Geology of the Leonides between the Bernesga and Porma rivers, Cantabrian Mountains, NW Spain

A stratigraphic analysis of the Lower Palaeozoic in the Bernesga-Porma area revealed relatively stable shelf conditions in the miogeosynclinal part of a geosyncline located further to the south (fig. 5). The Caledonian period might be represented by the synsedimentary volcanism (dolerites and tuffites) during the Ordovician and Silurian. With the onset of the Devonian, the shelf area became progressively less stable and is separated into the Bernesga and Esla sub-basins by the WSW-ENE trending Pardomino ridge (fig. 15). The Leonide facies south of the León line varies in composition and becomes thinner towards this line (fig. 14 and app. III and IV). The WNW-ENE trending Sabero-Gordón hinge line becomes apparent during the Upper Devonian; it separates an area of progressively steeper uplift and subsequent erosion in the northern Leonides from an area of continued subsidence and rapid sedimentation in the southern Alba and Corada sub-basins (fig. 16). The fundamental León, Sabero-Gordón and Pardomino lines were reactivated during the Lower Carboniferous (fig. 23) which is identified by shallow marine, condensed sequences and again during the deposition of the Upper Carboniferous flysch and molasse facies in tectonically controlled asymmetric basins (figs. 30 and 66). The Bretonic epeirogenic phase resulted in a ± 4° SSW tilt of the Leonides west of the Pardomino line (fig. 14) and further accentuated the outwedging of the strata. The geometry of the asymmetric folding and thrusting of the Leónides during the initial Sudetic folding phase is a direct consequence of the palaeogeography and facies boundaries recorded (fig. 63). Subsequent erosion of these culminations produced the coarse-grained material of the flysch facies, consisting of graded wackes and turbidites (fig. 65). The gradually climaxing Asturian folding phase migrated in time and space; the oldest tecto-facies are developed on top of the Visean Alba griottes south of the Sabero-Gordón line, while north of this line they are found on top of the Namurian (A) Caliza de Montaña Formation (fig. 31). The youngest fusulinid assemblages in the San Emiliano Formation south of the León line indicate the base of the Westphalian, while the youngest fossil determinations in the Lena Formation north of this line are indicative of the Upper Westphalian (app. V). The depositional environment changes from marine to paralic and finally to continental. In the Bernesga-Porma area, the Leonides consist of seven thrust units, which are more than 25 km long and approximately 2 km thick; the maximal recorded displacement is more than 3 km. These thrust units were back-folded and faulted before they were partly covered by Stephanian molasse facies, deposited in intramountainous basins in the back- and fore-deeps of the thrust-folds. In the disharmonically folded Piedrafita Unit parasitic and cascade folds occur (figs. 70 and 71); stretching of the competent beds (boudinage) and flattening of argillaceous beds (slaty cleavage) were also recorded (fig. 60). The Stephanian Matallana and Rucayo basins are mainly deformed by reversed faulting in the basement (fig. 72) during and after the molasse deposition (late-orogenic Saalic phase). Epithermal mineralizations and silicious mylonite lenses mark the most important fault zones. The large gap in the stratigraphic record (Stephanian-Upper Cretaceous) represents the period of structural adjustment and uplift of the Hercynian core of the Cantabrian Mountains. After the Upper Cretaceous transgression had ceased (fig. 39), a strong morphogenetic uplift (> 1 km) related to the Savian phase (Oligocene) took place along a narrow E-W trending flexure zone (fig. 76). The extensive erosion resulted in the deposition of limestone conglomerates in torrential piedmont fans towards the down-warped León Basin (fig. 74). The geometry of the associated mountain flank thrusting was also studied from detailed gravity profiles covering the entire area (fig. 75). The tectono-stratigraphical evolution of the Bernesga-Porma area is sketched in figure 77.