Earth Science Conservation Review
| Carboniferous Subarea-1; Derrygonnelly-Marble Arch-Cuilcagh | Fermanagh |
| Site Type: | Various |
| Site Status: | |
| Council area: | Fermanagh District Council |
| Grid Reference: | H1252 |
| Google maps: | 54.41689,-7.81514 |
| Rocks | |
|---|---|
| Rock Age: | Carboniferous (Namurian, Tournasian, Visean) |
| Rock Name: | Ballyshannon Limestone Formation, Bellavally Formation, Benbulben Shale Formation, Bundoran Shale Formation, Carraun Shale Formation, Dartry Limestone Formation, Glenade Sandstone Formation, Glencar Limestone Formation, Leitrim Group, Meenymore Formation, |
| Rock Type: | Limestone, Mudstone, Sand, Sandstone, Shale, Siltstone |
| Interest | |
| Minerals: | Micrite, Sparrite |
| Fossil Groups: | Bivalve, Brachiopod, Coral, Crinoid, Foraminifera, Gastropod, Goniatite, Nautiloid, Polyzoan, Trilobite |
| Other interest: | palaeokarst, Marine sediments |
Description:
INTRODUCTION
The geological mapping of the Carboniferous rocks in Subarea-1 has been completed by the Geological Survey of Northern Ireland. The information is published on the following geological maps:
GSNI, 1982. 1:50,000 Solid Geology of Sheet 45 (Enniskillen) GSNI, 1991. 1:50,000 Solid Geology of Sheet 44, 56 and 43 (Derrygonnelly & Marble Arch) GSNI, 1995. 1:50,000 Solid Geology of Sheet 57 and part of 58 (Lisnaskea) - in preparation.
The southwestern shore of Lower Lough Erne lies just outside the northwest corner of Sheet 44, 56 and 43 and is included on the 1:50,000 geological map of the Kesh area.
GSNI, 1994. 1:50,000 Solid Geology of Sheet 32 and 31 (Kesh).
All of the formations that constitute the Tyrone Group were erected by Oswald (1955) in the area between the Ox Mountains and Donegal Bay, in NW Ireland. In so doing he did not select strato- type sections but the geographical name of each formation offers sufficient clues as to the location of their best and most typical sections. Sites selected for these formations in Subarea-1 therefore represent a locality or localities where typical lithologies are displayed and, if possible, where diagnostic fossils may be collected.
New members, that occur within formations of the Tyrone Group in Subarea-1, are only recognised in the Dartry Limestone Formation. Stratotype sections selected by the GSNI for each member are thus accorded ASSI status and are described herein.
Within the Leitrim Group all the formations and members recognised in NW Ireland were erected by Brandon (1968, 1972) at stratotype sections in Cos. Cavan and Leitrim. The exceptions are the Glenade (Beds) Sandstone Formation (Oswald, 1955) and the Meenymore (Member) Formation, which West, Brandon and Smith (1968) included as part of the now obsolete Aghagrania Formation.
Each formation, and constituent members, and the selected ASSI are thus described sequentially commencing with the oldest unit. Although the lowest Carboniferous beds, the Basal Clastics, are considered to occur in this area, they are not exposed and therefore will not be described here.
This subarea is noteworthy for the occurrence (inter alia) of one of only two known exposures of Waulsortian Limestone in Northern Ireland, a fact which was not recognised until recently and which is recorded here for the first time.
I - TYRONE GROUP
In County Fermanagh the Tyrone Group comprises six formations consisting mainly of limestones and shales which overlie an unnamed and unexposed, in Subarea-1, basal sequence of dominantly clastic rocks that were deposited in terrestrial, peritidal and shallow marine environments. The sequence is concealed by drift deposits on the SW shore of Lower Lough Erne but is well exposed in the Clogher Valley. The basal beds were proven in deep boreholes drilled at Big Dog (1965), Slisgarrow (1984) and Kilcoo Cross (1985). The stratigraphy of these boreholes and detailed information on the basal beds is summarised in Philcox et al. (1992).
