Conodont biostratigraphy and depositional history of the Middle Devonian to Lower Carboniferous in the Cantabrian zone (Cantabrian Mountains, Spain)

Within the Cantabrian zone during the Devonian and Early Carboniferous three large palaeogeographical units were of importance: the Asturian geanticline, the Palencian basin and the Asturo-Leonesian basin. These units have a different history of sedimentation, particularly the Palencian basin. Further, in the Asturo-Leonesian basin there were some important structural elements: the Intra-Asturo-Leonesian facies line, formed by an active, normal fault parallel to the border of the basin, and two structural highs: the Pardomino high and the Somiedo high which divided the basin into three pieces. The Asturo-Leonesian basin was a narrow, shallow continental shelf which in the south and west passed into the deeper part which extended over the West Asturian-Leonese zone. With help of biostratigraphic correlations of the Middle Devonian to Early Carboniferous deposits in the Cantabrian zone, based on conodonts, an overview is given of the depositional history of the entire area. Several times during the Givetian and Frasnian a biostromal platform developed in the Asturo-Leonesian basin with small bioherms of stromatoporoids and corals at the southern border along the facies line and in the east along the León line, with a lagoon behind. In the Palencian basin the sediment supply was always smaller. There, shales and nodular limestones with pelagic faunas (Gustalapiedra Formation) were deposited. Carbonate sedimentation started simultaneously with the formation of the first carbonate platform of the Portilla Formation in the Asturo-Leonesian basin. Repeatedly the Asturian geanticline was uplifted leading to tilting of the Asturo-Leonesian basin and to erosion of the uplifted parts. The siliciclastic erosion products were deposited in the subsided parts leading to progradation of the coast with the formation of coastal barriers notably along the facies line. As soon as the supply of siliciclastics decreased, a new carbonate platform could form. The last stromatoporoid-coral biostromes formed such a platform at the end of the Frasnian in the Esla area (Crémenes Limestone in the Nocedo Formation). At the same time, in the west of the Asturo-Leonesian basin fan-deltas formed along the facies line, with conglomerates which originated from erosion at the geanticline, which then apparently extended to the facies line. The Asturian geanticline had extended gradually during the Devonian and the differences between the basins had increased. Uplift of the geanticline resulted in the emergence of the entire Asturo-Leonesian basin during the early Famennian. Then, erosion products from the geanticline were also transported into the Palencian basin by turbidity currents (Murcia Formation) interrupting the sedimentation of nodular limestones which recovered later on (Vidrieros Formation). At the end of the Famennian, due to a transgression the sea spread rapidly over the truncated geanticline. South of the Intra-Asturo-Leonesian facies line initially turbiditic storm deposits (Fueyo Formation) formed while a thin layer of sands and crinoidal grainstones (Ermita Formation) was deposited on the geanticline and in the major part of the Asturo-Leonesian basin. The entire Cantabrian zone may have been emergent during the early Tournaisian and, together with deformations in the West Asturian-Leonese zone movements lead to an inversion of the relief in the Cantabrian zone with the formation of a number of small basins in the platform. When, during the late Tournaisian a transgression lead to the spread of cold, nutrient-rich water over the entire area, some of this water stagnated in the basins so that black shales, radiolarites and phosphatic nodules (Vegamián Formation) could be deposited at the same time as the carbonate sedimentation continued on other parts of the platform. The continuation of the transgression lead to the formation of nodular limestones (Alba Formation), first only on the shallow platforms but later also in the deeper parts because circulation in the basins improved. The Variscan orogenesis lead to deformations of which some are discussed in this paper. Lithological data indicate that the Esla nappe and the Valsurvio area were displaced towards the north and later the South Cantabrian block was shifted about 15 to 20 km towards the east along the Sabero-Gordón line. The different formations and smaller lithological units are described and an informal subdivision of the Portilla Formation is proposed. The conodont faunas which were found are described. A number of interesting or problematic species is treated more extensively. The conodont biozonation is discussed. For the early Carboniferous the regional zonation of Higgins (1974) is adapted: the Polygnathus fauna and Gnathodus pseudosemiglaber Zone are introduced. Rather extensively the palaeoecology of the conodonts will be discussed.