Dolomitization of siliceous microfossils in Namurian concretionary limestones

Early diagenetic, micritic, carbonate concretions from Namurian goniatite bands in Staffordshire and Derbyshire contain Radiolaria and sponges excellently preserved owing to replacement by ankeritic dolomite anhedra. The microfossils are detectable only by acid etching of limestone surfaces and thin sections. Preservation by the dolomite is believed to occur under restricted conditions in which silica solution and dolomite crystallization post-date a phase of rapid calcite crystallization.