Earth Science Conservation Review
| Carboniferous Subarea-6; Armagh-Benburb | Armagh, Tyrone |
| Site Type: | Various |
| Site Status: | |
| Council area: | Armagh District Council, Dungannon District Council |
| Grid Reference: | H875455,H815523 |
| Google maps: | 54.35114,-6.6543 |
| Rocks | |
|---|---|
| Rock Age: | Carboniferous (Arundian, Asbian, Brigantian, Chadian, Courcyean, Holkerian, Tournasian, Visean) |
| Rock Name: | Annaclare Group, Armagh Group, Ballynahone Micrite Formation, Blackstokes Limestone Formation, Blackwater Limestone Formation, Carrickaness Sandstone Formation, Drumman More Sandstone Formation, Killuney Conglomerate Formation, Maydown Limestone Formation |
| Rock Type: | Conglomerate, Crinoidal limestone, Limestone, Mudstone, Sand, Sandstone |
| Interest | |
| Fossil Groups: | Arthropod, Bivalve, Brachiopod, Coral, Crinoid, Fish, Foraminifera, Gastropod, Miospore, Plant, Sponge, Trilobite |
| Other interest: | cross-bedding, dessication cracks, ripple marks, Deltaic sediments, Fluvial sediments, Marine sediments |
Description:
INTRODUCTION
The geology of the Armagh-Benburb area of Co. Armagh has been published by the Geological Survey of N. Ireland on 1:50,000 Sheet 47 (Armagh) in 1984. Carboniferous rocks outcrop in 2 main areas. In the south is the largest area of Lower Carboniferous rocks centred on Armagh City. These rest unconformably on the Lower Palaeozoic rocks of the Down-Longford Massif, at their southern margin, and are overlain unconformably to the north by red-beds of the Triassic Sherwood Sandstone Group. Isolated from this outcrop at Armagh by a broad area of Triassic rocks is the narrow outcrop of late Viséan rocks that extends northwards from Killylea (H79 44) to Benburb and almost to Moy (H85 56).
A great deal of information is available for the Carboniferous rocks at Benburb (Mitchell & Mitchell, 1982) and exposures are of such quality as to provide an almost continuous section through this relatively thin sequence. In the Armagh area however, despite a biostratigraphical database that contains a great deal of new information, for many parts of the succession either the exposure density is very poor or monotonous lithologies dominate the sequence and a comprehensive stratigraphy cannot be determined. On the published geological map of the Armagh area the strata are divided into a basal clastic sequence, consisting of three formations and a succeeding relatively poorly exposed marine sequence that was attributed to the Armagh Group. These distinctive sequences are separated by an unconformity and it has been decided to include the 3 basal formations in a new litho- stratigraphical unit, the Annaclare Group.
All the rocks exposed in the Benburb section are included in the Tyrone Group and assigned to 4 formations that do not form part of Oswald's standard nomenclature. Their outcrop is confined to Subarea-6 and the eastern part of Subarea-5 (Aughnacloy-Tynan). There is no lithological correlation between the geographically separate outcrops of Lower Carboniferous rocks at Benburb (Tyrone Group) and the Armagh Group at Armagh. The Armagh Group most closely resembles the Asbian and Brigantian strata exposed in the Cookstown area (Subarea-7) and at Desertmartin (Subarea-12).
I - TYRONE GROUP
In NW Co. Armagh, in Subarea-6, the outcrop of the Tyrone Group is confined solely to the section of late-Viséan strata that are superbly exposed in the gorge of the River Blackwater at Benburb. About 230m of strata that crop out between Benburb Bridge (H8193 5193) in the east and Milltown (H801 516) ~1.6km to the west, can be divided into 4 formations (Mitchell & Mitchell, op. cit.). The lowest beds of the Maydown Limestone Formation are succeeded by the Blackstokes Limestone Formation, the Carrickaness Sandstone Formation, with the Blackwater Limestone Formation at the top of the sequence. The Blackstokes and Blackwater formations can be subdivided into 2 and 6 members respectively.
