Earth Science Conservation Review
| Introduction to the Precambrian of Northern Ireland | Antrim, Fermanagh, Londonderry, Tyrone |
| Site Type: | Various |
| Site Status: | |
| Grid Reference: | |
| Google maps: | , |
| Rocks | |
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| Rock Age: | Precambrian (Archaean, Proterozoic) |
| Interest | |
Description:
1. THE PRECAMBRIAN STRATIGRAPHIC FRAMEWORK
PRECAMBRIAN STRATIGRAPHIC NOMENCLATURE AND TIME SCALE
The Precambrian Era spans the earliest interval of geological time, from the origin of the earth (currently estimated to be about 4600-5000 million years ago (Ma)) to the beginning of the Cambrian Period (570 Ma).
In common with most stratigraphic terminology, Precambrian nomenclature has changed throughout the years as geological knowledge increased and concepts evolved. The task of producing a standardised and globally acceptable stratigraphic scale has fallen to the International Union of Geological Sciences (IUGS) working through its largest constituent body, the International Commission of Stratigraphy (ICS) and many subcommissions. The chronometric divisions and terminology for the Precambrian as used in this report are based on the scheme proposed by Cowie et al. (1989) to the ICS, adopted by the Subcommission on Precambrian Stratigraphy and recommended to the International Commission on Stratigraphy in 1989.
The Precambrian comprises two major stratigraphic subdivisions or Eons (the Archaean and the Proterozoic). These divisions are each characterised and dominated by rocks and rock associations that reflect broadly contrasting geological processes.
THE ARCHAEAN EON (c. 4600 Ma to 2500 Ma)
In 1976, the Precambrian Stratigraphic Subcommission defined the Archaean as a standard time division ending at 2500 Ma but with no formally defined beginning.
Throughout the world the Archaean is represented by a characteristically complex association of high-grade metamorphic gneiss. It is commonly believed that the bulk of the earth's continental crust was formed during the Archaean. No Archaean rocks are known to exist in Ireland.
THE PROTEROZOIC EON (2500 Ma to 570 Ma)
The term Proterozoic was introduced by Emmons in 1888 and gained wide usage in the United States of America during the early part of the 20th century (Harland et al., 1990). In the United States Geological Survey, the term Proterozoic was used more or less synonymously with Precambrian until 1925.
In 1964, the Geological Survey of Canada defined the Proterozoic chronometrically as succeeding the Archaean at 2390 Ma. This boundary was subsequently set at 2500 Ma by the International Subcommission on the Precambrian (Sims, 1980).
CHRONOSTRATIGRAPHIC NOMENCLATURE
In recent years there has been an increased tendency towards applying chronostratigraphy in the field and attempts continue to devise a workable scheme of chronostratic subdivisions for use in the Precambrian (Proterozoic).
Chronostratic schemes face major difficulties, particularly in finding Precambrian rock successions that can be reliably correlated on the worldwide scale. Chronostratic correlation has not so far been widely applied in Precambrian geology of Northern Ireland or indeed in the wider context of the British Isles and at present there remains a strong argument in favour of using chronometric divisions alone for sub-dividing Precambrian time (Harland et al., 1980).
THE PROTEROZOIC
During the Proterozoic Eon, the majority of active geological processes involved recycling of the Archaean continental crust rather than adding to it. Sedimentary basins formed on the margins of the eroding Archaean cratons. The early Proterozoic (2000 Ma) was also a period of major tectonic deformation with associated, metamorphism and granitic intrusion. At least some of this activity was associated with plate tectonics. Between 1000-1200 Ma, intense deformation and metamorphism affected the Grenville Belt which extends from eastern Canada to Ireland, the Scottish Highlands and Scandinavia. Fragments of the Grenville Belt occur today as inliers within the later, Caledonian fold-belt in Ireland and Scotland.
THE PHANEROZOIC (PALAEOZOIC ERA)
Strictly speaking the Phanerozoic lies outside the remit of this report. However, it is useful to make brief reference to tectonic events during the lowermost Phanerozoic (Palaeozoic) since these events continued to affect the Dalradian rocks of the Sperrin Mountains and North-east Antrim.
In Northern Ireland the later phases of the Caledonian orogeny (D3 onwards) resulted in south-easterly-directed thrusting along the Omagh/Castle Archdale Fault system. In mid-Ulster, in the hanging wall of the Omagh Fault (Omagh Thrust), the Dalradian rocks of the Sperrin Mountains were translated south-eastwards across an Ordovician (about 470 Ma old, Hutton et al., 1985) ophiolite and island arc (Tyrone Plutonic and the Tyrone Volcanic Group (Alsop et al., 1993)). There is also evidence in the younger Phanerozoic rocks south of the Omagh/Castle Archdale Fault system in Fermanagh and west Tyrone, that brittle and cataclastic deformation continued intermittently throughout most of the Carboniferous Period probably until Westphalian B times (about 310 Ma) (Mitchell, 1992). It is assumed that much of these later (Variscan) tectonic events also effected the Dalradian rocks.
