REEVALUATION OF TETRAGRAPTUS PHYLLOGRAPTOIDES AND T. AKZHARENSIS BIOZONES (LOWER ORDOVICIAN, FLOIAN) FROM THE CORDILLERA ORIENTAL ARGENTINA

Abstract

This paper presents new accuracies and correlations of the Lower Ordovician (Floian) biostratigraphic scheme of the Cordillera Oriental. The Tetragraptus phyllograptoides and T. akzharensis biozones are reevaluated based on graptolite records from the lower levels of the Acoite Formation, outcropping in the Quebrada de Quinilicán (Sierra de Aguilar). The stratigraphic range of the characteristic taxa is discussed and compared with that observed in Los Colorados area. In the Tetragraptus phyllograptoides Biozone, it is worth mentioning the presence of Tetragraptus approximatus (Nicholson), Tetragraptus sp., Baltograptus geometricus (Törnquist), Cymatograptus demissus (Törnquist) and C. rigoletto (Maletz, Rushton and Lindholm) in addition to the index species. Tetragraptus amii (Elles y Wood), Cymatograptus protobalticus (Monsen), C. balticus (Tullberg) and Paradelograptus sp. appear in the lower part of the T. akzharensis Biozone, whereas Baltograptus vacillans (Tullberg), Corymbograptus v-fractus tullbergi (Monsen), Trichograptus dilaceratus (Herrmann), Expansograptus latus (Hall), E. holmi (Törnquist), E. constrictus (Hall), Acrograptus gracilis (Törnquist), and A. filiformis (Tullberg) are recorded in the upper portion of the same biozone. The regional correlation of both biozones is corroborated on the western margin of the Cordillera Oriental. The possible subdivision of the T. akzharensis Biozone is commented, according to the biostratigraphic scheme recently proposed for Scandinavia, and a more accurate intercontinental correlation of Northwestern Argentina Floian levels is proposed. In addition, the paleoecological implications of some studied taxa and the paleobiogeographical relationships between Northwestern Argentina and other Gondwanan regions are discussed, and the strong affinity with Baltoscandinavia during the Lower Ordovician is validated.