ESCR - Introduction to the Structural Geology of Northern Ireland

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Introduction to the Structural Geology of Northern Ireland	Antrim, Down, Fermanagh, Londonderry, Tyrone

Site number:	641
Locality Type:	Various
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County:	Antrim, Down, Fermanagh, Londonderry, Tyrone

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Introduction:
1. THE STRUCTURAL FRAMEWORK OF NORTHERN IRELAND 1.1 PLATE TECTONICS The development and general acceptance of plate tectonic theory, in particular the key mechanism of continental drift, has provided geologists with a mechanism to account for the worlds major structural and metamorphic terranes and the discontinuities that exist between adjacent terranes. North of the Iapetus suture a number of terranes, which now make up northern Britain and Ireland, formed at sites along the Laurentian (North American) continental margin in the Iapetus Ocean. These terranes were brought together into their present configuration as a result of large-scale sinistral transpressive movements. 1.2 TERRANES The rocks in Northern Ireland range in age from the early Proterozoic (Moinian) to Recent. The older and often structurally complex rocks (Early Proterozoic to Lower Palaeozoic) have been divided into a number of distinct tectono-stratigraphic ‘terranes’ which are separated by major NE-SW trending bounding faults (Fig. 1, after Bluck et al., 1992). Each terrane consists of rocks of similar geological provenance and a shared metamorphic and deformational history. These terranes coalesced or ‘docked’ in their present juxtapositions as a result of oblique compressional earth movements during late-Caledonian times. The nomenclature for the principal terranes and terrane boundaries in Northern Britain is derived mainly from the classic regions of Scottish geology where much of the pioneering research into structural geology was carried out. This nomenclature has been extended, and applied, with a few minor modifications, to Northern Ireland.
Description:
2. PRE-CALEDONIAN STRUCTURE 2.1 INTRODUCTION The oldest and structurally most complex rocks known to exist in Northern Ireland occur in separate inliers in County Fermanagh (part of the Lough Derg Inlier) and County Tyrone (the Central Tyrone Inlier). In all likelihood, these inliers represent allochthanous fragments of pre-Caledonian basement terranes and are not basement to the supercrustal Dalradian Succession of the orthotectonic Caledonides in northwestern Ireland (Max et al., 1985). 2.2 THE PRE-DALRADIAN INLIERS The Lough Derg Inlier (Lough Derg Group) lies within the SW extension of the Central Highlands Terrane into Northern Ireland, and the Central Tyrone Inlier (Corvanaghan Group) lies within the Tyrone-Girvan Sub-Terrane of the Midland Valley Terrane (Fig. 2). The Lough Derg Inlier consists of early Proterozoic (Moinian?) fluvial shelf deposits, which have undergone polyphase deformation and metamorphism. Deformation started pre-740Ma and ended at about 450Ma during the late stages of the Caledonian Orogeny. 2.3 LOUGH DERG INLIER (SLISHWOOD DIVISION, LOUGH DERG GROUP) Outcrop of the Lough Derg Inlier spans the Fermanagh-Donegal border between Lower Lough Erne and Lough Derg. The inlier covers an area of approximately 200km2 of which 35km2 is located within Northern Ireland. The Lough Derg Group, together with similar rocks at Rosses Point and the Ox Mountains in the Republic of Ireland, is believed to constitute a single tectono-stratigraphic terrane referred to as the Slishwood Division (Max and Long, 1985). Gravity and magnetic data indicate that this terrane extends well beyond the limits of outcrop in the present inliers. Whether or not the Slishwood Division forms basement to much of the Dalradian orthotectonic Caledonides across NW Ireland is less certain. All of the exposed contacts between the Slishwood Division and the orthotectonic Caledonides are syn-metamorphic slides or faults (Max and Long, 1985). Furthermore, late-Caledonian transpression may have been responsible for considerable lateral translation of the Slishwood Division rocks from their source area. The Lough Derg Group consists of a distinctive association of psammitic paragneisses, subsidiary pelite and boudinaged and tectonically disrupted tholeiite dykes. Along its northern margin, the inlier is overlain tectonically by Upper Dalradian strata, which were thrust southwards during the late stages of the Caledonian Orogeny. Elsewhere, the western and eastern margins of the Inlier have faulted contacts with Carboniferous rocks. Carboniferous rocks overlie the Lough Derg Inlier along its southern boundary. 2.3.1 Age of the Slishwood Division The age of the Slishwood Division (Lough Derg Group), and by implication the potential age limits of its deformation, have been subject of much debate. Anderson (1948) was first to propose a Moinian age on the basis of lithological similarities with the Moines in Scotland. Max et al. (1984) proposed a late-Grenvillian age for the Slishwood Division based on a Rb-Sr whole rock isochron, which corresponded to an age of 895+60Ma. This date is poorly constrained however and is based on a composite sample of semipelites taken from the Ox Mountains and the Lough Derg Inlier. In view of the high pressure granulite facies metamorphism of the inlier (a metamorphic grade that is unknown in the Dalradian rocks of Britain), it has been concluded that the rocks in the Lough Derg Inlier were deposited later than 1713Ma and prior to the Grenvillian Orogeny at c.1100-1000Ma (Long and McConnell, 1999). 2.3.2 Lough Derg Inlier: metamorphic history Rocks of the Lough Derg Inlier contain evidence of a long and varied thermo-tectonic history. An early, high-grade (eclogite facies) event was followed by a series of retrograde events related to exhumation. Various researchers have attempted to establish a chronology and relate the deformation and metamorphism of the Lough Derg Inlier / Ox Mountains to global orogenic events. In recent years, a general consensus has begun to emerge and has been summarized by Long and McConnell (1999). 2.3.2.1 Eclogite and Granulite Facies metamorphism Evidence of the earliest metamorphism of the Lough Derg Inlier is restricted to pods (augens) of early-formed metabasite sills and dykes. Mineral assemblages in these rocks include sodic clinopyroxenes (replaced by amphibole), plagioclase and pyrope garnet indicative of high temperature/high pressure eclogite facies metamorphism. The psammitic paragneisses, which are the dominant lithology in the Lough Derg Inlier, contains no mineralogical evidence of this eclogite facies event. In the Ox Mountains, alluminous lithologies within the Slishwood Division contain perthite, mesoperthite, sillimanite, kyanite, garnet and rutile. This assemblage has been interpreted as evidence of a separate high-pressure granulite facies metamorphism sometime after the eclogite facies metamorphism (Sanders, 1987). There is less definite evidence of a similar event in the Lough Derg Inlier. The Inlier contains neither mesoperthite nor sillimanite. Unitt (1997) has however suggested that inclusion trails (quartz, white mica, ilmenite and rutile) within flattened garnet poikiloblasts in psammite at Lough Derg indicate a high temperature anhydrous granulite facies event. 