Compositional variations of the "Levipustula fauna" in the Argentine Precordillera and its relationships with the carboniferous glacial event in the southwestern Gondwanan margin

CISTERNA, G.A. & STERREN, A.F.

Florencia

Congreso; 32nd International Geological Congress; 2004

The Namurian marine invertebrate "Levipustula fauna" is a conspicuous element inside of the Calingasta-Uspallata Basin in the Argentine Precordillera. This fauna appears typically associated with glacial-marine sequences related to the carboniferous glacial event that affected the southwestern Gondwanan margin. A detailed study of the brachiopods and bivalves that integrate this fauna in the classical localities of Hoyada Verde, La Capilla and Leoncito (San Juan province), suggests important taxonomic, paleoecological and taphonomic variation. In the Hoyada Verde Formation the "Levipustula fauna", located immediately above of glacial diamictite horizons, appears associated to mudstones facies interpreted as the postglacial transgression. There, the brachiopod assemblage is dominated by "Levipustula"-"Neospirifer"-Kitakamithyris with the species "Levipustula" levis Maxwell, "Neospirifer" ("Trigonotretidae" gen. nov.) leoncitensis (Harrington) and Kitakamithyris sp., usually accompanied by Beecheria sanjuanensis sp. nov. and Spiriferellina octoplicata (Sowerby). This assemblage has been identified in La Capilla Formation (Cerro El Morado locality) too. The bivalve fauna studied in these units shows more important taxonomic differences than the brachiopods. In the Hoyada Verde Formation the species identified are Phestia sp. aff. P. bellistriata (Stevens), Palaeolima retifera (Shumard), Streblochondria sanjuanensis Sterren and Streblochondria stappenbecki Reed, while in the La Capilla Formation (Cerro El Morado), Phestia sp., Aviculopecten barrealensis Reed, Schizodus sp., Oriocrassatella? sp. and Pleurophorella? sp. are recognized. Also, in both units, the associated fauna are represented by briozoos (fundamentally fenestellids), gastropods and crinoids.In the Leoncito Formation the "Levipustula fauna" occurs in sandstones horizons located below of the glacial diamictite beds. There, this fauna is poorly diversified, composed fundamentally by brachiopods characterized by the "Neospirifer"-Septosyringothyris assemblage, where Levipustula is very scarce. The fauna described from Hoyada Verde and La Capilla Formation is proposed like the typical "Levipustula fauna" associated to the postglacial transgression. On the contrary, the "Levipustula fauna" from Leoncito Formation could be considered a colder "pre-interglacial fauna".