Earth Science Conservation Review
| Carboniferous Subarea-5; Clogher-Aughnacloy | Tyrone |
| Site Type: | Various |
| Site Status: | |
| Council area: | Dungannon District Council |
| Grid Reference: | H540515,H666523 |
| Google maps: | 54.40968,-7.16833 |
| Rocks | |
|---|---|
| Rock Age: | Carboniferous (Arundian, Asbian, Chadian, Holkerian, Visean) |
| Rock Name: | Ballyshannon Limestone Formation, Benbulben Shale Formation, Bundoran Shale Formation, Carrickaness Sandstone Formation, Dartry Limestone Formation, Glencar Limestone Formation, Leitrim Group, Maydown Limestone Formation, Meenymore Formation, Mullaghmore |
| Rock Type: | Conglomerate, Limestone, Mudstone, Sand, Sandstone |
| Interest | |
| Minerals: | Calcite |
| Fossil Groups: | Bivalve, Brachiopod, Conodont, Coral, Crinoid, Foraminifera, Gastropod, Goniatite, Microfossil, Microfossils, Polyzoan, Trilobite |
| Other interest: | breccia, cross-bedding, net veins, Deltaic sediments, Fluvial sediments, Marine sediments |
Description:
INTRODUCTION
The geological mapping of the Carboniferous rocks in Subarea-5 has been completed by the Geological Survey of Northern Ireland. The information is published on the following geological maps:
GSNI, 1979, 1:50,000 Solid Geology of Sheet 34 (Pomeroy) GSNI, 1982, 1:50,000 Solid Geology of Sheet 45 (Enniskillen) GSNI, 1983, 1:50,000 Solid Geology of Sheet 46 (Clogher)
The geographical distribution of the formations and members that comprise the Tyrone and Leitrim groups in this area is depicted on these maps and will not be replicated here. Unfortunately the GSNI decided when publishing these maps, to erect a stratigraphy, for the Carboniferous rocks, that included a mixture of Oswald's (1955), now standard, lithostratigraphical names such as Ballyshannon Limestone and Dartry Limestone formations and new terms, such as Carnteel and Glennoo Limestone formations. They also correctly identified the occurrence of sedimentary rocks representative of the Leitrim Group in this area but then proceeded to assign them to a new lithostratigraphical unit, the Slieve Beagh Formation, when it is quite evident that they are typical of the Meenymore Formation, erected by Brandon in 1972.
Without doubt the most confusing part of the GSNI stratigraphy is the use of the term Carnteel Formation, and its constituent member, the Aughnacloy Sandstone. These units appear on all 3 maps but are most areally widespread on the Clogher map, in particular dominating the Carboniferous outcrop east of Aughnacloy. There is no doubt that the tripartite division of the Carnteel Formation into upper and lower parts, separated by the Aughnacloy Sandstone, correlates precisely with Oswald's standard formations, namely the Bundoran Shale, Mullaghmore Sandstone and Benbulben Shale formations. West of Aughnacloy this correlation holds true. Severe problems arise however in the outcrop east of Aughnacloy (Sheet 46) where it can be demonstrated that both the Carnteel Formation and the so-called Aughnacloy Sandstone Member are much younger than hitherto suspected, and thus do not correlate with the succession west of Aughnacloy.
The author has decided to partially revise the Carboniferous stratigraphy in Subarea-5, based on new biostratigraphical information, and to dispense with many of the terms coined by the GSNI, preferring instead to utilise the standard formation names.
-----------------------------|----------------------------------- GSNI (1982, 1983) | THIS REPORT -----------------------------|----------------------------------- Slieve Beagh Formation | Meenymore Formation -----------------------------|----------------------------------- | | Carrickaness Glennoo/Dartry |Dartry Lst. Fmn. | Sst. Fmn. Limestone Fmn. | |----------------- |-----------------| -----------------------------|Glencar Lst. Fmn.|Maydown Lst. Fmn. |-----------------| Carnteel Formation | Benbulben |- - - - - - - - - | Shale Fmn. | ? -----------------------------|----------------------------------- Aughnacloy Sst. Mbr. | Mullaghmore Sandstone Fmn. -----------------------------|----------------------------------- Carnteel Formation | Bundoran Shale Formation -----------------------------|----------------------------------- Ballyshannon Lst. Fmn. | Ballyshannon Lst. Fmn. -----------------------------|-----------------------------------
In Subarea-5 the lowest rocks of the Carboniferous succession belong to the Ballyness and Clogher Valley formations. No attempt is made here to either redefine these units or to identify ASSI for each of them.
