Abstract

Quartz geodes (cauliflower-like) appear in a specific level of the Late Cretaceous (north of Burgos). Due to their anhydrite relicts and spherulitic quartz (length-slow chalcedony) they are considered as pseudomorphs of anhydrite nodules, formed during early diagenesis. At the same level compact chert nodules sporadically appear, presenting a considerable amount of sponge spicules with fibrous texture (length-fast chalcedony). The silicification of the anhydrite nodules took place before total compaction and cementation of the sediment. The source of silica is principally biogenic ( sponge spicules). There is no link between the depositional environment and the existence in itself of anhydrite nodules. Indirectly it is possible to see the effects of an arid climate, which have not been recorded in the lithologic column here described.--Modified journal abstract.

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