Earth Science Conservation Review
| The Carboniferous System in N. Ireland - Introduction | Antrim, Armagh, Down, Fermanagh, Londonderry, Tyro |
| Site Type: | Various |
| Site Status: | |
| Grid Reference: | |
| Google maps: | , |
| Rocks | |
|---|---|
| Rock Age: | Carboniferous (Namurian, Stephanian, Tournasian, Visean, Westphalian) |
| Rock Name: | Armagh Group, Carboniferous Limestone, Coal Measures, Leitrim Group, Millstone Grit Group (Repl M. Grit), Omagh Sandstone Group, Slievebane Group, Tyrone Group |
| Rock Type: | Limestone, Sand, Sandstone, Shale, Siltstone |
| Interest | |
| Fossil Groups: | Bivalve, Brachiopod, Coral, Crinoid, Foraminifera, Gastropod, Goniatite, Microfossil, Microfossils, Nautiloid, Polyzoan, Trilobite |
| Other interest: | Marine sediments |
Description:
INTRODUCTION
The outcrop of Carboniferous rocks in Northern Ireland is largely confined to the area west of the River Bann and in particular to Counties Fermanagh and Tyrone. In order to facilitate the description of these rocks and the construction of a lithostratigraphical framework their outcrop, in Northern Ireland is divided into 12 geographical Sub-areas. This division is artificial and is based on a number of factors including:
a. The existing geographical fragmentation of their outcrop, for example, the Ballycastle area (Subarea-9), north and south Co. Down (Subareas-10 and 11), much of the outcrop in Co. Armagh (Subarea-6), the Kesh - Omagh area (Subarea-3) and the Newtownstewart Outlier (Subarea-8).
b. The existence of natural divisions, for example, the linear nature of Lower and Upper Lough Erne separates Subarea-1 (Derrygonnelly - Marble Arch - Cuilcagh Mountain) from Subareas-2 and 3, the Lisnaskea - Fivemiletown and Kesh - Omagh areas respectively.
c. Areas that are known to enclose Carboniferous rocks but which differ consistently lithologically from surrounding, but probably continuous strata, for example, the Dungannon - Coalisland - Cookstown area (Subarea-7), and the separate parts of Subarea-4 (Fintona Block) that are formed exclusively of red-beds.
It should always be remembered however that during Carboniferous times, in particular the Dinantian and early Silesian, the deposition of these sedimentary rocks was a virtually continuous process, with local or, less commonly, regional interruptions resulting from contemporaneous tectonic activity. Enormous advances have been made in recent years by the Geological Survey of Northern Ireland, particularly as a result of the complete remapping of the Carboniferous of Co. Fermanagh, in achieving a more comprehensive understanding of the biostratigraphy of these rocks. Without this information it would prove impossible to correlate accurately different lithological units that occur in separate parts of the systemic outcrop and thus to reconstruct the palaeogeographical evolution of the Carboniferous basins.
GEOGRAPHICAL SUBAREAS 1 - 12
The geological composition of the geographical subareas into which the Carboniferous outcrop in Northern Ireland is divided is outlined below, together with a brief listing of the main divisions, and the ages of the rocks contained therein.
