Facies and diagenesis of the Devonian Portilla limestone formation between the river Esla and the Embalse de la Luna, Cantabrian Mountains, Spain

In the central part of the Cantabrian Mountains, between the artificial lake in the rivei Luna in the west and the river Esla in the east, outcrops of the Portilla Limestone Formation were investigated. A fairly uniform development could be observed in four structurally different areas. Six lithostratigraphic units: four members and two beds, were recognized and correlations were established on the basis of lithostratigraphic characteristics. In these units six facies types could be distinguished. Lithological samples, characteristic of the facies types, served as a basis for the investigation of the diagenetic products and processes, especially the early diagenetic ones. Mainly in view of differences in lithofacies, and of the presence of biogenetic upgrowths related with the Sabero-Gordón line, this structural line is supposed to have acted already during deposition of the Portilla Limestone sediments. Arguments are presented in favour of an autochthonous origin of the Peña Corada Unit, earlier recognized as an allochthonous unit. A number of selected stratigraphic sections were systematically investigated on the faunal content, and a Givetian-Frasnian age could preliminarily be deduced for the Portilla Limestone Formation. Near Matallana only the base is possibly Eifelian; Frasnian fauna was observed nowhere. The field observations, the additional laboratory observations and the data collected during investigation of the diagenetic processes and products served as a basis for a regional sedimentation model (Fig. 19). Isopach-lithofacies maps (Figs. 15, 16, 17, 18) illustrate the basin-fill history. After a period in which mainly siliciclastic material was supplied by a hinterland situated in the north and in the vicinity of the Porma fault, carbonate sedimentation began. In the area east of the Porma fault, deposition of carbonate material started in the north and progressively shifted towards the south; in the area west of the Porma fault deposition of carbonate material progressed from the Matallana section on towards the west and the east. Both west and east of the Porma fault a vast shallow platform occurred, with prolific crinoid-bryozoan growth. This resulted in deposition of encrinic/encrinal grainstones and packstones, interrupted in some places by siliciclastic sediments. In the area east of the Porma fault a slope was present on which in Member B patchy reefs of the biostromal type developed. In the area west of the Porma fault enormous accumulations of bioclastic (coral) material (banks) were deposited. Bioturbated mudstones, wackestones and packstones show a relation to these biostromal and bank sediments. In the area east of the Porma fault this pattern was suddenly interrupted by supply of siliciclastic material, presumably coming from the north and the northwest. During the deposition of this siliciclastic material the flooded reefs could partly re-establish themselves, but were flooded afresh by a second pulse of siliciclastic material. In the area west of the Porma fault the siliciclastic material is interbedded and admixed in the carbonate sediments. Only in the extreme west is pure siliciclastic material present. The last phase of deposition of the Portilla Limestone sediments resulted in the development of bioherms and biostromes with associated sediments such as slightly dolomitized mudstones, and packstones containing pellets, ostracods, gastropods and fairly large quantities of bioturbation structures. The bioherms occurred mainly in places where reefs had previously developed. Due to the Upper Devonian erosion nothing can be said with certainty concerning the upper part of the sequence in the area north of Valdoré in the autochthonous area, and in the Pedroso syncline west of the Bernesga valley.