New invertebrate peri-glacial faunal assemblages in the Agua de Lucho Formation, Río Blanco Basin, Argentina. The most complete marine fossil record of the early Mississippian in South America

A new fossil record of early Mississippian marine faunas is described from the Agua de Lucho Formation in the Sierra de las Minitas (La Rioja province, Río Blanco Basin, western Argentina). Considered Tournaisian in age based on local palynological data and biostratigraphic correlations, this faunal record is notably diverse associated with glacially influenced deposits, in contrast with other early Mississippian assemblages from Argentina, traditionally described as developed in warm to temperate climates. The vertical distribution of bivalves and brachiopods, in particular, indicates significant faunal changes through the thick section studied, and their occurrences are compared with those described from the Zorritas Formation in northern Chile. The new records include some typical components of the regional Michiganites scalabrinii-Azurduya chavelensis assemblage Zone, but important vertical and lateral compositional variations of this biozone are discussed. It also highlights the importance of the species Azurduya chavelensis as a conspicuous component of the Early Mississippian deposits from South America, which would become a proper macrofaunal tool to intra and inter-basinal correlations.