Facies relations, depositional environments and diagenesis in a major early Proterozoic stromatolitic carbonate platform to basinal sequence, Campbellrand Subgroup, Transvaal Supergroup, Southern Africa

Abstract

The Campbellrand Subgroup and its correlative, the Malmani Subgroup of the Transvaal Supergroup, represent a major 2300–2600 Ma old carbonate buildup. It is on average between 1500 and 1700 m thick and covers an area of approximately 190,000 km 2 on the Kaapvaal craton. The original depository probably extended across the entire 600,000 km 2 surface area of the craton and beyond. The buildup consists essentially of two major lithofacies assemblages, namely a basinal, non-stromatolitic, laminated carbonate and shale sequence (with minor chert, iron-formation and mafic tuff interbeds) off the craton to the west, and a shallow-water stromatolitic carbonate sequence on the craton proper. Growth faults along the edge of the craton controlled the platform margin defined by columnar stromatolite, oolite and carbonate arenite shoal deposits. These are flanked basinward by “reef-like” giant subtidal stromatolitic mounds elongated perpendicular to the platform margin and grading into slope deposits consisting of laminated carbonates with interbeds of chaotic dolomite breccia and conophyton stromatolites. Platform lagoonal deposits consist of dark grey, laminoid fenestrate, stratified stromatolites interfingering to the interior of the craton with intertidal to supratidal, light grey cherty dolomite units with a variety of structures such as domal and stratified stromatolites, laminoid fenestrae, tepee structures and flat pebble conglomerates. The buildup consisted primarily of limestone which was replaced by early diagenetic dolomite. The chemical composition of the dolomite is related to depofacies. Platform margin and lagoonal dolomites are manganese-rich, basinal dolomites are iron-rich and intertidal to supratidal deposits are virtually free of iron and manganeses. Indications are that the carbonate buildup developed from an early carbonate ramp stage into a mature rimmed carbonate shelf. Eventually the carbonate shelf was drowned by a major transgression followed by the deposition of the Kuruman iron-formation.