Tectonic control on the evolution of the fluvial systems of the Vinchina Formation (Miocene), northwestern Argentina

Abstract

The Miocene Vinchina Formation accumulated in a large foreland basin is related to the uplift of the Andes Mountains. This 5100m thick unit was mostly deposited in fluvial environments, but short episodes of eolian and lacustrine sedimentation also occurred. Low-angle intraformational unconformities and dramatic facies changes define three depositional sequences. Sequence S1 is composed of sandstones and mudstones deposited in anastomosing river systems during a period of tectonic quiescence. Sequence S2, formed by conglomerates and sandstones with scarce intercalations of mudstones, rests on a low-angle erosional surface and represents deposition in fluvial braided systems. S2 probably resulted from progradation of clastic wedges after an episode of uplift of the fold-thrust belt. Sequence S3, dominated by sandstones, mudstones, and sparse intraformational conglomerates, was deposited in ephemeral braided and meandering rivers governed by a complex balance between subsidence and supply. Fluvial styles in the Vinchina Formation allowed the recognition of three major stages in the foreland basin evolution. During the underfilled stage (S1), high subsidence rates favored the development of axial anastomosing rivers with well-preserved floodplain sediments. Early post-tectonic sedimentation marks initial overfilled conditions (S2), characterized by high sediment supply and progradation of clastic wedges as braided rivers deposits. Finally, more advanced overfilled conditions (S3) were reached when sediment supply matched subsidence, giving birth to ephemeral braided and meandering systems.