Earth Science Conservation Review
| Carboniferous Subarea-2; Lisnaskea & Fivemiletown | Fermanagh, Tyrone |
| Site Type: | Various |
| Site Status: | |
| Council area: | Dungannon District Council, Fermanagh District Council |
| Grid Reference: | H4447 |
| Google maps: | 54.37022,-7.323 |
| Rocks | |
|---|---|
| Rock Age: | Carboniferous (Arundian, Asbian, Brigantian, Chadian, Courcyean, Holkerian, Tournasian, Visean) |
| Rock Name: | Ballyness Formation, Ballyshannon Limestone Formation, Benbulben Shale Formation, Bundoran Shale Formation, Clogher Valley Formation, Dartry Limestone Formation, Leitrim Group, Meenymore Formation, Tyrone Group |
| Rock Type: | Limestone, Mudstone, Sand, Sandstone, Shale |
| Interest | |
| Fossil Groups: | Bivalve, Brachiopod, Conodont, Coral, Crinoid, Echinoderm, Foraminifera, Goniatite, Polyzoan, Trilobite |
| Other interest: | Marine sediments |
Description:
INTRODUCTION
The geological mapping of the Carboniferous rocks in Subarea-2 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, 1995. 1:50,000 Solid Geology of Sheet 57 and part of 58 (Lisnaskea).
The Carboniferous succession in Subarea-2 is represented by the Tyrone and Leitrim groups. The geology of this area is divided conveniently into 2 main parts by the southwestern part of the Clogher Valley Fault. This separates the western end of the Clogher Valley, north of the fault, from higher ground of Slieve Beagh to the south and the Lisnaskea-Newtownbutler area of south Co. Fermanagh. North of the fault, in the Clogher Valley are rocks located in the lower part of the Carboniferous sequence. Conversely, Slieve Beagh consists primarily of a huge area of outcrop of the Meenymore Formation at the top of the sequence, and the Lisnaskea-Newtownbutler area is dominated by rocks in the middle part of the sequence between the Ballyshannon and Dartry limestones.
Formations and members that were recognised first in this Subarea are accorded 2-star (* *) ASSI status and those which form part of Oswald's standard lithostratigraphical sequence, as already described for Subarea-1, warrant a 1-star (*) rating. Stratotype sections for the Ballyness and Clogher Valley formations and for the Newtownbutler Limestone, Mullynagowan Sandstone, Carnmore Sandstone and Alderwood Mudstone members thus occur in Subarea-2.
I - TYRONE GROUP
In south and eastern Co. Fermanagh (Subarea-2) the Tyrone Group comprises six formations consisting mainly of limestones and shales. Unlike the western part of the county (Subarea-1) the basal sequence is exposed, in the Clogher Valley, and is divided into 2 formations (GSNI, 1982). The basal beds, the Ballyness Formation, consist mainly of coarse red conglomerates and sandstones. Conformably and gradationally overlying these clastic sediments is the Clogher Valley Formation which consists of peritidal carbonates and shales.
Based on new biostratigraphical information there does not appear to be a break between the top of the basal clastic sequence and the succeeding Ballyshannon Limestone Formation. Unfortunately, the transitional sedimentary rocks and their faunas are only poorly known and for the present the strata are assigned to the base of the latter formation.
The Ballyshannon Limestone is everywhere in N. Ireland succeeded by a sequence consisting of the Bundoran Shale, Mullaghmore Sandstone and Benbulben Shale formations. Throughout the Clogher Valley (Subarea-5) and in NW Co. Armagh (Subarea-6) the GSNI (1983) were unable to differentiate between the respective shale formations either lithologically or faunally. Both formations (i.e. the Bundoran and Benbulben Shale) therefore were grouped together and renamed the Carnteel Formation, and a sandstone which was thought to occur within the shale formation was thus redefined as the Aughnacloy Sandstone Member of the Carnteel Formation. Subsequent correlations of Lower Carboniferous sequences in N. Ireland have consequently presumed that the Aughnacloy Sandstone Member is the lateral equivalent of the Mullaghmore Sandstone Formation. In some instances this correlation is correct but in others it can be demonstrated that a sandstone unit, assigned to the Aughnacloy Member, is of late Viséan age and is therefore not the equivalent of the Arundian Mullaghmore Formation.
