Earth Science Conservation Review
| Carboniferous Subarea-7; Dungannon-Coalisland-Cookstown | Tyrone |
| Site Type: | Various |
| Site Status: | |
| Council area: | Cookstown District Council, Dungannon District Council |
| Grid Reference: | H7962,H8466,H8077 |
| Google maps: | 54.50072,-6.7806 |
| Rocks | |
|---|---|
| Rock Age: | Carboniferous (Namurian, Visean, Westphalian) |
| Rock Name: | Armagh Group, Carboniferous Limestone, Coal Measures, Leitrim Group, Millstone Grit Group (Repl M. Grit), Rockdale Limestone Formation, Rossmore Mudstone Formation |
| Rock Type: | Limestone, Mudstone |
| Interest | |
| Fossil Groups: | Bivalve, Brachiopod, Coral, Crinoid, Fish, Foraminifera, Gastropod, Goniatite, Nautiloid, Plant, Polyzoan, Trilobite |
| Other interest: | Marine sediments |
Description:
INTRODUCTION
The geological mapping of the Carboniferous rocks in the Dungannon-Coalisland-Cookstown area has been completed by the Geological Survey of Northern Ireland. The information for the Dungannon-Coalisland area is published on One Inch to One Mile Sheet 35 (Dungannon, 1960) and is accompanied by a geological memoir for that area (Fowler & Robbie, 1961). The geological map for the Cookstown area (1:50,000 Sheet 27) was published in 1983.
The Carboniferous rocks of the Dungannon-Coalisland area were mapped in the period from 1947-1956 prior to the introduction of the chronostratigraphical scheme of George et al. (1976) and the universal acceptance of the lithostratigraphical principles requiring the formal naming of individual units in a hierarchical system. The antiquated terms "Carboniferous Limestone Series, Millstone Grit and Coal Measures" were thus used on this map although certain lithological horizons, in particular coal seams, thin limestones and 'grit' beds were accorded specific names.
The geographical distribution of the Carboniferous rocks in Subarea-7 is depicted on the Dungannon and Cookstown maps. In the former area the largest outcrop is of the Carboniferous Limestone series of Viséan age and only in the areas between Dungannon and Coalisland and at Annaghone are younger, 'middle and upper', Carboniferous rocks preserved. In the Cookstown area, only the Carboniferous outcrop around this town is included in Subarea-7. These rocks were (1983) allocated ages according to the old- fashioned system utilising the letter 'd' and were labelled d2b-c and d2c, suggesting a late Viséan age, for the Derryloran and Rockdale formations respectively. The term 'Derryloran Grits' was used on the Dungannon map to represent an horizon near the base of the Carboniferous Limestone Series and it was thought to be of Arundian age by George et al. (1976, fig.16), on the basis of the occurrence of the brachiopod Stenoscisma isorhyncha. The occurrence of the coral Michelinia megastoma at a similar horizon in the Dungannon area would also indicate an Arundian age but no additional fauna of this age has been found in the area and no sites representing these strata have been selected. The use of the term Derryloran Formation on the Cookstown map for an horizon that is clearly much younger than the strata to which it was applied in the Dungannon area is problematical and the use of the term in the former area should be discontinued.
Much of the information on the Namurian (Millstone Grit) and Westphalian (Coal Measures) strata in Subarea-7 was derived from boreholes although it is possible to designate sites that are representative of some of these rocks.
I - ARMAGH GROUP
The geology of east Co. Tyrone is published as three maps.
GSNI, 1968. 1:63,360 Solid Geology of Sheet 35 (Dungannon) GSNI, 1979. 1:50,000 Solid Geology of Sheet 34 (Pomeroy) GSNI, 1983. 1:50,000 Solid Geology of Sheet 27 (Cookstown)
However the lithostratigraphical nomenclature used on these maps is confusing and no attempt was made to apply the chronostrati- graphical stages of the Dinantian (George et al., 1976) to these rocks. This raises acute problems in deciphering the geology of the Dinantian rocks of the Dungannon area, in particular. Apart from a unit, located near the base of the sequence, and referred to as the Derryloran Grits, if the Rossmore Mudstones are excluded, then the remainder of the "Carboniferous Limestone Series" is undivided. Thin "Grit" horizons were also noted (1:63,360 Sheet 35) as occurring higher in the sequence but very little information was available for them.
The northward extension of this part of the Dinantian outcrop in east Co. Tyrone was mapped by the GSNI (1979, 1983) in the Cookstown area on 1:50,000 Sheets 34 and 27. On the Pomeroy sheet the rocks were assigned to the Rockdale Limestone Formation with the Derryloran Grit Member comprising its basal unit. Subsequently on the Cookstown sheet the Rockdale Limestone Formation was retained but the Derryloran Grit was now elevated to formation status. The Rockdale Limestone and top part of the Derryloran Grit are depicted on the latter map as having the same age and an interdigitating relationship.
