Earth Science Conservation Review
| Carboniferous Subarea-3; Kesh-Omagh | Fermanagh, Tyrone |
| Site Type: | Various |
| Site Status: | |
| Council area: | Fermanagh District Council, Omagh District Council |
| Grid Reference: | H180638,H450725 |
| Google maps: | 54.52272,-7.722 |
| Rocks | |
|---|---|
| Rock Age: | Carboniferous (Arundian, Chadian, Courcyean, Holkerian, Tournasian, Visean) |
| Rock Name: | Ballyshannon Limestone Formation, Benbulben Shale Formation, Bin Mountain Sandstone Formation, Bundoran Shale Formation, Claragh Sandstone Formation, Drumchorick Siltstone Formation, Keenaghan Shale Formation, Mullaghmore Sandstone, Omagh Sandstone Group, |
| Rock Type: | Limestone, Sand, Sandstone, Shale |
| Interest | |
| Other interest: | bedding, cross-bedding, massive, palaeokarst, ripple marks, Marine sediments |
Description:
INTRODUCTION
The geological mapping of the Carboniferous rocks in Subarea-3 has been completed by the Geological Survey of Northern Ireland. The information is published on the following geological maps:
GSNI, 1994. 1:50,000 Solid Geology of Sheet 32 and 31 (Kesh) GSNI, 1995. 1:50,000 Solid Geology of Sheet 33 (Omagh).
The Omagh and Tyrone groups are represented in this area.
Although the GSNI have remapped the greater part of Subarea-3, a sliver of Carboniferous rocks, located at the northern edge of their outcrop, located within 1:50,000 Sheet 25 (Newtownstewart) have still to be mapped in detail. By far the bulk of this outcrop is occupied by rocks of the Omagh Sandstone Group and the Claragh Sandstone Formation, the basal unit of the Tyrone Group hereabouts and it is not believed that any new information of real consequence to this study, will be forthcoming.
The area was first surveyed in detail by the Geological Survey of Ireland and the Kesh and Omagh One Inch maps were published in 1885. Their interpretation of the stratigraphy of these Carboniferous rocks conformed precisely to the standard Geological Survey nomenclature of the times. This being so and having failed to recognise the importance of major strike faults causing repeated repetition of the sequence, the geologists were forced to assign what appeared to them to be stratigraphically higher and therefore younger limestones and sandstones, to the Upper Limestone and Yoredale Sandstone and Shale divisions.
In the Kesh area this raised slight problems in the southern part of the outcrop. However, in the Omagh area to the east, the problems created by this erroneous assignment were enormous and virtually all of their age determinations for the various limestone, sandstone or shale units are now known to be incorrect. This was unfortunate as their mapping of the outcrop of the various lithological units was extremely good and in some instances, particularly with the lowest horizon, now referred to the Omagh Sandstone Group, was invariably correct.
The Carboniferous geology of the Kesh-Omagh area was subsequently revised by Simpson (1955) who subdivided the strata into four lithostratigraphical groups, with a total thickness of between 3500 feet (1060m) and 8200 feet (2500m). He recognised the impact of strike faults on the distribution of the various lithologies and correctly correlated similar lithologies of the same age that occurred on opposite sides of these major faults. Of these groups only the Omagh Sandstone Group is retained as an integral part of the Carboniferous with its upper and lower boundaries being represented by unconformities. Lying stratigraphically above the Omagh Group are the multitude of formations and members that constitute the Tyrone Group. At its base is the Claragh Sandstone Formation, reduced from the Group status conferred by Simpson. The Tyrone Group also includes both the Pettigoe Limestone and Clonelly Sandstone groups of Simpson. It is now apparent that both groups are composite units encompassing many of Oswald's (1955) standard lithostratigraphical formations such as the Ballyshannon, Bundoran, Mullaghmore and Benbulben formations and new units recognised by the GSNI (1994, 1995).