I - TYRONE GROUP
In south Co. Tyrone, in Subarea-5, the Tyrone Group comprises a virtually complete sequence of formations that are representative of the Lower Carboniferous succession in the western part of N. Ireland. This is particularly true in the western half of the Subarea, west of Aughnacloy, which corresponds to the eastern part of the Clogher Valley. The Carboniferous geology of the Clogher Valley is split by the NE-SW trending Clogher Valley Fault. North of the fault, at the east end of the Clogher Valley, the rocks belong almost exclusively to the Ballyness and Clogher Valley formations. South of the fault, the wide upland area of Slieve Beagh consists mainly of the outcrop of the Meenymore Formation (Leitrim Group).
The Clogher Valley Fault is clearly a major structural break in Co. Tyrone and has a complex surface expression and history of movement. This is evident from the occurrence of isolated, fault- bounded lenticles of older Dinantian rocks located on the south side of the fault, in Subarea-5, within the outcrop of the Meenymore Formation. Examples include the exposure of Ballyshannon Limestone at Slatmore (H50 48) and the complete sequence between the Bundoran Shale and Dartry Limestone at Fardross (H52 48). East of Clogher the Clogher Valley Fault splits into several sub-parallel fractures that separate linear outcrops of various formations, of widely differing ages, north and east of Aughnacloy (1:50,000 Sheet 46). It is possible that an important splay of the Clogher Valley Fault passes through Aughnacloy because just east of the town at Drummond (H69 52) there is an isolated, fault-bounded, exposure of the Bundoran Shale. This is located 17km ENE of the Fardross section, the nearest outcrop of the shale and as such is the most easterly outcrop of this formation in N. Ireland.
In the eastern part of Subarea-5, between Aughnacloy, Eglish (H78 56) and Middletown (H75 38) the Carboniferous outcrop consists entirely of calcareous mudstones and thin and thick limestones, referred by the GSNI (1983) to the Carnteel Formation, and a sandstone, correlated (GSNI, 1983) with the Aughnacloy Sandstone Member. The former lithologies contain an Asbian fossil assemblage and correlate with the Benbulben Shale and Glencar Limestone formations. The stratigraphical position of this sandstone is unclear but there appears to be sufficient field evidence to suggest that it everywhere succeeds the Benbulben Shale Formation. Based on new biostratigraphical information the sandstone is early Asbian or younger. The precise correlation of this sandstone with other late Viséan sandstones in Subarea-6 at Benburb or near Armagh, is still not determined.
The Dartry Limestone Formation was recognised by the GSNI throughout Subarea-5 (1:50,000 Sheets 45 and 46). In the western part of the Clogher Valley it consists of lithologies identical to those around Lisnaskea and Newtownbutler in Subarea-2, and thus the assignment of that name is fully justified. In the east of Subarea-5, north of Aughnacloy, beds assigned to the Dartry Limestone by the GSNI (1983) are atypical of the formation and correspond most closely to the Maydown Limestone Formation (Mitchell & Mitchell, 1982), which was first recognised in the Benburb area.
(i) BALLYSHANNON LIMESTONE FORMATION
In N. Ireland the outcrop of the Ballyshannon Limestone Formation is confined to Co. Fermanagh, in particular west of Fivemiletown (Key Site 202), on the SW shore of Lower Lough Erne (Sites 174 to 176) and around Newtownbutler, in the south of the county. The only other occurrence of this formation is at Slatmore (H50 48), 11km east of the main formation outcrop at Mullaghsillogagh (Subarea-2). On the 1:50,000 geological map of the Clogher area (Sheet 46) the limestones at Slatmore are referred to the Dartry Limestone Formation. Based on lithologies and fauna present this is clearly incorrect.