At Benburb Bridge the Carboniferous rocks are faulted against red sandstones of Triassic age, while at Milltown red sandstones with a basal conglomerate, also of Triassic age, rest with angular unconformity on the Carboniferous strata. The Carboniferous rocks dip to the west at angles of<30 deg. except where they have been disturbed by the normal faults which cut the section into narrow blocks.
The Carboniferous rocks are of Dinantian age and belong to the Asbian and Brigantian stages of the Viséan. Previous authors, in particular Sheridan (1972) continued to use outdated terms such as 'Yoredale' and 'Calp' for various lithologies at Benburb, but this has been discontinued. These rocks were first described in detail by Portlock (1843) who noted the alternation of limestones and sandstones. Various palaeontologists notably Portlock, McCoy (1844) and Davis (1883) have subsequently established several species of fossils, particularly corals, echinoids, bryozoa and fish, based on specimens from Benburb.
The geological map of the Armagh-Benburb area (Sheet 47) was published first by Egan (1873), Geological Survey of Ireland, who divided the sequence into Lower Limestone, Calp and Upper Limestone. Hull (in Egan, 1873, p.13) considered the Island Sandstone and Outlet Limestone members of the Blackwater Formation, at the top of the sequence, to be of Lower Permian age, separated from the Carboniferous below and Triassic rocks above by unconformities, despite the occurrence of Carboniferous fossils (Baily in Egan, 1873, p.16) in these strata. Sheridan (1972) concluded that the Benburb section contained almost 500m of strata but his lithological subdivisions cannot be recognised as easily as Egan's simple grouping of 1873. The type section of the 4 formations, and their constituent members, recognised at Benburb, occur in this section.
For site specific information see; Key Site 261 - River Blackwater, Benburb.
II - ANNACLARE GROUP
The stratigraphy of the 'Basal Clastics' sequence of strata in the Armagh area was defined by the GSNI on the 1:50,000 geology map of that area (Sheet 47, 1984). The rocks were divided into 3 formations commencing at the base in the Killuney Conglomerate Formation, passing up into the Retreat Siltstone Formation with the Ballynahone Micrite Formation at the top. Their Tournaisian age was recognised as was the fact that they were separated by a stratigraphical break (disconformity - GSNI, 1984) from the basal beds of the succeeding Armagh Group. The latter rocks were regarded as being of Chadian age although new evidence now appears to indicate that they are slightly younger.
On the basis that the sediments forming the 3 formations named above are part of a single depositional sequence that evolved without obvious breaks it was decided to include them in a single hierarchial lithostratigraphical unit, the Annaclare Group, thus emphasising their separation from the younger Armagh Group.
The principal outcrop of the Annaclare Group is located at the southern margin of the Carboniferous outcrop in Co. Armagh (Sheet 47). Repeated faulting has dislocated their outcrop and the best exposures are now found in the outcrop east of Armagh City. Southwest of Armagh exposures are very rare and the existence of these rocks is neither proven, nor is the position of the sub- Carboniferous unconformity, with the Silurian rocks of the Down- Longford Massif, defined with any confidence. Type sections for each formation are located in the outcrop NE of Armagh City.
The thickness of the Annaclare Group is estimated to be ~300m with each of the three formations represented by ~100m of strata. Biostratigraphical evidence for the age of these rocks occurs in both the Retreat Siltstone and Ballynahone Micrite formations in the form of abundant miospore assemblages. With the exception of one sample recovered from the top of the latter formation, all the remaining samples indicate an assignment to the CM Biozone and a Tournaisian age. Because that one sample contains a few specimens of the miospore Lycospora pusilla it is assigned to the succeeding Pu Biozone (McPhilemy, 1988) and although it is probably of earliest Chadian age is still likely to occur within the Tournaisian Series (sensu Riley, 1993). The precise contact between the Annaclare and Armagh groups is not exposed but limestones located near the base of the latter group do contain a microbiota that can be used to date the rocks.
