Graptolite and conodont faunas from the Middle-Upper Ordovician Sierra de la Invernada Formation, Central Precordillera of San Juan, Argentina

ORTEGA, G; ALBANESI, G.L.; BEJERMAN, A.; VOLDMAN, G.G.; BANCHIG, A.L.

Londres

Congreso; 3th Internacional Palaeontological Congress; 2010

IPA

The Sierra de La Invernada Formation (Middle-Upper Ordovician) is exposed on the western flank ofthe 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 calcarenitesand subordinate conglomerates, which is intruded by Late Ordovician basic dikes. Its thickness variesbetween 1000 and 4000 m due to partial wedging out of the succession to the north and frequentrepetition due to faults in the south; its base is not exposed and the formation is overlain by theSilurian-Devonian Corralito Formation. The formation records several deepening-shallowing cyclesthrough the lower Darriwilian-Sandbian-lower Katian interval. Graptolites of the Undulograptusaustrodentatus, U. dentatus, "Holmograptus lentus", "Pterograptus elegans", Hustedograptusteretiusculus, and Nemagraptus gracilis zones together with conodonts of the Eoplacognathuspseudoplanus, E. suecicus, Pygodus serra, and P. anserinus zones are present in a deepeningsequence in the lower part of the formation. The Climacograptus bicornis, Diplacanthograptuscaudatus, and Climacograptus tubuliferus graptolite zones occur together with the Amorphognathustvaerensis and A. superbus conodont zones in a deepening sequence in the upper part of theformation. Disregarding non-significant stratigraphic gaps and lack of information, this formationrepresents one of the most fossiliferous successions of the Middle and Upper Ordovician of thePrecordillera and its conodont and graptolite lineages are most useful for world-wide high resolutioncorrelation. Paleoenvironmental analysis reveals a complex pattern of sea-level changes and relateddiversification and migration events.