Earth Science Conservation Review
| The Devonian System in N. Ireland - Introduction | Antrim, Fermanagh, Tyrone |
| Site Type: | Various |
| Site Status: | |
| Council area: | Cookstown District Council, Fermanagh District Council, Moyle District Council, Omagh District Council |
| Grid Reference: | |
| Google maps: | , |
| Rocks | |
|---|---|
| Rock Age: | Devonian (Emsian, Famennian, Frasnian, Gedinnian, Givetian, Sieginian) |
| Rock Name: | Cross Slieve Group, Fintona Group, Gortfinbar Conglomerate Formation, Raveagh Sandstone Formation, Red Arch Formation, Shanmaghery Sandstone Formation, Shanmullagh Formation (Form Blm), Tedd Formation |
| Rock Type: | Conglomerate, Mudstone, Sand, Sandstone |
| Interest | |
| Other interest: | cross-bedding, fault, alluvial fan, braided stream |
Description:
INTRODUCTION
The Devonian outcrop in Northern Ireland is confined to two areas which are, in Counties Tyrone and Fermanagh the Fintona Block, and in NE County Antrim the coastal sector between Cushendun and Red Bay.
Historically these rocks were ascribed to the Lower Old Red Sandstone and Upper Old Red Sandstone facies with their differentiation relying entirely on lithological criteria, there being no palaeontological evidence.
The discovery by Harper and Hartley in 1938 of two fragments of a fossil fish from the southern part of the Fintona Block, near Lisbellaw, was heralded as a major breakthrough and accepted as affirmation of the long suspected early Devonian age for the Fintona Group. However, the specimen is now considered to be reworked (Blieck, 1993) and palynological evidence (Mitchell & Owens, 1990) demonstrates that the rocks of this part of the Fintona Block are Carboniferous. Mitchell and Owens also revised the lithostratigraphy of much of the remainder of the Fintona Block and, using palynological techniques in some cases, identified separate fault-bounded segments of Devonian rocks, and both Lower and Upper Carboniferous rocks.
No palaeontological evidence for the age of the Devonian rocks in NE Antrim has been found yet and their ages are therefore based solely on comparisons with lithologically similar rocks of undoubted Devonian age in the Midland Valley of Scotland (Wilson, 1953; Simon, 1984).
A: RED-BED SEDIMENTARY ROCKS
All of the Devonian sedimentary rocks in Northern Ireland are red-beds. One definition of red-beds is as follows (Glossary of Geology, 1987).
"Sedimentary strata composed largely of sandstone, siltstone and shale, with locally thin units of conglomerate, limestone or marl, that are predominantly red in colour due to the presence of ferric oxide (hematite) usually coating individual grains. At least 60% of any given succession must be red before the term is appropriate, the interbedded strata being of any colour."
All of these sedimentary rocks are non-marine and were deposited in basins that developed on a land mass far removed from the influence of the sea. Deposition of sediments was therefore influenced strongly by gravity, in the case of very coarse rocks such as conglomerates and by water, whether it be in rivers or in lakes, in the case of fine-grained sediments.
The spectrum of colours typical of all of these rocks in both the Fintona Block and NE Antrim is concentrated in the pink-red- purple-brown range. Processes of "reddening" were essentially syndepositional and involved the ageing and dehydration of ferric oxyhydroxides during and after deposition. The rise and primarily the fall of the water table controls the depth to which penetrative oxidation ("reddening") is effective and thus the alteration of ferrous to ferric oxides of iron. Because these sediments are believed to have been deposited in a desert environment, and there are innumerable examples of desiccation cracks throughout the sequence illustrating periodic aridity, the lowering of the water table was probably a response to either episodic or prolonged and continuous evaporation. The process of pedogenesis (soil formation) is an important influence in red-bed sequences and many examples are found, particularly in the Fintona Block.
