Diagenetic Evolution of Mudstones: Black Shales to Laminated Limestones, an Example from the Lower Jurassic of SW Britain

The degree of carbonate cementation in argillaceous sediments represents one of the significant diagenetic evolutions in ancient mudstones. The alternation of the black shale and laminated limestone of the Lower Jurassic of SW Britain is an example, which typify extent of diagenetic modification of marine clay and organic-rich sediments. The Lower Jurassic mudstones of the SW Britain include black shales, marls and alternate with limestone beds and nodules. Laminated limestones are also present and are only enclosed between the black shales. Detailed petrography combined with geochemical analysis of the black shales and enclosed laminated limestoned have been carried out and compared to the typical isolated nodules in order to interpret the diagenetic history of mudstones. Displacive growth of the microspar crystals was an important process that occurred during the diagenetic evolution of these mudstones. The black shales show carbon (+0.9 to +1.9‰ (PDB)) and oxygen (−2.2 to −4.1‰ (PDB)) isotopic values, which is compared to their enclosed laminated limestones, that show more negative carbon (−0.2 to −1.2 ‰ (PDB)) and oxygen (−5.4 to −6.8‰ (PDB)) values. The stratigraphic position, petrography and isotopic relationship of the black shales and enclosed laminated limestones indicate a degree of diagenetic evolution and carbonate cementation of mustones during progressive burial.