I - OMAGH SANDSTONE GROUP
The oldest Carboniferous rocks in the Kesh-Omagh area are referred to the Omagh Sandstone Group (Simpson, 1955). Their outcrop is confined largely to the area north and NW of Omagh town and to the northern and western margins of the Lack Inlier (GSNI, 1995).
The Omagh Group is the one remaining unit of the four groups originally erected by Simpson. There are several reasons for retaining this nomenclature. The first and most important is that the lower and upper contacts are unconformities. At its base the group overlies metamorphic rocks of the Southern Highland Group in the SW Sperrin Mountains and Lack Inlier and at its top it is succeeded by the Claragh Sandstone Formation, the basal clastic unit of the Tyrone Group. At two places in the Kesh-Omagh area, the unconformable relationship between the Omagh Sandstone Group and Claragh Sandstone Formation is evident. The second reason for retaining the Omagh Sandstone Group as a separate unit is due to the likelihood of formation and member units being recognised within its outcrop. Strong facies changes occur within the group outcrop and detailed study might identify particular lithologies or marker horizons, that can be traced not only within the Kesh- Omagh area but to the Newtownstewart Outlier (Subarea-8). For these reasons it was decided (GSNI, 1995) to retain Simpson's Omagh Sandstone Group as a unit separate from the Tyrone Group.
The maximum thickness of the Omagh Sandstone Group is estimated to be about 250m. This is based on some very incomplete sections and the belief that gentle folding and faulting may have combined to amplify the outcrop width in some parts of the Omagh area.
The age of the Omagh Sandstone Group can be ascertained from the occurrence of abundant, moderately to well preserved miospores. These belong to the CM Biozone and contain specimens of the index taxa Auroraspora macra and Schopfites claviger. Most significant however is the absence, in most samples, of the miospore Lycospora pusilla, the basal Viséan (Pu Zone) index taxon.
The selection of one section that adequately reflects the many lithologies that comprise the Omagh Sandstone Group is difficult. Simpson (1955) did not nominate a stratotype but in choosing the group name clearly signalled the area within which lay its main outcrop. His lithological descriptions are based mainly on the long section in the Killyclogher Burn, located to the NE of Omagh. The basal unconformity with metamorphic basement is exposed (H4725 7437) in the bed of the river and the succeeding 100m of strata consist largely of red-beds, predominantly sandstones but with layers of coarse white quartz pebbles and pedogenic carbonate nodules. The only fine-grained sediments exposed in the Killyclogher Burn are exposed ~1.2km downstream of the basal unconformity, in the midst of a housing estate. The basal unconformity between the Omagh Sandstone Group and meta- morphic basement is also exposed (inter alia) at the SW end of the Lack Inlier. Simpson included these rocks in his Claragh Sandstone Group but they are lithologically quite dissimilar to that formation and contain abundant miospores of the CM Biozone and are undoubtedly Tournaisian.
For site specific information see; Key Site 217 - Glendurragh River, Lack.
II - TYRONE GROUP
Detailed geological mapping of the Kesh-Omagh area by the GSNI indicates that all of Oswald's standard lithostratigraphical formations, between and including the Ballyshannon Limestone and Benbulben Shale formations occur in that area. Because the litho- logical definition of these formations, namely the Ballyshannon, Bundoran, Mullaghmore and Benbulben formations conforms closely to their description from the type areas in Co. Sligo, and other sections in Co. Fermanagh (Subarea-1), sites selected for them are described only briefly.
During Viséan times in N. Ireland, the Kesh-Omagh area lay close(r) to the northern shoreline of the Northwest Carboniferous Basin. In consequence it not only received a greater amount of clastic sediment than, for example, the Derrygonnelly area of Co. Fermanagh, but was also continually affected by episodic changes in the bathymetry of the basin. As a result the deposition of the 'normal' sediments of the standard formations was continually interrupted by the influx of clastic debris and by the sudden and temporary introduction of very shallow water conditions that significantly altered the nature of the sediments being deposited and of the sedimentary processes operating at the time. The resultant effects on the Carboniferous succession in the Kesh- Omagh area were thus quite dramatic and numerous new formations, and particularly members, are therefore recognised in that area and their designated stratotype sections are located there. The areal and stratigraphical distribution of all of these new litho- stratigraphical units is depicted on the 1:50,000 geological maps of the Kesh (Sheet 32) and Omagh (Sheet 33) areas (GSNI, 1994,5).
