THE EFFECT OF LATE CENOZOIC ARIDIFICATION ON SEDIMENTATION IN THE EASTERN CORDILLERA OF NW ARGENTINA (ANGASTACO BASIN)

In the Central Andes of NW Argentina, late Cenozoic conglomerates are preserved along the margin of the Puna Plateau. One such basin, the Angastaco, has a well-understood structural history allowing for comparison between tectonics and the sedimentary record. This study evaluates the affect of climate on facies, grains size and sedimentation rates using sedimentology, geochronology and stable isotope geochemistry for Miocene-Pliocene deposits in the Angastaco basin (Eastern Cordillera, NW Argentina) in order to determine if there is a correlation between deposition of coarse Pliocene fluvial-alluvial deposits and climate, or if local tectonics can explain the increase in grain size. U-Pb zircon dates constrain the transition between the finer-grained fluvial-lacustrine Palo Pintado Formation to coarser-grained fluvial-alluvial San Felipe Formation to ~ 5.2 Ma and the first deposition of sediment derived from the present-day orographic barrier to ~ 4 Ma. δ¹³C values from pedogenic carbonate nodules range from -15.4 ‰ to -10.2 ‰ for the Palo Pintado Formation and from -9.5 ‰ to -8.2 ‰ for the San Felipe Formation, which can be best explained by increase, sustained aridity since ~ 5 Ma. The δ¹⁸O values range from -9.6 ‰ to -5.9 ‰ for the Palo Pintado Formation and from -6.1 ‰ to -5.2 ‰ for the San Felipe Formation, corroborating this interpretation. The shift towards more arid conditions correlates with a significant increase in grain size and no change in sedimentation rate. Since aridity precedes the development of an orographic effect, the authors interpret the grain size increase in the Angastaco basin since ~ 5 Ma to be a response of the sedimentary system to aridification resulting from regional climate change.