Linking time-equivalent paleosols and lacustrine rocks to reconstruct paleoclimate in the Ischigualasto Basin, NW Argentina

Abstract

Field relationships between the coeval Ischichuca and Chanares Formations in the southern part of the Ischigualasto rift basin, northwestern Argentina, combined with mineralogical, geochemical, and petrographic analyses demonstrates that the climate was semi-arid to semi-humid during the Middle Triassic. Although climate changes (sediment and water supply) controlled the types of sediment that accumulated in the basin, facies distribution is controlled by differential subsidence in the rift basin. Asymmetrical subsidence is inferred from the change in thickness of the rocks perpendicular to the western basin-bounding Valle Fertil fault. Braidplain and fluvially-dominated lacustrine deltaic deposition occurred closest to the Valle Fertil fault. The rivers drained the surrounding highlands composed of Precambrian metamorphic rocks and Paleozoic plutons. Fluvial systems tended towards the topographic minimum, in this case, the lake, which is 10 km from the Valle Fertil fault. The lack of Gilbert-style deltas suggests that the fluvial systems entering the lake were not on a steep gradient and the lake was not very deep. The widespread lateral extent and thin palustrine deposits and B horizons of moderately well-developed Vertisols along the southern and eastern margins illustrates slow aggradation of material. Vertic features in paleosols, pedogenic mud chips in fluvial and lacustrine deposits, and clastic-organic and carbonate-organic couplets in lacustrine facies indicate seasonality of precipitation. The Corg/Stotal ratios, presence of Unionids, charophytes, and detrital clay minerals, and lack of desiccation features or saline and saline alkaline minerals indicate the lake was fresh. Significant coal swamp deposition occurred along the lake margins. The combination of Vertisols, coals, and lack of pedogenic carbonate in the paleosols suggests the mean annual precipitation in the Ischigualasto Basin was between 120--250 cm. The carbon isotopic composition of terrestrial organic matter in fluvial and lacustrine deposits and Vertisols documents that plants were subjected to significant water stress.