(i) KILLUNEY CONGLOMERATE FORMATION
The oldest Carboniferous rocks that occur in the Armagh area consist of red-purple conglomerates and sandstones. These represent the basal beds of the Annaclare Group and are assigned to the Killuney Conglomerate Formation. At their base the strata overlie unconformably greywackes and slates of probable Ordovician and Silurian age that represent the northern edge of the Down-Longford Massif in N. Ireland. Although these rocks are exposed in several stream sections between the extremities of the formation outcrop in the Tall River (H947 484), 500m NE of Richhill and the Callan River (H86 42), 1.5km south of Milford, the basal unconformity is only exposed at one locality, in the type section, ENE of Armagh. No fossils, and no lithologies suitable for palynological analysis, have been recovered from this formation whose maximum thickness is estimated at ~90m.
(ii) RETREAT SILTSTONE FORMATION
The middle formation of the Annaclare Group is referred to as the Retreat Siltstone Formation. This consists mainly of fine-grained clastic sedimentary rocks that represent a transitional stage, in a depositional continuum, between red-bed subaerial deposits of the Killuney Formation and peritidal limestones and shales of the Ballynahone Formation. The outcrop of these strata mirrors closely that of the preceeding Killuney Formation but temporary exposures, in existence at the time of resurvey by the GSNI, extended the area of outcrop into poorly exposed parts of the Carboniferous outcrop where the presence of the Retreat Formation was not suspected (Sheet 47).
Miospore assemblages recovered from the Retreat Formation belong to the CM Biozone and are of Tournaisian age. Although no miospores were recovered in the type section in the Killuney Stream, a prolific and diagnostic assemblage was obtained from green mudstones that crop out in the Retreat Glen (H903 497), 5.2km NNE of the centre of Armagh City. This contained the following miospores:
Aurorospora macra Grandispora sp. Baculatisporites fusticulus Pulvinispora scolecophora Calamospora sp. Punctatisporites irrassus Colatisporites decorus P. cf. solidus C. denticulatus Retusotriletes incohatus Crassispora trychera Rugospora polyptycha Cyclogranisporites palaeophytus Schopfites claviger Discernisporites crenulatus S. delicatus
Both index taxa of the CM Biozone are present in this assemblage, namely A. macra and S. claviger, and, with the absence of Lycospora pusilla, the index taxon of the succeeding Pu Biozone, the strata of the Retreat Siltstone Formation clearly belong to the CM Biozone and are of Tournaisian age. The Retreat Siltstone Formation is believed to be ~90-100m thick and has gradational lower and upper boundaries, from the underlying Killuney Formation and into the succeeding Ballynahone Formation.
For site specific information see; Key Site 262 - Killuney Stream.
(iii) BALLYNAHONE MICRITE FORMATION
The highest strata of the Annaclare Group consist of alternating limestones and shales and are referred to the Ballynahone Micrite Formation. The type section and best exposure of these rocks is in the Ballynahone River, north of Dean's Bridge (H8850 4581) on the NE outskirts of Armagh City. The GSNI have not recovered any diagnostic fossils from this formation. However McPhilemy (1988) analysed a suite of lithologies, both for palynomorphs and foraminifera, from the core of a borehole drilled in the early 1980's at Wilson's Bridge (H882 475), 2km north of the Mall in Armagh. She recovered 17 separate assemblages of miospores from the basal part of the sequence of which 12 came from various levels in the Ballynahone Micrite Formation. These assemblages are all indicative of the CM Biozone (Tournaisian) with the exception of the top most sample that came from shales 4.2m below the top of the formation, in the borehole, and which contained specimens of Lycospora pusilla. This is the first record of this species in the sequence and marks the base of the Pu Biozone, probably early Chadian. The thickness of the Ballynahone Fmn. in outcrop is estimated at 90m (GSNI, 1984) and in the borehole at Wilson's Bridge was accurately measured at precisely 100m.
For site specific information see; Key Site 265 - Ballynahone River (Part A).
III - ARMAGH GROUP
The stratigraphy of the Armagh Group in Subarea-6 was defined by the GSNI on the 1:50,000 Solid Geology map (Sheet 47) of the Armagh area. Several problems conspired initially to complicate the construction of a stratigraphical column, not least the fractured and faulted nature of much of the outcrop and the repetition, by faulting, of one or two resistant limestone lithologies. With the exception of thin sequences of Brigantian strata at the top and Arundian strata at the base, all the intervening strata seemed to be Asbian and no well defined succession of formations, or members could be distinguished, unlike strata, of the same age, in the Tyrone Group.
