Burial dolomitization of the Woo Dale Limestones Formation (Lower Carboniferous), Derbyshire, England

Dolomite occurs in open-shelf packstones and grainstones of the Vincent House Member of the Woo Dale Limestones Formation (Lower Carboniferous) in the Wye Valley area, east of Buxton, Derbyshire, Central England. Two stages of dolomitization can be recognized. Stage 1 consists of limpid, non-luminescent dolomite lacking detectable iron and manganese. Stage 2 comprises cloudy, inclusion-rich, orange-red luminescing dolomite with significant iron and manganese. The contact between the two stages is irregular and suggests a hiatus between dolomite growth stages. Dolostones lacking any unreplaced limestone may contain up to 35% calcite cement rilling intercrystal porosity. This cement shows a characteristic zonal sequence and is normally in optical continuity with the surrounding dolomite crystals. In some cases, precementation compaction of the dolomite mosaic caused fracturing of the crystals. Sherds of dolomite on the floors of pore-spaces then provided the nucleus for growth of calcite cement. The petrographic character and stratigraphic distribution of the dolomite suggest that dolomitization occurred at depth, during burial of the Woo Dale Limestones in the Late Carboniferous. Dolomitizing solutions, expelled from basinal shales, moved up-dip along permeable limestones or along the sub-Carboniferous unconformity. Fluids responsible for the generation of stage 2 dolomite may have been contaminated by ions such as iron and manganese released from the alteration of volcanic rocks which occur beneath the Carboniferous Limestone in the Wye Valley area.