Rapid downdip textural and compositional maturation of Moodies Group siliciclastics, Barberton Greenstone Belt, South Africa and Eswatini

The Paleoarchean Moodies Group (ca. 3.22 Ga) of the Barberton Greenstone Belt (BGB) is the oldest known well-preserved, shallow-water siliciclastic sequence on Earth. Proximal-to-distal textural and petrographic examination of selected units across adjacent terrestrial, coastal, and shallow-marine depositional facies of this strongly deformed unit provides the opportunity to constrain Archean weathering conditions by source-to-sink analysis, complementing previous bulk analyses. We examined conglomerate, sandstone, and shale composition by detailed geologic mapping and facies analysis along progressive downdip sediment routing systems, after excluding regions of syndepositional hydrothermal alteration. The majority of quartz and feldspar grains in the central BGB was sourced from intermediate to felsic intrusives and (sub-)volcanics of the Hooggenoeg Formation of the Onverwacht Anticline (OA). Downdip textural and mineralogical maturation from immature, F-, L- and matrix-rich, coarse-grained debris-flow-style units to texturally and compositionally supermature fine-grained sandstone occurs within a few km; facies belts fringe the OA. Chemical weathering and mechanical disaggregation of latite-dominated clast populations in proximal facies accompanied decomposition of F to clay (now partially preserved as sericite grains) and of intermediate volcanic grains to quartz-sericite-mosaic grains within a few km from their source. Energetic reworking in wide coastal-facies belts winnowed the abundant clay from the sediment and generated supermature sand, possibly aided by high tides. We deduce that Moodies Group sediment generation appears to have occurred in an aggressive weathering environment; in the coastal zone, only sediment which was silicified surficially and pre-compaction escaped transformation to a sedimentary “restite”.