Petrological studies of Devonian rocks in Scotland and cretaceous rocks in Canada
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Sedimentological, petrological and geochemical studies have been carried out to analyse the process of calcretization and to understand the influence of environment of deposition on sandstone diagenesis. The sediments examined come from Lower Old Red Sandstones (Lr. Devonian) of eastern Scotland and hydrocarbon bearing Viking, Cardium and Belly River Formations (Cretaceous) of southcentral Canada. Some scepticism has been expressed regarding the occurrence of displacive calcite but this type of growth has been found in the calcrete profiles in Lower Old Red Sandstones of Carnoustie, east Scotland. This thesis presents fresh petrographic evidence of expansion and isolation of clastic grains by growing calcite crystals. The morphology of calcite crystals observed under cathodoluminescence provide compelling evidence that they are not a passive pore filling cement but have grown in confined spaces from supersaturated solutions by displacing the constraining medium. Field relationships suggest that the calcite was generated within the vadose zone where crystallization was promoted by rapid surface evaporation leading to supersaturation. A two-water model based on petrological and geochemical criteria has been suggested to explain the mechanism of growth of displacive calcite crystals (Chapter - IV). The recognition of displacive calcite is highly significant as it offers insight into paleoclimatic conditions and diagenetic history. Crystal morphologies observed under cathodoluminescence and microprobe data suggests that the displacive calcite was originally low Mg-calcite and grew from rapidly evaporating fresh pore waters with extremely low Mg/Ca ratios. The growth of originally low Mg-calcite and absence of palygorskite, sepiolite and dolomite in the Carnoustie calcretes is considered significant and it has been emphasized that no one process applies to all calcretes and the chemistry of pore waters and micro-environmental conditions within the pores are major significant factors controlling process of calcretization. The comparisons drawn between the diagenetic products observed in the Lower Old Red Sandstone and the Viking, Cardium and Belly River Formations suggest that the detrital mineralogy and aqueous solutions migrating through the pore system of sandstones are the two major factors that accomplish all the complex chemical reactions during diagenesis. In general, the Viking, Cardium and Belly River sediments show large variations in their pre-burial early diagenetic histories within small areas (even between adjoining wells) while the Lr. Old Red Sandstone show a monotonous paragenetic sequence over the entire area. This is compatible with the expected variations in the pore-fluid chemistry and micro-environmental conditions within the pores deduced from the depositional environments of these sediments. Hence it has been concluded that the pre-burial early diagenetic processes and products in clastic sediments are controlled by their environment of deposition which subsequently could influence the course of late stage diagenesis as well.
Thesis, PhD Doctor of Philosophy
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