Lisnaskea, Fermanagh-south Tyrone area, Carboniferous, Northern Ireland

From Earthwise
Jump to: navigation, search
Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.

W I Mitchell

Lisnaskea

Lithostratigraphy of the Tyrone Group in the four regions of Co. Fermanagh-south Co. Tyrone. (P947933)
Distribution of Carboniferous rocks in Co. Fermanagh - south Co. Tyrone. (P947821)

Tyrone Group

The outcrop of the Tournaisian (Courceyan-early Chadian) section of strata is located mainly in Counties Cavan and Monaghan in Ireland [1][2]. The basal Fearnaght Formation (P947933) rests unconformably on Ordovician rocks[2].

Despite the absence of the Mullaghmore Sandstone and Glencar Limestone formations in the area between Lisnaskea and Newtownbutler, Tyrone Group strata between the Ballyshannon Limestone and lower Dartry Limestone formations (P947933) are 1400 m thick compared to 1300 m in the Glennoo Borehole (P947821).

Loose blocks, and a small exposure, of the Ballyshannon Limestone Formation on the shore of Upper Lough Erne at Derryvore [H 341 229] are rich in corals including Clisiophyllum spissum, Koninckophyllum fragile, Siphonodendron martini and S. sociale and the brachiopod Pustula pyxidiformis. The upper ‘member’ of the Formation crops out at Mullynagowan [H 425 265] 1 km northeast of Newtownbutler. They are overlain by 20 m of grey to fawn grainstone of the Newtownbutler Limestone Member [3], the highest part of the Ballyshannon Limestone Formation in this area. Above the exposed conformable base [H 426 266] of the succeeding Bundoran Shale Formation the lowest beds belong to the Mullynagowan Sandstone Member and consist of 2 m of calcareous silty sandstone containing Chondrites and Rhizocorallium burrows. A coral fauna from near the base of the Benbulben Shale Formation is dominated by the solitary species Clisiophyllum vesiculosum, Koninckophyllum cyathophylloides, Siphonophyllia garwoodi and S. caninioides, and is associated with Lithostrotion and Holkerian foraminifera including concavus stage Paraarchaediscus. This coral fauna is more typically found in the Bundoran Shale Formation where it is associated with mid- to late Arundian foraminifera. In the Lisnaskea-Newtownbutler area the contact between the Dartry Limestone Formation and Benbulben Shale Formation is exposed in a road cutting [H 362 337] at Lisnaskea. The Dartry Limestone consists of bedded, fine-grained limestone with thin mudstone and grey-black chert. Near the top of the Formation the Ballagh Limestone Member consists of 6m of crinoidal grainstone and is exposed [H 404 310] in Rockfield quarry.

Leitrim Group

The disconformable contact between the Dartry Limestone Formation and overlying Meenymore Formation is exposed at the top of Rockfield quarry. Near the middle of the Meenymore Formation the Carnmore Sandstone Member [3] comprises about 75 m of pale greyish fawn, very coarse-to medium-grained pebbly sandstone. The youngest, Alderwood Mudstone Formation, crops out [4] in the stream at Tircar [H 486 470], 4.3 km ESE of Fivemiletown (P947821). It consists of fossiliferous mudstone, laminated (stromatolitic) limestone, peritidal limestone, bioclastic limestone and calcareous siltstone. The early Brigantian age is based on the occurrence of the trilobite Paladin bakewellensis.

References

  1. Geological Survey of Northern Ireland 2004. Lisnaskea, Northern Ireland Sheet 57 and 58. Solid Geology. 1:50 000. (Keyworth, Nottingham: British Geological Survey).
  2. 2.0 2.1 Geraghty, M. 1997. A geological description of Monaghan - Carlingford to accompany the Bedrock Geology 1:100 000 Scale Map Series, sheet 8 and 9, Monaghan and Carlingford, with contributions by J Farrelly, K Claringbold, C Jordan, R Meehan and M Hudson. Geological Survey of Ireland (GSI), 60pp.
  3. 3.0 3.1 Mitchell, W I. 1995. Geological description of the area around Lisnaskea and Newtownbutler, Co. Fermanagh. Part of 1:50 000 Geological Sheet 57 (Lisnaskea). Geological Survey of Northern Ireland. Technical Report GSNI 95/8.
  4. Geological Survey of Northern Ireland 1982. Enniskillen, Northern Ireland Sheet 45. Solid Geology. 1:50 000. (Southampton: Ordnance Survey for the Geological Survey of Northern Ireland).