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La fauna de graptolitos de la Formación Acoite, en el borde occidental de la Cordillera Oriental Argentina. Análisis bioestratigráfico

by B A Toro
Ameghiniana ()

Abstract

A biostratigraphic analysis of the graptolite fauna collected from the Acoite Formation, in the western side of the Argentine Cordillera Oriental is presented. The fossils come from eight stratigraphic sections, located in the Tumbaya, Humahuaca and Cochinoca departments, Jujuy Province. These sections were measured from south to north in Agua Blanca, Los Colorados, Chamarra, Ri?o Trancas, Los Perros, Ri?o Cajas, Agua Chica and Lumara creeks. A biozonation scheme has been established as a result of the vertical range of fortyfive species of graptoloids. This suggests a basal Arenigian-middle Arenigian age for the studied unit. The Tetragraptus phyllograptoides Zone (Earliest Arenigian, corresponding to La 3 of the Australian zonation) is developed in the basal levels of the Acoite Formation and is characterized, apart from the index fossil, for the following species: Didymograptus (s. 1.) rigoletto Maletz et al., D. (s. 1.) cf. D. rigoletto, Baltograptus geometricus (Tornquist), D. (s. 1.) uniformis Elles and Wood, D. (s. 1.) cf. D. demissus To?rnquist, Clonograptus multiplex (Nicholson) and Tetragraptus approximates approximatus Nicholson. The Tetragraptus akzharensis Zone (Early Arenigian, Be 1 and part of Be 2 of Australia), is represented by diagnostic species, such as: Tetragraptus akzharensis Tzaj., Didymograptus (Expansograptus) constrictus (J. Hall), D. (s. 1.) holmi To?rnquist, D. (s. 1.) protobalticus Monsen, Corymbograptus v-fractus (Salter) and Baltograptus vacillans (Tullberg); whereas other taxa persist from the lower biozone. The Baltograptus deflexus Zone (Early Arenigian, equivalent to part of Be 2-Be 4) is the most extense and contains: Baltograptus deflexus (Elles and Wood), B. cf. B. deflexus, B. vacillans, B. kurcki To?rnquist, B. turgidus (Lee), B. kunmingensis Ni, Pendeograptus fruticosus (J. Hall), Dichograptus octobrachiatus (J. Hall), Didymograptus (Expansograptus) similis (J. Hall), D. (E.) suecicus (Tullberg), D. (s. 1.) balticus (Tullberg), D. (s. 1.) uniformis, D. (s. 1.) holmi, D. (s. 1.) simulans Elles and Wood, D. (s. 1.) dilatans T.S. Hall, D. (s. 1.) sinensis Lee and Chen, D. (s. 1.) cf. D. aureus T.S. Hall, D. (s. 1.) cf. D. asperus Harris and Thomas, Tetragraptus reclinatus abbreviatus Boucek, T. reclinatus cf. reclinatus Elles and Wood, T. (s. 1.) quadribrachiatus (J. Hall), Acrograptus filiformis (Tullberg), A. cf. A. pusillus (Tullberg) and Kinnegraptus sp. Finally, in the uppermost Didymograptellus bifidus Zone (Middle Arenigian, equivalent to Ch 1-Ch 2), Baltograptus minutus (To?rnquist) and Phyllograptus anna J. Hall appear together with the index fossil; whereas Didymograptus (s. 1.) simulans, B. kurcki and B. deflexus persist from the previous zone. The recognized biozones allow the correlation of the Acoite Formation with other outcrops from the Lower Ordovician of northwestern Argentine. Moreover, a world-wide correlation scheme is proposed.

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