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Phosphates and Dolomites in Coastal Upwelling Sediments of the Peru Margin and the Monterey Formation (Naples Beach Section), California
The Monterey Formation in the Naples Beach section contains an exceptionally well developed array of phosphatic rocks, particularly in the carbonaceous marl member. Similar deposits occur in Neogene sediments along the continental margin (shelf-upper slope) of Peru. Phosphogenesis (= precipitation of authigenic carbonate fluorapatite or CFA) in both areas occurred in organic-rich host sediments deposited beneath zones of strong coastal upwelling. Both areas contain distinctive F-phosphates and D-phosphates. F-phosphates are friable and typically have a light color; they represent in situ precipitation of CFA in organic-rich sediments during early diagenesis just beneath the sea floor. D-phosphates are dense, well lithified nodules, conglomerates and hardgrounds of CFA which commonly have a dark color or dark external coatings. D-phosphates record cycles of phosphogenesis near the sea floor, exhumation by bottom currents, reworking, reburial and renewed phosphogenesis; evidence from the Naples Beach section indicates F-phosphates converted to D-phosphates during these cycles by repeated episodes of phosphogenesis. Some D-phosphate layers probably indicate extended periods of low net sediment accumulation, others are clearly products of transportation and redeposition. Associated with the phosphatic rocks in both the Naples Beach section and offshore Peru are thin (5-40 cm) layers and concretionary horizons of authigenic dolomites. These also probably formed during intervals of sharply reduced sedimentation and likewise represent a form of condensation.
Phosphate-dolomite bedding patterns are different in the Naples Beach and Peruvian sections. At Naples Beach, dolomitic intervals tend to lie above phosphatic horizons, whereas the reverse occurs along the Peru margin where graded D-phosphate gravels commonly lie above eroded or burrowed dolomitic layers. In the Naples Beach pattern, the D-phosphate conglomerates represent winnowed and reworked horizons, and the phosphate-dolomite pairs record progressive condensation during relative sea level rises. In the Peru margin pattern (typical also of other Neogene shelfal regions), condensed dolomitic intervals formed during relative sea level rises commonly became exposed as hardgrounds or firmgrounds which are overlain by transported and redeposited D-phosphate conglomerates deposited during the succeeding relative sea level fall. A further speculative interpretation of the carbonaceous marl member at Naples Beach is that the D-phosphate layers represent middle Miocene glacial to interglacial transitions characterized by substantial winnowing and reworking, whereas F-phosphates formed during low energy highstand conditions of interglacial periods.
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