Abstract

New zircon U-Pb ages from six volcanic ashes in the Subandean foothills provide for the first time well-constrained depositional ages of the late Cenozoic strata in this region. The radiometric ages indicate 12.4 ± 0.5 Ma, 7.93 ± 0.26 Ma, and 5.94 ± 0.20 Ma depositional ages for the bases of Yecua, Tariquia, and Guandacay Formations, respectively. We used these zircon ages to determine the accumulation rates of the late Cenozoic foreland sediments in the Subandes. Our results show a fourfold increase (from 130 to 628 m/m.y.) in sediment accumulation rates between ca. 7.9 and 6 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 paleodrainage networks in the central Andes. This correlation suggests that climate may have been an important factor between the late Miocene and Pliocene.

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