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Department of Geology, School of Geosciences, The Queen’s University of Belfast, Belfast, BT7 1NN, UK
The North Channel seaway trends NW-SE between Scotland and Northern Ireland and is underlain by the Larne and Portpatrick Basins (Fig. 1). The nearby Clyde Basin is split into the northeast and southwest Arran Troughs. All are structurally complex, with evidence for Caledonian, Variscan and Tetiary tectonic activity on a crustal scale. In addition, the Southern Uplands and Orlock Bridge major strike-slip faults underlie the basin, so an understanding of their kinematics is critical in assessing the controls on basin development.
Triassic sediments outcrop in the North Channel as a result of basin inversion, with the stratigraphy following the common succession of a lower, arenaceous-dominated Sherwood Sandstone Group, overlain by the saliferous/anhydritic Mercia Mudstone Group (Fig. 2). Permian sediments consist of an upper marl unit (Upper Permian Marls), underlain by a dolomitic limestone (Magnesian Limestone), and subsequently an arenaceous unit (Lower Permian Sandstone).
Permian volcanicity is of importance in the area: the discovery of 617 m of volcanics in the Larne No. 2 borehole (Penn et al. 1983) suggests that they are more than local in their extent, and are a direct indication of rift initiation. They have been isotopically dated at 245 ± 13 Ma, which is around 40 Ma younger than stratigraphically equivalent lavas in southwest Scotland and Islay (Fig. 3), possibly suggesting more than one rift event in the region. However, the stratigraphic evidence tends to suggest that this date is in error, as they underlie more than 600 m of Permian (i.e. older than 248 Ma) clastic and evaporitic
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