(i) KEENAGHAN SHALE FORMATION
In the SW of Subarea-3 the sub-Carboniferous basement consists either of the Upper Dalradian Southern Highland Group or of the Moinian Lough Derg Group (GSNI, 1994, 1:50,000 Sheet 32). The former metamorphic rocks outcrop along the northern margin of the Carboniferous outcrop between Lough Derg and the southwestern foothills of the Sperrin Mountains. In the east they are overlain unconformably by the Omagh Sandstone Group and in the west by the thick (up to 500m) coarse-grained and pebbly sandstones of the Claragh Formation. These clastic sediments only occur east of the Pettigoe Fault. West of the Pettigoe Fault, on the NW shore of Lower Lough Erne is the outcrop of the psammite-dominated sequence of the Lough Derg Group. The 'basal clastic' component, of the Lower Carboniferous succession, that rests unconformably on the Lough Derg Group consists of ~20m of fine-grained clastic and carbonate rocks of the Keenaghan Shale Formation, the Claragh Sandstone Formation not being represented. The former formation is overlain conformably by the Ballyshannon Limestone Formation.
For site specific information see; Key Site 218 - Keenaghan Lough.
(ii) CLARAGH SANDSTONE FORMATION
The Claragh Sandstone started life as one of Simpson's (1955) four lithostratigraphical group divisions of the Carboniferous rocks in an area, which he termed, the "Omagh Syncline", but which is now simply referred to as the Kesh-Omagh area. He estimated its thickness at between 300m and 1060m and correlated the unit with Vaughan's C1 Zone, equivalent to the basal Viséan Chadian Stage of George et al. (1977). Unfortunately he did not select a stratotype section for the unit.
Remapping by the GSNI of much of the outcrop of the Claragh Sandstone has identified slight variations in the nature of the component lithologies, but these are both thin and laterally impersistent and the unit consists mainly of very coarse to coarse-grained arkose. On this basis it was decided (GSNI, 1994) to retain the name Claragh Sandstone but to reduce its status to that of a formation and to place it within the Tyrone Group.
The outcrop of the Claragh Sandstone Formation extends in an arcuate zone from Omagh, northwestwards to Bin and Bolaght Mountains and west to the line of the Pettigoe Fault, about 6km east of Lough Derg. Virtually all the rocks are unfossiliferous but occasionally, beds of dark grey mudstone do occur in the sandstone sequence and these contain abundant, diverse and well preserved miospore assemblages that include the Pu Biozone index taxon Lycospora pusilla and thus are of Viséan age.
These mudstones occur in a stream section (H3036 7613) 3.3km NW of Drumquin. Cropping out in the same stream, some 350m to the SE (H3078 7589) ie. downstream, is a bed of limestone 0.5m thick. This consists of medium to dark grey, very coarse-grained pack- stone and oncolitic algal boundstone, slightly sandy in patches with bioclasts including brachiopods, gastropods, ostracods and serpulid debris. Abundant algae in the rock include the forms:
Girvanella sp. Koninckopora sp. (monolaminar) Koninckopora sp. (bilaminar) Shartymophycus fusus Ortonella sp.
Plurilocular foraminifera are restricted to two forms:
Earlandia sp. Spinobrunsiina sp.
The presence of the dasyclad alga Koninckopora, with both monolaminar and bilaminar examples is significant and favours a late Chadian to early Arundian age. In view of the late Chadian age for the beds overlying the Claragh Sandstone, a Chadian age is favoured for this formation.
