Estudio de las manifestaciones siliceas del Noroeste de la Provincia de Navarra

Abstract

The geological study of the Northern of the Aralar Range (N.W. of Navarra) shows the existence of different types of silicification in limenstones dated as Callovian-Oxfordian (Jurassic) and in the Valanginian (Lower Cretaceous). We have carried out four outcrops near Iribas, Madoz, Goldaraz and Leiza villages, with a systematic collection both of the "host-rock" and silicifications in order to establish the existing relations between them. Three general types of silicification have been differenciated: a) selective silicifications in aisolated compact nodules b) selective silicifications in Thalassinoides burrow system c) selective silicification in sulphate nodules, forming small quartz geodes filled sometimes with calcite and other times with asphalt. The source of silica is probably biogenic due to presence of numerous siliceous sponge spicules, partially or tatally calcefied. The crystallographic study of the cherts of the four otucrops has allowed us to confirm the existence of a dependence between the crystallinity Index (C.I.) with two methods and the volumen of the cell unit, indicating that a greater volume of cell unit corresponds with a studies of the density in which a direct relation between the density of the samples and the cristallinity has been observed