Earth Science Conservation Review
| Carboniferous Subarea-12; Draperstown-Glenshane Pass-Limavady | Londonderry |
| Site Type: | Various |
| Site Status: | |
| Council area: | Limavady Borough Council, Magherafelt District Council |
| Grid Reference: | H7894,C6722 |
| Google maps: | 54.78826,-6.7875 |
| Rocks | |
|---|---|
| Rock Age: | Carboniferous (Asbian, Tournasian, Visean) |
| Rock Name: | Altagoan Formation, Armagh Group, Desertmartin Limestone Formation, Drumard Member, Iniscarn Sandstone Formation, Leitrim Group, Meenymore Formation, Mormeal Member, Tyrone Group |
| Rock Type: | Evaporite, Limestone, Mudstone, Sand, Sandstone, Shale, Siltstone |
| Interest | |
| Minerals: | Anhydrite, Gypsum, Halite |
| Fossil Groups: | Bivalve, Crustacea, Miospore, Trace fossil |
| Other interest: | cross-bedding, dessication cracks, ripple marks |
Description:
INTRODUCTION
With the exception of three parts of the Carboniferous outcrop in Subarea-12, the geological mapping of these rocks by the GSNI has not been completed. New data are available for their distribution in the Limavady area (GSNI, 1981), extending southwards to a latitude ~8km south of the town, in the Draperstown area (GSNI, 1995) and west of Magherafelt (GSNI, 1983). For the central part of the outcrop between Bovevagh (C66 13) NW of Dungiven and Moneyneany (H75 97), near Draperstown, geological information was only gathered during a rapid reconnaissance survey.
The main aim of this survey was to review the 19th century maps of the Geological Survey of Ireland and to collect samples, suitable for palynology, at sites chosen throughout the entire outcrop and based on those six-inch maps. It was thus hoped to determine the age, or range of ages, of these strata, and on the basis of this experience, albeit brief, to nominate ASSI that are representative of the rocks in Subarea-12. This rapid survey was not designed as a substitute for the detailed geological resurvey of the Dungiven area, including the Carboniferous outcrop between Bovevagh and Moneyneany, which commenced in 1995.
Biostratigraphical information on the age of the 'Basal Clastics' is now available for all of their outcrop in Subarea-12, with the notable exception of the entire part of the outcrop west of Limavady, bordering the south shore of Lough Foyle, and south of Limavady lying east of the River Roe.
I - TYRONE GROUP
The distribution of Carboniferous rocks in County Londonderry is depicted on the 1:250,000 geological map of Northern Ireland (GSNI, 1977). All the strata are of Lower Carboniferous age and are assigned to the Tyrone Group. The bulk of the outcrop is formed of rocks formerly assigned to the 'Basal Clastics' and, following the 19th century nomenclature of the Geological Survey of Ireland, is labelled with the letter 'd'. Subsequently the GSNI (1983) subdivided these rocks into the basal Iniscarn Sandstone Formation overlain by the Altagoan Formation. Surveyor's notes, that were made during the resurvey of these rocks in the Draperstown-Magherafelt area (GSNI, 1983), indicate that the Altagoan Formation is readily divisible into two parts although this was not depicted on the geological map of the Cookstown area. The basal part of the formation was informally referred to as the Drumard Member and the upper part as the Mormeal Member. Based on the author's experience of these rocks it is clear that these subdivisions of the Altagoan Formation are indeed recognisable throughout much of this Carboniferous outcrop in Subarea-12, beyond the stratotype sections in the Altagoan River and Drumard Water, and warrant recognition as formal litho- stratigraphical units with separate ASSI designation.
Rocks of the Iniscarn Formation and the Drumard Member of the Altagoan Formation were deposited initially in a non-marine, terrestrial environment which gradually changed, in the Mormeal Member, to fluvial, brackish and hypersaline, intertidal conditions. This transition represents the early stages in a marine transgression and is closely comparable to the sequence and succession of events that are found in the Holywood Group at Cultra-Craigavad in Subarea-10. Abundant miospore assemblages in the Mormeal Member of the Altagoan Formation and elsewhere in the Carboniferous outcrop indicate a CM-Pu Zone assignment and a late Tournaisian age.
