TRIASSIC BIOTIC ASSEMBLAGES OF TWO PALEOLAKES OF ARGENTINA

BENAVENTE, CECILIA A.; MANCUSO, ADRIANA; CABALERI, NORA G.

Mendoza

Congreso; 4th INTERNATIONAL PALAEONTOLOGICAL CONGRESS; 2014

Asociación Paleontológica Internacional

The Triassic Cerro de las Cabras and Cerro Puntudo formations represent part of the continental sedimentary infilling of the Cuyana rift basin. The basin has two asymmetric half-grabens ? the North containing the Cerro Puntudo sub-basin with the Cerro Puntudo Formation (CPF) and the South where the Potrerillos sub-basin and the Cerro de las Cabras Formation (CCF) are identified. Both units conform to alluvial-fluvial-lacustrine successions of rift basins including carbonates with abundant microbialites. Sedimentology reveals playa lake systems with sandflat, mudflat, palustrine, and lacustrine subenvironments affected by subaerial exposure and pedogenesis. Geochemistry indicates hydrologically open paleolakes (through lack of a covariant trend between C and O stable isotopes), with meteoric water supply through springs and precipitation. The CCF is an aggradational succession that corresponds to the evaporative facies association type, diagnostic of underfilled lake basins. However, the lakes model links underfilled lakes with persistently closed surface hydrology, though groundwater supply may be open here. The CCF shows a negative C isotope signature indicating biologic effects attributed to organic matter (OM) decomposer heterotrophic bacteria. There are abundant stromatolitic limestones and rhizohaloes-rhizoliths representing paleosols. The CPF is an aggradational-minor progradational succession, pointing to a fluctuating profundal facies association diagnostic of balanced-fill lakes. However, the restricted areal extent of these deposits cannot confirm this interpretation. The CPF shows less negative excursions of the C isotopic signature interpreted as algal blooms with increased photosynthetic activity. The CPF contains a microbialite assemblage reflecting an aquatic paleoecosystem with a simple trophic network, composed of filamentous and coccoid algae with Cyanophyta affinities that constitute primary producers with lithified structures through bio-mediated carbonate precipitation. Charophytes have been identified fulfilling the same task. Ostracodes (heterotrophs) fed from detritus and bacteria as OM decomposers. Terrestrially associated components were hygrophytes and arthropods from the Skolithos-Scoyenia ichnofacies. In both paleolakes, biotic assemblages are freshwater-saline tolerant, as in balanced-fill lakes. However, they present low diversity and moderate abundance due to shallow-ephemeral paleolakes with high alkalinity, common lake level fluctuations, and subaerial exposure. These attributes certainly constrained the opportunistic biotic assemblages in unstable settings. For the CCF, biota supports an underfilled lake. Its open hydrology shows that underfilled lakes may be closed to surface water supply but open to groundwater movement. More studies are required to determine if the CPF can be analyzed under the current lake model and if indeed it can be interpreted as a balanced-fill lake type as biota suggests.