NEOARCHEAN (C. 2.58 GA) HALITE CASTS IN PERITIDAL SEDIMENTS OF THE BLACK REEF- CHUNIESPOORT GROUP TRANSITION BEDS, TRANSVAAL SUPERGROUP (SOUTH AFRICA)

Possibly the most extensive and best-preserved Archean halite casts have been discovered in ca. 2.58 Ga shales of the Black Reef and basal Oaktree Fms, Transvaal Sgp, in Mpumalanga Province, South Africa. The halite casts are found in mixed siliciclastic-carbonate peritidal sediments in the transition zone from the fluvial Black Reef Fm to marine (intertidal to subtidal) stromatolitic dolomites of the Oaktree Fm, in the carbonate-dominated Chuniespoort Gp. Halite casts occur in thin (0.5-1.0 cm) shale beds interbedded with rippled sandstones in the uppermost Black Reef Fm, and in a 10-40 cm-thick shale bed in the basal Oaktree Fm. Two exceptionally well-preserved pavement exposures (several 10’s of m2 in area) of halite casts are developed in subhorizontal, almost unmetamorphosed sediments of the basal Oaktree Fm. The halite cast-bearing pavements are interbedded with dololutites and rippled dolarenites, or with stromatolitic and microbial-laminated dolomites. The halite casts range in size from ca. 1 mm to 20 mm, and have cubic or triangular shapes, depending on the orientation. The sides of the crystal casts are concave inwards. Some of the casts display distinct hopper shapes characteristic of halite crystals. Associated sedimentary structures pointing to subaerial exposure include flat-topped ripples, desiccation cracks, adhesion ripples and adhesion warts, and tepee structures. Palaeocurrents determined from wave ripples and stromatolite elongation directions indicate N-S and NE-SW trends, possibly related to flood and ebb-tidal current activity. Associated with some of the halite casts are small randomly orientated euhedral rhombic brown carbonate crystals, which appear to be of diagenetic origin. We interpret the halite cast-bearing bed as having been deposited in a supratidal sabkha environment. Halite may have precipitated from a supersaturated residual brine after carbonate deposition. The absence of any indication of the former presence of gypsum or anhydrite supports previous models for a deficiency of sulphate in Neoarchaean seawater.