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Keywords:

  • Continental shelf;
  • hardground;
  • Miocene;
  • phosphorite;
  • sea-level;
  • Sr isotopes

Abstract

Detailed microstratigraphy and geochemistry of a single-layered phosphatic pebble from the Southern Rocky Plateau at the Head of the Cape Canyon on the western margin of South Africa are utilized to reveal a complex interplay between deposition, erosion and authigenesis in the Miocene. The phosphatic pebble has three micrite and carbonate fluorapatite cemented layers separated by Fe-rich hardground surfaces. The layers contain a diverse grain assemblage of reworked glauconite, phosphorite, detrital quartz and biogenic carbonate. Overlapping Sr-isotope derived ages of point-drilled samples from the three layers range from 19·1 to 17·6 Ma, although a single point near the outer edge of the uppermost layer has a Sr age of 12·8 to 10·4 Ma, and a reworked bivalve shell fragment from the middle layer has a Sr age of 26·3 to 24·6 Ma. The microstratigraphy of the phosphatic pebble provides evidence of multiple Baturin cycles, where periods of deposition and carbonate fluorapatite authigenesis are followed by reworking, erosion and precipitation of iron oxide hardground surfaces. The condensed microstratigraphy of the pebble is consistent with the regional depositional history of the area and indicates significant fluctuations in the depositional environment related to changes in sea-level, sediment supply and upwelling along the western margin of southern Africa during the early to middle Miocene.