ABSTRACT: The Cambrian-Ordovician boundary in NW Argentinawas originally proposed for the lower upper section of the Cardonal Formation in the Amarilla Creek, Cajas Range, Cordillera Oriental of Jujuy, based on the first appearance of the conodont Cordylodus lindstromi. After the finding of the index fossil Iapetognathus, the intersystemic boundary in this stratigraphic unit was recently verified. Through the upper 42 meters of the Cardonal Formation, which is 160 m in total thickness, conodonts of the Cordylodus lindstromi, Iapetognathus and Cordylodus angulatus zones were recovered. The lower upper section of the formation where is located the referred boundary consists of grey-greenish shales interbedded with calcareous sandstones that represent an upper off-shore environment which deepens upwards. The associated species belong to the genera Acanthodus, Cordylodus, Eoconodontus, Furnishina, Iapetognathus, Orminskia, Phakelodus, Problematoconites, Proconodontus and Teridontus. Importantly, the index species Iapetognathus fluctivagus and I. jilinensis were documented for the region in this study. The conodont species aremostly cosmopolitan, integrating a local fauna that is interpreted as representing the Transitional Faunal Realm of Mid-latitudes in the Cold Water Domain or to the lately defined Southwestern Gondwana Province from the Cold Domain in the Shallow-Sea Realm. The conodont elements exhibit a black color (CAI 5 = 300º and 480º), with a significant alteration caused by a Jurassic-Cretaceous boundary hydrothermal influence.

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