Conodont and Graptolite Biostratigraphy of the Ordovician System of Argentina

Abstract

Abstract The state of the art on conodont and graptolite biostratigraphy of the Ordovician System of Argentina is introduced. A composite biostratigraphic chart is assembled by precise ties of conodont and graptolite index species throughout the system. The conodont faunas of the Precordillera of western Argentina are dominated by components of the Tropical Domain during the Early Ordovician, but an increasing interplay of temperate to cold-water taxa is developed along the period. The conodont associations from northwestern Argentine basins (with major faunas in Cordillera Oriental and Sistema de Famatina) consist of mixed assemblages from both Domains characterizing transitional environments, always from the marine Shallow-Water Realm. Two series of biostratigraphic units, 29 conodont zones and 35 graptolite zones, are determined through the Ordovician System of Argentina, with several bizones divided into subzones. In the Argentine Precordillera, 17 conodont zones and 19 graptolite zones are determined. A number of these biozones are recognized in neighboring areas, such as San Jorge, Bloque de San Rafael, and Cordillera Frontal. In northwestern Argentine basins, the biozones are distributed in the Sistema de Famatina, Puna, Cordillera Oriental, and Sierras Subandinas, totalizing 12 conodont zones and 16 graptolite zones. Detailed graptolite zonations are achieved for the Lower Ordovician of northwestern Argentina and for the Middle and Upper Ordovician of the Precordillera. The conodont faunas from the Upper Ordovician are not well documented in Argentine basins instead. The graptolite assemblages of the Cordillera Oriental are characterized by faunas of high and medium paleolatitudes. Conversely, Middle and Upper Ordovician graptolite faunas of the Precordillera are referred to as low paleolatitudes.