Coarse Clastic Tidal and Fluvial Sedimentation During A Large Late Archean Sea-Level Rise: The Turffontein Subgroup in the Vredefort Structure, South Africa
B. G. Els, J. J. Mayer, 1998. "Coarse Clastic Tidal and Fluvial Sedimentation During A Large Late Archean Sea-Level Rise: The Turffontein Subgroup in the Vredefort Structure, South Africa", Tidalites: Processes & Products, Clark R. Alexander, Richard A. Davis, Vernon J. Henry
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Most of the economic gold placers of the late Archean Witwatersrand Supergroup of South Africa have been found to be of fluvial origin. A downstream transition from the fluvial paleoenvironment to coastal environments is expected, but such facies changes have not been observed within the previously studied areas. In this paper, the sedimentology of the uppermost formations of the Supergroup in a distal part of the preserved basin, namely the Vredefort structure, is described and evidence for transition from fluvial through tidal to shallow marine depositional conditions is submitted. The study area comprises the exposed arcuate northeastern to southwestern sector of the Vredefort structure, where overturned strata dip radially towards its center. In the Vredefort structure, the upper subgroup of the Witwatersrand Supergroup, the Turffontein has been subdivided into five formations, for which the codes Rt1 through Rt5 are used. The succession between formations Rt1 through Rt3 is the subject of this study.
Only the upper part of the basal formation, Rt1, is exposed in the study area and comprises coarse-grained, trough and planar cross-bedded arenites, which are texturally moderately mature. Paleocurrent distributions are mostly unimodal, except in the extreme eastern region, where the distribution is polymodal. Formation Rt2 disconformably overlies formation Rtl and consists of interbedded conglomerate and coarse-grained, cross-bedded arenites. Pebble size is largest in the northwestern parts of the study area and decreases towards the east. Paleocurrent distributions are unimodal in the western parts of the study area, but bimodal to polymodal in the central and eastern parts. Formation Rt3 conformably overlies formation Rt2 and comprises predominantly coarse-grained, moderately mature, trough and planar cross-bedded quartz wackes, with scattered small pebbles and thin conglomerates. Paleocurrent distributions of this formation, too, are unimodal in the western part of the study area and mostly bimodal in the remainder. A number of planar cross-bed sets with foreset packages separated by mudstone or immature sandstone laminae were found in the outcrops of the central and eastern parts of the study area. In some cases the thicknesses within series of consecutive foreset packages vary cyclically. A planar cross-bed set with mud-draped foreset packages was found towards the top of the formation in the eastern parts of the study area.
The mudstone-bounded foreset packages described are interpreted as tidal bundles and indicate a tidal setting for those paleogeographic parts of the formations where they occur. Bimodal paleocurrent distributions provide additional evidence for tidal deposition. West of an imaginary point near the center of the study area, no indicators of tidal sedimentation have been found and all the formations were probably deposited by fluvial processes here. However, to the east of this point, which indicates the position of a paleo-bayline, formations Rt2 and Rt3 exhibit characteristics indicative of tidal deposition. Paleocurrents, and the decrease in pebble size of the conglomerates of formation Rt2 towards the east, suggest that its sediment was derived from a western source area, most likely in response to tectonic uplift and tilting here, and transported into a tidal environment by fluvial processes. The basal disconformity of this formation is thought to be a type 2 unconformity and the result of progradation of coarse-grained gravel across a paleo-surface comprising sand.