Despite the absence of the Mullaghmore Sandstone, at outcrop, in the Lisnaskea - Newtownbutler part of Subarea-2, the Bundoran Shale and Benbulben Shale formations are distinguishable both lithologically and faunally and it has been decided to dispense with the terms Carnteel Formation and Aughnacloy Sandstone Member and revert to Oswald's (1955) traditional formation names for these shale-sandstone-shale divisions.
The Glennoo Borehole (1965-66), which was drilled in N. Ireland on the south flank of Slieve Beagh (H496 415), commenced in the Dartry Limestone and finished at a total depth of 6910ft, in the "Upper Old Red Sandstone" basal clastics. In the borehole the Mullaghmore Sandstone is about 230m thick. This borehole is located only 15km NNE of the Newtownbutler area where the sandstone is apparently not represented.
(i) BALLYNESS FORMATION/CLOGHER VALLEY FORMATION
In N. Ireland the lowest beds of the Carboniferous succession usually consist of coarse-grained clastic rocks. This sequence, in Subarea-2, crops out along the southern margin of the Fintona Block and extends from Lower Lough Erne, intermittently along the north side of the Clogher Valley to the Cookstown area of Co. Tyrone. The Geological Survey of Ireland (1884) referred to the 'basal clastics' as the "Lower Carboniferous Sandstones and Shales". More recently Griffith (1970) noted their lithological similarity to the Upper Old Red Sandstone and Cementstone Groups of the Midland Valley of Scotland and demonstrated a Tournaisian age, at least for the peritidal carbonates (Cementstone Group) of the Clogher Valley Formation. He recognised a disconformity between the red conglomerates and sandstones at the base of the Carboniferous and, although unfossiliferous, he decided that they were probably of Devonian age on the basis of their similarity to rocks of the Upper Old Red Sandstone facies in Scotland. In 1979 the terms Ballyness Formation and Clogher Valley Formation appeared, for the first time, on the 1:50,000 Solid Geology map of the Pomeroy area and subsequently, in 1982 and 1983, on the Enniskillen and Clogher maps respectively. On all these maps, both formations were assigned to the Carboniferous, without equivocation. This age assignment is supported by the present study.
For site specific information see;
Key Site 201 - Cole Bridge Stream.
(ii) BALLYSHANNON LIMESTONE FORMATION
With the exception of a large outcrop between Tempo and Brookeborough that straddles the Clogher Valley between the Tempo-Sixmilecross/Clabby faults and the Clogher Valley Fault, GSNI mapping indicates that the Ballyshannon Limestone is virtually unexposed or was not deposited in the remainder of the Clogher Valley. In the southern part of Subarea-2, between Newtownbutler and the border with the Republic of Ireland, the Ballyshannon Limestone occurs beneath the Bundoran Shale Formation but is very poorly and patchily exposed.
For site specific information see;
Key Site 202 - Mullaghsillogagh Quarries, Clabby.
(iii) BALLYSHANNON LIMESTONE - BUNDORAN SHALE FORMATIONS
In the southern part of Subarea-2, around Newtownbutler, typical lithologies of the Ballyshannon Limestone Formation occur in a few, poorly exposed stream sections. The formation base and the lower part of the formation are not exposed. The top of the Ballyshannon Limestone, the contact with the base of the over- lying Bundoran Shale Formation and fossiliferous beds in the lower part of that formation are exposed in the south-flowing stream forming the boundary between Mockbeggar and Mullynagowan townlands, NE of Newtownbutler. The complete section is some 350m long.
