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Onshore and Offshore North of Ireland |
Department of Geology, Trinity College, Dublin 2, Republic of Ireland
Permian and Mesozoic rocks are of limited distribution in Ireland, except for the Ulster Basin in the northeast of the island. This contrasts with the offshore basins, the widespread Mesozoic and Tertiary sequences of which are briefly reviewed. Onshore Permo-Triassic red-bed rocks occur in the Kingscourt Graben and in Wexford. Jurassic red clays are recorded in karstic solution hollows in Carboniferous limestones in south County Cork, and Upper Cretaceous chalk in a karstic collapse feature at Ballydeenlea, near Killarney. A growing number of Tertiary non-marine fissure and collapse deposits are being found, and where dated are of Oligocene and Neogene age. The stratigraphy and setting of the post-Carboniferous outliers are discussed. Published organic maturation data suggest that high maturation values in the Carboniferous over much of the country can be attributed to high heat flows during the Variscan episode, and to a cover of Silesian rocks. Post-Variscan burial depths appear to have been modest.
Available evidence suggests that Ireland has been a positive and largely emergent feature for much of post-Variscan time. Permo-Triassic deposits were probably never widespread onshore. The Early Jurassic and Late Cretaceous were periods of marine submergence, although rocks of this age were subsequently largely removed by erosion. Low topographic relief and the protective layers of Lias and Chalk have combined to restrict the amount of erosion of the landscape, the essential elements of which had been etched out by the end of the Triassic. The scattering of Mesozoic and Tertiary deposits at low topographic levels argue against the traditional view of the Irish landscape as a product of erosion of a high-level peneplain.
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