Abstract
In the Barrios Formation of the southern Cantabrian Mountains pyroclastic rocks occur which have an Ordovician age. They form the filling of several craters and pipes, which are unconformably overlain by Ordovician respectively Silurian sediments. The pyroclastic material mainly consists of fractured quartz grains which are mixed with angular hydrothermally altered glas lapilli. The strong fragmentation of the volcanoclastic components, even in the lower parts of the pipes, is caused by phreatomagmatic eruptions under shallow marine conditions. The Barrios Formation which is disrupted by the volcanic necks, was deposited in a shallow marine environment with a very low sedimentation rate.
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