2. PRECAMBRIAN LITHOSTRATIGRAPHIC FRAMEWORK IN N. IRELAND
INTRODUCTION
Throughout the world there are particular difficulties associated with stratigraphic correlation of the Precambrian rocks and in this respect, Northern Ireland is no exception. In common with most metamorphic terranes, the rocks contain no fossils and there is therefore no possibility of biostratigraphic correlation. The rocks are also structurally complex as a result of orogenic folding and thrusting and allochthonous movement of terranes. The effects and implications of the latter can be particularly difficult to recognise and quantify.
PRE-DALRADIAN ROCKS IN N. IRELAND
The Pre-Dalradian (Moinian?) strata in Northern Ireland provide perhaps the greatest challenges for correlation because of their apparently allochthonous nature and their probable status as "suspect terranes". The Pre-Dalradian Inliers of Lough Derg and Central Tyrone are isolated and fault-bounded. The rocks in each of these inliers have undergone pre-Grampian, high-grade (up to and including sillimanite grade) metamorphism and polyphase deformation. These mineral assemblages have subsequently been overprinted by a retrogressive thermo-tectonic event during the early Caledonian.
THE DALRADIAN SUPERGROUP
In Ireland and Scotland, major difficulties have also been encountered when detailed stratigraphic correlation has been attempted within the Dalradian Supergroup (Gibbons et al., 1994). The Dalradian successions tend to be lithologically diverse with rapid facies variations, and in certain areas parts of successions have been severely attenuated or removed by faulting. As a result, widespread correlations in the Dalradian are not considered reliable at the formation level and for the most part, formations are regarded as being of local significance only albeit remaining as valuable indicators of local changes in depositional conditions and the geometry of tectonic structures. Fortunately, some more widespread correlations are possible at group level in the Dalradian. The topmost part of the Argyll Group (Tayvallich Subgroup) is, for example, marked by a distinctive lithological association of limestone and volcanogenic rocks. This association can be traced from Scotland (Lough Tay Limestone), through Northern Ireland (Torr Head Formation & Dungiven Formation) to Donegal (Culdaff Limestone). Likewise, the base of the Argyll Group in Scotland and in Donegal is marked by a boulder bed or tillite (Schiehallion Boulder Bed, Port Askaig Tillite).
3. PRECAMBRIAN TERRANES
TERRANES & PLATE TECTONIC MODELS
An account of the palaeogeography and evolution of the Precambrian of the British Isles is best prefaced by a brief discussion of the concept of terranes and the important role played by plate tectonic processes in modelling and accounting for the distribution of the continents through geological time. For many years, major lithological, structural and metamorphic discontinuities have been recognised within the world's principal metamorphic belts. Likewise, in Northern Britain, major faults separate tracts of tectonized rocks of contrasting age (Archaean to Late Proterozoic) and metamorphic grade (Granulite Facies to Lower Greenschist Facies). Early interpretations of these rocks largely disregarded the global dimension and treated rocks as essentially autochthonous (formed essentially in the place they are found today).
Since the 1960's however, the evolution of the world's metamorphic belts (including Northern Britain) has been viewed in a global context. Plate tectonic theory, in particular the key mechanism of continental drift, has revolutionised our understanding of the evolution of the continents. Plate tectonic theory offers a mechanism whereby large areas of lithospheric crust (plates) can migrate over large distances across the surface of the earth through geological time. Oceans can open up, form sedimentary basins, then close, and in the process form mountain chains over destructive plate margins.
Because the earth's crust was much thinner during Precambrian times, and there were higher rates of geothermal heat flow, the rate at which lithospheric plates moved was much greater than it is today. Consequently, by the end of the Precambrian, some of the older rocks in the earth's crust had already experienced several episodes of deformation and metamorphism.
The net result of such movements is that areas of the earth's crust (terranes), characterised by shared lithological, structural and metamorphic features, may drift away from their area of origin. The process of movement and coming together (docking) of terranes often takes place along large-scale transform or transpressional faults. The net results of these processes are metamorphic belts which may consist of a series of accreted allochthonous and autochthonous terranes with boundaries defined by faults or fault complexes. Because of the faulted nature of most terrane boundaries, the tops and bottoms of geological successions in individual terranes are seldom seen.
