NEOGENE BASIN EVOLUTION IN THE LAS PEÑAS BASIN, SALAGASTA REGION, MENDOZA, ARGENTINA

A succession of coarse clastic strata in the Salagasta region, northwest of Mendoza, Argentina, was deposited in the southern Miocene Las Peñas basin. These strata represent the easternmost exposed block of Andean retroarc foreland basin strata unroofed by migration of the fold and thrust belt into the Precordillera. The stratigraphic section is >2000 m thick and consists of three formations: Mariño, La Pilona, and Rio de los Pozos Formations which are exposed in the lower plate of a thrust fault placing the Triassic Uspallata Group over synorogenic Miocene strata.

The Mariño Fm. (t>450 m) is composed primarily of thin-medium bedded, medium-coarse grained lithic arenite with distinct tuffaceous interbeds. The Mariño Fm. grades into the La Pilona Fm. (t~250 m), which is composed of a coarsening upwards succession of medium-thick bedded, coarse grained lithic arenite, intercalated with lesser granule to pebble conglomerate. The La Pilona Fm. is sharply overlain by the Rio de los Pozos Fm, a polymict, pebble-boulder conglomerate with abundant channel features and basal scours. Sedimentologic characteristics of the Salagasta section indicate a fluvial depositional environment and evolution from a distal to proximal retroarc foreland basin.

Detrital zircon samples from the Salagasta section include two from Mariño Fm. and one from Rio de los Pozos Fm. The samples display detrital zircon populations derived from syndepositional volcanism, Miocene Farallones Group, Permian Choiyoi Group, and early Paleozoic Villavencencio Fm. A tuffaceous sample from the lower Mariño Fm. contains syndepositional zircon indicating a maximum depositional age of 8.75 Ma and a major population derived from the Permian Choiyoi Group, suggesting clastic influx from the uplifted Frontal Cordillera at this time. The upper Mariño Fm. contains peaks representing the Permian Choiyoi Group, Miocene Farallones Group, and syndepositional volcanism (ca 8 Ma) as well as an introduction of early Palezoic Villavencencio Fm. suggesting uplift of the Precordillera in this region between 8.75-8 Ma. The Rio de los Pozos Fm. is dominated by a strong Miocene peak reflecting clastic influx from the Farallones Fm, and lacks syndepositional volcanic detritus, suggesting sediment derivation from the Cordillera Principal during a period of waning volcanism.