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Northern Ireland is Geologically Quite Special

Courtesy of Dr Rob Raine GSNI– lecture notes from BGS presentation (15th March 2021)

Northern Ireland is geologically quite special. It has examples of rocks from almost every time period, but all within a small geographical area. These rocks also represent many different environments, from tropical seas, deserts and even volcanic eruptions. Although there is a little bit of rock from most time periods, there is also an abundance of rocks from four geological time periods; Precambrian metamorphic rocks, Silurian marine rocks, Carboniferous rocks and Palaeogene lavas.

With many time periods, comes many gaps in time and most of these are related to different periods of uplift and erosion, associated with mountain building (orogenic) events, with rocks getting preserved during periods of sea-level rise or formation of rifts.

When we look at how rocks from each period are distributed on a geological map, the picture is complex. Large areas are represented by rocks from the four common time periods, with more rare rocks that represent other time periods preserved as slices, slithers and downfaulted areas, or barely poke out along the fringes of regions that have subsided over millions of years.

Whilst the magnetic maps show the faults clearly, the gravity shows the deeper structure and when looking at Northern Ireland several structural trends can be seen.

The principal ones are NE-SW and these represent the juxtaposition of terranes during the closure of the Iapetus Ocean and the ensuing Caledonian mountain building event.

The terranes mirror the structural makeup of Scotland.

Basement rocks

The basement rocks can be divided into 4 groups based on age or origin and mark the assembly of Northern Ireland crust during closure of the Iapetus Ocean.

Basement 1

The oldest rocks In Northern Ireland are probably Mesoproterozoic in age. Their precise age is not known, but because the rocks have experienced higher grade metamorphism than the Dalradian rocks in Northern Ireland they are presumed to be significantly older, probably Grenvillian. The rocks include banded gneissose psammites of the Lough Derg Group, found north of Belleek, Co. Fermanagh.

Basement 2

Younger, Neoproterozoic rocks of lower metamorphic grade comprise the Dalradian Supergroup, which represents a continuation of the Grampian terrane from Scotland. This is well exposed at Torr Head in the northeast and in the Sperrin mountains. The rocks are continuous underneath the younger Antrim lavas.

Basement 3

The third basement terrane The Midland Valley terrane in NI comprises a little bit of oceanic crust that was accreted via ophiolite obduction during the Grampian orogeny and co-eval Late Ordovician and early Silurian sediments that possess a fauna similar to Scottish Midland valley localities such as Girvan. The rocks are preserved in fault bound slices at Pomeroy and Lisbellaw that were uplifted to shallower depth during the Variscan orogeny.

The rocks of this terrane are distinct from the Southern Upland terrane because they have not been metamorphosed.

Basement 4

To the south of this terrane, the final suite of basement rocks forms the southern upland-Down Longford terrane and represents a late Ordovician to early Silurian succession of turbidites and shales. These are thought to represent an accretionary prism accumulating sediment on the Laurentian margin.


The bit of Devonian in Northern Ireland lies within fault bounded relicts of continental basins along the equivalent of the highland boundary fault. They are found in counties Co. Armagh and NE Co. Antrim. The Fintona Block in Co. Armagh is an area of uplifted Devonian.

This constitutes an inverted continental basin underlain by the Dromore gravity high. The gravity high is likely to be an extension of the Tyrone ophiolite complex or a concealed Palaeogene ultrabasic layered intrusion like Slieve Gullion. 


The second area of Devonian rocks is smaller but better exposed on the coast at Cushendall and Cushendun, Co. Antrim. The Devonian conglomerates of the Cross Slieve Group are well exposed in the Caves of Cushundun. To the South of the Devonian rocks a unit of younger sandstones and breccias are not well dates, and the Red Arch Formation was originally assigned to the ‘upper old red sandstone’ but is probably latest Carboniferous or Early Permian, based on palaeomagnetic data.

Carboniferous and the Variscan orogeny

The Carboniferous rocks across Northern Ireland are variable across Northern Ireland and there are well defined facies belts and different basins.


