A revised boundary between the Scoor Pelitic Gneiss and the Lagan Mòr Formation in SW Mull?

J Faithfull

The Moine rocks of SW Mull have played a key role in our understanding of the Moine sequence in the NW Highlands. The Mull sequence was thought uniquely, to contain a continuous sedimentary sequence relating the Morar and Glenfinnan units seen elsewhere in Scotland. However, recent work by Krabbendam et al. (2021) and others, suggests that the Mull sequence does contain a major tectonic break, analogous to the Sgurr Beag Thrust of the mainland. This has implications for the existing stratigraphy used on Mull which have not previously been considered. In particular, the Lagan Mòr Formation, as currently shown on published geological maps, has little or no intrinsic integrity as a unit. It spans two sequences of unrelated rocks, separated by a major tectonic break. This work looks at this contact in detail, and suggests that the stratigraphic definitions of the Scoor Pelitic Gneiss and the Lagan Mòr Formation are modified, so that the boundary between these units coincides with the tectonic break.