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Journal of the Geological Society

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The origin of late spar cements in the Lincolnshire Limestone, Jurassic of central England

¹ The British Petroleum Company plc, Britannic House, Moor Lane, London EC2Y 9BU, UK
² Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK
³ Department of Geology, University of Liverpool, Brownlow Street, P.O. Box 147, Liverpool L69 3BX, UK
⁴ Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

The carbon and oxygen isotopic compositions of the Lincolnshire Limestone (Bajocian) of the East Midlands are shown to be determined largely by two end-member components: marine precipitates and later sparry burial cements. The origin of major quantities of such burial cement is controversial, the extreme possibilities ranging from calcite precipitation from meteoric groundwaters to precipitation from highly saline brines, with contrasting flow regimes.

This study demonstrates how formation-wide sampling and mapping of stable isotopic compositions of burial cements from the Lincolnshire Limestone, combined with a detailed knowledge of its burial history and associated thermal evolution, can be employed to distinguish the influence of different water types. Two 'Provinces' are differentiated with respect to the stable isotopic compositions of the latest, volumetrically most significant, ferroan burial cements.

(1) A 'Southern Province' associated with thinning of the Limestone, shallower depths of maximum burial (approx. 300 m) and light ¹⁸O and ^I3C cement compositions;

(2) A 'Central Province' associated with deeper burial (approx. 550 m) but less ¹⁸O- and ¹³C-depleted cement compositions.

The relative isotopic depletion of the Southern Province ferroan burial cements is considered to be a function of their precipitation from meteoric waters, with a depleted ^I8O signature relative to seawater, at temperatures of approximately 24 °C. ¹³C depletion is tentatively ascribed to the persistence of ¹³C depleted soil-gas CO₂ in the precipitating waters.

In the Central Province, basinal waters of both meteoric and marine derivation are considered to have precipitated the late spar, with associated up-dip fluid movement at temperatures between 29 and 39 °C. ¹³C depletion is believed to be controlled by the input of CO₂ derived from thermal oxidation of organic matter.

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