Tectonic movements of the Variscan orogenic cycle and sea-level fluctuations associated with polar ice sheets had a massive impact on the duration and frequency of time gaps and also on the resulting sedimentary basin fill. 

In Northern Ireland there is little resemblance between the present Carboniferous outcrop and the configuration of the original sedimentary basins. Significant disruption to the Carboniferous lithostratigraphy is related to seven separate episodes of tectonic activity that define important stages in the evolution of the area and illustrate the variability of Variscan deformation.

Most of the carboniferous contains cycles of marine shales, deltaic sandstones and then carbonate platforms or a combination of these. In Fermanagh where the most complete carboniferous sedimentation is preserved these comprise the Tyrone, Armagh and Leitrim Groups. The carbonates of the Gelncar and Dartry Limestone formations have led to the most impressive landscapes and are the best known being exposed in the Fermanagh highlands and the Cuilcagh Lakelands UNESCO Global Geopark.

The top of the Leitrim Group is marked by a change to deltaic sedimentation of the ‘Millstone Grit’/coal measures and the Ballycastle Group. These rocks have been the source of coal mining in Northern Ireland. Only Westphalian A is present, but younger strata may have survived the Variscan orogeny in the basin depocentres as some westphalian B mudrocks are found in one of the basins that was inverted during the Variscan.

The Variscan orogeny brought about an end to sedimentation within carboniferous basins across Northern Ireland. In the SW of Ireland, rocks lay within the Variscan front and were intensely folded, cleaved and thrusted. Northern Ireland however, experienced gentle folds and major fault movement. Thermal maturation studies show the decreasing northeast trend. The faults activated along pre-existing Caledonoid trends and uplift of the Lack and Lisbellaw inliers either side of the Fintona Block Devonian occurred during this time. 

Moving on to the stratigraphy. It is a given that most of the geological record is composed of missing strata as we can see in this chronostratigraphic chart for the NE Atlantic rift system. 

Under typical conditions of base-level rise and fall only a fraction of geological time may actually be represented by accumulated sediment. This was emphasized by Ager (1973) who remarked that “the stratigraphic record is more gap than record.” and time gaps occur in an apparently fractal pattern from brief pauses to large unconformities. 

The records onshore are nowhere near as complete as the offshore basins and it could be said that over time Northern Ireland has suffered from a chronic lack of accommodation space or has had more than its fair share of uplift, but despite this, its presence on both the Iapetus and the Atlantic margin however has given it a little bit of rock from most time periods.

Map of Mines at Ballycastle
Ballyvoy Mine, Ballycastle Coalfield

Thick Permian and Triassic and the formation of sedimentary basins

Following the Variscan orogeny, a period of tectonic extension caused rift basins across much of NW Europe and in the NE of Northern Ireland, basins opened again along Caledonoid trends, but they were also controlled by older NNW-SSE and N-S lineaments. Rifting occurred during the Early Permian and then again during the Early Triassic. By late Triassic and Jurassic times, the basin bounding faults were less active, although the basins continued to accumulate sediment and there may have been an element of thermal subsidence.


The Permian stratigraphy in these basins follows a consistent broad pattern, but within that is quite variable. The Larne basin is somewhat of an anomaly, as pronounced rifting resulted in thick volcanic and volcaniclastic material. Besides this most basins have a lower Permian series of breccias overlain by sandstones (Enler Group). Then there is an unconformity overlain by Upper Permian marine carbonates and evaporates followed by floodplain and lacustrine mudstones (Belfast Group).

Deposition of the conglomerates of the Enler Group was controlled by nearby fault scarps along Palaeozoic basement rocks and deposits grade laterally to sandstones. They can be hundreds of metres thick, with deep boreholes in Antrim showing that it reaches more than 550 m in thickness.

Transgression occurred in Northern Ireland at a stage where topography was subdued and equates to the EZ1 cycle marked here by advancement of the Bakevillia Sea and deposition of the ‘magnesian limestone’ now called the Belfast Harbour Evaporite Formation.

