Geology and geochronology of type Chasicoan (late Miocene) mammal-bearing deposits of Buenos Aires (Argentina)

The late Miocene Chasicoan mammal-bearing deposits exposed along the lower reach of Arroyo Chasicَ are composed of cross-bedded, very fine sandstones interpreted as a channel-bar deposit (lithofacies association 1) grading upward into sandy siltstones (lithofacies association 2), probably accumulated through relatively high-density flows in a marginal channel and/or floodplain environment. The uppermost levels are dominantly composed of mudstones and sandy siltstones (lithofacies association 3) deposited in generally low-energy conditions of sedimentation in a swampy environment. Several paleosols (lithofacies P) are present, indicating that the succession was the result of episodic fluvial sedimentation. The volcaniclastic composition (primary and reworked pyroclastics) suggests that the fluvial system drained the westward region by the Andean foothills. An impact event dated at 9.23 ± 0.09 Ma and recorded by impact glasses (escorias) during deposition of lithofacies Sp enables the fine tuning of the chronology of the deposits through high-resolution magnetostratigraphic profiles, which indicate that the approximately 9.4 m thick succession recorded by lithofacies association 1 and 2 accumulated between 9.43 and 9.07 Ma. The lithofacial arrangement of the succession does not support the current differentiation of the Arroyo Chasicَ Formation into the Vivero and Las Barrancas members. Previous biostratigraphic interpretations contain significant inconsistencies in light of the revised stratigraphy proposed here.