The geology of Liébana, Cantabrian Mountains, Spain; deposition and deformation in a flysch area

The Nansa-Deva map sheet of the Geological Map of the Southern Cantabrian Mountains is published. The accompanying thesis deals with the stratigraphy and structures of the Devonian, Carboniferous, Permian and Mesozoic rocks which constitute the mapped area. A condensed sequence of nodular limestones and shales was deposited in the larger part of the area during Middle-Upper Devonian and Lower Carboniferous. The rocks of this time interval are preserved badly, occurring for a large part as exotics in Upper Carboniferous ‘Wildflysch’ deposits. The establishment of a stratigraphic record for these largely allochthonous occurrences was enabled by examination of their conodont content and by comparing them lithostratigraphically and biostratigraphically with the known sequence of the Cardano area south of the mapped area. The condensed sequence, the thickness of which normally does not exceed 500 m, is followed in the central part of the area by an Upper Carboniferous flysch sequence which may amount to a thickness of 10 km. This sequence becomes intercalated with, and gradually replaced by, a more shallow facies of siliciclastics and biogenetic limestone lenses to the south. Deposition of limestone in a stable shelf environment occurred in the northern part of the area, the Picos de Europa, during the same time interval. The stratigraphy of these Upper Carboniferous rocks was solved mainly by examination of the fusulinid content of the limestone bodies. Biostratigraphical dating of limestone pebbles occurring in conglomerate lenses in the flysch sequence and of exotics from ‘Wildflysch’ units resulted in a number of maximum ages providing the base for the stratigraphic record of this sequence. A pronounced unconformity separates the Upper Carboniferous and older rocks from the clearly postorogenic Permian and Mesozoic rocks. Upper Carboniferous sedimentary facies boundaries coincide with the boundaries between areas with a different structural development. The structural development in the Picos de Europa, characterized by the formation of large nappes, was independent of, and started much later than, the deformation in the remaining area. There, the Upper Carboniferous was not a time of undisturbed subsidence and sedimentation. Local uplift and erosion, faulting, bending and collapse folding are contemporaneous with the flysch sedimentation in the central part of the area. This contemporaneity is less obvious in the southern area. Several deformation types can be distinguished in the flysch rocks: sedimentary deformation (slumping), early tectonical synsedimentary deformation (bending and collapse) and eutectonical deformation (buckling, kinking and cleavage). All kinds of transitions between these main types were also observed. Unravelling the history of sedimentation and deformation, the spatial limitedness of individual deformation processes, the contemporaneity of deformation and sedimentation and the noncontemporaneity of deformation processes in different parts of the area have to be inferred. Most unconformities that originated during the structural development of the subject area have only a local extent. No unconformity could be traced over the entire area. The establishment of a number of folding phases does not seem an appropriate generalization for the structural history of the subject area.