THE TERRANES OF NORTHERN BRITAIN
The basement rocks of Northern Britain consist of a number of allocthonous terranes separated by major NE to SW trending faults. These terranes range in age from Early Proterozoic to Lower Palaeozoic age (about 2.9 Ga to 470 Ma) (Bluck et al., 1992).
HEBRIDEAN TERRANE
The oldest rocks in the British Isles occur in the Hebridean Terrane, which forms the Hebridean Islands and the adjacent NW coast of Scotland. This terrane represents a fragment of the original North American (Laurentian) craton and consists of Archaean and early Proterozoic (Lewisian) rocks, mainly acid and intermediate, high-grade metamorphic gneiss. These rocks are overlain by a succession of largely undeformed, mid-late Proterozoic, fluvio-lacustrine red-bed sediments and Cambro-Ordovician platform arenites and carbonates. No rocks of comparable age are known to exist in Northern Ireland.
NORTHERN HIGHLANDS TERRANE
The Northern Highlands Terrane lies directly SE of the Hebridean Terrane and is separated from it by the NW-directed Moine Thrust. This terrane consists of early Proterozoic (Moinian) fluvial shelf deposits which have undergone polyphase metamorphism and deformation starting ? pre-740 Ma and ending at about 450 Ma during the late stages of the Caledonian Orogeny (Mid to Late Ordovician). Comparable rocks occur in Northern Ireland in the Lough Derg and Central Tyrone Inliers. These rocks are demonstrably of pre-Dalradian age although they do not necessarily represent autochthonous basement to the Dalradian succession in the Central Highlands Terrane.
CENTRAL HIGHLANDS (GRAMPIAN) TERRANE
The Central Highlands (Grampian) Terrane is separated from the Northern Highlands by the Great Glen Fault. This is an extensive terrane which stretches from the Banffshire coast, through the Central Highlands of Scotland, to Northern Ireland, Donegal and Clew Bay in the west of Ireland. In Scotland, the southern boundary of this terrane is clearly delineated by the Highland Boundary Fault. In Ireland however, this southern boundary is less clearly defined and is marked by a series of faults and fault complexes most recently referred to as the Antrim-Galway Line (British Geological Survey, 1996). With the exception of the Central Tyrone and Lough Derg Inliers, all of the Precambrian rocks in Northern Ireland belong to the Dalradian Supergroup and lie within the Central Highland Terrane.
PRECAMBRIAN STRATIGRAPHIC NOMENCLATURE AND TIME SCALE
The Precambrian Era spans the earliest interval of geological time, from the origin of the earth (currently estimated to be about 4600-5000 million years ago (Ma)) to the beginning of the Cambrian Period (570 Ma).
In common with most stratigraphic terminology, Precambrian nomenclature has changed throughout the years as geological knowledge increased and concepts evolved. The task of producing a standardised and globally acceptable stratigraphic scale has fallen to the International Union of Geological Sciences (IUGS) working through its largest constituent body, the International Commission of Stratigraphy (ICS) and many subcommissions. The chronometric divisions and terminology for the Precambrian as used in this report are based on the scheme proposed by Cowie et al. (1989) to the ICS, adopted by the Subcommission on Precambrian Stratigraphy and recommended to the International Commission on Stratigraphy in 1989.
The Precambrian comprises two major stratigraphic subdivisions or Eons (the Archaean and the Proterozoic). These divisions are each characterised and dominated by rocks and rock associations that reflect broadly contrasting geological processes.
THE ARCHAEAN EON (c. 4600 Ma to 2500 Ma)
In 1976, the Precambrian Stratigraphic Subcommission defined the Archaean as a standard time division ending at 2500 Ma but with no formally defined beginning.
Throughout the world the Archaean is represented by a characteristically complex association of high-grade metamorphic gneiss. It is commonly believed that the bulk of the earth's continental crust was formed during the Archaean. No Archaean rocks are known to exist in Ireland.
THE PROTEROZOIC EON (2500 Ma to 570 Ma)
The term Proterozoic was introduced by Emmons in 1888 and gained wide usage in the United States of America during the early part of the 20th century (Harland et al., 1990). In the United States Geological Survey, the term Proterozoic was used more or less synonymously with Precambrian until 1925.
In 1964, the Geological Survey of Canada defined the Proterozoic chronometrically as succeeding the Archaean at 2390 Ma. This boundary was subsequently set at 2500 Ma by the International Subcommission on the Precambrian (Sims, 1980).