Unconformably succeeding these rocks, in the extreme southeastern corner of the outcrop, are fossiliferous marine limestones of the Desertmartin Limestone Formation. On the 1:250,000 geological map (1977) they are annotated with the symbol d2a, suggesting a Chadian-Arundian age, which is incorrect. However on the 1:50,000 geological map of the Cookstown area (GSNI, 1983) they are annotated with the symbol d2c which suggests a late Viséan age that is in fact correct since the limestones contain Asbian fossils.
(i) INISCARN FORMATION
By the end of Devonian times in N. Ireland the Sperrin Mountains did not exist as a topographical entity and the land surface was reduced to an undulating peneplain. During early Carboniferous times regional seismicity was responsible for the formation of steep scarp slopes, probably with a NE-SW orientation, in the landscape. Constant regeneration of the altitude differential between these scarps, by movement on fault lines, and the basin floor led to the erosion, predominantly by fluvial action, of the metamorphic bedrock. These coarse products of the denudation cycle were banked up against the scarp slopes as alluvial fans and their thickness is thus extremely variable, being thickest against the scarp and thinning rapidly away from the source of the debris.
The basal beds of the Carboniferous sequence in Subarea-12 always consist of conglomerates and very coarse sandstones which are usually stained a bright red to red-purple colour. They rest unconformably on a variety of rock types which are predominantly of metamorphic lithologies in the north but in the southern part of the outcrop between Moneymore and Magherafelt, fringe the margins of the Lower Palaeozoic Slieve Gallion 'Granite' complex and overlie a small outlier of ? Devonian rocks (GSNI, 1983). Clasts in the conglomerate are usually locally derived and where the formation reaches its maximum thickness are frequently inter- bedded with breccias containing angular clasts. In some parts of the outcrop of the Iniscarn Formation however, in particular where there is only a thin development, clasts are conspicuously rounded and consist mainly of micaceous quartzite and vein quartz. This may be interpreted as indicating a polycyclic origin for some of these clasts and more complex transport paths for the detritus.
The Iniscarn Formation is never completely exposed in any one section and its thickness is estimated at over 600m. In some parts of the outcrop located outside Iniscarn Townland the formation is very much reduced in thickness and may be only in the region of 20-100m. At its top the Iniscarn Formation is gradational into finer, red and grey lithologies of the Drumard Member of the Altagoan Formation.
The Iniscarn Formation is unfossiliferous.
For site specific information see; Key Site 298 - Iniscarn Townland, near Desertmartin Key Site 299 - Spincha Burn, Cluntygeeragh Townland.
(ii) ALTAGOAN FORMATION
Under the term Altagoan Formation (GSNI, 1983) are grouped a very variable sequence of thin-bedded, calcareous sedimentary rocks ranging from coarse pebbly sandstones to micaceous laminated sandstones, siltstones, mudstones and shales. Carbonate lithologies appear commonly in the upper part of the formation and consist of thin beds of grey micritic limestone, laminated stromatolitic (cyano-bacterial) limestones and several levels of relict evaporite beds. The formation is readily divisible into two parts which are referred to as the basal Drumard Member succeeded by the Mormeal Member. The total thickness is estimated at between 225-275m.
(1) DRUMARD MEMBER
This unit represents the lowest part of the Altagoan Formation and is between 150-200m thick. Lithologies forming the Drumard Member are broadly gradational between the underlying Iniscarn Sandstone Formation and the succeeding Mormeal Member. It is arguable as to whether the strata should be placed within the Altagoan or Iniscarn formation but for the moment, they are mapped as part of the former formation.
The dominant lithologies are red- and green-grey, unfossiliferous silty mudstones, laminated siltstones and fine-grained, parallel and cross-laminated sandstones. Beds of compact pebbly sandstone are numerous but are usually relatively thin. Although no limestone beds are present, many of the red-beds contain in situ nodules of pedogenic carbonate which, in some horizons have been winnowed from the enclosing sediment and redeposited as an intra- formational breccia. Though red-purple colours usually dominate over greens and greys, in some parts of the sequence the reverse is true. Light and dark shades of grey, so typical of the succeeding Mormeal Member are very rare.
Sedimentary structures such as ripple marks and desiccation cracks occur sporadically indicating deposition from shallow flowing water and subaerial exposure, respectively.
