The geology of the Valderrueda, Tejerina, Ocejo and Sabero coal basins (Cantabrian Mountains, Spain)

Four Upper Carboniferous limnic coal basins in the Cantabrian mountains are described. In the coal measures, which are known as the Cea formation and unconformably overlie the Older Palaeozoic, two sedimentary cycles are recognised. Accordingly, the unconformable sequence is subdivided into two members. The lower one, the Carrión member, starts with quartzite conglomerates and becomes gradually finer grained upwards. It yields anthracitic coal and upper Westphalian D floras. Its maximum thickness is 1200 m. The upper one, the Prado member, begins with limestone conglomerates and also grades to finer sediments upwards. It contains dry to fat coals and Stephanian A to B floras. Its maximum compiled thickness may be approximately 2500 m but a complete section is not found anywhere. The Cea formation shows onlap onto the Older Palaeozoic towards the west. The predominant structural trend in the Cea formation in the described area is east-west. A few structures with north-south axes were recognised in the Valderrueda and Ocejo basins. They are thought to have originated from differential compaction and to be the earliest structures of the Cea formation. The east-west structures are dominated by wide, asymmetric synclines, separated by narrow zones of disturbance instead of anticlines. They have originated as a secondary effect of block faulting in the underlying Older Palaeozoic formations. In the history of the Cea formation large east-west trending fundamental faults (terminology from de Sitter, 1956), probably separating basement blocks, play a dominant role in the deposition as well as in the deformation of the Cea rocks. Activity along these large east-west faults in the Older Palaeozoic rocks is proved to have continued intermittently from the upper Westphalian (and earlier, Rupke, 1965) to the middle Tertiary. Thus the deformation of the Cea deposits, which is dependent on the movement along these faults, must have been a long-lasting process and not a short-lived event like, for instance, the Permian Saalic phase, as was formerly believed.