Mineralogy and diagenesis of some South African coastal and marine carbonates

Abstract

The dominant carbonate minerals in South African dune, beach, coastal bay, continental shelf, and upper slope environments are aragonite and low-magnesian calcite. High-magnesian calcite, which is abundant in the “pure” carbonate depositional environments of other parts of the world, is found only as a minor constituent in these South African “non-carbonate” depositional environments. In the area studied, there is little difference between the carbonate mineralogy of the beaches and dunes, but the beach-dune environment contains considerably more aragonite and less low-magnesian calcite than the marine environment. Mean high-magnesian calcite values are strikingly uniform for all environments, differing less than 1% between the marine and beach-dune environments. Diagenetic alteration of the sand-size carbonate fraction of these sediments has been investigated quantitatively. The present carbonate mineralogy of a sample was determined by X-ray diffraction. The pre-diagenetic carbonate mineralogy of the same sample was obtained by point-counting individual grains and assigning to them their original mineralogic composition. Statistically significant differences between the two mineralogic values indicate diagenetic alteration. High-magnesian calcite, which is considered the most sensitive mineral to diagenetic changes, shows no mineralogic alteration in the beach-dune environment, but does in sediments presently in the marine environment. Diagenetic alteration of high-magnesian calcite to low-magnesian calcite occurred as a result of prolonged subaerial exposure of the continental shelf during Pleistocene sea-level fluctuations. Recent beach-dune sediments have not been sufficiently isolated from the marine environment to allow alteration of high-magnesian calcite.