In the Dungannon-Cookstown area small quarries occur in many places. By far the biggest quarry in that area is however the Cement Works Quarry at Cookstown. This affords a continuous section through more than 100m of limestones in the upper part of the Carboniferous sequence and illustrates the relationship between carbonate and clastic lithologies that presumably could be referred to the Rockdale Limestone Formation and Derryloran Grit 'Formation' (GSNI, 1983).
CONFIDENTIALITY OF INFORMATION
Access to the Cement Works Quarry was kindly granted by Mr. G. McCartney, Quarry Manager. Mr. McCartney requested that detailed information regarding the stratigraphy and composition/quality of the limestones should be kept to a minimum. He also requested that the Company should be permitted to see and vete any document written about the quarry and this has been adhered to by the GSNI.
Although a great deal of borehole information is available to the Company, the GSNI constructed its own stratigraphical section for the quarry. This is not published in detail but only those parts relating to important fossil collections/occurrences or to unusual lithologies or sedimentary structures are described.
For site specific information see; Key Site 280 - Cement Works Quarry, Cookstown.
II - LEITRIM GROUP
In Subarea-7 the outcrop of the Rossmore Mudstone Formation, the only representative of the Leitrim Group in this area, is confined to the area between Dungannon and Coalisland (GSNI, 1960). In that area the strata are inclined to the NE at ~10 deg. and the formation is about 150m thick. Based on the occurrence of Brigantian macro- and microfaunas below the Rossmore Mudstones these strata are of late Brigantian age. The precise age of the top of the formation is less well defined but the base of the Namurian (the Brigantian-Pendelian Stage boundary) is located (George et al., 1978) at the base of a thick sandstone 17-28m below the Main Coal. Strata equivalent in age and lithology to the Meenymore and Glenade Sandstone formations of the Leitrim Group in the western part of N. Ireland are thus not represented in Subarea-7.
(i) ROSSMORE MUDSTONE FORMATION
The term Rossmore Mudstones was first used by Robbie (1955, p.31) to represent the highest beds in the Carboniferous Limestone Series in the Dungannon area (1:63,360 Sheet 35). Good exposures of the mudstones do not occur naturally but they are exposed in brick pits in the Dungannon area. Fossils are abundant in certain beds and include bivalves, brachiopods, bryozoa, crinoids, trilobites and goniatites.
Numerous boreholes in this area penetrated various parts of the Rossmore Mudstones succession and although no complete section through the formation was obtained a great deal of lithological information is available. At the base the Rossmore Mudstones gradationally succeed bioclastic limestones and the fauna changes from assemblages of coral-brachiopods to one dominated by gastropods and bivalves. The remainder of the formation is dominated by dark grey mudstones interbedded with laminae of paler siltstone and occasionally of sandstone, lateral continuity of these coarser laminae being a feature of the formation.
According to George et al. (1976) the goniatite fauna, which includes Sudeticeras crenistriatum, an index taxon of the P2a Biozone (mid-Brigantian), is of Brigantian age but there are indications of a possible early Namurian age from higher levels. This problem is still not fully resolved.
For site specific information see; Key Site 281 - Derraghadoan Pit, Dungannon.
(ii) 'COAL MEASURES' (GROUP)
In Subarea-7 Coal Measures strata of Westphalian age are confined to two small areas at Coalisland and near Annaghone (1:50,000 Sheet 35 - Dungannon). The Coalisland Coalfield is bounded on two sides by faults and to the east is overlain unconformably by Triassic rocks. At Annaghone, 6.5km to the north, the coalfield is faulted on three sides and overlain unconformably on its east side by Permian rocks. According to the geological memoir for the Dungannon area (Sheet 35, 1961) very few exposures occur and virtually all the detailed information, from both coalfields, is derived from boreholes. The one exception is the outcrop of the basal beds of the Coal Measures in the working brick pit at Coalisland.
Excavations at Coalisland, in search of brick clays, cover a vast area and have resulted in a devastated landscape of tipped material, and active and abandoned workings on the SW side of the town (Photographs 143 & 144). By the very nature of the quarrying activities it is very difficult to construct an accurate picture of the geology of the rocks. In earlier times it was possible to measure sections in this pit and the GSNI, in the geological memoir of the Dungannon area recognised the marine band at the base of the Westphalian (Coal Measures) sequence and the succeeding 33m of strata. The marine band contains abundant fossils including bivalves, brachiopods, gastropods, nautiloids, fish remains and goniatites, in particular Gastrioceras subcrenatum which is the index taxon for the Subcrenatum Marine Band, which marks the base of the Westphalian in England, Wales and NW Europe.
