Discussion on evidence for a major Neoproterozoic orogenic unconformity within the Dalradian Supergroup of NW Ireland

Abstract

Geoff Tanner writes: Firstly, I would like to congratulate the authors for presenting a very carefully documented, and splendidly illustrated, account of the origin of the Stralinchy conglomerates. The aim of this contribution is to focus on one important question: is their controversial model for the Donegal Dalradian supported by studies of the stratigraphically equivalent Dalradian rocks of the SW Highlands of Scotland? If an orogenic unconformity is present in the Dalradian Supergroup, evidence for this should be found at the base of, or within, the Easdale Subgroup in this region. The field relationships seen at this stratigraphical level in Scotland are reviewed briefly, and an attempt made to reconcile the results with the Donegal model. Jura Quartzite–Easdale Subgroup relationship in SW Highlands of Scotland. The contact between the Jura Quartzite and the Scarba Conglomerate Formation (which represents the base of the Easdale Subgroup) is best seen in excellent coastal exposures between Jura and Loch Creran. It is exposed on the SE side of the island of Jura; is excised by faulting on the islands of Scarba and Lunga; and re-appears on the SE side of the Rubha Aird Peninsula, 7 km north of Oban. At Lussagiven on Jura [NR 6387 8666], the top of the Jura Quartzite is marked by cross-bedded quartzite followed by 23 m of thinly bedded, commonly gritty, sandstones with conglomerate lenses. These beds, which display cross-lamination, graded bedding and channel structures, pass upwards with perfect conformity into a 36 m thick unit of laminated slate with sandstone ribs, the Jura Slate member of the Scarba Conglomerate Formation. Gritty sandstone and conglomerate beds up to 1 m thick, that characterize the Scarba Conglomerate, appear 280m farther up sequence They contain pebbles to 3 cm; rip-up clasts of mudrock; and less common rafts of sandy sediment up …