STABLE ISOTOPIC RECORDS FROM MIOCENE PEDOGENIC CARBONATES IN THE SOUTHERN CENTRAL ANDES: IMPLICATIONS FOR THE ELEVATION OF THE ANDES NEAR 33°S

The oxygen isotopic composition of pedogenic carbonates are potential passive recorders of important information about elevation in and adjacent to the sedimentary basins where they formed. Similarly, the stable carbon isotopic composition of these carbonates provides insight to the vegetation type and, by extension, water-stress conditions. We use stable isotope data from pedogenic carbonates collected from three Miocene foreland basin sequences to explore temporal and spatial changes in elevation and vegetation type between 34 and 32 degrees S latitude. The basins examined in this study are within the present day frontal and/or principal cordillera regions and today fall under the influence of the westerlies. In general our data do not show a systematic covariation between d13C and d18O values suggesting evaporative enrichment of soil water has a minimal effect on our samples. The middle to latest Miocene Alto Tunuyán Basin lies between the Frontal and Principal Cordilleras at 34°S. The persistent d18O values of pedogenic carbonates point to lower middle to late Miocene elevations than present suggesting significant (>1000 m) post-Miocene surface uplift of the Frontal and Principal cordilleras occurred at this latitude, while vegetation was dominated by C4 type plants throughout the entire period sampled. The Uspallata Basin at 33 degrees south lies at the southernmost terminus of the Precordillera near an eastward step in the Frontal Cordillera. Sediments from the Uspallata Basin show a pronounced shift in the d18O values to more negative values and d13C to more positive values. We tentatively interpret this change to reflect increased elevation of the Principal and Frontal Cordilleras and a resulting increase in plant water-stress conditions. Age constraints on these sediments are pending. At 32 degrees S latitude, the middle to early late Miocene Manantiales Basin is nested within the Frontal Cordillera. The stable isotopic record from this basin suggests elevations similar to todays with a mixed C3-C4 vegetation environment.