Carboniferous of County Londonderry, Northern Ireland
|Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.|
W I Mitchell
The outcrop of the Roe Valley Group and the Tyrone Group is separated by the Tow Valley Fault (P947816).
The Roe Valley Group (P947930) crops out on the west side of White Mountain and Mullaghmore (P947816) and along the south side of Lough Foyle. The Spincha Burn Conglomerate Formation rests on the Dalradian and the lower part (P947997) of the succeeding Barony Glen Formation in Barony Glen [C 731 016] is dominated by calcrete, mudstone and siltstone, interpreted as pedogenic and lacustrine deposits. The upper part consists of grey mudstone and thin limestone deposited in a marginal marine environment. The Roe Valley Group contains the miospores Schopfites claviger and Auroraspora macra, of late Tournaisian (CM Biozone) age.
The outcrop of the Tyrone Group occurs in the Draperstown district. The basal Iniscarn Formation crops out in the Altalacky River (P947816). At its base [H 746 981], boulder conglomerates representing half the Formation contain rounded clasts up to 2 m long. The upper part is exposed on Slievemoyle and consists of feldspathic breccias. It fines up into the succeeding Altagoan Formation with the Drumard and Mormeal members (P947930). The Mormeal Member, seen south of Altagoan Bridge [H 799 944], consists of mudstone and channelised sandstone . It includes five evaporite beds up to 0.43 m thick with halite and gypsum or anhydrite now pseudomorphed by calcite . Grey mudstone with a macrofauna of bivalves and ostracods also contains the miospores Auroraspora macra, Schopfites claviger and rare Lycospora pusilla (Fossil 3) indicating the basal Pu Biozone and an early Chadian age.
The local unconformity between the Tyrone and Armagh groups (P947930) is equivalent to 2000 m of early Chadian to late Asbian strata in Co. Fermanagh and south Co. Tyrone. Sandy limestone of the Desertmartin Limestone Formation contains corals and the late Asbian brachiopod Gigantoproductus cf. semiglobosus in the exposure at Cullion [H 813 909] (P947816). Exposures of the Meenymore Formation (Leitrim Group) occur some 40 0m to the south.
- Geological Survey of Northern Ireland 1983. Cookstown, Northern Ireland Sheet 27. Solid Geology. 1:50 000. (Southampton: Ordnance Survey for the Geological Survey of Northern Ireland).
- Cameron, I B, and Old, R A. 1997. Geology of the country around Cookstown, Co. Tyrone. Geological Sheet 27 (Cookstown). Geological Survey of Northern Ireland. Technical Report GSNI 97/7.