Evolución paleogeográfica y paleoambiental de los depósitos continentales del Jurásico Medio en el sector austral de la Cuenca Neuquina, República Argentina

in the southern part of the neuquén basin (west-central argentina) the cuyo group (lower to middle jurassic) ends with continental deposits of the challacó formation (de ferraríis, 1947). despite the numerous stratigraphic schemes proposed during the last years (gulisano et al., 1984; legarreta y gulisano, 1989; legarreta y uliana, 1996, among others) these deposits are still poorly studied, not only in terms of depositional environments but also of their relationship with the marine facies of the cuyo group and the overlying loteniano-chacayano cycle. the aim of this paper is to present a paleoenvironmental study of the challacó formation together with a paleogeographic evolution for the southern margin of the neuquén basin during the middle jurassic, in order to complete the stratigraphic evolution for the southwestern margin of the neuquén basin during these times. the study area is located in the central part of the neuquén province in west central argentina (fig. 2), and the outcrops of the challacó formation are related to a series of structures associated with the huincul wrench fault zone (dorsal neuquina). two areas of study were defined due to the lack of a direct correlation between them. in the eastern sector, located 50 km south-east of zapala (figure 2), the challacó formation is more than 300 m thick and unconformably overlies marine deposits of the lajas formation. the basal section is characterised by conglomerates and coarse-grained sandstones (fig. 3) and the upper section by lenticular sandstone bodies, red/green mudstones and a few conglomerates. in this part of the basin tithonian black shales of the vaca muerta formation overlie the challacó formation. in the western sector the challacó formation is between 40 and 120 m thick and comprises lenticular sandstone bodies and minor conglomerates intercalated with tabular red/green mudstones (fig. 4). in this region, the challacó formation overlies marine deposits of the lajas formation and is in tu