BASIN EVOLUTION AND PROVENANCE VARIATIONS IN THE MIOCENE ATUEL BASIN, EL SOSNEADO REGION, MENDOZA, ARGENTINA

Stratigraphic successions in the southern Atuel basin record progressive orogenic exhumation and basin subsidence in the Miocene retroarc foreland basin related to the structural evolution of the Malargüe fold and thrust belt, northwest of Malargüe, Argentina (~35°S). Miocene strata in the eastern Atuel basin include thin-medium bedded lithic arenite of the Agua de la Piedra Fm. unconformably overlying tilted strata of the Cretaceous Malargüe Group. The basal contact of the Loma Fiera Fm. is an angular unconformity with the underlying Agua de la Piedra Fm. These successive angular unconformities show progressive deformation of synorogenic retroarc foreland basin strata in response to movement on the Malargüe fold and thrust belt.

Analysis of the Loma Fiera Fm. constrains the depositional setting and provenance of these synorogenic strata. The base of the Loma Fiera Fm. (>550 m) is a parallel-bedded coarse pebble-boulder conglomerate; this begins a cyclic succession of four fining upward sequences grading from a basal lenticular cobble-boulder conglomerate into well-stratified coarse lithic arenite and overlying siltstone. Abundant structures (channel fills, basal scour marks, parallel and trough-cross stratification) indicate fluvial and alluvial fan deposition in the retroarc foreland basin.

Clast counts reveal episodic variations in clastic input. The first clast count collected 200 m from the base of the section shows abundant rhyolite detritus derived from the Permian-Triassic Choiyoi Group of the Frontal Cordillera. The second count collected 275 m from the base of the section shows an increase in sandstone and limestone clasts distinguished by ammonite fossils, characteristic of the Jurassic Tordillo Formation. Giambiagi et. al. (2008) inferred the maximum age for the Loma Fiera Fm. is 9.5 Ma based on ⁴⁰Ar/³⁹Ar ages on conglomerate boulders. Clast data coincides with the exposure of both the Frontal Cordillera and Jurassic strata of the Neuquén basin at this time. Data from two detrital zircon samples indicate sediment derived from both the Permian-Triassic Choiyoi Group of the Cordillera Frontal and populations of Paleozoic and Mesozoic grains presumably derived from the Neuquén basin. Syndepositional grains were not recognized, implying a lack of volcanism during deposition of the Loma Fiera Fm.