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Valderez Ferreira, Federal University of Pernambuco (Brazil)
Alcides Sial, Federal University of Pernambuco (Brazil)
Silvio Peralta, National University of San Juan (Argentina)
Claudio Gaucher, Facultad de Ciencias (Uruguay)
Alejandro Toselli, National University of Tucuman (Argentina)
Miguel Parada, University of Chile (Chile)
Marcio Pimentel, University of Brasília (Brazil)
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A global positive excursion of d13C (∼+5‰) for the Steptoean stage, Upper Cambrian (SPICE) carbonates represents a major perturbation of the Cambrian carbon cycle at ∼500Ma. C-isotope variation curves for the Cambrian-Ordovician boundary in Australia and US reveal a pronounced negative C-isotope excursion (-4‰) in the uppermost Cambrian (Sunwaptan stage). We have recognized both anomalies in Upper Cambrian platformal carbonates of the Precordillera, Argentina. This anomaly has been called SNICE. C and Sr-isotope profiles of the Precordillera (Zonda, La Flecha and La Silla Fms) were constructed for four sections: (a) Quebrada de la Flecha, and Quebrada de Juan Pobre and (b) Cerro La Silla and Quebrada de La Angostura. At this latter, upper part of the La Flecha Fm, d13Ccarb values vary up-section from -2.0 to +5.6‰ (PDB) and records the SPICE anomaly; the peak of this excursion is characterized by intercalated 2 m thick beds of black shale with marl and limestone, recording the onset of a sea-level change. Profile for the Zonda Fm at the Quebrada de Juan Pobre shows d13C values from ∼ -2‰ to ∼ + 1‰ with two important excursions, probably SPICE and SNICE. This indicates that Zonda Fm was probably deposited during the Steptoean at the Quebrada de Zonda, diachronically with the deposition of the La Flecha Fm at Quebrada de La Angostura. The Steptoean Zonda Fm dolostones at the Quebrada de la Flecha exhibit a d13C range from -2.7 to +0.6 ‰ with discrete positive anomaly about 200 m below the transition to the overlying Sunwaptan La Flecha Fm. Pronounced anomaly (-5.6‰) is found in the La Flecha Fm at ∼ 300 m below the transition to the La Silla Fm. At the Cerro La Silla, Zonda carbonates shows d13C values of ∼ -1‰, increasing slightly at the transition with the La Flecha Fm (-1 to 0 ‰). The transition of the La Flecha to the La Silla Fms presents alternation of black shales and dolomitic limestone with discrete positive C-isotope excursion (SPICE?). At the Quebrada de La Flecha, 87Sr/86Sr for the Zonda Fm are 0.70924 - 0.70955 and for the La Flecha Formation, 0.70908 - 0.70942. At the Quebrada de Juan Pobre, La Flecha carbonates show values from 0.70875 to 0.70963 whereas at Cerro La Silla, this ratio varies from 0.70914 to 0.70923 for the La Flecha Fm,and from 0.70898 to 0.70980 for the La Silla Fm. At the Quebrada de La Angostura, ratios for the La Flecha carbonates range from 0.70918 to 0.70993. The overall variation of 87Sr/86Sr is consistent with globally reported Upper Cambrian seawater values at ca. 500Ma. The unambiguous record of SPICE in the La Flecha Formation at the Quebrada de La Angostura supports deposition Steptoean age and allows precise regional stratigraphic correlation. The pronounced SNICE recorded in the La Flecha Fm carbonates at the Quebrada de La Flecha is likely equivalent to that registered in Sunwaptan carbonates of NA and Australia, and might be tied to a global event, as a valuable tool in stratigraphic correlation.
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