(i) BALLYSHANNON LIMESTONE FORMATION
Oswald (1955) included the basal conglomerate and sandstones within the Ballyshannon Limestone but these were removed by Brunton and Mason (1979) and the formation name restricted only to the limestones. The formation crops out along the SW side of Lower Lough Erne and supposedly extends southeastwards under Upper Lough Erne although exposure is limited to few localities.
The main formation outcrop occurs however in the Derrygonnelly area and several quarries, which expose the lower levels, but not the base, are developed near Blaney at the base of the north and NE facing escarpment. On higher ground to the SW of the quarries, successive strike-parallel SE trending ridges, with karstic features, expose the highest beds of limestone below the over- lying Bundoran Shale Formation. Typically the top beds of limestone form long southwesterly inclined dip slopes covered by a thin, patchy mantle of till and the outcrop of the Bundoran Shale may be identified by the appearance, and development, of till drumlins and ridges on that lithology, in preference to the relatively drift-free limestone terrain. The precise contact between the Ballyshannon and Bundoran formation is apparently not exposed but the highest limestone bed encountered in the area, at Inisway Townland (H1628 5184), is a foraminifera-rich grainstone and differs completely from the normal medium-grained packstones. As in the Kesh-Omagh area lithologies at the top of the Ballyshannon Formation in Subarea-1 record an episode of bathymetric shallowing which resulted, in the former area, in partial exposure and erosion and widespread micritisation and instigated, in the latter area, a change from open shelf to high energy shallow water.
In the Big Dog Borehole the formation is 345m thick whereas between Blaney (H165 528) and Roosky (H185 498) it is an estimated 200m.
Microfaunas recovered from the Ballyshannon Limestone indicate a late Chadian age for the basal part of the formation and an Arundian age for the upper part.
In the Blaney-Roosky area the Ballyshannon Limestone is divided into three informal members.
For site specific information see;
Key Site 174 - Blaney Quarry Key Site 175 - Carrickreagh Quarry Key Site 176 - Inisway Quarry Key Site 177 - Bellanaleck Quarry.
(ii) BUNDORAN SHALE FORMATION
This formation, described by Oswald (1955), is spectacularly exposed on the west coast of Ireland at Bundoran. In Co. Fermanagh the formation is rarely well exposed and its outcrop tends to be confined to low, drift-covered ground, sandwiched between the harder limestones of the Ballyshannon Formation below and succeeding sandstones of the Mullaghmore Formation. In Subarea-1 the principal outcrop occupies much of the alluvium- covered land adjacent to Lough Macnean Lower and west of Upper Lough Erne.
The formation consists of dark grey, calcareous mudstone with thin limestones and yields of rich macrofauna dominated by solitary and rugose corals and by large brachiopods. Limestone beds are usually thin and lenticular and may be derived by turbidite flow. They frequently contain the concentrated debris of crinoid ossicles, broken bryozoa, brachiopods and corals and, an abundant microbiota including archaediscid foraminifera such as Paraarchaediscus at the involutus stage which indicates a mid to late Arundian age.
Of all the formations in the Tyrone Group the Bundoran shale is one of the few that maintains its lithological integrity over much of Northern Ireland and significant variations are usually confined to a thin sandstone member at, or just above, the formation base. The deposition of the shales seems to have accompanied an episode of marine transgression northwards and eastwards onto metamorphic rocks in the Southern Sperrin Mountains and along the north side of the Down-Longford Lower Palaeozoic massif. Its thickness is extremely variable ranging from 555m in the Big Dog Borehole, to 438m and 427m at the Kilcoo Cross and Slisgarrow boreholes, to a minimum of 60m in the area north of Derrygonnelly.
At the top of the Bundoran Formation the fossiliferous shales and thin limestones are gradually replaced by siltstones, very fine sandy siltstones, heterolithic siltstones and sandstones and finally by dark grey fine-grained silty sandstones. These changes record the progressive influence of deltaic sedimentation on the basin and herald the regional influence of increased erosion and uplift of the hinterland leading to the deposition of the Mullaghmore Sandstone Formation.
For site specific information see;
Key Site 178 - Claragh Townland.
