New geochronological data and evolution of the Late Paleozoic formations in the western Andean Precordillera, San Juan, Argentina

COLOMBO, F.; LIMARINO, C.O.; SPALLETTI, L.; GALLASTEGUI, GLORIA; RUBIO ORDOÑEZ, A.; CUESTA, A.; BUSQUEST, P.; CARDÓ, R.; CÉSARI, S. N.; MÉNDEZ BEDIA, I.; HEREDIA, N.

Madrid

Conferencia; Gondwana 15; 2014

Late Palaeozoic formations in the western Precordillera (km 114 of the N-20 road) were revisited (Colombo et al., in press). The lower member of the Del Ratón Fm overlies the Devonian basement with marked angular unconformity (D1) and is interpreted as a fjord deposit. It consists of conglomerates with igneous clasts, in which Gallastegui et al. (in press) obtained an early Carboniferous (late Tournaisian) U-Pb zircon age of 348±2 Ma. The upper member covers another smooth unconformity (D2), and is interpreted as the deposit of an alluvial fan system which evolved into a coastal fan delta. A sharp rise in relative sea-level resulted in the deep-sea fan Carboniferous deposits of the El Planchón Fm at the end of Chanic orogeny. During the rest of the Carboniferous, these successions were incorporated into the proto-Precordillera high, a remnant of the Chanic cordillera, and a new (D3) angular unconformity is recorded between Carboniferous and Permian deposits. The early Permian (Asselian) Del Salto Fm, which accumulated after the collapse of the proto-Precordillera (Gondwanan orogeny), is essentially composed of sandstone and shale interpreted as coastal and shallow marine deposits. A new tectonic event is recorded at the top of the Del Salto Fm (D4 unconformity), and the coarse-grained alluvial deposits of the Quebrada del Alumbre Fm accumulated during the early Permian according to a new U-Pb zircon age of 293±5 Ma (data) from a pyroclastic fall deposit. Subsequently, a new expansive coastal system (Escombrera Fm) was deposited over a smooth unconformity (D5). Barredo et al. (2012) published a middle Permian U-Pb zircon age of 267±7 Ma from an andesite located within this unit. The Quebrada de la Arena Fm rests on another smooth unconformity (D6). This unit consists of siliciclastic deposits accumulated in continental environments (playa-lake, high-sinuosity and braided fluvial systems, and alluvial fans). In the lower part of the Quebrada de la Arena Fm, a new middle Triassic (Ladinian) age of 238±2 Ma (U-Pb zircon) is obtained from a pyroclastic fall deposit. Thus, the D6 unconformity records the limit between the Permian and the Triassic deposits. The Rincón Blanco Group (?sensu lato?), which lies on an angular unconformity (D7), was deposited during the Middle?Late Triassic and displays another angular unconformity (D8) in its lower part.