Sediments on the continental margin off the west coast of South Africa

Abstract

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On land, erosion of quartzitic phosphorites has resulted in extensive pelletal deposits. Similar phosphorite occurrences on the outer shelf are a southerly extension of the richer pelletal deposits off South West Africa. Quartzitic phosphorites have formed in nearshore areas of intense surface upwelling during transgressive and regressive periods. The accumulation and decay of siliceous phytoplankton in these regions has resulted in anaerobic bottom conditions and interstitial apatite precipitation. The calcareous and glauconitic phosphorites which cover extensive areas of the shelf have formed in regions, of less intense subsurface upwelling. Here the input of P he bottom sediment has been sufficient to cause replacement; of calcareous mud and lithification of the upper sedimentary layer. The ubiquitous presence of finely divided glauconite in the matrix of phosphorites and in foraminiferal tests is explained by co-replacement of clay minerals and micrite. Migration of deposition belts during changes in sea level has resulted in a heterogeneous mixture of phosphorite components and conglomeratic textures are produced by inclusion of limestone fragments in the bottom sediment

Microprobe analyses indicate that glauconite is rich in K20 (9, 01 percent) and MgO (5, 45 percent. Fe is probably emplaced in the glauconite structure independently and prior to the fixation of K and it is possible that some of the Mg is located in the interlayer position. Most of the "glauconite" is composed of a mixture of apatite and glauconite minerals. Inclusion of quartz and calcareous material in some glauconite and mixed mineral pellets mitigates against a colloidal aggregation theory for their origin. Instead, evidence is presented which indicates that some glauconite and mixed pellets are derived from in situ alteration of bottom sediment and/or by glauconitization subsequent to fragmentation of the bedrock and formation of the pellet

Holocene clay in glauconite-rich sediment contains insufficient Fe for present-day glauconite diagenesis. The reason for this is unclear, but it may be due to reduced upwelling and curtailed biogenic deposition. The concomitant reduction in the influx of P would also account for the absence of modern-day phosphatization. Phosphorites/formed during the U. Eocene and M. Miocene and glauconite diagenesis occurred during M. Miocene/L. Miocene times