105 

 Phosphorite 



The only authigenic rock occurring in the bay is phos- 

 phorite. Almost every sample collected from the rock and 

 gravel area on the outer shelf, as well as samples from other 

 parts of the bay, contained phosphorite (Fig. 32). The phos- 

 phorite generally is in the form of nodules ranging in size 

 from less than 2 mm to more than 10 mm. Some phosphatized 

 mammal bones and shark's teeth were also recovered (Fig. 33). 

 Dietz, Emery, and Shepard (1942, p. 929) report that the only 

 shelves off southern California having phosphorite are off 

 Santa Monica and San Diego, although it is common on banks and 

 topographic highs far from shore. They report that the 

 shallowest sample of phosphorite obtained was from the Santa 

 Monica shelf in 240 feet of water, but one sample recovered 

 during the present survey came from 140 feet of water. This 

 may have been transported following formation, however. 



Dietz, Emery, and Shepard (1942), and Dietz and Emery 

 (1942) have shown that the phosphorite found off the coast of 

 southern California is formed in situ by chemical precipi- 

 tation of phosphate from sea water. They also note that 

 Foraminifera of lower Middle Miocene age occur in large 

 nodules on the outer shelf in Santa Monica Bay, and in others 

 from the same general area there are fossils of Middle Miocene 

 age (Luisian or Relizian), 



Significance and Origin of Phosphorite 



Phosphorite forms by the chemical precipitation of tri- 

 Calcium phosphate from sea water. The precipitation depends 



