Investigating sediment source, supply and evolution of the Northwest Carboniferous Basin: A multi-proxy provenance approach
Anders, Bébhinn
Anders, Bébhinn
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Publication Date
2022-06-14
Type
Thesis
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Abstract
Provenance analysis investigates the origin of sediment and its pathways into sedimentary basins. Such analysis must consider certain external controls (e.g. location and scale of the transportsystem feeding a basin, climate, topography) and active processes (e.g. sedimentary mixing and storage), which affect sediment from initial generation, through transport, to eventual burial and diagenesis. This study investigates both spatial and temporaltrends in provenance signals, and, by implication, sediment supply,using the intracratonic Northwest Carboniferous Basin (NWCB) in Ireland as a study site, with particular focus on theMullaghmore Sandstone Formation (MSFm).This mid-Viséan siliciclasticunit represents afluvial/deltaic systemdepositedduring a regional lowstandwithin a largely carbonate-dominant basin filling sequence. The extent to which depositional environment, manifested in sedimentary facies (e.g. shoreface facies compared to channel sandstone facies), influences the provenance signals recorded in sedimentary rocks can be directly addressed. An integrated approach assessing field sedimentology and multiple geochemical techniques has been employed. Four lithostratigraphic sectionsthrough the MSFm across northwest Ireland were logged, sampled and analysed using a multi-proxyprovenanceapproach. Optical microscopy, U-Pb zircon geochronology,apatite U-Pb geochronology, trace elements in apatite and Pb-in-K-feldspar analysis are used to constrain the sources supplying sediment to the NWCB and the processes by which these source signals might be modified.Sedimentology, paleoflow data and geochemical dataallsuggest the sources of the MSFm layto the north/northwest.TheArchean–PaleoproterozoicLewisianComplexand equivalent Nagssugtoqidian Beltof Greenlandcomprise the oldest and most distal sources.Paleoproterozoic–Mesoproterozoic sources such asthe Rhinns Complex, bathymetrichighs offshore Ireland and Scotland(e.g. Rockall Bank),and the Annagh Gneiss Complex (or an equivalent) arethe main sediment contributors. Neoproterozoic metasedimentary sources, especially the Dalradian Supergroup, delivereda significant proportion of recycled sediment to the basin. Finally, sources associated with the Caledonian Orogenic Cycle such as the granites of the Donegal Batholith and associated lamprophyre dykes comprise the youngest, and likely most proximal sources.Provenance signals arenot homogenous across the MSFm, with highest variability between the most western and eastern sections, suggesting that the depositional system feeding the basin comprised of multiple discrete fluvial systems. Variations in overall depositional environment (i.e. terrestrially influenced, proximal to source versus distal shoreface environments) likely play a role in the amount of mixing the sediments experienced prior to final deposition. Additionally, provenance signals vary temporally through the stratigraphyof the MSFm. Five successive channel sandstone facies, from one location, display fluctuating sediment supply within each channel unit. These fluctuations are attributed to a combination of a monsoonal climate, repetitive wildfire events (which decrease sediment stability and increase erosion in the hinterland), catchment expansion,or basement unroofing and channel migration.Pb-in-K-feldspar analysis allows for the identification of first cycle sediment populations and has the ability to discern sources of similar U-Pb ages which zircon, for example, cannot differentiate. Combined U-Pb apatite geochronology and trace elementsoffers information on both source age and lithology and is extremely powerful in identifying recycled grain populations. Zircongrains with both core and rim agescaneffectively record multiple phases of magmatism and metamorphism. Core–rim analysis has helped decode the complex crustal reworkinghistory of a hinterland which in Viséan times was locatednear the margin of the ancientLaurussiancontinent.Evidence of the amalgamation and break-up of the palaeo-supercontinents Nuna, Rodinia and Laurussia is recorded in zircon grains from the MSFm. Such detailed intra-grain analysis has also aided in the precise identification of discrete source terranes.This study highlights the intrabasinal variability in sediment supply within a medium-sized basin, in spatial, temporal and depositional environmental terms. Integrated sedimentological assessment combined with a multi-proxy provenance approach employing both stable and labile mineral phases is essential for identifying all source terranes in any provenance investigation, and more accurate synthesisofthe provenance data obtained.
Publisher
NUI Galway