The occurrence of thin beds of green or grey mudstone or shale is thus a rare event in such a thick sequence of predominantly red-coloured rocks. It indicates the existence of a standing body of water, for example a lake, and persistently high water table after burial of the lacustrine sediments thus preventing oxidation of iron compounds. In the Fintona Block the occurrence of such green beds probably represents less than 1% of the total succession. Nevertheless these beds, whose thickness is usually measured in millimetres although beds up to 1m thick do occur, are extremely important in that they alone preserve the plant pollen (miospores) by which the rocks can be dated. Miospores formerly occurred in all the fine-grained sediments but were destroyed by the process of oxidation and are thus now absent from the red rocks.
Beds of green mudstone occur commonly in the Fintona Block but only those examples in the Shanmullagh Formation (Irvinestown Segment) have yielded miospores. No miospores have been found south of the Tempo-Sixmilecross Fault or in NE Antrim.
B: DEVONIAN PALAEOGEOGRAPHY
Based on the unsubstantiated early Devonian age assigned historically to rocks of Lower Old Red Sandstone facies in the Fintona Block and northeast Antrim, the traditional, and now undoubtedly erroneous, view of early Devonian palaeogeography in the north of Ireland invariably associated sediment accumulation with orogenesis. Fashionable reconstructions envisaged a mountain range of Himalayan proportions (Simon & Bluck, 1982; Simon, 1984) located to the north of the Devonian sedimentary basins and consisting largely of older Caledonian, principally metamorphic, rocks. At their southern margin these mountains, which supposedly extended from western Ireland across Scotland, north of the Midland Valley, were bounded by the then active Highland Boundary Fault and various extensions. Major river systems draining from the front of the mountains were apparently comparable (Simon & Bluck, 1982, p.12) in size to the present day Ganges-Brahmaputra drainage system in the Indian subcontinent.
The sediments that accumulated in the Devonian basins were thus derived by erosion of this rapidly rising mountain range. As a consequence of this activity it was envisaged that thick sequences of conglomerates and sandstones, that thickened and coarsened northwards, were deposited on alluvial fans which banked up against the fault-controlled mountain front. Finer- grained sediments that were contemporaneous with and eventually succeeded the conglomerates were deposited at some distance south of the mountains on alluvial floodplains and in temporary playa lakes as the finest mud, silt and sand fraction of this on-going cycle of erosion.
There is indeed a substantial body of evidence supporting this palaeogeographical reconstruction of early Devonian times in the Midland Valley of Scotland and at Kintyre in terms of the age of the strata and their environments of deposition, sequence, provenance and sediment dispersal patterns. This model cannot however be applied unequivocally to all the Devonian rocks in Northern Ireland as will be demonstrated later.
C: CUSHENDUN-CUSHENDALL-RED BAY, NORTHEAST ANTRIM
(i) Early Devonian - Lower Old Red Sandstone facies
Although the precise age of the Devonian Cross Slieve Group of NE Antrim is not known there are striking similarities with the sedimentary rocks of the early Devonian (Lower Old Red Sandstone) outcrop of Scotland (Simon, 1984). The preponderance of very coarse conglomerates in the lower half of the 1300m thick sequence in NE Antrim, resting on metamorphic basement, suggests that these northerly derived sediments were deposited in a rapidly subsiding, fault-controlled basin. Strata in the upper part of this sequence are partly conglomeratic and also of alluvial fan origin but may not necessarily have been deposited at the fault-controlled margin of a mountain range. These upper conglomerates, which are almost totally volcanoclastic, were probably derived by the erosion of a contemporaneous, but now concealed or eroded, volcanic centre emitting dacitic lavas, that lay to the south of the present outcrop. These patterns of sediment type, dispersal and provenance in the Cross Slieve Group precisely parallel those found on the north side of the Midland Valley of Scotland where there is contemporaneity between the sedimentary rocks of the Strathmore Basin and the volcanic rocks of the Ochil-Sidlaw Hills.
(ii) Late Devonian - Upper Old Red Sandstone facies
To the south of Cushendall the Cross Slieve Group is overlain unconformably (inter alia) by red-pink conglomerates of the Red Arch Formation, of late Palaeozoic or early Mesozoic age (Simon, 1984). The formation is unfossiliferous and thus its precise age is not known. It is 400m thick and comprises 5 units of which only the upper part of Unit 5, at the top of the sequence, is non-conglomeratic, consisting of sandstones and thick red-brown mudstones with thin green beds.
