Graptolite–conodont biostratigraphy and biofacies of the Middle Ordovician Cerro Viejo succession, San Juan Precordillera, Argentina

High resolution graptolite–conodont biostratigraphy and biofacies analysis have been carried out for the lower member of the Los Azules Formation exposed in the Cerro Viejo area, San Juan Precordillera, western Argentina. The rock succession is composed of alternating rich fossiliferous argillites and virtually barren volcanic ash beds (K-bentonites) of Darriwilian (Da2) age. Graptolite and conodont collections from the Los Azules and Amarilla creeks sections are studied. Graptolites are the most common fossils in the lower member of the Los Azules Formation and represent the Undulograptus dentatus and Holmograptus lentus zones. A diverse conodont fauna recovered from the uppermost limestones of the underlying San Juan Formation, and conodont elements preserved on bedding plane surfaces in the lower Los Azules Formation, indicate the Lenodus variabilis and Eoplacognathus suecicus zones. Micaceous siltstone with Bergstroemograptus crawfordi (Harris) and abundant biserial graptolites close to the top of the lower member is notable. This bed yielded the first specimens of Polonodus magnum Albanesi. This species precedes the entrance of Pygodus anitae Bergström, which is typical from the upper part of the E. suecicus Zone. Characteristic graptolites of the Diplograptus? decoratus Zone (Da3) or equivalent faunas were not recorded in the studied sections suggesting the presence of a hiatus between the lower and middle members, which may indicate a critical event in the history of the basin. A rich conodont assemblage dominated by planktic–pelagic forms (Periodon–Paroistodus–Protopanderodus–Drepanoistodus), an abundant shelly fauna, and scarce graptolites are recorded in the upper San Juan Limestone. After drowning of carbonate platform the conodont diversity dramatically dropped becoming graptolites the dominant elements of the faunas in the lower member of the Los Azules Formation. The Periodon–Pygodus biofacies represents a low diversity deep-water conodont community present both in the upper part of the lower member and the middle member. Graptolite faunas represent the isograptid biofacies, including pandemic and Pacific forms of the epipelagic and deep-water biotopes. They are related to a transgressive event that diachronically flooded the Precordilleran carbonate platform during the early Middle Ordovician.