2.3.2.2 Amphibolite Facies metamorphism Subsequent metamorphism of the Lough Derg Inlier involved retrogression of the high-temperature, high-pressure mineral assemblages. During the early Caledonian (Grampian) Orogeny, the Lough Derg Inlier lay at a higher structural level within the earth’s crust and was subject to amphibolite facies metamorphism. In the metabasite augens, pyroxene and garnet were replaced by poikiloblastic hornblende and in the psammite, muscovite grew as a result of rehydration and garnet formed in semipelite beds in the more micaceous domains. 2.3.2.3 Lower Greenschist Facies metamorphism The final phase of metamorphic retrogression in the Lough Derg Inlier took place during the post-Grampian uplift (Acadian Orogeny) about 400Ma. During this event, garnets were chloritized and there was widespread sericitization of plagioclase feldspar. 2.3.3 Lough Derg Inlier: structural history Anderson (1948) published the earliest detailed structural map of the Lough Derg Inlier (Fig. 3). The map indicates a broad, large-scale, NE plunging structure known as the Lough Derg Anticline. Anderson noted the presence of numerous minor fold structures that were "too irregular to be made the basis of any tectonic conclusions". The part of the Lough Derg Inlier that lies within Northern Ireland is on the SE, more steeply dipping limb of the anticline. Long and McConnell (1999) summarized the structural framework and deformation chronology for the Slishwood Division. This framework is largely based on research in the Ox Mountains where a fuller record of the earliest granulite facies deformations (D1 and D2) is preserved (Sanders, 1987, 1994; Church, 1961, 1969; Unitt, 1997). In the Lough Derg Inlier, preservation of these earliest structures (D1 and D2) is limited to some of the metabasite augens. The most conspicuous feature of the psammites in the Lough Derg Group is the strong lithological banding. This feature is now believed to be due largely to tectono/metamorphic segregation (Church, 1961, 1969; Ford, 1980; Unitt, 1977; Sanders et al., 1987) and not the straightforward lithological banding as suggested by Borradaile (1974). Church (1961, 1969) recognized three phases of deformation in the SW part of the Lough Derg Inlier. D1 and D2 structures were shown to be early recumbent isoclinal folds while D3 were upright folds related to the Lough Derg Anticline. Borradaile (1974) confirmed a similar structural pattern when he mapped the remaining part of the inlier. A recent study of the Lough Derg Inlier (Unitt, 1997), treats the amphibolite facies metamorphic event as a chronostratigraphic marker (Dn). Compared with the Ox Mountains, the Lough Derg Inlier contains comparatively little evidence of early (pre-Dn) structures. Unitt did, however, confirm the existence of the early D1 and D2 isoclinal structures in the Lough Derg Inlier and designated them as Dn-1 and Dn respectively. Dn-2 Structures relating to the earliest eclogite facies deformation (Dn-2) are believed to have been largely obliterated by subsequent deformations. The pale quartz and feldspar leucosomes, which give the psammitic rocks their distinctive gneissose appearance, were produced by early partial melting and segregation and are believed to lie parallel to the now obliterated Dn-2 fabric. Dn-1 Dn-1, the oldest discernible structures in the Lough Derg Inlier, deform the early leucosomes and form minor (centimetre-scale) intrafolial folds and an axial-planar foliation. These structures are equivalent to the folds and foliation designated D1 by Church (1961) and Borradaile (1974). Dn In Unitt’s classification, Dn can be equated with D2 as defined by Church (1961) and Borradaile (1974). Dn refolds the Dn-1 intrafolial folds and forms the dominant foliation within the inlier. This foliation is a variably intense, flattened and ribboned quartz mylonitic fabric that is axial-planar to tight isoclinal (Dn) folds. Dn+1 Dn+1 deformation consists of brittle and semi-brittle extensional structures that are believed to be related to an extensional orogenic collapse. Dn+2 Dn+2 deformation produced open, upward facing folds of earlier structures and resulted in the formation of the Lough Derg Anticline as defined by Anderson (1948). This deformation has also been taken as equivalent to D7 in the Dalradian rocks further north in Central Donegal. The Dn+2 structure plunges gently towards the north. For site specific information on the Lough Derg Inlier see Key Site 642 - Croagh Mountain. 2.4 THE CENTRAL TYRONE INLIER (CORVANAGHAN FORMATION) 2.4.1 Introduction and previous research The Central Tyrone Inlier (Corvanaghan Formation) has a lenticular outcrop extending over an area of about 20km2 between Omagh and Cookstown in mid County Tyrone. The inlier forms part of the Tyrone-Girvan Sub-Terrane of the Midland Valley Terrane (Bluck, 1992, Fig. 1). Surrounding the Central Tyrone Inlier are basic plutonic and volcanic rocks (Tyrone Plutonic Complex and Tyrone Volcanic Group) which are believed to be fragments of an Ordovician ophiolite complex and a volcanic arc succession respectively (Hutton et al., 1985) (Fig. 5). Contacts between the Tyrone Inlier and the ophiolite are very poorly exposed but are in all cases believed to be tectonic. There is little or no direct evidence of the age or geological affinity of the Central Tyrone Inlier (Max and Long, 1985) and therefore it can be regarded as a ‘suspect terrane’. Portlock (1843) produced one of the earliest maps of the Central Tyrone Inlier and distinguished the inlier from the ‘Metamorphic Rocks (hornblendic type)’ which surround it (i.e. the Tyrone Volcanic Group and Tyrone Plutonic Complex, Geological Survey of Northern Ireland, 1995, 1997). Portlock did not however differentiate between the ‘Mica Schists’ of the inlier and the Dalradian strata of the nearby Sperrin Mountains. On the first edition of Geological Sheets 26 and 34 (Geological Survey of Ireland, 1882, 1877), the rocks of the Tyrone Central Inlier were described as ‘Schists and Gneisses’. Although these early maps differentiated the inlier from the ‘Pyroxene Rocks’ of the Tyrone Plutonic Complex which surround it, all strata were categorized as "Probably of Pre-Cambrian or Upper Laurentian age". Hartley (1933) considered the Central Tyrone Inlier to be of Dalradian age and