ESCR - The Triassic System in N. Ireland

Earth Science Conservation Review

Summary Full report
The Triassic System in N. Ireland - Introduction	Antrim, Armagh, Down, Londonderry, Tyrone

Site number:	1192
Locality Type:	Various
Grid Reference:
Google maps:	,
County:	Antrim, Armagh, Down, Londonderry, Tyrone
Period:	Triassic
Stages:	Anisian, Carnian, Ladinian, Lower Triassic, Middle Triassic, Norian, Rhaetian, Upper Triassic
Lithostrat:	Mercia Mudstone Group, Penarth Group, Sherwood Sandstone Group

Site Description
Introduction:
The name 'Trias' was proposed by Alberti (1834) to cover a sequence already known in fair detail at outcrop, particularly in Lower Saxony. Central to the classification was the recognition of the Muschelkalk, a part of the succession characterised by dolomites and limestones carrying a marine fauna. The strata below the Muschelkalk were termed `Bunter', those above `Keuper', thus creating the three-fold division which is still in use and from which the system name is derived. The stages of the system are shown in the following table: TABLE TO BE ADDED
Description:
CORRELATION OF TRIASSIC STRATA The customary description of the British Trias deposits as `unfossiliferous' is inappropriate as diverse and locally abundant assemblages are known but correlation is hampered by the absence of Triassic ammonites. Palynomorphs, particularly miospores, which potentially occur in deposits representing a wide range of different but contemporaneous environments, are widespread and are the principal means of correlation. Over the last 25 years, sufficient palynological evidence has accrued to enable a beginning to be made in the objective extension of the European stage nomenclature to Britain. Palynomorphs from the isolated Permo-Triassic sequence at Kingscourt in the Republic of Ireland currently form the only biostratigraphic evidence available from the entire British Isles for the approximate location of the Permian-Triassic boundary (Warrington et al., 1980). So for practical purposes in N. Ireland the base of the Trias is located at the base of a sequence of sandstones which rest on mudstones (Connswater Marl Formation) which have evaporites at their base. Some of the Connswater Marl Formation may be Triassic in age. Although the relationships of British palynomorph assemblages to the ammonite-based stages of the European Triassic are imprecisely known, it seems likely that all the European stages are represented in the British Isles. The first occurrence of ammonites of the genus Psiloceras is the criterion for defining the base of the Hettangian and therefore of the top of the Triassic. The effect of this is that the basal few metres of the Lias (Waterloo Mudstone Formation) are regarded as Triassic in age. In this report, the whole of the Waterloo Mudstone Formation will be considered in the Jurassic section. No stratigraphically useful fossils have been reported from the Sherwood Sandstone Group in N. Ireland. The Mercia Mudstone Group is also devoid of significant macrofossils but useful miospore assemblages have been reported in Langford Lodge borehole (Manning et al., 1970), Larne No. 1 borehole (Warrington & Harland, 1975), and Port More borehole (Wilson & Manning, 1978). More recently, Warrington (1995) has described palynomorphs from the Larne No. 2, Ballytober, Ballymacilroy, Killary Glebe and Mire House boreholes. He concluded that the lower part of the group, comprising the halite-bearing sequences below the Knocksoghey Formation is Anisian in age; there are indications of the presence of Ladinian and Carnian deposits above. The Penarth Group deposits contain a predominantly shelly marine macrofauna which has been documented from Larne No. 1 borehole and exposures in east Co. Antrim (Ivimey-Cook, 