1: DERRYGONNELLY - MARBLE ARCH - CUILCAGH MOUNTAIN
Leitrim Group - late Viséan to early Namurian Tyrone Group - late Tournaisian to late Viséan
2: LISNASKEA - FIVEMILETOWN AREA
Leitrim Group - late Viséan Tyrone Group - Viséan
3: KESH - OMAGH AREA
Tyrone Group - Viséan Omagh Sandstone Group - late Tournaisian
4: FINTONA BLOCK
Slievebane Group - Westphalian A, B Greenan Sandstone Formation - late Viséan to early Namurian Kilskeery Group - late Viséan to early Namurian
5: CLOGHER - AUGHNACLOY AREA
Leitrim Group - late Viséan Tyrone Group - late Tournaisian - Viséan
6: ARMAGH - BENBURB AREA
Leitrim Group - late Viséan Tyrone Group - late Viséan Armagh Group - Viséan Annaclare Group - late Tournaisian
7: DUNGANNON - COALISLAND - COOKSTOWN AREA
'Coal Measures' - Westphalian A 'Millstone Grit' - early and late Namurian Rossmore Mudstone Formation - late Viséan Rockdale Limestone Formation - late Viséan 'Carboniferous Limestone Series' - Viséan
8: NEWTOWNSTEWART OUTLIER
Carboniferous, undivided - late Tournaisian
9: BALLYCASTLE AREA
Upper Carboniferous - early Namurian Lower Carboniferous - late Viséan
10: NORTH COUNTY DOWN
A. CASTLE ESPIE Castle Espie Group - late Viséan
B. CULTRA - CRAIGAVAD AREA Holywood Group - late Tournaisian
11: CRANFIELD - GREENCASTLE AREA
Carlingford Limestone Group - Viséan
12: DRAPERSTOWN - GLENSHANE PASS - LIMAVADY AREA
Leitrim Group - late Viséan Desertmartin Limestone Formation - late Viséan Altagoan Formation } } - late Tournaisian Iniscarn Formation }
In the majority of the 12 Subareas the strata have been named according to the modern rules of lithostratigraphical nomenclature i.e. in a hierarchical arrangement of members, formations and groups. However in a few Subareas, that were remapped by the GSNI between 1947 and 1970, the terminology is archaic and does not conform to the new principles. These include Subarea-7 (GSNI 1960, 1961) and Subarea-9 (1963, 1966). The remapping of other parts of the Carboniferous outcrop has still to be commenced by the GSNI so whilst it is possible to determine quickly the age of the strata in that area, details of their lithostratigraphy are as yet unknown; this applies to Subareas-8 (Newtownstewart) and 12 (Draperstown - Glenshane Pass, but not the Limavady segment which was published by the GSNI in 1981).
In other parts of Northern Ireland that are covered by the new 1:50,000 geological maps of the GSNI the strata are assigned correctly to various members and formations. Unfortunately in many instances the geological memoir that should accompany these maps is still awaiting publication and detailed information on the location of stratotype sections for the lithostratigraphical units is either unpublished or was not selected in the first place. Where possible the appropriate field maps that cover the known outcrop of a particular unit have been consulted and it was attempted, after a rapid field inspection, to select a section that best represents its lithological character.
Sections or sites selected in this manner are not stratotypes and may be challenged by other geologists who are more familiar either with an area of N. Ireland or a particular group of rocks. This problem is particularly evident in Subareas-5 (Clogher - Aughnacloy; GSNI, 1982) and 7 (Cookstown; GSNI, 1983).
In addition to the problem of archaic lithostratigraphical nomenclature, for all the areas listed above, i.e. 5, 7, 8, 9, 11 and 12 there is an additional problem relating to the age of the Carboniferous rocks and the means by which this information was depicted on the published maps. Dating of the rocks was achieved, prior to the mid-1970s, by determination of their coral - brachiopod faunas which were then compared to sequences of faunal zones in the Bristol area (Vaughan, 1905) and northern England (Garwood, 1907, 1913). Unfortunately these schemes did not take account of either the facies control exerted on such benthic faunas or on the effects of diachronism and could not recognise discontinuities in the rock sequence.
In the 19th century the Geological Survey of Ireland adopted a tripartite classification for the Carboniferous rocks of NW Ireland comprising Lower Limestone, Calp or Middle Limestone and Upper Limestone, the middle division also being subdivided into Lower Calp Shale, Calp Sandstone, Calp Shale and Calp Limestone.
On their published One Inch to One Mile geological maps these divisions were assigned various colour schemes and a notation based on the letter 'd', for Carboniferous strata, as outlined below:
NOTATION GSNI STRATIGRAPHICAL NAMES PRESENT NAMES ------------------------------------------------------------- d4 Millstone Grit } } Leitrim Group } d3 Yoredale Shale and Sandstone } ------------------------------------------------------------- d2c Upper Limestone } } d2b Middle or Calp Limestone, } sandstone and shale } } Tyrone Group d2a Lower Limestone } } d1 Lower Carboniferous beds; } sandstone and shale } -------------------------------------------------------------
Unfortunately, biostratigraphical control of the true age of various horizons was poor at that time, with the result that beds assigned, for example, to the Millstone Grit (d4) ranged in age from the upper Asbian (late Viséan) on Slieve Beagh, near Clogher to the Arnsbergian stage (early Namurian) on Cuilcagh Mountain.
Despite these serious problems with the dating of much of the Carboniferous and thus the correlation between different parts of the Northwest Basin, the Geological Survey of Northern Ireland continued to use this notation into the late-1970s.
