Early late Albian (Cretaceous) of the central region of the Basque-Cantabrian Basin, northern Spain: biostratigraphy based on ammonites and orbitolinids

Sections in Albian platform and basin sediments in the Karrantza Valley, Trucíos and Balmaseda region to the west of Bilbao, northern Spain, only briefly described hitherto, have yielded late Albian foraminifera, mainly orbitolinids, and ammonites of Hysteroceras varicosum Zone and early Mortoniceras inflatum Zone age. They rest unconformably in this area, on basinal carbonates and marls of late early Albian age with an example of Douvilleiceras mammillatum dating its summit. The hiatus involves all of the middle Albian and possibly the lowermost upper Albian. The lithostratigraphy of the sections is described in outline and the ammonite and orbitolinid evidence of dating is recorded and illustrated. Correlation is made with the Armintza area to the north where contiguous basin sediments are also of varicosum Zone age resting, unconformably, on lower middle Albian sediments. Comparisons are made with similar Tethyan province ammonite faunas described from Sardinia and Venezuela.