Fossils are normally absent or very scarce in the Drumard Member although occasional dark grey shales containing the bivalve Modiolus and ostracods occur in the highest transitional beds to the succeeding Mormeal Member and, much less commonly, lower in the sequence. The late Tournaisian (CM Zone) age of the Drumard Member is however, clearly demonstrable on the basis of miospore assemblages from several localities that include an abundant and diverse assemblage characterised by the presence of Schopfites claviger and Auroraspora macra, the zonal index taxa of the CM Biozone and the absence of the Pu zonal taxon Lycospora pusilla.
The stratotype of the Drumard Member is in the Drumard Water (H79 92 to 79 93) where all strata exposed belong to this member. Neither the upper nor lower contacts are seen here. One of the best exposed and most accessible sections through the Drumard Member of the Altagoan Formation was discovered during the reconnaissance survey of the Glenshane Pass area, in Barony Glen on the west flank of White Mountain and Mullaghmore, 9km SSE of Dungiven. It is proposed to nominate this section as an ASSI.
For site specific information see; Key Site 300 - Drumard Water Key Site 301 - Barony Glen.
(2) MORMEAL MEMBER
The upper part of the Altagoan Formation is referred to the Mormeal Member and is best exposed in the Altagoan River, the stratotype section, where the contact with the Drumard Member is faulted.
Virtually all lithologies in the Mormeal Member are coloured in shades of dark and light grey and are thus distinguishable immediately from the purple-red-brown and green colours of the Drumard Member. The main lithologies are medium grey shales and mudstones which are frequently crowded with shells of Modiolus, ostracods, serpulid worm tubes and abundant carbonaceous fragments. Fine-grained brown-grey sandstones and grey-green siltstones are common and thin beds of micritic limestone occur at several horizons. By far the most distinctive lithology in the Altagoan Formation is found in the Mormeal Member and consists of thin, finely laminated stromatolitic (cyanobacterial) limestones which are associated with thin beds of evaporite minerals, in particular calcitised gypsum and/or anhydrite. These beds occur at 5 separate horizons in the Altagoan River. The evaporite minerals listed above are also associated with sphalerite and hollow pseudomorphs after cubic halite (salt) crystals in some of the micrites. Red-beds are very rare but have been noted.
Sedimentary structures are common and include, in particular, ripple marks on siltstone and sandstone surfaces and desiccation cracks, which are common on the surfaces of laminated limestones and non-laminated micrites.
The late Tournaisian age of the Mormeal Member, and the upper part of the Altagoan Formation, has been admirably demonstrated by Owens et al. (1977). They recovered an extremely diverse miospore assemblage comprising 68 taxa from a locality (H8000 9425) 320m SSE of Altagoan Bridge in the stratotype Altagoan River section although the probability of a very low Pu Zone age was stressed by Higgs et al. (1988).
For site specific information see; Key Site 303 - Altagoan River.
With the exception of three parts of the Carboniferous outcrop in Subarea-12, the geological mapping of these rocks by the GSNI has not been completed. New data are available for their distribution in the Limavady area (GSNI, 1981), extending southwards to a latitude ~8km south of the town, in the Draperstown area (GSNI, 1995) and west of Magherafelt (GSNI, 1983). For the central part of the outcrop between Bovevagh (C66 13) NW of Dungiven and Moneyneany (H75 97), near Draperstown, geological information was only gathered during a rapid reconnaissance survey.
The main aim of this survey was to review the 19th century maps of the Geological Survey of Ireland and to collect samples, suitable for palynology, at sites chosen throughout the entire outcrop and based on those six-inch maps. It was thus hoped to determine the age, or range of ages, of these strata, and on the basis of this experience, albeit brief, to nominate ASSI that are representative of the rocks in Subarea-12. This rapid survey was not designed as a substitute for the detailed geological resurvey of the Dungiven area, including the Carboniferous outcrop between Bovevagh and Moneyneany, which commenced in 1995.
Biostratigraphical information on the age of the 'Basal Clastics' is now available for all of their outcrop in Subarea-12, with the notable exception of the entire part of the outcrop west of Limavady, bordering the south shore of Lough Foyle, and south of Limavady lying east of the River Roe.
