Phosphorite 



69 



Figure 63. Distribution of phosphorite nodules. Samples are shown by dots; areas probably containing nodules are 

 cross-hatched. The letter M denotes samples containing Foraminifera of Miocene age; P is Pliocene to Recent age. 



been sampled. The rare small pieces of 

 phosphorite that have been obtained within 

 basin or slope muds deeper than about 3000 

 feet have almost certainly been transported 

 from shallow areas by sHdes or turbidity 

 currents. 



The total area that is known or believed 

 to be covered by phosphorite is about 6000 

 square miles. If the average thickness of 

 phosphorite is 1 inch, the region contains 

 approximately 1 bilhon tons of rock, about 

 one-tenth of the other phosphate rock re- 

 serves of the United States according to 

 Mansfield's (1940) estimate. 



Characteristically, the phosphorite consists 



of nodules and slabs commonly having flat 

 bottoms and nodular tops (Fig. 64). The 

 largest piece recovered to date is 2 feet long 

 and weighs 150 pounds, but doubtlessly 

 larger dredges could obtain larger pieces. 

 Small grains or oolites occur in nearly every 

 sample of coarse sediment seaward of the 

 mainland shelf that has been examined, 

 strongly suggesting their proximity to the 

 larger nodules. In addition, phosphorite 

 commonly forms a thin coating on other 

 kinds or rock. The nodules and grains are 

 hard and dense with a smooth glazed brown 

 surface which in deep water is usually cov- 

 ered by a film of manganese oxide (Dietz, 



