Palaeoenvironmental significance of middle Oxfordian deep marine deposits from La Manga Formation, Neuquén Basin, Argentina

PALMA, RICARDO M.; BRESSAN, GRACIELA S.; KIETZMANN, DIEGO A.; RICCARDI, ALBERTO C.; MARTIN-CHIVELET, J.; LÓPEZ-GÓMEZ, JOSÉ

JOURNAL OF IBERIAN GEOLOGY

SERVICIO PUBLICACIONES

Lugar: Madrid; Año: 2014 vol. 40 p. 507 - 520

1698-6180

The Callovian-Oxfordian of the Neuquén Basin (Argentina) is characterized by an extensive marine carbonate system (La Manga Formation)with a predominance of shallow and middle ramp deposits, although locally in tectonically controlled settings, deeper depositsalso formed. These middle Oxfordian deeper deposits consist of mudstone-wackestone carbonates alternating with black shales and showmarked centimetre-scale rhythmicity, probably controlled by the Earth?s orbit parameters, mainly precession and eccentricity. The presentstudy was designed to examine these deposits in terms of their sedimentology, geochemistry, and small-scale stratigraphy in Arroyo LosBlancos, southern Mendoza Province, where they are exceptionally well exposed and preserved. Results indicate that the sediments weredeposited in an anoxic to dysoxic, relatively deep, sea-floor environment as revealed by the presence of: a) organic-rich shales, b) undisruptedlamination in most facies, c) pyrite framboids, and d) absence or scarcity of trace fossils and benthic fauna. Thin beds of gradedwackestone-packstone and accumulations of thin shells of Bositra bivalves indicate sporadic reworking of the sea floor by weak currents.Organic petrology, Rock-Eval pyrolysis and thermal maturity indicators were used to characterize TOC and different types of organic matter.Low pyrolysis S yields along with low hydrogen indices suggest poor kerogen convertibility. Vitrinite reflectance (%Ro) ranged from2.39 to 2.97 with an average of 2.70. The thermal alteration index (TAI) was 4+(5), indicating overmaturity. A tectono-sedimentary modelis proposed for these deposits. According to this model, the relatively deep, organic-matter rich facies of La Manga Formation would havebeen deposited in the deepest zones of topographic lows controlled by tilting and differential subsidence of tectonic blocks bounded bynormal faults.These faults were probably inherited from previous extensional tectonics of Late Triassic - Early Jurassic age.