Earth Science Conservation Review
| Carboniferous Palaeogeography of N. Ireland | Antrim, Armagh, Down, Fermanagh, Londonderry, Tyro |
| Site Type: | Various |
| Site Status: | |
| Grid Reference: | |
| Google maps: | , |
| Rocks | |
|---|---|
| Rock Age: | Carboniferous (Namurian, Stephanian, Tournasian, Visean, Westphalian) |
| Rock Name: | Armagh Group, Carboniferous Limestone, Coal Measures, Leitrim Group, Millstone Grit Group (Repl M. Grit), Omagh Sandstone Group, Slievebane Group, Tyrone Group |
| Rock Type: | Limestone, Mudstone, Sand, Sandstone, Shale, Siltstone |
| Interest | |
| Fossil Groups: | Brachiopod, Conodont, Coral, Crinoid, Foraminifera, Goniatite, Microfossil, Microfossils, Polyzoan, Trilobite |
| Other interest: | Marine sediments |
Description:
INTRODUCTION
By the end of Devonian times in Ireland the northern shoreline of the Rheic Ocean was located at the north end of Cork Harbour. The rest of the country was a low-lying arid to semi-arid alluvial plain traversed by southward-flowing ephemeral rivers and with localised temporary playa lakes. Mountain areas, if present, were located far to the north, beyond the area of the Fintona Block, in which are exposed the most northerly dated (Upper) Devonian sedimentary rocks in Ireland. The alluvial plain was blanketed by fine-grained fluvial and lacustrine red-bed sediments and the early Carboniferous (Tournaisian) sea advanced northwards over this land surface.
There has always been an assumption amongst geologists in Ireland that the northward spread of the Carboniferous marine transgression reached the north of Ireland in early Viséan times and basal Carboniferous sequences were therefore of post- Tournaisian age. We now know that this is incorrect and indeed late Tournaisian sedimentary rocks, far from being absent or thin in N. Ireland, represent, in places, a significant part of the total sequence and consist of sediments deposited in non-marine, peritidal and marine environments. Their deposition and the northward advance of the marine transgression was however only temporary and regional regression of that sea and exposure of a new land surface led to the development of a slight angular unconformity between those rocks and the succeeding early Viséan sequences. These sediments were deposited during the next marine transgression whose influence continued into early Namurian times in N. Ireland.
A. DINANTIAN
I - Late Tournaisian
In N. Ireland the oldest Carboniferous rocks are of late Tournaisian age and generally belong to non-marine and peritidal facies. Miospores recovered from them belong to the CM Zone and the recognition of this distinctive assemblage, without the base Viséan index taxon Lycospora pusilla, has dramatically extended the outcrop of late Tournaisian strata in N. Ireland.
The occurrence of rocks of this age is now confirmed throughout the Kesh-Omagh area (Subarea- 3), in Co. Armagh (Subarea-6), at Cultra in north Co. Down (Subarea-10), in the Clogher Valley - Enniskillen area (Subarea-5) and in deep boreholes in Subarea-5 and in the Lough Allen Basin in NW Ireland (Philcox et al., 1992). In general the sequence is thin, in outcrop rarely exceeding 300m although in the Clogher Valley it is estimated at over 1000m, albeit in a badly exposed area. Thick sequences are also present on the Sperrin Mountains around Draperstown and the Glenshane Pass (Subarea-12), and in the Newtownstewart Outlier (Subarea-8) which although not yet remapped by the GSNI is estimated to contain up to 1500m of non-marine CM Zone lacustrine and fluviatile sediments.
With the exception of the Newtownstewart Outlier and parts of the Glenshane Pass area, all other late Tournaisian sequences commence in red-beds of the Upper Old Red Sandstone facies, pass through a transitional suite of sediments of deltaic, brackish and hypersaline environments and culminate in peritidal and, very rarely, fully marine facies. In N. Ireland the age of these late Tournaisian beds is not obviously diachronous and appears to have been deposited on a relatively subdued land surface with changing facies related to the effects of one marine transgression.
