I 



Phosphorite 



California waters contain three times as 

 much phosphate ion as was originally as- 

 sumed, and thus the degree of saturation is 

 greater than computed. The great difference 

 between phosphorite and simple tricalcium 

 phosphate vitiates further refinement of the 

 original calculations. Although not proved, 

 it seems likely that phosphorite may owe its 

 origin to direct precipitation as colloids from 

 sea water in areas of strong upwelling, that 

 is, areas where cool deep water containing 

 much phosphate ion and other nutrients rises 

 to the surface and undergoes an increase of 

 pH and temperature and a decrease of pres- 

 sure. This would explain much more satis- 

 factorily than other hypotheses the presence 

 of phosphorite in the oxidizing environments 

 of southern California and off other coasts, 

 at least some of which are also areas of 

 upwelling. 



Age 



Fossils within the nodules include bones 

 and teeth of fishes and bones of sea mam- 

 mals. The most abundant and useful fossils, 

 however, are the tests of Foraminifera. 

 These appear to belong to two distinct age 

 groups. Foraminifera in most of the dark- 

 brown nodules, some of which are cemented 

 together by later phosphorite, contain only 

 Foraminifera that are characteristic of Mid- 

 dle to early Late Miocene (Relizian, Luisian, 

 and Mohnian). Most are Luisian and Moh- 

 nian, the same as the age of most determin- 

 able mudstone, shale, and limestone of the 

 continental borderland. No Early Miocene 

 (Saucesian) or late Late Miocene (Delmon- 

 tian) Foraminifera have been recognized in 

 the phosphorite. The second age group of 

 Foraminifera is represented by those found 

 mostly in light-brown nodules and in the 

 matrix of phosphorite conglomerates. These 

 Foraminifera are Late Pliocene to Recent 

 forms, whose age is supported on Coronado 

 Bank by the presence of the phosphorite atop 

 sedimentary strata of Late or post-Late 

 Pliocene age. Where determinable, these 

 Foraminifera are benthonic and characteristic 

 of the same general depth of water as that in 

 which their enclosing nodules were dredged. 



73 



When the problem of the phosphorite off 

 southern California was first studied (Dietz, 

 Emery, and Shepard, 1942), the Miocene 

 age of it was recognized to be strongly op- 

 posed by the wide distribution on the sea 

 floor of large nodules, in contrast to their 

 scarcity on the adjacent land and by the 

 obvious in situ position of many of the nod- 

 ules atop banks that must have been eroded 

 in post-Miocene times. The post-Late Plio- 

 cene age was opposed by the frequency of 

 pure Miocene faunas in the nodules, al- 

 though it was favored by the Late Pliocene 

 to Recent and mixed faunas in other nodules. 

 As a result, the pro and con evidence for 

 both ages was presented, but no very strong 

 stand on either age could be taken. 



The age question was reviewed in 1950 by 

 Emery and Dietz in the light of some new 

 evidence. This new evidence was the pres- 

 ence of large nodules similar to those of the 

 sea floor on an unconformity between the 

 Valmonte diatomite and the Malaga mud- 

 stone in Malaga Cove of the Palos Verdes 

 Hills, as described by Woodring, Bramlette, 

 and Kew (1946). This phosphorite is only 

 slightly younger (Mohnian-Delmontian) than 

 the sea floor material, and it shows conclu- 

 sively that large nodules did form during the 

 Miocene Epoch. The conclusion was reached 

 that most of the sea floor phosphorite was 

 deposited during Miocene time but that the 

 nodules during their subsequent exposure on 

 the sea floor served as nuclei for renewed 

 Quaternary deposition of phosphorite. 



Later, a study of Coronado Bank by 

 Emery, Butcher, Gould, and Shepard (1952) 

 showed an abundance of phosphorite atop 

 strata that is no older than Late Pliocene. 

 This phosphorite is quite obviously of Late 

 or post-Late Pliocene age. It is now evident 

 that the original uncertainty about whether 

 the phosphorite is Miocene or post-Late 

 Pliocene age must be resolved in favor of 

 both ages. It is believed that much of the 

 phosphorite, especially the dark-brown nod- 

 ules containing only Miocene Foraminifera, 

 was deposited during the Middle or early 

 Late Miocene Epoch. Perhaps a slowing of 

 the rate of deposition of detrital sediments 

 allowed the accumulation of large nodules 



