Blanca A. Toro


"A biostratigraphic analysis of the graptolite fauna collected from the Acoite Formation, in the westem 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, Río Trancas, Los Perros, Río 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 basallevels of the Acoite Formation and is characterized, apart from the index fossil, for the following species: Didymograptus (s. l.) rigoletto MaJetz et al., D. (s. l.) cf. D. rigoletto, Baltograptus geometricus (Tornquist), D. (s. l.) uniformis Elles and Wood, D. (s. l.) cf. D. demissus Tornquist, Clonograptus multiplex (Nicholson) and Tetragraptus approximatus 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. l.) holmi Tornquist, D. (s. l.) 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 pan of Be 2-Be 4) is the most extense and contains: Baltograptus deflexus (ElIes and Wood), B. cf. B. deflexus, B. vacillans, B. kurcki Tornquist, B. turgidus (Lee), B. kunmingensis Ni, Pendeograptus fruticosus (1. Hall), Dichograptus octobrachiatus (J. Hall), Didyniograptus (Expansograptus) similis (J. Hall), D. (E.) suecicus (Tullberg), D. (s. l.) ba/ticus (Tullberg), D. (s. l.) uniformis, D. (s. l.) /1O/lI/i, D. (s. l.) simulans Elles and Wood, D. (s. l.) dilatans T.S. Hall, D. (s. l.) sinensis Lee and Chen, D. (s. l.) cf. D. aureus T.S. Hall, D. (s. l.) cf. D. asperus Harris and Thomas, Tetragraptus reclinatus abbreviatus Boucek, T. reclinatus cf. reclinatus ElIes and Wood, T. (s. l.) 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 I-Ch 2), Baltograptus minutus (Tornquist) and Phyllograptus anna J. Hall appear together with the index fossil; whereas Didymograptus (s. l.) 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. "

Full Text:

 Subscribers Only