In 1977 six mineral exploration boreholes were drilled near Wilson's Bridge (H88 47), just over 2km NNE of the Mall in Armagh City. The deepest, borehole WB3, penetrated 360m of strata and revealed the presence of a hitherto unsuspected sandstone referred to by the GSNI as the Drumman More Sandstone Formation, ~117m thick, beneath a thin sequence of younger limestones. Below the sandstone, in the borehole, are conglomerates and sandstones which succeed the full sequence of the Ballynahone Micrite Formation. The borehole was terminated some 80m into the Retreat Siltstone Formation (McPhilemy, 1988). This borehole (H882 475) provided McPhilemy with a wealth of palynological samples from which she was able to define the biostratigraphical range of the formations and other discrete lithological units in this Carboniferous sequence. She also demonstrated that limestone beds occurring below and above the sandstone contained foraminifera of Lower Arundian-Holkerian and Asbian-Brigantian ages respectively. Inadvertently she defined the spread of ages of the limestone dominated sequence that succeeds the Drumman More Sandstone, but the Asbian section is much thicker than the evidence from the boreholes would indicate.
Due to the scattered nature of exposures in the limestone outcrop NE of Armagh it is still not possible to construct a precise stratigraphy and to define the stratigraphical position either of each locality or of every lithological change. Based on mappingg by the GSNI the limestone sequence overlying the Drumman More Sandstone is ~300m thick of which only the upper 80-90m are of Brigantian age. By far the greater number of quarries located between Armagh and Loughgall are developed in Asbian limestones.
West of Armagh, the Carboniferous outcrop is some 4km wide but exposures, with the exception of quarries, e.g. Navan (H849 452), Carrickaloughan (H858 468) and Lisadian (H835 466) are isolated and of limited areal and stratigraphical extent. In the area north of Killylea (Sheet 47; H79 44) the GSNI depict a facies change between sediments of the Armagh and Tyrone groups. There is very little information on the nature of this change since there are virtually no exposures in the thickly drift-covered ground between Killylea and the Benburb section, some 7km to the north.
On the southern outskirts of Armagh City, in the townlands of Cargagh and Drumarg Downs (H8643 & 8743) are numerous, abandoned quarries and natural sections which show an intriguing variety of lithologies.
In the absence of a formalised lithostratigraphical nomenclature for much of the Armagh Group, the selection of sites that are truly representative of the lithologies of different ages that occur in Subarea-6 is unfortunately purely subjective. Nevertheless it is a starting point and may be modified in the future depending on the availability of new or existing data.
The geology of the Armagh-Benburb area of Co. Armagh has been published by the Geological Survey of N. Ireland on 1:50,000 Sheet 47 (Armagh) in 1984. Carboniferous rocks outcrop in 2 main areas. In the south is the largest area of Lower Carboniferous rocks centred on Armagh City. These rest unconformably on the Lower Palaeozoic rocks of the Down-Longford Massif, at their southern margin, and are overlain unconformably to the north by red-beds of the Triassic Sherwood Sandstone Group. Isolated from this outcrop at Armagh by a broad area of Triassic rocks is the narrow outcrop of late Viséan rocks that extends northwards from Killylea (H79 44) to Benburb and almost to Moy (H85 56).
A great deal of information is available for the Carboniferous rocks at Benburb (Mitchell & Mitchell, 1982) and exposures are of such quality as to provide an almost continuous section through this relatively thin sequence. In the Armagh area however, despite a biostratigraphical database that contains a great deal of new information, for many parts of the succession either the exposure density is very poor or monotonous lithologies dominate the sequence and a comprehensive stratigraphy cannot be determined. On the published geological map of the Armagh area the strata are divided into a basal clastic sequence, consisting of three formations and a succeeding relatively poorly exposed marine sequence that was attributed to the Armagh Group. These distinctive sequences are separated by an unconformity and it has been decided to include the 3 basal formations in a new litho- stratigraphical unit, the Annaclare Group.