1975; Griffith & Wilson, 1982), Port More borehole (Wilson & Manning, 1978), from exposures in the Belfast area (Tate, 1864) and from Langford Lodge borehole (Manning et al., 1970), and from the Mire House borehole (Fowler & Robbie, 1961). The Westbury Formation contains the bivalve Rhaetavicula contorta while the palynomorph assemblage is dominated by miospores which are Rhaetian on the basis of the character of the miospore associations and the presence of Rhaetogonyaulax rhaetica, which is the index fossil of the latest Triassic Rr dinoflagellate cyst biozone. DISTRIBUTION OF TRIASSIC STRATA IN NORTHERN IRELAND Triassic rocks outcrop in the Rathlin and Larne-Lough Neagh basins, to the north and south of the Dalradian ridge which stretches from Torr Head to the Sperrin Mountains. The Rathlin Basin succession crops out principally in eastern Co. Londonderry from near Dungiven northwards to Magilligan and Lough Foyle, and is also known from the Magilligan borehole. Farther east, in north Co. Antrim, the succession is known mainly from the Port More borehole; only small outcrops occur at Murlough Bay. The main outcrops of Triassic rocks in N. Ireland are in the Larne-Lough Neagh Basin. These crop out discontinuously in east Co. Antrim, from Cushendall south-eastwards to Larne and Whitehead, but the main outcrop is in southern Co. Antrim and north Co. Down (Manning et al., 1970; Griffith & Wilson, 1982; Smith et al., 1991) with other outcrops SW and west of Lough Neagh. LITHOSTRATIGRAPHY OF THE TRIASSIC The nomenclature used in N. Ireland follows that proposed by Warrington et al. (1980) and is shown in the following table: TABLE TO BE ADDED The formations assigned to the Triassic in N. Ireland include representatives of the groups recognised in Britain. The base of the system is arbitrarily located, in the absence of palaeonto- logical evidence, in a sequence of mudstones, overlain by sandstones, which rest upon sediments containing late Permian faunas. The mudstones are part of the `Permian Upper Marl' of Smith et al. (1974) and are unfossiliferous; the overlying sandstones are assigned to the Sherwood Sandstone Group. The Sherwood Sandstone Group is well known from outcrop and comprises deposits formerly variously referred to as `Bunter Sandstone' (Egan, 1882; Manning et al., 1970) or `Lower Keuper Sandstone' (Egan, 1884; Nolan & Egan, 1884; 1885) or both (Hull et al., 1871a; Egan, 1881; Fowler & Robbie, 1961). Sporadic references are also made to `Pebble Beds' and the `Upper Mottled Sandstone' (Hull et al., 1871b; Egan, 1881). The Sherwood Sandstone Group is largely undifferentiated except where described from the Port More borehole (Wilson & Manning, 1978), Belfast Harbour borehole (Smith, 1986), Armagh (GSNI, 1985) and Cookstown (GSNI, 1983) districts. It is very variable in thickness, from about 10m near Murlough Bay to 513m at Port More, principally in relation to the Dalradian ridge and predominantly comprises reddish brown sandstones with mudstone intercalations. Conglomerates are present locally. The deposits of the Sherwood Sandstone Group overstep those assigned to the Permian, particularly on the flanks of the Dalradian ridge and also to the west and south of Permian deposits in counties Armagh and Tyrone. The Mercia Mudstone Group is poorly represented at outcrop and is principally known from deep boreholes in Co. Antrim. The Port More borehole succession (Wilson & Manning, 1978), 653m thick, lacks halite but six formations are distinguished within the predominantly mudstone sequence. In the Larne boreholes the sequence, 967m thick, (Manning & Wilson, 1975) includes significant beds of halite within the Glenstaghey and Craiganee formations; up to 5 halite beds are present within the sequence around Carrickfergus, with some