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Along the western coast of Baja California, between approximately 24.5° and 27° N. lat., extensive fine-grained phosphate deposits occur on a 100-km.-wide continental shelf. They are found from a depth of 100 m. to the shore where carbonate apatite makes up to 25 per cent by weight of present and more ancient barriers built up in front of coastal lagoons. Their similarity with ancient phosphorites, believed to have developed on large shallow submarine platforms, prompted a study of these sediments. Intense seasonal upwelling of deep nutrient-rich waters from the California Current controls the hydrography of the whole shelf, and shallow ridges form an effective sill restricting circulation over it. A reducing environment has developed and iron sulfides are prese t nearly everywhere. Locally produced organic matter is being supplied continuously to the bottom through the oxygen-deficient waters, this at a fairly high rate because of shallow depths. Conditions favorable to the preservation of large quantities of non-oxidized inorganic phosphate in the sediments are in this way maintained. Whether or not this situation results in the present formation of calcium phosphate minerals cannot be ascertained. Apatite could be measured quantitatively by X-ray diffraction down to very low levels. This shows the persistence of a rich zone over 20-30 km. along the coast between depths of 60-100 m. From several evidences these deposits represent ancient barriers reworked during the last transgression. Onshore similar phosphatic sands over 50 ft. thick are kno n to exist in the vicinity of the lagoons. From geological considerations and geochemical arguments the average age of these deposits is Pliocene to Pleistocene. They could be as old as Miocene. They appear in equilibrium with their modern environment and, if so, could continue to develop, but unambiguous evidences concerning their present growth have been difficult to obtain.
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