Earth Science Conservation Review
| Introduction to the Dalradian rocks of Northern Ireland | Antrim, Londonderry, Tyrone |
| Site Type: | Various |
| Site Status: | |
| Grid Reference: | |
| Google maps: | , |
| Rocks | |
|---|---|
| Rock Age: | Precambrian (Dalradian) |
| Interest | |
Description:
1. INTRODUCTION
PALAEOGEOGRAPHY OF THE DALRADIAN SUPERGROUP
In Northern Britain, during the late Proterozoic Period, the rocks which constitute the Dalradian Supergroup were deposited in a series of tectonically controlled sedimentary basins along the eastern continental margin of the Laurentian Archaean Basement of North America (Harris et al., 1978). These ensialic basins pre-dated the formation of the Iapetus Ocean and are believed to have been geographically separated from the developing Iapetus Ocean by a continental ridge or landmass.
The Dalradian sediments accumulated in predominantly marine and intertidal environments. Sedimentation probably spanned a period of approximately 300 million years and ended during late-Cambrian to early-Ordovician times when a splitting apart of the Laurentian Plate culminated in the opening of the Iapetus Ocean. The Dalradian sedimentary basins were subjected periodically to rapid subsidence and this enabled them to accommodate a thick and rapidly accumulating sedimentary pile (overall thickness estimated to be 25 Km).
Dalradian sedimentation took place within an ever-changing tectonic framework, which resulted in changing depositional environments. The lowermost Groups in the Dalradian, (Grampian Group and Appin Group), neither of which is represented in Northern Ireland, were deposited in comparatively stable platform environments underpinned by stable, pre-Dalradian metamorphic basement. The Grampian and Appin Groups are characterised by shelf and near shoreline carbonate deposits (limestone, phyllite), tidal shelf and deltaic sands (quartzite) and basinal muds (slates). The top of the Appin Group is marked by a shallowing of the depositional basin and the formation of stromatolitic limestone.
At the base of the Argyll Group (Middle Dalradian) there is evidence of the onset of a glacial episode. A widespread tillite horizon is exposed right across the Dalradian belt in the British Isles (excluding Northern Ireland where strata of this age are not exposed or missing). Thereafter, during much of the remaining Argyll Group times there was a dramatic change in the nature and pattern of sedimentation. A major rifting episode occurred during this period, resulting in the initiation of large syn-depositional faults across the Dalradian depositional belt. From Argyll times onwards, a set of syn-depositional Caledonide and trans-Caledonide trending faults divided the area of Dalradian deposition into a series of north-north-east - south-south-west trending blocks and basins. The existence of so many separate sedimentary basins accounts in part for some of the difficulties experienced in establishing detailed lithological correlations across the Dalradian outcrop. In the Argyll Group, and in the succeeding Southern Highland Group, much of the sedimentation was of a turbiditic nature and took place in deep water basinal environments. This change was brought about by active syn-depositional marginal faults which allowed basins to subside and accommodate the rapidly accumulating sediment (Anderton, 1979, 1982). There is also evidence of increased tectonic instability during this period with deposition taking place both in rapidly deepening and in shallowing basins.
The principal source of sediment in the Argyll Group lay to the north-west of the depositional basins, on the eroded continental margin which provided the predominantly mature and quartz-rich sediments deposited as deep sea fans and turbidites.
VOLCANIC ACTIVITY IN THE DALRADIAN
Throughout the Dalradian succession, there is evidence of both extrusive and intrusive volcanic activity in or adjacent to the depositional basins. This volcanic activity manifests itself in the form of metabasite sills and dykes and at particular stratigraphic horizons as basaltic pillow lavas, hyaloclastites and volcanogenic sediments (green beds) which contain a significant component of basic to intermediate volcaniclastic sediment.
Volcanic activity reached a climax in the Tayvallich Subgroup towards the top of the Argyll Group. In Scotland these periods of localised vulcanism were generally associated with phases of basin deepening in the Middle Dalradian and the basaltic magmas erupted at this time had predominantly tholeiitic petrological and geochemical characteristics. These factors have led to the conclusion that this volcanism probably represented the early phase of an igneous cycle associated with crustal distension and possible separation of continental crust in lower Cambrian times (Graham, 1979; Graham et al., 1981).
