A MULTIPROXY APPROACH TO PALEOHYDROLOGIC AND PALEOCLIMATE RECONSTRUCTION OF TRIASSIC LACUSTRINE BASINS OF ARGENTINA

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 basin in central-west Argentina. They have been radioisotopically dated to the Anisian Stage, a time interval during which paleoclimate in the south-west margin of Gondwana is poorly known for terrestrial environments. 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. We calculated subaqueous lake margin paleotemperatures and estimated annual δ¹⁸O_w. This was done considering the two extremes of the range of estimated 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 estimates for each locality.

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 range between 8 and 26 °C for CC and between 6 and 31 °C for CP. 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 most likely reflects within-lake processes. Calculated paleotemperatures for this paleolake range between 3.5 and 30°C. Interpretation of relative thermal ranges instead of 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. These data provide the first detailed paleoclimate proxy data for mid-latitude Gondwana during the Middle Triassic.