A multiproxy approach to paleohydrologic and paleoclimate reconstruction of Triassic lacustrine basins of Argentina

BOHACS, KEVIN MICHAEL; MANCUSO, ADRIANA CECILIA; BENAVENTE, CECILIA ANDREA; IRMIS, RANDALL

Phoenix

Encuentro; GSA Annual Meeting; 2019

The Middle Triassic Cerro de las Cabras (CC), Cerro Puntudo (CP) and Santa Clara Arriba (SCA) Formations are part of the continental sedimentary fill of different sub-basins of the Cuyana rift in central-west Argentina. They have been dated as Anisian, a particular time lapse during which paleoclimate in the south-west margin of Gondwana is scarcely known for terrestrial ecosystems. A detailed study of the stable-carbon (C) and oxygen-isotope (O) composition of the lacustrine carbonates, integrated with sedimentological and mineralogical evidence, enabled interpretation of their complex hydrology and paleoclimate conditions. The paleolakes studied correspond to balanced-fill lake type systems with calcitic and dolomitic carbonates. We calculated subaqueous lake margin paleotemperatures and estimated annual δ18Ow. This was done considering the two extremes of the range of possible paleolatitude for the Cuyana Basin during Anisian times and the two extremes of the likely paleoaltitude based on the elevation of modern rift lake systems developed within the East Africa Rift. These parameters yielded a range of four possible paleotemperature ranges. Lack of correlation between C and O stable-isotope values for both CC and CP paleolakes strongly indicates open hydrology with short water-residence times. Paleotemperatures calculated for CC range between 8 and 26 °C and for CP between 6 and 31 °C. Although a suspected season-specific signal possibly resulted in underestimation of the coldest thermal endmember for both systems. Strong correlation between C and O stable-isotope values of the SCA carbonates indicates a long residence time for lake waters and the tendency to hydrological closure perhaps linked to seasonal rainfall. Therefore, its C and O stable isotope signature reflects most likely within lake processes. Calculated paleotemperatures for this paleolake range between 3.5 and 30°C. Interpretation of thermal ranges instead of calculated absolute paleotemperatures suggests intermediate arid to sub-humid paleoclimate conditions which is consistent with previously reported sedimentary and mineralogy data from these sub-basins that pointed to seasonality in the Cuyana rift basin and with global paleoclimate models for the Triassic.