Geologic framework and first results from ICDP BASE drilling in the Moodies Group (~3.22 Ga), Barberton Greenstone Belt, South Africa

Abstract

Units of the up to 3.7 km thick Moodies Group (~3.22 Ga) in the Barberton Greenstone Belt, South Africa and Eswatini, comprise some of the oldest well-preserved sedimentary strata on Earth, deposited within only a few million years in pro-deltaic to alluvial settings, with a dominance of tidal deltas and coastal plains. They consist of widespread quartzose, lithic, tuffaceous and arkosic sandstones, polymict conglomerates, common siltstones and shales, and rare BIFs and jaspilites, all interbedded with rare dacitic air-fall tuffs and several lavas. Moodies strata preserve abundant sedimentary structures and represent a very-high-resolution record of Paleoarchean surface processes. Microbial mats, early diagenetic vadose-alteration zones and tidal rhythmites are locally common. Moodies strata provide a unique opportunity to investigate the conditions under which bacterial life spread and thrived in coastal-zone and terrestrial settings on early Earth.The ICDP Barberton Archean Surface Environments (BASE) Project drilled November 2021 to July 2022 eight inclined boreholes of 280 to 497 m length each through steeply inclined or overturned Moodies Group strata. The unweathered and continuous core record was complemented by sampling in three several-km-long tunnels and by detailed surface mapping. Two to three rigs operated concurrently, delivering twenty to sixty m of high-quality core daily. This core was processed in a large, publicly accessible hall in downtown Barberton. An exhibition provided background explanations for visitors and related this fundamental-geoscience research project to the geology of the Barberton-Makhonjwa Mountains World Heritage Site. The archive half of the core, nearly 3 km total, remained in South Africa, the working half is curated at the ICDP core repository in Berlin, Germany. We show preliminary cross sections, overall core photographs and representative lithologic descriptions.