Provenance of Triassic sandstones in NW European Margin Basins - reconstructing palaeogeography using a multi-proxy approach
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Provenance studies endeavour to unravel the history of a sediment, from investigating potential source rocks, to assessing the processes which may have affected sediment during weathering, transport, burial and diagenesis, to evaluating sediment supply over time. The range of information acquired from provenance studies aid reconstruction of source to sink models. These models can help better predict reservoir sandstone distribution and quality in the sub-surface and also help reconstruct palaeogeography. Lower-Middle Triassic sandstones in NW European margin basins form several proven hydrocarbon reservoirs; Wytch Farm oil field in the Wessex Basin, Morecambe gas field in the East Irish Sea Basin, various oil and gas fields in the North Sea, and the Corrib gas field, in the Slyne Basin offshore west of Ireland. As a result, these basins have been extensively studied such that Triassic palaeogeography on and offshore Britain is reasonably well constrained. However, peripheral basins such as the Slyne Basin, offshore western Ireland and the various basins across Northern Ireland are not as well understood. Previous provenance analysis of the Triassic sandstones in the Slyne Basin revealed supply from the north of the basin. These results suggest that basins offshore western Ireland were supplied with sediment from a large-scale fluvial system separate to the Budleighensis river system which flowed north from the Variscan Uplands delivering detritus to the basins on and offshore Britain. The nature and position of the divide between these two Triassic sediment supply systems is unknown. Provenance analysis of the basins across Northern Ireland may offer further insight into Triassic palaeodrainage in NW European margin basins and may help constrain the location of a potential drainage divide. This study applies a multi-proxy approach, utilising standard petrographic analysis, Pb-isotopic analysis of K-feldspar, U-Pb zircon and apatite geochronology, and trace element geochemistry, to investigate the provenance of the Lower-Middle Triassic sandstones in the Slyne Basin and basins across Northern Ireland. Detailed optical petrography provides an insight into the nature of the sandstones in these basins, in particular the diagenetic processes which may have affected the sediment during deposition and burial. This information provides an understanding of the sedimentary environment where these sandstones were deposited and also provides a foundation for the provenance information gathered by the other more sophisticated techniques. Provenance analysis of Lower-Middle Triassic sandstones in the Slyne Basin, offshore western Ireland, utilising U-Pb apatite and zircon geochronology and Pb-isotopic analysis of K-feldspar, revealed mixed Archaean-Proterozoic sources, as well as a minor Caledonian source for the Triassic sandstones in the Slyne Basin. U-Pb zircon and apatite geochronology also revealed a Permo-Triassic source which was not identified by the Pb-isotopic analysis. These results imply palaeodrainage from sources in NW Scotland and Greenland, with minor input from the Irish Massif, confirming that a southerly flowing fluvial system existed offshore Ireland. Although a definitive source is difficult to identify for the Permo-Triassic material, there is evidence that potential sources existed to the north, and detritus may have been supplied to the Slyne Basin through a series of linked basins. The discrepancy between the results from each method emphasises the need for a multi-proxy approach to provenance studies. U-Pb apatite geochronology, apatite trace element geochemistry and Pb-isotopic K-feldspar analysis of Triassic sandstones in basins across Northern Ireland also revealed mixed Archaean-Proterozoic sources with significant input from Variscan sources also detected. Furthermore, the provenance signal showed intermittent switching between these two source areas, to the north and south respectively, over time. These results imply that the basins across Northern Ireland were supplied with sediment by two fluvial systems flowing north and south respectively, with the southerly system probably being an extension of the Budleighensis river system. This infers that the basins across Northern Ireland may have been temporarily connected to the basins on and offshore Britain during the Triassic. In this way provenance results from the Slyne Basin and basins across Northern Ireland provide fresh insight into Triassic palaeodrainage and also help reconstruct and constrain palaeogeography for NW Europe during the Middle-Upper Triassic, while highlighting the importance of a multi-proxy provenance approach which utilises signals from sand grains with contrasting 1) abundance in source areas; and 2) stability in the sedimentary environment.
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