All the rocks exposed in the Benburb section are included in the Tyrone Group and assigned to 4 formations that do not form part of Oswald's standard nomenclature. Their outcrop is confined to Subarea-6 and the eastern part of Subarea-5 (Aughnacloy-Tynan). There is no lithological correlation between the geographically separate outcrops of Lower Carboniferous rocks at Benburb (Tyrone Group) and the Armagh Group at Armagh. The Armagh Group most closely resembles the Asbian and Brigantian strata exposed in the Cookstown area (Subarea-7) and at Desertmartin (Subarea-12).
I - TYRONE GROUP
In NW Co. Armagh, in Subarea-6, the outcrop of the Tyrone Group is confined solely to the section of late-Viséan strata that are superbly exposed in the gorge of the River Blackwater at Benburb. About 230m of strata that crop out between Benburb Bridge (H8193 5193) in the east and Milltown (H801 516) ~1.6km to the west, can be divided into 4 formations (Mitchell & Mitchell, op. cit.). The lowest beds of the Maydown Limestone Formation are succeeded by the Blackstokes Limestone Formation, the Carrickaness Sandstone Formation, with the Blackwater Limestone Formation at the top of the sequence. The Blackstokes and Blackwater formations can be subdivided into 2 and 6 members respectively.
At Benburb Bridge the Carboniferous rocks are faulted against red sandstones of Triassic age, while at Milltown red sandstones with a basal conglomerate, also of Triassic age, rest with angular unconformity on the Carboniferous strata. The Carboniferous rocks dip to the west at angles of<30 deg. except where they have been disturbed by the normal faults which cut the section into narrow blocks.
The Carboniferous rocks are of Dinantian age and belong to the Asbian and Brigantian stages of the Viséan. Previous authors, in particular Sheridan (1972) continued to use outdated terms such as 'Yoredale' and 'Calp' for various lithologies at Benburb, but this has been discontinued. These rocks were first described in detail by Portlock (1843) who noted the alternation of limestones and sandstones. Various palaeontologists notably Portlock, McCoy (1844) and Davis (1883) have subsequently established several species of fossils, particularly corals, echinoids, bryozoa and fish, based on specimens from Benburb.
The geological map of the Armagh-Benburb area (Sheet 47) was published first by Egan (1873), Geological Survey of Ireland, who divided the sequence into Lower Limestone, Calp and Upper Limestone. Hull (in Egan, 1873, p.13) considered the Island Sandstone and Outlet Limestone members of the Blackwater Formation, at the top of the sequence, to be of Lower Permian age, separated from the Carboniferous below and Triassic rocks above by unconformities, despite the occurrence of Carboniferous fossils (Baily in Egan, 1873, p.16) in these strata. Sheridan (1972) concluded that the Benburb section contained almost 500m of strata but his lithological subdivisions cannot be recognised as easily as Egan's simple grouping of 1873. The type section of the 4 formations, and their constituent members, recognised at Benburb, occur in this section.
For site specific information see; Key Site 261 - River Blackwater, Benburb.
II - ANNACLARE GROUP
The stratigraphy of the 'Basal Clastics' sequence of strata in the Armagh area was defined by the GSNI on the 1:50,000 geology map of that area (Sheet 47, 1984). The rocks were divided into 3 formations commencing at the base in the Killuney Conglomerate Formation, passing up into the Retreat Siltstone Formation with the Ballynahone Micrite Formation at the top. Their Tournaisian age was recognised as was the fact that they were separated by a stratigraphical break (disconformity - GSNI, 1984) from the basal beds of the succeeding Armagh Group. The latter rocks were regarded as being of Chadian age although new evidence now appears to indicate that they are slightly younger.
On the basis that the sediments forming the 3 formations named above are part of a single depositional sequence that evolved without obvious breaks it was decided to include them in a single hierarchial lithostratigraphical unit, the Annaclare Group, thus emphasising their separation from the younger Armagh Group.
The principal outcrop of the Annaclare Group is located at the southern margin of the Carboniferous outcrop in Co. Armagh (Sheet 47). Repeated faulting has dislocated their outcrop and the best exposures are now found in the outcrop east of Armagh City. Southwest of Armagh exposures are very rare and the existence of these rocks is neither proven, nor is the position of the sub- Carboniferous unconformity, with the Silurian rocks of the Down- Longford Massif, defined with any confidence. Type sections for each formation are located in the outcrop NE of Armagh City.