Elsewhere in the outcrop of the Claragh Sandstone the dominant lithology is a thin to massively bedded, very coarse-grained, gritty and often pebbly subarkose, arkose and lithicarenites. Sedimentary structures are very common and include parallel and cross laminations, normally graded beds and channelised bedforms with erosional conglomeratic bases. Colours are concentrated in the pale grey, white and fawn range although iron-rich carbonate cements, when present, give rise to rusty brown staining. These lithologies may be seen throughout the outcrop of this formation. The GSNI estimate the thickness of the Claragh Sandstone Formation to vary between 325-500m.
For site specific information see; Key Site 220 - Binnawooda Spring.
(iii) TERMON RIVER LIMESTONE FORMATION
This is a new lithostratigraphical formation first recognised by the GSNI in the Kesh area. It is ~35m thick and consists mainly of carbonates of the peritidal facies and is laterally equivalent to the lower part of the Bin Mountain Sandstone Formation. The main outcrop of the formation occurs towards the northern edge of the Kesh-Omagh area (Subarea-3) along the course of the Termon River, NE of Pettigoe and on the east face of Dunnaree Hill, west of Drumquin. A sparse macrofauna and relatively abundant micro- biota indicate a late Chadian age for this formation. Both the upper and lower contacts and the top 18m of the formation are exposed in the stratotype section.
For stratutype section site specific information see; Key Site 219 - Giant's Grave section.
(iv) BIN MOUNTAIN SANDSTONE FORMATION
This new lithostratigraphical formation first recognised by the GSNI in the Omagh area is known to extend southwestwards into the Kesh area. The existence of "calcareous horizons near the middle of the (Claragh Sandstone Group) succession" was known to Simpson (1955, p.396) who pointed out that they were well exposed on the scarp face of Bin Mountain in the form of arenaceous limestones, being both conglomeratic and oolitic. Sheridan (1972, p.48) demonstrated that this horizon was Viséan in age and that the youngest beds in the section, from an unknown horizon, contained archaediscids. His correlation however of the Bin Mountain horizon with the Lower Limestone (Ballyshannon Formation) and the Lower Calp Shale (Bundoran Formation) is incorrect.
The formation is at least 53m thick at the stratotype section on Bin Mountain and is exposed at the emergence of Binnawooda spring and in the rough lane leading south from the spring. Some 6.8km SW of the stratotype, in the Giant's Grave section the member is reduced in thickness to ~6m and is now separated from the top of the Claragh Sandstone Formation by another new lithostrati- graphical unit, the Termon River Limestone Formation. At the Binnawooda spring section the latter formation is absent, this being explained entirely in terms of lateral facies variation.
Palaeontological evidence indicates a late Chadian age. Towards the top of the formation there is a gradational transition to the Ballyshannon Limestone Formation and within this section are the Chadian-Arundian and early-late Arundian boundaries.
For site specific information see; Key Site 220 - Binnawooda Spring.
(1) TULLYARD CONGLOMERATE MEMBER
The occurrence of white quartz pebbles in the Bin Mountain Sandstone Formation is concentrated in thin discontinuous layers in the lowest 12m of the stratotype section. Towards the eastern extremity of the formation outcrop the occurrence of white quartz pebbles is concentrated into a section of strata about 4m thick, although scattered pebbles do occur above and below this horizon in bioclastic sandstones. This horizon is referred to as the Tullyard Conglomerate Member and represents a new lithostrati- graphical unit within the Bin Mountain Sandstone Formation.
For stratotype section site specific information see; Key Site 222 - Tullyard Townland stream.
(2) RUSHINDOO OOLITE MEMBER
During the remapping of the Kesh area (1:50,000 Sheet 32) by the GSNI the sandstone unit near the top of the Giant's Grave section was believed to represent a new member, the Gortnaree Sandstone Member, and to be the lowest part of the Ballyshannon Limestone Formation. However on completion of the regional remapping of the entire Carboniferous outcrop in the Kesh-Omagh area it became apparent that this sandstone is neither a new member, nor part of the Ballyshannon Limestone, but is simply a thin development of the Bin Mountain Sandstone Formation. Accordingly the use of the term Gortnaree Sandstone Member is now discontinued. Although the Gortnaree Sandstone Member does not exist, the Rushindoo Oolite does represent a separate and mappable lithology, not at the base of the Ballyshannon Limestone, but within the upper part of the Bin Mountain Formation.
