CONICET | Buscador de Institutos y Recursos Humanos

The Sierra de La Invernada Formation (Middle-Upper Ordovician) is exposed on the western flank of the La Invernada Range along the western border of the Central Precordillera, San Juan Province, western Argentina. The formation is a heterolithic siliciclastic succession with interbedded calcarenites and subordinate conglomerates, which is intruded by Late Ordovician basic dykes. Its thickness varies between 1000 and 4000 m due to partial wedging out of the succession to the north and frequent repetition due to faults in the south; its base is not exposed and the formation is overlain by the Silurian-Devonian Corralito Formation. The formation records several deepening-shallowing cycles through the lower Darriwilian-Sandbian-lower Katian interval. Graptolites of the Undulograptus austrodentatus, U. dentatus, Holmograptus lentus, Pterograptus elegans, Hustedograptus teretiusculus, and Nemagraptus gracilis zones together with conodonts of the Eoplacognathus pseudoplanus, E. suecicus, Pygodus serra, and P. anserinus zones are present in a deepening sequence in the lower part of the formation. The Climacograptus bicornis, Diplacanthograptus caudatus, and Climacograptus tubuliferus graptolite zones occur together with the Amorphognathus tvaerensis and A. superbus conodont zones in a deepening sequence in the upper part of the formation. Disregarding non-significant stratigraphic gaps and lack of information, this formation represents one of the most fossiliferous successions of the Middle and Upper Ordovician of the Precordillera and its conodont and graptolite lineages are most useful for world-wide high resolution correlation. Palaeoenvironmental analysis reveals a complex pattern of sea-level changes and related diversification and migration events.