D: THE FINTONA BLOCK - COs. TYRONE & FERMANAGH
The distribution of Devonian rocks of the Fintona Group in the Fintona Block is based on the geological maps of the Geological Survey of Northern Ireland (1979, 1982a,b) and on the paper by Mitchell and Owens (1990). Previous correlations and age assignments (Simon, 1984) were shown to be incorrect, by these authors, based on new biostratigraphical information. In addition since 1990 further palynology has shown that the Tedd Formation is undoubtedly Devonian and that the Lower Devonian age, postulated by Richardson (in Mitchell & Owens, 1990) for the Shanmullagh Formation is too old. Unfortunately there is still no palynological data from the rocks in the Fintona Block located south of the Tempo-Sixmilecross Fault, and their mid-late Devonian age is based solely on the K-Ar wholerock dating of weathered contemporaneous volcanic rocks.
Based on these dates there is apparently no correlation between the rocks of the Fintona Group, in the Fintona Block (Middle- Upper Devonian) and the presumed Lower Devonian Cross Slieve Group of NE Antrim. In addition there are stark contrasts between the sediment types, facies, provenance and origins of the Cross Slieve Group and the restricted suite of sedimentary rocks now assigned to the Tedd and Shanmullagh formations, in particular, of the Fintona Group.
The Fintona Block is divided into two parts by the Tempo- Sixmilecross Fault (Wilson, 1953) and the rocks in each part will be described separately.
(i) North of the Tempo-Sixmilecross Fault-Irvinestown/Tedd Cross Roads Segments
The rocks in these segments occupy the northern part of the Fintona Block, north of the Tempo-Sixmilecross and Killadeas- Seskinore faults and south of the Castle Archdale Fault - Omagh Fault complex. In the southwest adjacent to Lower Lough Erne, the Devonian rocks are faulted against the Lower Carboniferous Tyrone Group and in the northeast, near Carrickmore, rest unconformably on the Tyrone Volcanic Group.
All of the rocks in the Tedd Cross Roads Segment are assigned to the Tedd Formation, and in the Irvinestown Segment to the Shanmullagh Formation. They consist exclusively of red-beds in which there are no conglomerates and very few examples of coarse- grained sandstones. The bulk of the rocks consist of fine-grained fluvial sandstones and siltstones, playa lake mudstones with abundant pedogenic (soil) horizons. Beds of green and dark green mudstones frequently contain miospores of Upper Devonian age and in particular the Frasnian and Lower Famennian Stages. Because of the gross similarity of red-bed lithologies throughout the outcrop of the Shanmullagh Formation and the apparent absence of widespread marker horizons the overall thickness of these rocks is not known. It is however evident that they were deposited in the axial region of the late Devonian sedimentary basin in this part of Northern Ireland in a location far removed from sediment source areas.
(ii) South of the Tempo-Sixmilecross Fault
These rocks of the Fintona Group are bounded on their north side by this fault and are located at least 10km south of the supposed position of the southwesterly extension, in Ireland, of the Highland Boundary Fault. They are divided into three formations and consist of about 3500m of red-beds and volcanic rocks although exposure over much of the outcrop is poor and the possible repetitive effects of concealed faulting should not be discounted.
The basal conglomerate of the lowest formation, the Shanmaghery Sandstone is exposed south of Pomeroy resting unconformably on Llandovery (early Silurian) rocks of that eponymous inlier. It is a thin conglomerate suggesting that the local provenance of the clasts had a subdued topography. Contrast this with the very thick, and consistently very coarse, alluvial fan conglomerates found at the base of, and frequently throughout, older Devonian sequences in NE Antrim and Scotland.
Succeeding volcanoclastic conglomerates of the Gortfinbar Conglomerate Formation were apparently derived largely by the erosion of different centres of contemporaneous volcanic lavas that erupted in the east of the outcrop around Cappagh. Two separate phases of volcanism occur within the formation but importantly, there is no evidence of the development of contemporaneous and diuturnal fault scarps that would have provided a significant supply of clasts from pre-existing non-volcanic rocks.
At the top of the sequence is the Raveagh Sandstone Formation which consists entirely of sandstones but is very poorly exposed.
