“Isotope Geochemistry of the lacustrine facies of Cañadón Asfalto Formation at Quebrada Las Chacritas, Cerro Cóndor, Chubut”.

RAMOS, A.; CAGNONI, M.; CABALERI, N.; ARMELLA, C.; PANARELLO, H.; VOLKHEIMER, W. AND SILVA NIETO, D

San Carlos de Bariloche

Simposio; VI South American Symposium on Isotope Geology; 2008

INGEIS-CIG

This work presents the first results of the carbon and oxygen isotopic analyses of the lacustrine carbonates from the lower section of Cañadón Asfalto Formation at Quebrada Las Chacritas, Cerro Cóndor, central Chubut province. The carbonate facies indicate different cycles of expansion (humid periods) and contraction (arid periods) which characterize the evolution of the lake related with the local tectonic history. Lake sedimentation is the result of local variations in the water level during the development of the rift system.  Mn and Fe concentrations are associated with compositional variations between the different carbonate facies and evidence syn- and postdepositional modifications which were recorded in carbon and oxygen isotopic trends. δ13C and δ 18O values of these lacustrine carbonates (-6.2‰≤ δ13C ≤0.3‰ and -10.7‰ ≤ δ18O ≤ -3.4‰) maintain a positive correlation with r = 0.64 in agreement with dominantly closed hydrological conditions.  Higher isotope ratios recorded during shallowing agree with dominant evaporative conditions and subaereal exposure. Local increases in fluvial inputs changed the isotopic signals to more depleted ratios. Two small segments of the profile show negative correlation between δ13C and δ 18O associated with subaereal exposure periods. In these cases, while the δ13C values decrease due to oxidation of the organic matter, oxygen isotope ratios increase due to evaporation. The isotope signals maintain a correlation which allows the interpretation of the paleohydrological and paleoclimatic variations. The obtained isotope data demonstrate a good agreement between the δ13C and δ18O variations and the expansion and contraction cycles of the lake, defined by the sedimentological study.