viewed the inlier simply as a repetition of the ‘Sperrin Schists’. He mapped the inlier in some detail and described the rocks as "granulitic quartz-feldspar biotite schists and muscovite-biotite gneisses" which he believed most likely originated as, "a recrystallised and highly folded series of banded arenaceous and argillaceous sediments". Hartley claimed that the higher (sillimanite) metamorphic grade found locally within the inlier was the result of impregnation by granitic veins. Hartley did not describe the structure or the deformational history in any significant detail. The second edition of Sheet 34 (Pomeroy) (Geological Survey of Northern Ireland, 1979) made a clear distinction between "the siliceous granulite and quartzo-feldspathic gneiss" of the Central Tyrone Inlier (referred to for the first time as the Corvanaghan Formation), and the surrounding Tyrone Volcanic rocks. As a result of this work, Cobbing et al. (1965) proposed that the Central Tyrone Inlier was of Lower Dalradian age and that in parts the Ordovician Volcanics (Tyrone Volcanic Group) rested unconformably upon it. The second edition of Sheet 26 (Draperstown) (Geological Survey of Northern Ireland, 1995) covers most of the outcrop of the Central Tyrone Inlier and shows the Corvanaghan Formation as being of ‘Moinian?’ age. 2.4.2 Central Tyrone Inlier: age and provenance The Central Tyrone Inlier remains a relatively under-studied and enigmatic part of Northern Ireland’s geology. The rocks are structurally complex having experienced polyphase deformation and regional metamorphism locally up to sillimanite grade. Little detail is known about contacts with the surrounding geology. The inlier is believed to be a fragment of pre-Caledonian Proterozoic continental crust. The most recent data available (Earls et al., 1996) indicates a minimum Rb-Sr isotope age for the inlier in the region of 611Ma. This age is for muscovite and feldspars taken from two pegmatites, which cut most of the structural elements in the inlier. Hutton has also pointed to the presence of significant hornblende, biotite, chlorite (green bed?) assemblages and suggested that these may indicate an affinity with the Grenville Terranes of Northern America rather than with other northern British Moinian Terranes. For site specific information on the Central Tyrone Inlier see Key Site 643 - Oughtmore Mountain. 3. THE CALEDONIAN STRUCTURE 3.1 INTRODUCTION Prior to the opening up of the North Atlantic Ocean, an extensive mountain chain, known as the Caledonian-Appalachian Mountains, extended from Scandinavia through northern Britain and Ireland into Greenland and along the eastern seaboard of North America. The tectonic deformations responsible for the Caledonian Orogeny had their origin in the convergence and collision of three major continental blocks: 1. Gondwana (Africa, South America, India, Australia and Antarctica) 2. Laurentia (North America, Greenland and NW Scotland) 3. Baltica (NW Europe including southern Britain). In Ireland, the Caledonian Orogeny spanned the period between 475Ma and 385Ma. During this time there were two principal mountain-building episodes: 1. Acadian Orogeny: c. 385Ma (Late Silurian to Early Devonian) 2. Taconic (Grampian) Orogeny: 475Ma to 463Ma (Ordovician) There is evidence of an earlier Caledonian (Early Grampian) event at about 600Ma in Scotland (Tanner and Leslie, 1994) where an early fabric (designated D1) is cut by the Ben Vuirich Granite (emplacement date of 590Ma). The significance of this early deformation event has lead to much debate amongst proponents of different models for the British and Irish Caledonides. 3.2 THE CALEDONIDES IN NORTHERN BRITAIN The British-Irish sector of the Caledonian Orogen consists of a series of distinctive tectonostratigraphic ‘terranes’ separated by major fault lines (Bluck, 1992; Hutton, 1987) (Fig. 1). Hutton suggested that these terranes accreted as the continents converged and Gondwanaland rotated clockwise causing Laurentia to collide with Baltica. Thereafter, Gondwanaland broke free and created a major sinistral strike-slip zone during late-Silurian to early-Devonian times. The most important implication of this strike-slip model is that the terranes of the British Caledonides are considered to be allocthonous and have been transported many kilometres from their place of origin. The ‘Caledonian’ rocks in Northern Ireland can be assigned to one of three 3 distinctive terranes each having a unique tectonic history and its own characteristic structures: 1. PERIOD: Ordovician-Silurian STRATIGRAPHY: Gala, Hawick & Moffat Shale Groups LITHOLOGY: Turbidite sequence TERRANE: Southern Uplands Terrane 2. PERIOD: Ordovician STRATIGRAPHY: Tyrone Volcanic & Plutonic Groups LITHOLOGY: Ophiolite and island arc volcanic suite TERRANE: Midland Valley Terrane, including the Tyrone-Girvan Sub-Terrane 3. PERIOD: Dalradian STRATIGRAPHY: Southern Highland & Argyll Groups LITHOLOGY: Basinal meta-sedimentary and meta-igneous rocks TERRANE: Grampian (Central Highland) Terrane 3.3 GRAMPIAN (CENTRAL HIGHLANDS) TERRANE: DALRADIAN The Grampian (Central Highlands) Terrane (Bluck, 1992) extends SW into Northern Ireland from the Central Highlands of Scotland. This terrane consists of Dalradian strata and has also been referred to as the ‘Orthotectonic Caledonides’ (Holland, 1981). The principal areas of outcrop of the Dalradian rocks in Northern Ireland are the Sperrin Mountains and the smaller inliers at Lack, County Tyrone and NE Antrim. During the Caledonian Orogeny, the basin sediments and contemporaneous basic igneous rocks of the Dalradian succession were subjected to intense polyphase deformation which produced several superimposed phases of cleavage and folds. The rocks also experienced regional metamorphism and developed mineral assemblages characteristic of the Greenschist Facies of the high-pressure Barrovian Facies Series. In Scotland, the southern part of the Grampian Terrane is characterized structurally by a series of large recumbent fold structures (Phillips, 1981). These structures have produced regional stratigraphic repetitions and inversions of large tracts of the succession along the southern edge of the belt. In Northern Ireland, comparable major folds and structural inversions have been mapped in NE Antrim and the Sperrin Mountains (Sperrin Nappe syn. Claudy Anticline). 