Abstract
Middle Darriwilian to lower Sandbian conodonts were recorded from the Las Aguaditas Formation at its type section in the Argentine Precordillera. A total of 9,974 conodont specimens were recovered from 46 carbonate samples, which represent 68 species of 38 genera. A biostratigraphic study verified a middle Darriwilian age for the interval spanning the contact between the San Juan and the Las Aguaditas formations. The following zones are determined in the study section: the Lenodus variabilis Zone, with the Periodon gladysae and Paroistodus horridus subzones following the Precordilleran scheme; the L. variabilis, Yangtzeplacognathus crassus, and the Eoplacognathus pseudoplanus zones with the Microzarkodina hagetiana and M. ozarkodella subzones, and the Pygodus anserinus Zone, according to the Scandinavian scheme; the Periodon macrodentatus Zone, with the Histiodella sinuosa, H. holodentata and H. cf. holodentata subzones, and the P. zgierzensis Zone with the H. kristinae Subzone that correlates the North American scheme. A stratigraphic gap was recognized between the lower and middle members of the Las Aguaditas Formation. It comprises the Eoplacognathus suecicus and Pygodus serra zones, and the lower subzone of the Pygodus anserinus Zone. The variation of conodont diversity through the study section conforms to shallowing and deepening patterns, which accompanies the changes of the provenance lithology. Three conodont assemblages were quantitatively recognized: a) Diverse conodont association, b) Low diversity conodont association and c) Recovery phase association. We propose to use the North American biozonal scheme of conodonts for the Central Precordillera because of the affinity of documented index taxa, which provides a more accurate intercontinental correlation for the global Middle Ordovician Series.
Highlights
The Precordillera of western Argentina is bordered by the Sierras Pampeanas to the east and by the Cordillera Frontal to the west
The present study agrees with Albanesi (1998) because in the equivalent levels the diagnostic M and Pa elements of L. variabilis are identified in association with P. gladysae
M. ozarkodella is identified in the upper part of the formation and is derived from M. hagetiana, which is consistent with the phylogeny of the genus described by Löfgren and Tolmacheva (2008) and associated conodont index species
Summary
The Precordillera of western Argentina is bordered by the Sierras Pampeanas to the east and by the Cordillera Frontal to the west. The nature and origin of the Precordillera has been the subject of diverse studies (e.g., Astini et al, 1995; Aceñolaza et al, 2002; Benedetto, 2004; Voldman et al, 2009). It is interpreted as part of a major exotic terrane. Some authors suggest an allochthonous origin for this terrane, with rifting from Laurentia, beginning drift across the Iapetus Ocean in the Cambrian and accretion to the western margin of Gondwana by the Mid or mid-Late Ordovician (Benedetto, 1993; Benedetto et al, 2009; Astini et al, 1995; Bordonaro and Banchig, 1995; Albanesi et al, 1995a; Albanesi and Bergström, 2010). An alternative hypothesis considers the Precordillera a para-autochthonous Gondwanan block, which drifted along the Gondwana margin from its original position close to Antarctica and South Africa (Aceñolaza et al, 2002; Finney, 2007)
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