The southernmost record of the Aseptella-Tuberculatella/Rhipidomella-Micraphelia Fauna (late Serpukhovian-Bashkirian) in the Calingasta-Uspallata basin, Argentina

Abstract

The Agua del Jagüel Formation crops out in the southernmost part of the Calingasta-Uspallata basin, in central western Argentina. The lower part of the unit is characterized by a glacigenic sequence with diamictites and mudstones with dropstones. In the latter, elements of the Aseptella-Tuberculatella/Rhipidomella-Micraphelia Fauna (AT/RM Fauna), such as the brachiopods Rhipidomella discreta Cisterna, Micraphelia indianae Simanauskas and Cisterna and Orbiculoidea? sp., the gastropods Murchisonia? sp., and Glabrocingulum (Stenozone)? sp., the bivalve Nuculanidae indet., rugose corals, and indeterminate fragments of nautiloids and hyolithids, have been identified. The importance of this fauna mainly resides in its paleoenvironmental and biostratigraphic implications. AT/RM Fauna is characteristic of restricted environments with relatively low concentrations of oxygen and nutrients in the seafloor, which is consistent with the glaciomarine sequences in fjord-type coasts suggested for the Agua del Jagüel Formation. The relatively low diversity of the fauna in this unit compared to that defined in the El Paso Formation, located further north in the basin, might suggest more restricted sectors for benthic colonization, related to the paleovalle’s isolation from oceanic waters. The postglacial mudstones with marine invertebrate faunas of late Serpukhovian-Bashkirian age would have been deposited in relatively restricted (palaeofjord) part of the Uspallata-Calingasta basin as well as in open shelf environments. The marine flooding over drastically different coast configurations and the availability of nutrient and oxygen in the water column would have propitiated the development of faunas with important differences in the taxonomic composition and the paleoecological structure (AT/RM and Levipustula Faunas) occurring at the same time interval. Radiometric data in Agua del Jagüel Formation and paleontological records in the glacial-postglacial sedimentary succession in the basin (marine invertebrate faunas, palynomorphs and plants) are the most important tools to adjust the timing of the postglacial transgression. This information herein presented complements the scheme proposed for the Carboniferous sequences throughout the central western of Gondwana but is not sufficient to assign a more precise age of the fauna studied within the late Serpukhovian-Bashkirian interval.