3.3.1 The Sperrin Mountains To some extent all of the Dalradian rocks in Northern Ireland contain evidence of polyphase deformation. Under favourable circumstances, at least three distinct deformation phases can be demonstrated and it is possible to relate regional and/or zonal cleavages to at least two of these phases. There is a divergence of opinion surrounding the detailed interpretation of the large-scale structure of the Sperrin Mountains (Arthurs, 1994; Smith and Johnston, 1994; Alsop and Hutton, 1994; Long, 1997). This controversy centres on the precise location of the axis of the major SE facing fold, which inverts strata in the southern Sperrins. Hutton and Alsop located this fold axis in Glenelly (Fig. 7) whereas, the structural model favoured by Hartley (1933), Arthurs (1976) and GSNI (1995, 1997) locates the axis in the vicinity of Donemana to the north of the main Sperrin Ridge (Fig. 8). There is a significant strain-gradient north to south across the Dalradian outcrop in County Londonderry and the Sperrin Mountains. The most intensely deformed and metamorphosed rocks are found within the Mullaghcarn Formation in the southern Sperrins and the Lack Inlier close to the Omagh Fault. The least deformed rocks occur in the Ballykelly and Londonderry Formations in the Foyle Syncline (Fig. 8). 3.3.1.1 Deformation phases D1 The earliest recognizable deformation is a fine-grained slaty cleavage (S1) parallel to bedding (S0) throughout the area (Alsop and Hutton, 1993(a)). No F1 fold structures have been recognized. D2 The Sperrin Mountains are dominated by a major regional overfold (F2) analogous to the large-scale recumbent structures mapped in NE Antrim, the Cowal Anticline and the Tay Nappe in the Southern Highlands of Scotland. The Sperrin structure was originally referred to as the ‘Claudy Anticline’ and designated as an F1 structure (Pitcher and Berger, 1972). More recent research has revealed this fold to be an F2 structure and it is currently being referred to as the ‘Sperrin Fold’ or the ‘Sperrin Nappe’ (Arthurs, 1976; Alsop et al., 1993(a); Geological Survey of Northern Ireland, 1995). In the structural interpretation favoured by the Geological Survey of Northern Ireland, the axis of the Sperrin Fold lies north of the main Sperrin watershed. The axial trace trends ENE-WSW between Dungiven and Butterlope before it swings to a NE-SW orientation between Butterlope and Baronscourt. The principle structure in Alsop and Hutton’s (1993) interpretation is also a regional-scale, SE facing overfold (the Sperrin Nappe). The axis of this structure is postulated to lie in Glenelly (Fig. 7) some distance south of the Sperrin watershed. The structure as outlined by Alsop and Hutton has also involved the renaming and regrouping of the Dalradian stratigraphy which sees a stratigraphic repetition of the Sperrin Formation (incorporating the Glenelly Formation, Glengawna Formation and Mullaghcarn Formation) either side of the axis in Glenelly. Perhaps the most radical implication of the Alsop and Hutton model is that all strata south of the Dungiven Formation have been designated as Argyll Group (Middle Dalradian) rather than Southern Highlands Group (Upper Dalradian) as in the model favoured by the Geological Survey of Northern Ireland. Throughout the Sperrins and north Londonderry, minor F2 folds and a penetrative axial-planar fabric (S2) are closely associated with the major F2 structure. On the limbs of the Sperrin Nappe and the Lough Foyle Syncline, S2 is subparallel to bedding (S0) and S1, and often transposes these earlier surfaces. Bedding-cleavage relationships are clearly visible in the north of the region where strain is least intense. Here for example, on the upwards facing limb of the Sperrin Nappe and the southern limb of the Foyle Syncline, cleavages and bedding are all subparallel with S2 dipping towards NNW at a consistently higher angle. Minor folds on the northern limb of the Sperrin Nappe verge southwards and have a geometry consistent with translation of top towards the SW. In the southern Sperrins, on the inverted limb of the Sperrin Nappe, strain levels are significantly higher especially approaching the Omagh Fault. In this region, cleavage and bedding are more parallel and the surfaces are often difficult to distinguish. D3 (including the Omagh Fault) D3 deformation in the Sperrins is related to an ESE-directed compression, which formed medium-scale, re-folds of the F2 structures. This deformation phase produced regional swings in the orientation of S2 and the composite (S2, S1 and S0) foliations. Good examples of meso-scale F3 folds occur on the southern limb of the Sperrin Nappe on Mullaghcarbatagh and in Butterlope Glen. Throughout the region, minor F3 folds plunge gently towards the NE and SW. A variably intense S3 cleavage is axial-planar to these structures and forms strong crenulation of S2 surfaces particularly in pelitic lithologies. The Omagh Fault (also referred to as the Omagh Thrust, Sperrin Fault or Sperrin Thrust) is part of a complex zone of late-Caledonian faults extending from south Londonderry to Fermanagh. The Omagh Fault, together with the Castle Archdale Fault along the southern boundary of the Lack Inlier, dips gently NW and marks the southern boundary of the Dalradian outcrop in Northern Ireland. This fault system is a relatively late-formed (D3) feature along which there was oblique ESE translation of Dalradian rocks across a remnant of Ordovician island arc (Tyrone Volcanic Group) (Fig. 9) (Alsop and Hutton, 1993(a), 1993(b)). Carboniferous strata are incorporated into and deformed within this fault complex that in west County Tyrone see rocks as young as Westphalian up-ended and inverted. The fact that Carboniferous strata are effected indicates that movement along these faults persisted into the Variscan. In the zone directly NW of the Omagh Thrust, there is evidence that a period of extensional deformation followed close after the crustal thickening and thrusting associated with the D3 deformation. The formation of a series of steep to vertical E-W trending shear zones in the vicinity of Rousky [2572 3862] has been attributed to gravitational collapse of the Dalradian strata on a thrust ramp at the edge of the Tyrone Igneous Complex (Tyrone Volcanic Group) (Alsop and Hutton, 1993(b)). These shear zones have a predominantly dextral sense of shear movement and host quartz veins which are in places auriferous e.g. at Curraghinalt [2570 3868] (see Key Site 521 - Curraghinalt Burn, near Gortin). 3.3.1.2 Structural interpretations There is a broad consensus amongst the protagonists in the structural debate that the Dungiven Formation (Dungiven Limestone Formation in Alsop’s stratigraphy) belongs to the Tayvallich Subgroup (Argyll Group) and marks the transition between the Upper and Middle Dalradian. Significant differences do however remain over the interpretation of the remaining stratigraphy in the Sperrins (stratigraphic correlations are, as follows): 1. GROUP: Southern Highland Group SUBGROUP: Hartley (1938), Arthurs (1976), GSNI (1995): Mullaghcarn Fm, Glengawna Fm, Glenelly Fm, Dart Fm Alsop and Hutton (1993): Newtownstewart Fm 2. GROUP: Argyll Group SUBGROUP: Tayvallich Hartley (1938), Arthurs (1976), GSNI (1995): Dungiven Fm Alsop and Hutton (1993): Dungiven Limestone Fm 3. GROUP: SUBGROUP: Crinan Easdale Hartley (1938), Arthurs (1976), GSNI (1995): Newtownstewart Fm Alsop and Hutton (1993): Sperrin Fm For site specific information on the Dalradian rocks (Grampian (Central Highlands) Terrane) of the Sperrin Mountains see: Key Site 644 - Prehen Quarry Key Site 645 - Butterlope Glen Key Site 646 - Mountfield Quarry Key Site 647 - McNally’s Burn, Broughderg. 