Age Constraints on the Portezuelo Del Tontal Formation (Middle–Upper Ordovician) of the Western Argentine Precordillera: First Insights from Conodont Biostratigraphy

The first conodont fauna recovered from the Portezuelo del Tontal Formation (PTF) in the Sierra del Tontal, San Juan Province, is analysed, providing a precise biostratigraphic control for the scarcely fossiliferous, thick Ordovician siliciclastic sequences of the western Argentine Precordillera. In the studied Cerro Cóndores stratigraphic section, the lower outcropping levels of the PTF are characterised by sandstone beds, with a few having minor calcareous acidic reaction, deposited under medium energy, open-marine, siliciclastic shelf conditions. The recovered assemblage consists of 50 conodont elements, including Ansella jemtlandica, Costiconus costatus, Drepanodus arcuatus, Parapaltodus simplicissimus, Paroistodus originalis, Periodon macrodentatus, Protopanderodus graeai, and Spinodus spinatus as the most conspicuous species. After the biostratigraphic assessment of species of the Paroistodus horridus complex, the PTF conodont asemblage is referred to the Darriwilian (Middle Ordovician) Yangtzeplacognathus crassus Zone, a critical interval in the tectonostratigraphic history of the Precordillera. The collection also contains reworked conodont specimens whose biostratigraphic ranges coexist during the middle-upper Floian Stage (Lower Ordovician). The conodont assemblage from the PTF allows for a precise correlation with the eastern and central domains of the Precordillera, as well as other regions of the world, improving the knowledge on the dynamics of the Ordovician Precordilleran basin and the palaeogeographic distribution of its faunas.