The Lanseria palaeosol: Implications for palaeoweathering and terrestrial life at ∼2.6 Ga on the Kaapvaal Craton

Abstract

Archaean palaeosols are a rare record of surface processes on the early Earth. We report on the discovery of a palaeoweathering profile developed at the contact between Mesoarchaean basement and the Neoarchaean to Palaeoproterozoic Transvaal Supergroup along the northern margin of the Johannesburg Dome, Kaapvaal Craton. Granodiorite ∼3.1 Ga old was subjected to weathering just prior to the deposition of the ∼2.6 Ga Black Reef Formation. The ∼25 m-thick profile is dominated by a palaeosaprolite and organic matter-rich palaeosol remnant at the top that experienced erosion upon marine transgression on the Kaapvaal Craton. The Al concentration of the profile, sericite content and chemical index of alteration (CIA) values progressively increase from the parent granodiorite towards the nonconformity as Fe, Mg, Na and Ca concentrations decrease. The decrease of Co and Zn together with Fe and the retention of U, V and Cr collectively suggest a reducing and acidic environment at the time of weathering. Carbonaceous matter with negative δ13C isotopic values (−27 to −23 ‰V-PDB) in the palaeosol is compatible with the former presence of photoautotrophic microbes. Copper depletion and mobility of P may indicate the presence of organic acids that enhanced chemical weathering of the bedrock. The palaeoweathering profile was subjected to diagenetic-hydrothermal K-metasomatism, regional metamorphism and deformation, which collectively transformed the regolith into a quartz-sericite schist, as it presently occurs. Despite these modifications, the original record of palaeoweathering and microbial colonisation is well preserved. Colonisation of a large swath of the Kaapvaal Craton by terrestrial soil-dwelling microbiota at 2.60 Ga is recorded in abundant soil-derived carbonaceous matter in transgressive marine conglomerates that erosively overlie the palaeosol.