20 trask. PRINCIPLES OF SEDIMENTATION [Ch. 1 



Deep Sea 



The ocean contains several distinct environments of deposition with 

 respect to supply of mechanical and chemical constituents, distance 

 from land, character of water, and configuration of the basin of dep- 

 osition (Murray and Renard, 1891). In the deep sea far from land, 

 the rate of supply of mechanical debris is low. The sediments consist 

 principally of material of organic or wind-blown (eolian) and cosmic 

 origin. Relatively little material is now being precipitated in the deep 

 ocean. Water shallower than 15,000 feet, however, apparently is satu- 

 rated with calcium carbonate in some areas. Thus calcium carbonate 

 perhaps is now being precipitated chemically, though most of the car- 

 bonate seems to be of organic origin. The ocean water below a depth 

 of 20,000 feet at present appears to be undersaturated with calcium 

 carbonate. As a result, particles of calcium carbonate seemingly dis- 

 solve as they settle to the bottom. The sediments contain little cal- 

 cium carbonate. The undersaturation of the deep water in the ocean, 

 however, should not be considered a reliable index of past condi- 

 tions, because the present state almost certainly is due to the addi- 

 tion of cold and dense water to the bottom of the ocean during the Ice 

 age. As this cold water sank during Pleistocene time it must have 

 displaced upward, or cooled, the water that formerly lay at the bottom 

 of the ocean. The deep water in other geologic periods may well have 

 been materially warmer and more thoroughly saturated with calcium 

 carbonate than is the present water. Consequently some ancient lime- 

 stones may be of deep-water origin. 



Radiolarian chert is considered by some people to be of deep-sea 

 origin, because at present radiolarian deposits are found principally 

 in deep water. Siliceous deposits of this type represent a relatively 

 rapid rate of deposition of silica with respect to terrigenous debris 

 (Davis, 1918). Such conditions most certainly can prevail in deep 

 water far from shore, but they also could exist in basins near shore 

 if the adjoining land areas supplied little debris and if an excess of 

 silica were supplied the sea from submarine sources. Some of the 

 radiolarian deposits of the Franciscan formation of California may be 

 of shallow-water origin. 



Coarse sediments can be found far from shore in deep water, as indi- 

 cated by the poorly sorted glacial marine deposits reported by Bradley 

 and Bramlette (Bradley et al., 1942) from the North Atlantic. These 

 sediments contain terrigenous constituents of ice-raft origin. 



