Conference Contribution Details
Mandatory Fields
J. Murray
43rd Irish Geological Research Meeting.
Enigmatic Striped Limestones from the late Visťan of the Shannon Basin, southwest Ireland.
University College Cork
Poster Presentation
Optional Fields
18-FEB-00
20-FEB-00
A distinctive unit of striped limestone (Parsonage formation, late Visťan) occurs over a wide area within the Shannon Basin, southwest Ireland. The 'stripes' consist essentially of alternating light and dark laminations of carbonate. The lamination varies from planar to crinkly: localised folding and brecciation is common. The paler bands tend to be thicker and are composed predominantly of micro-spar. The thinner, darker interlayers are finer grained and contain concentrations of dispersed organic material, intercrystalline clay minerals, framboidal pyrite and fine-grained silica. Grumuleuse textures, probably microbial in origin, are common in the dark bands. The striped limestones appear to be largely barren of fossils, except for the rare occurrence of ostracodes. Striped limestone is particularly well exposed near the top of the succession at Monument Hill Quarry near Lixnaw in northwest Co. Kerry. Dark calcilutitic limestones and shales with rarer crinoidal calcarenites dominate the section here. Features such as graded bedding, large-scale sedimentary slumps and breccias, commonly containing derived mud-mound clasts, suggest a basinal setting with local slopes. The striped limestones are underlain and overlain by such basinal facies. They themselves exhibit folding, brecciation and at some horizons even occur as rafts. It is possible that they were once evaporites, which have since been replaced by carbonate. The enclosing facies suggest a deep water origin, but a bed of oncoids, immediately below the striped unit, is probably of shallow water origin. †
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