The thickness of the Annaclare Group is estimated to be ~300m with each of the three formations represented by ~100m of strata. Biostratigraphical evidence for the age of these rocks occurs in both the Retreat Siltstone and Ballynahone Micrite formations in the form of abundant miospore assemblages. With the exception of one sample recovered from the top of the latter formation, all the remaining samples indicate an assignment to the CM Biozone and a Tournaisian age. Because that one sample contains a few specimens of the miospore Lycospora pusilla it is assigned to the succeeding Pu Biozone (McPhilemy, 1988) and although it is probably of earliest Chadian age is still likely to occur within the Tournaisian Series (sensu Riley, 1993). The precise contact between the Annaclare and Armagh groups is not exposed but limestones located near the base of the latter group do contain a microbiota that can be used to date the rocks.
(i) KILLUNEY CONGLOMERATE FORMATION
The oldest Carboniferous rocks that occur in the Armagh area consist of red-purple conglomerates and sandstones. These represent the basal beds of the Annaclare Group and are assigned to the Killuney Conglomerate Formation. At their base the strata overlie unconformably greywackes and slates of probable Ordovician and Silurian age that represent the northern edge of the Down-Longford Massif in N. Ireland. Although these rocks are exposed in several stream sections between the extremities of the formation outcrop in the Tall River (H947 484), 500m NE of Richhill and the Callan River (H86 42), 1.5km south of Milford, the basal unconformity is only exposed at one locality, in the type section, ENE of Armagh. No fossils, and no lithologies suitable for palynological analysis, have been recovered from this formation whose maximum thickness is estimated at ~90m.
(ii) RETREAT SILTSTONE FORMATION
The middle formation of the Annaclare Group is referred to as the Retreat Siltstone Formation. This consists mainly of fine-grained clastic sedimentary rocks that represent a transitional stage, in a depositional continuum, between red-bed subaerial deposits of the Killuney Formation and peritidal limestones and shales of the Ballynahone Formation. The outcrop of these strata mirrors closely that of the preceeding Killuney Formation but temporary exposures, in existence at the time of resurvey by the GSNI, extended the area of outcrop into poorly exposed parts of the Carboniferous outcrop where the presence of the Retreat Formation was not suspected (Sheet 47).
Miospore assemblages recovered from the Retreat Formation belong to the CM Biozone and are of Tournaisian age. Although no miospores were recovered in the type section in the Killuney Stream, a prolific and diagnostic assemblage was obtained from green mudstones that crop out in the Retreat Glen (H903 497), 5.2km NNE of the centre of Armagh City. This contained the following miospores:
Aurorospora macra Grandispora sp. Baculatisporites fusticulus Pulvinispora scolecophora Calamospora sp. Punctatisporites irrassus Colatisporites decorus P. cf. solidus C. denticulatus Retusotriletes incohatus Crassispora trychera Rugospora polyptycha Cyclogranisporites palaeophytus Schopfites claviger Discernisporites crenulatus S. delicatus
Both index taxa of the CM Biozone are present in this assemblage, namely A. macra and S. claviger, and, with the absence of Lycospora pusilla, the index taxon of the succeeding Pu Biozone, the strata of the Retreat Siltstone Formation clearly belong to the CM Biozone and are of Tournaisian age. The Retreat Siltstone Formation is believed to be ~90-100m thick and has gradational lower and upper boundaries, from the underlying Killuney Formation and into the succeeding Ballynahone Formation.
For site specific information see; Key Site 262 - Killuney Stream.
