The Portilla formation (Middle Devonian) of the Alba Syncline, Cantabrian Mountains, Prov. Leon, Northwestern Spain: carbonate facies and rhynchonellid palaeontology

Three members are informally distinguished in this formation (A, B, and C from base to top). They are present at the western part of the outcrop (thickness ca. 246 m). On the eastern and southeastern part, only member A and basal part of member B are present and the thickness is reduced to ca. 20 m. A sharp surface of discontinuity separates member A from member B. The Portilla Formation abounds in reef-building elements associated with other groups. Five major carbonate facies types are established that belong to a complex biostromal ‘reef’ facies. Vertical and lateral facies changes are demonstrated. The carbonate facies was deposited in a shallow-marine environment. Towards end of deposition of member A, sharp changes in depositional conditions occurred, soon followed by a notable influx of siliciclastics. A distinctive barrier ‘reef’ pattern was established during deposition of member B. It protected a back-reef area from the open shallow sea. This back-reef environment was separated from an area of dominantly siliciclastic deposition in the southeast by an extremely shallow marine or shoal area which might have been emergent. During deposition of member B there occurred a rhythmic alternation of the back-reef carbonates and the carbonates continuous with the ‘reef’ barrier, probably reflecting minor changes in sea level likely due to epeirogenetic movements of the bottom. Eventually organic growth and associated carbonate sedimentation exceeded the rate of subsidence and as a result the ‘reefs’ laterally shifted seawards, followed by the back-reef facies. The facies pattern suggests an increasingly emergent tendency of the marginal part of the carbonate basin due to bottom movements. The barrier ‘reef’ pattern of member B probably terminated due to changes in relative subsidence during deposition of member C. A strong supply of siliciclastics during the deposition of the Nocedo Formation brought an end to the carbonate sedimentation of the Portilla Formation. The variation in thickness in the Portilla Formation has been mainly due to a slow and prolonged differential subsidence of the carbonate depositional basin. The absence of a large part of member B and member C in the easterly and southeasterly directions is probably largely due to non-deposition of sediments. Seventeen species are described of rhynchonellid brachiopods, out of which four species are new. Three new genera are established. Wherever available some critical German rhynchonellid species have been sectioned for comparison. The rhynchonellid and atrypid brachiopod fauna from the Portilla Formation show a great affinity with the Middle Devonian fauna of Eifel region, Germany. The Spanish fauna could be assigned to the mixed or Eifel facies, or close to this type. Striking similarity exists also between the Spanish fauna and the Middle Devonian fauna from the Holy Cross Mountains, Poland. The rhynchonellids and atrypids strongly suggest that the Eifelian — Givetian boundary lies in the basal part of member B. It is suggested that member A is of Eifelian age and that members B and C, apart from the basal part of member B are of Givetian age.