3.3.2 The NE Antrim Inlier An inlier of Dalradian rocks covers an area of approximately 200km2 in NE Antrim. The inlier forms part of the Grampian (Central Highlands) Terrane, which extends into Northern Ireland from Scotland. The Antrim Inlier has faulted contacts and in part is overlain unconformably by various igneous and sedimentary rocks ranging in age from Devonian to Palaeogene. Because of its location, midway between the Dalradian of the Sperrin Mountains and Scotland, the Antrim Inlier has been pivotal in establishing stratigraphic correlations. 3.3.2.1 Stratigraphy of NE Antrim Inlier The NE Antrim Inlier contains Upper and Middle Dalradian strata, which have been divided into several formations (see below). The inlier is lithologically varied but consists mainly of quartz schist and psammite. Of particular note is the Torr Head Formation (Tayvallich Subgroup, Argyll Group) a mixed assemblage of carbonate rocks interbedded with siliciclastic and greenbed lithologies. This formation marks the transition between the Argyll Group (Middle Dalradian) and the Southern Highland Group (Upper Dalradian). The Torr Head Formation can be correlated directly along strike with the Dungiven Formation in the Sperrins and the Tayvallich and Loch Tay Limestones in Scotland. Dalradian Stratigraphy of the NE Antrim Inlier: 1. GROUP: Southern Highland Group - SUBGROUP: FORMATION: Glendun MEMBER: 2. GROUP: Southern Highland Group SUBGROUP: FORMATION: Runabay Head MEMBER: Carnaneigh Green Beds 3. GROUP: Southern Highland Group SUBGROUP: FORMATION: MEMBER: Leckpatrick Green Beds 4. GROUP: Southern Highland Group SUBGROUP: FORMATION: Altmore MEMBER: 5. GROUP: Argyll Group SUBGROUP: Tayvallich FORMATION: Torr Head MEMBER: 6. GROUP: Argyll Group SUBGROUP: Crinan FORMATION: Owencam MEMBER: 3.3.2.2 Previous research Compared with Donegal and Scotland, the NE Antrim Inlier remains a relatively under-researched area of Dalradian geology. There has been little modern mapping or study of regional and small-scale structure in the area. Bedrock exposure is generally poor or is inaccessible. The Geological Survey of Northern Ireland mapped the northern part of the inlier during revision of the Ballycastle Sheet (Geological Survey of Northern Ireland, 1963; Wilson and Robbie, 1966). The southern part of the inlier has not yet been resurveyed apart from some reconnaissance mapping by Arthurs (1976). Knowledge of the structure of the Inlier is therefore heavily reliant on early research by Bailey and McCallien (1934), Goldring (1961), and Wilson and Robbie (1966). 3.3.2.3 Structure of the NE Antrim Inlier The structural interpretation of the Grampian Caledonides of Scotland is based on pioneering work by E. B. Bailey who relied heavily on the primary mapping carried out by the then Geological Survey of Great Britain (Stephenson and Gould, 1995). Bailey and McCallien (1934) regarded the NE Antrim Inlier as a SW structural continuation of the Grampian Caledonides of Scotland. In Bailey’s structural model for the Southern Highlands of Scotland, the late Proterozoic (Upper Dalradian) strata lie on the lower, inverted limb of, a regional-scale, SE facing recumbent fold, the Tay Nappe. During the Caledonian orogeny the original Tay Nappe Structure was further deformed and modified by flattening, tightening and extension. This resulted in formation of a series of locally important secondary folds. Roberts (1974) considered the structure of the SW Highlands of Scotland in terms of ‘primary’ and ‘secondary’ deformations. He concluded that the ‘primary deformation’ consisting of the earliest three and possibly four phases of deformation resulted in formation of the primary structures which are major regional folds including the Tay Nappe. A ‘secondary deformation’ consisting of all subsequent deformation phases was largely responsible for modification of these primary structures. Notable amongst the secondary structures are the monoformal modifications of the Tay Nappe such as the Cowal Anticline. In the SW Highlands, the Tay Nappe forms a broad upright arch structure, the Cowal Antiform. Further south, approaching the Highland Boundary Fault, the Tay Nappe hinge zone is folded downwards into the Highland Border Steep Belt becoming a downwards facing synformal anticline, the Aberfoyle Anticline (Henderson, 1938; Shackleton, 1958) (Fig. 13). Most of the Dalradian strata in NE Antrim lie on the inverted lower limb of the extension of the Tay Nappe into Ireland. In the southern part of the Inlier the strata are also disposed about a major upwards-facing structure, the Altmore Anticline, which is equivalent to the Cowal Anticline of SW Scotland (Wilson and Robbie, 1966). The hinge of the Tay Nappe continuation is not exposed in Antrim and the axis of the Altmore Anticline strikes NE and can be traced from just south of Loughaveema to the coast near Leckpatrick (Arthurs, 1976). 3.3.2.4 Details of the Caledonian deformation D1 In NE Antrim, the earliest deformation produced a penetrative cleavage (S1), which is generally parallel to bedding and resulted in grain flattening and a fine metamorphic striping. This cleavage also forms an intersection lineation (L1) defined by S1 crenulations of earlier (S0 and S1) surfaces. No F1 folds have been recognized. D2 The S2 schistosity associated with D2 is a spaced cleavage which crenulates S1. S2 forms microlithons in psammitic lithologies and a composite S1/S2 striping in the finer-grained pelitic beds. S2 is axial-planar to a number of tight isoclinal minor folds (F2). The axes of the minor F2 folds are parallel to quartz rods, segregations and a crenulation intersection lineation (L2). Sense of vergence in the F2 folds indicates that they are situated on the lower limb of a major recumbent fold that closes to the SE or E. D3 D3 related folds and cleavages occur sporadically in NE Antrim. In Glenmakeeran, an S3 schistocity similar in style to S2 is seen to cut and crenulate S2 and is axial-planar to a fold in S2. Arthurs (1976) noted that in the south of the inlier, in Glendun, Glenaan and Lisbawn, the third schistocity is ubiquitous and is axial-planar to F3 folds which deform a composite S1/S2 schistocity. The F3 folds are generally upright in Glenaan and flat-lying in upper Glendun. The axes of these folds strike north across the normal NE-SW regional Caledonian trend. For site specific information on the Dalradian rocks (Grampian (Central Highlands) Terrane) of the NE Antrim Inlier see: Key Site 