For site specific information see; Key Site 251 - Slatmore Quarry, Clogher.
(ii) BUNDORAN SHALE FORMATION
On the 1:50,000 geological map of the Clogher area (Sheet 46) the distribution of the Ballyshannon Limestone is restricted to a linear outcrop, some 3km long and 0.5km wide, located just east of Aughnacloy. The outcrop is entirely fault-bounded. The northern bounding fault is depicted on the Clogher map as an easterly continuation of the Clogher Valley Fault or one of the major splays. Surrounding strata were assigned by the GSNI either to the Carnteel Formation (now the Benbulben Shale Formation) or to the Aughnacloy Sandstone Member.
By far the best exposures of lithologies that are representative of this inlier of earlier Dinantian sediments are found in the disused quarry, and adjacent roadside cutting, in Drummond Townland (H690 519) 2.6km ESE of the centre of Aughnacloy. The sequence here and at other localities consists exclusively of medium to dark grey, very fossiliferous calcareous mudstones and thin limestones. Such lithologies are characteristic of the Bundoran Shale Formation and do not occur in the Ballyshannon Limestone, to which they were wrongly assigned (GSNI, 1983).
For site specific information see; Key Site 252 - Drummond Quarry, Aughnacloy.
(iii) BUNDORAN SHALE FORMATION; MULLAGHMORE SANDSTONE FORMATION; BENBULBEN SHALE FORMATION; GLENCAR LIMESTONE FORMATION & DARTRY LIMESTONE FORMATION
In N. Ireland it is extremely rare to find an exposure of sufficient length to reveal more than a small part of a formation of the Tyrone Group and indeed it is even more unusual for any section to expose a contact between two adjacent formations or between a member and its parent formation. The usual exception to this rule most commonly occurs in river sections draining steep mountain sides where strata are virtually horizontal and thick sequences may thus be exposed over a relatively short length of the rivers course. Examples include the section in Barony Glen (Key Site 301) on the west side of White Mountain (Subarea-12) and the numerous sections on the NE face of Cuilcagh Mountain, in strata of the Leitrim Group (Subarea-1).
On the relatively flat to undulating, often thickly drift-covered land of the Clogher Valley in Subarea-5, such an occurrence would therefore be quite exceptional. The section of strata exposed in the stream, and on the valley sides, flowing northwards through Fardross Forest, 3.3km SSW of Clogher Cathedral, represents a succession that commences at the base, in the top of the Bundoran Shale Formation and is succeeded by all of the Mullaghmore Sandstone, Benbulben Shale and Glencar Limestone formations and ends in the lower part of the Dartry Limestone Formation. Exposure is not completely continuous but is nevertheless so good that the author is not aware of any comparable section in N. Ireland, nor of any other section that exposes such a range of formations of the Tyrone Group.
For site specific information see; Key Site 253 - Fardross Stream, Clogher.
(iv) MAYDOWN LIMESTONE FORMATION
In Co. Armagh (Subarea-6) a sequence of late Viséan (Tyrone Group) sedimentary rocks, ~230m thick, is completely exposed in the gorge of the River Blackwater, Benburb (Mitchell & Mitchell, 1982). The Maydown Limestone Formation is the lowest unit in the sequence and is ~130m thick. The formation is characterised by the gradual upward disappearance of shales, from the base of the section where they are either dominant or are interbedded in equal proportions with limestones to a shale-free sequence, or one with shales restricted to thin laminae, at the top. In this respect the Maydown Formation mimics the lithological changes that occur between the Benbulben Shale, Glencar Limestone and Dartry Limestone formations in Cos. Tyrone and Fermanagh (Key Site 253). For this reason the Geological Survey of Ireland (Egan, 1873) assigned these strata to the Calp Limestone and Shale, but not to the Upper Limestone, despite the appearance of such limestones at the top of this sequence which is now referred to the Maydown Limestone Formation.
The age of the Maydown Limestone Formation is undoubtedly Asbian, based on a prolific fauna of corals and brachiopods which occur throughout the sequence except in the thickest shale horizons at the base.
