The Petrology and Origin of Some Glauconitic and Glauco-Conglomeratic Phosphorites from the South African Continental Margin

Abstract

ABSTRACT Glauconitic and glauco-conglomeratic phosphorite rocks are areally widespread on the South African continental margin. The glauconitic phosphorites ( 18% P2O5) have packstone or wackestone textures and are composed of sand-sized grains of glauconite, with lesser amounts of microfossils and terrigenous grains set in a matrix of collophane-francolite and micrite. The glauco-conglomeratic phosphorites ( 17% P2O5) are composed of variably phosphatized limestone pebbles set in a matrix very similar in texture and composition to the glauconitic phosphorites. Fifteen representative samples were selected for major element analysis and the relatively low P2O5 concentrations (15-21%) determined for these rocks reflect the diluent effects of allogenic grains. Substitution of CO32- for PO43- is indicated by the relatively high F/P2O5 ratios and the high apatite CO2 levels determined. Coupled substitution of Na+ and S6+ for Ca2+ and P5+ is also indicated, and high Fe2O3, K2O and MgO levels reflect the presence of glauconite. Many of the sedimentary features exhibited by these texturally heterogeneous rocks are incompatible under normal hydrodynamic conditions, suggesting an unusual depositional environment. It is proposed that the matrix cement was originally lime mud and that under the influence of a regressive coastline/shallowing sea, tidal and storm wave currents were responsible for the accumulation of the proto-phosphorite sediments in file relatively shallow and flat continental shelf areas. In deeper and steeper continental slope areas it is suggested that sediment accumulation resulted from the action of turbidity currents and mudflows. Lithification of the sediments is considered to have resulted from the phosphatization of lime mud by interstitial waters rich in phosphates. A shallow water lago nal/estuarine environment is indicated for the diagenesis of those rocks found in shallow continental shelf areas, whereas an open ocean environment is indicated for those rocks found in deeper water on the continental slope.