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

Abstract

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 Phases 1 and 2. The Peñascal Limestones (late Early Aptian) represent carbonate platform Phase 3 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.

The Early Aptian volcanic episode of Gutiolo (N Spain): Expression of the Bilbao Rift Fault Zone

“…This stratigraphic correlation is reliable, both by lateral continuity and stratigraphic position. The Bolintxu‐Abrisketa sections have been dated with many ammonites as belonging to the upper part of D. furcata Zone, i.e., in the Ch (Ch) meyendorffi Subzone (Fernández‐Mendiola et al, ). In summary, palaeontologically, the Gutiolo volcanics correspond to the uppermost part of the early Aptian.…”

“…The drowning of the underlying rudist limestone unit in Seberetxe is testified by the record of an 8‐m‐thick unit of marl and marly limestone with open sea fauna (ammonites), inmediately replaced by black lutites and sandstones. These later still contain ammonites, some of them from a Boreal provenance ( Tropaeum, Australiceras ) with diameters up to 50 cm (Fernández‐Mendiola et al, ).…”

“…The stratigraphy of the Aptian series south of Bilbao was first described by Rat () and later on by García‐Mondéjar and García Pascual () and Garrote Ruiz et al (). Recent studies on the same subject are those of García‐Mondéjar, Fernández‐Mendiola, Millán, and Mendicoa () and Fernández‐Mendiola, Mendicoa, Owen, and García‐Mondéjar (). In general terms, the south of Bilbao Aptian consists of three formations that are from older to younger: Ereza Formation (sandstones and lutites; García‐Mondéjar & García Pascual, ), Peñascal Formation (limestones; Fernández‐Mendiola et al, ), and Bilbao Formation (marlstones; García‐Mondéjar & García Pascual, ).…”

“…Taking into account that our 123 ± 3 Ma date gives an ample interval that can be nearly 3 Myr younger than the 123.16 Ma at top of the OAE1a and that the stratigraphy shows that the Gutiolo volcanics are younger than the OAE1a in Bilbao (Fernández‐Mendiola et al, ), the precise isotopic age of the Gutiolo volcanism must be considered younger than 123.16 Ma, i.e., in the range of our interval 123–120 Ma. OAE1a deposits lie 400 m below the Gutiolo volcanics in the Bilbao area.…”

“…Recent studies on the same subject are those of García‐Mondéjar, Fernández‐Mendiola, Millán, and Mendicoa () and Fernández‐Mendiola, Mendicoa, Owen, and García‐Mondéjar (). In general terms, the south of Bilbao Aptian consists of three formations that are from older to younger: Ereza Formation (sandstones and lutites; García‐Mondéjar & García Pascual, ), Peñascal Formation (limestones; Fernández‐Mendiola et al, ), and Bilbao Formation (marlstones; García‐Mondéjar & García Pascual, ). All of them are dated with ammonites and orbitolinids, the first two in the early Aptian and the third one in the early and late Aptian (Fernández‐Mendiola et al, ).…”