Late Tournaisian sediments rest directly on Lower Palaeozoic rock of the Down-Longford massif at two places along its northern margin, in Co. Armagh and near Belfast, at Cultra. Whether or not the transgression that introduced hypersaline/peritidal conditions into these areas migrated northeastwards along the northern margin of the massif or transgressed southwestwards from the Midland Valley of Scotland, where there are rocks of identical age and facies, is not known. Based on the evidence of later Viséan limestones resting on Lower Palaeozoic basement in south Co. Down and at Castle Espie in north Co. Down, it is tempting to conclude that the bulk of the Down-Longford massif was land during late Tournaisian and early Viséan times and did not contribute significant quantities of detritus to the later Carboniferous sedimentary sequences.
II - Visean
(i) Chadian With the exception of SW Fermanagh and the Clogher Valley area, where there appears to be a continuous section from the late Tournaisian into the early Viséan, in all other areas to the north and east there is evidence of an unconformity separating rocks of these ages. In the Kesh-Omagh area, for example, the lowest marine beds are of late Chadian age but exhibit rapid facies variations from oolites and sandstones in the north which pass laterally into peritidal carbonates and ultimately crinoidal packstones of the Ballyshannon Limestone Formation in the south. All these lithologies are underlain by up to 500m of very coarse clastic sediments of early Viséan (Pu Zone) age. The generation of such a thickness of coarse debris was clearly related to contemporaneous movement on faults bordering the Dalradian rocks to the north, and the Moinian Lough Derg Psammites to the west. Near Ballyshannon, late Chadian marine sedimentary rocks rest almost directly on the Moinian, the Claragh Sandstone Formation being absent west of the Pettigoe Fault.
Viséan strata overlying the basal clastics (late Tournaisian) in Armagh and in the Dungannon-Coalisland area are no older than early Arundian and there is therefore no evidence of the Chadian transgression having reached any other part of N. Ireland outside Fermanagh and Tyrone.
The occurrence of an area of Waulsortian build up in SW Fermanagh at Bellanaleck was a new and totally unexpected discovery and complements the previously known outcrop of identical lithologies in the Belleek area (1:50,000 Sheet 32, GSNI, 1994). The age of the Bellanaleck bioherm is late Tournaisian which in the strict sense is equivalent to the late early-Chadian and correlates with the major development of the Waulsortian complex in central and southern Ireland. Biostratigraphical data from the Belleek area indicates that these Waulsortian bioherms, that occur just above the base of the Ballyshannon Limestone Formation succeed a thin basal clastic sequence that contains miospores of the Pu Zone and rests on the Moinian basement. The best exposed bioherm here is allochthonous and has clearly moved downslope from its point of accumulation. However little can be said about the Bellanaleck build up because it occurs in a very badly exposed area of low ground adjacent to Lough Erne.
(ii) Arundian The Arundian is one of two periods during Dinantian times when enormous thicknesses of carbonate and clastic sediments accumulated in the NW Basin. This may reflect the importance of contemporaneous faulting, generating uplift and a continuous sediment supply to fault-bounded marine basins. Based on the distribution of Viséan rocks in N. Ireland there is sound evidence that the late Tournaisian transgression was a temporary episode over much of the Province and areas in which there was continuity of marine sedimentation into the Chadian were severely restricted and confined to the area south and west of Lough Erne and in the Clogher Valley.
The extent of the land area created during this early Chadian regression included all of the remainder of N. Ireland and only gradually, in late Chadian times, in the Kesh-Omagh area, and in the early Arundian, in Co. Armagh, were the first effects of the returning marine transgression evident. However thereafter, during the Arundian, marine sedimentation continued unabated with only minor breaks recognisable in near shore sequences. In the Kesh area the Chadian-Arundian boundary is conformable and the sediments reflect a change from shallow water sands and oolitic sands to a deeper water mud and carbonate mid-shelf environment. Southwards the Ballyshannon Limestone is dominantly carbonate, having lost the fine mud element that characterised the more proximal, near shore environment in the north.
