Origin and age of phosphorite from the Last Glacial Maximum to Holocene transgressive succession off the Orange River, South Africa

Abstract

A major erosional unconformity between upper Cretaceous sediments and the Last Glacial Maximum (LGM) transgressive succession occurs on the inner continental shelf off the Orange River. Vibracores from 122 m water-depth consist of upper Pleistocene shelly, phosphatic sands deposited in a high-energy shoreface environment during the LGM lowstand between 22 and 19 ka. The gravel-rich shoreface sand grades into condensed shell-rich delta-front sands as sea level rose between 19 and 14.3 ka. An abrupt facies transition to prodelta muddy sands corresponds to the rapid rise in sea level associated with Melt Water Pulse (MWP) 1A at 14.3 ka and the facies transition to clayey silt corresponds to MWP 1B at around 11 ka. A large decrease in macro- and microfossil diversity indicates low-oxygen conditions as the sea level rose. Phosphorite recovered from throughout the succession includes large interior mollusc moulds, sand-size peloids and friable nodules. Sr-derived ages of the phosphorite range from early Miocene to Quaternary. X-ray amorphous friable Ca-phosphate nodules occur in organic-rich Holocene mud deposits. Quaternary sediment on the shelf represents a complex condensed section, in which reworked phosphorite pebbles show multiple episodes of phosphogenesis often separated by millions of years within individual pebbles. The phosphorite indicates that organic-rich mud deposition on the shelf was prevalent during the early to middle Miocene marine transgressions and sea-level highstands that were associated with increased upwelling as indicated by the extensive early to middle Miocene eolianites of the Tsondab Sandstone Formation. Most friable phosphorite that formed during Quaternary marine transgressions and highstands was probably fragmented and transported off shelf during succeeding marine regressions.