The uppermost Albian–lower Cenomanian Bielba Formation of the type-area (Cantabria, northern Spain): facies, biostratigraphy, and benthic Foraminifera

Abstract

The lithofacies, microfacies, depositional sequences, and benthic Foraminifera of the uppermost Albian–lower Cenomanian Bielba Formation in its type-area in northern Cantabria, Spain, consisting of a lower siliciclastic member and an upper Cobreces Member, have been investigated. The investigated section constitutes a hypostratotype for the Bielba Formation, and the lower siliciclastic member is here formally defined as the Somocuevas Member. The obtained data are framed in the major global paleoenvironmental and paleoceanographic changes that occurred during the Albian–Cenomanian transition. Eleven lithofacies types have been identified from facies analysis and vertical log characterization. These lithofacies accumulated in various depositional settings such as storm, wave, and tidal influenced delta front and shoreface, shallow-water carbonate ramp, and offshore marine environments. Within an overall transgressive trend, the succession of the Bielba Formation comprises two depositional transgressive–regressive sequences corresponding to the Somocuevas and Cobreces Members, bounded by a major relative sea-level fall at the end of the Albian. The sea-level fall was followed by a rapid marine transgression and shifting from siliciclastic-dominated deposition of the Somocuevas Member to carbonate-dominated deposition of the Cobreces Member. The microfauna of the Cobreces Member is characterized by omnipresent orbitolinids, trocholinids, and a variety of other mostly small-sized foraminifera that so far have not attracted much attention. Among them, taxa with biostratigraphic importance and taxonomic–phylogenetic importance can yet be found. The benthic Foraminifera are associated with calcareous green algae (dasycladaleans, udoteaceans) and occur in calcarenites of poorly diversified microfacies containing variable amounts of siliciclastics. The microfauna closely compares to that of the overlying late early to middle Cenomanian Altamira Formation.