CHRONOSTRATIGRAPHIC NOMENCLATURE
In recent years there has been an increased tendency towards applying chronostratigraphy in the field and attempts continue to devise a workable scheme of chronostratic subdivisions for use in the Precambrian (Proterozoic).
Chronostratic schemes face major difficulties, particularly in finding Precambrian rock successions that can be reliably correlated on the worldwide scale. Chronostratic correlation has not so far been widely applied in Precambrian geology of Northern Ireland or indeed in the wider context of the British Isles and at present there remains a strong argument in favour of using chronometric divisions alone for sub-dividing Precambrian time (Harland et al., 1980).
THE PROTEROZOIC
During the Proterozoic Eon, the majority of active geological processes involved recycling of the Archaean continental crust rather than adding to it. Sedimentary basins formed on the margins of the eroding Archaean cratons. The early Proterozoic (2000 Ma) was also a period of major tectonic deformation with associated, metamorphism and granitic intrusion. At least some of this activity was associated with plate tectonics. Between 1000-1200 Ma, intense deformation and metamorphism affected the Grenville Belt which extends from eastern Canada to Ireland, the Scottish Highlands and Scandinavia. Fragments of the Grenville Belt occur today as inliers within the later, Caledonian fold-belt in Ireland and Scotland.
THE PHANEROZOIC (PALAEOZOIC ERA)
Strictly speaking the Phanerozoic lies outside the remit of this report. However, it is useful to make brief reference to tectonic events during the lowermost Phanerozoic (Palaeozoic) since these events continued to affect the Dalradian rocks of the Sperrin Mountains and North-east Antrim.
In Northern Ireland the later phases of the Caledonian orogeny (D3 onwards) resulted in south-easterly-directed thrusting along the Omagh/Castle Archdale Fault system. In mid-Ulster, in the hanging wall of the Omagh Fault (Omagh Thrust), the Dalradian rocks of the Sperrin Mountains were translated south-eastwards across an Ordovician (about 470 Ma old, Hutton et al., 1985) ophiolite and island arc (Tyrone Plutonic and the Tyrone Volcanic Group (Alsop et al., 1993)). There is also evidence in the younger Phanerozoic rocks south of the Omagh/Castle Archdale Fault system in Fermanagh and west Tyrone, that brittle and cataclastic deformation continued intermittently throughout most of the Carboniferous Period probably until Westphalian B times (about 310 Ma) (Mitchell, 1992). It is assumed that much of these later (Variscan) tectonic events also effected the Dalradian rocks.
2. PRECAMBRIAN LITHOSTRATIGRAPHIC FRAMEWORK IN N. IRELAND
INTRODUCTION
Throughout the world there are particular difficulties associated with stratigraphic correlation of the Precambrian rocks and in this respect, Northern Ireland is no exception. In common with most metamorphic terranes, the rocks contain no fossils and there is therefore no possibility of biostratigraphic correlation. The rocks are also structurally complex as a result of orogenic folding and thrusting and allochthonous movement of terranes. The effects and implications of the latter can be particularly difficult to recognise and quantify.
PRE-DALRADIAN ROCKS IN N. IRELAND
The Pre-Dalradian (Moinian?) strata in Northern Ireland provide perhaps the greatest challenges for correlation because of their apparently allochthonous nature and their probable status as "suspect terranes". The Pre-Dalradian Inliers of Lough Derg and Central Tyrone are isolated and fault-bounded. The rocks in each of these inliers have undergone pre-Grampian, high-grade (up to and including sillimanite grade) metamorphism and polyphase deformation. These mineral assemblages have subsequently been overprinted by a retrogressive thermo-tectonic event during the early Caledonian.
THE DALRADIAN SUPERGROUP
In Ireland and Scotland, major difficulties have also been encountered when detailed stratigraphic correlation has been attempted within the Dalradian Supergroup (Gibbons et al., 1994). The Dalradian successions tend to be lithologically diverse with rapid facies variations, and in certain areas parts of successions have been severely attenuated or removed by faulting. As a result, widespread correlations in the Dalradian are not considered reliable at the formation level and for the most part, formations are regarded as being of local significance only albeit remaining as valuable indicators of local changes in depositional conditions and the geometry of tectonic structures. Fortunately, some more widespread correlations are possible at group level in the Dalradian. The topmost part of the Argyll Group (Tayvallich Subgroup) is, for example, marked by a distinctive lithological association of limestone and volcanogenic rocks. This association can be traced from Scotland (Lough Tay Limestone), through Northern Ireland (Torr Head Formation & Dungiven Formation) to Donegal (Culdaff Limestone). Likewise, the base of the Argyll Group in Scotland and in Donegal is marked by a boulder bed or tillite (Schiehallion Boulder Bed, Port Askaig Tillite).
