（エントリー）A hot, hydrothermally-fed microbial tidal flat of the Paleoarchean? - Hints and evidence from the Moodies Group, Barberton Greenstone Belt, South Africa

抄録

The Paleoarchean Moodies Group (ca. 3,220 Ma) in the central Barberton Greenstone Belt comprises up to around 3.7km thick, sandy alluvial- to tidal-facies sediments interspersed with diverse, syn- to shortly post-depositional intermediate to mafic (sub-)volcanic units. Renown for harbouring the up-to-date oldest known record of macroscopic mappable biomats in a siliciclastic tidal environment on earth, these densely biolaminated sub- to supra-tidal sandstones are pervaded by abundant, up to 6m high fluid-escape structures. Feeding small sand volcano ridges, their distribution is largely restricted to a single, ca. 150m thick, highly silicified sandstone unit overlying the Moodies-aged Lomati River Sill, ca. 1km below. Semiquantitative μXRF-mapping of slabbed fluid-escape structures suggests substantial enrichments of Fe, Mg, Ti, and Cr between the conduits and their surrounding beds, implying influences past mere microbial fluid-retention and overpressure build-up from decaying biomats in the shallow subsurface. Supported by sediment textures indicative of argillaceous and sericitic alteration, Raman temperatures ca. 50 – 100°C above the regional maximum, and past field observations of hydrothermal alteration and peperites nearby, we propose the substantial involvement of hydrothermal fluids generated in the thermal aureole of the cooling Lomati River Sill. Thermal and chemical gradients may have boosted microbial growth, while surficial pre-compaction silicification favoured the preservation of delicate microbial remains.