Abstract
The stratigraphic record of the Pygodus serra conodont Zone in the Cuyania terrane of western Argentina is discussed in this contribution. Three well-known sections were sampled in Precordillera and the San Rafael Block. The studied successions are composed mainly by clastic rocks with variable amount of carbonate. The lower part of Ponón Trehué Formation of the San Rafael Block and the La Cantera Formation of the Eastern Precordillera are composed of conglomerate and represent the input of coarse clastic deposits to the Cuyania basin. The Los Azules Formation, in Los Amarillitos section of the Central Precordillera, has a massive sandstone bed with carbonate nodules in a section largely of black shale. Key conodonts recovered from these sections indicate a correlation to the E. robustus and E. lindstroemi subzones of the Pygodus serra Zone of the upper Darriwillian Stage (Middle Ordovician Series). The species in the Ponón Trehué Formation are almost all the same as those in the Precordillera sections. Correlation of the clastic sedimentary successions between the three sections indicates that vertical facies changes were not controlled by eustasy. More likely, they were controlled by differential tectonic subsidence.
Highlights
Middle Ordovician outcrops of the Precordillera terrane extend from latitude 29o S to 33o S, and correlative rocks crop out near San Rafael City in the south of Mendoza Province, Western Argentina (Fig. 1)
The lower part of Ponón Trehué Formation of the San Rafael Block and the La Cantera Formation of the Eastern Precordillera are composed of conglomerate and represent the input of coarse clastic deposits to the Cuyania basin
Key conodonts recovered from these sections indicate a correlation to the E. robustus and E. lindstroemi subzones of the Pygodus serra Zone of the upper Darriwillian Stage (Middle Ordovician Series)
Summary
Middle Ordovician outcrops of the Precordillera terrane extend from latitude 29o S to 33o S, and correlative rocks crop out near San Rafael City in the south of Mendoza Province, Western Argentina (Fig. 1) These latter exposures occur in the Sierra Pintada range, in the San Rafael Block of Criado Roqué and Ibáñez (1979), and they are composed of Darriwillian carbonate strata known as the Ponón Trehué Formation (Heredia, 2006). A Greenville-type basement (Ramos et al, 1996; Cingolani and Varela, 1999) is present in the eastern part of the San Rafael Block and is partially covered by the Ponón Trehué Formation, which is composed of fossiliferous carbonate-siliciclastic strata of Ordovician age (Núñez, 1979; Bordonaro et al, 1996; Lehnert et al, 1999; Heredia, 2003 and references mentioned there) Biostratigraphic correlation of these Ordovician strata is based on conodonts, and the correlation of the lower part of the type section of the Ponón Trehué Formation is constrained by the conodont species Eoplacognathus robustus Bergström and Eoplacognathus lindstroemi (Hamar). New interpretations of biostratigraphic data allow us to compare the sedimentary successions where the E. robustus and E. lindstroemi subzones (Pygodus serra Zone) are recorded, and on that basis we propose that tectonic subsidence controlled the stratigraphic successions
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