3. PRECAMBRIAN TERRANES
TERRANES & PLATE TECTONIC MODELS
An account of the palaeogeography and evolution of the Precambrian of the British Isles is best prefaced by a brief discussion of the concept of terranes and the important role played by plate tectonic processes in modelling and accounting for the distribution of the continents through geological time. For many years, major lithological, structural and metamorphic discontinuities have been recognised within the world's principal metamorphic belts. Likewise, in Northern Britain, major faults separate tracts of tectonized rocks of contrasting age (Archaean to Late Proterozoic) and metamorphic grade (Granulite Facies to Lower Greenschist Facies). Early interpretations of these rocks largely disregarded the global dimension and treated rocks as essentially autochthonous (formed essentially in the place they are found today).
Since the 1960's however, the evolution of the world's metamorphic belts (including Northern Britain) has been viewed in a global context. Plate tectonic theory, in particular the key mechanism of continental drift, has revolutionised our understanding of the evolution of the continents. Plate tectonic theory offers a mechanism whereby large areas of lithospheric crust (plates) can migrate over large distances across the surface of the earth through geological time. Oceans can open up, form sedimentary basins, then close, and in the process form mountain chains over destructive plate margins.
Because the earth's crust was much thinner during Precambrian times, and there were higher rates of geothermal heat flow, the rate at which lithospheric plates moved was much greater than it is today. Consequently, by the end of the Precambrian, some of the older rocks in the earth's crust had already experienced several episodes of deformation and metamorphism.
The net result of such movements is that areas of the earth's crust (terranes), characterised by shared lithological, structural and metamorphic features, may drift away from their area of origin. The process of movement and coming together (docking) of terranes often takes place along large-scale transform or transpressional faults. The net results of these processes are metamorphic belts which may consist of a series of accreted allochthonous and autochthonous terranes with boundaries defined by faults or fault complexes. Because of the faulted nature of most terrane boundaries, the tops and bottoms of geological successions in individual terranes are seldom seen.
THE TERRANES OF NORTHERN BRITAIN
The basement rocks of Northern Britain consist of a number of allocthonous terranes separated by major NE to SW trending faults. These terranes range in age from Early Proterozoic to Lower Palaeozoic age (about 2.9 Ga to 470 Ma) (Bluck et al., 1992).
HEBRIDEAN TERRANE
The oldest rocks in the British Isles occur in the Hebridean Terrane, which forms the Hebridean Islands and the adjacent NW coast of Scotland. This terrane represents a fragment of the original North American (Laurentian) craton and consists of Archaean and early Proterozoic (Lewisian) rocks, mainly acid and intermediate, high-grade metamorphic gneiss. These rocks are overlain by a succession of largely undeformed, mid-late Proterozoic, fluvio-lacustrine red-bed sediments and Cambro-Ordovician platform arenites and carbonates. No rocks of comparable age are known to exist in Northern Ireland.
NORTHERN HIGHLANDS TERRANE
The Northern Highlands Terrane lies directly SE of the Hebridean Terrane and is separated from it by the NW-directed Moine Thrust. This terrane consists of early Proterozoic (Moinian) fluvial shelf deposits which have undergone polyphase metamorphism and deformation starting ? pre-740 Ma and ending at about 450 Ma during the late stages of the Caledonian Orogeny (Mid to Late Ordovician). Comparable rocks occur in Northern Ireland in the Lough Derg and Central Tyrone Inliers. These rocks are demonstrably of pre-Dalradian age although they do not necessarily represent autochthonous basement to the Dalradian succession in the Central Highlands Terrane.
CENTRAL HIGHLANDS (GRAMPIAN) TERRANE
The Central Highlands (Grampian) Terrane is separated from the Northern Highlands by the Great Glen Fault. This is an extensive terrane which stretches from the Banffshire coast, through the Central Highlands of Scotland, to Northern Ireland, Donegal and Clew Bay in the west of Ireland. In Scotland, the southern boundary of this terrane is clearly delineated by the Highland Boundary Fault. In Ireland however, this southern boundary is less clearly defined and is marked by a series of faults and fault complexes most recently referred to as the Antrim-Galway Line (British Geological Survey, 1996). With the exception of the Central Tyrone and Lough Derg Inliers, all of the Precambrian rocks in Northern Ireland belong to the Dalradian Supergroup and lie within the Central Highland Terrane.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Introduction to the Precambrian of Northern Ireland, County Antrim, Fermanagh, Londonderry, Tyrone, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=122. Accessed on 2024-12-26 |
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