In 1955 Oswald, working in the Sligo area of northwest Ireland recognised all the divisions of the Lower Carboniferous erected by the Geological Survey of Ireland but, for the first time, attached a geographical name to the lithological qualifier for each of their units, and thus took a great step forward on the road to a modern lithostratigraphical nomenclature. Whilst working in County Fermanagh, from 1980 onwards, the GSNI adopted Oswald's classification of the Carboniferous rocks and has applied his divisions throughout the western part of Northern Ireland. Significantly, new mappable units that occur within Oswald's standard sequence of sediments are recognised by the GSNI in many separate geographical areas, and at different stratigraphical levels, and are allocated either member or formational status.
CHRONOSTRATIGRAPHY
One of the most significant conclusions of Ramsbottom (1973) was the dramatic illustration of the incompleteness of the Avon Gorge sequence at Bristol, through the recognition of numerous stratigraphical gaps and in so doing rendered the scheme of Vaughanian Zones (1905) no longer acceptable.
Subsequently the formal proposal for chronostratigraphical subdivision of the British Lower Carboniferous (Dinantian) by George et al. (1976) was a great step forward. It provided the first formal chronostratigraphical classification of the British Dinantian, against which all the biozonal schemes could be compared. It also fulfilled the need to separate the conceptual principles of biostratigraphy and chronostratigraphy. This resulted in subdivision of the British Dinantian into six regional stages; the Courceyan, Chadian, Arundian, Holkerian, Asbian and Brigantian. Each stage contains a suite of characteristic fossils and is defined at a basal boundary stratotype.
The correlation between Ramsbottom's major transgressive episodes or cycles and the chronostratigraphical scheme proposed by George et al. was clearly evident. Significantly Ramsbottom recognised that each major transgression was accompanied by a new migratory fauna that is used to recognise the different stage boundaries and by implication deduced that established biostratigraphical zone boundaries correlated with his cycle boundaries. Thus this approach conveniently married the new chronostratigraphical framework with existing biostratigraphical zonation, in particular that of Vaughan (1905).
In addition to the chronostratigraphical subdivision of the Dinantian, both the Namurian and Westphalian Series of the Silesian are also subdivided.
SITE RATING
A notation used to categorise each site is based on a 3, 2 and 1 star system for International, National and Local interest sites respectively. Each site will be annotated with the appropriate rating on the first page of the description as follows; * * * (denoting a site of International interest) * * (denoting a site of National interest) * (denoting a site of Local interest).
The outcrop of Carboniferous rocks in Northern Ireland is largely confined to the area west of the River Bann and in particular to Counties Fermanagh and Tyrone. In order to facilitate the description of these rocks and the construction of a lithostratigraphical framework their outcrop, in Northern Ireland is divided into 12 geographical Sub-areas. This division is artificial and is based on a number of factors including:
a. The existing geographical fragmentation of their outcrop, for example, the Ballycastle area (Subarea-9), north and south Co. Down (Subareas-10 and 11), much of the outcrop in Co. Armagh (Subarea-6), the Kesh - Omagh area (Subarea-3) and the Newtownstewart Outlier (Subarea-8).
b. The existence of natural divisions, for example, the linear nature of Lower and Upper Lough Erne separates Subarea-1 (Derrygonnelly - Marble Arch - Cuilcagh Mountain) from Subareas-2 and 3, the Lisnaskea - Fivemiletown and Kesh - Omagh areas respectively.
c. Areas that are known to enclose Carboniferous rocks but which differ consistently lithologically from surrounding, but probably continuous strata, for example, the Dungannon - Coalisland - Cookstown area (Subarea-7), and the separate parts of Subarea-4 (Fintona Block) that are formed exclusively of red-beds.
It should always be remembered however that during Carboniferous times, in particular the Dinantian and early Silesian, the deposition of these sedimentary rocks was a virtually continuous process, with local or, less commonly, regional interruptions resulting from contemporaneous tectonic activity. Enormous advances have been made in recent years by the Geological Survey of Northern Ireland, particularly as a result of the complete remapping of the Carboniferous of Co. Fermanagh, in achieving a more comprehensive understanding of the biostratigraphy of these rocks. Without this information it would prove impossible to correlate accurately different lithological units that occur in separate parts of the systemic outcrop and thus to reconstruct the palaeogeographical evolution of the Carboniferous basins.
