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Diagenesis of the Kimmeridge Clay Formation, onshore UK

Department of Geology, University of Sheffield, Sheffield S3 7HF, UK

Study of sedimentology, carbonate geochemistry and clay mineralogy of four laterally continuous beds from the outcrop of the Kimmeridge Clay Formation indicates strong lateral variations in the depositional and early diagenetic processes, reflecting the basin and swell topography of the depositional basin.

The mudstones from the swell environment with low sedimentation and subsidence rates are shelly and carbonate–rich with thin, poorly developed oil shales. They were deposited under dysaerobic conditions with consequent poor organic-matter preservation. Early diagenetic processes were limited to the sulphate reduction and methanogenesis zones with the formation of discrete horizons of concretions. The shallow burial depths attained are reflected by the immaturity of the organic matter in the mudstones although ordered illite-smectites are present.

Sediments from other locations comprise black, organic-rich mudstones, the thick beds and good organic-matter preservation indicating deposition under high sedimentation and subsidence rates with anoxic bottom waters in a basinal environment. These indurated mudstones contain a few shell fragments and much pyrite, indicating intense reaction in the sulphate reduction zone. Carbonates were precipitated in the methanogenesis and decarboxylation zones as ferroan calcites and dolomites. The much greater burial depths are indicated by the higher maturity of the organic matter and by the ordered nature of the illite–smectites and the loss of kaolinite.

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