(iii) MULLAGHMORE SANDSTONE FORMATION
The Mullaghmore Sandstone was described by Oswald (1955) from coastal exposures west of the Ox Mountains in NW Ireland. It is stratigraphically bounded by the Bundoran Shale Formation below and Benbulben Shale Formation above. Mullaghmore Head, 10km west of Bundoran, is the type locality. Shales within this section have yielded a rich miospore assemblage of the TS Biozone and are assigned a late Arundian to mid-Holkerian age (Higgs, 1984) although Buckman (1992) whilst studying the trace fossils in the sandstone there, reiterated the erroneous correlation of the Mullaghmore Formation and the Holkerian Stage, in its entirety.
The sandstones of the Mullaghmore Formation were derived by uplift and erosion of land located to the north of Subarea-1, in Co. Fermanagh. Its geometry in the basin is a wedge that thins to the south from a source in the northwest and northeast. Analysis of the diverse ichnofauna and sedimentary structures by Buckman (op. cit.) are indicative of a shallow water marine environment.
In Subarea-1 despite the extension of the formation outcrop south from Derrygonnelly to the eastern end of Lough Macnean Lower, the existence of these rocks south of a west-trending fault at Drumscollop (H140 482) is unproven. Numerous exposures do occur however in the formation outcrop east of Lough Melvin, in the Ninny's Hill area (G985 565) and in the district between Derrygonnelly (H12 52) and Monea (H15 49) 5km to the southeast. The selection of a single site that is representative of the Mullaghmore Formation in Subarea-1 is therefore somewhat problematical but a locality near Derrygonnelly has been chosen.
When unweathered, the sandstones are typically grey, fine- to medium-grained, moderately well-sorted and calcareous, and they are in beds up to 0.3m thick. Bedding surfaces are commonly rippled and bioturbated. Most surface exposures of these rocks are deeply weathered however and consequently are stained by an orange-brown crust on the buff to white decalcified sandstone. Coarsening-upwards cycles consist of fine-grained sandstones with parallel and convolute laminations, and ripples and grey siltstones at the base, succeeded by medium-grained, well-sorted cross-laminated subarkosic sandstones with coarse bioclastic sandstones at the top. The bioclasts include brachiopods, gastropods, nautiloids, bivalves and crinoid ossicles with pellets and radial-spar ooliths. Based on this lithological variation the environment of deposition seems to shift from a prodelta location to a more proximal sandflat situation. Periodic incursions of brackish to fully marine conditions occurred periodically and introduced a marine benthos to the delta top.
The thickness of the formation varies from about 200m at the type locality to 204m in the Slisgarrow Borehole and 217m at Kilcoo Cross, both in Subarea-1. The base of the formation is not exposed in this subarea and only at one locality (Key Site 179) is the conformable transition with the overlying Benbulben Shale Formation exposed.
For site specific information see;
Key Site 179 - Derrygonnelly Road Cutting.
(iv) BENBULBEN SHALE FORMATION
This formation was described by Oswald (1955) from outcrops in Co. Sligo and its type area is located there in the Benbulben Range and the Dartry Mountains. In Subarea-1, in Co. Fermanagh, its outcrop mostly occupies low-lying drift covered ground thus limiting exposure to deeply incised streams. Nevertheless in a few places, in particular on the north side of Cuilcagh Mountain, the SE face of Belmore Mountain and NW of the Glen Syncline to the NE of Garrison, there are extensive exposures demonstrating the lithological uniformity of the Formation. The Benbulben Shale is lithologically similar to the Bundoran Shale Formation but is darker coloured, often less calcareous but more fossiliferous. Its thickness is highly variable, ranging from 365m in the Slisgarrow Borehole (H026 518), to 317m at Big Dog (H019 497) and 198m at Kilcoo Cross (G970 481). At outcrop its thickness is consistently less than that recorded in boreholes, ranging from ~150m in the area SE of Belmore Mountain to 90m at Derrygonnelly.
