Abstract
The present study deals with the conodont biostratigraphy from the middle and upper parts of the San Juan Formation (Lower-Middle Ordovician) exposed at the Los Gatos creek section, west of the cerro Viejo de Huaco, Central Precordillera of San Juan Province. The numerous conodonts recovered, corresponding to 55 species, allow to recognize a series of biozones in the studied section. The biostratigraphic analysis carried out herein allows determining associations of conodonts assignable to the Oepikodus evae, Oepikodus intermedius, Baltoniodus triangularis-Tripodus laevis and Lenodus variabilis zones that correspond to the middle Floian to lower Darriwilian. The Baltoniodus navis and Microzarkodina parva zones were not identified due to insufficient information provided by this stratigraphic section. In general, the levels corresponding to the Baltoniodus triangularis-Tripodus laevis Zone contain more conodonts, than the records of other localities from Precordillera. These specimens are well preserved with a color alteration index (CAI) of 2-2.5, indicating overburden paleotemperatures ranging from 60° and 155° C. The large presence of fragmented conodonts, with recrystallized surfaces and crystal overgrowth, could indicate the effect of distinctive diagenetic fossil processes on the bearer limestone. The analysis of the diversity and abundance of genera and species of conodonts by chronostratigraphic intervals presents a great percentage of cosmopolitan taxa, 18% and 38% in common, at species level, only with the North-American Midcontinent Province (NAMP), and with a small percentage with the North-Atlantic Province (NAP) and the Precordillera, which allows to approximate a greater paleobiogeographic affinity of the conodonts recovered with those of the NAMP than those of the NAP for the entire interval studied. On the other hand, the Precordillera is verified as a province with its own characteristics as identified by several authors.
Highlights
La Precordillera se localiza en el sector centrooccidental del territorio argentino, limitando al oeste con la Cordillera Frontal y al este con las Sierras Pampeanas
The biostratigraphic analysis carried out allows determining associations of conodonts assignable to the Oepikodus evae, Oepikodus intermedius, Baltoniodus triangularis-Tripodus laevis and Lenodus variabilis zones that correspond to the middle Floian to lower Darriwilian
The Baltoniodus navis and Microzarkodina parva zones were not identified due to insufficient information provided by this stratigraphic section
Summary
La Precordillera se localiza en el sector centrooccidental del territorio argentino, limitando al oeste con la Cordillera Frontal y al este con las Sierras Pampeanas. Benedetto (1993, 2004) y Astini et al (1995) proponen el desmembramiento del terreno de Precordillera a partir de los Apalaches, relacionando el arco magmático del Sistema de Famatina con la acreción de la Precordillera a Gondwana durante el Darriwiliano-Sandbiano. Thomas y Astini (1996) y Astini et al (1996) realizan una síntesis de estas hipótesis, las que proponen que el desprendimiento de la Precordillera provendría de los Apalaches del sur y la región austral de Marathon, Texas, durante el Cámbrico temprano-medio (Ramos, 2004). Finney (2007) postula la hipótesis de un origen para-autóctono de la Precordillera, según la cual, ésta, como parte del terreno de Cuyania, habría migrado a través de una falla transformante desde el margen sur del oeste de Gondwana hacia la posición actual acaecida en el Devónico Thomas y Astini (1996) y Astini et al (1996) realizan una síntesis de estas hipótesis, las que proponen que el desprendimiento de la Precordillera provendría de los Apalaches del sur y la región austral de Marathon, Texas, durante el Cámbrico temprano-medio (Ramos, 2004). Finney (2007) postula la hipótesis de un origen para-autóctono de la Precordillera, según la cual, ésta, como parte del terreno de Cuyania, habría migrado a través de una falla transformante desde el margen sur del oeste de Gondwana hacia la posición actual acaecida en el Devónico
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