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Postgraduate Research Institute for Sedimentology, Reading University, Whiteknights, Reading RG6 2AB, UK
The Orcadian Basin of northern Scotland (largely of Middle Devonian age) is a major lacustrine rift basin, with the widely correlateable, Achanarras/Sandwick lake bed extending for at least 800 km along the rift. Above this lake bed, there are about 540 m of cyclic lacustrine sediments, with lake sedimentation terminated by alluvial fan progadation. The Upper Stromness Flagstone Formation and the Rousay Flagstone Formation making up this lacustrine sequence are given revised and precise definitions within four reference sections from the Orkney Islands. The sequence consists of about 45 first-order cycles averaging about 12 m in thickness, which resulted from long-term climatic fluctuations in rainfall with a cycle time-scale of about 25 000 years. Longer climatic fluctuations of about 100000 and 400000 years are also present. These periods correspond to the precession and two eccentricity orbital cycles.
Detailed knowledge of the stratigraphy constrains the structural and maturation history of the basin. The basin formed in a series of extensional half graben, and the Orkney area suffered later inversion. The lake sediments first reached the oil window at relatively shallow depths (c. 2000 m), probably in the early Carboniferous, implying very high geothermal gradients during the early part of the basin history as corroborated by the presence of Devonian volcanics in the region. In the Orkney area, uplift probably started in the late Carboniferous.
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