and limestone may form 

 in this way (Lotze, 1938). Alternations of the level of water above 

 or below sill level or specially favored areas of evaporation, like the 

 Gulf of Kara Bugaz in the Caspian Sea, favor the influx of chemical 

 ingredients and ensuing concentration to the point of precipitation. 

 Unfortunately examples of basins of many of the types of chemical 

 deposits formed in the past do not exist today. Certainly no areas are 

 known where dolomite is now forming (Sander, 1936). Geologists in 

 endeavoring to determine the mode of origin of saline and other chemi- 

 cal deposits would do well to consider carefully the fundamental prin- 

 ciples of oceanography as manifested by the present ocean. 



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