648 - Torr Head Key Site 649 - Benvan, Murlough Key Site 650 - Carnaneigh Key Site 651 - Cushendun Bay. 3.4 THE SOUTHERN UPLANDS TERRANE: PARATECTONIC CALEDONIDES Sometime during late Cambrian to early-Ordovician times, the Laurentian continental plate split apart. The North American Plate (Laurentia) detached from the North European Plate (Baltica) and the Central European Plate (Avalonia) which at that time incorporated Nova Scotia and Newfoundland. Separation of these plates resulted in the formation of the Iapetus Ocean, also referred to as the Proto-Atlantic Suture. During the time that the Iapetus Ocean was open, turbidite facies (greywacke and shale) sediments accumulated in an open geosynclinal setting (Fig. 19). In Northern Ireland, the term ‘Paratectonic Caledonides’ refers to the broad belt of Ordovician and Silurian (Lower Palaeozoic) turbidite facies rocks which form the Southern Uplands Terrane and the Northern Belt Sub-Terranes (Bluck et al., 1992). This terrane extends from the Southern Uplands of Scotland, across much of Counties Down and Armagh and into central Ireland. The northern boundary of the Southern Uplands Terrane is the Southern Uplands Fault in Scotland and its less clearly defined continuation in Northern Ireland (Anderson, 1965). The southern boundary of the Southern Uplands Terrane is the Iapetus Suture, the line of final closure of the Iapetus Ocean. 3.4.1 Regional structure The Iapetus Ocean was a transient feature and sometime during middle-Ordovician times, it started to close as the Laurentia and Baltica/Avalonia plates converged. In Northern Britain and the north of Ireland, closure of Iapetus was accommodated by a process of northerly-directed subduction along the Laurentian Plate margin. This situation was mirrored in Southern Britain and Ireland where Iapetus crust and sediment were subducted in a SE direction along the northern margin continental margin of Avalonia. The Southern Uplands Terrane consists of a number of discrete tectono-stratigraphic blocks separated by major NE-SW trending strike faults. In a N-S traverse across the Southern Uplands Terrane, successive ‘blocks’ containing increasingly younger rocks are encountered. At the same time, within each block the strata young consistently towards the north. McKerrow et al. (1977) proposed an ‘imbricate thrust’ or ‘accretionary prism’ tectonic model to account for the stratigraphical and structural patterns observed across the Southern Uplands Terrane in Scotland. The structural and stratigraphic characteristics noted in Scotland extend into Northern Ireland and have been confirmed following detailed surveys and research in Counties Down and Armagh (Anderson, 1962; Cameron, 1981; Geological Survey of Northern Ireland, 1985, 1987). 3.4.2 Deformation The primary deformation (D1) in the Paratectonic Caledonides produced tight to moderately tight asymmetrical folds (F1) and a single, penetrative, broadly axial-planar cleavage (S1). The intersection of S1 on bedding (S0) results in a conspicuous intersection lineation (L1) in the more argillaceous lithologies. On the Ards Peninsula, F1 fold hinges plunge overall towards the NE. Asymmetrical folds display NW younging fold limbs that are generally longer and broader than their counterparts which young in the opposite direction. Overall, this is the geometry of a fold envelope descending towards the NW with fold vergence dominantly towards the SE. Anderson (1962) identified a series of fault-bounded structural and stratigraphic blocks in the Gala Group (Silurian) rocks on the Ards Peninsula. Subsequent research by Dalzell (incorporated in the 1:250,000 scale Solid Geological Map, Geological Survey of Northern Ireland, 1997) extended mapping of these blocks SW across County Down and into County Armagh. Each block is bounded by NE-SW trending strike-parallel faults and contains strata which young predominantly towards the NW. Overall however, blocks are progressively younger towards the SE. This pattern is consistent with an accretionary prism model of terrane formation related to plate margin subduction (Fig. 20). 3.4.3 ESCR Sites Outcrops along the eastern side of the Ards Peninsula offer an accessible if incomplete dip section through the central portion of the Southern Uplands Terrane. The principle stratigraphic and structural elements of this Terrane are well represented and can be demonstrated at a number of key localities. These sites have previously been documented in an excursion guide (Anderson et al., 1978). For site specific information on the Paratectonic Caledonides (Southern Uplands Terrane) see: Key Site 652 - Orlock Bridge Key Site 653 - Whiskin Rocks Key Site 654 - White House Port, Cloghy Key Site 655 - Knockinelder Bay and Kearney Point Key Site 656 - Millin Bay. 4. THE VARISCAN OROGENY 4.1 INTRODUCTION During Devonian to late-Carboniferous times, Europe was affected by a series of major tectonic events brought about by the convergence of continental plates. By about 250Ma, almost all of these plates had come together to form a single supercontinent known as Pangea (Osborne et al., 1995). In Europe, these events culminated in the Variscan Orogeny also referred to as the Hercynian Orogeny or the Amorican Orogeny. The time equivalent event in North America is referred to as the Appalachian or Alleghenian Orogeny. In Northern Ireland and Britain, Variscan earth movements persisted into late-Carboniferous times as borne out by the fact that Coal Measures strata (Westphalian) and Stephanian strata were tilted prior to deposition of the Permian. The most intense deformation in the Variscan Orogeny took place in the Hartz Mountains of Central Europe. There is a perceptible tailing-off in the intensity and effects of the orogen northwards across Europe into Britain. This effect is noticeable in Ireland where Variscan structures change in character from north to south (Fig. 26) across three broad zones defined by Gill (1962). Each of Gill’s zones is characterized by the predominance of a particular style of deformation and structures. Boundaries between adjacent zones are gradational and arbitrarily drawn. The most southerly zone in Ireland (Zone 1) typically contains E-W orientated, open, parallel folds accompanied by northerly directed overthrusting and thrust stacking. Rocks in this zone have a cleavage related to the deformation and have experienced low-grade regional metamorphism. Zone 2, which runs through the Irish Midlands, represents a transition between Zone 1 and Zone 3, the most northerly zone, which includes all of Northern Ireland. Here the structures are mainly reactivations of pre-existing northeast-southwest trending Caledonian Faults. Folding in Zone 3 is restricted to open flexuring and rollovers in the vicinity of major faults. 