Abstract
The paleozoic stratigraphic sequence outcropping on the southern border of the Sierra de la Demanda, whose age ranges from the Cambrian to the Ordovician, has been divided into six different formations. According to their facies, these are equivalent to the those outcropping in the westernmost sector of the West Asturian-Leonese Zone (WALZ). The rocks have been affected by two hercynian deformation phases and a late hercynian fracturation. The first hercynian phase originated folds and all knids associated with a primary foliation; the second phase gave rise to overridings and occasionally a crenulation cleavage. The deformation is accompanied by low and very low-grade regional metamorphism with intensity decreasing in the general terms towards the SE. Cristallochemical data of the samples obtained by means of X-ray powder diffractometry, have revealed the epizone-anchizone limit and, in sorne cases, have evidenced the metamorphic jump produced by the faults.
Highlights
The rocks have been affected by two hercynian deformation phases and a late hercynian fracturation
The deformation is accompanied by low and very low-grade regional metamorphism with intensity decreasing in the general terms towards the SE
Cristallochemical data of the samples obtained by means of X-ray powder diffractometry, have revealed the epizone-anchizone limit and, in sorne cases, have evidenced the metamorphic jump produced by the faults
Summary
Por facies, a las que aparecen en el sector más occidental de la Zona Asturoccidental-Leonesa del Macizo Hespérico y, según Colchen (1974), pueden ser divididos en tres grandes conjuntos litológicos:. - Por un lado, la disminución de los niveles arenosos hacia la parte occidental de la sierra, que ya había sido puesta de manifiesto por Colchen (1974), dificulta en este sector el establecimiento de límites entre dichas formaciones. NIETO d) Calcoesquistos de Mansilla (Cámbrico Medio): Esta formación está constituida por una sucesión de areniscas dolomíticas y pizarras. E) Alternancias de la Demanda (Cámbrico Medio-Superior I Ordovícico): Se trata de una potente serie de más de 4.000 m de potencia de alternancias pelítico-samíticas con intercalaciones de areniscas cuarcíticas en bancos de decimétricos a métricos.
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