The Devonian outcrop in Northern Ireland is confined to two areas which are, in Counties Tyrone and Fermanagh the Fintona Block, and in NE County Antrim the coastal sector between Cushendun and Red Bay.
Historically these rocks were ascribed to the Lower Old Red Sandstone and Upper Old Red Sandstone facies with their differentiation relying entirely on lithological criteria, there being no palaeontological evidence.
The discovery by Harper and Hartley in 1938 of two fragments of a fossil fish from the southern part of the Fintona Block, near Lisbellaw, was heralded as a major breakthrough and accepted as affirmation of the long suspected early Devonian age for the Fintona Group. However, the specimen is now considered to be reworked (Blieck, 1993) and palynological evidence (Mitchell & Owens, 1990) demonstrates that the rocks of this part of the Fintona Block are Carboniferous. Mitchell and Owens also revised the lithostratigraphy of much of the remainder of the Fintona Block and, using palynological techniques in some cases, identified separate fault-bounded segments of Devonian rocks, and both Lower and Upper Carboniferous rocks.
No palaeontological evidence for the age of the Devonian rocks in NE Antrim has been found yet and their ages are therefore based solely on comparisons with lithologically similar rocks of undoubted Devonian age in the Midland Valley of Scotland (Wilson, 1953; Simon, 1984).
A: RED-BED SEDIMENTARY ROCKS
All of the Devonian sedimentary rocks in Northern Ireland are red-beds. One definition of red-beds is as follows (Glossary of Geology, 1987).
"Sedimentary strata composed largely of sandstone, siltstone and shale, with locally thin units of conglomerate, limestone or marl, that are predominantly red in colour due to the presence of ferric oxide (hematite) usually coating individual grains. At least 60% of any given succession must be red before the term is appropriate, the interbedded strata being of any colour."
All of these sedimentary rocks are non-marine and were deposited in basins that developed on a land mass far removed from the influence of the sea. Deposition of sediments was therefore influenced strongly by gravity, in the case of very coarse rocks such as conglomerates and by water, whether it be in rivers or in lakes, in the case of fine-grained sediments.
The spectrum of colours typical of all of these rocks in both the Fintona Block and NE Antrim is concentrated in the pink-red- purple-brown range. Processes of "reddening" were essentially syndepositional and involved the ageing and dehydration of ferric oxyhydroxides during and after deposition. The rise and primarily the fall of the water table controls the depth to which penetrative oxidation ("reddening") is effective and thus the alteration of ferrous to ferric oxides of iron. Because these sediments are believed to have been deposited in a desert environment, and there are innumerable examples of desiccation cracks throughout the sequence illustrating periodic aridity, the lowering of the water table was probably a response to either episodic or prolonged and continuous evaporation. The process of pedogenesis (soil formation) is an important influence in red-bed sequences and many examples are found, particularly in the Fintona Block.
The occurrence of thin beds of green or grey mudstone or shale is thus a rare event in such a thick sequence of predominantly red-coloured rocks. It indicates the existence of a standing body of water, for example a lake, and persistently high water table after burial of the lacustrine sediments thus preventing oxidation of iron compounds. In the Fintona Block the occurrence of such green beds probably represents less than 1% of the total succession. Nevertheless these beds, whose thickness is usually measured in millimetres although beds up to 1m thick do occur, are extremely important in that they alone preserve the plant pollen (miospores) by which the rocks can be dated. Miospores formerly occurred in all the fine-grained sediments but were destroyed by the process of oxidation and are thus now absent from the red rocks.
Beds of green mudstone occur commonly in the Fintona Block but only those examples in the Shanmullagh Formation (Irvinestown Segment) have yielded miospores. No miospores have been found south of the Tempo-Sixmilecross Fault or in NE Antrim.
