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

This paper discusses isotopic and geochemical trends in shallow burial (< 1500 in) 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 Fort-nation 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 (delta(13)C = -1.2 parts per thousand to + 1.7 parts per thousand PDB) show that organic matter breakdown did not contribute significant carbon to the shallow burial pore fluids. Oxygen isotope data (delta(18)O -4.5parts per thousand to - 11.2 parts per thousand 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 Fe and 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 palacoaquifers 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. (C) 2002 Elsevier Science B.V. All rights reserved.