4.2 VARISCAN STRUCTURES IN NORTHERN IRELAND In Northern Britain, Variscan structures take the form of faults and gentle, broad amplitude open folds. These folds and their subsequent erosion resulted in the separation of the Carboniferous Coal Measures into the present coalfields in the British Isles (Bennison et al., 1969). In Northern Ireland, Variscan deformation ranged from Devonian to end-Carboniferous times when the orogen reached its local culmination. The strong, parallel E-W orientated folding and penetrative deformation that characterizes the Variscan ‘belt’ in southern Britain and Ireland is absent in Northern Ireland. Instead, small outliers of gently folded Carboniferous (Namurian and Westphalian) strata are preserved in fault-bounded troughs, which form the coalfields at Coalisland, Dungannon, Stewartstown and Ballycastle. The principal manifestation of the Variscan Orogen in Northern Ireland is faulting rather than folding. Much of the deformation was accommodated by reactivation of earlier formed ESE-trending Caledonian faults. Structures such as the Tempo-Sixmilecross, Pettigo, Cool and Castle Archdale faults in County Fermanagh are examples of pre-existing Caledonian faults modified during the Variscan (Fig. 26). These normal and transform structures remained active during the development of the NW Carboniferous Basin in County Fermanagh and adjoining counties. Examples of more extreme folding and tilting of Carboniferous strata have been noted along the northern margin of the NW Carboniferous basin in County Fermanagh and tilting of Carboniferous strata is a common feature in the vicinity of Variscan faults. Mitchell (1992) used his detailed knowledge of stratigraphy in the northern part of the Lough Allen Basin in Fermanagh and County Tyrone to propose detailed chronology for Variscan tectonic events in Northern Ireland. The late-Viséan succession contains evidence of the effects that tectonic instability had on sedimentation and the formation of localized depo-centres. Stratigraphic sequences, which thicken or thin rapidly over short distances, are indicators of the existence of localized ramps formed by uplift. Slumped horizons and debris flows provide evidence of rapid basin subsidence (Mitchell, 1992) (Fig. 27). For site specific information on Variscan structure see: Key Site 657 - Ederny Quarry Key Site 658 - Straduff Quarry Key Site 659 - Fardross Stream. 5. POST-VARISCAN DEFORMATION 5.1 INTRODUCTION Deformation continued in Britain and Northern Ireland during post-Variscan times producing a diverse range of mainly regional and meso-scale structures. This deformation exercised major controls on the siting and formation of sedimentary basins and deposition during the Mesozoic and Cenozoic eras. The regional and non-site specific nature of the structures formed during this period, coupled with a general lack of geological literature have made it impractical to designate post-Variscan sites in the current site network. 5.2 THE MESOZOIC BASINS Early Mesozoic times saw widespread crustal subsidence that led to the formation of fault-controlled sedimentary basins in both onshore and offshore Britain (Kent, 1978). This period of tectonism was mainly tensional and was dominated by vertical movements within an otherwise stable cratonic environment. During the earliest stages of basin formation, rift systems developed in many cases exploiting pre-existing Caledonian (NE-SW) faults and lines of crustal weakness. The fault-bounded Mesozoic basins in NW Britain can be divided into two distinct groups (McLean, 1978). An outer group referred to as the ‘Marginal Belt’, stretches approximately 150km offshore to the edge of the continental shelf, and consists of basins with a predominantly NE-SW orientation. An inner group of basins, with a characteristically NW-SE or NNW-SSE orientation, is referred to as the ‘Clyde Belt’, and includes the basins found in Northern Ireland. In NE Ireland, remnants of several Mesozoic basins outcrop around the edge of the Antrim Plateau. Outcrop is generally limited and is restricted to the coastal areas and occasional stream sections. 5.2.1 Rathlin and Lough Foyle Basin The Rathlin Basin is a half-graben structure which deepens southwards towards the Tow Valley Fault and extends north, offshore beneath Rathlin. The Lough Foyle Basin is fault-bounded along the NW shore of Lough Foyle and extends SE beneath Lough Foyle and the thick drift deposits of Magilligan. Exposure is terminated along the scarp, which marks the edge of the basalt plateau east of the Roe Valley. 5.2.2 Larne Basin The Larne Basin in SE Antrim, contains up to 3000m of Mesozoic strata. The basin geometry is strongly controlled by a system of NNW-SSE trending faults. Mesozoic rocks also extend in a series of shallower structures beneath Belfast Lough, Belfast and the Lagan Valley. 5.2.3 Newtownards Trough The Newtownards Trough underlies much of the area between Comber and Newtownards. The trough extends southwards beneath the northern part of Strangford Lough where it forms a half-graben structure deepening eastwards towards and truncating against the Newtownards Fault. 5.2.4 Concealed Mesozoic basins (Lough Neagh and South Antrim) Regional gravity surveys of Northern Ireland indicate the presence of a number of sedimentary basins and sub-basins lying concealed by basalt lavas and under Lough Neagh, SW Antrim and east Tyrone (Geological Survey of Northern Ireland, 1981; Bennett, 1983; Thompson, 1978; Carruthers et al., 1987) (Fig. 31). 5.2.5 Mesozoic structural history The Rathlin Trough developed as a result of sinistral transtension producing extension on NNW-SSE faults in a compartmentalized area between the Tow Valley and Lough Foyle Faults (Kerr, 1987). The study of basement/cover relationships in north Antrim has enabled reconstruction of the post-Carboniferous stress history for the area, in particular the effects of Tertiary stresses on the underlying Mesozoic strata. By analysis of mesofractures across the north Antrim area, Kerr was able to distinguish four principal mesofracture systems, which modified rocks in the Rathlin Trough mainly through transtensional movements on the Tow Valley Fault. Kerr identified four main tectonic systems in the late-Carboniferous to Tertiary structural history of north Antrim and was able to subdivide the Tertiary Fractures (System 4) into two stages, as follows: SYSTEM 1: TIMING: end Carboniferous STRUCTURE: E-W to ESE-WNW compression SYSTEM 2: TIMING: Triassic STRUCTURE: ENE-WSW transtension SYSTEM 3: TIMING: late-Jurassic to early-Cretaceous STRUCTURE: ESE-WNW compression SYSTEM 4 (Stage 1): TIMING: Tertiary (late-Palaeogene to early-Eocene) STRUCTURE: ENE-WSW extension producing normal faults SYSTEM 4 (Stage 2): TIMING: Tertiary (mid-Eocene to mid-Miocene) STRUCTURE: NNE-SSW compression producing strike-slip faults and reactivation of Stage 1 structures 5.3 OPENING