Petrography and Diagenesis of Silcrete from the Kalahari Basin And Cape Coastal Zone, Southern Africa

Abstract

ABSTRACT Silcrete is widely distributed in southern Africa and occurs both in genetic association with deeply weathered bedrock (weathering profile type) and as a significant component of the Cenozoic Kalahari Beds where it has formed through silicification of a variety of continental deposits (nonweathering profile type). Silcrete fabrics are classified into four major types: GS (grain-supported--skeletal quartz grains in contact), F (floating--skeletal grains `floating' in matrix), M (matrix skeletal grains < 5% of volume), and C (conglomeratic--detrital component containing pebbles). F and M fabric types are subdivided on the presence/absence of glaebules. Replacement by silica of a precursor matrix can account for the development of all four fabric types, but at least some GS and C fabr c silcretes have formed by passive silica precipitation in packing voids. Glaebules and colloform features predominantly composed of concentrations of anatase and iron oxide are common in weathering profile silcretes, but their precise origin is uncertain. Authigenic silica is present as megaquartz, microquartz, length-slow and length-fast chalcedony (predominantly in void-fills), cryptocrystalline silica and opal-CT depending on the silica and ionic concentrations of pore waters, pH, and host sediment characteristics. Diagnostic petrographic features, including glaebules, colloform structures, and chalcedonic vugh-fills, may be used in conjunction with geochemical data and sedimentary associations to interpret silcretes in the stratigraphic record with respect to weathering profile types (formed in humid, low pH environments) or nonweathering profile types (formed in high-pH ? semiarid to arid environments).