Phosphorite pellets and rock from the western continental margin and adjacent coastal terrace of South Africa

Abstract

Well-rounded, spherical and ovoidal phosphorite pellets and irregular phosphatic rocks are located on the western continental margin and on the adjacent coastal terrace of South Africa. These minor (<1% of total unconsolidated sediment) and isolated offshore occurrences of pelletal phosphorite are the southernmost extension of rich phosphate deposits off South West Africa. The onland phosphorites formed in a sheltered, shallow-water lagoon or estuary during the Miocene. Wind-generated upwelling and high biological productivity in the estuary and in the immediately adjacent offshore region provided a plentiful supply of P to the bottom sediments. Fine layers (2–15 mm) of pure phosphorite were precipitated directly from the overlying water column, whereas phosphatic sandstones were formed by apatite precipitating within the interstices of quartzitic sediment mantling the estuary floor. Because the solubility of apatite decreases with increasing temperature, a process of cyclic precipitation may have been operative in shallow water during low tides when solar heating was at a maximum. The phosphorite pellets originated by accretion, usually around nuclei of quartz, feldspar or biogenic material at the sediment—water interface while rolling back and forth in shallow water. Pellets were also derived by fragmentation of partially lithified apatitic bedrock. Phosphorite pellets and rocks on the middle and outer shelf probably formed under conditions similar to the onland deposits during a later regression.