The geology of San Isidro-Porma area (Cantabrian Mountains, Spain)

The age of the non-metamorphic sedimentary series in the Cantabrian mountains ranges from the Cambrian through the Tertiary. The Lower Palaeozoic deposits mainly show a sandy or quartzitic facies, and were deposited in an extensive Cambro-Ordovician geosyncline of which only the miogeosynclinal part is accessible for investigation, the orthogeosynclinal part now being covered by the Mesozoic-Tertiary deposits of the Spanish Meseta. The Caledonian orogeny is expressed in phenomena resulting from igneous activity. The Hercynian orogeny started with epeirogenetic movements during the Upper Devonian (Bretonic phase of Stille). The uplift caused a varying hiatus on which Upper Famennian transgressive sediments are found unconformably overlying anything between the Lower Famennian and the Middle Cambrian. The E-W-trending León line played an important role during these epeirogenetic movements. In the Asturides north of it, a Central Asturian dome shows an almost constant amount of uplift. Over a very wide area the Upper Famennian rests unconformably on the Lower Ordovician quartzite. Two main stratigraphic and tectonic units are distinguished: 1. The Isidro-Tarna-Ponton subarea. 2. The Mampodre-Fontasguera-Ten subarea. The Upper Famennian transgression was followed by the sedimentation of a uniform limestone during the Visean and Namurian. In the Central Asturian dome region a period of non-deposition or slow sedimentation set in after the sedimentation of the Namurian limestone. Locally, even this limestone was deposited under such conditions. South of the León line, the sedimentation continued with a thick series of mainly clastic deposits (San Emiliano Formation). During the Upper Namurian-Lower Westphalian, tectonic forces (Sudetic phase of Stille) caused nappe and overthrust structures south of the León line, whereas the areas north of this line, the Central Asturian dome region included, acted as a marginal trough to this orogene. The Asturian folding phase started in the Upper Westphalian, during which the Carboniferous basins north of the León line became folded. In the Central Asturian dome region, for example, SW-NE-moving thrust sheets developed, and were refolded in a predominantly E-W direction. Stephanian intramontanous coal basins are found in the present area along the León line. These basins were folded in an E-W direction at some time between the Triassic and Upper Stephanian (Saalic phase of Stille). Features indicative of a Würm glaciation are frequently found in the area under consideration.