CENOZOIC SHIFTS IN PROVENANCE, DEPOSITIONAL SYSTEMS, AND SUBSIDENCE ASSOCIATED WITH A MAJOR HIATUS IN THE NEUQUEN FORELAND BASIN, WEST-CENTRAL ARGENTINA

The northern Neuquén Basin in Argentina contains unique structural and stratigraphic records of Cenozoic Andean shortening. Debate surrounds a ~20 Myr hiatus that marks a significant shift in foreland basin evolution in terms of sediment accumulation and paleodrainage, and provenance. To test various hypotheses for the cause of the observed stratigraphic shift from ~40 to 20 Ma, detailed measured stratigraphic sections were evaluated in conjunction with detrital zircon U-Pb geochronological analyses in the Cerro Butaló and Agua Botada localities at 36°S. Stratigraphic and sedimentologic analyses of a ~1100 m thick succession reveal a highly condensed section (Rodados Lustrosos unit) distinguished by very well rounded, highly polished cobbles that sustained major wind-blown abrasion. These unique clasts include commonly faceted surfaces diagnostic of ventifacts, and are interpreted to have endured significant weathering and eolian abrasion for up to 20 Myr. Eolian ablation was concurrent with minimal deposition over a >500 km along strike within the Andean foreland basin. Locally preserved eolian sandstones may represent minor deposition that is partially contemporaneous with a non-depositional hiatus across adjacent basin segments. Detrital zircon U-Pb age signatures record a pronounced shift in sediment sources before and after the hiatus, with a change from a dominantly Andean magmatic arc (40-70 Ma) to high-diversity fold-thrust belt provenance (10-25, 40-70, 150-200, and 250-290 Ma). This flourishing of sediment sources across the condensed section indicates major drainage reorganization and landscape change during the ~20 Myr hiatus. Shortening-related growth strata are observed in the coarse-grained Miocene Agua de la Piedra and Loma Fiera Formations that post-date the hiatus, indicating rapid Andean deformation and basin subsidence. These findings point to tectonic rather than climatic drivers of basin evolution, with an important mid-Cenozoic pause in Andean shortening and tectonic subsidence. However, Oligocene global cooling and regional aridification may have promoted eolian conditions. Further integration of geochemical data, sediment dispersal patterns, and geochronological results will help clarify the influence of climate and tectonics on Andean foreland basin evolution.