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Ancient Fan-Delta Systems
Dagbreek Fan-Delta: An Alluvial Placer to Prodelta Sequence in the Proterozoic Welkom Goldfield, Witwatersrand, South Africa
The Dagbreek Formation represents a fan-delta which covers an area of approximately 400 km2. The fan is wedge-shaped in section and thickens from 25 m in the west to over 200 m some 30 km further east. Its base, marked by the Leader placer, rests unconformably on a debris flow arenite of the Harmony Formation. The Leader placer consists of two conglomerate placers, namely the lower oligomictic, mature, Alma placer and the overlying polymictic, less mature, Bedelia placer. Their combined thickness varies between 30 cm to more than 3 m. Intercalated conglomerate and quartzite beds occur within the lower 14 m of the formation, and these fine upwards into khaki-coloured, coarse-grained protoquartzites with rare pebble lags. Small scale trough cross-bedding is ubiquitous. The fining-upward megasequence continues through medium- to fine-grained subgraywackes and thin khaki to black shale beds near the top. The subgraywackes in the top 20 m display medium to large scale cross-bedding and form fining-upward units which contain shale intraclasts at their bases. Palaeocurrent measurements and pebble size distribution patterns indicate a unimodal fan-like distribution trending eastward. The sandy and silty shale on top is plane-bedded, ripple cross-laminated, and graded bedded, these facies being arranged in coarsening-upward sequences.
The placers are interpreted as braided channel-fill sands and gravels. The overlying sands represent the more distal braided equivalents of the alluvial fan whereas facies sequences in the topmost part suggest single channels on the lower fan area. The shaly sequence on top represents shallow subaqueous sediments deposited in a lacustrine prodelta area.
The model presented here is that of a fan-delta produced by rejuvenation of the source followed by lowering of relief and recession of the source area.
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