USING OXYGEN ISOTOPE VALUES OF MODERN WATER AND ANCIENT CARBONATE TO RECONSTRUCT THE PALEOELEVATION HISTORY OF THE INTER-ANDEAN VALLEY, ECUADOR

The Inter-Andean Valley (IAV) of Ecuador is located between the Eastern and Western Cordillera at an elevation of approximately 2-3 km above sea-level. To reconstruct the paleoelevation history of this region and to better understand the tectonic and geodynamic mechanisms responsible for its formation we analyzed the stable isotope values of ancient nonmarine carbonate from Neogene strata exposed in the IAV. In addition, we sampled meteoric waters and modern carbonate throughout the Ecuadorian Andes to better characterize the oxygen isotope system and constrain paleoelevation interpretations. Oxygen isotope values of meteoric water in the foreland basin, Eastern Cordillera, IAV, and Western Cordillera of Ecuador are between -6 and -12‰ (VSMOW); values of meteoric water at specific locations are consistent with the average annual δ¹⁸O values of local precipitation. As expected, the δ¹⁸O values of meteoric waters in Ecuador vary inversely with elevation, although the calculated lapse rate of -1.4‰/km is relatively low compared to global averages. Lake waters in the northern IAV have δ¹⁸O values of -9‰, whereas lake waters within the more arid southern IAV have enriched δ¹⁸O values of -3‰ and do not lie on the global meteoric water line, suggesting evaporative effects. Oxygen isotope values of nonmarine carbonate from Neogene strata (ca. 15-3 Ma) are between -6 and -11‰ (VPDB). Lacustrine carbonate from the ca. 5-3 Ma Chota Formation exposed in the northern IAV yields oxygen isotope values between -9 and -11‰, suggesting these carbonates precipitated from relatively depleted waters with δ¹⁸O values similar to those of modern lakes and rivers in the area. The δ¹⁸O values of the Chota Formation are consistent with the hypothesis that elevations in this area 5-3 Myrs ago were similar to modern elevations, although other factors can also explain the observed δ¹⁸O values. Data from this investigation help clarify spatial trends in the modern oxygen isotope system of the Ecuadorian Andes and can be used to evaluate competing tectonic models of IAV formation.