RETROARC BASIN REORGANIZATION AND DIACHRONOUS ARIDIFICATION DURING PALEOGENE UPLIFT OF THE SOUTHERN CENTRAL ANDES

Tectonic development of the Andean Cordillera has profoundly changed the topography, climate, and vegetation patterns of the southern Central Andes. The Oligocene-Pliocene Bermejo Basin in Argentina provides key data on fold-thrust belt kinematics of the orogenic belt and paleoclimate south of the high-elevation Puna Plateau. Ongoing debate about the timing of initiation of upper plate contraction and uplift in the region persists, precluding a more complete understanding of the tectonic and climatic drivers on basin evolution and changes in sediment dispersal. We present new sedimentology, detrital geochronology, sandstone petrography, and subsidence analysis from the Bermejo Basin that reveal siliciclastic-evaporative fluvial, lacustrine, and eolian environments prior to the main documented phase of Oligo-Miocene construction of the Frontal Cordillera and Argentine Precordillera. We report the first radiometric dates from detrital zircons collected in the Ciénaga del Río Huaco Fm. that confirm a Late Cretaceous maximum depositional age (95-93 Ma) from strata previously mapped as Permian. Detrital zircon U-Pb ages of ca. 38-37 Ma from the overlying redbeds, here assigned to the correlative Puesto La Flecha Fm., further support sedimentation prior to onset of Oligocene eolian deposition and extend back the incipient foreland basin record into at least Eocene time. Provenance and paleocurrent data from Upper Cretaceous fluvial strata indicate that detritus was derived from deeply dissected arc and cratonic sources in the northeast. By Late Eocene time, sediment dispersal shifted from axial sediment dispersal from the north to dominantly transversal from the Andean arc and Frontal Cordillera in the west. Subsidence and flexural analysis of the basin fill is compatible with deposition in a slowly subsiding distal foredeep position that maintained fluvial connectivity to the hinterland during topographic uplift and unroofing of the Frontal Cordillera. Maximum depositional ages of the overlying Vallecito Fm. indicate early Oligocene onset of eolian conditions at 30°S. A regional synthesis of Cenozoic dune deposits spanning ~32-25°S suggests a southward younging of inception of eolian conditions; by early Miocene time, the Andean retroarc foredeep encompassed an extensive network of dune fields.