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 the Yangtzeplacognathus crassus, Eoplacognathus pseudoplanus (Microzarkodina hagetiana and M. ozarkodella subzones) and Eoplacognathus suecicus zones of Darriwilian age. Index species and co-occurrences of graptolites and conodonts were recorded in the Las Aguaditas Formation allowing the identification of the Nemagraptus gracilis and the Pygodus anserinus zones, which represent the Sandbian Stage. These data indicate a hiatus between the Las Chacritas and the Las Aguaditas formations, corresponding to the Pygodus serra Zone and the Pterograptus elegans and Hustedograptus teretiusculus zones (upper Darriwilian). A total of 7287 identifiable conodont elements were recorded from the study section. The species frequency registered for each zone shows that Periodon and Paroistodus are the most abundant taxa, which are indicative of open marine environments. The records of particular conodont taxa, such as Histiodella, Periodon, Microzarkodina, Eoplacognathus and Baltoniodus, allow a precise global correlation with other regions such as southcentral 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.

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This study includes 3 localities from the Jáchal area in the Central Precordillera of San Juan Province, from north to south: the Oculta creek, Las Aguaditas creek, and Cerro La Chilca sections. We deal with the graptolite faunas and conodont index species recorded from units that overlie the carbonate San Juan Formation, spanning the lower part of the Los Azules, Las Aguaditas, and Gualcamayo formations in their respective areas. The index graptolites and associated species are reported, which enable the recognition of the Levisograptus dentatus Zone in the Central Precordillera. The presence of graptolites in the limestones from the top of the San Juan Formation at the Cerro La Chilca section is documented for the first time. The record of representatives of the Lenodus variabilis, Yangtzeplacognathus crassus, Eoplacognathus pseudoplanus, Histiodella, and Periodon lineages recognized in these units, linked with respective graptolite zones, provide precise information for global correlation purposes. The use of conodont and graptolite zones from different areas of the Central Precordillera enables the verification of the diachronous contact between the San Juan Formation and overlying units, which spans the lower to middle Darriwilian, in our investigated sections and classical localities previously documented.