SOUTHERN HIGHLAND GROUP (UPPER DALRADIAN)
The Southern Highland Group (Upper Dalradian) is characterised by a major influx of comparatively immature clastic sediments. These include potash-feldspathic grits with fresh metamorphic clasts and granitic materials. Much of this material was deposited in the form of proximal turbidites. It is unlikely that such material could have been derived from the same source as the bulk of the earlier, more mature Dalradian sediment and therefore, it has been suggested that during the Upper Dalradian, these sedimentary basins were separated from the Iapetus Ocean? which lay some distance to the south, by a continental ridge or landmass (Harris et al., 1978).
2. DEFORMATION AND METAMORPHISM
DEFORMATION
Deposition of the Dalradian rocks came to an end sometime during late-Cambrian to early Ordovician times. From the end of the Cambrian, the Iapetus Ocean began to close as a result of convergence of the North America and European Plates. Plate convergence and continental collision resulted in a series of tectonic and metamorphic events during which the Dalradian rocks were buried, deformed and metamorphosed. These mountain building events lasted from about 510-400 million years ago. Peak deformation occurred at around 500 million years ago during the Grampian Orogeny at which time major (F2) fold structures formed in the Dalradian of the Sperrins and North-east Antrim. The early-formed structures in the Dalradian were then modified by a series of later tectonic events culminating around 400 million years ago with the Caledonian Orogeny, a major mountain building event which deformed and uplifted the Ordovician and Silurian rocks of the Southern Uplands of Scotland and the Down-Longford Belt.
METAMORPHISM
The Dalradian rocks in North-west Ireland have been affected by Barrovian style regional metamorphism associated with polyphase deformation. In the Sperrins and North-east Antrim metamorphic mineral assemblages generally indicate peak metamorphism at garnet grade (epidote amphibolite facies). However, peak metamorphic conditions were probably not uniform and there is abundant evidence, for example in the Sperrins, of a strain and metamorphic grade increase south-eastwards from the lower Greenschist Facies, low strain rocks in the Londonderry Syncline (Londonderry Formation) to the more highly strained, garnet grade rocks in the southern Sperrins along the Omagh Fault (Mullaghcarn Formation).
Peak metamorphism was followed by a widespread retrogressive metamorphic phase probably related to structural uplift of strata right across the Dalradian belt. This resulted in an overprinting of lower to middle Greenschist Facies mineral assemblages with chlorite across much of the outcrop.
3. DALRADIAN LITHOSTRATIGRAPHIC FRAMEWORK AND CORRELATION
Detailed correlations between the Dalradian of the British Isles and supposed equivalent successions on the North-east American seaboard remain open to question. Kennedy (1975) proposed a correlation between the Dalradian of the British Isles and the Fleur de Lys Supergroup in Newfoundland, although (Hibbard, 1988) cast some doubts on Kennedy's correlation.
The Dalradian Supergroup consists of a lithologically diverse succession of metasedimentary and metavolcanic rocks. The boundaries of the Dalradian outcrop in Northern Britain (Central Highlands Terrane) are structurally controlled and neither the top nor the bottom of the succession is exposed. Nevertheless the aggregate thickness of the Dalradian Supergroup in the British Isles (making no allowances for tectonic thickening) has been estimated at 25 kms (Gibbons et al., 1994).
Many rapid facies variations occur within the Dalradian succession and account for some of the difficulties in correlation. However, despite these difficulties, there is now a comprehensive lithostratigraphic framework for the Dalradian rocks of Northern Ireland. At group level at least, and more rarely at the formation level, there are now sustainable correlations across the entire Dalradian belt in Ireland and Scotland. These correlations are based on published scientific papers and maps produced by the Geological Survey of Northern Ireland (Gibbons et al. (eds.), 1994).
