Estudio geológico del área de Peñacerrada (Alava) : petrografía y cristalografía de las silificaciones existentes

Abstract

The geological study of the Peñacerrada (Alava) area shows the existence of different types of silicification in the Dogger (Jurassic) Age and in the Thanetian (Paleocene) Age. We have drawn stratigraphic sections in three places: Montoria, Arroyo de Santiama (Peñacerrada) and Loza, with relations between them. The silica soruce may be biogenci, because a big number of siliceous sponge spicules included in the nodular chert ara appreciated. The Dogger carbonate sediments were deposited in an open platform (neritic) while the Thanetian sediments were in a lagoon environment. The movements of the shoreface generated sequences of "shallowing upward" where the mixing continental and marine waters played an important role for silicification and dolomitization processes observed in Loza. We have differentiated foru types of general silicification: Äutigenic quartzs, nodular chert, irregular silidifications and lenticular chert bands. The Crystallographic and Mineralogic study of the cherts show the general relation between the Crystalinity index values of the cherts and the limestone or dolomitic replacement. The values obtained for the crystallinity and unit cell volume allows us to corfirm the inverse relation existing between them