Sediment and its Formation 



The materials that constitute the sea floor are 

 derived from various sources. Organic and inorganic 

 matter falls from the water column or moves in laterally 

 to find places of accumulation. Most inorganic matter 

 comes from the erosion of subaerial rocks; from atmospheric 

 fallout including dust, ash, and meteoritic spherules; and 

 from volcanic activity. Ice rafting drops variable amounts 

 of inorganic and organic material in the high latitudes, but 

 in tropical and midlatitudes the quantity is not significant. 

 Most organic matter comes from the tests and debris of 

 organisms living in the upper layers of the oceans. Deep- 

 water pelagic fish supply a varying amount of organic 

 material to bottom sediments. 



Turbidity flows help to move sediments rapidly into 

 deep water toward their final resting places. s 



Normally, in deeper water, the organic content of 

 sediments is highly variable. Sediments, if rich in organic 

 matter, support large colonies of surface and subsurface 

 feeders leading, in turn, to churning and the development 

 of microrelief. Where little organic material is present, 

 churning is insignificant, microrelief is minimal, and the 

 bottom approaches flatness. Nearshore sediments are made 

 up largely of subaerial runoff, are usually extremely rich 

 in organic matter, and form maximum microrelief. 



Sediments are usually characterized according to 

 their composition. If 30 percent or more is calcium car- 

 bonate, the sediment is designated a calcareous ooze. If 

 less than 30 percent carbonate is present, the sediment is 

 called a clay. When the sediment consists mostly of silica, 

 it is designated a siliceous ooze. 



In spite of wide differences in environment and 

 origin, bottom sediments may still be acoustically similar 

 as far as their microrough upper surfaces are concerned. 

 Differences in color have no influence on sound reflection. 

 Organic content of sediments may have some effect, but 

 this has not been determined. 



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