Dolomitization Models from Early Precambrian Sequence in South Africa: ABSTRACT

TL;DR: A 1,500m-thick chemical sedimentary unit within the approximately 2,300m.y.-old Transvaal Supergroup contains more than 75% dolomite as discussed by the authors.

Abstract: A 1,500-m-thick chemical sedimentary unit within the approximately 2,300-m.y.-old Transvaal Supergroup contains more than 75% dolomite. A paleogeographic reconstruction for this stratigraphic interval has delineated a passive southeastern basin margin which was a low-relief, stromatolitic tidal flat; this is distinguished from an active northwestern basin margin along which banded iron formations precipitated in lagoons behind clastic-textured carbonate barriers. Large stromatolitic mounds occupied the intervening shallow, subtidal environment. Two generations of dolomitization are recognizable in the tidal-flat facies; these are considered to reflect schizohaline conditions. Palisade pseudomorphs, probably after gypsum, indicate hypersalinity which is considered to have resulted in the penecontemporaneous development of micritic dolomites. Later, coarse-grained, limpid dolomites with associated secondary cherts are interpreted as a response to a freshwater, low-pH meteoric overprint. The barrier facies comprises limestones and replacement dolomites which contain up to 10% FeO and 4% MnO in the carbonate lattice. The degree of dolomitization decreases away from the banded iron formations, suggesting the lagoon as a potential source of a refluxing magnesium-iron-manganese brine. The widespread occurrence of both iron and mang nese in their divalent states lends support to previous contentions of a reducing atmosphere during the early Precambrian. The stromatolitic mounds, which have an average stratigraphic thickness of 200 m and a basinal extent of hundreds of kilometers, consist of uniformly pure and fine-grained dolomites which are characteristically devoid of chert. These dolomites may be primary or, conversely, could have formed through reaction with the ambient waters of an intracratonic epeiric sea which had high concentrations of magnesium and sodium. End_of_Article - Last_Page 446------------