I - TYRONE GROUP
The distribution of Carboniferous rocks in County Londonderry is depicted on the 1:250,000 geological map of Northern Ireland (GSNI, 1977). All the strata are of Lower Carboniferous age and are assigned to the Tyrone Group. The bulk of the outcrop is formed of rocks formerly assigned to the 'Basal Clastics' and, following the 19th century nomenclature of the Geological Survey of Ireland, is labelled with the letter 'd'. Subsequently the GSNI (1983) subdivided these rocks into the basal Iniscarn Sandstone Formation overlain by the Altagoan Formation. Surveyor's notes, that were made during the resurvey of these rocks in the Draperstown-Magherafelt area (GSNI, 1983), indicate that the Altagoan Formation is readily divisible into two parts although this was not depicted on the geological map of the Cookstown area. The basal part of the formation was informally referred to as the Drumard Member and the upper part as the Mormeal Member. Based on the author's experience of these rocks it is clear that these subdivisions of the Altagoan Formation are indeed recognisable throughout much of this Carboniferous outcrop in Subarea-12, beyond the stratotype sections in the Altagoan River and Drumard Water, and warrant recognition as formal litho- stratigraphical units with separate ASSI designation.
Rocks of the Iniscarn Formation and the Drumard Member of the Altagoan Formation were deposited initially in a non-marine, terrestrial environment which gradually changed, in the Mormeal Member, to fluvial, brackish and hypersaline, intertidal conditions. This transition represents the early stages in a marine transgression and is closely comparable to the sequence and succession of events that are found in the Holywood Group at Cultra-Craigavad in Subarea-10. Abundant miospore assemblages in the Mormeal Member of the Altagoan Formation and elsewhere in the Carboniferous outcrop indicate a CM-Pu Zone assignment and a late Tournaisian age.
Unconformably succeeding these rocks, in the extreme southeastern corner of the outcrop, are fossiliferous marine limestones of the Desertmartin Limestone Formation. On the 1:250,000 geological map (1977) they are annotated with the symbol d2a, suggesting a Chadian-Arundian age, which is incorrect. However on the 1:50,000 geological map of the Cookstown area (GSNI, 1983) they are annotated with the symbol d2c which suggests a late Viséan age that is in fact correct since the limestones contain Asbian fossils.
(i) INISCARN FORMATION
By the end of Devonian times in N. Ireland the Sperrin Mountains did not exist as a topographical entity and the land surface was reduced to an undulating peneplain. During early Carboniferous times regional seismicity was responsible for the formation of steep scarp slopes, probably with a NE-SW orientation, in the landscape. Constant regeneration of the altitude differential between these scarps, by movement on fault lines, and the basin floor led to the erosion, predominantly by fluvial action, of the metamorphic bedrock. These coarse products of the denudation cycle were banked up against the scarp slopes as alluvial fans and their thickness is thus extremely variable, being thickest against the scarp and thinning rapidly away from the source of the debris.
The basal beds of the Carboniferous sequence in Subarea-12 always consist of conglomerates and very coarse sandstones which are usually stained a bright red to red-purple colour. They rest unconformably on a variety of rock types which are predominantly of metamorphic lithologies in the north but in the southern part of the outcrop between Moneymore and Magherafelt, fringe the margins of the Lower Palaeozoic Slieve Gallion 'Granite' complex and overlie a small outlier of ? Devonian rocks (GSNI, 1983). Clasts in the conglomerate are usually locally derived and where the formation reaches its maximum thickness are frequently inter- bedded with breccias containing angular clasts. In some parts of the outcrop of the Iniscarn Formation however, in particular where there is only a thin development, clasts are conspicuously rounded and consist mainly of micaceous quartzite and vein quartz. This may be interpreted as indicating a polycyclic origin for some of these clasts and more complex transport paths for the detritus.
The Iniscarn Formation is never completely exposed in any one section and its thickness is estimated at over 600m. In some parts of the outcrop located outside Iniscarn Townland the formation is very much reduced in thickness and may be only in the region of 20-100m. At its top the Iniscarn Formation is gradational into finer, red and grey lithologies of the Drumard Member of the Altagoan Formation.
The Iniscarn Formation is unfossiliferous.
For site specific information see; Key Site 298 - Iniscarn Townland, near Desertmartin Key Site 299 - Spincha Burn, Cluntygeeragh Townland.
(ii) ALTAGOAN FORMATION
Under the term Altagoan Formation (GSNI, 1983) are grouped a very variable sequence of thin-bedded, calcareous sedimentary rocks ranging from coarse pebbly sandstones to micaceous laminated sandstones, siltstones, mudstones and shales. Carbonate lithologies appear commonly in the upper part of the formation and consist of thin beds of grey micritic limestone, laminated stromatolitic (cyano-bacterial) limestones and several levels of relict evaporite beds. The formation is readily divisible into two parts which are referred to as the basal Drumard Member succeeded by the Mormeal Member. The total thickness is estimated at between 225-275m.
