Mineral diagenesis and inferred fluids in basinal mudstones: The Carboniferous Morridge Formation, Widmerpool Gulf, UK

Abstract

An appreciation of the diagenetic processes impacting basinal mudstones is important for understanding the properties of mudstones and the nature of fluids which might have been expelled onto adjacent carbonate platforms. This study utilizes petrographic and mineralogical data from mudstones from the Serpukhovian Morridge Formation in the Widmerpool Gulf, UK to investigate the paragenetic succession and associated mineral precipitation during diagenesis of mudstones. Authigenic minerals in the studied samples contain kaolinite, calcite, non-ferroan and ferroan dolomite, pyrite, ankerite, and quartz. The Si/Al ratios in most samples are above 2.5, indicating that most of the studied samples contain excess silica. The widespread presence of authigenic kaolinite and quartz provides important information on the sources for dissolved silica in pore water. The presence of pyritized radiolarians indicates that the dissolution of siliceous radiolarian skeletons is the most likely principle silica source within the system. The illitization of kaolinite also provided a possible silica source for quartz overgrowth during burial diagenesis.It has been long-proposed that the supply of base metals (i.e. Ba, Zn and Mg) for mineralization on the adjacent Derbyshire Platform came from juxtaposed basins, but evidence for this has been limited. The results of this study show that the diagenetic evolution of mudstones in the late Carboniferous links closely to the mineralization observed on the Derbyshire Platform. In the Widmerpool Gulf, the source of magnesium incorporated into dolomite is likely from basinal brine that also provided a source of barium for the barite mineralization. Based on the burial history model, the studied samples were buried to 1.3 km with a temperature of 82 °C in the late Carboniferous. Hydrocarbon was generated during maturation of organic matter and migrated with basinal brines through factures into surrounding formations during basin inversion. This mixed fluid likely contributed to the mineralization on the Derbyshire Platform.