The top beds of the Ballyshannon Limestone Formation consist of pale grainstones that are referred to as the Newtownbutler Limestone Member, a new lithostratigraphical unit that is succeeded abruptly by the Mullynagowan Sandstone Member, a new member at the base of the Bundoran Shale Formation. The section described in Key Site 204 is the stratotype for both new members.
For site specific information see;
Key Site 204 - Newtownbutler Stream.
(iv) BENBULBEN SHALE FORMATION
In Subarea-2 the outcrop of the Benbulben Shale Formation is located predominantly south of the Clogher Valley Fault with the exception of a small area around Maguiresbridge in Co. Fermanagh.
As already described the Mullaghmore Sandstone Formation is apparently absent in the Lisnaskea-Newtownbutler area of south Co. Fermanagh although it is 220m thick in the Glennoo Borehole, located 18km NNE of Newtownbutler. There are difficulties therefore in south Fermanagh in recognising the top of the Bundoran Shale and base of the Benbulben Shale, given their lithological similarity. Fortunately there are significant biostratigraphical differences, in the macro- and microfaunas of the formations. In particular the first appearance of the cerioid coral Lithostrotion is restricted, throughout Ireland, to the Benbulben Shale Formation and equivalent horizons. In most areas, including south Fermanagh, this occurs in association with Cf5 (Holkerian) foraminifera such as Archaediscus and concavus stage archaediscids. The coral does not occur in either the Bundoran Shale or Mullaghmore Sandstone formations although curiously, in the Kesh area, its lowest occurrence, in the Benbulben Shale, is with involutus stage paraarchaediscids of late Arundian age. Other significant additions to the prolific coral fauna in the Benbulben Shale include the first appearance of the colonial form Siphonodendron pauciradiale. In south Fermamagh the thickness of the Benbulben Shale may be in the order of 4-500m compared to 300m in the Glennoo Borehole.
The Benbulben Shale consists exclusively of dark grey-brown calcareous and micaceous shaley mudstones with thin beds of lenticular limestones and sandstones and rare thick limestone beds. The sandstones and some of the limestones are confined to wide shallow channels that erode the underlying mudstones and frequently show a basal lag of larger bioclasts. Sedimentary structures such as planar and ripple laminations, grading, tool marks and an erosive base all indicate transport and a probable origin from turbidites.
Faunally the mudstones contain abundant solitary corals such as:
Clisiophyllum vesiculosum Hexaphyllia sp. Koninckophyllum cyathophylloides Palaeosmilia murchisoni Siphonophyllia caninoides S. garwoodi
and a variety of colonial forms including:
Lithostrotion araneum L. portlocki Siphonodendron martini S. pauciradiale S. sociale
In the mudstones Siphonodendron occurs rarely as intact colonies and virtually always as debris bands in thin lenticular beds with a lag deposit containing numerous juvenile solitary corals in a silt grade matrix.
The limestone usually are thin and are packed with broken and frequently graded crinoid ossicles, bryozoa and brachiopod shell fragments. All of the above corals are associated with limestones that contain foraminifera including Archaediscus and concavus stage paraarchaediscids, which are undoubted Holkerian or younger indicators.
For site specific information see;
Key Site 205 - Kilmore-Drumbullog townlands ditch.
(v) BENBULBEN SHALE FORMATION-DARTRY LIMESTONE FORMATION
In Subarea-2 in Co. Fermanagh, there is only one location where it is possible to see the boundary between the Benbulben Shale and Dartry Limestone formations. This is located on the west side of Lisnaskea.
For site specific information see;
Key Site 206 - Lisnaskea road cutting.
(vi) DARTRY LIMESTONE FORMATION
In Subarea-2 the outcrop of the Dartry Limestone Formation only occurs south of the Clogher Valley Fault. In south Co. Fermanagh the formation is folded about a synclinal axis plunging gently to the northeast. The outcrop in the western fold limb is located between Donagh, in the south, Lisnaskea and the area east of Maguiresbridge and strata are inclined eastwards at between 10-14 deg. Isolated, fault-bounded exposures of the Dartry Limestone also occur on the south flank of Slieve Beagh near the border with the Republic of Ireland.
