Permian stratigraphy, Northern Ireland

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Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.

W I Mitchell

Stratigraphy

Geology of the Cultra foreshore, north Co. Down; A. location map (1:50 000 Ordnance Survey sheet 15), B. geological map of the Carboniferous and Permian outcrop (7), C. detailed geological map of the Permian outcrop. Connswater Marl Formation 'Magnesian Limestone' (P947842)
Divisions of the Permian rocks in the Belfast area. (P947938)
The Permian outcrop at Cultra and the contact between the ‘Magnesian Limestone’ (foreground) and the Connswater Marl Formation (‘Permian Upper Marls’). Southeast side of Belfast Lough [J 412 809] at Cultra, Co. Down. (Hammer 36cm long). (P948005)
Configuration of terranes in the northern parts of Ireland and Britain (15). (P947789)
Comparative vertical sections of the Permian rocks in the Belfast and Newtownards areas of Co. Down (9). (P947843)
Distribution of Permian, Triassic and Jurassic rocks in northeast Northern Ireland. (P947841)
Detailed lithostratigraphy of the Permian section in the Larne No.2 borehole (12). (P947844)
Fossils 1–2 Late Permian (Lopingian) miospores from the Larne No. 2 borehole: 1 Lueckisporites virkkiae 2 Perisaccus granulosus (P948075)
Litho- and chronostratigraphy of the Permian, Triassic and Jurassic rocks. (P947937)
Distribution of Permian, Triassic and Jurassic rocks in northeast Northern Ireland. (P947841)
Lithostratigraphy of the Permian and Triassic rocks at Armagh (9). (P947939)

Belfast Lough

The Permian outcrop at Cultra (P947842), the most extensive in Northern Ireland, comprises about 13 m of strata [1] with the Permian section in the Belfast Harbour borehole [2] nearly 135 m thick (P947938). Fossils are rare in the Belfast Harbour Evaporite Formation in the borehole. At Cultra, the Magnesian Limestone consists of 9 m of yellow, coarse-grained dolomite but cannot be assigned to one particular member in the Belfast Harbour Evaporite Formation (P947938). At Cultra, shells are either dispersed or occur in layers up to 0.3 m thick composed of the bivalve Bakevellia (B.) binneyi and gastropods. The junction with the succeeding Connswater Marl Formation is sharp and conformable (P948005).

During the Early Permian, rivers draining high ground on the Southern Uplands-Down-Longford Terrane (P947789) deposited clastic detritus (Enler Group) on alluvial fans. The overlapping Late Permian marine transgression initially deposited fine-grained sandstone and siltstone of the Musgrave Clastic Member. Succeeding carbonate beds accumulated on a coastal fringe with fluctuating water depths and salinity. Near-shore shoals of oolitic and shelly carbonate sands protected areas of algal mat and laminated micrite mud on a sabkha. Hot, evaporitic climatic conditions resulted in the crystallisation of primary anhydrite, gypsum and halite. Sediment of the Connswater Marl Formation washed from the early Palaeozoic landmass that still basked in a hot arid climate.

Newtownards

Permian rocks are exposed at only one locality in this area but the succession is established in boreholes [3]. The rocks were deposited in the Newtownards trough and show lithological differences with the adjacent Belfast area [4]. The rocks consist entirely of coarse clastic and argillaceous lithologies and, in the absence of a correlative of the Magnesian Limestone, are undated (P947843).

East Co. Tyrone

The poorly exposed Permian outcrop at Grange (P947841) is better known from boreholes and consists of three units [5]. The ‘Basal Sands’, 0.1–7.8 m of reddish brown pebbly sandstone, the fossiliferous (bivalves, gastropods and bryozoa) ‘Magnesian Limestone’ (18–23 m) with a lower dolomitic siltstone overlain by dolomitic and oolitic limestone and the ‘Permian Upper Marls’, less than 10 m thick.

Armagh

Information on the unfossiliferous Permian rocks in this outlier (P947841), (P947939), is mostly from boreholes [6]. A Permian age is unsubstantiated and is based on the historical identification of the ‘Magnesian Limestone’. The only exposure is of 3.5 m of the Drumarg Conglomerate Formation on the west side of the Keady Road [H 871 439], south of Armagh. The outcrop of presumed Triassic rocks is confined to the northern edge of the outlier but other exposures of the constituent formations do occur in the main Triassic outcrop in north Co. Armagh.