For site specific information see; Key Site 223 - Rushindoo Townland quarry.
The geological mapping of the Carboniferous rocks in Subarea-3 has been completed by the Geological Survey of Northern Ireland. The information is published on the following geological maps:
GSNI, 1994. 1:50,000 Solid Geology of Sheet 32 and 31 (Kesh) GSNI, 1995. 1:50,000 Solid Geology of Sheet 33 (Omagh).
The Omagh and Tyrone groups are represented in this area.
Although the GSNI have remapped the greater part of Subarea-3, a sliver of Carboniferous rocks, located at the northern edge of their outcrop, located within 1:50,000 Sheet 25 (Newtownstewart) have still to be mapped in detail. By far the bulk of this outcrop is occupied by rocks of the Omagh Sandstone Group and the Claragh Sandstone Formation, the basal unit of the Tyrone Group hereabouts and it is not believed that any new information of real consequence to this study, will be forthcoming.
The area was first surveyed in detail by the Geological Survey of Ireland and the Kesh and Omagh One Inch maps were published in 1885. Their interpretation of the stratigraphy of these Carboniferous rocks conformed precisely to the standard Geological Survey nomenclature of the times. This being so and having failed to recognise the importance of major strike faults causing repeated repetition of the sequence, the geologists were forced to assign what appeared to them to be stratigraphically higher and therefore younger limestones and sandstones, to the Upper Limestone and Yoredale Sandstone and Shale divisions.
In the Kesh area this raised slight problems in the southern part of the outcrop. However, in the Omagh area to the east, the problems created by this erroneous assignment were enormous and virtually all of their age determinations for the various limestone, sandstone or shale units are now known to be incorrect. This was unfortunate as their mapping of the outcrop of the various lithological units was extremely good and in some instances, particularly with the lowest horizon, now referred to the Omagh Sandstone Group, was invariably correct.
The Carboniferous geology of the Kesh-Omagh area was subsequently revised by Simpson (1955) who subdivided the strata into four lithostratigraphical groups, with a total thickness of between 3500 feet (1060m) and 8200 feet (2500m). He recognised the impact of strike faults on the distribution of the various lithologies and correctly correlated similar lithologies of the same age that occurred on opposite sides of these major faults. Of these groups only the Omagh Sandstone Group is retained as an integral part of the Carboniferous with its upper and lower boundaries being represented by unconformities. Lying stratigraphically above the Omagh Group are the multitude of formations and members that constitute the Tyrone Group. At its base is the Claragh Sandstone Formation, reduced from the Group status conferred by Simpson. The Tyrone Group also includes both the Pettigoe Limestone and Clonelly Sandstone groups of Simpson. It is now apparent that both groups are composite units encompassing many of Oswald's (1955) standard lithostratigraphical formations such as the Ballyshannon, Bundoran, Mullaghmore and Benbulben formations and new units recognised by the GSNI (1994, 1995).
I - OMAGH SANDSTONE GROUP
The oldest Carboniferous rocks in the Kesh-Omagh area are referred to the Omagh Sandstone Group (Simpson, 1955). Their outcrop is confined largely to the area north and NW of Omagh town and to the northern and western margins of the Lack Inlier (GSNI, 1995).
