Chronology of Neogene sedimentation, stratigraphy, and tectonism in the Campo de Talampaya region, La Rioja Province, Argentina

Abstract

Chronostratigraphic, paleoenvironmental, and tectonic analysis of a 2320 m thick Neogene continental section was conducted along the southern limit of the Campo de Talampaya. Magnetic polarity stratigraphy and fission track ages were used to develop a relatively precise geochronology. The strata record a period of mixed evaporitic and terrigenous elastic sedimentation (∼20−6.1 Ma) that persisted in the distal Bermejo Basin during eastward migration of the Precordillera fold-thrust belt. A critical transition of depositional environments and a change from western to eastern source areas occurred between 12.9 and 12.3 Ma. The saline mud-flat environment of the Río Mañero Formation, derived from a western source and characterized by fine-grained sandstones, mudstones, and evaporites, yielded to a short period of playa lake deposition. This was soon overwhelmed by deposition of the Desencuentro Formation, composed of sandstones and mudstones with an eastern source area and deposited in a sand-flat/mud-flat environment. We propose that this stratigraphic transition resulted from initiation of uplift of the Sierra de Velasco, a range of the Sierras Pampeanas, located ∼65 km to the east-northeast of the section. Basement clasts, probably derived from the Sierra de Velasco, are found in conglomeratic lenses beginning in the lower part of the Desencuentro Formation. The modern Campo de Talampaya basin formed after deposition of the top of the Alto de San Nicolás paleomagnetic section (6.1 Ma) when west-verging reverse faults uplifted the Sierra Morada and Sierra de los Tarjados. All strata in the section were tilted eastward during these culminating uplift events.