Afsettingstoestande van die Pretoria Groep gesteentes in die Pretoria-Bronkhorstspruit-Delmas gebied

Abstract

The Pretoria Group in the studyarea comprises nine formations: the basal Rooihoogte, Timeball Hill, Boshoek, Hekpoort, Strubenkop, Daspoort, Silverton, Magaliesberg and Rayton Formations. These formations consist of alternating mudrocks and quartzitic sandstones, with minor andesitic lawa. The Rooihoogte Formation overlies the dolomites of the Chuniespoort Group unconformably. A basal conglomerate layer, quartzitic sandstones and minor mudrocks characterize this formation. The basal carbonaceous shales of the Timeball Hill Formation grade upwards into a rhythmic rnudstone – siltstone facies. Minor ironrich sandstones and diamictites occur in the middle to upper part of this formation. Thin lenses of crossbedded sandstones form the Boshoek Formation, and a thick sequence of prdominantly andesitic lava characterizes the Hekpoort Formation. Rhythmicically interbedded muds tones and siltstones predominate in the Strubenkop Formation, with minor iron-rich sandstones and siltstones. The Daspoort Formation consists of horizontally bedded and cross-bedded arkosic sandstones, and carbonaceous shales predominate in the overlying Silverton Formation. A sequence of coarse-grained horizontally stratified and cross-bedded sandstones, with abundant ripplemarks characterizes the Magaliesberg Formation. The Rayton Formation consists of preedominant horizontally bedded and cross-bedded sandstones, with minor slitstones, limestones and andesites. Chemical analyses of mudrocks indicate a high degree of weathering for the source rocks, which were predominantly granitic in composition. The palaeosalinity of the depositional environment of the different formations fluctuated between freshwater and salt water conditions. The latter were probably associated with subsiding water levels. A fan-delta palaeoenvironment is postulated for the Rooihoogte Formation, and deltaic deposition for the Timeball Hill Formation. Braided channels on alluvial fans are envisaged for the Boshoek Formation, and partial melting of lower crustal material for the Hekpoort Formation, with subaerial extrusion of the lavas. The Strubenkop Formation probably represents the distal portions of the fan-deltas which are thought to have formed the Daspoort Formation. A prodelta delta-front model is proposed for the Silverton Formation, which in turn grades upwards into the proposed lacustrine coastline deposits of the Magaliesberg Formation. Braided channel deposits on alluvial fans/fan-deltas are envisaged for the Rayton Formation. The alluvi-al fans/fan-deltas and deltas proposed for the Pretoria Group most likely prograded into the lacustrine basin from the north. A central palaeohigh, postulated by previous workers, situated to the north of Pretoria, could have acted as a source for the sediments.