Geochemical and stable isotope patterns of calcite cementation in the Upper Cretaceous Chalk, Uk: Direct evidence from calcite-filled vugs in brachiopods

Abstract

The Upper Cretaceous Chalk of the British Isles is famous for its whiteness, thickness (800 m) and the magnificent cliffs it forms on the coast of Yorkshire, Kent, Sussex, Isle of Wight and Dorset. Compositionally its sediments are dominated by the low-Mg calcite skeletons of the Coccolithophoridae, usually admixed with minor but varying amounts of foraminifera, bivalves and the calcareous skeletal fragments of other marine invertebrates, as well as the siliceous skeletons of sponges and radiolaria. Traces of continentally de-

Regional hardening of Upper Cretaceous Chalk in eastern England, UK: trace element and stable isotope patterns in the Upper Cenomanian and Turonian Chalk and their significance

“…This has not been recognised in the field although there are nests of contours associated with low δ 13 C values (down to 0.6%) at Elsham and Tetford that could be related to patches of localised anoxic cementation. The model stable isotope values are comparable in their general range to those reported by Hu et al (2012) from the cement-filled cavities inside brachiopods from Speeton and Hunstanton. An indication of the changing isotopic values of the cement responsible for the hardening is provided by Terebratulid T11 from Unit A at Speeton …”

“…O values (-4.0‰ to -10.2‰) of the model cement are always negative and are comparable to the values (-4.7‰ to -11.6‰) reported by Hu et al (2012) in the cement inside brachiopods from eastern England. These authors argued that the range of values reflected increased temperature (30 to 75ºC) during cementation within marine pore fluid with little or no influence from meteoric water.…”

“…This suggests that the pore-space loss is again due to a cement with isotope values that indicate its precipitation over a range of temperatures and at stages of diagenesis comparable to those described by Hu et al (2012). The explanation of the negative values of the model trace elements in Unit B is dealt with later.…”

“…A direct indication of the trace element geochemistry and the sequence of cements responsible for the hardening is provided by the description by Hu et al (2012) of the calcite filling the vug within the body cavity of Terebratulid T11 from Unit A at Speeton (Text-fig 18). Three growth zones are recognised, the earliest (zone A; only observed but not analysed) is very thin and irregularly developed, consisting of minute non-luminescing crystals probably of a Mg-rich calcite that line the interior of the shell.…”

“…The pattern is dominated by two chalk bodies of low Sr cement -a northern one defined by the 500 ppm contour, this stretches from Speeton with the lowest Sr values (143-174 ppm) to South Ferriby and Elsham; a southern one defined by the 300 ppm contour that includes most of the sections at Tetford with the lowest value (99 ppm), South Thoresby and Welton. Hu et al (2012) from calcite cements precipitated from the early to the late stages of Chalk diagenesis in eastern England.…”