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

Abstract

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 δ 18 O value of+0·5 to ‐1·5‰ PDB. As such, the calcite probably precipitated from marine pore fluids shortly after turbidite deposition. Carbon isotopes (δ 13 C=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 <2 mol% MgCO 3 , despite its marine parentage. Production of this cement is ascribed to a combination of an elevated aragonite saturation depth and a lowered marine Mg 2+ /Ca 2+ ratio in early Cretaceous ‘calcite seas’, relative to modern oceans. Scapa cement compositions concur with published models in suggesting that Hauterivian ocean water had a Mg 2+ /Ca 2+ ratio of ≤1. This is also supported by consideration of the spatial distribution of early calcite cement in terms of concretion growth kinetics. In contrast to the dominant early cement, late‐stage ferroan, 18 O‐depleted calcites were sourced outwith the Scapa Member and precipitated after 1–2 km of burial. Our results emphasize that bioclast dissolution and low‐Mg calcite cementation in sandstone reservoirs should not automatically be regarded as evidence for uplift and meteoric diagenesis.