Sedimentation and tectonics in the Scottish Dalradian

Synopsis Sedimentological research on the Scottish Dalradian has progressed from the recognition of sedimentary structures in the 1930s, via the identification of sedimentary facies from the 1950s onwards, to the integration, in the 1970s, of sedimentological data with that from studies of stratigraphy, tectonics and volcanism. This has now led to an understanding of the pre-orogenic evolution of the Dalradian terrane in terms of progressive lithospheric stretching associated with the break-up of the Proterozoic Supercontinent. The Appin and Argyll Groups were deposited on the NW side of a late Precambrian marine gulf which developed over a complex zone of crustal thinning between the Laurentian and Baltic parts of the Super-continent. As extension accelerated, subsidence rates increased and the Dalradian area of the gulf evolved from a relatively shallow shelf into a series of turbidite basins. Thinning of the lithosphere gave rise, in Argyll Group times, to locally intense igneous activity. Subsequently, complete continental rupture along the gulf axis led to the birth of the Iapetus Ocean. By Southern Highland Group times the Dalradian terrane had become part of the new, thermally-subsiding, Laurentian continental margin. One can envisage the geometry and facies variations of many horizons within the Dalradian in terms of a pattern of numerous fault blocks defined by listric normal faults, dipping SE towards the site of continental rupture, and NW–SE trending transfer faults, which divided the gulf and subsequent margin into a series of compartments. It was movements on these faults that largely controlled Dalradian stratigraphic evolution. For example, pulses of rapid stretching, and consequent fault activity, produced basin-deepening sequences which mark the base of the Easdale and Crinan Subgroups.