The Carboniferous Courceyan-Chadian) Sedimentary Facies Mosaic of the Keel -Ardagh Area of County Longford, Eire

  • Stuart Baskerville

    Student thesis: Doctoral Thesis


    The stratigraphic relationships and sedimentary environments of the Silurian, Devonian and Lower Carboniferous lithologies in the Keel area are deduced and described. This was accomplished by the analysis of over 10.000M. of diamond drilled core from the area.

    The oldest rocks are Silurian shales with turbidites. Deposition was in forearc basins in an active subduction zone on the north margin of lapetus.

    The area was one of net erosion in the Lower Devonian following the continental collision which closed lapetus.

    The Upper Devonian of the area is represented by the Microconglomerate lying unconformably on the Silurian. This lithology was produced by braided streams draining the immediate Keel area.

    The earliest Carboniferous sediments are an assemblage of sandstones and conglomerates.

    The Lower Quartz Sandstone was laid down in sandy braided streams. The drainage basin of the Keel rivers was considerably larger by this time and sediment was derived from western Ireland.

    Uplift in western Ireland made quartz pebbles and higher stream velocities to transport them available. These pebbles formed the Quartz Pebble Conglomerate deposited in pebbly braided streams.

    Lowering of the source area by erosion resulted in reduced sediment grain size. Sandy braided streams again dominated the Keel area and deposited the Upper Quartz Sandstone.

    Owing to the sea transgressing from the south marginal marine sediments were then deposited in the area, these being represented by the Lower Mixed Beds.

    During the advancing transgression facies belts were moving northwards. Lagoonal (Navan Micrite) deposits were laid down behind a barrier complex now represented by the Upper Mixed Beds.

    To seaward of the barrier shallow marine calcareous sediments were deposited on a homoclinal ramp. These are represented in the Keel area by the Shaley Pales and the Bioclastic Limestone Unit.

    Further to seaward Waulsortian type mudmounds were deposited on the deeper ramp. Owing to the transgressive regime existing in the Lower Carboniferous each facies belt moved northwards over the Keel area.

    At the close of the Courceyan partial subsidence of the ramp took place and basinal/slope shales with turbidites (Calp) were deposited as lateral equivilants of the shallower Oakport Limestone on the surrounding shelf remnent.
    Date of AwardFeb 1998
    Original languageEnglish
    Awarding Institution
    • Polytechnic of Wales

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