Conodont and Graptolite Biostratigraphy of the Ordovician System of Argentina

Abstract

The state of the art on conodont and graptolite biostratigraphy of the OrdovicianSystem of Argentina is introduced. A composite biostratigraphic chart is assembledby precise ties of conodont and graptolite index species throughout the system.The conodont faunas of the Precordillera of western Argentina are dominated bycomponents of the Tropical Domain during the Early Ordovician, but an increasinginterplay of temperate to cold-water taxa is developed along the period. Theconodont associations from northwestern Argentine basins (with major faunas inCordillera Oriental and Sistema de Famatina) consist of mixed assemblages fromboth Domains characterizing transitional environments, always from the marineShallow-Water Realm. Two series of biostratigraphic units, 29 conodont zones and35 graptolite zones, are determined through the Ordovician System of Argentina,with several bizones divided into subzones. In the Argentine Precordillera, 17conodont zones and 19 graptolite zones are determined. A number of these biozonesare recognized in neighboring areas, such as San Jorge, Bloque de San Rafael, andCordillera Frontal. In northwestern Argentine basins, the biozones are distributed inthe Sistema de Famatina, Puna, Cordillera Oriental, and Sierras Subandinas, totalizing12 conodont zones and 16 graptolite zones. Detailed graptolite zonations areachieved for the Lower Ordovician of northwestern Argentina and for the Middleand Upper Ordovician of the Precordillera. The conodont faunas from the UpperOrdovician are not well documented in Argentine basins instead. The graptoliteassemblages of the Cordillera Oriental are characterized by faunas of high andmedium paleolatitudes. Conversely, Middle and Upper Ordovician graptolite faunasof the Precordillera are referred to as low paleolatitudes.