In late Arundian times, however, an influx of deltaic sediments, that occurred throughout the Carboniferous basins in the north of Ireland reflected a significant change in their bathymetry and an increased input of clastic sediment. With the exception of the Lisnaskea area, a thick development of deltaic sandstone/ siltstone, mudstone and thin coals conformably succeeds the Bundoran Shale Formation. In the Kesh area the transitional sediments at the top of the Bundoran Shale show a gradual increase in silt and sand grade sediment and a reduction in the great variety of benthic animals that typified the formation. At the base of the Mullaghmore Sandstone there is a clean, thick marine sandstone probably deposited in subtidal conditions that is everywhere succeeded by a thin, very distinctive, sequence of dark shales and mudstones with thin laminated stromatolitic limestones, peritidal carbonates and evaporite beds that represent a further shallowing and introduction of supratidal, hypersaline sabkha-type environment throughout the Kesh-Omagh area. Thereafter deltaic conditions dominated the marginal marine environment in the Kesh area. This rapid transition from deeper water offshore mud facies of the Bundoran Shale to shallow marine and supratidal facies of the Mullaghmore Formation, without major northwards coarsening of clastic sediments may indicate a further episode of eustatic control on sea level changes.
Periodic exposure of the delta top led either to the formation of thin coal seams or to the development of red-beds with palaeosols and channelised fluvial sandstones. On other occasions minor fluctuations in sea-level caused the inundation of large areas of the delta top and introduced marine benthos into an otherwise non-marine environment. Such horizons usually consist of thin crinoidal limestones but, at one unique locality, comprises a grey mudstone bedding surface crowded with small specimens of starfish and brittle stars.
At the end of the deltaic sedimentation in most parts of the NW Basin there was a rapid return to the deposition of basinal muds very similar to those of the older Bundoran Shale. However, these belong to the Benbulben Shale Formation and represent a return to fully marine basinal sedimentation with a rich benthos. Thin beds of sandstone and very fossiliferous limestone occupy shallow channels in the mudstone and probably originated as turbidity currents derived from adjacent areas of shallow water.
(iii) Holkerian In the north of Ireland Holkerian zonal taxa are usually lacking and the presumption that this stage is represented, in any Lower Carboniferous sequence, is usually intuitive and is rarely supported by indisputable biostratigraphical evidence. Several species of brachiopods, corals and trilobites are thought, in Great Britain, to be either confined to the Holkerian or to enter at this horizon. In particular are species of the distinctive cerioid rugose coral Lithostrotion. Unfortunately, in the Kesh area this coral occurs with involutus stage archaediscids, thus indicating a late Arundian age. However in south Co. Fermanagh, in the Lisnaskea area specimens of L. araneum and L. portlocki occur with cf. Archaediscus sp. and concavus stage archaediscid foraminifera respectively, forms that are Cf5 (Holkerian) entries. In many parts of the Carboniferous outcrop in N. Ireland the biostratigraphical data is insufficient to permit recognition of strata of this age. The exception is however in Co. Fermanagh, including the Kesh-Omagh area, where the full extent of Holkerian strata is confined to a portion of the Benbulben Shale Formation. Diagnostic Asbian taxa are commonly found in the upper part of this formation. It thus appears that in the late Arundian- Holkerian and earliest Asbian over much of N. Ireland, deposition of dark marine shales was taking place in offshore marine basins. Fossils are very common throughout the Benbulben Shale Formation in most parts of its outcrop. The exception to this is the Lisnaskea area where the upper part of the formation is not only devoid of macrofauna but the microfauna is restricted to very rare specimens of stunted foraminifera and no algae. The latter observation is particularly significant since these plants only inhabited marine environments within the photic zone. Their absence presumably indicates mud deposition in very deep water, at depths in which the flora could not survive.