Within Northern Ireland, the Dungiven Formation (Sperrin Mountains) and the Torr Head Limestone Formation (North-east Antrim) are the key marker horizons which form the basis of current Dalradian correlation throughout Northern Ireland. More significantly these distinctive limestonevolcanogenic rock associations have also been correlated with the Lough Tay Limestone Formation in the Central Highlands of Scotland and with the Culdaff Limestone in Donegal. On the basis of these correlations, the Dungiven and Torr Head Formations are equivalent to the Tayvallich Subgroup of the Argyll Group (Middle Dalradian) and as a consequence of folding and overturning, the majority of the remaining Dalradian Strata in the Sperrins and North-east Antrim belong to the Southern Highlands Group.
TABLE TO BE ADDED The Dalradian Stratigraphy of Northern Ireland.
4. DALRADIAN GEOLOGY: PREVIOUS RESEARCH
From the earliest days of the development of geology as a scientific discipline, the Dalradian metamorphic rocks of the British Isles have been the subject of intense research interest. Academic interest in the Highlands of Scotland has continued largely unabated throughout this century and this area has become unquestionably one of the classic areas (of world-wide renown) for the study of regional metamorphism and structural geology. Barrow (1893) introduced the concept of metamorphic minerals as indicators of metamorphic grade in pelitic rocks at the south-eastern end of the Central Highlands, a concept subsequently extended (with some refinements) by Tilley (1925) into the western part of the Central Highlands. Barrow and Tilley promoted the concept of "isograds", thermal surfaces which could be inferred on the basis of the presence or absence of index minerals. Read (1952) postulated another metamorphic zonal scheme, known as Buchan type metamorphism in order to account for unusual mineral assemblages along a narrow zone in the north-east Highlands.
HISTORY OF MAPPING OF THE DALRADIAN ROCKS IN IRELAND
In Ireland, as in the rest of the British Isles, the earliest systematic geological mapping was carried out by publicly financed organisations. In 1838 and 1839, the Ordnance Survey inscribed geological maps of Ireland (at one inch to one mile and one quarter inch to one mile scales) for the Railway Commission which had been established to report on rail communications in Ireland. These maps were based on work by Richard Griffith. During the 1830's, Griffith had worked in the office of the General Survey and Valuation of Rateable Property in Ireland where he had unofficially collected geological data in tandem with his official duties. In later years, as Commissioner of the Valuation Office, Griffith continued to encourage collection of geological data and was in effect running an unofficial Geological Survey. On Griffith's 1839 (1/4 inch to one mile) map, the Dalradian rocks were described as "Primary sedimentary Rocks" and subdivided into three lithological categories, "Mica Slate", "Primary Limestone" and "Quartz Rock". The bulk of the Dalradian outcrop in Northern Ireland was represented as Mica Slates with only the limestone units at Torr Head (County Antrim) and in the northern Sperrins being shown as discrete units.
From the 1820's onwards, under the directorship of Thomas Colby, the Ordnance Survey in Ireland, also acted as an unofficial precursor of the Geological Survey of Ireland (Herries Davies, 1995). The Ordnance Survey recorded geological data at the same time as surveying for the 1:10,560 scale topographic map of Ireland. These geological activities became more formalised in 1830 when Joseph Portlock was appointed to the Ordnance Survey as a geologist.
In Ireland, the first complete coverage of topographic maps at the six inch to one mile scale became available for County Londonderry and consequently Portlock concentrated his early geological surveying in the north of Ireland. In 1843, Portlock's "Report on the Geology of the County of Londonderry etc." was published. This volume included a « inch to one mile scale geological map covering all of the Dalradian outcrop of the Sperrin Mountains (Portlock, 1843). Portlock described the Dalradian rocks as "Crystalline Schists, Gneiss, Mica Schist & Hornblendic Gneiss". Although Portlock recognised the relative antiquity of these strata, he did not assign them to any particular geological system and with the exception of showing limestone and "trap rocks" (metabasites) there was no attempt to subdivide the rocks into lithological formations.
