The Cambrian-Ordovician boundary in the Cordillera Oriental, NW Argentina

ALBANESI, GUILLERMO L.; GIULIANO, MARÍA EUGENIA; PACHECO, FERNANDA E.; ORTEGA, GLADYS; MONALDI, RUBÉN C.

Harrisonburg, Virginia

Simposio; 12th International Conference on the Ordovician System; 2015

SEPM Society for Sedimentary Geology

The global Cambrian/Ordovician inter-systemic boundary was a controversial problem regarding thefossil record and reference stratigraphic section. The Cambrian/Ordovician Boundary Working Group ofthe International Subcommission on Ordovician Stratigraphy (ICS-IUGS) selected the conodont speciesIapetognathus fluctivagus as the index fossil for establishing the boundary after comprehensivetaxonomic studies. Nicoll et al. (1999) monographed I. fluctivagus and other species of Iapetognathus aswell as its purported ancestor Iapetonudus ibexensis, and they documented their known occurrences,ranges and correlations with other fossils considered significant to define the base of the Tremadocian.Cooper et al. (2001) proposed the section of Green Point, western Newfoundland, Canada, for the GSSPfor the Cambrian/Ordovician (C/O) boundary determined there by the first appearance of I. fluctivagus,which was later approved by the IUGS executive. Terfelt et al. (2012) revisited the taxonomy, stratigraphic ranges and lineage of Iapetognathus, withinterpretations that were refuted by Miller et al. (2014). The latter authors concluded that the homotaxialsuccession of Iapetognathus species, as defined by Nicoll et al. (1999), should be maintained fordetermining the base of the Ordovician System globally. The definition of the C/O boundary in Argentinawas evaluated following the newly conodont biostratigraphic concepts (Nicoll et al. 1999; Cooper et al.2001) in the Volcancito Formation at the Famatina Range of western Argentina (Albanesi et al. 2005).Previous studies about this boundary by means of conodonts were carried out in the Cajas Range of theCordillera Oriental, northwestern Argentina, by Rao (1999) according to the definition of the C/Oboundary by Barnes (1988) on the base of the Cordylodus lindstromi Zone. Recently, other study areas,such as Lari in the Puna region (Giuliano et al. 2013) or new localities in the Cordillera Oriental; e. g.,Santa Victoria, Nazareno, El Moreno, Alfarcito (e. g., Zeballo and Albanesi 2009), were surveyed aspotential sites for the location of the C/O boundary (Figure 1). Albanesi and Pacheco (2010) recorded I.fluctivagus in the Amarilla Creek section of the Cajas Range, close to the base of the upper CardonalFormation. This stratigraphic interval (44. 5 m thick) consists of grey-greenish shales interbedded withcalcareous sandstones that represent an upper off-shore environment. The present contribution attempts abetter approximation of the C/O boundary considering a resampling for conodonts through the criticalinterval, with the definition of successive conodont zones and the record of the FAD of I. fluctivagus.