The strata lying conformably below the Dartry Limestone belong to the Benbulben Shale Formation and the one exposure of this boundary in Subarea-2 is described in Key Site 206. The limestone is succeeded by the varied clastic and carbonate sequence of the Meenymore Formation. This upper contact is clearly conformable but in view of the abrupt nature of the change from sediments representative of a deep water environment to sediments deposited in shallow water or intertidally, it is probably disconformable. The Dartry Limestone - Meenymore contact is also exposed at only one locality, in Rockfield (Mitton's) Quarry, described in Key Site 209.
Lithologically the Dartry Limestone Formation in the area east of Upper Lough Erne consists of alternating thin-thick beds of medium grey to grey-brown and blue-grey, very fine- to fine- grained bituminous packstones from 0.08-0.3m thick and splintery, calcareous silty shales up to 0.4m thick. Nodular and bedded blue-black chert occurs throughout the formation although it is only well developed in the upper part of the formation. Bedding surfaces are characteristically undulose and this condition is frequently exacerbated by the growth of large secondary nodules of chert. Shale thickness is variable but in both of the working quarries in the Lisnaskea area, at Slush Hill (H37 35) and at Rockfield (H405 312), shale layers are very thin, rarely more than 0.01-0.02m thick.
The precise age of the Dartry Limestone in Subarea-2 is difficult to pinpoint because of the virtual absence of any diagnostic fossils. The macrofauna is impoverished, being restricted to fragmented brachiopods, bryozoa, bivalves, crinoid ossicles, echinoderm plates and no corals. Very occasionally a rich macrofauna is well-preserved in the decalcified, claystone core of large former chert nodules. This is the case at one exceptional locality (H3938 3088) near Donagh where the fauna includes a small number of specimens of the trilobites Eocyphinium brevis and E. seminiferum, which suggest an Asbian age for the Dartry Limestone. Bearing in mind that the overlying Meenymore Formation contains goniatites of the B2 Zone (late Asbian) this is hardly surprising.
The relatively rare microbiota of stunted foraminifera and scarce algae, with numerous sponge spicules suggests that the limestones were deposited in a relatively deep water carbonate facies with a transported flora and fauna.
The Ballagh Limestone Member is the only member that is distinguished in the Dartry Limestone Formation and outcrop is confined to the area southeast of Lisnaskea.
For site specific information see;
Key Site 208 - Ballagh Quarry.
The geological mapping of the Carboniferous rocks in Subarea-2 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, 1995. 1:50,000 Solid Geology of Sheet 57 and part of 58 (Lisnaskea).
The Carboniferous succession in Subarea-2 is represented by the Tyrone and Leitrim groups. The geology of this area is divided conveniently into 2 main parts by the southwestern part of the Clogher Valley Fault. This separates the western end of the Clogher Valley, north of the fault, from higher ground of Slieve Beagh to the south and the Lisnaskea-Newtownbutler area of south Co. Fermanagh. North of the fault, in the Clogher Valley are rocks located in the lower part of the Carboniferous sequence. Conversely, Slieve Beagh consists primarily of a huge area of outcrop of the Meenymore Formation at the top of the sequence, and the Lisnaskea-Newtownbutler area is dominated by rocks in the middle part of the sequence between the Ballyshannon and Dartry limestones.
Formations and members that were recognised first in this Subarea are accorded 2-star (* *) ASSI status and those which form part of Oswald's standard lithostratigraphical sequence, as already described for Subarea-1, warrant a 1-star (*) rating. Stratotype sections for the Ballyness and Clogher Valley formations and for the Newtownbutler Limestone, Mullynagowan Sandstone, Carnmore Sandstone and Alderwood Mudstone members thus occur in Subarea-2.
