The Bowland Shale (UK): development of new sedimentological and diagenetic models for a Palaeozoic fine-grained succession

UoM administered thesis: Phd

  • Authors:
  • Sarah Newport

Abstract

Organic-rich mudstones are complex in terms of their sedimentology and diagenesis. Here, a multi-disciplinary approach was used to focus on understanding the active depositional processes during the mud-rich sedimentation, variations in organofacies within a mud-rich slope succession and the complexity of the paragenesis of organic-rich mudstone diagenesis using the Bowland Shale (Carboniferous, Craven Basin UK). A total of eleven lithofacies, comprising of claystone (calcareous and argillaceous), mudstone (calcareous, argillaceous and siliceous), siltstone (calcareous, siliceous and authigenic), sandy mudstone (calcareous and siliceous), sandstone (micaceous) and matrix supported conglomerate, are identified using macro- and micro-scale analysis of variation in composition, texture and sedimentary features of two mud-rich successions (Marl Hill-11 and Preese Hall-1 boreholes). All lithofacies are dominated by turbidites and/or debrites. Interpreted depositional environments vary from a sediment-starved slope, a slope dominated by turbidites and a slope dominated by debrites (marginal succession) to a lateral peripheral lobe fringe, a frontal peripheral lobe fringe and a frontal lobe fringe (basinal succession). Within the slope succession, two end-member organofacies were determined based on the abundance of the 56 kcal/mol activation energy peak derived from One-Run-Fixed-Arrhenius kerogen kinetic analysis: an absence (or < 5 %) indicating ‘organofacies A’, which contains the highest proportion of algal material and, therefore, Type I kerogen and a presence (> 15 %) indicating ‘organofacies B’, which contains the highest proportion of sporomorphs and, therefore, Type II kerogen. Fifteen types of authigenic minerals have been identified. Early diagenetic precipitates include calcite, quartz, pyrite, barite, kaolinite and phosphate. Burial diagenetic precipitates include calcite, ferroan and non-ferroan dolomite, quartz, illitisation of smectite, kaolinite, chlorite and Pb-Zn mineralisation. Diagenetic processes include coupled dissolution-precipitation reactions, sulphate reduction, methanogenesis, clay mineral transformations and organic matter catagenesis. Burial diagenetic precipitates of calcite, dolomite, quartz and Pb-Zn mineralisation in mudstones, sandstones and limestones within the basin are inherently linked (both compositionally and temporally) indicating open-system geochemical behaviour.

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Original languageEnglish
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Award date31 Dec 2019