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Title: The diagenesis of the Corallian Group (Upper Jurassic), southern Dorset, UK
Author: Bishop, S. N.
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2002
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The mixed carbonate-siliciclastic sediments of the Upper Jurassic Corallian Group of southern Dorset were deposited within the Wessex Basin, in a variety of shallow marine environments. The early diagenesis of these sediments was dominated by bacterially mediated reactions taking place in a modified marine pore fluid. Corallian limestones contain early calcite cements that post-date authigenic pyrite. These calcites incorporated 13C-poor carbon released by the breakdown of organic matter by sulphate-reducing bacteria; other sources of carbon were seawater and the dissolution of marine shell material. Siderite, occurring in limestones, sandstones and mudrocks, formed within the zone of bacterial methanogenesis. Pyritferous calcite-cemented concretions in the Nothe Grit Member formed at the base of the sulphate reduction zone where anaerobic methane oxidation was taking place. Septarian cracks developed in the concretions during shallow burial; cementation of these fractures by calcite continued into deep burial. Skeletal aragonite within Corallian limestones and sandstones started to dissolve during early diagenesis, but some survived into deeper burial. Neomorphism of skeletal aragonite to calcite also started during early diagenesis and continued into deeper burial. Rhaxella sponge spicules, composed of opaline silica, began to dissolve during early diagenesis, providing silica for the silicification of allochems and early calcite cements, and for local quartz cementation. Late calcite cements within Corallian limestones were sourced from the dissolution of aragonitic skeletal grains and also from pressure solution of carbonate particles and early cements. Calcite-cemented concretions of "doggers" within the Bencliff Grit Member formed during burial diagenesis and were sourced from shell material. δ18O data for late calcite cements are consistent with precipitation from a fluid with the oxygen isotopic composition of Jurassic seawater at temperatures not exceeding the estimated maximum burial temperature of 65oC. The Corallian rocks of southern Dorset contain a complex array of different calcite veins, many of which can be shown to be related to repeated movement on extensional faults in late Jurassic-Cretaceous times and also to compressional reactivation of these structures during the inversion of the Wessex Basin in the Tertiary.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available