The formation consists primarily of dark grey, almost black, calcareous fissile shales with interbedded thin, nodular bioclastic limestones and lenticular sandstones. Philcox et al. (1992) identified substantial lateral interfingering between shales of the Benbulben Formation and limestones of the succeeding Glencar Formation in wireline logs from the deep boreholes. They demonstrated that the relatively thin succession in Macnean-2 is laterally equivalent to only the lower half of the much thicker sequence in Slisgarrow-1 where, in the top of the shale, there are several distinct limestone tongues up to 6m thick which correlate with similar thick limestones in the base of the Glencar Formation in the Kilcoo Cross Borehole.
In the Northwest Basin the Benbulben Shale, like the Bundoran Shale, maintains its uniform lithology over the entire region and shows no significant variations on the dominant mudstone - thin limestone character. The deposition of the shales was accompanied by a cessation of sand entering the basin, uniform rate of subsidence and a transgression, and bathymetric deepening, of the sea over former delta complexes of the Mullaghmore Sandstone.
All lithologies are extremely fossiliferous. The mudstones in particular contain abundant remains of fenestellid bryozoa and brachiopods but it is the large specimens of the solitary coral Siphonophyllia benburbensis and, to a lesser extent of fragmented colonies of various species of Siphonodendron that is so characteristic. This formation also contains the first occurrence of the compound coral Lithostrotion, which in many localities are inverted and show evidence of transport. Limestones, though thin and often lenticular, commonly contain a graded transported assemblage of bioclast fragments, in particular crinoid ossicles and may reflect an origin as tempestites or from turbidity currents. Thin sandstone beds are usually channelised and erode the underlying mudstone indicating sediment flow from distant shallow water and land areas.
Foraminifera are particularly important in defining the precise age of this formation especially at its base. The occurrence of involutus stage paraarchaediscids and Uralodiscus, some 4m above the formation base, in the Kesh-Omagh area indicates a mid- to late-Arundian age. Higher in the formation there are records of concavus stage archaediscids and Archaediscus sp, both Holkerian entries and Endothyra archerbecki, together with a profusion of relatively primitive archaediscids, which reflects a position within the early Asbian.
Sites, exposing the Benbulben Shale Formation, which are selected for ASSI status occur in the Lough Macnean Lower part of Subarea-1 and demonstrate lithologies and macrofauna typical of the formation (Site 180) in one case and its conformable upper contact with the succeeding Glencar Limestone Formation at the other (Site 181). The formation base and conformable lower contact are described in the section for the Mullaghmore Sandstone Formation under Key Site 179.
For site specific information see;
Key Site 180 - Rahallan Townland Key Site 181 - Tullyhona stream.
The geological mapping of the Carboniferous rocks in Subarea-1 has been completed by the Geological Survey of Northern Ireland. The information is published on the following geological maps:
GSNI, 1982. 1:50,000 Solid Geology of Sheet 45 (Enniskillen) GSNI, 1991. 1:50,000 Solid Geology of Sheet 44, 56 and 43 (Derrygonnelly & Marble Arch) GSNI, 1995. 1:50,000 Solid Geology of Sheet 57 and part of 58 (Lisnaskea) - in preparation.
The southwestern shore of Lower Lough Erne lies just outside the northwest corner of Sheet 44, 56 and 43 and is included on the 1:50,000 geological map of the Kesh area.
GSNI, 1994. 1:50,000 Solid Geology of Sheet 32 and 31 (Kesh).
All of the formations that constitute the Tyrone Group were erected by Oswald (1955) in the area between the Ox Mountains and Donegal Bay, in NW Ireland. In so doing he did not select strato- type sections but the geographical name of each formation offers sufficient clues as to the location of their best and most typical sections. Sites selected for these formations in Subarea-1 therefore represent a locality or localities where typical lithologies are displayed and, if possible, where diagnostic fossils may be collected.
New members, that occur within formations of the Tyrone Group in Subarea-1, are only recognised in the Dartry Limestone Formation. Stratotype sections selected by the GSNI for each member are thus accorded ASSI status and are described herein.
