Stable isotope composition of the Cerro Puntudo lacustrine limestones (Triassic, Argentina)

BENAVENTE, CECILIA; CAGNONI, MARIANA; FINKELSTEIN,DAVID; CABALERI, N.G

Sao Pablo

Simposio; 9 South American Symposium on Isotope Geology; 2014

CPGeo

The Triassic Cerro Puntudo Formation represents the sedimentary infilling of the Cuyana Rift Basin in its northern most depocenter. The sedimentary unit contains continental deposits, including alluvial, fluvial and palustrine-lacustrine sediments. The palustrine-lacustrine system is mainly carbonate with abundant microbialites and has been identified from the following facies: Mottled limestones, Nodular limestones, Stromatolitic limestones and Oncolitic boudstones. The characteristics of these facies allow inferring an ephemeral playa lake depositional system with common features of subaerial exposure and pedogenesis. The stable isotope composition of the carbonate facies was determined in order to establish the main paleohidrological and paleoenvironmental characteristics of the paleolake. All the facies mentioned were sampled with a micro-drill to obtain precisely and accurately the δ13C and δ18O values for the micrites. The δ13C values of the limestones of the Cerro Puntudo unit range between -8.93? and -0.75?. The δ18O values range between -11.53? and -5.04?.These isotope values show very low covariation indicating hydrological open conditions. The δ13C data show a positive excursion interpreted as the result of algal blooms with increased photosynthetic activity in the paleolake. This might as well correlate with seasonal climate fluctuations. The δ18O data present tendencies that fluctuate between evaporative and dilution conditions. These trends could be related to regional climate changes. Along with the sedimentary features, the stable isotope composition of the lacustrine limestones of the Triassic succession provided useful information to reconstruct the paleoenvironmental and paleohidrological conditions during deposition.