(iv) Asbian During Dinantian times in N. Ireland the Asbian period was one of unparalleled tectonic activity that profoundly affected the sedimentary record and lead to enormous variations in the thickness and types of sediment deposited. During earliest Asbian times the sedimentary record appears to indicate a progressive and uniform bathymetric shallowing that may have been in response to regional uplift, and a gradual cessation of the transport of mud into the basin. This is particularly evident over much of Fermanagh, Tyrone and parts of Armagh where the upward transition from the Benbulben Shale to the Glencar Limestone Formation is rapid and shales are interbedded with limestones in almost equal proportions in the latter formation. Thickness variations of these formations appear to result from diachronism and not from fault control on local depocentres (Philcox et al., 1992). A similar transition is also apparent in Armagh at the base of the Benburb section, within the Maydown Limestone Formation, and in the Clogher Valley, where it is typified by the Fardross section. The local exception to this regional standard pattern is found in the Lisnaskea area. There the Glencar Limestone is absent and the transition from deep water shales of the Benbulben Formation into equally faunally and florally impoverished cherty limestones and mudstones of the Dartry Limestone Formation is abrupt.
In the post-Glencar Limestone times the tectonic pattern was transformed by rapid and sustained uplift on faults, exposure of metamorphic basement adjacent to the Kesh area, formation of in-basinal ramps along new fault lines, rapid subsidence of non-marine basins and accumulation of thick red-bed sequences and regional marine regression. These events affected all of N. Ireland but the sedimentary record, by which they are defined, is concentrated mainly in the western part of the Province.
(v) Brigantian As a result of refined biostratigraphical information the palaeo- geography of Brigantian times is now seen to be more complex than hitherto suspected. In the eastern part of N. Ireland there is evidence, at Castle Espie in north Co. Down (Subarea-10), of the final inundation of the Down-Longford massif, although this may indeed represent only a local, rift-controlled incursion. At Ballycastle (Subarea-9), new palynological evidence indicates that basin subsidence only commenced there in the Brigantian but continued into the early Namurian.
Moving westwards the Greenan Sandstone Formation was deposited on Dalradian basement (Subarea-4), in a new non-marine basin that subsided along the present line of the Castle Archdale Fault. This shallow basin received at least 600m of medium-grained sandstones and existed during Brigantian and early Namurian times. Localised rhyolitic and dacitic volcanism occurred in this basin in Co. Tyrone and is matched by the basaltic lavas that were erupted near the base of the Ballycastle sequence.
An increase in the rate of subsidence, in the Brigantian, of the main non-marine basin located south of the southwestern extension of the Tyrone Igneous Complex, in the southern part of the Fintona Block, caused the deposition of coarse sandstones and conglomerates of the Ballyreagh Formation of the Kilskeery Group. It is likely that, given the reduction in tectonism evident in other Brigantian strata in N. Ireland, there was also a slowing down in the rate of subsidence of this basin and the lower levels of the fine-grained Ballinamallard Mudstone Formation may well reflect this eventuality and are Brigantian.
The remaining Brigantian strata in N. Ireland occur in the west of the Province and universally reflect a marked decline in tectonic activity and an increasing influence of thermal subsidence over wide areas. In particular Leitrim Group strata of the Bellavally and Carraun Shale formations of this age crop out over an area of ~1400km2. Minor cyclicity of thin limestones and other marine bands is evident in the latter formation and their regional persistence, combined with the absence of major facies variations and the almost uniform thickness of these and later lithostratigraphical units testifies to the relative stability of this slowly subsiding basin. The occurrence of sandstone dykes in the shales indicates that minor seismic disturbances affected the sediments.