THE GEOLOGICAL SURVEY OF IRELAND
The Geological Survey of Ireland was formed in 1845 and thereafter embarked upon an ambitious programme of geological mapping which was to lead, by the end of the century, to a complete geological survey of Ireland at the six inch to one mile scale. The Dalradian rocks in the north of Ireland were surveyed as part of this programme and during the period 1882 to 1890, the Geological Survey of Ireland published complete map coverage for the Dalradian in Northern Ireland (contained on 12 one inch to the mile scale sheets).
With the exception of some of the more remote areas in the Sperrin Mountains, the Dalradian outcrop in Northern Ireland was surveyed at a scale of the six inch to one mile. In all of the first edition one inch maps, the Dalradian strata are referred to as "altered Silurian rocks" or simply as metamorphic schist and gneiss. Little attempt was made to subdivide these rocks into lithological formations although, limestones, quartzites and quartz schists were mapped as were "basalt intrusives".
THE GEOLOGICAL SURVEY OF NORTHERN IRELAND
The Geological Survey of Northern Ireland was formed in 1947 with the intention of providing a complete revision of the one inch (later 1:50,000) geological maps of Northern Ireland.
Part of the Dalradian outcrop in north-eastern County Antrim was remapped during the resurvey of the Ballycastle area (Sheet 8) (Geological Survey of Northern Ireland, 1963) and described in the accompanying memoir (Wilson et al., 1966).
During the early 1970's the Geological Survey of Northern Ireland carried out geochemical surveys across the Sperrins Mountains and North-east Antrim, aware of the metalliferous potential of the Dalradian rocks, in particular gold mineralization. The results of these surveys were published in two Special Reports (Arthurs, 1976a, 1976b). These reports contained geological maps based on mapping carried out in tandem with the geochemical survey. Despite limitations imposed by the reconnaissance style of this mapping, these maps and their more rigorous approach to stratigraphy added significantly to the earlier published maps. Furthermore, Arthurs' maps represent some of the earliest attempts to employ modern structural techniques in mapping the Dalradian, particularly in the Sperrin Mountains.
The period since the publication of Arthurs' work, has seen a sustained period of commercial exploration for gold and precious minerals in the Sperrin Mountains and adjacent Dalradian outcrops. Since 1986, the Geological Survey of Northern Ireland's principal mapping programme has concentrated on the Sperrin Mountains to help underpin commercial exploration.
Many of the sites described in this review were first discovered in the course of the mapping carried out by the Geological Survey of Northern Ireland. The 1:50,000 scale Draperstown map (Sheet 26) (Geological Survey of Northern Ireland, 1995) covers much of the outcrop of the Dalradian in the Southern Sperrins including the Curraghinalt gold occurrence. The Omagh map (Sheet 33) (Geological Survey of Northern Ireland, 1995) includes the Dalradian inlier at Lack and the gold occurrence at Cavanacaw.
5. ACADEMIC RESEARCH
Although the Dalradian rocks of Ireland have been described in many published research papers dating back to the early years of the 19th century, these rocks have never received the same attention and intensive academic research as equivalent strata in Scotland.
Within Ireland, research efforts have tended to concentrate on the well-exposed successions of Donegal whereas the Dalradian in Northern Ireland has been comparatively neglected. It was only as a result of the burgeoning interest in the Dalradian as a site for potential gold mineralization, encouraged by the discovery of vein gold deposits near Gortin (Curraghinalt) and Omagh (Cavanacaw), that research interest has been re-awakened in recent times.
Following the publication of the Geological Survey of Ireland's one inch to one mile maps of the Sperrin Mountains in the 1880's, there was a long period during which little substantive research was carried out in the area.
Potentially the most significant geologist to work in the area during the early years of the 20th century was G. Barrow who embarked on a systematic study of the Sperrin Mountains following his retirement from mapping in the Scottish Highlands for the Geological Survey of Scotland. For reasons unknown, this work was however short lived, the only data published being a brief report to the British Association meeting in Belfast (Barrow, 1902), in which Barrow drew attention to the similarity between rocks at the southern margin of the Dalradian Belt in the Sperrins (in the vicinity of the Omagh Thrust) and the Highland Boundary and Highland Border Series in Scotland.