Studies of the sequence of non-marine bivalves that occur throughout these coalfields strata shows that the strata are not younger than Westphalian A.
For site specific information see; Key Site 282 - Coalisland Brick Pit.
The geological mapping of the Carboniferous rocks in the Dungannon-Coalisland-Cookstown area has been completed by the Geological Survey of Northern Ireland. The information for the Dungannon-Coalisland area is published on One Inch to One Mile Sheet 35 (Dungannon, 1960) and is accompanied by a geological memoir for that area (Fowler & Robbie, 1961). The geological map for the Cookstown area (1:50,000 Sheet 27) was published in 1983.
The Carboniferous rocks of the Dungannon-Coalisland area were mapped in the period from 1947-1956 prior to the introduction of the chronostratigraphical scheme of George et al. (1976) and the universal acceptance of the lithostratigraphical principles requiring the formal naming of individual units in a hierarchical system. The antiquated terms "Carboniferous Limestone Series, Millstone Grit and Coal Measures" were thus used on this map although certain lithological horizons, in particular coal seams, thin limestones and 'grit' beds were accorded specific names.
The geographical distribution of the Carboniferous rocks in Subarea-7 is depicted on the Dungannon and Cookstown maps. In the former area the largest outcrop is of the Carboniferous Limestone series of Viséan age and only in the areas between Dungannon and Coalisland and at Annaghone are younger, 'middle and upper', Carboniferous rocks preserved. In the Cookstown area, only the Carboniferous outcrop around this town is included in Subarea-7. These rocks were (1983) allocated ages according to the old- fashioned system utilising the letter 'd' and were labelled d2b-c and d2c, suggesting a late Viséan age, for the Derryloran and Rockdale formations respectively. The term 'Derryloran Grits' was used on the Dungannon map to represent an horizon near the base of the Carboniferous Limestone Series and it was thought to be of Arundian age by George et al. (1976, fig.16), on the basis of the occurrence of the brachiopod Stenoscisma isorhyncha. The occurrence of the coral Michelinia megastoma at a similar horizon in the Dungannon area would also indicate an Arundian age but no additional fauna of this age has been found in the area and no sites representing these strata have been selected. The use of the term Derryloran Formation on the Cookstown map for an horizon that is clearly much younger than the strata to which it was applied in the Dungannon area is problematical and the use of the term in the former area should be discontinued.
Much of the information on the Namurian (Millstone Grit) and Westphalian (Coal Measures) strata in Subarea-7 was derived from boreholes although it is possible to designate sites that are representative of some of these rocks.
I - ARMAGH GROUP
The geology of east Co. Tyrone is published as three maps.
GSNI, 1968. 1:63,360 Solid Geology of Sheet 35 (Dungannon) GSNI, 1979. 1:50,000 Solid Geology of Sheet 34 (Pomeroy) GSNI, 1983. 1:50,000 Solid Geology of Sheet 27 (Cookstown)
However the lithostratigraphical nomenclature used on these maps is confusing and no attempt was made to apply the chronostrati- graphical stages of the Dinantian (George et al., 1976) to these rocks. This raises acute problems in deciphering the geology of the Dinantian rocks of the Dungannon area, in particular. Apart from a unit, located near the base of the sequence, and referred to as the Derryloran Grits, if the Rossmore Mudstones are excluded, then the remainder of the "Carboniferous Limestone Series" is undivided. Thin "Grit" horizons were also noted (1:63,360 Sheet 35) as occurring higher in the sequence but very little information was available for them.
The northward extension of this part of the Dinantian outcrop in east Co. Tyrone was mapped by the GSNI (1979, 1983) in the Cookstown area on 1:50,000 Sheets 34 and 27. On the Pomeroy sheet the rocks were assigned to the Rockdale Limestone Formation with the Derryloran Grit Member comprising its basal unit. Subsequently on the Cookstown sheet the Rockdale Limestone Formation was retained but the Derryloran Grit was now elevated to formation status. The Rockdale Limestone and top part of the Derryloran Grit are depicted on the latter map as having the same age and an interdigitating relationship.
In the Dungannon-Cookstown area small quarries occur in many places. By far the biggest quarry in that area is however the Cement Works Quarry at Cookstown. This affords a continuous section through more than 100m of limestones in the upper part of the Carboniferous sequence and illustrates the relationship between carbonate and clastic lithologies that presumably could be referred to the Rockdale Limestone Formation and Derryloran Grit 'Formation' (GSNI, 1983).