The Omagh Group is the one remaining unit of the four groups originally erected by Simpson. There are several reasons for retaining this nomenclature. The first and most important is that the lower and upper contacts are unconformities. At its base the group overlies metamorphic rocks of the Southern Highland Group in the SW Sperrin Mountains and Lack Inlier and at its top it is succeeded by the Claragh Sandstone Formation, the basal clastic unit of the Tyrone Group. At two places in the Kesh-Omagh area, the unconformable relationship between the Omagh Sandstone Group and Claragh Sandstone Formation is evident. The second reason for retaining the Omagh Sandstone Group as a separate unit is due to the likelihood of formation and member units being recognised within its outcrop. Strong facies changes occur within the group outcrop and detailed study might identify particular lithologies or marker horizons, that can be traced not only within the Kesh- Omagh area but to the Newtownstewart Outlier (Subarea-8). For these reasons it was decided (GSNI, 1995) to retain Simpson's Omagh Sandstone Group as a unit separate from the Tyrone Group.
The maximum thickness of the Omagh Sandstone Group is estimated to be about 250m. This is based on some very incomplete sections and the belief that gentle folding and faulting may have combined to amplify the outcrop width in some parts of the Omagh area.
The age of the Omagh Sandstone Group can be ascertained from the occurrence of abundant, moderately to well preserved miospores. These belong to the CM Biozone and contain specimens of the index taxa Auroraspora macra and Schopfites claviger. Most significant however is the absence, in most samples, of the miospore Lycospora pusilla, the basal Viséan (Pu Zone) index taxon.
The selection of one section that adequately reflects the many lithologies that comprise the Omagh Sandstone Group is difficult. Simpson (1955) did not nominate a stratotype but in choosing the group name clearly signalled the area within which lay its main outcrop. His lithological descriptions are based mainly on the long section in the Killyclogher Burn, located to the NE of Omagh. The basal unconformity with metamorphic basement is exposed (H4725 7437) in the bed of the river and the succeeding 100m of strata consist largely of red-beds, predominantly sandstones but with layers of coarse white quartz pebbles and pedogenic carbonate nodules. The only fine-grained sediments exposed in the Killyclogher Burn are exposed ~1.2km downstream of the basal unconformity, in the midst of a housing estate. The basal unconformity between the Omagh Sandstone Group and meta- morphic basement is also exposed (inter alia) at the SW end of the Lack Inlier. Simpson included these rocks in his Claragh Sandstone Group but they are lithologically quite dissimilar to that formation and contain abundant miospores of the CM Biozone and are undoubtedly Tournaisian.
For site specific information see; Key Site 217 - Glendurragh River, Lack.
II - TYRONE GROUP
Detailed geological mapping of the Kesh-Omagh area by the GSNI indicates that all of Oswald's standard lithostratigraphical formations, between and including the Ballyshannon Limestone and Benbulben Shale formations occur in that area. Because the litho- logical definition of these formations, namely the Ballyshannon, Bundoran, Mullaghmore and Benbulben formations conforms closely to their description from the type areas in Co. Sligo, and other sections in Co. Fermanagh (Subarea-1), sites selected for them are described only briefly.
During Viséan times in N. Ireland, the Kesh-Omagh area lay close(r) to the northern shoreline of the Northwest Carboniferous Basin. In consequence it not only received a greater amount of clastic sediment than, for example, the Derrygonnelly area of Co. Fermanagh, but was also continually affected by episodic changes in the bathymetry of the basin. As a result the deposition of the 'normal' sediments of the standard formations was continually interrupted by the influx of clastic debris and by the sudden and temporary introduction of very shallow water conditions that significantly altered the nature of the sediments being deposited and of the sedimentary processes operating at the time. The resultant effects on the Carboniferous succession in the Kesh- Omagh area were thus quite dramatic and numerous new formations, and particularly members, are therefore recognised in that area and their designated stratotype sections are located there. The areal and stratigraphical distribution of all of these new litho- stratigraphical units is depicted on the 1:50,000 geological maps of the Kesh (Sheet 32) and Omagh (Sheet 33) areas (GSNI, 1994,5).