PALAEOGEOGRAPHY OF THE DALRADIAN SUPERGROUP
In Northern Britain, during the late Proterozoic Period, the rocks which constitute the Dalradian Supergroup were deposited in a series of tectonically controlled sedimentary basins along the eastern continental margin of the Laurentian Archaean Basement of North America (Harris et al., 1978). These ensialic basins pre-dated the formation of the Iapetus Ocean and are believed to have been geographically separated from the developing Iapetus Ocean by a continental ridge or landmass.
The Dalradian sediments accumulated in predominantly marine and intertidal environments. Sedimentation probably spanned a period of approximately 300 million years and ended during late-Cambrian to early-Ordovician times when a splitting apart of the Laurentian Plate culminated in the opening of the Iapetus Ocean. The Dalradian sedimentary basins were subjected periodically to rapid subsidence and this enabled them to accommodate a thick and rapidly accumulating sedimentary pile (overall thickness estimated to be 25 Km).
Dalradian sedimentation took place within an ever-changing tectonic framework, which resulted in changing depositional environments. The lowermost Groups in the Dalradian, (Grampian Group and Appin Group), neither of which is represented in Northern Ireland, were deposited in comparatively stable platform environments underpinned by stable, pre-Dalradian metamorphic basement. The Grampian and Appin Groups are characterised by shelf and near shoreline carbonate deposits (limestone, phyllite), tidal shelf and deltaic sands (quartzite) and basinal muds (slates). The top of the Appin Group is marked by a shallowing of the depositional basin and the formation of stromatolitic limestone.
At the base of the Argyll Group (Middle Dalradian) there is evidence of the onset of a glacial episode. A widespread tillite horizon is exposed right across the Dalradian belt in the British Isles (excluding Northern Ireland where strata of this age are not exposed or missing). Thereafter, during much of the remaining Argyll Group times there was a dramatic change in the nature and pattern of sedimentation. A major rifting episode occurred during this period, resulting in the initiation of large syn-depositional faults across the Dalradian depositional belt. From Argyll times onwards, a set of syn-depositional Caledonide and trans-Caledonide trending faults divided the area of Dalradian deposition into a series of north-north-east - south-south-west trending blocks and basins. The existence of so many separate sedimentary basins accounts in part for some of the difficulties experienced in establishing detailed lithological correlations across the Dalradian outcrop. In the Argyll Group, and in the succeeding Southern Highland Group, much of the sedimentation was of a turbiditic nature and took place in deep water basinal environments. This change was brought about by active syn-depositional marginal faults which allowed basins to subside and accommodate the rapidly accumulating sediment (Anderton, 1979, 1982). There is also evidence of increased tectonic instability during this period with deposition taking place both in rapidly deepening and in shallowing basins.
The principal source of sediment in the Argyll Group lay to the north-west of the depositional basins, on the eroded continental margin which provided the predominantly mature and quartz-rich sediments deposited as deep sea fans and turbidites.
VOLCANIC ACTIVITY IN THE DALRADIAN
Throughout the Dalradian succession, there is evidence of both extrusive and intrusive volcanic activity in or adjacent to the depositional basins. This volcanic activity manifests itself in the form of metabasite sills and dykes and at particular stratigraphic horizons as basaltic pillow lavas, hyaloclastites and volcanogenic sediments (green beds) which contain a significant component of basic to intermediate volcaniclastic sediment.
Volcanic activity reached a climax in the Tayvallich Subgroup towards the top of the Argyll Group. In Scotland these periods of localised vulcanism were generally associated with phases of basin deepening in the Middle Dalradian and the basaltic magmas erupted at this time had predominantly tholeiitic petrological and geochemical characteristics. These factors have led to the conclusion that this volcanism probably represented the early phase of an igneous cycle associated with crustal distension and possible separation of continental crust in lower Cambrian times (Graham, 1979; Graham et al., 1981).
