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Title: The sedimentology and diagenesis of the Charlestown Main Limestone (Brigantian), eastern Midland Valley of Scotland
Author: Pickard, Neil A. H.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1990
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Abstract:
The Charlestown Main Limestone, of the Dinantian Lower Limestone Group (Brigantian), in the eastern Midland Valley is generally the thickest marine limestone developed within a basin fill otherwise dominated by fluvio-deltaic sediments and volcanics. Field descriptions, enhanced and refined through the collection of samples and subsequent petrographic microfacies analysis, were used to define and map facies, including those comprising buildup complexes. The regional distribution of these facies was found to be controlled by (i) differential subsidence and (ii) the geographical location of two major penecontemporaneous fluvio-deltaic systems. Carbonate buildups present within the Charlestown Main Limestone are restricted to central Fife, an area of reduced subsidence (the Burntisland High) during the Brigantian. Buildup geometry and local facies mosaics reflect the relative water depth and position on the Burntisland High which, because of its slight topographical relief and more distal position with respect to the location of fluvio-deltaic systems, became an area of increased carbonate productivity, away from this structure, in the Midlothian Low, more monotonous argillaceous limestone facies, of low-energy and somewhat deeper water aspect, accumulated. Influxes of terrigenous mud associated with delta progradation from the NE curtailed carbonate deposition earlier in eastern than in western areas and accounts for the thinning of the Charlestown Main Limestone towards East Fife. Moreover, differential subsidence north of the Pentland-Firth of Forth fault zone allowed a thick sequence of prodeltaic sediments to accumulate above the Charlestown Main Limestone in Fife, while delta top sediments succeeded the limestone in Midlothian. Significant diagenetic modification of the Charlestown Main Limestone began immediately after deposition, and continued through shallow and intermediate burial into the deep bural réime. Using a range of integrated petrographic and geochemical techniques, broadly comparable cement stratigraphies (designed Stage A, B etc.) have been identified at several localities. These cement stratigraphies enabled the relative timing of other diagenetic events such as hydrocarbon migration, styolitization, fracturing and dolomitization to be placed within a diagenetic history related to the burial of the Charlestown Main Limestone. The earliest phases of diagenesis, while the Charlestown Main Limestone was still influenced by pore fluids of marine derivation, included several phases of dissolution (of biogenic silica and carbonate) which locally (i.e. within buildup facies) produced large volumes of secondary porosity. Stage A cement, comprising sub-isopachous layers of inclusion rich calcite, are best developed within these early dissolution voids. They are considered to represent altered fibrous or acicular marine cements. Clear, blocky to prismatic calcites succeed Stage A. While the exact sequence of cements varies between each locality, the cements are, nevertheless, considered to have formed during shallow burial from phreatic waters initially of sea-water composition which were gradually diluted by mixing with fresh water associated with delta advance and development of a meteoric lens. Geochemical variation within these cements reflects not only mixing with fresh water associated with delta advance and development of a meteoric lens, but also progressive burial through oxic, suboxic and then reducing zones in the shallow subsurface.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.660600  DOI: Not available
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