LAUSR

Abstract In the southern Central Andes of Argentina, between 34° and 41°S, the Neuquen Group records Upper Cretaceous nonmarine deposits that provide direct evidence of synorogenic sedimentation associated with the development of an early foreland basin. The configuration of this basin and its relation to the Malargue fold and thrust belt are analyzed in this article using sedimentological analyses, U-Pb dating of detrital zircons and seismic and surface structural data. The remnant wedge-top depozone preserved in the Argentinian Andean slope exposes sedimentological information that indicates that braided fluvial systems, which may have formed part of broader fluvial fans, transported and deposited the sediments from an extensive relief area located in the present western Andean slope to the Chilean Coastal Range zones. The eastern border of this depositional system was defined by a basement uplift, interpreted either as a forebulge or an incipient broken foreland system, from where gravel-sandy braided river systems fed the foredeep depozone transporting and depositing sediments from the exposed cratonic area in the east. Consequently, an initial underfilled foreland basin was configured. The evolution of this early basin and the areas of sediment supply were controlled by the eastern expansion of the Malargue fold and thrust belt, through the development of thick-skinned anticlines, which progressively fragmented the wedge-top zone. The evidence collected suggests that the Late Cretaceous uplift was a major tectonic event that exposed the entire Mesozoic column of the Neuquen basin up to the Permian-Triassic basement, which implies more than 2,500 m of denudation at that time. U-Pb determinations yielded a maximum depositional age of ca. 91 ± 2 Ma (Late Turonian), which suggests either non-deposition or cannibalization of a sedimentary interval of ca. 10 My at 36°S, when compared to the age of the basal sections of the Neuquen Group at other latitudes.