The Early Aptian (Cretaceous) stratigraphy of Mount Pagasarri (N Spain): Oceanic anoxic event-1a

Sedimentological, stratigraphical, palaeontological, and geochemical evidence from outcrops of the Early Aptian (Cretaceous) succession in Mount Pagasarri (northern Spain) dated by ammonites is described to elucidate the origin of organic carbon-rich sediments and carbonate platforms in the Early Aptian of the northern margin of the Tethyan Ocean. A 1,600-m-thick section spans the latest Barremian to the end of the Early Aptian and is divided into 3 units: a lower marine nearshore, a middle rudist-coral shallow-water carbonate platform, and a deeper-water outer shelf. Deshayesites oglanlensis, D. forbesi, D. deshayesi and Dufrenoyia furcata ammonite zones are represented. Three organic carbon-rich intervals are recognised. The oldest (top Barremian) is correlated with the Taxy Level of France/Germany. A second organic layer in the lower part of the Deshayesites deshayesi Zone sediments correlates with the Selli Event of Italy. A third organic-carbon enriched layer in the D. furcata Zone sediments post-dates the Aparein Level of the Aralar area in Spain. Carbon isotope curves reveal a brief negative excursion followed by a more prolonged positive trend during the Early Aptian interpreted as oceanic anoxic event 1a (OAE1a). Three carbonate platform growth phases are recognised in the Basque-Cantabrian Basin with OAE1a being developed between carbonate platform Phases1 and 2. The Penascal Limestones (late Early Aptian) represent carbonate platform Phase3 of the Basque-Cantabrian Basin succeeded by ammonite-bearing marls of the latest Early Aptian, reflecting platform demise and subsequent marine flooding. The Mount Pagasarri area is a key locality for study of the oceanic anoxic events in the Greenhouse Earth of the Early Aptian.