Abstract
Carboniferous lithostratigraphic units are analysed for the Sierra del Brezo area south of the Ruesga Fault at which the northern branch of the Cantabric-Asturian orogen overrides the southern branch in northern Palencia. These belong to the Asturian-Leonese facies realm and prolong the northward facing thrust units of northern León eastwards in the most tightly compressed part of the Palaeozoic orogen of the Cantabrian Mountains (Figs. 1, 2). Two subareas are the ValsurbioAlmonga anticlinal structure and the Rebanal-Ventanilla subarea where the axial planes are reversed and considerable shearing took place. As in northern León, the uppermost Famennian - lowermost Tournaisian Ermita Formation overlies progressively older Devonian strata northwards with an erosional contact (Fig. 3). The upper Tournaisian to lowermost Namurian Genicera Formation (with three constituent members) follows all over the area after a generalised stratigraphic break. A 600 to 800 m thick development of Namurian “caliza de montaña” (Barcaliente and Valdeteja formations) in the South passes northwards into much thinner (c. 40-85 m) limestones with a partial (sometimes total) elimination of the Barcaliente Formation (Figs. 4, 5). Carbonate debris flows and bioclastic limestones in shales characterise the top of the Valdeteja Formation. An erosional contact separates the Valdeteja Limestone Formation in the northern subarea from overlying shales and turbidites of the Carmen Formation. The marked stratigraphic breaks noted in the northern subarea match those found at exactly the same levels in the Revilla Nappe, some 30 km eastwards in northern Palencia. The Revilla Nappe succession, with low-angle unconformities separating an even thinner Valdeteja Formation (30 m) from Genicera Fm underneath and Carmen Fm above, apparently corresponds to the same palaeogeographic area characterised by repeated uplift as the northern subarea of the Sierra del Brezo (Fig. 6). It probably represents a strip of terrain suppressed by the Ruesga Fault which represents very considerable tectonic shortening attributable to the pre-Curavacas Palentian phase of deformation (dated as late Langsettian).
