Nodular carbonates in the Lower Carboniferous, Cementstone Group of the Tweed Embayment, Berwickshire: evidence for a former sulphate evaporite facies

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Abstract
Synopsis: A small number of coarsely crystalline nodular carbonate beds of calcite and dolomite are intercalated within lacustrine sediments in the Lower Carboniferous Cementstone Group of the Tweed area. Textural and mineralogical features indicate that they represent former nodular anhydrite beds which developed in lake marginal sabkhas. The close association of these replaced sulphates with beds of cementstone (finely crystalline dolomites) favours a sabkha model for cementstone dolomitisation in which sulphate precipitation helps to promote the requisite Mg/Ca ratios in the dolomitising brines. Two main stages of diagenesis, each dominated by mineral replacement, appear to have affected the sulphate facies subsequent to its formation: initially the anhydrite was subject to partial gypsification and then, at a later stage of diagenesis, dolomite and calcite replacements of sulphate occurred.