(i) KEENAGHAN SHALE FORMATION
In the SW of Subarea-3 the sub-Carboniferous basement consists either of the Upper Dalradian Southern Highland Group or of the Moinian Lough Derg Group (GSNI, 1994, 1:50,000 Sheet 32). The former metamorphic rocks outcrop along the northern margin of the Carboniferous outcrop between Lough Derg and the southwestern foothills of the Sperrin Mountains. In the east they are overlain unconformably by the Omagh Sandstone Group and in the west by the thick (up to 500m) coarse-grained and pebbly sandstones of the Claragh Formation. These clastic sediments only occur east of the Pettigoe Fault. West of the Pettigoe Fault, on the NW shore of Lower Lough Erne is the outcrop of the psammite-dominated sequence of the Lough Derg Group. The 'basal clastic' component, of the Lower Carboniferous succession, that rests unconformably on the Lough Derg Group consists of ~20m of fine-grained clastic and carbonate rocks of the Keenaghan Shale Formation, the Claragh Sandstone Formation not being represented. The former formation is overlain conformably by the Ballyshannon Limestone Formation.
For site specific information see; Key Site 218 - Keenaghan Lough.
(ii) CLARAGH SANDSTONE FORMATION
The Claragh Sandstone started life as one of Simpson's (1955) four lithostratigraphical group divisions of the Carboniferous rocks in an area, which he termed, the "Omagh Syncline", but which is now simply referred to as the Kesh-Omagh area. He estimated its thickness at between 300m and 1060m and correlated the unit with Vaughan's C1 Zone, equivalent to the basal Viséan Chadian Stage of George et al. (1977). Unfortunately he did not select a stratotype section for the unit.
Remapping by the GSNI of much of the outcrop of the Claragh Sandstone has identified slight variations in the nature of the component lithologies, but these are both thin and laterally impersistent and the unit consists mainly of very coarse to coarse-grained arkose. On this basis it was decided (GSNI, 1994) to retain the name Claragh Sandstone but to reduce its status to that of a formation and to place it within the Tyrone Group.
The outcrop of the Claragh Sandstone Formation extends in an arcuate zone from Omagh, northwestwards to Bin and Bolaght Mountains and west to the line of the Pettigoe Fault, about 6km east of Lough Derg. Virtually all the rocks are unfossiliferous but occasionally, beds of dark grey mudstone do occur in the sandstone sequence and these contain abundant, diverse and well preserved miospore assemblages that include the Pu Biozone index taxon Lycospora pusilla and thus are of Viséan age.
These mudstones occur in a stream section (H3036 7613) 3.3km NW of Drumquin. Cropping out in the same stream, some 350m to the SE (H3078 7589) ie. downstream, is a bed of limestone 0.5m thick. This consists of medium to dark grey, very coarse-grained pack- stone and oncolitic algal boundstone, slightly sandy in patches with bioclasts including brachiopods, gastropods, ostracods and serpulid debris. Abundant algae in the rock include the forms:
Girvanella sp. Koninckopora sp. (monolaminar) Koninckopora sp. (bilaminar) Shartymophycus fusus Ortonella sp.
Plurilocular foraminifera are restricted to two forms:
Earlandia sp. Spinobrunsiina sp.
The presence of the dasyclad alga Koninckopora, with both monolaminar and bilaminar examples is significant and favours a late Chadian to early Arundian age. In view of the late Chadian age for the beds overlying the Claragh Sandstone, a Chadian age is favoured for this formation.
Elsewhere in the outcrop of the Claragh Sandstone the dominant lithology is a thin to massively bedded, very coarse-grained, gritty and often pebbly subarkose, arkose and lithicarenites. Sedimentary structures are very common and include parallel and cross laminations, normally graded beds and channelised bedforms with erosional conglomeratic bases. Colours are concentrated in the pale grey, white and fawn range although iron-rich carbonate cements, when present, give rise to rusty brown staining. These lithologies may be seen throughout the outcrop of this formation. The GSNI estimate the thickness of the Claragh Sandstone Formation to vary between 325-500m.
For site specific information see; Key Site 220 - Binnawooda Spring.
