Biostratigraphy and palaeoecology of Middle–Late Ordovician conodont and graptolite faunas of the Las Chacritas River section, Precordillera of San Juan, Argentina

Abstract A conodont-graptolite biostratigraphic study was carried out on the top strata of the San Juan, Las Chacritas and Las Aguaditas formations in the La Trampa Range, Precordillera of San Juan in western Argentina. Significant conodont records in the San Juan and Las Chacritas formations allow for the recognition of theYangtzeplacognathus crassus, Eoplacognathus pseudoplanus(Microzarkodina hagetianaandM. ozarkodellasubzones) andEoplacognathus suecicuszones of Darriwilian age. Index species and co-occurrences of graptolites and conodonts were recorded in the Las Aguaditas Formation allowing the identification of theNemagraptus gracilisand thePygodus anserinuszones, which represent the Sandbian Stage. These data indicate a hiatus between the Las Chacritas and the Las Aguaditas formations, corresponding to thePygodus serraZone and thePterograptus elegansandHustedograptus teretiusculuszones (upper Darriwilian). A total of 7287 identifiable conodont elements were recorded from the study section. The species frequency registered for each zone shows thatPeriodonandParoistodusare the most abundant taxa, which are indicative of open marine environments. The records of particular conodont taxa, such asHistiodella, Periodon, Microzarkodina, EoplacognathusandBaltoniodus, allow a precise global correlation with other regions such as south-central China, Baltoscandia, North America, Great Britain, Southern Australia and New Zealand. The graptolite fauna identified here are recognized worldwide in equivalent strata in the Baltic region, Great Britain, North America, China, southern Australia and New Zealand. The presence of graptolites in the ribbon limestones of the Las Chacritas Formation is documented for the first time.