SOUTHERN HIGHLAND GROUP (UPPER DALRADIAN)
The Southern Highland Group (Upper Dalradian) is characterised by a major influx of comparatively immature clastic sediments. These include potash-feldspathic grits with fresh metamorphic clasts and granitic materials. Much of this material was deposited in the form of proximal turbidites. It is unlikely that such material could have been derived from the same source as the bulk of the earlier, more mature Dalradian sediment and therefore, it has been suggested that during the Upper Dalradian, these sedimentary basins were separated from the Iapetus Ocean? which lay some distance to the south, by a continental ridge or landmass (Harris et al., 1978).
2. DEFORMATION AND METAMORPHISM
DEFORMATION
Deposition of the Dalradian rocks came to an end sometime during late-Cambrian to early Ordovician times. From the end of the Cambrian, the Iapetus Ocean began to close as a result of convergence of the North America and European Plates. Plate convergence and continental collision resulted in a series of tectonic and metamorphic events during which the Dalradian rocks were buried, deformed and metamorphosed. These mountain building events lasted from about 510-400 million years ago. Peak deformation occurred at around 500 million years ago during the Grampian Orogeny at which time major (F2) fold structures formed in the Dalradian of the Sperrins and North-east Antrim. The early-formed structures in the Dalradian were then modified by a series of later tectonic events culminating around 400 million years ago with the Caledonian Orogeny, a major mountain building event which deformed and uplifted the Ordovician and Silurian rocks of the Southern Uplands of Scotland and the Down-Longford Belt.
METAMORPHISM
The Dalradian rocks in North-west Ireland have been affected by Barrovian style regional metamorphism associated with polyphase deformation. In the Sperrins and North-east Antrim metamorphic mineral assemblages generally indicate peak metamorphism at garnet grade (epidote amphibolite facies). However, peak metamorphic conditions were probably not uniform and there is abundant evidence, for example in the Sperrins, of a strain and metamorphic grade increase south-eastwards from the lower Greenschist Facies, low strain rocks in the Londonderry Syncline (Londonderry Formation) to the more highly strained, garnet grade rocks in the southern Sperrins along the Omagh Fault (Mullaghcarn Formation).
Peak metamorphism was followed by a widespread retrogressive metamorphic phase probably related to structural uplift of strata right across the Dalradian belt. This resulted in an overprinting of lower to middle Greenschist Facies mineral assemblages with chlorite across much of the outcrop.
3. DALRADIAN LITHOSTRATIGRAPHIC FRAMEWORK AND CORRELATION
Detailed correlations between the Dalradian of the British Isles and supposed equivalent successions on the North-east American seaboard remain open to question. Kennedy (1975) proposed a correlation between the Dalradian of the British Isles and the Fleur de Lys Supergroup in Newfoundland, although (Hibbard, 1988) cast some doubts on Kennedy's correlation.
The Dalradian Supergroup consists of a lithologically diverse succession of metasedimentary and metavolcanic rocks. The boundaries of the Dalradian outcrop in Northern Britain (Central Highlands Terrane) are structurally controlled and neither the top nor the bottom of the succession is exposed. Nevertheless the aggregate thickness of the Dalradian Supergroup in the British Isles (making no allowances for tectonic thickening) has been estimated at 25 kms (Gibbons et al., 1994).
Many rapid facies variations occur within the Dalradian succession and account for some of the difficulties in correlation. However, despite these difficulties, there is now a comprehensive lithostratigraphic framework for the Dalradian rocks of Northern Ireland. At group level at least, and more rarely at the formation level, there are now sustainable correlations across the entire Dalradian belt in Ireland and Scotland. These correlations are based on published scientific papers and maps produced by the Geological Survey of Northern Ireland (Gibbons et al. (eds.), 1994).
