Sea-floor phosphorites along the Central California continental margin

Abstract

Early to middle Miocene phosphorites are patchily distributed along the continental margin of central California, at water depths of 200 to 2,000 m, between Cape San Martin and Point Reyes. Greater concentrations of phosphorites are present along the lower continental slope off Pescadero Point, on Sur Knoll, and on Twin Knolls, at water depths of 1,000 to 1,400 m. No phosphorites were found on Cordell Bank or in Monterey Bay, nor along the walls of adjacent submarine canyons.The dominant mineral of these Miocene phosphorites is carbonate fluorapatite (francolite). Associated minerals include glauconite, dolomite, detrital silicates, and minor pyrite. Most of the central California margin phosphorites are conglomeratic and contain a multitude of microdiscontinuity surfaces implying repeated alternations of deposition, erosion, reworking, and cementation. Both orthochemical and allochemical phosphorites are recognized petrographically; samples from offshore Pescadero are mostly oolitic phosphorites whereas those collected from Sur and Twin Knolls are predominantly microsphorites. Many samples also contain volcanic glass shards as well as opaque organics; they have been extensively silicified and are intensely bioturbated. SEM observations reveal micron-size, euhedral crystals of apatite that are distinctly hexagonal, and commonly are intergrown or stacked indicating in situ precipitation and/or recrystallization of an amorphous precursor. Bacterialike structures were also observed suggesting the possibility of microbially mediated precipitation of apatite.Geochemically, the central California margin samples are phosphorites. P 2 O 5 content ranges from 11.5 to 31.0 percent and averages 24.4 percent. Major element (Si, Al, Fe, Ca, Mg, K, C) analyses are consistent with published values for phosphorites but do indicate slight, relative enrichments. Trace element concentrations are also typical for marine phosphorites with the exception of Mn which is significantly depleted and Sr which is greatly enriched, whereas uranium values range from 47 to 170 ppm, averaging 90 ppm. Organic carbon values range from 0.45 to 2.65 percent with darker colored phosphorites enriched in organic carbon relative to lighter colored samples.The Miocene central California margin phosphorites originated primarily by direct physio- and/or biochemical precipitation of apatite within oxygen-depleted, near-surface sediments, although subsequent recrystallization during burial may also have occurred. Polytaxic, paleoceanographic conditions appear to have dominated during phosphogenesis but may have been repeatedly interrupted by episodes of increased oceanic circulation during glacial events. Times of transition from interglacial to glacial modes are believed to have been critical to phosphogenesis. Diagenetic formation of these phosphorites occurred along impinging dysaerobic (0.1-1.0 ml/l O 2 ) oxygen minimum zones where internal nutrient recycling may have provided sufficient in situ organic material (phosphorous) and sufficiently high pH values to allow apatite precipitation.Central California margin phosphorites are a potential natural resource. However, their patchy distribution and occurrence at relatively great water depths make them economically less attractive than well-known, on-land accumulations in central California, as well as submarine deposits off the southern California shore that occur at much shallower (<330 m) water depths and in higher concentrations.