Geological Setting of the Campbellrand-Malmanic Carbonate Platform

Dawn Y. Sumner

(The links "*" connect to definitions of the preceeding word.)

The Campbellrand-Malmani carbonate* platform is part of the Transvaal Supergroup in South Africa. It is 2521+/- Ma* (Sumner and Bowring, 1996). It covers 190,000 km2 and probably originally covered the entire Kaapvaal Craton*, >600,000 km2 (Beukes, 1987). The platform is over 1 km thick in the predominantly shallow water to exposed Malmani Subgroup and in the predominantly shallow but submerged Campbellrand Subgroup (Button, 1973b; Beukes, 1987). Approximately 500 km2 of deep water sediments equivalent to the shallower water sediments are preserved near Prieska and are only about 500 m thick (Beukes, 1987). (Map)

The Campbellrand-Malmani carbonate platform is extremely well preserved. Structural disruption of preserved strata is limited to gentle warping over most of the craton. Structural disruption is limited to locally steeper dips around the Bushveld Complex and to intense folding and faulting in the Kheis Belt and Dooringberg Fault Zone coincident with the western boundary of the Kaapvaal craton (Beukes and Smit, 1987; Stowe, 1986). The present limits of preserved strata are related to post depositional erosion and do not reflect the extent and shape of the original depositional basin. Metamorphic alteration has been limited with most outcrops not heated above 200íC (Button, 1973b; Miyano and Beukes, 1984). Locally, the mineral amphibole is present due to heating from the Bushveld Complex magma intrusions in the Malmani Subgroup, and hydrothermal fluid alteration during late fluid flow produced local Pb-Zn, fluorite, and gold deposits in both the Malmani and Campbellrand subgroups. Early, fabric retentive dolomite* replaced most of the Malmani Subgroup, particularly very shallow water facies* which are also associated with chert* replacement (Button, 1973b; Eriksson, et al., 1975, 1976). However, significant amounts of the Campbellrand Subgroup still consist of limestone* (Beukes, 1987).

In the transition from rocks that formed in shallow waters to those that formed in deep water, are rocks containing delicate microbial structures (Figures 3-5 to 3-23). They formed below the depth at which waves impinge on the sea floor as demonstrated by the lack of evidence for erosion during deposition and their relationships to shallower and deeper water rock types. These rocks also lack evidence for the washing in of external sand and mud suggesting that no other areas of local erosion were present.

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Dawn Y. Sumner
Department of Geology
University of California
Davis, CA 95616