From Mid Argyll to Mull: the sea bed geology of the Firth of Lorne (lecture)

Dr Roger Anderton

The geology of the sea bed between Mull, Lorne and Mid Argyll has been interpreted using a new high-resolution bathymetric survey produced by SAMS (Scottish Association for Marine Science). Bedrock ridges are visible over much of the area because of the erosive action of fast tidal currents, although care has to be taken with their interpretation as glacial erosion can produce misleading structural features. As one might expect, the rock units seen on the adjacent mainland and islands are present offshore. Dalradian rocks of the Appin and Argyll Groups underlie the whole area. In the south the distribution of Dalradian formations seen onshore can be inferred, whilst in the north tighter folds and more homogeneous lithologies make interpretation much more ambiguous. Overlying the Dalradian are two areas of ORS (Old Red Sandstone); a small one SW of Kerrera which is an outlier of the Lorne Basin and a large one which underlies much of the western Firth. The latter shows a complex internal stratigraphy which is up to 1.5 km thick and probably forms a half-graben bounded by the Great Glen Fault. The Triassic/Jurassic rocks of Mull extend only a short distance offshore where they unconformably overlie the ORS, the SE coastline of Mull being approximately defined by the base of the Tertiary lava pile. In detail, the sea bed geomorphology of the area is very complex being influenced by the distribution of the bedrock units, faults, fracture systems, Tertiary and other dykes as well as by Quaternary erosional and depositional processes.

Roger has worked extensively on the sedimentology and tectonics of the Dalradian following his Ph.D. on the Dalradian of Islay, Jura and mid-Argyll. He now lives near Lochgilphead following a career at the British Geological Survey, Strathclyde University and in the oil industry with BP. He is a member of the Geological Society of Glasgow.