Abstract In 1970, the National Committee for Geological Sciences accepted in principle the ISSC recommendations for stratigraphic classification and a South African Committee for Stratigraphy (SACS) was appointed. A local code was published in 1971 and, through working groups, SACS has revised and completely reclassified the South African stratigraphic column. The former and the revised principal stratigraphic subdivisions are presented in tables. The principal lithostratigraphic units in the main Precambrian basins of deposition are being termed supergroups, groups, and sequences; the former Swaziland, Witwatersrand, and Ventersdorp Systems" are now Supergroups; the Pongola, Dominion Reef, and Waterberg "Systems are Groups; and the Transvaal System" is now the Transvaal and Griqualand-West sequences with component groups. The term Bushveld Complex replaces Bushveld Igneous Complex. The Gariep and Nosib Groups (formerly "Systems") have been placed in the Damara Supergroup. The Nama "System" which spans the Precambrian-Phanerozoic boundary has become a Group; its counterpart in the southwestern Cape Province, which is intruded by the Cape Granite, is now termed the Malmesbury Group. For the Phanerozoic, SACS decided that the Cape "System" becomes the Cape Supergroup and the Karoo "System" a sequence. In the Cape Supergroup the major units are now termed the Table Mountain, Bokkeveld, and Witteberg Groups. In the Karoo sequence, the Dwyka Tillite Formation forms the base followed by the Ecca, Beaufort, and Drakensberg Groups. The widespread dolerites intrusive into the Karoo are still collectively referred to as Karoo Dolerites. In the Jurassic-Cretaceous, the term Beds has been abandoned and Suurberg, Uitenhage, and Zululand Groups are defined. The problem of subdividing the Precambrian chronostratigraphically is discussed. In South Africa there are unrivalled, relatively undisturbed, and only slightly metamorphosed Precambrian successions that cover, with only minor hiatuses, the time span from 3,750 m.y. to the Phanerozoic. A wide span of reliable radiometric age determinations, and absence of major facies changes, has led to their formal subdivision into the following erathems: Swazian (3,750+ to 2,870 m.y.), Randian (2,870 to 2,630 m.y.), Vaalian (2,630 to 2,070 m.y.), Mogolian (2,070 to 1,080 m.y.) and Namibian (1,080 to 570 m.y.).

Abstract A review of 60 isotopic age measurements on southern African rocks indicates that many of these data have a bearing on important Precambrian stratigraphic problems. In 1948 Arthur Holmes first used age measurements in deciphering the structural history of this region. His study emphasized dated pegmatite bodies in metamorphic terrains as indicating ancient orogenic belts of different ages. Recent work has focused on dating the relatively flat-lying and unmetamorphosed Precambrian systems, and a revised stratigraphic time scale for the Transvaal Province is proposed here. Certain relatively flat-lying systems in southern Africa have ages greater than 2000 millio n years, exceeding the ages determined in the high-grade metamorphic terrains. These old systems include the Ventersdorp, Witwatersrand, and Dominion Reef. The Transvaal System may be ˜2000 m.y. old. The “ancient orogenic belt” theory implies near-contemporaneous stablep l atformdeposition and possible formation of post-orogenic arkose troughs. Reliable times of deposition for the adjacent undeformed systems can thus supply critical evidence concerning this theory. The region paralleling the Orange River in northernCape Province provides an example: there are many age measurements on pegmatites and metamorphic rock of ˜950m.y., and Holmes postulated the existence of a 1000 m.y.-old orogenic belt; this implies the existence of ˜1000 m.y.-old stratigraphic systems, both as a geosynclinal sequence and as the “stable-platform” equivalent. However, the accepted geologic column for this region does not reveal stratigraphic systems that can be clearly linked to the history of the hypothetical ˜ 1000 m.y.-old orogen. Either the accepted geologic column is in error and part of the Kheis System is much younger than hitherto supposed, or a widespread regional metamorphism and pegmatization affected the Kheis rocks some 2000 m.y. after their deposition. There are many determinations close to 500 m.y. in a belt extending through South-West Africa, the Central African Federation, and Mozambique. Widespread metamorphism, with emplacement of granitic rocks, took place in parts of southern Africa at the close of the Precambrian and during the Cambrian Period. Arcuate synclinal belts of low-grade metavolcanic rock w ith an age > 2500 m.y. are characterized by the “Steinmann” geosynclinal association of rock types; rocks and related mineral deposits of this association were evidently not formed i n the subsequent history of southern Africa. Younger Precambrian metamorphic terrains have a different tectonic style, showing frequent alternation of metasedimentary units with massive and gneissose granites, and occasional highgrade regional metamorphism. The characteristic differences i n structure may be related to thermal history o f the crust. Many of the age measurements impose restraints on stratigraphic correlations between the various isolated Precambrian areas.

The Ventersdorp Contact Reef, one of the major gold-bearing conglomerate horizons in the Witwatersrand Basin, occurs as a distinct horizon between the overlying Klipriviersberg Group lavas (2714 Ma) and the underlying Central Rand Group rocks (<2894 to >2714 Ma). The Ventersdorp Contact Reef has been metamorphosed and hydrothermally altered under greenschist facies conditions (290–350 °C and 0.2–0.3 GPa). A study of S, O, and H isotopes was carried out to constrain the sources of hydrothermal fluids and gold mineralization in the Ventersdorp Contact Reef. A narrow range of δ 34 S values (−1.5‰ to +1.8‰) for authigenic pyrite, pyrrhotite, chalcopyrite, sphalerite, and galena from the Ventersdorp Contact Reef and its hanging-wall and footwall lithologies suggests a reconstitution of detrital sulfides during fluid circulation at peak metamorphic conditions. The δ 18 O values of authigenic quartz and calcite range from +8.9‰ to +11.3‰, and the δ 18 O and δD values of muscovite separates are between +7.2‰ and +8.4‰ and −62‰ and −31‰ respectively. The estimated δ 18 O (+4.8‰ to +6.1‰) and δD (−27‰ to −39‰) values of the fluids in equilibrium with these minerals under the relevant P-T conditions suggest a metamorphic origin for the fluid. It is concluded that the metamorphic fluids involved were probably derived from the Basin itself, and allogenic sulfides in the Ventersdorp Contact Reef were reconstituted during fluid circulation at peak metamorphic conditions. Accordingly, gold appears to have been locally remobilized and possibly derived from pre-existing placer concentrations.