SUBAREA-5: The Aughnacloy-Tynan Area
In the western part of Subarea-5 the 'Aughnacloy Sandstone Member', as defined by the GSNI (1983), is exposed only in the Fardross section (Key Site 253) and being demonstrably of late Arundian age, is the correlative of the Mullaghmore Sandstone Formation and is now referred to by this name.
In the eastern part of Subarea-5, between Aughnacloy (H66 52) and Tynan (H76 43), the GSNI depict a tripartite outcrop for a sandstone unit that they referred to as the Aughnacloy Sandstone Member (1:50,000 Sheet 46, 1983). On this assumption the sandstone, which should be late Arundian in age, should also be underlain by the mid-Arundian Bundoran Shale Formation and overlain by the Arundian-Asbian Benbulben Shale Formation. New biostratigraphical and lithological evidence from the Aughnacloy- Tynan area shows that none of these assumptions is correct.
1. New miospore evidence demonstrates conclusively that the sandstone is Asbian and is thus not equivalent to the Mullaghmore Sandstone Formation.
2. Arundian strata, with the exception of the fault-bounded inlier of the Bundoran Shale Formation at Annagh and Drummond Quarry (Key Site 252), do not occur in the Aughnacloy-Tynan area.
3. Throughout this area the sandstone unit is everywhere underlain by the so-called 'Carnteel Formation' (GSNI, 1983). This comprises a mixed suite of lithologies including mudstones, interbedded limestones and limestones separated only by thin shales. In all respects this association is identical to the definition of the Maydown Limestone Formation. Prolific assemblages of macrofauna and abundant microbiota recovered from these lithologies are exclusively of Asbian age thus reinforcing the correlation with the Benburb sequence.
North of Aughnacloy the GSNI (1983) depict a fault-bounded lenticular outcrop of the Dartry Limestone Formation. Faunally and lithologically these strata are quite atypical of the Dartry Limestone, as seen in Subarea-2 and in the Fardross section (Subarea-5), but are identical to the limestone-dominated section at the top of the Maydown Limestone Formation.
All the available new evidence from the Aughnacloy-Tynan area now supports a correlation of strata cropping out in that part of Subarea-5, formerly assigned to the Carnteel Formation, with the Asbian Maydown Limestone Formation. The sandstone unit, formerly referred to the Aughnacloy Sandstone Member of the Carnteel Formation, is now correlated with the Asbian Carrickaness Sandstone Formation (Mitchell & Mitchell, 1982) of the Benburb area.
For site specific information see; Key Site 256 - Plaister Quarry, Aughnacloy.
The geological mapping of the Carboniferous rocks in Subarea-5 has been completed by the Geological Survey of Northern Ireland. The information is published on the following geological maps:
GSNI, 1979, 1:50,000 Solid Geology of Sheet 34 (Pomeroy) GSNI, 1982, 1:50,000 Solid Geology of Sheet 45 (Enniskillen) GSNI, 1983, 1:50,000 Solid Geology of Sheet 46 (Clogher)
The geographical distribution of the formations and members that comprise the Tyrone and Leitrim groups in this area is depicted on these maps and will not be replicated here. Unfortunately the GSNI decided when publishing these maps, to erect a stratigraphy, for the Carboniferous rocks, that included a mixture of Oswald's (1955), now standard, lithostratigraphical names such as Ballyshannon Limestone and Dartry Limestone formations and new terms, such as Carnteel and Glennoo Limestone formations. They also correctly identified the occurrence of sedimentary rocks representative of the Leitrim Group in this area but then proceeded to assign them to a new lithostratigraphical unit, the Slieve Beagh Formation, when it is quite evident that they are typical of the Meenymore Formation, erected by Brandon in 1972.
