Argyll Group, Dalradian Supergroup, Central Highlands (Grampian) Terrane, Northern Ireland
|Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.|
M R Cooper and T P Johnston
The oldest Dalradian rocks in the Sperrin Mountains belong to the Newtownstewart Formation and are exposed in the core of the recumbent Sperrin Fold (P947793) and (P948093). They consist of pale grey, thickly bedded, quartzose psammite with thin pelite interbeds which outcrop between Ballynamallaght and Newtownstewart, with some of the best exposures in quarries at Letterbrat [H 471 923], 2 km northwest of Plumbridge, and Cashty Wood [H 371 814] in the Baronscourt Estate.
The Dungiven Limestone Formation (P947914), the highest formation in the Argyll Group, is a regionally important marker which correlates with limestone formations in northeast Co. Antrim, Co. Donegal and Scotland. The formation is lithologically diverse and although limestone is a major constituent it is associated with pelite, semi-pelite, psammite, quartzite, basaltic pillow lavas and volcaniclastic sediments. All of these lithologies are exposed on the east flank of Butterlope Glen [H 493 947], 5 km north of Plumbridge (P947961). A lithologically distinctive limestone composed of black calcite crystals, up to 1cm in diameter, within clear calcite vein-breccia matrix occurs in a disused quarry in Banagher Glen [C 669 048] 6 km SSW of Dungiven (P947962), and in the banks of the River Roe [C 693 082] upstream of Dungiven Priory. Primary bedding in the limestone is defined by lines of graphite particles that were slightly disrupted by early diagenetic and near surface crystal growth and by variations in the amount of clastic sediment present.
Basaltic pillow lavas occur at several localities. The best example is at Craig [H 523 981], about 8 km northeast of Plumbridge, where the pillows, individually up to 1 m in diameter and consisting of fine-grained, equigranular, metabasalt, retain the characteristic triple junction, chilled margins, gas vesicles and radial fracture pattern. They probably equate with lavas in the Dungiven and Banagher Glen areas of the Sperrin Mountains and are correlated with lavas at Artigarvan and Strabane .
- Hartley, J J. 1938. The Dalradian rocks of the Sperrin Mountains and adjacent areas in Northern Ireland. Proceedings of the Royal Irish Academy. 44B, 141–71.