Trilobites and sedimentary facies of the upper Coquena Formation (late Tremadocian; Notopeltis orthometopa Zone), Cordillera Oriental, northwestern Argentina

Abstract

The area west of Purmamarca is a classic locality of the upper Tremadocian (Lower Ordovician) of the central Cordillera Oriental, northwestern Argentina. There, the upper member of the Coquena Formation documents the vertical passage of offshore to shoreface settings affected by storm activity. Updated information on the stratigraphic distribution of the trilobites from the Notopeltis orthometopa Zone in the Quebrada de Chalala is provided. Offshore facies from the lower part of the succession contain high diversity assemblages including Asaphellus jujuanus, Geragnostus callaveiformis, G. callaveiformis?, G. nesossii, Conophrys sp., Bienvillia rectifrons, Parabolinella triarthroides, Apatokephalus tibicen, Ceratopyge forficuloides, Pyrimetopus pyrifrons and Rossaspis? sp. In contrast, shoreface deposits of the upper part of the formation have very low diversity faunas, which are composed almost exclusively of Notopeltis orthometopa and Mekynophrys nanna. The FAD of N. orthometopa and M. nanna are located well above the first records of Asaphellus jujuanus and other trilobites. This fact may assist in refining the current biostratigraphic scheme of the upper Tremadocian of Argentina.