I - TYRONE GROUP
In south and eastern Co. Fermanagh (Subarea-2) the Tyrone Group comprises six formations consisting mainly of limestones and shales. Unlike the western part of the county (Subarea-1) the basal sequence is exposed, in the Clogher Valley, and is divided into 2 formations (GSNI, 1982). The basal beds, the Ballyness Formation, consist mainly of coarse red conglomerates and sandstones. Conformably and gradationally overlying these clastic sediments is the Clogher Valley Formation which consists of peritidal carbonates and shales.
Based on new biostratigraphical information there does not appear to be a break between the top of the basal clastic sequence and the succeeding Ballyshannon Limestone Formation. Unfortunately, the transitional sedimentary rocks and their faunas are only poorly known and for the present the strata are assigned to the base of the latter formation.
The Ballyshannon Limestone is everywhere in N. Ireland succeeded by a sequence consisting of the Bundoran Shale, Mullaghmore Sandstone and Benbulben Shale formations. Throughout the Clogher Valley (Subarea-5) and in NW Co. Armagh (Subarea-6) the GSNI (1983) were unable to differentiate between the respective shale formations either lithologically or faunally. Both formations (i.e. the Bundoran and Benbulben Shale) therefore were grouped together and renamed the Carnteel Formation, and a sandstone which was thought to occur within the shale formation was thus redefined as the Aughnacloy Sandstone Member of the Carnteel Formation. Subsequent correlations of Lower Carboniferous sequences in N. Ireland have consequently presumed that the Aughnacloy Sandstone Member is the lateral equivalent of the Mullaghmore Sandstone Formation. In some instances this correlation is correct but in others it can be demonstrated that a sandstone unit, assigned to the Aughnacloy Member, is of late Viséan age and is therefore not the equivalent of the Arundian Mullaghmore Formation.
Despite the absence of the Mullaghmore Sandstone, at outcrop, in the Lisnaskea - Newtownbutler part of Subarea-2, the Bundoran Shale and Benbulben Shale formations are distinguishable both lithologically and faunally and it has been decided to dispense with the terms Carnteel Formation and Aughnacloy Sandstone Member and revert to Oswald's (1955) traditional formation names for these shale-sandstone-shale divisions.
The Glennoo Borehole (1965-66), which was drilled in N. Ireland on the south flank of Slieve Beagh (H496 415), commenced in the Dartry Limestone and finished at a total depth of 6910ft, in the "Upper Old Red Sandstone" basal clastics. In the borehole the Mullaghmore Sandstone is about 230m thick. This borehole is located only 15km NNE of the Newtownbutler area where the sandstone is apparently not represented.
(i) BALLYNESS FORMATION/CLOGHER VALLEY FORMATION
In N. Ireland the lowest beds of the Carboniferous succession usually consist of coarse-grained clastic rocks. This sequence, in Subarea-2, crops out along the southern margin of the Fintona Block and extends from Lower Lough Erne, intermittently along the north side of the Clogher Valley to the Cookstown area of Co. Tyrone. The Geological Survey of Ireland (1884) referred to the 'basal clastics' as the "Lower Carboniferous Sandstones and Shales". More recently Griffith (1970) noted their lithological similarity to the Upper Old Red Sandstone and Cementstone Groups of the Midland Valley of Scotland and demonstrated a Tournaisian age, at least for the peritidal carbonates (Cementstone Group) of the Clogher Valley Formation. He recognised a disconformity between the red conglomerates and sandstones at the base of the Carboniferous and, although unfossiliferous, he decided that they were probably of Devonian age on the basis of their similarity to rocks of the Upper Old Red Sandstone facies in Scotland. In 1979 the terms Ballyness Formation and Clogher Valley Formation appeared, for the first time, on the 1:50,000 Solid Geology map of the Pomeroy area and subsequently, in 1982 and 1983, on the Enniskillen and Clogher maps respectively. On all these maps, both formations were assigned to the Carboniferous, without equivocation. This age assignment is supported by the present study.
For site specific information see;
Key Site 201 - Cole Bridge Stream.