Within the Leitrim Group all the formations and members recognised in NW Ireland were erected by Brandon (1968, 1972) at stratotype sections in Cos. Cavan and Leitrim. The exceptions are the Glenade (Beds) Sandstone Formation (Oswald, 1955) and the Meenymore (Member) Formation, which West, Brandon and Smith (1968) included as part of the now obsolete Aghagrania Formation.
Each formation, and constituent members, and the selected ASSI are thus described sequentially commencing with the oldest unit. Although the lowest Carboniferous beds, the Basal Clastics, are considered to occur in this area, they are not exposed and therefore will not be described here.
This subarea is noteworthy for the occurrence (inter alia) of one of only two known exposures of Waulsortian Limestone in Northern Ireland, a fact which was not recognised until recently and which is recorded here for the first time.
I - TYRONE GROUP
In County Fermanagh the Tyrone Group comprises six formations consisting mainly of limestones and shales which overlie an unnamed and unexposed, in Subarea-1, basal sequence of dominantly clastic rocks that were deposited in terrestrial, peritidal and shallow marine environments. The sequence is concealed by drift deposits on the SW shore of Lower Lough Erne but is well exposed in the Clogher Valley. The basal beds were proven in deep boreholes drilled at Big Dog (1965), Slisgarrow (1984) and Kilcoo Cross (1985). The stratigraphy of these boreholes and detailed information on the basal beds is summarised in Philcox et al. (1992).
(i) BALLYSHANNON LIMESTONE FORMATION
Oswald (1955) included the basal conglomerate and sandstones within the Ballyshannon Limestone but these were removed by Brunton and Mason (1979) and the formation name restricted only to the limestones. The formation crops out along the SW side of Lower Lough Erne and supposedly extends southeastwards under Upper Lough Erne although exposure is limited to few localities.
The main formation outcrop occurs however in the Derrygonnelly area and several quarries, which expose the lower levels, but not the base, are developed near Blaney at the base of the north and NE facing escarpment. On higher ground to the SW of the quarries, successive strike-parallel SE trending ridges, with karstic features, expose the highest beds of limestone below the over- lying Bundoran Shale Formation. Typically the top beds of limestone form long southwesterly inclined dip slopes covered by a thin, patchy mantle of till and the outcrop of the Bundoran Shale may be identified by the appearance, and development, of till drumlins and ridges on that lithology, in preference to the relatively drift-free limestone terrain. The precise contact between the Ballyshannon and Bundoran formation is apparently not exposed but the highest limestone bed encountered in the area, at Inisway Townland (H1628 5184), is a foraminifera-rich grainstone and differs completely from the normal medium-grained packstones. As in the Kesh-Omagh area lithologies at the top of the Ballyshannon Formation in Subarea-1 record an episode of bathymetric shallowing which resulted, in the former area, in partial exposure and erosion and widespread micritisation and instigated, in the latter area, a change from open shelf to high energy shallow water.
In the Big Dog Borehole the formation is 345m thick whereas between Blaney (H165 528) and Roosky (H185 498) it is an estimated 200m.
Microfaunas recovered from the Ballyshannon Limestone indicate a late Chadian age for the basal part of the formation and an Arundian age for the upper part.
In the Blaney-Roosky area the Ballyshannon Limestone is divided into three informal members.
For site specific information see;
Key Site 174 - Blaney Quarry Key Site 175 - Carrickreagh Quarry Key Site 176 - Inisway Quarry Key Site 177 - Bellanaleck Quarry.
(ii) BUNDORAN SHALE FORMATION
This formation, described by Oswald (1955), is spectacularly exposed on the west coast of Ireland at Bundoran. In Co. Fermanagh the formation is rarely well exposed and its outcrop tends to be confined to low, drift-covered ground, sandwiched between the harder limestones of the Ballyshannon Formation below and succeeding sandstones of the Mullaghmore Formation. In Subarea-1 the principal outcrop occupies much of the alluvium- covered land adjacent to Lough Macnean Lower and west of Upper Lough Erne.
