Abstract

Ooliths and pisoliths (öoids) formed of concentrically laminated hydroxy-carbonate-fluorapatite with variable proportions of calcite, siderite and goethite characterize horizons in the Cephalopod Limestones Member of Toarcian age (Upper Lias) in central England. These öoids occur, with framboidal pyrite, in micritic limestone containing reworked sideritic biocalcarenite and grains of phosphorite and micrite. The öoids are mainly spheroids and ellipsoids but some are broken or compactionally distorted. Carbonate-apatite was co-precipitated with ferric hydroxide in a high-energy environment and probably recurrently replaced primary aragonite-öoids. The phosphatic öoids were reworked together with biocalcarenite and redeposited in a lower energy, deeper, offshore environment in which diagenesis occurred under progressively lower pH and Eh conditions.

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