The tectonic and stratigraphic history of the Cardaño area, Cantabrian mountains, Northwest Spain

The Devonian and Carboniferous rock-sequence in the Cantabrian Mountains is developed in two different facies which are separated by an E-W tectonic line, the León Line, and called the Leonide and Palentian facies, respectively to the S and N. The Leonide facies is widely exposed throughout the Cantabrian Mountains and its tectonic history is now well known. The most complete sequence of the Palentian facies occurs in the present area, which lies across the boundaries of the provinces of Palcncia, León and Santander. Towards the west and north the Devonian and Lower Carboniferous gradually plunge below the younger strata. The Palentian facies in this region is only found north of a second fundamental tectonic feature, the SE-NW Cardaño Line. This line joins the León Line in the SE of the area near the village of Santibañez but can be traced far to the NW into the province of Oviedo. Immediately south of the Cardaño Line there is the Sicro basin filled with Upper Carboniferous elastics which even overlap in places the León Line further south. The pre-Westphalian rocks of the present area are limited to the north by the Peña Prieta Line, the fundamental nature of which is emphasised by a large granite intrusion. To the east the Polentinos fault separates the present area from the mainly Westphalian, Pisuerga basin. The fundamental lines are partly expressed at the surface as large fault systems. It is remarkable that in the Palentian facies sedimentation was not interrupted by the major erosional periods known from the Leonide facies. The post-Silurian and pre-Westphalian sequence of the Palentian facies is on average about 850 m thick and consists mainly of shales with thin limestone intervals. The corresponding sequence of the Leonide facies is on average about 1750 m thick and shows important, thick Devonian reef limestone intervals. Only the lower and uppermost of the Devonian contain clearly higher-energy deposits (sandstone sequences). Both the lithology and fossil association confirm that the Palentian facies sequences developed in a more offshore (middle to outer neritic) environment of deposition than indicated for the Leonide facies (littoral to inner neritic), Important epeirogenetic movements in the late Namurian and in the Westphalian disturbed the preceding long period of quiet sedimentation. Between the Cardaño Line to the south, the Peña Prieta Line to the north and the Polentinos fault to the east the Cardaño block was subjected to a regional tilting during the deposition of the rocks of the Yuso Group. The maximum uplift and erosion are indicated in the N and E whereas the deepest subsidence and maximum deposition have been detected in the S and W of the Cardaño block. The pre-Westphalian rocks, where unconformably overlain by those of the Yuso Group, show local, gentle, pre- or synsedimentary folded structures. The present study has not revealed pre-Westphalian structures that would justify the use of the term orogenic phase (i.e. Sudetic) for their origin. They are rather interpreted as the results of local compression accompanying pre- and synsedimentary epeirogenetic movements. The unconformable Yuso Group consists of a conglomerate facies — the Curavacas Formation — up to about 700 m thick and a sandstone-shale facies — the Lechada Formation — of at least 750 m N of Cardaño de Arriba (probably up to 2000 m to the W). The Westphalian rocks were deformed during the main compressive phase, which therefore is thought to correspond with the Asturian folding phase (pre-Stephanian). The tectonic transport here was from north to south in contrast to the Leonides where it was from south to north. This correlates with the theory that the folding of the two areas took place at different times; Asturian in the Asturides (Palentian facies) and Sudetic in the Leonides. The inhomogeneity of the Palentian facies rock sequence is reflected in the very complicated final tectonic picture. The Cardaño Area can be subdivided into 4 subareas (Northern, Central, Southern and Arauz), in each of which a different lithofacies is related with a corresponding minor tectofacies. Simultaneous cross folding can be related to the rapid facies changes in the affected rocks. The present area gives very instructive examples of the close interaction of tectonics and sedimentation. Epeirogenetic movements between fundamental tectonic lines controlled the deposition of the sedimentary sequence. These heterogeneous rocks were then acted upon by a relatively short compressive tectonic phase which created out of them the present architecture of the Cardaño Area.