Without doubt the most confusing part of the GSNI stratigraphy is the use of the term Carnteel Formation, and its constituent member, the Aughnacloy Sandstone. These units appear on all 3 maps but are most areally widespread on the Clogher map, in particular dominating the Carboniferous outcrop east of Aughnacloy. There is no doubt that the tripartite division of the Carnteel Formation into upper and lower parts, separated by the Aughnacloy Sandstone, correlates precisely with Oswald's standard formations, namely the Bundoran Shale, Mullaghmore Sandstone and Benbulben Shale formations. West of Aughnacloy this correlation holds true. Severe problems arise however in the outcrop east of Aughnacloy (Sheet 46) where it can be demonstrated that both the Carnteel Formation and the so-called Aughnacloy Sandstone Member are much younger than hitherto suspected, and thus do not correlate with the succession west of Aughnacloy.
The author has decided to partially revise the Carboniferous stratigraphy in Subarea-5, based on new biostratigraphical information, and to dispense with many of the terms coined by the GSNI, preferring instead to utilise the standard formation names.
-----------------------------|----------------------------------- GSNI (1982, 1983) | THIS REPORT -----------------------------|----------------------------------- Slieve Beagh Formation | Meenymore Formation -----------------------------|----------------------------------- | | Carrickaness Glennoo/Dartry |Dartry Lst. Fmn. | Sst. Fmn. Limestone Fmn. | |----------------- |-----------------| -----------------------------|Glencar Lst. Fmn.|Maydown Lst. Fmn. |-----------------| Carnteel Formation | Benbulben |- - - - - - - - - | Shale Fmn. | ? -----------------------------|----------------------------------- Aughnacloy Sst. Mbr. | Mullaghmore Sandstone Fmn. -----------------------------|----------------------------------- Carnteel Formation | Bundoran Shale Formation -----------------------------|----------------------------------- Ballyshannon Lst. Fmn. | Ballyshannon Lst. Fmn. -----------------------------|-----------------------------------
In Subarea-5 the lowest rocks of the Carboniferous succession belong to the Ballyness and Clogher Valley formations. No attempt is made here to either redefine these units or to identify ASSI for each of them.
I - TYRONE GROUP
In south Co. Tyrone, in Subarea-5, the Tyrone Group comprises a virtually complete sequence of formations that are representative of the Lower Carboniferous succession in the western part of N. Ireland. This is particularly true in the western half of the Subarea, west of Aughnacloy, which corresponds to the eastern part of the Clogher Valley. The Carboniferous geology of the Clogher Valley is split by the NE-SW trending Clogher Valley Fault. North of the fault, at the east end of the Clogher Valley, the rocks belong almost exclusively to the Ballyness and Clogher Valley formations. South of the fault, the wide upland area of Slieve Beagh consists mainly of the outcrop of the Meenymore Formation (Leitrim Group).
The Clogher Valley Fault is clearly a major structural break in Co. Tyrone and has a complex surface expression and history of movement. This is evident from the occurrence of isolated, fault- bounded lenticles of older Dinantian rocks located on the south side of the fault, in Subarea-5, within the outcrop of the Meenymore Formation. Examples include the exposure of Ballyshannon Limestone at Slatmore (H50 48) and the complete sequence between the Bundoran Shale and Dartry Limestone at Fardross (H52 48). East of Clogher the Clogher Valley Fault splits into several sub-parallel fractures that separate linear outcrops of various formations, of widely differing ages, north and east of Aughnacloy (1:50,000 Sheet 46). It is possible that an important splay of the Clogher Valley Fault passes through Aughnacloy because just east of the town at Drummond (H69 52) there is an isolated, fault-bounded, exposure of the Bundoran Shale. This is located 17km ENE of the Fardross section, the nearest outcrop of the shale and as such is the most easterly outcrop of this formation in N. Ireland.
In the eastern part of Subarea-5, between Aughnacloy, Eglish (H78 56) and Middletown (H75 38) the Carboniferous outcrop consists entirely of calcareous mudstones and thin and thick limestones, referred by the GSNI (1983) to the Carnteel Formation, and a sandstone, correlated (GSNI, 1983) with the Aughnacloy Sandstone Member. The former lithologies contain an Asbian fossil assemblage and correlate with the Benbulben Shale and Glencar Limestone formations. The stratigraphical position of this sandstone is unclear but there appears to be sufficient field evidence to suggest that it everywhere succeeds the Benbulben Shale Formation. Based on new biostratigraphical information the sandstone is early Asbian or younger. The precise correlation of this sandstone with other late Viséan sandstones in Subarea-6 at Benburb or near Armagh, is still not determined.