(iii) BALLYNAHONE MICRITE FORMATION
The highest strata of the Annaclare Group consist of alternating limestones and shales and are referred to the Ballynahone Micrite Formation. The type section and best exposure of these rocks is in the Ballynahone River, north of Dean's Bridge (H8850 4581) on the NE outskirts of Armagh City. The GSNI have not recovered any diagnostic fossils from this formation. However McPhilemy (1988) analysed a suite of lithologies, both for palynomorphs and foraminifera, from the core of a borehole drilled in the early 1980's at Wilson's Bridge (H882 475), 2km north of the Mall in Armagh. She recovered 17 separate assemblages of miospores from the basal part of the sequence of which 12 came from various levels in the Ballynahone Micrite Formation. These assemblages are all indicative of the CM Biozone (Tournaisian) with the exception of the top most sample that came from shales 4.2m below the top of the formation, in the borehole, and which contained specimens of Lycospora pusilla. This is the first record of this species in the sequence and marks the base of the Pu Biozone, probably early Chadian. The thickness of the Ballynahone Fmn. in outcrop is estimated at 90m (GSNI, 1984) and in the borehole at Wilson's Bridge was accurately measured at precisely 100m.
For site specific information see; Key Site 265 - Ballynahone River (Part A).
III - ARMAGH GROUP
The stratigraphy of the Armagh Group in Subarea-6 was defined by the GSNI on the 1:50,000 Solid Geology map (Sheet 47) of the Armagh area. Several problems conspired initially to complicate the construction of a stratigraphical column, not least the fractured and faulted nature of much of the outcrop and the repetition, by faulting, of one or two resistant limestone lithologies. With the exception of thin sequences of Brigantian strata at the top and Arundian strata at the base, all the intervening strata seemed to be Asbian and no well defined succession of formations, or members could be distinguished, unlike strata, of the same age, in the Tyrone Group.
In 1977 six mineral exploration boreholes were drilled near Wilson's Bridge (H88 47), just over 2km NNE of the Mall in Armagh City. The deepest, borehole WB3, penetrated 360m of strata and revealed the presence of a hitherto unsuspected sandstone referred to by the GSNI as the Drumman More Sandstone Formation, ~117m thick, beneath a thin sequence of younger limestones. Below the sandstone, in the borehole, are conglomerates and sandstones which succeed the full sequence of the Ballynahone Micrite Formation. The borehole was terminated some 80m into the Retreat Siltstone Formation (McPhilemy, 1988). This borehole (H882 475) provided McPhilemy with a wealth of palynological samples from which she was able to define the biostratigraphical range of the formations and other discrete lithological units in this Carboniferous sequence. She also demonstrated that limestone beds occurring below and above the sandstone contained foraminifera of Lower Arundian-Holkerian and Asbian-Brigantian ages respectively. Inadvertently she defined the spread of ages of the limestone dominated sequence that succeeds the Drumman More Sandstone, but the Asbian section is much thicker than the evidence from the boreholes would indicate.
Due to the scattered nature of exposures in the limestone outcrop NE of Armagh it is still not possible to construct a precise stratigraphy and to define the stratigraphical position either of each locality or of every lithological change. Based on mappingg by the GSNI the limestone sequence overlying the Drumman More Sandstone is ~300m thick of which only the upper 80-90m are of Brigantian age. By far the greater number of quarries located between Armagh and Loughgall are developed in Asbian limestones.
West of Armagh, the Carboniferous outcrop is some 4km wide but exposures, with the exception of quarries, e.g. Navan (H849 452), Carrickaloughan (H858 468) and Lisadian (H835 466) are isolated and of limited areal and stratigraphical extent. In the area north of Killylea (Sheet 47; H79 44) the GSNI depict a facies change between sediments of the Armagh and Tyrone groups. There is very little information on the nature of this change since there are virtually no exposures in the thickly drift-covered ground between Killylea and the Benburb section, some 7km to the north.
On the southern outskirts of Armagh City, in the townlands of Cargagh and Drumarg Downs (H8643 & 8743) are numerous, abandoned quarries and natural sections which show an intriguing variety of lithologies.
In the absence of a formalised lithostratigraphical nomenclature for much of the Armagh Group, the selection of sites that are truly representative of the lithologies of different ages that occur in Subarea-6 is unfortunately purely subjective. Nevertheless it is a starting point and may be modified in the future depending on the availability of new or existing data.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Carboniferous Subarea-6; Armagh-Benburb, County Armagh, Tyrone, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1179. Accessed on 2024-12-26 |
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