The formation consists of dark grey, calcareous mudstone with thin limestones and yields of rich macrofauna dominated by solitary and rugose corals and by large brachiopods. Limestone beds are usually thin and lenticular and may be derived by turbidite flow. They frequently contain the concentrated debris of crinoid ossicles, broken bryozoa, brachiopods and corals and, an abundant microbiota including archaediscid foraminifera such as Paraarchaediscus at the involutus stage which indicates a mid to late Arundian age.
Of all the formations in the Tyrone Group the Bundoran shale is one of the few that maintains its lithological integrity over much of Northern Ireland and significant variations are usually confined to a thin sandstone member at, or just above, the formation base. The deposition of the shales seems to have accompanied an episode of marine transgression northwards and eastwards onto metamorphic rocks in the Southern Sperrin Mountains and along the north side of the Down-Longford Lower Palaeozoic massif. Its thickness is extremely variable ranging from 555m in the Big Dog Borehole, to 438m and 427m at the Kilcoo Cross and Slisgarrow boreholes, to a minimum of 60m in the area north of Derrygonnelly.
At the top of the Bundoran Formation the fossiliferous shales and thin limestones are gradually replaced by siltstones, very fine sandy siltstones, heterolithic siltstones and sandstones and finally by dark grey fine-grained silty sandstones. These changes record the progressive influence of deltaic sedimentation on the basin and herald the regional influence of increased erosion and uplift of the hinterland leading to the deposition of the Mullaghmore Sandstone Formation.
For site specific information see;
Key Site 178 - Claragh Townland.
(iii) MULLAGHMORE SANDSTONE FORMATION
The Mullaghmore Sandstone was described by Oswald (1955) from coastal exposures west of the Ox Mountains in NW Ireland. It is stratigraphically bounded by the Bundoran Shale Formation below and Benbulben Shale Formation above. Mullaghmore Head, 10km west of Bundoran, is the type locality. Shales within this section have yielded a rich miospore assemblage of the TS Biozone and are assigned a late Arundian to mid-Holkerian age (Higgs, 1984) although Buckman (1992) whilst studying the trace fossils in the sandstone there, reiterated the erroneous correlation of the Mullaghmore Formation and the Holkerian Stage, in its entirety.
The sandstones of the Mullaghmore Formation were derived by uplift and erosion of land located to the north of Subarea-1, in Co. Fermanagh. Its geometry in the basin is a wedge that thins to the south from a source in the northwest and northeast. Analysis of the diverse ichnofauna and sedimentary structures by Buckman (op. cit.) are indicative of a shallow water marine environment.
In Subarea-1 despite the extension of the formation outcrop south from Derrygonnelly to the eastern end of Lough Macnean Lower, the existence of these rocks south of a west-trending fault at Drumscollop (H140 482) is unproven. Numerous exposures do occur however in the formation outcrop east of Lough Melvin, in the Ninny's Hill area (G985 565) and in the district between Derrygonnelly (H12 52) and Monea (H15 49) 5km to the southeast. The selection of a single site that is representative of the Mullaghmore Formation in Subarea-1 is therefore somewhat problematical but a locality near Derrygonnelly has been chosen.
When unweathered, the sandstones are typically grey, fine- to medium-grained, moderately well-sorted and calcareous, and they are in beds up to 0.3m thick. Bedding surfaces are commonly rippled and bioturbated. Most surface exposures of these rocks are deeply weathered however and consequently are stained by an orange-brown crust on the buff to white decalcified sandstone. Coarsening-upwards cycles consist of fine-grained sandstones with parallel and convolute laminations, and ripples and grey siltstones at the base, succeeded by medium-grained, well-sorted cross-laminated subarkosic sandstones with coarse bioclastic sandstones at the top. The bioclasts include brachiopods, gastropods, nautiloids, bivalves and crinoid ossicles with pellets and radial-spar ooliths. Based on this lithological variation the environment of deposition seems to shift from a prodelta location to a more proximal sandflat situation. Periodic incursions of brackish to fully marine conditions occurred periodically and introduced a marine benthos to the delta top.
The thickness of the formation varies from about 200m at the type locality to 204m in the Slisgarrow Borehole and 217m at Kilcoo Cross, both in Subarea-1. The base of the formation is not exposed in this subarea and only at one locality (Key Site 179) is the conformable transition with the overlying Benbulben Shale Formation exposed.
