Geochemical trends and palaeohydrological significance of shallow burial calcite and ankerite cements in Middle Jurassic strata on the East Midlands Shelf (onshore UK)

Abstract

This paper discusses isotopic and geochemical trends in shallow burial (<1500 m) carbonate cements in limestones from a Middle Jurassic shale-dominated marine to paralic succession situated on the East Midlands Shelf. The results are used to investigate carbonate–shale interactions between the near-surface realm of microbially mediated diagenesis and the deep thermobaric regime of clay mineral transformations and hydrocarbon generation. They are also combined with published data from cements in stratigraphically and geographically adjacent strata to better understand the Late Mesozoic–Cenozoic palaeohydrology of central–eastern England (East Midlands Shelf–Midland Platform). Shallow burial cements sampled from the Blisworth Limestone Formation consist of ferroan calcite and calcic ankerite, which are associated with ferroan neomorphic calcite and “beef” (cone-in-cone) calcite veins. Calcite cements are petrographically complex and variable, yet record consistent compositional trends. Carbon isotope values (δ13C=−1.2‰ to +1.7‰ PDB) show that organic matter breakdown did not contribute significant carbon to the shallow burial pore fluids. Oxygen isotope data (δ18O=−4.5‰ to −11.2‰ PDB) are consistent with precipitation from modified marine pore fluids that displaced the original brackish depositional fluids from the limestone during Palaeocene burial. The same fluids accompanied cementation in underlying Bajocian limestones. Ferroan calcite cements, neomorphic spar and “beef” record a surprising correlation in Fe2+ and Mg2+ concentration that suggests a common source for the ions in the adjacent shales. This trend is not present in the Bajocian limestones and indicates that ionic mass transfer was on a smaller scale than the regional “plumbing system”. The results suggest that Middle Jurassic limestone palaeoaquifers on the East Midlands Shelf hosted a westerly-directed, up-dip flow from compacting Upper and Lower Jurassic marine mudrocks in the Sole Pit Basin during the Cretaceous and Early Tertiary. However, Early Cretaceous meteoric groundwater palaeoaquifers recorded from cements in laterally equivalent Middle Jurassic limestones on the Midlands Platform and in the Weald Basin appear not to have extended northward into the East Midlands Shelf.