Geology of Johannesburg, Republic of South Africa

Abstract

Payable gold was discovered in 1886 on the farm Langlaagte some 5 km to the west of what is now the Central Business District (CBD) of Johannesburg. A village grew around the diggings and in 1897 the Johannesburg Municipality was constituted. Johannesburg is now by far the largest city in South Africa. It is the centre of the world9s largest gold-mining industry and has become the biggest engineering and manufacturing centre on the African continent. The rocks of four major stratigraphic sequences are represented in Johannesburg. The oldest rocks in the area comprise some of the most primitive assemblages on earth. These are isolated remnants of greenstones which survived assimilation by the Archaean Basement granitic rocks. The residual and transported soils derived from greenstones are often highly expansive. The Archaean Basement granitic rocks occur in the northern suburbs of Johannesburg and on weathering have produced soils with a collapsible fabric. Sediments of the Witwatersrand Supergroup partly cover the granitic rocks and associated greenstones. The major portion of the Central Business District and adjacent areas are occupied by this east-west-striking and southward-dipping succession of shales, quartzites and conglomerates. A characteristic geotechnical problem associated with the Witwatersrand rocks is the extreme variability over relatively short distances in a north-south direction. There are two occurrences of rocks of the Ventersdorp Supergroup in the Johannesburg area. The northern-most one is in the form of a graben, down-faulted between Witwatersrand rocks to the north and south of it. Residual soils within the graben are regarded as some of the most highly compressible soils on the Highveld. The second occurrence of the Ventersdorp Supergroup is on the southern boundary where transported and residual soils derived from the andesites are highly expansive. Mafic dykes, generally deeply weathered, occur in abundance in all rock types in the city. A variety of transported soils of the Quaternary Period occur as an ubiquitous blanket covering the underlying residual soils or bedrock. Aggregates from crushed waste rock from the mine dumps account for 74 percent of the total aggregate consumption in the Johannesburg area. The quartzite, however, contains very small amounts of pyrite which, under certain conditions, gives rise to an aggregate with potentially deleterious chemical properties. Granite and andesite are the remaining aggregate sources. Mining residue deposits in the form of slimes dams and sand dumps are common in many residential and industrial townships of Johannesburg. They pose severe and complex environmental problems such as water and dust pollution as well as long term erosion and slope stability problems. Restrictions have been imposed by the Government Mining Engineer on the development of sites which have been undermined to depths less than 244 m. This has affected a strip of ground 500 m wide, very close to the CBD. There is, however, a move by property owners, developers and consultants towards a relaxation of this constraint on development.