Análisis sedimentológico e icnológico en la corona ECO-5 de la formación Magallanes inferior (Cretácico superior - Eoceno inferior), Cuenca Austral.

Abstract

The sedimentological, ichnological and petrographic analysis of the ECO-5 core of the Lower Magallanes Formation (Upper Cretaceous-Lower Eocene) of the Austral basin, allowed the identification of seven sedimentary facies. These facies are integrated by fine to very fine massive sandstones (F1), very fine sandstones with hummocky cross-stratification (F2), very fine laminated sandstones (F3), very fine to fine calcareous massive sandstones (F4), calcareous massive mudstones (F5) and massive and laminated siltstones (F6-F7, respectively), with variable amount of glauconite and with the development of three ichnoassociations corresponding to the ichnofacies of Cruziana, Skolithos and Glossifungites. The analysis of the sedimentary facies and ichnofacies allowed an assignation of the deposits of the core ECO-5 to an updrift environment of a wave delta dominated. In this position of the deltaic system, the stress conditions are lower than in the downdrift, and thus presenting a greater abundance and diversity of fossil traces. The petrographic analysis allowed to observe a significant abundance of glauconite and pyrite in the samples. Glauconite ranges from incipient to highly evolved, with the highest degrees of evolution occurring in fine-grained sandstones. The porous material that includes the formation of glauconite has a purely biological origin, linked to the production of fecal pellets by the benthic organisms. In the ECO-5 core, autochthonous glauconites that did not undergo transport and were formed in situ, were identified as well as allochthonous glauconites of the parautochthonous type, that underwent minimal transport within the depositional system. The pyrite observed in the craquelure of glauconite or disseminated in this mineral, is subsequently formed to the greenish mineral. The presence of the Glossifungites ichnofacies, in the very fine and fine sandstones, together with the high degree of glauconite evolution, indicate the beginning of a transgressive system track.