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

Abstract

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 δ 18 O and δ I3 C cement compositions; (2) A 'Central Province' associated with deeper burial (approx. 550 m) but less 18 O- and 13 C-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 I8 O signature relative to seawater, at temperatures of approximately 24 °C. 13 C depletion is tentatively ascribed to the persistence of 13 C depleted soil-gas CO 2 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. 13 C depletion is believed to be controlled by the input of CO 2 derived from thermal oxidation of organic matter.