(ii) BALLYSHANNON LIMESTONE FORMATION
With the exception of a large outcrop between Tempo and Brookeborough that straddles the Clogher Valley between the Tempo-Sixmilecross/Clabby faults and the Clogher Valley Fault, GSNI mapping indicates that the Ballyshannon Limestone is virtually unexposed or was not deposited in the remainder of the Clogher Valley. In the southern part of Subarea-2, between Newtownbutler and the border with the Republic of Ireland, the Ballyshannon Limestone occurs beneath the Bundoran Shale Formation but is very poorly and patchily exposed.
For site specific information see;
Key Site 202 - Mullaghsillogagh Quarries, Clabby.
(iii) BALLYSHANNON LIMESTONE - BUNDORAN SHALE FORMATIONS
In the southern part of Subarea-2, around Newtownbutler, typical lithologies of the Ballyshannon Limestone Formation occur in a few, poorly exposed stream sections. The formation base and the lower part of the formation are not exposed. The top of the Ballyshannon Limestone, the contact with the base of the over- lying Bundoran Shale Formation and fossiliferous beds in the lower part of that formation are exposed in the south-flowing stream forming the boundary between Mockbeggar and Mullynagowan townlands, NE of Newtownbutler. The complete section is some 350m long.
The top beds of the Ballyshannon Limestone Formation consist of pale grainstones that are referred to as the Newtownbutler Limestone Member, a new lithostratigraphical unit that is succeeded abruptly by the Mullynagowan Sandstone Member, a new member at the base of the Bundoran Shale Formation. The section described in Key Site 204 is the stratotype for both new members.
For site specific information see;
Key Site 204 - Newtownbutler Stream.
(iv) BENBULBEN SHALE FORMATION
In Subarea-2 the outcrop of the Benbulben Shale Formation is located predominantly south of the Clogher Valley Fault with the exception of a small area around Maguiresbridge in Co. Fermanagh.
As already described the Mullaghmore Sandstone Formation is apparently absent in the Lisnaskea-Newtownbutler area of south Co. Fermanagh although it is 220m thick in the Glennoo Borehole, located 18km NNE of Newtownbutler. There are difficulties therefore in south Fermanagh in recognising the top of the Bundoran Shale and base of the Benbulben Shale, given their lithological similarity. Fortunately there are significant biostratigraphical differences, in the macro- and microfaunas of the formations. In particular the first appearance of the cerioid coral Lithostrotion is restricted, throughout Ireland, to the Benbulben Shale Formation and equivalent horizons. In most areas, including south Fermanagh, this occurs in association with Cf5 (Holkerian) foraminifera such as Archaediscus and concavus stage archaediscids. The coral does not occur in either the Bundoran Shale or Mullaghmore Sandstone formations although curiously, in the Kesh area, its lowest occurrence, in the Benbulben Shale, is with involutus stage paraarchaediscids of late Arundian age. Other significant additions to the prolific coral fauna in the Benbulben Shale include the first appearance of the colonial form Siphonodendron pauciradiale. In south Fermamagh the thickness of the Benbulben Shale may be in the order of 4-500m compared to 300m in the Glennoo Borehole.
The Benbulben Shale consists exclusively of dark grey-brown calcareous and micaceous shaley mudstones with thin beds of lenticular limestones and sandstones and rare thick limestone beds. The sandstones and some of the limestones are confined to wide shallow channels that erode the underlying mudstones and frequently show a basal lag of larger bioclasts. Sedimentary structures such as planar and ripple laminations, grading, tool marks and an erosive base all indicate transport and a probable origin from turbidites.
Faunally the mudstones contain abundant solitary corals such as:
Clisiophyllum vesiculosum Hexaphyllia sp. Koninckophyllum cyathophylloides Palaeosmilia murchisoni Siphonophyllia caninoides S. garwoodi
and a variety of colonial forms including:
Lithostrotion araneum L. portlocki Siphonodendron martini S. pauciradiale S. sociale
In the mudstones Siphonodendron occurs rarely as intact colonies and virtually always as debris bands in thin lenticular beds with a lag deposit containing numerous juvenile solitary corals in a silt grade matrix.
