Palynomorphs, Conodonts and Associated Graptolites from the Ordovician Los Azules Formation, Central Precordillera, Argentina

Abstract

Palynomorphs and conodonts are reported from the Ordovician Los Azules Formation, Argentine Precordillera. The formation is mainly made up of argillites, shales and mudstones, and is divided into three informal members. Fourteen species of acritarchs including Gorgonisphaeridiumm granulobrachium Ottone, Lophosphaeridium ruidum Ottone, and Polygonium echinatus Ottone that are proposed as new; two species of prasinophytes, one of cryptospores and three different morphotypes of phytodebris are identified throughout the middle and upper members of the formation. Two distinct palynomorph assemblages are recognized: the Lower Los Azules Palynological Assemblage (middle member) and the Upper Los Azules Palynological Assemblage (upper member). Eight species of conodonts are preserved as casts on bedding planes of the lower shales of the middle member. This conodont association is correlated with the upper division of the Eoplacognathus suecicus Zone from the late early Llanvirn. The upper part of the middle member contains the eponymous species of the Pygodus serra Zone of Llanvirn age. A calcirudite sample from the upper member yielded a rich and diverse microfauna that includes ten conodont species representing the lower part of the Amorphognathus tvaerensis Zone (early Caradoc). Conodonts and palynomorphs confirm previous graptolite-based dating of the Los Azules Formation (early Llanvirn-early Caradoc). The palynological assemblage exhibits similarities with coeval assemblages from the Northern Hemisphere and Gondwanaland.