In the Dungannon-Cookstown area (Subarea-7) the early Brigantian is represented by shallow water limestones and shales with grainstones and thin sandstones. Succeeding strata consist almost exclusively of shales and mudstones of the Rossmore Mudstone Formation which closely resemble the upper part of the Carraun Shale Formation in Co. Fermanagh. Shorelines at this time clearly lay some distance from the present outcrop in Co. Fermanagh and the land area was peneplained thus providing only fine mud detritus to the basin.
By the end of Devonian times in Ireland the northern shoreline of the Rheic Ocean was located at the north end of Cork Harbour. The rest of the country was a low-lying arid to semi-arid alluvial plain traversed by southward-flowing ephemeral rivers and with localised temporary playa lakes. Mountain areas, if present, were located far to the north, beyond the area of the Fintona Block, in which are exposed the most northerly dated (Upper) Devonian sedimentary rocks in Ireland. The alluvial plain was blanketed by fine-grained fluvial and lacustrine red-bed sediments and the early Carboniferous (Tournaisian) sea advanced northwards over this land surface.
There has always been an assumption amongst geologists in Ireland that the northward spread of the Carboniferous marine transgression reached the north of Ireland in early Viséan times and basal Carboniferous sequences were therefore of post- Tournaisian age. We now know that this is incorrect and indeed late Tournaisian sedimentary rocks, far from being absent or thin in N. Ireland, represent, in places, a significant part of the total sequence and consist of sediments deposited in non-marine, peritidal and marine environments. Their deposition and the northward advance of the marine transgression was however only temporary and regional regression of that sea and exposure of a new land surface led to the development of a slight angular unconformity between those rocks and the succeeding early Viséan sequences. These sediments were deposited during the next marine transgression whose influence continued into early Namurian times in N. Ireland.
A. DINANTIAN
I - Late Tournaisian
In N. Ireland the oldest Carboniferous rocks are of late Tournaisian age and generally belong to non-marine and peritidal facies. Miospores recovered from them belong to the CM Zone and the recognition of this distinctive assemblage, without the base Viséan index taxon Lycospora pusilla, has dramatically extended the outcrop of late Tournaisian strata in N. Ireland.
The occurrence of rocks of this age is now confirmed throughout the Kesh-Omagh area (Subarea- 3), in Co. Armagh (Subarea-6), at Cultra in north Co. Down (Subarea-10), in the Clogher Valley - Enniskillen area (Subarea-5) and in deep boreholes in Subarea-5 and in the Lough Allen Basin in NW Ireland (Philcox et al., 1992). In general the sequence is thin, in outcrop rarely exceeding 300m although in the Clogher Valley it is estimated at over 1000m, albeit in a badly exposed area. Thick sequences are also present on the Sperrin Mountains around Draperstown and the Glenshane Pass (Subarea-12), and in the Newtownstewart Outlier (Subarea-8) which although not yet remapped by the GSNI is estimated to contain up to 1500m of non-marine CM Zone lacustrine and fluviatile sediments.
With the exception of the Newtownstewart Outlier and parts of the Glenshane Pass area, all other late Tournaisian sequences commence in red-beds of the Upper Old Red Sandstone facies, pass through a transitional suite of sediments of deltaic, brackish and hypersaline environments and culminate in peritidal and, very rarely, fully marine facies. In N. Ireland the age of these late Tournaisian beds is not obviously diachronous and appears to have been deposited on a relatively subdued land surface with changing facies related to the effects of one marine transgression.
Late Tournaisian sediments rest directly on Lower Palaeozoic rock of the Down-Longford massif at two places along its northern margin, in Co. Armagh and near Belfast, at Cultra. Whether or not the transgression that introduced hypersaline/peritidal conditions into these areas migrated northeastwards along the northern margin of the massif or transgressed southwestwards from the Midland Valley of Scotland, where there are rocks of identical age and facies, is not known. Based on the evidence of later Viséan limestones resting on Lower Palaeozoic basement in south Co. Down and at Castle Espie in north Co. Down, it is tempting to conclude that the bulk of the Down-Longford massif was land during late Tournaisian and early Viséan times and did not contribute significant quantities of detritus to the later Carboniferous sedimentary sequences.