Within Northern Ireland, the Dungiven Formation (Sperrin Mountains) and the Torr Head Limestone Formation (North-east Antrim) are the key marker horizons which form the basis of current Dalradian correlation throughout Northern Ireland. More significantly these distinctive limestonevolcanogenic rock associations have also been correlated with the Lough Tay Limestone Formation in the Central Highlands of Scotland and with the Culdaff Limestone in Donegal. On the basis of these correlations, the Dungiven and Torr Head Formations are equivalent to the Tayvallich Subgroup of the Argyll Group (Middle Dalradian) and as a consequence of folding and overturning, the majority of the remaining Dalradian Strata in the Sperrins and North-east Antrim belong to the Southern Highlands Group.
TABLE TO BE ADDED The Dalradian Stratigraphy of Northern Ireland.
4. DALRADIAN GEOLOGY: PREVIOUS RESEARCH
From the earliest days of the development of geology as a scientific discipline, the Dalradian metamorphic rocks of the British Isles have been the subject of intense research interest. Academic interest in the Highlands of Scotland has continued largely unabated throughout this century and this area has become unquestionably one of the classic areas (of world-wide renown) for the study of regional metamorphism and structural geology. Barrow (1893) introduced the concept of metamorphic minerals as indicators of metamorphic grade in pelitic rocks at the south-eastern end of the Central Highlands, a concept subsequently extended (with some refinements) by Tilley (1925) into the western part of the Central Highlands. Barrow and Tilley promoted the concept of "isograds", thermal surfaces which could be inferred on the basis of the presence or absence of index minerals. Read (1952) postulated another metamorphic zonal scheme, known as Buchan type metamorphism in order to account for unusual mineral assemblages along a narrow zone in the north-east Highlands.
HISTORY OF MAPPING OF THE DALRADIAN ROCKS IN IRELAND
In Ireland, as in the rest of the British Isles, the earliest systematic geological mapping was carried out by publicly financed organisations. In 1838 and 1839, the Ordnance Survey inscribed geological maps of Ireland (at one inch to one mile and one quarter inch to one mile scales) for the Railway Commission which had been established to report on rail communications in Ireland. These maps were based on work by Richard Griffith. During the 1830's, Griffith had worked in the office of the General Survey and Valuation of Rateable Property in Ireland where he had unofficially collected geological data in tandem with his official duties. In later years, as Commissioner of the Valuation Office, Griffith continued to encourage collection of geological data and was in effect running an unofficial Geological Survey. On Griffith's 1839 (1/4 inch to one mile) map, the Dalradian rocks were described as "Primary sedimentary Rocks" and subdivided into three lithological categories, "Mica Slate", "Primary Limestone" and "Quartz Rock". The bulk of the Dalradian outcrop in Northern Ireland was represented as Mica Slates with only the limestone units at Torr Head (County Antrim) and in the northern Sperrins being shown as discrete units.
From the 1820's onwards, under the directorship of Thomas Colby, the Ordnance Survey in Ireland, also acted as an unofficial precursor of the Geological Survey of Ireland (Herries Davies, 1995). The Ordnance Survey recorded geological data at the same time as surveying for the 1:10,560 scale topographic map of Ireland. These geological activities became more formalised in 1830 when Joseph Portlock was appointed to the Ordnance Survey as a geologist.
In Ireland, the first complete coverage of topographic maps at the six inch to one mile scale became available for County Londonderry and consequently Portlock concentrated his early geological surveying in the north of Ireland. In 1843, Portlock's "Report on the Geology of the County of Londonderry etc." was published. This volume included a « inch to one mile scale geological map covering all of the Dalradian outcrop of the Sperrin Mountains (Portlock, 1843). Portlock described the Dalradian rocks as "Crystalline Schists, Gneiss, Mica Schist & Hornblendic Gneiss". Although Portlock recognised the relative antiquity of these strata, he did not assign them to any particular geological system and with the exception of showing limestone and "trap rocks" (metabasites) there was no attempt to subdivide the rocks into lithological formations.
THE GEOLOGICAL SURVEY OF IRELAND
The Geological Survey of Ireland was formed in 1845 and thereafter embarked upon an ambitious programme of geological mapping which was to lead, by the end of the century, to a complete geological survey of Ireland at the six inch to one mile scale. The Dalradian rocks in the north of Ireland were surveyed as part of this programme and during the period 1882 to 1890, the Geological Survey of Ireland published complete map coverage for the Dalradian in Northern Ireland (contained on 12 one inch to the mile scale sheets).
