Petrographic examination and re-interpretation of concretionary carbonate horizons from the Kimmeridge Clay, Dorset

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Abstract
Sections through two concretionary carbonate horizons (Yellow Ledge and Maple Ledge) from the Kimmeridge Clay type locality of the Dorset coast have been examined using back-scattered imaging electron microscopy, X-ray microanalysis and X-ray diffraction. The Yellow Ledge has experienced a complex cementation history involving precipitation of several carbonate phases. Samples from both the centre and margins of this ledge contain juxtaposed low Mg, low Fe calcite and ferroan dolomite. This indicates that the ledge did not simply grow from the centre outwards during progressive burial. By contrast, the Maple Ledge was found to be compositionally and texturally homogeneous, being cemented throughout by interlocking subhedral ferroan dolomite crystals. The observations made in this study indicate that a detailed petrographic examination of concretionary layers must be undertaken if geochemical and isotopic data obtained from the analysis of bulk samples are to be interpreted correctly.