The concretions of the Bearreraig Sandstone Formation: geometry and geochemistry

The sandbodies of the Bearreraig Sandstone Formation (Inner Hebrides, UK) are cemented by two generations of calcite. The first generation, an inhomogeneous ferroan calcite (0.05?3.28 mol% FeCo₃) formed during sulphate reduction (δ¹³C =?24 to ?32%o PDB) in marine porewaters (δ¹⁸O of cement from ?1 to ?4%o PDB) at very shallow burial depths (a few centimetres). These cements are rare but form millimetre-scale clusters of crystals which acted as nuclei to the later, concretionary cements. The second generation of cements are more homogeneous ferroan calcites (mean 1?58% mol% FeCo₃) which evolve to progressively higher Fe/Mg ratios. They are sourced by shell dissolution (δ¹³C of cement from +1 to ?3%o PDB) into meteoric (δ¹⁸O of cement from ?6 to ?10%o PDB) or mixed marine meteoric waters (δ¹⁸O of cement from ?4 to ?6%o SMOW). These were introduced into the formation either during Bathonian times as a freshwater lens, or, subsequent to partial inversion, by confined aquifer flow. Corroded feldspars within the concretions suggest that an interval of at least 8 Ma separated the deposition of the sediments from the onset of concretion growth. Abundant concretions are preferentially developed at certain horizons within the sandbodies, where the early generation of ferroan calcite cements provided nuclei. The latter formed close to the sediment-water interface, the concentration of cement within the sediment being related to sedimentation rate. The relatively high concentrations of the first generation of cement, upon which the concretionary horizons are nucleated, formed during periods of minimal sedimentation.