Facies and sequential organisation of a mudstone-dominated slope and basin floor succession: the Gull Island Formation,Shannon Basin,Western Ireland |
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| Institution: | 1. Norsk Hydro Research Centre, P.O. Box 7190, N-5020 Bergen, Norway;2. Roger Walker Consulting, 83 Scimitar View, Calgary, Alta., Canada;3. Delos, Knowl Wall, Beech, Staffordshire ST4 8SE, UK;1. MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing, 100037, China;2. China Deep Exploration Center-SinoProbe Center, China Geological Survey & Chinese Academy of Geological Sciences, Beijing, 100037, China;3. National Research Center for Geoanalysis, Chinese Academy of Geological Sciences, Beijing, 100037, China;4. Institute of Geology, Chinese Academy of Geological Sciences, Beijing, 100037, China;1. Dr. Moses Strauss Department of Marine Geosciences, Charney School of Marine, Sciences University of Haifa, Israel;2. Geological Survey of Israel, Jerusalem, Israel. 30 Malkhei Israel Street, Jerusalem, 95501, Israel;3. Department of Geophysical, Atmospheric and Planetary Sciences, Tel Aviv University, Israel;1. Qatar Carbonates and Carbon Storage Research Centre, Department of Earth Sciences and Engineering, Imperial College London, Prince Consort Road, London, SW7 2BP, United Kingdom;2. Department of Earth Sciences and Engineering, Imperial College London, Prince Consort Road, London, SW7 2BP, United Kingdom |
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| Abstract: | The lower part of the Carboniferous Shannon Basin of Western Ireland contains a deep-water succession which exceeds 1200 m in thickness that comprises five lithologically different units deposited within a confined, relatively narrow basin: (i) a calciclastic debris-flow and turbidite unit formed by resedimentation from nearby carbonate platforms, (ii) a siliciclastic black shale succession with former source potential which onlaps basin margins (Clare Shales), (iii) a sandstone-dominated turbidite formation, controlled by ponded accommodation and deposited axially in the basin (Ross Formation), (iv) a mudstone-rich turbidite-bearing succession, which onlaps basin margins (lower Gull Island Formation), and (v) a mudstone-dominated prograding slope succession (upper Gull Island Formation and lower Tullig Cyclothem), which grades transitionally upwards into deltaic deposits. The top unit records progradation at a time when basin differential subsidence had diminished significantly and local basin topography did not control deposition. The two upper mudstone-dominated units are different in terms of both sandstone content and their genetic significance within the overall basin-fill, and their potential relevance as reservoir analogues.The lower part of the Gull Island Formation contains three principal facies associations: (a) shallow turbidite channels and sheets representing channel margin and levee deposits, (b) mud-rich slumps, and (c) less than 1 m thick, rare, hemipelagic shales. More than 75% is deformed by soft-sediment deformation, but only to a smaller degree affecting sandstone units. The turbidites record transport to the ENE, along the axis of the basin, while the slumps were derived from an unstable northern slope and transported transversely into the basin towards the southeast. The distribution of turbidite sandstone and slumps is inversely proportional. Sandstones decrease in importance away from the basin axis as slumps increase in number and thickness. The lower part of the Gull Island Formation is interpreted to record progressive fill of a deep basin controlled by local, healed slope accommodation with onlap/sidelap of the basin margins. The instability resulted from a combination of fault-controlled differential subsidence between basin margin and basin axis, and high rates of sedimentation.The upper part of the Gull Island Formation is entirely dominated by mudstones, which grade upwards into siltstones. It contains rare, up to 15 m thick, isolated channels filled by turbidites, showing transport towards the east. The upper part records easterly progradation of a deep-water slope genetically tied to overlying deltaic deposits, and controlled by regional accommodation.The contrasts between the lower and upper parts of the Gull Island Formation show that onlapping/sidelapping turbidite successions have reservoir potential near basin axes, but that prograding deep-water slopes are less likely to have reservoir potential of significance. A suggested regional downlap surface between the two parts is a significant break and marker in terms of reservoir potential. |
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