For site specific information see;
Key Site 179 - Derrygonnelly Road Cutting.
(iv) BENBULBEN SHALE FORMATION
This formation was described by Oswald (1955) from outcrops in Co. Sligo and its type area is located there in the Benbulben Range and the Dartry Mountains. In Subarea-1, in Co. Fermanagh, its outcrop mostly occupies low-lying drift covered ground thus limiting exposure to deeply incised streams. Nevertheless in a few places, in particular on the north side of Cuilcagh Mountain, the SE face of Belmore Mountain and NW of the Glen Syncline to the NE of Garrison, there are extensive exposures demonstrating the lithological uniformity of the Formation. The Benbulben Shale is lithologically similar to the Bundoran Shale Formation but is darker coloured, often less calcareous but more fossiliferous. Its thickness is highly variable, ranging from 365m in the Slisgarrow Borehole (H026 518), to 317m at Big Dog (H019 497) and 198m at Kilcoo Cross (G970 481). At outcrop its thickness is consistently less than that recorded in boreholes, ranging from ~150m in the area SE of Belmore Mountain to 90m at Derrygonnelly.
The formation consists primarily of dark grey, almost black, calcareous fissile shales with interbedded thin, nodular bioclastic limestones and lenticular sandstones. Philcox et al. (1992) identified substantial lateral interfingering between shales of the Benbulben Formation and limestones of the succeeding Glencar Formation in wireline logs from the deep boreholes. They demonstrated that the relatively thin succession in Macnean-2 is laterally equivalent to only the lower half of the much thicker sequence in Slisgarrow-1 where, in the top of the shale, there are several distinct limestone tongues up to 6m thick which correlate with similar thick limestones in the base of the Glencar Formation in the Kilcoo Cross Borehole.
In the Northwest Basin the Benbulben Shale, like the Bundoran Shale, maintains its uniform lithology over the entire region and shows no significant variations on the dominant mudstone - thin limestone character. The deposition of the shales was accompanied by a cessation of sand entering the basin, uniform rate of subsidence and a transgression, and bathymetric deepening, of the sea over former delta complexes of the Mullaghmore Sandstone.
All lithologies are extremely fossiliferous. The mudstones in particular contain abundant remains of fenestellid bryozoa and brachiopods but it is the large specimens of the solitary coral Siphonophyllia benburbensis and, to a lesser extent of fragmented colonies of various species of Siphonodendron that is so characteristic. This formation also contains the first occurrence of the compound coral Lithostrotion, which in many localities are inverted and show evidence of transport. Limestones, though thin and often lenticular, commonly contain a graded transported assemblage of bioclast fragments, in particular crinoid ossicles and may reflect an origin as tempestites or from turbidity currents. Thin sandstone beds are usually channelised and erode the underlying mudstone indicating sediment flow from distant shallow water and land areas.
Foraminifera are particularly important in defining the precise age of this formation especially at its base. The occurrence of involutus stage paraarchaediscids and Uralodiscus, some 4m above the formation base, in the Kesh-Omagh area indicates a mid- to late-Arundian age. Higher in the formation there are records of concavus stage archaediscids and Archaediscus sp, both Holkerian entries and Endothyra archerbecki, together with a profusion of relatively primitive archaediscids, which reflects a position within the early Asbian.
Sites, exposing the Benbulben Shale Formation, which are selected for ASSI status occur in the Lough Macnean Lower part of Subarea-1 and demonstrate lithologies and macrofauna typical of the formation (Site 180) in one case and its conformable upper contact with the succeeding Glencar Limestone Formation at the other (Site 181). The formation base and conformable lower contact are described in the section for the Mullaghmore Sandstone Formation under Key Site 179.
For site specific information see;
Key Site 180 - Rahallan Townland Key Site 181 - Tullyhona stream.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Carboniferous Subarea-1; Derrygonnelly-Marble Arch-Cuilcagh, County Fermanagh, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1174. Accessed on 2024-12-26 |
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