II - Visean
(i) Chadian With the exception of SW Fermanagh and the Clogher Valley area, where there appears to be a continuous section from the late Tournaisian into the early Viséan, in all other areas to the north and east there is evidence of an unconformity separating rocks of these ages. In the Kesh-Omagh area, for example, the lowest marine beds are of late Chadian age but exhibit rapid facies variations from oolites and sandstones in the north which pass laterally into peritidal carbonates and ultimately crinoidal packstones of the Ballyshannon Limestone Formation in the south. All these lithologies are underlain by up to 500m of very coarse clastic sediments of early Viséan (Pu Zone) age. The generation of such a thickness of coarse debris was clearly related to contemporaneous movement on faults bordering the Dalradian rocks to the north, and the Moinian Lough Derg Psammites to the west. Near Ballyshannon, late Chadian marine sedimentary rocks rest almost directly on the Moinian, the Claragh Sandstone Formation being absent west of the Pettigoe Fault.
Viséan strata overlying the basal clastics (late Tournaisian) in Armagh and in the Dungannon-Coalisland area are no older than early Arundian and there is therefore no evidence of the Chadian transgression having reached any other part of N. Ireland outside Fermanagh and Tyrone.
The occurrence of an area of Waulsortian build up in SW Fermanagh at Bellanaleck was a new and totally unexpected discovery and complements the previously known outcrop of identical lithologies in the Belleek area (1:50,000 Sheet 32, GSNI, 1994). The age of the Bellanaleck bioherm is late Tournaisian which in the strict sense is equivalent to the late early-Chadian and correlates with the major development of the Waulsortian complex in central and southern Ireland. Biostratigraphical data from the Belleek area indicates that these Waulsortian bioherms, that occur just above the base of the Ballyshannon Limestone Formation succeed a thin basal clastic sequence that contains miospores of the Pu Zone and rests on the Moinian basement. The best exposed bioherm here is allochthonous and has clearly moved downslope from its point of accumulation. However little can be said about the Bellanaleck build up because it occurs in a very badly exposed area of low ground adjacent to Lough Erne.
(ii) Arundian The Arundian is one of two periods during Dinantian times when enormous thicknesses of carbonate and clastic sediments accumulated in the NW Basin. This may reflect the importance of contemporaneous faulting, generating uplift and a continuous sediment supply to fault-bounded marine basins. Based on the distribution of Viséan rocks in N. Ireland there is sound evidence that the late Tournaisian transgression was a temporary episode over much of the Province and areas in which there was continuity of marine sedimentation into the Chadian were severely restricted and confined to the area south and west of Lough Erne and in the Clogher Valley.
The extent of the land area created during this early Chadian regression included all of the remainder of N. Ireland and only gradually, in late Chadian times, in the Kesh-Omagh area, and in the early Arundian, in Co. Armagh, were the first effects of the returning marine transgression evident. However thereafter, during the Arundian, marine sedimentation continued unabated with only minor breaks recognisable in near shore sequences. In the Kesh area the Chadian-Arundian boundary is conformable and the sediments reflect a change from shallow water sands and oolitic sands to a deeper water mud and carbonate mid-shelf environment. Southwards the Ballyshannon Limestone is dominantly carbonate, having lost the fine mud element that characterised the more proximal, near shore environment in the north.
