A palaeoenvironmental interpretation of the Early Proterozoic Malmani dolomite from Zwartkops,South Africa |
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| Authors: | JF Truswell KA Eriksson |
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| Institution: | Department of Geology, University of the Witwatersrand, Johannesburg South Africa |
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| Abstract: | The Malmani Subgroup northwest of Johannesburg consists of dolomite and chert with only minor clastic sediments.A precise upper intertidal to marginal supratidal analogy and the associated relationship of varied structures suggest that much of the succession represents a tidal flat to intertidal complex formed within differing semiprotected to protected conditions. The dolomites from these environments are recrystallized, reflecting a meteoric influence, and the cherts which are commonly developed within them are also related to prevailing lower pH's. This dolomitization is considered to have been enhanced by the influx of meteoric waters which however resulted in the dolomites having undersaturated iron-manganese ratios. Rare colour-banded dolomites containing columnar stromatolites are thought to represent more steeply shelving intertidal conditions than are normally encountered in the epeiric sea. These dolomites contain quartz crystals rather than chert, suggestive of a lower concentration of silica in the original alkaline solutions. The exposure is part of a very widespread carbonate unit, dated at ca. 2.250 m.y.Subtidal conditions in which large elongate stromatolitic domes developed can be related to a marine transgression across a basal clastic beach sand; and secondly to a progradational tidal flat seawards of which a talus breccia developed on a steepened slope leading down to the subtidal regime. These dolomites formed by interaction with marine waters saturated with respect to iron and manganese, while the absence of chert reflects persisting alkaline conditions.A dark chert-free dolomitic facies with high iron and manganese contents of saturated ratios is considered to have developed in an alkaline lagoonal environment behind a subaqueous bar that is now represented by an overlying thick oolitic unit.The succession contains numerous chert breccias with which shales are associated. The breccias represent subaerial exposure phenomena related to regressions which were followed by periods of short-lived terrigenous influx. |
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