CONFIDENTIALITY OF INFORMATION
Access to the Cement Works Quarry was kindly granted by Mr. G. McCartney, Quarry Manager. Mr. McCartney requested that detailed information regarding the stratigraphy and composition/quality of the limestones should be kept to a minimum. He also requested that the Company should be permitted to see and vete any document written about the quarry and this has been adhered to by the GSNI.
Although a great deal of borehole information is available to the Company, the GSNI constructed its own stratigraphical section for the quarry. This is not published in detail but only those parts relating to important fossil collections/occurrences or to unusual lithologies or sedimentary structures are described.
For site specific information see; Key Site 280 - Cement Works Quarry, Cookstown.
II - LEITRIM GROUP
In Subarea-7 the outcrop of the Rossmore Mudstone Formation, the only representative of the Leitrim Group in this area, is confined to the area between Dungannon and Coalisland (GSNI, 1960). In that area the strata are inclined to the NE at ~10 deg. and the formation is about 150m thick. Based on the occurrence of Brigantian macro- and microfaunas below the Rossmore Mudstones these strata are of late Brigantian age. The precise age of the top of the formation is less well defined but the base of the Namurian (the Brigantian-Pendelian Stage boundary) is located (George et al., 1978) at the base of a thick sandstone 17-28m below the Main Coal. Strata equivalent in age and lithology to the Meenymore and Glenade Sandstone formations of the Leitrim Group in the western part of N. Ireland are thus not represented in Subarea-7.
(i) ROSSMORE MUDSTONE FORMATION
The term Rossmore Mudstones was first used by Robbie (1955, p.31) to represent the highest beds in the Carboniferous Limestone Series in the Dungannon area (1:63,360 Sheet 35). Good exposures of the mudstones do not occur naturally but they are exposed in brick pits in the Dungannon area. Fossils are abundant in certain beds and include bivalves, brachiopods, bryozoa, crinoids, trilobites and goniatites.
Numerous boreholes in this area penetrated various parts of the Rossmore Mudstones succession and although no complete section through the formation was obtained a great deal of lithological information is available. At the base the Rossmore Mudstones gradationally succeed bioclastic limestones and the fauna changes from assemblages of coral-brachiopods to one dominated by gastropods and bivalves. The remainder of the formation is dominated by dark grey mudstones interbedded with laminae of paler siltstone and occasionally of sandstone, lateral continuity of these coarser laminae being a feature of the formation.
According to George et al. (1976) the goniatite fauna, which includes Sudeticeras crenistriatum, an index taxon of the P2a Biozone (mid-Brigantian), is of Brigantian age but there are indications of a possible early Namurian age from higher levels. This problem is still not fully resolved.
For site specific information see; Key Site 281 - Derraghadoan Pit, Dungannon.
(ii) 'COAL MEASURES' (GROUP)
In Subarea-7 Coal Measures strata of Westphalian age are confined to two small areas at Coalisland and near Annaghone (1:50,000 Sheet 35 - Dungannon). The Coalisland Coalfield is bounded on two sides by faults and to the east is overlain unconformably by Triassic rocks. At Annaghone, 6.5km to the north, the coalfield is faulted on three sides and overlain unconformably on its east side by Permian rocks. According to the geological memoir for the Dungannon area (Sheet 35, 1961) very few exposures occur and virtually all the detailed information, from both coalfields, is derived from boreholes. The one exception is the outcrop of the basal beds of the Coal Measures in the working brick pit at Coalisland.
Excavations at Coalisland, in search of brick clays, cover a vast area and have resulted in a devastated landscape of tipped material, and active and abandoned workings on the SW side of the town (Photographs 143 & 144). By the very nature of the quarrying activities it is very difficult to construct an accurate picture of the geology of the rocks. In earlier times it was possible to measure sections in this pit and the GSNI, in the geological memoir of the Dungannon area recognised the marine band at the base of the Westphalian (Coal Measures) sequence and the succeeding 33m of strata. The marine band contains abundant fossils including bivalves, brachiopods, gastropods, nautiloids, fish remains and goniatites, in particular Gastrioceras subcrenatum which is the index taxon for the Subcrenatum Marine Band, which marks the base of the Westphalian in England, Wales and NW Europe.
Studies of the sequence of non-marine bivalves that occur throughout these coalfields strata shows that the strata are not younger than Westphalian A.
For site specific information see; Key Site 282 - Coalisland Brick Pit.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Carboniferous Subarea-7; Dungannon-Coalisland-Cookstown, County Tyrone, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1180. Accessed on 2024-12-26 |
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