LATE CENOZOIC TECTONIC AND CLIMATIC CONTROL ON THE ACCUMULATION OF THE CHACO FORELAND SEQUENCE

The 200 km wide Chaco foreland basin is an integral part of the Andean orogenic system of south-central Bolivia that is related to the thin-skinned basement-involved shortening and uplift and the subsequent eastward propagation of the Andean deformation. The basin contains up to 7.5 km thick of mainly late Oligocene to present continental sediments. To unravel the development history of Andes in this area, we used stratigraphic, sedimentary facies analysis, seismic stratigraphy, radiometric dating, and stable isotope approach on the hitherto poorly-constrained foreland strata. We used this dataset to reconstruct the Paleoenvironment, paleoclimate, structural sequence, and estimate the accumulation rates, thus allowing evaluating the role and influence of tectonic and climatic processes in controlling the sedimentation of the foreland sequence. Our new Zircon U-Pb ages from 10 volcanic ash samples intercalated in sedimentary rocks constrain the onset of thrusting in the Subandes at 12.4 Ma. Additionally, the radiometric ages indicate 12.41 ± 0.24 Ma, 7.93 ± 0.13 Ma, and 5.94 ± 0.08 Ma depositional ages for the bases of Yecua, Tariquia, and Guandacay formations respectively. Detailed sedimentologic studies as well as subsidence analysis and sediment accumulation rates show that this area was characterized by relatively high subsidence and very low sedimentation rates of craton-sourced ephemeral fluvial systems that coexisted with lacustrine depositional environments between 27 to ~8 Ma. Our observations suggest that the creation of accommodation space resulted from the eastward advance of thrusting and coeval tectonic loading. In addition, our results show rapid sediment supply and a four-fold (from 130 to 628 m/Ma) increase in sediment accumulation rates between ~7.9 and 5.9 Ma during the deposition of Andean-derived coarsening- and thickening-upward sandstone-dominated Tariquia strata. This increased accumulation rate correlates well with monsoon intensification and climate variability in South America, which was accompanied by the development of fluvial megafan paleo-drainage networks in the Central Andes. These environmental changes toward a monsoon climate are synchronous with changes in paleo-vegetation cover as indicated by trends in C3/C4 derived from pedogenic carbonates.