Larne No. 2 borehole

At least 1265 m of Permian strata [7] are recorded in the borehole (P947844). The lowest strata of the Enler Group, the Inver Volcanic Formation [8], consist of about 60m of breccio-conglomerate and sandstone overlain by 554m of heavily altered lavas and tuffs. Though difficult to classify petrographically they range from basaltic to trachyandesitic and trachytic [9]. It has been suggested [9] that the source of the Larne lavas, and the other trachyte-dominated lava centre within the Midland Valley of Scotland in East Lothian, are located over a sub-surface extension of the Southern Upland Fault. They are overlain by 440 m of arenaceous sediments of the Ballytober Sandstone Formation.

The overlying Belfast Group is divided into the ‘Magnesian Limestone Formation’ (14 m) that has a 6 m bed of anhydrite at the top and White Brae Mudstone Formation [10], which includes a 113 m thick halite at its base. A miospore assemblage [11] including Lueckisporites virkkiae (P948075) Fossil 1 and Perisaccus granulosus (P948075) Fossil 2 indicates a Wordian (Middle Permian, Guadalupian) to Late Permian (Lopingian) age (P947937). The precise position of the Permian-Triassic system boundary in Northern Ireland is not known.

North Co. Antrim

At the base of the Portmore borehole, north of the Tow Valley Fault (P947846), unfossiliferous conglomeratic sandstone (51 m) is succeeded by dolomitic mudstone and siltstone (14 m). These rocks probably correlate with the Enler Group and may be Early Permian.

After the transitory marine transgression, in which the Belfast Group was deposited, continental conditions became the major control on Late Permian and Triassic climates in northwest Europe.

References

  1. Griffith A E, and Wilson, H E. 1982. Geology of the country around Carrickfergus and Bangor. Memoir of the Geological Survey of Northern Ireland, Sheet 29 (Northern Ireland).
  2. Smith, R A. 1986. Permo-Triassic and Dinantian rocks of the Belfast Harbour Borehole. Report of the British Geological Survey, 18, No. 6.
  3. Smith, R A, Johnston, T P, and Legg, I C. 1991. Geology of the country around Newtownards. Memoir of the Geological Survey of Northern Ireland, Sheet 37 and part of sheet 38 (Northern Ireland).
  4. Geological Survey of Northern Ireland 1989. Newtownards, Northern Ireland Sheet 37 and part of sheet 38. Solid Geology. 1:50 000. (Southampton: Ordnance Survey for the Geological Survey of Northern Ireland).
  5. Fowler, A, and Robbie, J A. 1961. Geology of the country around Dungannon. Memoir of the Geological Survey of Northern Ireland, Sheet 35 (Northern Ireland).
  6. Geological Survey of Northern Ireland 1985. Armagh, Northern Ireland Sheet 47. Solid Geology. 1:50 000. (Southampton: Ordnance Survey for the Geological Survey of Northern Ireland).
  7. Penn, I E, Holliday, D W, Kirby, G A, Kubala, M, Sobey, R A, Mitchell, W I, Harrison, R K, and Beckinsale, R D. 1983. The Larne No. 2 Borehole: discovery of a new Permian volcanic centre. Scottish Journal of Geology, 19, 333–46.
  8. Geological Survey of Northern Ireland 2001. Ballymena, Northern Ireland Sheet 20. Solid Geology. 1:50 000. (Keyworth, Nottingham: British Geological Survey).
  9. 9.0 9.1 Harrison, R K, Styles, M T, Penn, I E, and Davis, A E. 1985. Petrology of Lower Permian volcanic rocks from the Larne No. 2 (Geothermal) Borehole, Co. Antrim, Northern Ireland. Report of the British Geological Survey, 17, No. 6.
  10. Geological Survey of Northern Ireland 2001. Ballymena, Northern Ireland Sheet 20. Solid Geology. 1:50 000. (Keyworth, Nottingham: British Geological Survey).
  11. Warrington, G. 1995. The Permian, Triassic and Jurassic in Northern Ireland: a palynological study with special reference to the hydrocarbon prospectivity of the Larne-Lough Neagh Basin. Geological Survey of Northern Ireland Technical Report GSNI/95/7.