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BRITAIN'S OFFSHORE OIL & GAS

*Foundations


F20: Relation of basin trend to basement 'Grain'

The Precambrian basement forms two-thirds of the British crust. The northern half (F21) was part of the supercontinent of Laurasia (Canada, Greenland and Scandinavia) and has had a long, eventful history dating back nearly 3000 million years. Most of it has been repeatedly reformed at high temperatures and pressures. The southern half, composed of less altered rocks, was a northern 'outpost' of the supercontinent of Gondwana (Africa, South America, India and Antarctica). The two halves were brought together when the Iapetus Ocean closed, creating the next tier, the Caledonides. The early Caledonides formed before Iapetus closed. They are metamorphic rocks with an incredibly complicated structure caused by repeated compression. The late Caledonides are sedimentary and volcanic rocks folded after Iapetus closed. The Norwegian Caledonides are intermediate in age and have a thrust-sheet structure unlike the British Caledonides. The North Sea is underlain at depth by all three types of Caledonides.

The Caledonian plate collisions in the Devonian Period produced a land of lakes and wide river plains in which the Old Red Sandstone was deposited. A seaway extended across southern Britain and into central Europe at this time. In the Carboniferous Period a warm coral sea flooded the Old Red Continent, but as the continent drifted northwards towards the equator, the sea retreated and humid swamplands developed, clothed in luxuriant vegetation. These, the coal forests, were periodically flooded when the polar ice-caps melted. Late in the Carboniferous Period, the southern seaway closed and Laurasia became fused to the main mass of Gondwana, with the formation of the Variscan mountain ranges - the Variscides - along the collision zone (F23). By the Permian Period all continental masses had come together to form the vast supercontinent of Pangaea which continued to drift northwards.

F21: The precambian basement F22: The caledonian basement F23: The older cover and variscides


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