over 20m thick being mined for the rock salt. The Penarth Group comprising the Westbury and Lilstock formations, is widely recognised (Fowler & Robbie, 1961; Manning et al., 1970; Ivimey-Cook, 1975; Manning & Wilson, 1975; Wilson & Manning, 1978; GSNI, 1981) and rests non-sequentially upon the Mercia Mudstone Group. As elsewhere in Britain, it is separated from the horizon defining the base of the Jurassic (the lowest occurrence of Psiloceras) by the basal beds of the Lias Group. It is 20.55m thick in the Larne No. 1 borehole. The Westbury Formation is comprised of dark grey mudstones with thin limestones and sandstones typical of the formation elsewhere in Britain. The succeeding beds of the Lilstock Formation comprise grey, locally reddish, calcareous mudstones overlain by calcareous mudstones and shales with thin limestones, respectively comparable with the Cotham and Langport members of Britain. The latter alternating shales and limestones pass conformably upwards into similar beds in the basal Jurassic, though locally there may be a non-sequence. TRIASSIC PALAEOGEOGRAPHY During latest Permian times, northerly-sourced epeiric marine environments, in which the Zechstein and Bakevellia evaporite- bearing sequences formed, were replaced by predominantly clastic sedimentary sequences. The succeeding Triassic deposits, representing about 45 million years, are largely continental. They succeed the Permian deposits conformably and overstep them to rest unconformably upon older rocks. Over much of the region the youngest Triassic deposits represent a transgression that re- established marine environments before the end of the Period. The region lay in Laurasia, the northern part of the Pangaean supercontinent, some 15-20 deg. north of the Equator and close to the northern margin of the Tethys ocean but separated from that ocean by the Variscan mountain chain, denudation of which continued from Permian into Triassic times. In the early Triassic (early to mid-Scythian) fish remains indicate aqueous environments, the salinity of which is unclear. Foraminifera, acritarchs and tintinnids are associated with the early Triassic microfloras at Kingscourt. If in situ these indicate marine conditions which appear anomalous in a context of regional continental sedimentation. In the mid Triassic (earliest Anisian) fluvial deposition that is evident in the Sherwood Sandstone Group gave way to evaporite bearing mudstones of the Mercia Mudstone Group. The water bodies were shallow with little current or tidal disturbance. Halite was probably precipitated in shallow bodies of hypersaline water undergoing evaporation with steady replenishment from a marine source (the Tethys ocean) rather than by evaporation of deep isolated water bodies. In the late Triassic (latest Norian) crustacea (Euestheria) are indicative of fresh to brackish water conditions in ephemeral pools as the sea retreated from the area. In the latest Triassic (Rhaetian) generally argillaceous and calcareous deposits, commonly richly fossiliferous, formed following a major marine transgression and in the succeeding marine environments. These beds include the Penarth Group and the lowest few metres of the Lias Group. The ultimate extent of the transgression in the north of Ireland is conjectural (Warrington & Ivimey-Cook, 1992).
Notes:
SITE RATING A notation used to categorise each site is based on a 3, 2 and 1 star system for International, National and Local interest sites respectively. For site specific information see Key Site 1197 - Brown's Bay, Island Magee Key Site 1198 - Cloghfin Port, Island Magee Key Site 1199 - Collin Glen Key Site 1200 - Scrabo Hill - Triassic Key Site 1201 - Milltown Key Site 1202 - Drapersfield Quarry Key Site 1203 - Lissan Water Key Site 1204 - Curraghlane.

