Graptolites and conodonts from the Middle-Upper Ordovician in the La Invernada Range, San Juan Precordillera, Argentina.

ORTEGA, G.; GUILLERMO LUIS ALBANESI; BANCHIG, A. L.; PERALTA, G. L.

Geologica Acta

Lugar: Barcelona; Año: 2008 vol. 6 p. 161 - 180

1695-6133

Outcrops on the western slope of the La Invernada Range, San Juan Precordillera (W Argentina), were surveyedfor lithofacial analysis and graptolite-conodont high resolution biostratigraphy. The study profile for the upperSierra de La Invernada Formation is approximately 500 m thick and is located in the northern part of the LaInvernada Range. Successive shallowing-deepening sedimentary cycles crop out throughout the upper part ofthe formation. The dominantly lower turbiditic sandy deposits alternate with shaly pelagic sedimentation, andoccasional coarser intervals that record gravitational flows. Hummocky structures occur in the uppermost partof the formation, indicating sedimentation under storm weather conditions. Rich graptolite faunas representingsix upper Middle to lower Upper Ordovician biozones have been recorded from this section: Pterograptus elegans(tentative assignment), Hustedograptus teretiusculus, Nemagraptus gracilis, Climacograptus bicornis,Diplacanthograptus caudatus, and Climacograptus tubuliferus biozones. This graptolite biostratigraphy is complementedby the conodont records from successive productive levels, which yielded conodonts that representthe Pygodus serra, P. anserinus, Amorphognathus tvaerensis, and A. superbus biozones. The recorded conodont-graptolite assemblages allow for a correlation with the upper Darriwilian, Sandbian and lower Katian globalstages, and provide preliminary biofacies records for the upper Sierra de La Invernada Formation.