(iii) TERMON RIVER LIMESTONE FORMATION
This is a new lithostratigraphical formation first recognised by the GSNI in the Kesh area. It is ~35m thick and consists mainly of carbonates of the peritidal facies and is laterally equivalent to the lower part of the Bin Mountain Sandstone Formation. The main outcrop of the formation occurs towards the northern edge of the Kesh-Omagh area (Subarea-3) along the course of the Termon River, NE of Pettigoe and on the east face of Dunnaree Hill, west of Drumquin. A sparse macrofauna and relatively abundant micro- biota indicate a late Chadian age for this formation. Both the upper and lower contacts and the top 18m of the formation are exposed in the stratotype section.
For stratutype section site specific information see; Key Site 219 - Giant's Grave section.
(iv) BIN MOUNTAIN SANDSTONE FORMATION
This new lithostratigraphical formation first recognised by the GSNI in the Omagh area is known to extend southwestwards into the Kesh area. The existence of "calcareous horizons near the middle of the (Claragh Sandstone Group) succession" was known to Simpson (1955, p.396) who pointed out that they were well exposed on the scarp face of Bin Mountain in the form of arenaceous limestones, being both conglomeratic and oolitic. Sheridan (1972, p.48) demonstrated that this horizon was Viséan in age and that the youngest beds in the section, from an unknown horizon, contained archaediscids. His correlation however of the Bin Mountain horizon with the Lower Limestone (Ballyshannon Formation) and the Lower Calp Shale (Bundoran Formation) is incorrect.
The formation is at least 53m thick at the stratotype section on Bin Mountain and is exposed at the emergence of Binnawooda spring and in the rough lane leading south from the spring. Some 6.8km SW of the stratotype, in the Giant's Grave section the member is reduced in thickness to ~6m and is now separated from the top of the Claragh Sandstone Formation by another new lithostrati- graphical unit, the Termon River Limestone Formation. At the Binnawooda spring section the latter formation is absent, this being explained entirely in terms of lateral facies variation.
Palaeontological evidence indicates a late Chadian age. Towards the top of the formation there is a gradational transition to the Ballyshannon Limestone Formation and within this section are the Chadian-Arundian and early-late Arundian boundaries.
For site specific information see; Key Site 220 - Binnawooda Spring.
(1) TULLYARD CONGLOMERATE MEMBER
The occurrence of white quartz pebbles in the Bin Mountain Sandstone Formation is concentrated in thin discontinuous layers in the lowest 12m of the stratotype section. Towards the eastern extremity of the formation outcrop the occurrence of white quartz pebbles is concentrated into a section of strata about 4m thick, although scattered pebbles do occur above and below this horizon in bioclastic sandstones. This horizon is referred to as the Tullyard Conglomerate Member and represents a new lithostrati- graphical unit within the Bin Mountain Sandstone Formation.
For stratotype section site specific information see; Key Site 222 - Tullyard Townland stream.
(2) RUSHINDOO OOLITE MEMBER
During the remapping of the Kesh area (1:50,000 Sheet 32) by the GSNI the sandstone unit near the top of the Giant's Grave section was believed to represent a new member, the Gortnaree Sandstone Member, and to be the lowest part of the Ballyshannon Limestone Formation. However on completion of the regional remapping of the entire Carboniferous outcrop in the Kesh-Omagh area it became apparent that this sandstone is neither a new member, nor part of the Ballyshannon Limestone, but is simply a thin development of the Bin Mountain Sandstone Formation. Accordingly the use of the term Gortnaree Sandstone Member is now discontinued. Although the Gortnaree Sandstone Member does not exist, the Rushindoo Oolite does represent a separate and mappable lithology, not at the base of the Ballyshannon Limestone, but within the upper part of the Bin Mountain Formation.
For site specific information see; Key Site 223 - Rushindoo Townland quarry.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Carboniferous Subarea-3; Kesh-Omagh, County Fermanagh, Tyrone, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1176. Accessed on 2024-12-26 |
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