Geochemistry of early siderite cements from the Eocene succession of Whitecliff Bay, Hampshire Basin, UK

Siderite cements in the Eocene of Whitecliff Bay (Hampshire Basin, U.K.) occur in a range of depositional facies and morphological associations. Most siderites are associated with in situ glaucony or occur beneath conspicuous lithological breaks. These associations illustrate the importance of slow deposition for early diagenetic carbonate precipitation. Siderite in the Eocene sediments of Whitecliff Bay is impure and in many cases comprises zoned crystals, with an overall decrease in substitution of Ca, Mg, and Mn for Fe from crystal core to rim. Trends in Ca, Mg, and Mn from early to later siderite vary between samples, with no evident environmental control. Siderite concretions without calcite cement occur only beneath lithological breaks where meteoric water may have been introduced. Precipitation temperatures have been calculated from siderite d18O data and are based on the assumption that precipitation occurred from seawater. These temperatures are reasonable for typical microbial siderite precipitated in marine sediment but are slightly low for marine siderite where there may have been meteoric overprinting. If the precipitation temperature for siderite cements is assumed (from d18O data for timeequivalent biogenic carbonate) rather than the composition of the precipitating fluid, the calculated isotopic composition of the precipitating fluid is generally compatible with the inferred marginal marine depositional environments.