The sedimentology and diagenesis of the Raisby Formation (Z1 carbonate), Northern England

Abstract

The Raisby Formation is the basal Zechstein (Upper Permian) carbonate, that crops out in northeast England. It was deposited in environments ranging from shallow water, above wave base, into a deeper water carbonate slope. The shallow water sediments are skeletal wackestones and packs tones which contain some sandstone horizons, whereas those deposited in deeper waters are mudstones, incorporating resedimented carbonates of variable thickness and extent. Many slope sediments were lithified early, forming nodular horizons. These cements, dominantly high magnesian calcite and aragonite, also occluded intra-skeletal pores, and were followed by the precipitation of framboidal pyrite. Early lithification of slope sediments had a significant influence on their stability, and response to resedimentation. Much of the Raisby Formation which was deposited on lower parts of the slope, was removed during an episode of catastrophic slope failure close to the end of first cycle carbonate deposition. During early burial, some sediments which were lithified early, neomorphosed to low-magnesian calcite microspar, which at a few localities was accompanied by the formation of coarsely crystalline calcite concretions. At the same time, or slightly earlier, sediments which did not lithify early, and some of those which did, were dolomitized and replaced by gypsum and anhydrite. Replacement by both dolomite and calcium sulphates was directly related to the deposition of the Hartlepool Anhydrite Formation. During burial, following calcium sulphate precipitation, the formation was relatively diagenetically inactive. Patchy Cu-Pb-Zn mineralization may have resulted from thermochemical reduction of anhydrite, in association with methane, and base metal-rich fluids derived from the Palaeozoic basement. During Tertiary uplift of the formation, a meteoric aquifer was established, which led to dissolution and calcitization of dolomite, anhydrite, and gypsum, and the precipitation of calcite cements. The calcite cements are associated with different types of iron and clay minerals and internal sediments. Luminescence petrography and stable isotope geochemistry demonstrate that most calcitization and calcite cementation was concentrated in two areas of the aquifer; a distal zone where fluids of elevated temperatures calcitized dolomite, gypsum and anhydrite, and precipitated pyrite, marcasite, barite and fluorite, and a proximal zone in which calcite cementation and dolomite calcitization was driven by CO₂degassing of calcite saturated, oxic groundwaters.