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Middle Ordovician biostratigraphy, paleoenvironments and δ C isotopic records in the Argentine Precordillera 1
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Guillermo L. Albanesi , Stig M. Bergström , Birger Schmitz , Gladys Ortega , Gustavo G. 1
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Voldman , Nicolás A. Feltes and Fernanda Serra 1
CONICET, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de Córdoba, Av. Vélez Sarsfield 299, Córdoba X5000FCO, Argentina. Email 2
[email protected]; School of Earth Sciences, Division of Earth History, The Ohio State 3 University, 125 S Oval Mall Columbus, Ohio 43210. USA; Department of Geology, University of Lund, Sölvegatan 12, SE-22362 Lund, Sweden. The conodont and graptolite biostratigraphy, depositional environments, and the C isotope changes through the Middle-Upper Ordovician Las Aguaditas and Las Chacritas formations in the Argentine Precordillera were investigated. These middle Darriwilian to early Sandbian units, which are exposed south of the town of Jachal, consist mostly of distal ramp or platform margin carbonates that overlie horst and graben structures. In both units their basal strata and the top part of the underlying San Juan Limestone belongs to the Yangtzeplacognathus crassus Zone. 13 More than 200 C isotope samples were analyzed from the 56 m thick Las Chacritas Formation and the lower 170 m of the Las Aguaditas Formation, and more than 60 conodont samples were processed from the same intervals. The Y. crassus, Eoplacognathus pseudoplanus, and E. suecicus zones were recognized in the Las Chacritas Formation. Apart from these zones, the Pygodus serra and P. anserinus zones were identified in the Las Aguaditas Formation, which also contains graptolites representative of the Paraglossograptus tentaculatus and Nemagraptus 13 gracilis zones. The peak C values recorded in ribbon limestones through the E. suecicus Zone in the Las Chacritas succession occur in the same chronostratigraphic interval as in Baltoscandia, North America, and China and are interpreted to represent the middle Darriwilian Isotope Carbon Excursion (MDICE), a high-resolution chronostratigraphic tool useful for global correlation. 13