Keywords
Minerals:	Halite
Rocks:	Evaporite, Limestone, Mudstone, Sand, Sandstone
Paleoenv:	desert, sabkha, shelf

References (24)
Alberti, F.Von, 1834. Beitrag zu einer Monographie des Bunten Sandsteins, Muschelkalks und Keupers, und die Verbindung dieser Gebilde zu einer Formation., pp.366, Cotta, Stuttgart and Tubingen.
Egan, F.W., 1881. Geology of the country around Moneymore. Memoirs of the Geological Survey of Ireland.
Egan, F.W., 1882. Geology of the country around Maghera. Memoirs of the Geological Survey of Ireland.
Egan, F.W., 1884. Geology of the country around Ballymoney. Memoirs of the Geological Survey of Ireland.
Fowler, A. & Robbie, J.A., 1961. Geology of the country around Dungannon. Memoirs of the Geological Survey Northern Ireland, HMSO, Belfast.
Griffith, A.E. & Wilson, H.E., 1982. Geology of the country around Carrickfergus and Bangor. Memoir of the Geological Survey of Northern Ireland, Sheet 29 (N. Ireland).
Hull, E., Warren, J.L. & Leonard, W.B., 1871. Explanatory memoir to accompany sheets 37, 38 and part of 29 including the country around Bangor, Newtownards, Comber and Saintfield in the County of Down. Memoirs of the Geological Survey of Ireland.
Hull, E., Warren, J.L. & Leonard, W.B., 1871. The country around Belfast, Lisburn and Moira. Memoirs of the Geological Survey of Ireland.
Ivimey-Cook, H.C., 1975. The stratigraphy of the Rhaetic and Lower Jurassic in East Antrim. Bulletins of the Geological Survey of Great Britain, numb. 50, pp.51-70.
Manning, P.I. & Wilson, H.E., 1975. The stratigraphy of the Larne Borehole, County Antrim. Bulletins of the Geological Survey of Great Britain, numb. 50, pp.1-50.
Manning, P.I., Robbie, J.A. & Wilson, H.E., 1970. Geology of Belfast and the Lagan Valley. Memoirs of the Geological Survey Northern Ireland.
Nolan, J. & Egan, F.W., 1884. Geology of the country around Dungiven. Memoirs of the Geological Survey of Ireland.
Nolan, J. & Egan, F.W., 1885. Geology of the country around Limavady. Memoirs of the Geological Survey of Ireland.
Penn, I.E., Holliday, D.W., Kirkby, G.A., Kubala, M., Sobey, R.A., Mitchell, W.I., Harrison, R.K. & Beckinsale, R.D., 1983. The Larne No. 2 Borehole; discovery of a new Permian volcanic centre. Scottish Journal of Geology, vol. 19, pp.333-346.
Smith, D.B., Brunstom, R.G.W., Manning, P.I., Simpson, S. & Shotton, F.W., 1974. A correlation of Permian rocks in the British Isles. Journal of the Geological Society of London, vol. 130, pp.1-45.
Smith, R.A., 1986. Permo-Triassic and Dinantian rocks of the Belfast Harbour Borehole. vol. 18 numb. 6 British Geological Survey.
Smith, R.A., Johnston, T.P. & Legg, I.C., 1991. Geology of the country around Newtownards Memoirs of the Geological Survey Northern Ireland, pp.42 PP.
Tate, R., 1864. On the Liassic strata of the neighbourhood of Belfast. Quarterly Journal of the Geological Society of London, vol. 20, pp.103-114.
Warrington, G., 1995. The Permian, Triassic and Jurassic in N. Ireland: a palynological study with special reference to the hydrocarbon prospectivity of the Larne-Lough Neagh Basin. numb. GSNI/95/7 Geological Survey of Northern Ireland.
Warrington, G. & Harland, R., 1975. Appendix 5. Palynology of the Trias and Lower Lias of the Larne Borehole. Bulletins of the Geological Survey of Great Britain, numb. 50 In The Stratigraphy of the Larne Borehole, County Antrim., pp.37-50.
Warrington, G. & Ivimey-Cook, H.C., 1992. Memoirs of the Geological Society of London, numb. 13 In Atlas of Palaeogeography and Lithofacies.
Warrington, G., Audley-Charles, M.G., Elliott, R.E., Evans, W.B., Ivimey-Cook, H.C., Kent, P., Robinson, P.L., Shotton, F.W. & Taylor, F.M., 1980. A correlation of Triassic rocks in the British Isles. Geological Society of London Special Report, numb. 13 Geological Society of London.
Wilson, H.E. & Manning, P.I., 1978. Geology of the Causeway coast. Memoir for one-inch geological sheet 7. Memoirs of the Geological Survey Northern Ireland, HMSO, Belfast.
Wilson, H.E. & Robbie, J.A., 1966. Geology of the country around Ballycastle. Memoirs of the Geological Survey Northern Ireland.

Map(s):	GSNI (1981) 1:50,000 Geological Sheet 12 (Limavady); GSNI (1983) 1:50,000 Geological Sheet 27 (Cookstown); GSNI (1985) 1:50,000 Geological Sheet 47 (Armagh); GSNI (1994) 1:50,000 Geological Sheet 21 (Larne).
Record type:	ESCR report
Entered by:	E M Porter

Enlander, I., Dempster, M. & Doughty, P., 2026. The Triassic System in N. Ireland - Introduction, County Antrim, Armagh, Down, Londonderry, Tyrone, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=1192. Accessed on 2026-11-07.