Diagenetically formed ooids and intraclasts in South African calcretes

Certain South African calcrete (caliche) beds contain structures that very closely resemble normal marine ooids and intraclasts. The ooids/intraclasts are actually non-marine in origin, and were formed by an in situ process of calcretization and selective neomorphism. The term ‘diagenetic ooids/intraclasts’ is thus applied to distinguish these grains from marine ooids/intraclasts. Quaternary calcretes contain the only known ooids/intraclasts in any South African Cretaceous or Cainozoic rocks. The diagenetic grains are believed to have formed in the following manner. Rain water percolates through an unlithified calcareous sand deposit, dissolving carbonate along its downward path. This carbonate-laden water is eventually checked in its descent and drawn upward somewhat by capillary action. Evaporation and soil suction in the uppermost zones of the sediment body cause precipitation of a concentric coating of carbonate mud around individual grains (incipient ooids) or composites of grains (incipient intraclasts). As this dissolution-precipitation cycle is repeated the micritic coatings thicken and locally push grains apart. At this stage three things can happen: (a) sparry calcite may be introduced as an intergranular cement (very rare in these calcretes), (b) wet carbonate mud may fill the pore spaces and lithify to micrite (common), or (c) wet carbonate mud may fill the pore spaces and neomorphose to microspar (common). The last process causes ooids/intraclasts to stand out distinctively, since they are opaque, micrite-coated grains set in a fabric of clear microspar. This selective neomorphism of the micritic mud in pore spaces, rather than the micritic mud forming ooids/intraclasts, is probably a function of rapidity of lithification. The micrite of the coatings must have lithified quickly, before neomorphic crystal enlargement could occur, whereas lithification of the intergranular mud was delayed long enough for the growth of microspar crystals.