Structural geometry and development of the Witwatersrand Basin, South Africa

Abstract

Abstract The Witwatersrand Basin is an Archaean basin situated on the Kaapvaal Craton of Southern Africa. The results presented here focus on the structural geometry and development of the basin. Detailed structural sections across the western and northwestern parts of the basin are presented, using seismic data integrated with borehole, mine and outcrop information. The structural development of the basin can be expressed in simple terms, and in a manner that is standard practice within the oil industry. There are several clearly identified stages in basin evolution. The basin was initiated as a rift during Dominion Group times ( c. 3074 Ma), with post-rift thermal subsidence during the early part of West Rand Group times. Thermal subsidence would have been completed by late West Rand Group times ( c. 2900 Ma). Minor volcanic interludes within the West Rand Group sequence may testify to the existence of phases of extension during West Rand Group times. Onset of compression and thrusting outside the thermal basin generated a flexural load and clastic input into the basin as it evolved from thermal sag to foreland basin during late West Rand Group times ( c. 2950 Ma). Foreland basin development culminated in Central Rand Group times, with an increasingly coarse-grained clastic input, and thrust systems that progressively encroached on the basin margins, profoundly influencing structural styles. Thrusting was interrupted during Klipriviersberg Group times (2714 Ma) by the accumulation of basic volcanic rocks. Further thrusting occurred at the end of Klipriviersberg Group times. The basin then returned to an extensional tectonic setting during the Platberg Group rifting (2709 Ma), a major rift event across much of the area. Thermal subsidence related to this rift phase occurred during late Platberg Group times. The overprint of Platberg Group extensional faults breaks up the structural continuity of the Central and West Rand Group sediments within, and adjacent to, the basin, and it is difficult to elucidate the earlier compressional thrust-fold structures. Careful integration of seismic, borehole, mine and outcrop data allows structures from the thrusting event be recognized and more complete sections drawn. These present a larger-scale view of the structure of the basin than has previously been obtained.