The limestone usually are thin and are packed with broken and frequently graded crinoid ossicles, bryozoa and brachiopod shell fragments. All of the above corals are associated with limestones that contain foraminifera including Archaediscus and concavus stage paraarchaediscids, which are undoubted Holkerian or younger indicators.
For site specific information see;
Key Site 205 - Kilmore-Drumbullog townlands ditch.
(v) BENBULBEN SHALE FORMATION-DARTRY LIMESTONE FORMATION
In Subarea-2 in Co. Fermanagh, there is only one location where it is possible to see the boundary between the Benbulben Shale and Dartry Limestone formations. This is located on the west side of Lisnaskea.
For site specific information see;
Key Site 206 - Lisnaskea road cutting.
(vi) DARTRY LIMESTONE FORMATION
In Subarea-2 the outcrop of the Dartry Limestone Formation only occurs south of the Clogher Valley Fault. In south Co. Fermanagh the formation is folded about a synclinal axis plunging gently to the northeast. The outcrop in the western fold limb is located between Donagh, in the south, Lisnaskea and the area east of Maguiresbridge and strata are inclined eastwards at between 10-14 deg. Isolated, fault-bounded exposures of the Dartry Limestone also occur on the south flank of Slieve Beagh near the border with the Republic of Ireland.
The strata lying conformably below the Dartry Limestone belong to the Benbulben Shale Formation and the one exposure of this boundary in Subarea-2 is described in Key Site 206. The limestone is succeeded by the varied clastic and carbonate sequence of the Meenymore Formation. This upper contact is clearly conformable but in view of the abrupt nature of the change from sediments representative of a deep water environment to sediments deposited in shallow water or intertidally, it is probably disconformable. The Dartry Limestone - Meenymore contact is also exposed at only one locality, in Rockfield (Mitton's) Quarry, described in Key Site 209.
Lithologically the Dartry Limestone Formation in the area east of Upper Lough Erne consists of alternating thin-thick beds of medium grey to grey-brown and blue-grey, very fine- to fine- grained bituminous packstones from 0.08-0.3m thick and splintery, calcareous silty shales up to 0.4m thick. Nodular and bedded blue-black chert occurs throughout the formation although it is only well developed in the upper part of the formation. Bedding surfaces are characteristically undulose and this condition is frequently exacerbated by the growth of large secondary nodules of chert. Shale thickness is variable but in both of the working quarries in the Lisnaskea area, at Slush Hill (H37 35) and at Rockfield (H405 312), shale layers are very thin, rarely more than 0.01-0.02m thick.
The precise age of the Dartry Limestone in Subarea-2 is difficult to pinpoint because of the virtual absence of any diagnostic fossils. The macrofauna is impoverished, being restricted to fragmented brachiopods, bryozoa, bivalves, crinoid ossicles, echinoderm plates and no corals. Very occasionally a rich macrofauna is well-preserved in the decalcified, claystone core of large former chert nodules. This is the case at one exceptional locality (H3938 3088) near Donagh where the fauna includes a small number of specimens of the trilobites Eocyphinium brevis and E. seminiferum, which suggest an Asbian age for the Dartry Limestone. Bearing in mind that the overlying Meenymore Formation contains goniatites of the B2 Zone (late Asbian) this is hardly surprising.
The relatively rare microbiota of stunted foraminifera and scarce algae, with numerous sponge spicules suggests that the limestones were deposited in a relatively deep water carbonate facies with a transported flora and fauna.
The Ballagh Limestone Member is the only member that is distinguished in the Dartry Limestone Formation and outcrop is confined to the area southeast of Lisnaskea.
For site specific information see;
Key Site 208 - Ballagh Quarry.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Carboniferous Subarea-2; Lisnaskea & Fivemiletown, County Fermanagh, Tyrone, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1175. Accessed on 2024-12-26 |
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