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The Hebridean Basins and Adjacent Areas |
Department of Geology, Leicester University, University Road, Leicester, LE1 7RH, UK
Department of Geology and Applied Geology, Glasgow University, Glasgow G12 8QQ, UK
Minor element and stable isotope data are presented for calcite cements within seven decimentre-scale concretions from a single bedding plane within the Middle Jurassic Valtos Sandstone Formation of Eigg. Stable oxygen isotope data (
The concretions are shown to be clustered within the bedding plane; concretion nucleation is interpreted to have been controlled by the availability of suitable substrates within the sediment. The concretions probably nucleated upon aragonitic Neomiodon shells which had previously transformed to calcite.
18O = –9.3 to –11.1
) suggest precipitation at burial depths of between 200 and 300 m at temperatures of 31–46°C, within static pore waters of meteoric origin. Mg/Ca and Mn/Ca ratios (2.5 ± 1.2 and 0.062 ± 0.008 mole percent respectively) imply that growth was predominantly transport-controlled. An average sized concretion from the horizon (r = 11.8 cm) is predicted to have grown in 0.36–0.84 Ma. Stratigraphic constraints show concretion growth to have been delayed by at least 15 Ma after the deposition of the sediments.