The BHEF is a succession of dolomitic micrites, oolites with evaporate cycles. In the centre of the Larne Basin there is also between 100-200 m of pure halite. The outcrops of the mag limestone are not as impressive as the cliffs in Co. Durham, and only a little bit is exposed on the muddy foreshore of Belfast Lough, most of the best material seems to have been removed by the Normans to use in Carrickfergus castle across the water.

Arid climates persisted into the Triassic and large fluvial systems developed in the basins. Fluvial sandstones and then red mudstones with salt were deposited. The large river systems and connected basins led to more uniformity in the stratigraphy however and the Sherwood Sandstone Group and the Mercia Mudstone Group are widespread. Northern Ireland was at the intersection of two long distance sediment supply routes, the northern system carrying older Laurentian derived sediments and the Budleighensis River system, with a largely Variscan derived sediment source.

The best exposure in Northern Ireland is at Scrabo Hill where the sandstones are preserved in quarries beneath a Palaeogene sill.

The SSG is up to 650 in Larne 2

Overlying this unit, like the rest of the UK are Triassic mudstones of the MMG, up to 950 m in Larne 2. In the Larne Basin we have deposits of halite (rock salt). This Northern Irish halite lies at the far reaches of the larger saline basins that formed across Europe.

The mine at Kilroot produces 500,000 tonnes annually.

Permian ‘brockram’ from the GSNI Ballyalton No.3 Borehole drilled near Newtownards, Co. Down in 1958.
Scrabo Tower

A Little Bit of Jurassic

And now, for me as a fossil collector we come to my personal favourite rocks (the Jurassic). Fringing the Antrim plateau at the margins of the Permo-Triassic basins the boundary between the Triassic and Jurassic is preserved. In the SW are of the plateau it has been removed by post Jurassic uplift and erosion but it is well exposed around the coast of NE Antrim.

There is a good record across the T-J boundary strata, recording the change from continental lacustrine of the MMG to open marine of the Lias Group, with a well preserved Penarth group of restricted marine sediments in between. Typically only 100-200 m of Lias is present above the boundary but up to 560 m is recorded in the centre of the Rathin basin and preserving the youngest sediments of Pliensbachian age.

Recent studies have provided high resolution records across the T-J boundary interval of organic isotopes, microfossils, palynology and ammonites.

But the Jurassic doesn’t go higher than the Pliensbachian in any of the basins. Other basins offshore have a more complete record, even the outcrops in the Scottish Islands.

The Jurassic rocks are locally rich in fossil remains.

Mitchell, 2004

And marine vertebrates, such as the ichthyosaur from Minnis mudflow. Nicknamed the Minnis monster. On one of the Jurassic exposures around the Co. Antrim coast, 20 years of searching by one fossil collector has produced three dinosaur bones. A Scelidosaur, a Megalosaur and an unidentified fragment.

Chalk seas, uplift and eruption

Following the Early Jurassic there was a big stratigraphic break and all we know is that the Jurassic was at some point uplifted and tilted because an angular unconformity exists beneath the Upper Cretaceous rocks. This is probably the result of a series of non-orogenic episodes that are termed the Cimmerian uplift.

No individual section through the Cretaceous is very thick, but all the records can be pieced together, and it shows that sedimentation extended into the latest part of the Cretaceous (Early Maastrichtian).

The lowest unit of Cretaceous age is the Hibernian Greensand Formation. Here the greensand is younger than equivalent rocks in Great Britain (Cenomanian as opposed to Albian). The succession is similar in age to Cretaceous in Scotland, but the chalk facies persisted for longer in Antrim. 

And overlying this, the Ulster White Limestone Formation is divided into 14 members, the chalk in Northern Ireland. It contains lots of small unconformities but on the whole, presents a good record of sedimentation from the Santonian to the Early Maastrichtian. To call it a chalk is somewhat misleading since it is well compacted and now has a specific gravity of 2.6 g/cc compared to 1.95. 5% porosity rather than 50%.

Northern Ireland has an array of beautifully exposed Palaeogene volcanic rocks and features. Most notable is the UNESCO World Heritage Site, the Giant’s Causeway.


