Abstract

Phosphate-rich rocks (av. 17,8% P 2 O 5 ) mantle extensive parts of the shelves off the west and south coasts of southern Africa. These deposits comprise three non-conglomeratic and two conglomeratic rock types. The non-conglomeratic types are phosphatized foraminiferal lime packstones which are either poor in glauconite and quartz; rich in goethite; or highly glauconitic. The first conglomeratic type is also rich in glauconite, but contains pebble inclusions of phosphatized foraminiferal limestone. The second conglomeratic type is similar to the first, but is distinguished by its low glauconite content and high macrofossil and goethite abundance. The bulk mineralogy and the geochemistry of these rock types are dominated by the main non-phosphatic diluent present in each group. Extensive reworking of phosphate-rich rocks during repeated Tertiary regressive/transgressive cycles has concentrated glauconite and apatite in the overlying unconsolidated sediments. The diamictic texture exhibited by these phosphatic rocks suggests a complex depositional history. However, the heterogeneous mixture of authigenic, terrigenous and biogenic components and varied textures can be simply explained in terms of migrating zones of deposition and erosion during repeated transgressive/regressive cycles. Deposition of large quantities of calcareous zooplankton on the outer margin provided sufficient phosphorous and calcium carbonate to the sea floor for the replacement of lime mud (micrite) to form carbonate apatite (francolite) cement. Thus all components of the bottom sediment were lithified to form a phosphatic capping or 'pavement,' which, in some areas, became reworked during subsequent regressive periods. Renewed carbonate deposition and a further period of phosphatization occurred when the zones of deposition migrated back across the shelf in response to a rising sea level. Pebbles of phosphatized limestone and glauconite left behind after the previous regressive cycle became incorporated into the second-generation phosphatic deposits, thereby forming the conglomeratic rock types. The principal diagenetic mechanism responsible for phosphatization and lithification of all these 'open-shelf' deposits is the replacement of lime mud (produced by the disaggregation of calcareous foraminiferal and coccolithophorid debris) by francolite. Although petrographic evidence indicates many periods of phosphatization there is only sufficient stratigraphic data to support a Middle Miocene period of mineralization.

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