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

Abstract

Late Palaeozoic formations in the western Precorclillera (km 114 of the N-20 road) were revisitcd (Colombo el aL, in press). The lower mcmber of the Del Ratón Fm overlies the Devonian basemen t with marked angular unconformity (O1 ) and is interpreted as a uord deposit. It consists of conglomerares with igneous clasts, in which Gallastegui el aL (in press) obtained an early Carboniferous 0ate Tournaisian) U-Pb zi rcon age of 348±2 Ma. The u pper member covcrs another smooth unconformity (D2), ancl is interpreted as the deposi t of an alluvial fan system which evolved into a coastal fan del ta. A sharp rise in relative sealevel resulted in thc dcep-sea fan Ca rboniferous deposits of the El Planchón Fm at the end of Chanic orogen y. During the rest of the Carboniferous, these succcssions were incorporated into the proto­ Precordillera high, a remnant of the Chanic cordillera, ancl a new (03) angular unconformity is recorded between Carboniferous ancl Permian dcposi ts. The early Permjan (Asselian) Del Salto Fm, which accumuJated after the collapse of thc proto-PrecordiUera (Gondwanan orogeny), is essen tially composed of sandstone and shalc i nterpreted as coastal and shallo´w marine deposi ts. A new tectonic event is recorded at the top of the Del Salto Fm (04 unconformity), and thc coarse-grained alluvial deposits of the Quebrada del Alumbre Fm accumulatcd d u ri ng the carly Permian according toa new U-Pb zi rcon age of 293±5 Ma (data) from a pyrodastic fall dcposit. Subsequently, a new expansive coastal systcm (Escom brera Fm) was deposi ted over a smoot h u nconformity (05).Barredo el al. (201 2) 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 rcsts on anotht´r smooth unconformity (06).This unit consists of siliciclastic deposits accum ulated in continen tal environments (playa-lake, high-si nuosi ty and braided fluvial systems, and alluvial fans). In the lower part of the Quebrada de la Arena Fm, a new mjddle Triassic (Ladinian) age of 2 R±2 Ma (U-Pb zircon) is obtained from a pyroclastic fall dt´posit. Thus, the D6 unconfor mity recorcls the limit between the Permian and the Triassic deposits. The Rincón Blanco Group ("sensu lato"), which líes on an angular unconformity (07), was deposited during the Middle-Late Triassic and displays another angular unconfor mi ty (08) in its lmver part.