Limestone Formation and Dolomitization in a Lower Proterozoic Succession from South Africa

Abstract

ABSTRACT Limestones, dolomitic limestones and dolomites of the Transvaal Dolomite which accumulated in intertidal, high energy and subtidal environments in an epeiric sea have been chemically investigated, with the view to a further understanding of Lower Proterozoic carbonate formation. The carbonates studied are characterized by unusually high Fe and Mn contents. Both metals are present in their lower oxidation states. This suggests a less oxidizing atmosphere at the time of formation of these rocks relative to the present. The limestones in the area studied apparently formed under conditions of saturation of iron, manganese and calcium. The Fe/Mn ratios of these limestones are contrasted with those of ferruginous dolomitic limestones and ferruginous dolomites elsewhere in the sedimentary basin which formed when manganese was undersaturated in solution. Contemporary dolomitization is most intense in upper intertidal to supratidal zones, but in the section considered this process has been most complete in the subtidal region. The dolomitization process is related to the nature of the depository on the Precambrian craton. This process is considered to be an early one, resulting from the action of waters in contact with limestone. A further influence, involving meteoric water with a lower pH has possibly been responsible for the dolomitization and also recrystallization of some carbonates.