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

Abstract

the history of research into the cementation of the Upper Cretaceous Chalk of the UK is reviewed. Calcitefilled vugs within the shell cavities of terebratulid brachiopods from the Cenomanian Chalk of eastern england have been investigated by cathodoluminesence imaging, staining, electron microprobe and stable isotope analysis. this has provided the first detailed analysis of the geochemistry of the Chalk’s cement. two cement series, suboxic and anoxic, are recognized. both start with a Mg-rich calcite with positive δ13C values considered to have been precipitated under oxic conditions influenced by aerobic ammonification. the suboxic series is characterized by positive δ13C values that became increasingly so as cementation progressed, reaching values of 3.5‰. Manganese is the dominant trace element in the earlier cement, iron in the later cement. Mnand fe-reducing microbes influenced cement precipitation and the trace element and δ13C patterns. the anoxic series is characterized by δ13C values that became increasingly negative as cementation progressed, reaching values of –6.5‰. trace elements are dominated by iron and manganese. sulphate-reducing microbes influenced cement precipitation and the trace element and δ13C patterns. both cement series are related closely to lithofacies and early lithification pre-dating the regional hardening of the Chalk. the suboxic series occurs in chalk which was continuously deposited and contained hematite pigment and limited organic matter. the anoxic series was associated with slow to nil deposition and hardground development in chalks that originally contained hematite pigment but no longer do so, and an enhanced supply of organic matter.