Sedimentology and stable isotope composition of freshwater cyanobacterial carbonates from the Oligocene / Miocene of central Patagonia (Argentina): environmental and climatic signals

CABALERI, N.G; CAGNONI, M.

Zaragoza

Congreso; 28th IAS Meeting of Sedimentology; 2011

International Association of Sedimentologists

The palaeoenvironmental evolution of La Pilila lake from Oligocene-Miocene of the Carinao Formation is presented after isotopic and facies analysis. The outcrops at the east side of the Sierra de Tecka represent a shallow lacustrine system interconnected by rivers and small streams. Extensive precipitation of carbonate was favoured by paleogroundwater discharge following flow paths through pre-existing structures related with active faulting which occured during the previous Andean Cordillera uplift. The system is represented by the distributional changes of the lacustrine and fluvial facies. The following microbialitic facies which characterized the different lacustrine environments have been identified: a) shallow water ponds developed on a hard substratum of gabbro volcanic clasts (Radial domal microbialites), b) shallow environment with clear and calm water and scarce fluvial input (Bulbous radial microbialite), c) littoral environment where the growing pattern of the bioherm was produced when the lake level increased (Columnar branched stromatolites). Furthermore, littoral bars (Microlaminated microbialites built up by radial sparite)were recognized. Microbialites built up by homogeneous micrite have developed in protected environments within these bars. Between biohermal algal structures small ponds formed by skeletal packstones, grainstones and rudstones were originated. The stabilization facies formed in a shallow environment with a climatic seasonal variation (Microbialites with incipient lamination). The associated microbialitc facies with fluvial channels are represented by Microbialitic mudstones with intercalations of oncosiliciclastic wackestone related to the mouth channels and Bulbous microbialites built up by the growth of communities on the channel margins. The fluvial facies are represented by sandy grainstones. The lake received groundwater and freshwater discharges. Bicarbonate groundwater probably contained abundant dissolved CO2 from a deep source (δ13C~ -6.0?) while the water contributions of rivers and streams had depleted soil CO2 (δ13C~ -27.0?). The range of values of the δ18OPDB in carbonates (-7.2? to -10.0?) indicates that groundwaters cooled and mixed with meteoric water. On the other hand, the isotopic signal δ13CPDB recorded in the carbonates (-4.6? to -1.2?) was affected by microenvironmental effects originated by photosynthetic activity of cyanobateria and algae. The biological activity removes the 12CO2 from the lake water leaving carbonates enriched in 13C. The biogenic precipitation was favoured by humid and warm climatic conditions. The absence of δ13C- δ18O covariance indicates that the basin was hydrologically opened.