The Dartry Limestone Formation was recognised by the GSNI throughout Subarea-5 (1:50,000 Sheets 45 and 46). In the western part of the Clogher Valley it consists of lithologies identical to those around Lisnaskea and Newtownbutler in Subarea-2, and thus the assignment of that name is fully justified. In the east of Subarea-5, north of Aughnacloy, beds assigned to the Dartry Limestone by the GSNI (1983) are atypical of the formation and correspond most closely to the Maydown Limestone Formation (Mitchell & Mitchell, 1982), which was first recognised in the Benburb area.
(i) BALLYSHANNON LIMESTONE FORMATION
In N. Ireland the outcrop of the Ballyshannon Limestone Formation is confined to Co. Fermanagh, in particular west of Fivemiletown (Key Site 202), on the SW shore of Lower Lough Erne (Sites 174 to 176) and around Newtownbutler, in the south of the county. The only other occurrence of this formation is at Slatmore (H50 48), 11km east of the main formation outcrop at Mullaghsillogagh (Subarea-2). On the 1:50,000 geological map of the Clogher area (Sheet 46) the limestones at Slatmore are referred to the Dartry Limestone Formation. Based on lithologies and fauna present this is clearly incorrect.
For site specific information see; Key Site 251 - Slatmore Quarry, Clogher.
(ii) BUNDORAN SHALE FORMATION
On the 1:50,000 geological map of the Clogher area (Sheet 46) the distribution of the Ballyshannon Limestone is restricted to a linear outcrop, some 3km long and 0.5km wide, located just east of Aughnacloy. The outcrop is entirely fault-bounded. The northern bounding fault is depicted on the Clogher map as an easterly continuation of the Clogher Valley Fault or one of the major splays. Surrounding strata were assigned by the GSNI either to the Carnteel Formation (now the Benbulben Shale Formation) or to the Aughnacloy Sandstone Member.
By far the best exposures of lithologies that are representative of this inlier of earlier Dinantian sediments are found in the disused quarry, and adjacent roadside cutting, in Drummond Townland (H690 519) 2.6km ESE of the centre of Aughnacloy. The sequence here and at other localities consists exclusively of medium to dark grey, very fossiliferous calcareous mudstones and thin limestones. Such lithologies are characteristic of the Bundoran Shale Formation and do not occur in the Ballyshannon Limestone, to which they were wrongly assigned (GSNI, 1983).
For site specific information see; Key Site 252 - Drummond Quarry, Aughnacloy.
(iii) BUNDORAN SHALE FORMATION; MULLAGHMORE SANDSTONE FORMATION; BENBULBEN SHALE FORMATION; GLENCAR LIMESTONE FORMATION & DARTRY LIMESTONE FORMATION
In N. Ireland it is extremely rare to find an exposure of sufficient length to reveal more than a small part of a formation of the Tyrone Group and indeed it is even more unusual for any section to expose a contact between two adjacent formations or between a member and its parent formation. The usual exception to this rule most commonly occurs in river sections draining steep mountain sides where strata are virtually horizontal and thick sequences may thus be exposed over a relatively short length of the rivers course. Examples include the section in Barony Glen (Key Site 301) on the west side of White Mountain (Subarea-12) and the numerous sections on the NE face of Cuilcagh Mountain, in strata of the Leitrim Group (Subarea-1).
On the relatively flat to undulating, often thickly drift-covered land of the Clogher Valley in Subarea-5, such an occurrence would therefore be quite exceptional. The section of strata exposed in the stream, and on the valley sides, flowing northwards through Fardross Forest, 3.3km SSW of Clogher Cathedral, represents a succession that commences at the base, in the top of the Bundoran Shale Formation and is succeeded by all of the Mullaghmore Sandstone, Benbulben Shale and Glencar Limestone formations and ends in the lower part of the Dartry Limestone Formation. Exposure is not completely continuous but is nevertheless so good that the author is not aware of any comparable section in N. Ireland, nor of any other section that exposes such a range of formations of the Tyrone Group.