In late Arundian times, however, an influx of deltaic sediments, that occurred throughout the Carboniferous basins in the north of Ireland reflected a significant change in their bathymetry and an increased input of clastic sediment. With the exception of the Lisnaskea area, a thick development of deltaic sandstone/ siltstone, mudstone and thin coals conformably succeeds the Bundoran Shale Formation. In the Kesh area the transitional sediments at the top of the Bundoran Shale show a gradual increase in silt and sand grade sediment and a reduction in the great variety of benthic animals that typified the formation. At the base of the Mullaghmore Sandstone there is a clean, thick marine sandstone probably deposited in subtidal conditions that is everywhere succeeded by a thin, very distinctive, sequence of dark shales and mudstones with thin laminated stromatolitic limestones, peritidal carbonates and evaporite beds that represent a further shallowing and introduction of supratidal, hypersaline sabkha-type environment throughout the Kesh-Omagh area. Thereafter deltaic conditions dominated the marginal marine environment in the Kesh area. This rapid transition from deeper water offshore mud facies of the Bundoran Shale to shallow marine and supratidal facies of the Mullaghmore Formation, without major northwards coarsening of clastic sediments may indicate a further episode of eustatic control on sea level changes.
Periodic exposure of the delta top led either to the formation of thin coal seams or to the development of red-beds with palaeosols and channelised fluvial sandstones. On other occasions minor fluctuations in sea-level caused the inundation of large areas of the delta top and introduced marine benthos into an otherwise non-marine environment. Such horizons usually consist of thin crinoidal limestones but, at one unique locality, comprises a grey mudstone bedding surface crowded with small specimens of starfish and brittle stars.
At the end of the deltaic sedimentation in most parts of the NW Basin there was a rapid return to the deposition of basinal muds very similar to those of the older Bundoran Shale. However, these belong to the Benbulben Shale Formation and represent a return to fully marine basinal sedimentation with a rich benthos. Thin beds of sandstone and very fossiliferous limestone occupy shallow channels in the mudstone and probably originated as turbidity currents derived from adjacent areas of shallow water.
(iii) Holkerian In the north of Ireland Holkerian zonal taxa are usually lacking and the presumption that this stage is represented, in any Lower Carboniferous sequence, is usually intuitive and is rarely supported by indisputable biostratigraphical evidence. Several species of brachiopods, corals and trilobites are thought, in Great Britain, to be either confined to the Holkerian or to enter at this horizon. In particular are species of the distinctive cerioid rugose coral Lithostrotion. Unfortunately, in the Kesh area this coral occurs with involutus stage archaediscids, thus indicating a late Arundian age. However in south Co. Fermanagh, in the Lisnaskea area specimens of L. araneum and L. portlocki occur with cf. Archaediscus sp. and concavus stage archaediscid foraminifera respectively, forms that are Cf5 (Holkerian) entries. In many parts of the Carboniferous outcrop in N. Ireland the biostratigraphical data is insufficient to permit recognition of strata of this age. The exception is however in Co. Fermanagh, including the Kesh-Omagh area, where the full extent of Holkerian strata is confined to a portion of the Benbulben Shale Formation. Diagnostic Asbian taxa are commonly found in the upper part of this formation. It thus appears that in the late Arundian- Holkerian and earliest Asbian over much of N. Ireland, deposition of dark marine shales was taking place in offshore marine basins. Fossils are very common throughout the Benbulben Shale Formation in most parts of its outcrop. The exception to this is the Lisnaskea area where the upper part of the formation is not only devoid of macrofauna but the microfauna is restricted to very rare specimens of stunted foraminifera and no algae. The latter observation is particularly significant since these plants only inhabited marine environments within the photic zone. Their absence presumably indicates mud deposition in very deep water, at depths in which the flora could not survive.