GEOGRAPHICAL SUBAREAS 1 - 12
The geological composition of the geographical subareas into which the Carboniferous outcrop in Northern Ireland is divided is outlined below, together with a brief listing of the main divisions, and the ages of the rocks contained therein.
1: DERRYGONNELLY - MARBLE ARCH - CUILCAGH MOUNTAIN
Leitrim Group - late Viséan to early Namurian Tyrone Group - late Tournaisian to late Viséan
2: LISNASKEA - FIVEMILETOWN AREA
Leitrim Group - late Viséan Tyrone Group - Viséan
3: KESH - OMAGH AREA
Tyrone Group - Viséan Omagh Sandstone Group - late Tournaisian
4: FINTONA BLOCK
Slievebane Group - Westphalian A, B Greenan Sandstone Formation - late Viséan to early Namurian Kilskeery Group - late Viséan to early Namurian
5: CLOGHER - AUGHNACLOY AREA
Leitrim Group - late Viséan Tyrone Group - late Tournaisian - Viséan
6: ARMAGH - BENBURB AREA
Leitrim Group - late Viséan Tyrone Group - late Viséan Armagh Group - Viséan Annaclare Group - late Tournaisian
7: DUNGANNON - COALISLAND - COOKSTOWN AREA
'Coal Measures' - Westphalian A 'Millstone Grit' - early and late Namurian Rossmore Mudstone Formation - late Viséan Rockdale Limestone Formation - late Viséan 'Carboniferous Limestone Series' - Viséan
8: NEWTOWNSTEWART OUTLIER
Carboniferous, undivided - late Tournaisian
9: BALLYCASTLE AREA
Upper Carboniferous - early Namurian Lower Carboniferous - late Viséan
10: NORTH COUNTY DOWN
A. CASTLE ESPIE Castle Espie Group - late Viséan
B. CULTRA - CRAIGAVAD AREA Holywood Group - late Tournaisian
11: CRANFIELD - GREENCASTLE AREA
Carlingford Limestone Group - Viséan
12: DRAPERSTOWN - GLENSHANE PASS - LIMAVADY AREA
Leitrim Group - late Viséan Desertmartin Limestone Formation - late Viséan Altagoan Formation } } - late Tournaisian Iniscarn Formation }
In the majority of the 12 Subareas the strata have been named according to the modern rules of lithostratigraphical nomenclature i.e. in a hierarchical arrangement of members, formations and groups. However in a few Subareas, that were remapped by the GSNI between 1947 and 1970, the terminology is archaic and does not conform to the new principles. These include Subarea-7 (GSNI 1960, 1961) and Subarea-9 (1963, 1966). The remapping of other parts of the Carboniferous outcrop has still to be commenced by the GSNI so whilst it is possible to determine quickly the age of the strata in that area, details of their lithostratigraphy are as yet unknown; this applies to Subareas-8 (Newtownstewart) and 12 (Draperstown - Glenshane Pass, but not the Limavady segment which was published by the GSNI in 1981).
In other parts of Northern Ireland that are covered by the new 1:50,000 geological maps of the GSNI the strata are assigned correctly to various members and formations. Unfortunately in many instances the geological memoir that should accompany these maps is still awaiting publication and detailed information on the location of stratotype sections for the lithostratigraphical units is either unpublished or was not selected in the first place. Where possible the appropriate field maps that cover the known outcrop of a particular unit have been consulted and it was attempted, after a rapid field inspection, to select a section that best represents its lithological character.
Sections or sites selected in this manner are not stratotypes and may be challenged by other geologists who are more familiar either with an area of N. Ireland or a particular group of rocks. This problem is particularly evident in Subareas-5 (Clogher - Aughnacloy; GSNI, 1982) and 7 (Cookstown; GSNI, 1983).
In addition to the problem of archaic lithostratigraphical nomenclature, for all the areas listed above, i.e. 5, 7, 8, 9, 11 and 12 there is an additional problem relating to the age of the Carboniferous rocks and the means by which this information was depicted on the published maps. Dating of the rocks was achieved, prior to the mid-1970s, by determination of their coral - brachiopod faunas which were then compared to sequences of faunal zones in the Bristol area (Vaughan, 1905) and northern England (Garwood, 1907, 1913). Unfortunately these schemes did not take account of either the facies control exerted on such benthic faunas or on the effects of diachronism and could not recognise discontinuities in the rock sequence.
