Climatic and geochemical implications of Archean pedogenic gypsum in the Moodies Group (similar to 3.2 Ga), Barberton Greenstone Belt, South Africa
- Lithic sandstones of braided-fluvial to supratidal facies in the Paleoarchean Moodies Group (similar to 3.22 Ga, Barberton Greenstone Belt, South Africa) include several regionally traceable units with common to abundant, in places rock-forming, nodular concretions of megaquartz pseudomorphs after gypsum, barite and calcite. Concretionary accumulations are stratiform and commonly associated with aqueously reworked, fine-grained, tuffaceous sediment of originally rhyodacitic composition and can grow to fist sized agglomerates in crusts tens of m in lateral extent. Weathering of tuffaceous material and feldspar delivered alkali cations such as Ca, Ba, and K, while carbonates were likely supplied by silicate weathering of mafic to ultramafic volcanic rocks during exposure to a CO2-rich atmosphere. Sulfate ions were partly delivered by oxidative pyrite dissolution which may have included microbial and abiotic disproportionation of volcanic S or SO2. Concretionary growth apparently took place under pedogenic to early diagenetic conditionsLithic sandstones of braided-fluvial to supratidal facies in the Paleoarchean Moodies Group (similar to 3.22 Ga, Barberton Greenstone Belt, South Africa) include several regionally traceable units with common to abundant, in places rock-forming, nodular concretions of megaquartz pseudomorphs after gypsum, barite and calcite. Concretionary accumulations are stratiform and commonly associated with aqueously reworked, fine-grained, tuffaceous sediment of originally rhyodacitic composition and can grow to fist sized agglomerates in crusts tens of m in lateral extent. Weathering of tuffaceous material and feldspar delivered alkali cations such as Ca, Ba, and K, while carbonates were likely supplied by silicate weathering of mafic to ultramafic volcanic rocks during exposure to a CO2-rich atmosphere. Sulfate ions were partly delivered by oxidative pyrite dissolution which may have included microbial and abiotic disproportionation of volcanic S or SO2. Concretionary growth apparently took place under pedogenic to early diagenetic conditions within unconsolidated granular sediment in the vadose zone, dominated by seasonal fluctuations of the groundwater level under evaporitic conditions. The concretions likely represent the oldest terrestrial evaporites known to date and form part of the oldest known compound paleosols. Their formation and composition constrain the local occurrence of sulfate in the Archean atmo- and hydrosphere, their interaction with the emerging biosphere, Archean weathering regime, local climate, and vadose-zone hydrodynamics. (C) 2016 Elsevier B.V. All rights reserved.…
