Geology Of The Makgabeng Formation

The generally mature, fine- to medium-grained quart arenites (and lesser finer rocks as well as pebbly sandstones) of the Makgabeng Formation are interpreted as eolian dune, playa, saline pan and interdune facies within an overall desert setting. The eolian facies (inversely-graded wind ripple strata unequivocally support such an origin) is predominant, both stratigraphically and geographically, with contained cross-bedding patterns suggesting either barchanoid or straight-crested dune bedforms (Eriksson et al., 2000; Simpson et al., 2002). Within the lower portions of these eolian cross-strata are localised, erosively-based massive sandstones which broaden out into sheet sandstones, laid down on the paleodune plinths; their strong resemblance to modern deposits from catastrophic rainfall events resulting in the failure of the front faces of high dunes, suggests an origin through similar processes during Makgabeng deposition (Simpson et al., 2002). Thin (few metres thick) lenses of texturally and mineralogically immature sandstones within the predominant eolianites, up to 5 km in lateral extent and characterised by water-lain structures, are ascribed to an origin as playas, and saline pan deposits also occur locally within the eolian succession (Simpson et al., 2004).

The final Makgabeng facies, comprising thinner (generally <1 m) lenses of quartzose sandstone and minor mudrocks, extending up to about 100 m laterally within the eolianites, bear evidence for wind, current, wave and combined flow ripples, and also exhibit mudcracks and evaporite casts, with trace amounts of evaporate minerals detected through X-ray diffraction (Eriksson et al., 2000; Simpson et al., 2004). These interpreted interdune deposits were allied to the extreme precipitation events inferred above, thus being subject to high-energy flash flood events which would have reworked the desiccation products developed over long intervals between such high energy aqueous occurrences (Eriksson et al., 2000). It is this facies which contains a suite of sedimentary structures, described next, and ascribed to an origin through microbial mat growth, desiccation and destruction, as longer intervals of shallow water interdune pools and increasing desiccation alternated with extreme rainfall events.

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