The spatial distribution and geochemical variation of fault and fracture hosted calcite and gypsum cements in the eastern Bristol Channel Basin

Abstract

Abstract The spatial distribution and geochemical variation of fault- and fracture-hosted cements can be used to reconstruct the evolution and migration of pore fluids in sedimentary basins. Geochemical data, from stable oxygen and carbon isotope and strontium isotope analyses, of calcite and gypsum cements provide valuable insights into the source of pore fluids and their evolution in the eastern Bristol Channel Basin. Overall, at both an outcrop-scale and a basin-scale there were no observable relationships between the dip angle and direction of fractures with the oxygen and carbon isotopic composition of calcite cements, indicating that the movement and distribution of fluids in fractures is not controlled by the orientation of fracture opening. Radiogenic strontium isotopic compositions of calcite and gypsum cements indicate at least two different mineralising fluid compositions that are strongly controlled by the lithologies (fossiliferous limestones and shales, and marls with gypsum nodules, respectively) in which they occur. These fluids originated as connate pore fluids that were altered during the diagenesis of their host lithologies. Mineralising fluids within the Lower Jurassic Lias Group are characterised by enriched oxygen and strontium isotopic compositions and relatively depleted carbon isotopic compositions, associated with the alteration of labile minerals and carbonaceous fossils. Mineralising fluids within the Triassic Mercia Mudstone Group have enriched strontium isotopic compositions as a result of the interaction of fluids with evaporitic intervals. The mineralising fluids were redistributed through migration along faults and their associated damage zones. This study highlights the importance of integrating a range of geochemical analyses when attempting to understand the source of fluids and their evolution through time. The findings from this study can also be applied to nearby petroleum producing basins, e.g. the Wessex Basin, where equivalent rocks are economically significant source rocks and caprocks and have experienced similar structural histories.