In the 19th century the Geological Survey of Ireland adopted a tripartite classification for the Carboniferous rocks of NW Ireland comprising Lower Limestone, Calp or Middle Limestone and Upper Limestone, the middle division also being subdivided into Lower Calp Shale, Calp Sandstone, Calp Shale and Calp Limestone.
On their published One Inch to One Mile geological maps these divisions were assigned various colour schemes and a notation based on the letter 'd', for Carboniferous strata, as outlined below:
NOTATION GSNI STRATIGRAPHICAL NAMES PRESENT NAMES ------------------------------------------------------------- d4 Millstone Grit } } Leitrim Group } d3 Yoredale Shale and Sandstone } ------------------------------------------------------------- d2c Upper Limestone } } d2b Middle or Calp Limestone, } sandstone and shale } } Tyrone Group d2a Lower Limestone } } d1 Lower Carboniferous beds; } sandstone and shale } -------------------------------------------------------------
Unfortunately, biostratigraphical control of the true age of various horizons was poor at that time, with the result that beds assigned, for example, to the Millstone Grit (d4) ranged in age from the upper Asbian (late Viséan) on Slieve Beagh, near Clogher to the Arnsbergian stage (early Namurian) on Cuilcagh Mountain.
Despite these serious problems with the dating of much of the Carboniferous and thus the correlation between different parts of the Northwest Basin, the Geological Survey of Northern Ireland continued to use this notation into the late-1970s.
In 1955 Oswald, working in the Sligo area of northwest Ireland recognised all the divisions of the Lower Carboniferous erected by the Geological Survey of Ireland but, for the first time, attached a geographical name to the lithological qualifier for each of their units, and thus took a great step forward on the road to a modern lithostratigraphical nomenclature. Whilst working in County Fermanagh, from 1980 onwards, the GSNI adopted Oswald's classification of the Carboniferous rocks and has applied his divisions throughout the western part of Northern Ireland. Significantly, new mappable units that occur within Oswald's standard sequence of sediments are recognised by the GSNI in many separate geographical areas, and at different stratigraphical levels, and are allocated either member or formational status.
CHRONOSTRATIGRAPHY
One of the most significant conclusions of Ramsbottom (1973) was the dramatic illustration of the incompleteness of the Avon Gorge sequence at Bristol, through the recognition of numerous stratigraphical gaps and in so doing rendered the scheme of Vaughanian Zones (1905) no longer acceptable.
Subsequently the formal proposal for chronostratigraphical subdivision of the British Lower Carboniferous (Dinantian) by George et al. (1976) was a great step forward. It provided the first formal chronostratigraphical classification of the British Dinantian, against which all the biozonal schemes could be compared. It also fulfilled the need to separate the conceptual principles of biostratigraphy and chronostratigraphy. This resulted in subdivision of the British Dinantian into six regional stages; the Courceyan, Chadian, Arundian, Holkerian, Asbian and Brigantian. Each stage contains a suite of characteristic fossils and is defined at a basal boundary stratotype.
The correlation between Ramsbottom's major transgressive episodes or cycles and the chronostratigraphical scheme proposed by George et al. was clearly evident. Significantly Ramsbottom recognised that each major transgression was accompanied by a new migratory fauna that is used to recognise the different stage boundaries and by implication deduced that established biostratigraphical zone boundaries correlated with his cycle boundaries. Thus this approach conveniently married the new chronostratigraphical framework with existing biostratigraphical zonation, in particular that of Vaughan (1905).
In addition to the chronostratigraphical subdivision of the Dinantian, both the Namurian and Westphalian Series of the Silesian are also subdivided.
SITE RATING
A notation used to categorise each site is based on a 3, 2 and 1 star system for International, National and Local interest sites respectively. Each site will be annotated with the appropriate rating on the first page of the description as follows; * * * (denoting a site of International interest) * * (denoting a site of National interest) * (denoting a site of Local interest).
| Enlander, I., Dempster, M. & Doughty, P., 2024. The Carboniferous System in N. Ireland - Introduction, County Antrim, Armagh, Down, Fermanagh, Londonderry, Tyro, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1171. Accessed on 2024-12-26 |
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