For site specific information see; Key Site 253 - Fardross Stream, Clogher.
(iv) MAYDOWN LIMESTONE FORMATION
In Co. Armagh (Subarea-6) a sequence of late Viséan (Tyrone Group) sedimentary rocks, ~230m thick, is completely exposed in the gorge of the River Blackwater, Benburb (Mitchell & Mitchell, 1982). The Maydown Limestone Formation is the lowest unit in the sequence and is ~130m thick. The formation is characterised by the gradual upward disappearance of shales, from the base of the section where they are either dominant or are interbedded in equal proportions with limestones to a shale-free sequence, or one with shales restricted to thin laminae, at the top. In this respect the Maydown Formation mimics the lithological changes that occur between the Benbulben Shale, Glencar Limestone and Dartry Limestone formations in Cos. Tyrone and Fermanagh (Key Site 253). For this reason the Geological Survey of Ireland (Egan, 1873) assigned these strata to the Calp Limestone and Shale, but not to the Upper Limestone, despite the appearance of such limestones at the top of this sequence which is now referred to the Maydown Limestone Formation.
The age of the Maydown Limestone Formation is undoubtedly Asbian, based on a prolific fauna of corals and brachiopods which occur throughout the sequence except in the thickest shale horizons at the base.
SUBAREA-5: The Aughnacloy-Tynan Area
In the western part of Subarea-5 the 'Aughnacloy Sandstone Member', as defined by the GSNI (1983), is exposed only in the Fardross section (Key Site 253) and being demonstrably of late Arundian age, is the correlative of the Mullaghmore Sandstone Formation and is now referred to by this name.
In the eastern part of Subarea-5, between Aughnacloy (H66 52) and Tynan (H76 43), the GSNI depict a tripartite outcrop for a sandstone unit that they referred to as the Aughnacloy Sandstone Member (1:50,000 Sheet 46, 1983). On this assumption the sandstone, which should be late Arundian in age, should also be underlain by the mid-Arundian Bundoran Shale Formation and overlain by the Arundian-Asbian Benbulben Shale Formation. New biostratigraphical and lithological evidence from the Aughnacloy- Tynan area shows that none of these assumptions is correct.
1. New miospore evidence demonstrates conclusively that the sandstone is Asbian and is thus not equivalent to the Mullaghmore Sandstone Formation.
2. Arundian strata, with the exception of the fault-bounded inlier of the Bundoran Shale Formation at Annagh and Drummond Quarry (Key Site 252), do not occur in the Aughnacloy-Tynan area.
3. Throughout this area the sandstone unit is everywhere underlain by the so-called 'Carnteel Formation' (GSNI, 1983). This comprises a mixed suite of lithologies including mudstones, interbedded limestones and limestones separated only by thin shales. In all respects this association is identical to the definition of the Maydown Limestone Formation. Prolific assemblages of macrofauna and abundant microbiota recovered from these lithologies are exclusively of Asbian age thus reinforcing the correlation with the Benburb sequence.
North of Aughnacloy the GSNI (1983) depict a fault-bounded lenticular outcrop of the Dartry Limestone Formation. Faunally and lithologically these strata are quite atypical of the Dartry Limestone, as seen in Subarea-2 and in the Fardross section (Subarea-5), but are identical to the limestone-dominated section at the top of the Maydown Limestone Formation.
All the available new evidence from the Aughnacloy-Tynan area now supports a correlation of strata cropping out in that part of Subarea-5, formerly assigned to the Carnteel Formation, with the Asbian Maydown Limestone Formation. The sandstone unit, formerly referred to the Aughnacloy Sandstone Member of the Carnteel Formation, is now correlated with the Asbian Carrickaness Sandstone Formation (Mitchell & Mitchell, 1982) of the Benburb area.
For site specific information see; Key Site 256 - Plaister Quarry, Aughnacloy.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Carboniferous Subarea-5; Clogher-Aughnacloy, County Tyrone, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1178. Accessed on 2024-12-26 |
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