Low-Mg calcite marine cement in Cretaceous turbidites: origin, spatial distribution and relationship to seawater chemistry

ABSTRACT Lower Cretaceous (Hauterivian) bioclastic sandstone turbidites in the Scapa Member (North Sea Basin) were extensively cemented by low‐Mg calcite spars, initially as rim cements and subsequently as concretions. Five petrographically distinct cement stages form a consistent paragenetic sequence across the Scapa Field. The dominant and pervasive second cement stage accounts for the majority of concretions, and is the focus of this study. Stable‐isotope characterization of the cement is hampered by the presence of calcitic bioclasts and of later cements in sponge spicule moulds throughout the concretions. Nevertheless, trends from whole‐rock data, augmented by cement separates from synlithification fractures, indicate an early calcite δ18O value of+0·5 to ‐1·5‰ PDB. As such, the calcite probably precipitated from marine pore fluids shortly after turbidite deposition. Carbon isotopes (δ13C=0 to ‐2‰ PDB) and petrographic data indicate that calcite formed as a consequence of bioclastic aragonite dissolution. Textural integrity of calcitic nannoplankton in the sandstones demonstrates that pore fluids remained at or above calcite saturation, as expected for a mineral‐controlled transformation. Electron probe microanalyses demonstrate that early calcite cement contains