A palaeoenvironmental study of the Pretoria group in the Eastern Transvaal

Abstract

The Pretoria Group in the major outcrops of the study area comprises fourteen formations, the five uppermost only being developed in the eastern part of the Transvaal Basin. The fifteenth, the Makeckaan Formation occurs in the Makeckaan 'fragment' to the northwest of Pretoria. The basal Rooihoogte Formation, comprising chert breccia, chert conglomerate, cross-bedded sandstone and minor mudrocks, unconformably overlies the dolomites and banded iron-formations of the Chuniespoort Group. Only the conglomerate lithofacies is developed at Marble Hall and Dennilton. The bulk of the succeeding Timeball Hill Formation consists of laminated, ferruginous mudrocks, with basal carbonaceous mudrocks and a medial sandstone member. An ironstone lithofacies is associated with the latter, whereas arkose and diamictite lithofacies occur locally within the upper shale zone. The Boshoek Formation overlies the Timeball Hill Formation with an erosive contact, and comprises an overall upward-fining arrangement of conglomerates, cross-bedded sandstones and mudrocks. It is not encountered at Marble Hall or Dennilton. Andesitic lavas, with minor pyroclastic material and mudrocks, characterize the Hekpoort Formation, which is covered by a thin bed of sericitic mudrock in the study area, and throughout large parts of the Transvaal Basin. T_he Dwaa1= Heuwel Formation,-consisting of major cross-bedded.sandstone.s, with interlc:J,yered mudrocks, and, in the northern part of the study area, conglomerates, rests unconformably on the Hekpoort Formation, and is succeeded by the rhythmically interlayered mudstones and siltstones and minor sandstones of the Strubenkop Formation. Mature quartz arenites characterize the Daspoort Formation in the south of the main eastern Transvaal outcrop area, as well as at Marble Hall and Dennilton; further north, mudrocks and less mature sandstones predominate. U_p _ ~Q.._?Q90m_of mudro~~s, with a medial volcanic unit, comprise th:._3verlying Silverton Formation, whereas mostly mature, cross-bedded sandstones, intercalated with minor mud rocks, characterize the Magaliesberg Formation. Predominant mudrocks occur in the Vermont Formation, with subordinate carbonate rocks, sandstones and chert. The cross-bedded and planar-bedded sandstones of the succeeding Lakenvlei Formation are mostly subarkosic, but quartzitic varieties also occur; minor mudrocks are present. A mudrock lithofacies characterizes the basal part of the Nederhorst Formation, whereas the upper portion consists of fine-grained arkoses, with minor quartzitic to argillaceous sandstones. Mature and subarkosic, cross-bedded sandstones comprise most of the Steenkampsberg Formation; in the Houtenbek Formation, the uppermost unit of the Pretoria Group in the eastern Transvaal, major mudrocks are interlayered with mature sandstones and, locally, with carbonate rocks and arkoses. The five post-Magaliesberg formations are developed only in the southern half to two thirds of the main eastern Transvaal outcrop area; they are absent at Potgietersrus and Marble Hall, and are poorly preserved at Dennilton. The Makeckaan Formation, outcroping as an isolated occurrence to the north of Marble Hall, consists of major feldspathic arenites, with minor quartzitic, argillaceous and lithic sandstones. Geochemical analyses of the Pretoria Group mudrocks point to intensive weath-ering in the source areas of the Timeball Hill, Strubenkop and Silverton Formations, whereas fresher source materials probably were available during later stages of sedimentation in the Pretoria Group depository. The source area composition is thought to have been mixed granitic-basaltic. Boron concentrations indicate a relatively low palaeosalinity in the basin, especially during pre-Magaliesberg times. Alluvial fan depositional settings are proposed for the Rooihoogte, Boshoek and Dwaal Heuwel Formations, whereas the Time ball Hill Formation is thought to have been laid down in a deltaic palaeoenvironment. The Klapperkop Sandstone Member of the Timeball Hill Formation, the Daspoort Formation and the Steenkampsberg Formation probably represent delta front deposits. The subaerial extrusion of the Hekpoort lavas and pyroclastics was succeeded by a period of subaerial weathering. A palaeoenvironmental setting basinal to the Dwaal Heuwel fans is postulated for the Strubenkop mudrocks, which overlie the former conformably. Prodelta deposition is envisaged for the Silverton Formation, with subaquaeous extrusion of the interbedded lavas and pyroclastics. The Magaliesberg Formation is thought to represent regressive shoreline deposits, which were succeeded by the mudflats of the Vermont Formation. Fan-delta or delta sedimentation is suggested for the overlying Lakenvlei Formation. In the Nederhorst Formation, basal mudflats were succeeded by a fluvio-deltaic palaeoenvironment. A complex, regressive shoreline - mudflat - shallow water depositional setting is suggested for the uppermost Houtenbek Formation. The Makeckaan Formation, which is ten ta ti vely correlated with the Lakenvlei Formation, or, alternatively, with the five uppermost formations of the Pretoria Group as a whole, only developed in the eastern Transvaal, and is thought to have been laid down in a fluvio-deltaic palaeoenvironment. The alluvial fans, deltas and fan-deltas proposed here for the Pretoria Group in the eastern Transvaal, probably were fed from source areas situated to the north, northeast, east, and, to a smaller extent, to the south. For the occurrences of Pretoria Group rocks at Marble Hall and Dennilton, provenance regions to the northwest and southwest probably existed, possibly related to a central palaeohigh situated to the north of Pretoria, and separating western and eastern Transvaal sub-basins.