OF THE NORTH ATLANTIC AND PALAEOGENE MAGMATISM The last major tectonic event to effect Northern Ireland took place during the Palaeogene and was associated with the opening of the North Atlantic Ocean. North of the Azores, this process took place as the American and European continental plates were separating in a complex sequence of events involving multiple spreading centres. Throughout northern Britain, this was also a time of intense magmatism fueled by hotspots (plumes) in the earth’s upper mantle. The combined effects of sea-floor spreading and magmatism put the continental crust along the NW margin of Europe, including Northern Ireland, under a tensional tectonic regime. At the same time the crust was subject to doming and uplift as a result of upwelling of magma throughout the British Tertiary Igneous Province. In Northern Ireland, during the early Palaeogene, the crust experienced brittle extensional tectonic deformation. Geoffroy et al. (1996) recognized two phases of deformation affecting the Cretaceous chalk and early Tertiary igneous rocks. Each of these phases was characterized by a different stress regime with similar trends of the maximum component of horizontal stress. The Palaeogene in Northern Ireland was a time of extensive extrusive and intrusive magmatic activity. According to Anderson et al. (1968), a number of categories of structure formed at this time and can be directly linked to the magmatism. These include: a. Ring fractures associated with ring-dykes and cone sheets b. Doming and concentric folds due to magmatic intrusion c. Dilation due to magma upwelling and crustal dilation 5.3.1 Intrusions controlled by ring fractures Large circular or annular fractures are closely associated with the intrusion of the Tertiary central igneous intrusions and complexes in Northern Ireland. The Mourne Mountain Granites were emplaced as five main injections (G1-G5) in two centres; G1-G3 in the eastern Mournes and G4-G5 in the western Mournes. According to Richey’s (1927) intrusion model, all five 5 major granites were emplaced in ring fractures and were accommodated by subsidence of the encircled Silurian country rocks. In the Slieve Gullion Ring Complex where some of the igneous activity reached the surface, granophyre and porphyritic felsite ring dykes were emplaced in concentric (diameter of about 11km) ring fractures at the southwestern end of the Newry Granodiorite Complex (Caledonian). Elsewhere in south Down, for example at Glasdrumman Port [3380 3221] (see Key Site 1111 - Mournes - Glasdrumman Port Cone-sheet), composite acid cone-sheets occur in the country rocks surrounding the Mourne Granite Complex. These intrusions were emplaced in outwards-fanning, conical fractures generated by the magmatic upsurge associated with the emplacement of the Mourne Granites. 5.3.2 Doming associated with magmatism In Northern Ireland there is limited evidence of doming associated with magmatism during the Palaeogene. There is some indication of structural drag and strike swing in the Silurian country rocks close to the outer eastern contact of the Mourne Granites, for example at Ballagh Park [3385 3289], although this is not as pronounced as the doming and concentric folding noted around the Northern Granite Complex on the Isle of Arran and the early caldera on Mull. 5.3.3 Crustal dilation Swarms of NW-SE trending dykes occur across Northern Ireland. These swarms increase in intensity in the vicinity of intrusive centres such as Slieve Gullion and the Mourne Mountains and in parts of County Fermanagh. Dykes fill fractures, which are orientated normal to the major direction of rifting that took place between Greenland and Northern Europe (Fig. 32) (Preston, 2001). 5.3.4 Late to post Palaeogene faulting A glance at the outcrop pattern of the basalts in County Antrim indicates that significant faulting took place in Northern Ireland post-eruption and that there are two distinct groups of late to post Palaeogene faults. This pattern has been borne out by regional geophysical surveys and images which confirm the occurrence of these mapped faults along with many hitherto hidden ones (Carruthers et al., 1997). 5.3.4.1 NE-SW trending faults The most numerous and readily mappable group of faults comprise NE-SW trending normal faults including the Tow Valley Fault, Sixmilewater Fault and the Carnlough Fault. These structures shared an orientation with the much earlier formed Caledonian faults and probably represent a series of reactivations of these pre-existing lines of weakness. There is evidence that at least some faults in this group remained active well into the Oligocene Period playing an important role in defining the basins at Ballymoney, Crumlin and Coagh in which the clay-and lignite-bearing Lough Neagh Group was deposited (Legg, 1992) (Fig. 33). 5.3.4.2 NNW-SSE trending faults The second group of late to post Palaeogene faults trend NNW-SSE. These faults help compartmentalize the basalt outcrop and also cut the intrusive centres of the Mourne Mountains and Slieve Gullion. Faults in this group appear in general to be formed earlier and are frequently cross-cut by the NE trending faults. For site specific information on post-Variscan structure see: Key Site 660 - Camlough Quarry Key Site 661 - Eden Lodge.
Notes:
For site specific information on the Lough Derg Inlier see Key Site 642 - Croagh Mountain. For site specific information on the Central Tyrone Inlier see Key Site 643 - Oughtmore Mountain. For site specific information on the Dalradian rocks (Grampian (Central Highlands) Terrane) of the Sperrin Mountains see: Key Site 644 - Prehen Quarry Key Site 645 - Butterlope Glen Key Site 646 - Mountfield Quarry Key Site 647 - McNally’s Burn, Broughderg. For site specific information on the Dalradian rocks (Grampian (Central Highlands) Terrane) of the NE Antrim Inlier see: Key Site 648 - Torr Head Key Site 649 - Benvan, Murlough Key Site 650 - Carnaneigh Key Site 651 - Cushendun Bay. For site specific information on the Paratectonic Caledonides (Southern Uplands Terrane) see: Key Site 652 - Orlock Bridge Key Site 653 - Whiskin Rocks Key Site 654 - White House Port, Cloghy Key Site 655 - Knockinelder Bay and Kearney Point Key Site 656 - Millin Bay. For site specific information on Variscan structure see: Key Site 657 - Ederny Quarry Key Site 658 - Straduff Quarry Key Site 659 - Fardross Stream. For site specific information on post-Variscan structure see: Key Site 660 - Camlough Quarry Key Site 661 - Eden Lodge.

Keywords


Record type:	ESCR report	Recorded by:	T.P. Johnston
Entered by:	E M Porter
Last updated:	23/09/03

Enlander, I., Dempster, M. & Doughty, P., 2026. Introduction to the Structural Geology of Northern Ireland, County Antrim, Down, Fermanagh, Londonderry, Tyrone, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=641. Accessed on 2026-11-06.