Mineralogical and geochemical evolution of a basalt-hosted fossil soil (Late Triassic, Ischigualasto Formation, northwest Argentina): Potential for paleoenvironmental reconstruction

摘要

Reconstruction of paleoclimatic conditions in the Ischigualasto basin, northwestern Argentina, has been constrained by field studies coupled with mineralogic, whole-rock, and fine-fraction chemical and stable isotope analysis of a Triassic (Carnian) basalt-hosted fossil soil. Field evidence, such as wedge-shaped aggregate structure and slickensides, indicate this was likely a paleo-Vertisol. Whole-rock analysis defines down-profile trends in clay mineralogy and chemical composition that are consistent with modern soils developed upon basalt parent material. X-ray diffraction analysis indicates that the basaltic parent material is dominated by plagioclase with trace amounts of weathered 2:1 phyllo-silicate. Overlying weathered horizons show a progressive loss of plagioclase and an increase in phyllosilicates with minor amounts of kandite clays and detrital quartz. X-ray diffraction analysis of the <2 µm fraction shows that the weathered layers are dominated by dioctahedral smectite (montmorillonite) with a minor fraction of kaolinite in the upper layers of the profile. There is a progressive loss of basic cations in conjunction with an increase in concentration, on a wt% basis, of conservative elements from the basalt upward through the weathering profile. The combined data indicate that this soil likely formed on a stable landscape in a cool and humid climate. In addition, the presence of quartz in the paleosol profile suggests an eolian contribution of sediment during pedogenesis.