Silicification and bentonites in the Silurian of Gotland

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Abstract
A list of 25 localities with silicified fossils is given. Silicification was selective. At most of the localities, only the bryozoans and brachiopods are affected. Their skeletons originally consisted of low-magnesium calcite. Silicified gastropods (original shells mainly aragonite) and crinoids (original skeletons probably high-magnesium calcite) occur only at three localities. Seven other taxa are also represented. The origin of the silica was tephra, as is indicated by the close connection between the bentonites and the levels with siHcified fossils. The source area for the tephra was probably south of Gotland.