Palaeoenvironmental evolution of the glacial-postglacial transition in the Agua Colorada Formation (Carboniferous, Sierra de Narvez, northwest Argentina).

Abstract

The Carboniferous sequences (lower section of the Paganzo Group) outcroping in the central part of the Sierra de Narvez (Las Angosturas area, northwest Argentina) are described in this paper. From the regional point of view, two major Carboniferous sequences have been identified. Firstly, diamictites, conglomerates, sandstones, carbonaceous mudstones and thin coal beds that were included in the Agua Colorada Formation. A second group comprises monotonous sequences of shales and interstratified mudstones and sandstones which were excluded from the Agua Colorada Formation and identified with the informal name of "Marine Late Paleozoic rocks". Sedimentological characteristics of the Agua Colorada Formation allow recognizing seven facies associations in the Angosturas area: 1) monomictic diamictites, 2) polymictic diamictites and conglomerates, 3) shale-sandstone cycles, 4) dark shales, 5) greenish monomictic conglomerates, 6) channelised polymictic conglomerates and 7) coarse-grained sandstones, shales and thin coal beds. Facies associations 1 and 2, including both massive and resedimented diamictites, are interpreted as related to the well known Namurian gondwanic glacial event in western basins of Argentina. Facies associations 3, 4 and 5 record the evolution of a fjord environment formed as a consequence of the postglacial transgression (the so-called Namurian postglacial transgression) that flooded glacial valleys in the major part of the Paganzo Basin. A regional relative sea level fall marked by a regional incision surface occurred at the base of facies association 6, and the fjord environment was sharply replaced by fluvial sedimentation (facies associations 6 and 7). A regional correlation model, based on the postglacial transgression key level, is proposed among the Sistema del Famatina, Precordillera and Sierras Pampeanas.