LINKAGE BETWEEN TWO LACUSTRINE MODELS APPLYING STABLE ISOTOPES TO TRIASSIC CARBONATE PALEOLAKES FROM ARGENTINA

The Triassic Cuyana and Ischigualasto-Villa Unión rift Basins in central-west Argentina contain continental strata including prominent carbonate-rich lacustrine sequences. Detailed studies of the stable carbon- and oxygen-isotope composition of these carbonates, together with additional sedimentological and mineralogical evidence enabled interpretation of the complex hydrology of these paleolakes. Furthermore, these lacustrine systems were analyzed in the light of existing models for playas hydrology (Rosen, 1994) and lake-basin types (Carroll and Bohacs, 1999; Bohacs et al., 2000) allowing the integration of these classification schemes.

The Cerro de las Cabras and Cerro Puntudo Formations are coeval and represent two ephemeral carbonate-rich playa-lake systems with abundant microbialites that lack correlation between carbon and oxygen stable-isotope values. This strongly indicates short water-residence time, implying open hydrology. Negative d¹⁸O signatures suggest meteoric-water input. Less negative values of d¹⁸O point to evaporative conditions. Stable-isotopic values for the Cerro de las Cabras paleolake together with sedimentology data indicate a recharge playa-lake in contrast to a through-flow playa-lake for Cerro Puntudo.

The incorporation of groundwater hydrology into the classification of lake-basin types distinguishes five new lake-basin subtypes for playa systems. For balanced-fill lakes there are recharge and through-flow lake subtypes. For underfilled lakes there are recharge, through-flow, and discharge lake subtypes. This implies that underfilled-lake types can have closed surficial hydrography but open groundwater hydrology when considering residence time (water inflow – outflow balance) and interactions between surface and subsurface.

The Los Rastros Formation is a fluvial-lacustrine facies association that contains carbonates, interpreted as an overfilled-lake basin. Nevertheless, carbon and oxygen stable-isotope values show correlation in the basal interval of the formation and lack of correlation towards the upper part suggesting a change from closed to open hydrology through time.