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.) oc cur in a range of depositional facies and morphological associations. Most siderites are associated with ill situ glaucony or occur beneath conspicuou s lithological breaks. These associations illustrate the importance of slow deposition for early diagenetic carbonate precipitation. Siderite in the E ocene sediments of Whitecliff Bay is impure and in many cases comprises zon ed 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 temper atures have been calculated from siderite delta(18)O data and are based on the assumption that precipitation occurred from seawater. These temperature s are reasonable for typical microbial siderite precipitated in marine sedi ment but are slightly low for marine siderite where there may have been met eoric overprinting. If the precipitation temperature for siderite cements i s assumed (from delta(18)O data for time-equivalent biogenic carbonate) rat her than the composition of the precipitating fluid, the calculated isotopi c composition of the precipitating fluid is generally compatible with the i nferred marginal marine depositional environments.