PALEOGEOGRAPHY AND DEPOSITIONAL SYSTEMS OF CRETACEOUS-OLIGOCENE STRATA: EASTERN PRECORDILLERA, ARGENTINA

We present new sedimentology, detrital geochronology, and mapping from continental strata in the Argentine Precordillera fold-and-thrust belt in the Andes at 30°S. The ‘red strata’ of the Cuesta de Huaco anticline reside beneath Oligocene foredeep deposits and previously were assigned Paleozoic ages. This work contributes the first Paleocene radiometric ages from the pre-Oligocene Bermejo Basin and reinterprets timing of sedimentation for an important interval in Andean retroarc foreland basin history. We provide key constraints that elucidate possible drivers for basin subsidence and connect this Late Cretaceous-Paleocene sedimentation to Andean deformation, rather than post-rift thermal subsidence. This finding contrasts with the commonly accepted onset of foreland basin sedimentation at this latitude, ca. 22 Ma.

The Cenomanian-Turonian Ciénaga del Río Huaco (CRH) Formation is 120-250 m thick and represents a longitudinal braided fluvial channel belt that was up to 25 km wide and received sediment predominantly from the eastern interior craton or recycled orogenic sources to the northeast. The most prominent detrital zircon U-Pb age population is Proterozoic Grenville/Sunsás with lesser contributions from Carboniferous and Permo-Triassic batholiths. We propose that the depocenter responded to Andean tectonic activity during deposition of the upper CRH coeval with a reversal in sediment dispersal direction from northeast to southwest, based on paleoflow measurements and appearance of Late Jurassic-Miocene arc-derived zircons.

The Danian-Priabonian Puesto La Flecha (PLF) Formation is 20-70 m thick in the study area and represents a shallow, ephemeral hypersaline lake with minor clastic input and influx of western-derived, Andean arc sediment within the Bermejo Basin. Major detrital zircon populations are from Paleocene-Eocene Andean magmatism, Permo-Triassic batholiths, and Grenville/Sunsás cratonic sources. During Paleocene-Eocene time, the region experienced aridification and the sediment routing system shifted to western sources. The restored position of the PLF is consistent with low subsidence/sedimentation rates during lake formation within an incipient distal foredeep depozone, prior to major foredeep sedimentation of the Oligocene-Miocene eolian strata.