The geology of the Palaeozoic rocks of the South-Western part of the Cantabrian Cordillera, North Spain

Abstract

The Cantabrian mountains of north Spain contain an unmetamorphosed shallow-water succession ranging in age from late Pre-Cambrian to late Carboniferous. The Pre-Cambrian is represented by the predominantly clastic Mora Formation, and is overlain unconformably by the probably early Cambrian Herrería Formation, deposited in a fluviatile environment. Mid-Cambrian deposits are represented by the Láncara Formation which contains algal stromatolites and a thin but distinctive griotte. The overlying Oville Formation is trilobite-rich. In Arenig times the beach sandstones of the Barrios Quartzite were deposited—these contain Cruziana and Rusophycus. Lower Palaeozoic deposition closed with the graptolite-bearing Formigoso Formation. The Upper Palaeozoic successions are richer in carbonates than those of the Lower Palaeozoic. After the ferruginous San Pedro sandstones which span the Silurian—Devonian boundary the limestones and shales of the La Vid formation were laid down, followed by the prominent brachiopod and coral-rich limestones of the Santa Lucía Formation. Clastic conditions returned with the Huergas Formation, but reverted to carbonate in the Portilla Formation. Slight movements before the Famennian gave rise to a basal unconformity to the Ermita Formation. The Carboniferous rocks begin with a second griotte, the Alba griotte, followed by a thick development of Caliza de Montana. Westphalian clastics indicate the early phases of the Hercynian orogeny, whose main activity in this region was between the Westphalian and the Stephanian. The rocks of the pre-Stephanian succession were folded and thrust to stand vertical or overturned. Stephanian conglomerates and coals then filled in the valleys. Throughout the Mesozoic and Cainozoic, the mountains acted as a landmass. The region was not refolded during the Alpine orogeny but a reversed fault developed along the southern margin of the mountains and raised the main part of the chain. Very rapid erosion of the chain took place and torrential conglomerates and fluviatile sands spread to the south over the Meseta region.