Preservation of shell microstructure in silicified brachiopods from the Upper Cretaceous Wilmington Sands of Devon

Abstract

Summary From a study of silicified fossils, and in particular the microstructure of brachiopods, from the Wilmington Sands (Upper Cretaceous) of Devon, a model of skeletal silicification is proposed. Three distinct morphologies of silica were formed, controlled by the relative rates of silica supply and carbonate dissolution: ( a ) a fine-scale replacement of the original shell microstructure where silica was abundant; ( b ) a concentric ring morphology called ‘beekite’ where silica supply was limited, and ( c ) a granular white crust formed where carbonate dissolution was restricted. Silicification occurred during early diagenesis as a result of bacterial decay of organic matter intimately associated with skeletal fragments, within a sediment of restricted permeability. A build-up of CO 2 probably caused dissolution of skeletal carbonate, and bicarbonate released from this caused local precipitation of silica. The proposed mechanism is belived to be of general applicability to micrite-rich carbonate sediments.