Co.Antrim - Part of the North Atlantic Igneous Province

The unconformity with the underlying cretaceous represents around 5 million years of uplift, karstification and weathering of the chalk before the Antrim Lava Group was erupted on the land surface. The clay with flints formation is the product of that weathering.

Giants Causeway
Cliffs of vent deposits, Carrick a rede rope bridge, N Antrim

Northern Ireland lies on the margin of the NAIP. This represents an area of flood basalts and other volcanic rocks associated with a hotspot called the Icelandic plume. During the Palaeogene the plume was located under NI and it has the oldest recorded lavas and intrusions of the igneous province.

The volcanic rocks form part of a more localised group called the British and Irish Palaegene Igneous Province (BIPIP), which Includes Mull, Skye, Ardnamurchan, Arran and Northern Ireland. Out of these, Northern Ireland has the most extensive area of onshore extrusive volcanics. These lavas form the imposing Antrim Plateau, stretching from cave Hill near Belfast to Binevenagh in Co. Londonderry and forming a rugged coastal landscape in east and north Antrim. The lavas display fantastic examples of igneous textures and contain numerous zeolites, some of which were first named in Northern Ireland (e.g. garronite and gobbinsite). The eruption of the basalts was initially associated with explosive activity on the North coast and vents are filled by agglomerate and associated with tuffs over several kilometres extent. The site of Carrick-a-rede is a good example of one of these vents.

The Interbasaltic Formation is a lateritic and bauxitic fossil soil horizon resulting from long duration weathering of the basalt surface during a pause in eruptions of around 1 million years. The mining of this deposit for iron ore became an extensive industry in Co. Antrim from the mid-1800’s. The mines declined in the 1920s but during WW2 when the war effort required bauxite for the production of aircraft. With Co. Antrim being the only known source in the UK, many of the mines were reopened producing almost 300,000 tonnes of ore from 1941-1945.

Sitting within the Interbasaltic Formation in Mid Antrim, the acidic volcanic rocks of Tardree (rhyolite dome) and Sandy Braes (obsidian volcanic agglomerate) are surrounded by pyroclastic deposits suggesting that this was a significant volcanic centre during the pause in basalt eruption.

The North Atlantic Igneous Province Cooper in Mitchell, 2004.

There are also a number of other igneous centres including the eastern and western Mournes granites and the basic layered intrusion and ring dyke of Slieve Gullion. Other smaller plugs are dotted around the Plateau. Tievebulliagh is one that has baked a large xenolith of laterite into a rock called porcellanite and this hill became the site of a Neolithic axe factory. Other examples of dolerite plugs include Slemish and the Bendoo plug.

A number of large dolerite sills up to 200 m thick are also present across Northern Ireland and are Palaeogene age, but intruded into carboniferous, Triassic and Jurassic sediments. The Fair Head Sill is the most prominent, occupying the whole headland next to Ballycastle. At Portrush a dolerite sill forms Ramore Head and a series of islands called the Skerries. The whole region is cross-cut by dykes that formed in several events, showing just how much stretching of the crust there was to accommodate the magma intrusion. Some of this extension may have been related to the ocean spreading and some due to thermal doming.

Formation of today’s landscape

After the periods of volcanic eruption, the thick pile of lavas was dissected by faults, uplifted and eroded during the Eocene, with the landscape developing into a peneplain. Studies show that at least one kilometre of lavas were eroded on Mull and it is likely that NI saw similar amounts of erosion. In the Neogene, a period of extension allowed the formation of pullapart basins such as in Lough Neagh, Ballymoney and Agivey and these were filled by clays and lignites of the Lough Neagh Group. The area around Lough Neagh remains a low area and is accumulating sediment once again.

The present land surface of Northern Ireland has largely then been eroded and sculpted by ice, producing erratics and rugged U-shaped valleys. Northern Ireland has large areas covered in the glacial sediment called till, obscuring a lot of the underlying geology, but producing a beautiful landscape of drumlins.