(1) DRUMARD MEMBER
This unit represents the lowest part of the Altagoan Formation and is between 150-200m thick. Lithologies forming the Drumard Member are broadly gradational between the underlying Iniscarn Sandstone Formation and the succeeding Mormeal Member. It is arguable as to whether the strata should be placed within the Altagoan or Iniscarn formation but for the moment, they are mapped as part of the former formation.
The dominant lithologies are red- and green-grey, unfossiliferous silty mudstones, laminated siltstones and fine-grained, parallel and cross-laminated sandstones. Beds of compact pebbly sandstone are numerous but are usually relatively thin. Although no limestone beds are present, many of the red-beds contain in situ nodules of pedogenic carbonate which, in some horizons have been winnowed from the enclosing sediment and redeposited as an intra- formational breccia. Though red-purple colours usually dominate over greens and greys, in some parts of the sequence the reverse is true. Light and dark shades of grey, so typical of the succeeding Mormeal Member are very rare.
Sedimentary structures such as ripple marks and desiccation cracks occur sporadically indicating deposition from shallow flowing water and subaerial exposure, respectively.
Fossils are normally absent or very scarce in the Drumard Member although occasional dark grey shales containing the bivalve Modiolus and ostracods occur in the highest transitional beds to the succeeding Mormeal Member and, much less commonly, lower in the sequence. The late Tournaisian (CM Zone) age of the Drumard Member is however, clearly demonstrable on the basis of miospore assemblages from several localities that include an abundant and diverse assemblage characterised by the presence of Schopfites claviger and Auroraspora macra, the zonal index taxa of the CM Biozone and the absence of the Pu zonal taxon Lycospora pusilla.
The stratotype of the Drumard Member is in the Drumard Water (H79 92 to 79 93) where all strata exposed belong to this member. Neither the upper nor lower contacts are seen here. One of the best exposed and most accessible sections through the Drumard Member of the Altagoan Formation was discovered during the reconnaissance survey of the Glenshane Pass area, in Barony Glen on the west flank of White Mountain and Mullaghmore, 9km SSE of Dungiven. It is proposed to nominate this section as an ASSI.
For site specific information see; Key Site 300 - Drumard Water Key Site 301 - Barony Glen.
(2) MORMEAL MEMBER
The upper part of the Altagoan Formation is referred to the Mormeal Member and is best exposed in the Altagoan River, the stratotype section, where the contact with the Drumard Member is faulted.
Virtually all lithologies in the Mormeal Member are coloured in shades of dark and light grey and are thus distinguishable immediately from the purple-red-brown and green colours of the Drumard Member. The main lithologies are medium grey shales and mudstones which are frequently crowded with shells of Modiolus, ostracods, serpulid worm tubes and abundant carbonaceous fragments. Fine-grained brown-grey sandstones and grey-green siltstones are common and thin beds of micritic limestone occur at several horizons. By far the most distinctive lithology in the Altagoan Formation is found in the Mormeal Member and consists of thin, finely laminated stromatolitic (cyanobacterial) limestones which are associated with thin beds of evaporite minerals, in particular calcitised gypsum and/or anhydrite. These beds occur at 5 separate horizons in the Altagoan River. The evaporite minerals listed above are also associated with sphalerite and hollow pseudomorphs after cubic halite (salt) crystals in some of the micrites. Red-beds are very rare but have been noted.
Sedimentary structures are common and include, in particular, ripple marks on siltstone and sandstone surfaces and desiccation cracks, which are common on the surfaces of laminated limestones and non-laminated micrites.
The late Tournaisian age of the Mormeal Member, and the upper part of the Altagoan Formation, has been admirably demonstrated by Owens et al. (1977). They recovered an extremely diverse miospore assemblage comprising 68 taxa from a locality (H8000 9425) 320m SSE of Altagoan Bridge in the stratotype Altagoan River section although the probability of a very low Pu Zone age was stressed by Higgs et al. (1988).
For site specific information see; Key Site 303 - Altagoan River.
| Enlander, I., Dempster, M. & Doughty, P., 2024. Carboniferous Subarea-12; Draperstown-Glenshane Pass-Limavady, County Londonderry, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1185. Accessed on 2024-12-26 |
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