With the exception of some of the more remote areas in the Sperrin Mountains, the Dalradian outcrop in Northern Ireland was surveyed at a scale of the six inch to one mile. In all of the first edition one inch maps, the Dalradian strata are referred to as "altered Silurian rocks" or simply as metamorphic schist and gneiss. Little attempt was made to subdivide these rocks into lithological formations although, limestones, quartzites and quartz schists were mapped as were "basalt intrusives".
THE GEOLOGICAL SURVEY OF NORTHERN IRELAND
The Geological Survey of Northern Ireland was formed in 1947 with the intention of providing a complete revision of the one inch (later 1:50,000) geological maps of Northern Ireland.
Part of the Dalradian outcrop in north-eastern County Antrim was remapped during the resurvey of the Ballycastle area (Sheet 8) (Geological Survey of Northern Ireland, 1963) and described in the accompanying memoir (Wilson et al., 1966).
During the early 1970's the Geological Survey of Northern Ireland carried out geochemical surveys across the Sperrins Mountains and North-east Antrim, aware of the metalliferous potential of the Dalradian rocks, in particular gold mineralization. The results of these surveys were published in two Special Reports (Arthurs, 1976a, 1976b). These reports contained geological maps based on mapping carried out in tandem with the geochemical survey. Despite limitations imposed by the reconnaissance style of this mapping, these maps and their more rigorous approach to stratigraphy added significantly to the earlier published maps. Furthermore, Arthurs' maps represent some of the earliest attempts to employ modern structural techniques in mapping the Dalradian, particularly in the Sperrin Mountains.
The period since the publication of Arthurs' work, has seen a sustained period of commercial exploration for gold and precious minerals in the Sperrin Mountains and adjacent Dalradian outcrops. Since 1986, the Geological Survey of Northern Ireland's principal mapping programme has concentrated on the Sperrin Mountains to help underpin commercial exploration.
Many of the sites described in this review were first discovered in the course of the mapping carried out by the Geological Survey of Northern Ireland. The 1:50,000 scale Draperstown map (Sheet 26) (Geological Survey of Northern Ireland, 1995) covers much of the outcrop of the Dalradian in the Southern Sperrins including the Curraghinalt gold occurrence. The Omagh map (Sheet 33) (Geological Survey of Northern Ireland, 1995) includes the Dalradian inlier at Lack and the gold occurrence at Cavanacaw.
5. ACADEMIC RESEARCH
Although the Dalradian rocks of Ireland have been described in many published research papers dating back to the early years of the 19th century, these rocks have never received the same attention and intensive academic research as equivalent strata in Scotland.
Within Ireland, research efforts have tended to concentrate on the well-exposed successions of Donegal whereas the Dalradian in Northern Ireland has been comparatively neglected. It was only as a result of the burgeoning interest in the Dalradian as a site for potential gold mineralization, encouraged by the discovery of vein gold deposits near Gortin (Curraghinalt) and Omagh (Cavanacaw), that research interest has been re-awakened in recent times.
Following the publication of the Geological Survey of Ireland's one inch to one mile maps of the Sperrin Mountains in the 1880's, there was a long period during which little substantive research was carried out in the area.
Potentially the most significant geologist to work in the area during the early years of the 20th century was G. Barrow who embarked on a systematic study of the Sperrin Mountains following his retirement from mapping in the Scottish Highlands for the Geological Survey of Scotland. For reasons unknown, this work was however short lived, the only data published being a brief report to the British Association meeting in Belfast (Barrow, 1902), in which Barrow drew attention to the similarity between rocks at the southern margin of the Dalradian Belt in the Sperrins (in the vicinity of the Omagh Thrust) and the Highland Boundary and Highland Border Series in Scotland.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Introduction to the Dalradian rocks of Northern Ireland, County Antrim, Londonderry, Tyrone, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=124. Accessed on 2024-12-26 |
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