Kesh-Omagh, Fermanagh-south Tyrone area, Carboniferous, Northern Ireland
|Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.|
Carboniferous rocks in this area rest unconformably on the Dalradian to the north and in the south are faulted (P947821) . The Omagh Sandstone Group and Tyrone Group ( (P947927) (P947933) ) are separated by an unconformity. Newly described formations and clastic members in the Tyrone Group help resolve the cause, effect and timing of marine transgression and regression and reflect the proximity of the northern shoreline in early and mid-Viséan times.
Omagh Sandstone Group
Uplift, folding and erosion in the late Courceyan and early Chadian reduced the outcrop of the Omagh Sandstone Group which consists of strata deposited prior to, and during, a marine transgression. The basal unconformity in Killyclogher Burn [H 473 744] is overlain by 100 m of unfossiliferous red sandstone with calcrete nodules and pebbles of white quartz. The section in the Glendurragh River [H 275 671] near Lack includes channel sandstone and siltstone and mudstone with Courceyan-early Chadian (CM Biozone) miospores. Thin algal (cyanobacterial) limestone, evaporite replacement textures, serpulid worm traces and crinoidal sandstone with rare brachiopods are also present. A small outlier [H 559 807] of unfossiliferous and undated reddish brown conglomerate and coarse-grained sandstone, overlying the Dalradian of the Central Highlands (Grampian) Terrane in the hanging wall of the Omagh Thrust Fault, may belong to this Group  (P947815).
In the north of this area the sub-Tyrone Group unconformity and Claragh Sandstone Formation (P947933) overstep the Omagh Sandstone Group westwards and rest on the Dalradian. Coarse-grained, pebbly lithic arkose of the Claragh Sandstone Formation, with channels and planar cross-laminations, form the twin ridges [H 213 740] 250 m southwest of Scraghy quarry. Its late Chadian age is confirmed by miospores including Lycospora pusilla in mudstones and of mono- and bilaminar Koninckopora in an oncolitic boundstone near the top of the Formation. The succeeding Termon River Limestone and Bin Mountain Sandstone formations (P947933) are partly contemporaneous (P947823).
The upper part of the Termon River Limestone Formation is exposed in the Giants Grave section [H 190 731] (P947821) and contains the late Chadian foraminifera Eoparastaffella but no archaediscids and the coral Dorlodotia pseudovermiculare. At Binnawooda Spring [H 247 769] the Formation comprises 1.2 m of mudstone, siltstone and peritidal limestone (P947823). The overlying Bin Mountain Sandstone Formation is bioturbated with a similar micro- and macrofauna and is late Chadian, except in the upper part where the early Arundian foraminifera Glomodiscus, Uralodiscus and Viseidiscus umbogmaensis are present. Scattered pebbles of vein quartz occur in the lowest 12 m of strata but at Tullyard [H 301 764] 5.5 km to the east, are restricted to the 4 m thick Tullyard Conglomerate Member . The Rushindoo Oolite Member crops out at the western end of the Formation outcrop [H 159 719], 8 km north of Kesh, and consists of 6 m of oolitic limestone and calcareous sandstone (P947936).
In Drummahon Burn [H 238 762] (P947821) 5m of strata at the contact between the Bin Mountain Sandstone Formation and Ballyshannon Limestone Formation contain foraminifera and the corals Dorlodotia pseudovermiculare, Dorlodotia cf. briarti and Siphonodendron martini which define the position of the Chadian-Arundian and early to mid-Arundian boundaries. The upper 60m of the Ballyshannon Limestone Formation (P947936) is exposed in Moffatt’s quarry [H 316 717], 3 km southwest of Drumquin (P947821), overlain by 40 m of the Bundoran Shale Formation (P948004). The Crockanaver Limestone Member is exposed in quarries at the west end of the ridge at Carn [H 222 635], 1.5 km south of Ederney (P947936). In contrast, the laterally equivalent Drumowen Sandstone Member is only developed near Drumquin. The Ederny Limestone Member represents the top of the Ballyshannon Limestone Formation and outcrops widely in the Kesh-Omagh area .
The Bundoran Shale Formation (P947936) thins northwards from 100 m in the Kesh-Ederney area to 40 m at Drummahon Burn. At its base the Skea Sandstone Member thickens northwards from 1.4 m of sandy, bioclastic limestone in Ederney quarry [H 224 640], to 4.5 m of sandstone at Shanvin [H 212 733] and at least 20 m of conglomerate at Drummahon Lane [H 227 761], 13 km NNE of Kesh. The conglomerate consists of vein quartz and quartzite clasts in an orange-stained, coarse-grained feldspathic sand matrix. Dolomitised limestone clasts contain mid- to late Arundian corals. The sandstone is absent in Moffatt’s quarry and 3m of mudstone, siltstone and fenestral micrite with rhizoliths mark the base of the Bundoran Shale Formation.
The basal beds of the Mullaghmore Sandstone Formation (P947936) in contact with the Bundoran Shale Formation in White Glen [H 292 731] are orange-stained sandstone of the Dromore Sandstone Member (30 m). The Member is also exposed in the nearby roadside quarry [H 293 731] as 12 m of green to fawn micaceous sandstone with parallel laminations. The southerly inclined dip-slope at Tullynashammer [H 187 713] represents the top surface of this Member, overlain by black mudstone and algal limestone of sabkha and lagoonal environments. The Drumskinny Sandstone Member (40 m) consists of fawn to white sandstone with rare brachiopods and burrows. The upper part of the Formation in the Bannagh River [H 169 675] includes thin limestones with the late Arundian foraminifera Eoparastaffella, Eotextularia and Paraarchaediscus at involutus stage.
The basal bed of the overlying Drumchorick Siltstone Formation (P947936) in the Bannagh River [H 169 675] is a bioturbated calcareous silty sandstone with brachiopods. Higher limestones with corals possessing features of both Siphonodendron and Lithostrotion, a ‘cerioid tendency’, and foraminifera including Eotextularia and Paraarchaediscus at involutus and possibly concavus stages, occupy a position just below the Arundian-Holkerian boundary . Crinoidal limestone turbidites are exposed at Drumkeeran [H 207 670], 2.5 km northwest of Ederney.
A rich macrofauna in the Benbulben Shale Formation includes brachiopods and colonial and solitary corals. A limestone bed located 4m above the base in a stream [H 209 675] 4.3 km northeast of Kesh contains the coral Lithostrotion portlocki and late Arundian foraminifera Pararchaediscus at involutus stage. Higher strata contain Holkerian foraminifera. The youngest formation, the Tubbrid Sandstone Formation, is probably Holkerian (P947936).
- Geological Survey of Northern Ireland 1995. Omagh, Northern Ireland Sheet 33. Solid Geology. 1:50 000. (Keyworth, Nottingham: British Geological Survey).
- Geological Survey of Northern Ireland 1997. Northern Ireland. Solid Geology (second edition). 1:250 000. (Keyworth, Nottingham: British Geological Survey).
- Geological Survey of Northern Ireland, 1995. Draperstown, Northern Ireland Sheet 26. Solid Geology. 1:50 000. (Keyworth, Nottingham: British Geological Survey).
- Geological Survey of Northern Ireland 1994. Kesh, Northern Ireland Sheet 32. Solid Geology. 1:50 000. (Keyworth, Nottingham: British Geological Survey).
- Riley, N J. 1993. Dinantian (Lower Carboniferous) biostratigraphy and chronostratigraphy in the British Isles. Journal of the Geological Society, London, 150, 427–46.