Late Cenozoic phosphogenesis on the western shelf of South Africa in the vicinity of the Cape Canyon

Abstract

The strontium isotope ( 87Sr/ 86Sr) composition of phosphorite (carbonate fluorapatite) cemented grains (PCG), phosphate-cemented limestone (PCL), skeletal phosphorite grains (SPG) and biogenic calcite (mollusc shell, benthic foraminifera and echinoid spines) from sediment on the outer continental shelf in the vicinity of the Cape Canyon ranges from 0.707981 to 0.709167 and corresponds to an age range of late early Oligocene to late Pleistocene. Most biogenic carbonate Sr ages agree with biostratigraphic ages, but some samples contain significantly older and reworked robust (>250 μm) benthic foraminifera. Although the Sr ages of biogenic and phosphorite grains indicate that sedimentation was fairly continuous, most sediment is reworked with only lower Miocene, upper Pliocene and upper Pleistocene successions generally preserved in the study area. The initial phosphogenic episode from the latest Oligocene to early early Miocene (21–26 Ma) was the largest, with additional phosphogenic episodes in the late early Miocene (16–19 Ma) and middle Miocene (10–15 Ma). Most phosphorite occurs as PCG that formed during marine transgressions and highstands from the late Oligocene warming event until the mid-Miocene climatic optimum, a period of episodic Antarctic glaciation and several positive marine δ 13C excursions. Upper Oligocene to lower Miocene PCG have been extensively reworked and are associated with major erosional unconformities. PCG vary from peloidal sands to gravel mollusc moulds and contain variable amounts of quartz, glauconite and pyrite inclusions. PCL formed from 9 to 17 Ma by cementation and replacement of older calcareous sediment during the middle to late Miocene marine regression. Phosphorite, as well as associated glauconite and pyrite, indicate that periods of rapid accumulation of organic-rich, terrigenous muds derived from the focusing of the suspended load of rivers and organic matter produced in areas of coastal upwelling have occurred episodically on the margin since the late Oligocene. The relative paucity of phosphorite younger than the middle Miocene may result from frequent erosional events as documented by pronounced unconformities on the shelf that reflect lowered eustatic sea level, tectonic uplift and high-frequency, high-amplitude Quaternary sea-level fluctuations.