Oligocene to Holocene glauconite–phosphorite grains from the Head of the Cape Canyon on the western margin of South Africa

Abstract

Abstract Glauconite and carbonate–fluorapatite (CFA) are significant components of the condensed Upper Oligocene to Holocene sediment succession recovered from the western margin of southern Africa. The detailed texture and geochemistry of these glauconite–phosphorite grains are reported here within the stratigraphic framework established on the outer continental shelf in the Head of the Cape Canyon area. Three texturally and stratigraphically distinct glauconite grain types are recognised: Oligo/Miocene infilled foraminiferal tests, Mio/Pliocene green and Quaternary dark green to black grains. Most of these glauconitic grains include a minor to significant amount of CFA. The Upper Oligocene to Lower Miocene sediment succession is dominated by fine sand-sized benthic and planktic foraminifera. Glauconite grains constitute 20–50% of the sediment and represent tests infilled by variable proportions of glauconite and CFA cement with the test wall replaced by CFA only. Glauconite–CFA formation during the Late Oligocene to Early Miocene was associated with warm-water upwelling coupled with third-order marine transgressions. Rare-earth element (REE) chemistry indicates that sub-oxic conditions prevailed during diagenesis. Mio/Pliocene green and Quaternary dark green to black medium sand-sized glauconite grains constitute between 20% and 95% of these predominantly siliciclastic sediments. The green to black glauconite grains are texturally different, but have similar major and REE chemistry. Since the mid-Miocene, formation of glauconite–CFA grains is associated with cold-water upwelling and detrital, Fe-rich terrigenous mud deposition in oxic to suboxic bottom waters analogous to the Holocene coast-parallel, organic-rich mudbelt on the shelf. The texture of these grains indicates repeated cycles of glauconite and CFA authigenesis during marine transgressions and winnowing and reworking during marine regressions.