(iv) Asbian During Dinantian times in N. Ireland the Asbian period was one of unparalleled tectonic activity that profoundly affected the sedimentary record and lead to enormous variations in the thickness and types of sediment deposited. During earliest Asbian times the sedimentary record appears to indicate a progressive and uniform bathymetric shallowing that may have been in response to regional uplift, and a gradual cessation of the transport of mud into the basin. This is particularly evident over much of Fermanagh, Tyrone and parts of Armagh where the upward transition from the Benbulben Shale to the Glencar Limestone Formation is rapid and shales are interbedded with limestones in almost equal proportions in the latter formation. Thickness variations of these formations appear to result from diachronism and not from fault control on local depocentres (Philcox et al., 1992). A similar transition is also apparent in Armagh at the base of the Benburb section, within the Maydown Limestone Formation, and in the Clogher Valley, where it is typified by the Fardross section. The local exception to this regional standard pattern is found in the Lisnaskea area. There the Glencar Limestone is absent and the transition from deep water shales of the Benbulben Formation into equally faunally and florally impoverished cherty limestones and mudstones of the Dartry Limestone Formation is abrupt.
In the post-Glencar Limestone times the tectonic pattern was transformed by rapid and sustained uplift on faults, exposure of metamorphic basement adjacent to the Kesh area, formation of in-basinal ramps along new fault lines, rapid subsidence of non-marine basins and accumulation of thick red-bed sequences and regional marine regression. These events affected all of N. Ireland but the sedimentary record, by which they are defined, is concentrated mainly in the western part of the Province.
(v) Brigantian As a result of refined biostratigraphical information the palaeo- geography of Brigantian times is now seen to be more complex than hitherto suspected. In the eastern part of N. Ireland there is evidence, at Castle Espie in north Co. Down (Subarea-10), of the final inundation of the Down-Longford massif, although this may indeed represent only a local, rift-controlled incursion. At Ballycastle (Subarea-9), new palynological evidence indicates that basin subsidence only commenced there in the Brigantian but continued into the early Namurian.
Moving westwards the Greenan Sandstone Formation was deposited on Dalradian basement (Subarea-4), in a new non-marine basin that subsided along the present line of the Castle Archdale Fault. This shallow basin received at least 600m of medium-grained sandstones and existed during Brigantian and early Namurian times. Localised rhyolitic and dacitic volcanism occurred in this basin in Co. Tyrone and is matched by the basaltic lavas that were erupted near the base of the Ballycastle sequence.
An increase in the rate of subsidence, in the Brigantian, of the main non-marine basin located south of the southwestern extension of the Tyrone Igneous Complex, in the southern part of the Fintona Block, caused the deposition of coarse sandstones and conglomerates of the Ballyreagh Formation of the Kilskeery Group. It is likely that, given the reduction in tectonism evident in other Brigantian strata in N. Ireland, there was also a slowing down in the rate of subsidence of this basin and the lower levels of the fine-grained Ballinamallard Mudstone Formation may well reflect this eventuality and are Brigantian.
The remaining Brigantian strata in N. Ireland occur in the west of the Province and universally reflect a marked decline in tectonic activity and an increasing influence of thermal subsidence over wide areas. In particular Leitrim Group strata of the Bellavally and Carraun Shale formations of this age crop out over an area of ~1400km2. Minor cyclicity of thin limestones and other marine bands is evident in the latter formation and their regional persistence, combined with the absence of major facies variations and the almost uniform thickness of these and later lithostratigraphical units testifies to the relative stability of this slowly subsiding basin. The occurrence of sandstone dykes in the shales indicates that minor seismic disturbances affected the sediments.
In the Dungannon-Cookstown area (Subarea-7) the early Brigantian is represented by shallow water limestones and shales with grainstones and thin sandstones. Succeeding strata consist almost exclusively of shales and mudstones of the Rossmore Mudstone Formation which closely resemble the upper part of the Carraun Shale Formation in Co. Fermanagh. Shorelines at this time clearly lay some distance from the present outcrop in Co. Fermanagh and the land area was peneplained thus providing only fine mud detritus to the basin.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Carboniferous Palaeogeography of N. Ireland, County Antrim, Armagh, Down, Fermanagh, Londonderry, Tyro, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1173. Accessed on 2024-12-26 |
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