El Paso Formation: A key unit for the correlation of the Carboniferous glaciation in the Calingasta-Uspallata basin of western Argentina

Abstract

The Late Paleozoic successions exposed along the western flank of the Precordillera at the latitude of Barreal in western Argentina are characterized by mid-Carboniferous glacial marine and glacially-influenced sediments (San Eduardo Group sensu M´esigos, 1953), followed by shallow marine, sand-rich deposits (Barreal Group).The El Paso Formation, exposed in the homonymous brachyanticline, is here subdivided into six facies associations (FAs), in ascending stratigraphic order: Boulder Conglomerate Complex FA, Sandstone Complex FA, Mudstones with Gastropods FA, Diamictite Complex FA, and Mudstones and Fine Sandstones FA. This analysis focuses on climatically sensitive facies related to the late Paleozoic Ice Age (LPIA), evidence of which is recorded in the adjacent Hoyada Verde Formation. The Boulder Conglomerate Complex FA includes boulders with striations on polished surfaces, indicative of glacially derived material. It grades up to sandstones with tangential cross-bedding and wave ripples, indicative of deposition as delta-scale sand-prone subaqueous clinoforms along a slope. The Diamictite Complex FA includes ice-rafted detritus (IRD) in two of its facies (dropstone mudstones and thin-bedded diamictites). The postglacial flooding, identified in various basins of western Argentina, is also present in the uppermost interval of the El Paso Formation (Mudstones and Fine Sandstones FA). It includes mudstones and rippled sandstones with abundant invertebrate fauna mainly brachiopods, bivalves, and gastropods, accompanied by conulariids, nautiloids, ostracods and corals, associated with palynomorphs. This study demonstrates through several lines of evidence that the upper interval of the El Paso and the Hoyada Verde Formations are time equivalent and represent key localities for the mid-Carboniferous (Serpukhovian-Early Bashkirian) event of the LPIA in the Calingasta Uspallata basin. Furthermore, unlike in the Hoyada Verde Formation, where the Carboniferous/pre-Carboniferous ("Pre-Tillítico") unconformity contact is concealed, the El Paso basal coarse fill rests on basement. The lower interval of the El Paso Formation exhibits similar facies characteristics and fining-upwards stacking pattern to the lower interval of the Majaditas Formation exposed further south.