Delamaran trilobites from the La Laja Formation, San Juan, Argentina

Abstract

The La Laja Formation lies exposed almost continuously along the western margin of two hill ranges, i.e., the Sierra Chica de Zonda and Sierra de Villicum. This limestone unit bears a remarkable fauna of Cambrian trilobites. In this work we analyse the Delamaran trilobite fauna from the upper part of the El Estero Member and the lower third of the Soldano Member of the La Laja Formation. Taxa described are Glossopleura walcotti Poulsen, Glossopleura lodensis (Clark), Fieldaspis sp., and Prozacanthoides zondaensis n. sp. The Poliella denticulata, Mexicella mexicana?, and Glossopleura walcotti Zones are documented; these had been previously included in a hiatus. Trilobites-based correlations are established between the La Laja Formation and olistoliths of the Western Precordillera. Additional correlations are also suggested between the Precordillera and different localities in Laurentia. Finally, we discuss the biogeographic implications of the described material. Delamaran taxa reported from the La Laja Formation are characteristic and so far restricted to Laurentia. Analysis of the trilobite fauna from this unit supports the allochtonous origin of the Precordillera.