Middle Oxfordian sponges-microbialites mounds from La Manga Formation, Neuquén Basin, Argentina

The Neuquén basin, Argentina, shows magnificent outcrops of Jurassic sedimentary sequences, both of continental and marine origin, including some carbonate platforms. Few attention has been paid, within those platforms, to sponge-microbialite mounds. This work addresses their characterization in the La Manga Formation, from both outcrop stratigraphy and thin sections petrography. Data from ammonites place these mounds within the Perisphinctes Araucanites Zone, belonging to the Middle Oxfordian. The framework of La Manga Fm. mounds consists of hexactinellid sponges and microbialites. The latter grew as a thin crust on the walls of the sponges and can be dense, peloidal, or microlaminated. Microbialites together with micritic matrix, microencrusters, and microbial crusts, played a significant role in the development on mound structure. The laminated or massive crust grew on already lithified surfaces, as suggested the presence of encrusting organisms in some of the mounds, as is the case of encrusting serpulids and thin colonies of bryozoans. Based on their size, shape and lack of internal bedding, two types of mounds were differentiated. Based on the nature of the skeleton and the type of organism growing in their framework, the development of Hexactinellid sponges can be related to low energy mid-distal carbonate ramp environments. They grew under controlling factors such as nutrients, alkalinity, and oxygenation. A comparison with other described Jurassic microbialites has been carried out with special focus on the western Tethys area.