SECT. 3] BASIN SEDIMENTATION AND DIAGENESIS 591 



distribution results because the primary mode of transport is by suspension 

 and coarser detritus settles out faster. The increase in grain size of the sediments 

 along the continental slope is partly due to the effects of currents removing 

 finer material and partly due to the contribution of relatively coarse foramini- 

 feral tests. 



b. Calcium carbonate distribution 



It is difficult to predict the distribution of carbonates in basins since their 

 origin can be organic, relict, residual and even authigenic. In the Baltic Sea 

 residual carbonates are present and hence their distribution is erratic. In areas 

 with limestone deposits, reworked carbonates will be introduced by run-off, 

 streams or wave abrasion. Thus, the Danube and other rivers carry CaCOs 

 and Ca(HC03)2 into the Black Sea (Caspers, 1957, p. 830) along with terrigenous 

 material. The bulk of the calcium carbonate in Black Sea sediments is thought 

 to be chemically precipitated, although some believe deposition by bacterial 

 activity is also imjDortant. 



Kuenen's (1943) data for the Moluccan basins show first a decrease of car- 

 bonate content with distance from shore and then a sharp increase at about 

 100 km. In the southern California basins there is a continuous increase with 

 distance from shore (Fig. 2), the maximum being reached at the continental 

 shelf break. The explanation is based on relative supply. Near shore, product- 

 ivity may be lowered because of turbidity in the water preventing photosyn- 

 thesis, but calcium carbonate is introduced with reworked material. The supply 

 of vast amounts of terrigenous matter generally dilutes the carbonate when 

 relative analyses are made. In the offshore basins, especially in areas of high 

 productivity, calcium carbonate deposition is high and masking by detrital 

 material will be minimal. 



c. Organic matter 



The organic matter is generally closely related to the grain size and often to 

 the calcium carbonate content. In land-locked basins this relationship is due 

 almost entirely to relative rates of settling and supply of detrital material, 

 which in turn is related to the sources of supply and to the current circulation 

 patterns. The organic content is usually greatest in the part of the basin where 

 settling is the slowest and not necessarily in its deepest part (e.g. Lake Mara- 

 caibo). 



In continental-shelf basins the pattern is different. Here there is first an 

 increase in organic matter in going from the nearshore basins to the outlying 

 basins and then a decrease in the basins furthest from shore. This pattern, 

 shown in Fig. 2, can be explained on the basis of relative rates of supply of 

 organic matter and terrigenous material. Away from the coast, detrital material 

 decreases and hence the organic component becomes relatively more imjjortant. 

 Still further offshore the depth of the basin bottom below the euphotic zone 

 becomes so great that the settling organic debris is probably largely decomposed 

 by aerobic bacterial action before it reaches the bottom. 



