Palaeoenvironment and conodonts from the southern end of the Lower Ordovician carbonate platform of the Cuyania terrane, Central Argentina

Abstract

This study documents the sedimentology and conodont fauna of the Lower Ordovician carbonate platform from the southernmost end of the Cuyania terrane, belonging to the Las Matras block, La Pampa Province, Argentina. This carbonate platform is represented by the San Jorge Formation that includes a metamorphic and a sedimentary member. The latter sedimentary member includes dominant limestones and subordinate dolostones comprising peloidal grainstone and peloidal/aggregate grainstone, with subordinate lime mudstone, and aggregate/oncoidal rudstone. Dominant lithofacies are microbial laminite, thrombolite boundstones, and laminated and massive mudstone, with subordinate cross-bedded grainstone, and intraclastic conglomerate. Metre-scale shallowing-upward cycles reflecting shallow subtidal to supratidal depth in a peritidal environment were recognized. The conodont association consists of twelve species including Cordylodus angulatus and Ventricodus spurius as index species. This conodont association allows correlation with the Baltoscandian Paltodus pristinus Subzone of the Paltodus deltifer Zone and the Northamerican Ventricodus spurius Subzone of the Rossodus manitouensis Zone, thus suggesting a middle Tremadocian age for the sampled levels. This biostratigraphic information permits correlation of the San Jorge Formation with the upper section of the La Silla Formation from the well-known sequence of the Argentine Precordillera from the Cuyania Terrane. Some important differences with the La Silla Formation are the common presence of microbialites (stromatolites and thrombolites), development of metre-scale shallowing upward cycles and absence of ooidal facies in the San Jorge Formation. Tuff levels are older and of different composition than those recorded in the Precordillera, thus suggesting a closer location to the SW Gondwana margin.