B: DEVONIAN PALAEOGEOGRAPHY
Based on the unsubstantiated early Devonian age assigned historically to rocks of Lower Old Red Sandstone facies in the Fintona Block and northeast Antrim, the traditional, and now undoubtedly erroneous, view of early Devonian palaeogeography in the north of Ireland invariably associated sediment accumulation with orogenesis. Fashionable reconstructions envisaged a mountain range of Himalayan proportions (Simon & Bluck, 1982; Simon, 1984) located to the north of the Devonian sedimentary basins and consisting largely of older Caledonian, principally metamorphic, rocks. At their southern margin these mountains, which supposedly extended from western Ireland across Scotland, north of the Midland Valley, were bounded by the then active Highland Boundary Fault and various extensions. Major river systems draining from the front of the mountains were apparently comparable (Simon & Bluck, 1982, p.12) in size to the present day Ganges-Brahmaputra drainage system in the Indian subcontinent.
The sediments that accumulated in the Devonian basins were thus derived by erosion of this rapidly rising mountain range. As a consequence of this activity it was envisaged that thick sequences of conglomerates and sandstones, that thickened and coarsened northwards, were deposited on alluvial fans which banked up against the fault-controlled mountain front. Finer- grained sediments that were contemporaneous with and eventually succeeded the conglomerates were deposited at some distance south of the mountains on alluvial floodplains and in temporary playa lakes as the finest mud, silt and sand fraction of this on-going cycle of erosion.
There is indeed a substantial body of evidence supporting this palaeogeographical reconstruction of early Devonian times in the Midland Valley of Scotland and at Kintyre in terms of the age of the strata and their environments of deposition, sequence, provenance and sediment dispersal patterns. This model cannot however be applied unequivocally to all the Devonian rocks in Northern Ireland as will be demonstrated later.
C: CUSHENDUN-CUSHENDALL-RED BAY, NORTHEAST ANTRIM
(i) Early Devonian - Lower Old Red Sandstone facies
Although the precise age of the Devonian Cross Slieve Group of NE Antrim is not known there are striking similarities with the sedimentary rocks of the early Devonian (Lower Old Red Sandstone) outcrop of Scotland (Simon, 1984). The preponderance of very coarse conglomerates in the lower half of the 1300m thick sequence in NE Antrim, resting on metamorphic basement, suggests that these northerly derived sediments were deposited in a rapidly subsiding, fault-controlled basin. Strata in the upper part of this sequence are partly conglomeratic and also of alluvial fan origin but may not necessarily have been deposited at the fault-controlled margin of a mountain range. These upper conglomerates, which are almost totally volcanoclastic, were probably derived by the erosion of a contemporaneous, but now concealed or eroded, volcanic centre emitting dacitic lavas, that lay to the south of the present outcrop. These patterns of sediment type, dispersal and provenance in the Cross Slieve Group precisely parallel those found on the north side of the Midland Valley of Scotland where there is contemporaneity between the sedimentary rocks of the Strathmore Basin and the volcanic rocks of the Ochil-Sidlaw Hills.
(ii) Late Devonian - Upper Old Red Sandstone facies
To the south of Cushendall the Cross Slieve Group is overlain unconformably (inter alia) by red-pink conglomerates of the Red Arch Formation, of late Palaeozoic or early Mesozoic age (Simon, 1984). The formation is unfossiliferous and thus its precise age is not known. It is 400m thick and comprises 5 units of which only the upper part of Unit 5, at the top of the sequence, is non-conglomeratic, consisting of sandstones and thick red-brown mudstones with thin green beds.
D: THE FINTONA BLOCK - COs. TYRONE & FERMANAGH
The distribution of Devonian rocks of the Fintona Group in the Fintona Block is based on the geological maps of the Geological Survey of Northern Ireland (1979, 1982a,b) and on the paper by Mitchell and Owens (1990). Previous correlations and age assignments (Simon, 1984) were shown to be incorrect, by these authors, based on new biostratigraphical information. In addition since 1990 further palynology has shown that the Tedd Formation is undoubtedly Devonian and that the Lower Devonian age, postulated by Richardson (in Mitchell & Owens, 1990) for the Shanmullagh Formation is too old. Unfortunately there is still no palynological data from the rocks in the Fintona Block located south of the Tempo-Sixmilecross Fault, and their mid-late Devonian age is based solely on the K-Ar wholerock dating of weathered contemporaneous volcanic rocks.
Based on these dates there is apparently no correlation between the rocks of the Fintona Group, in the Fintona Block (Middle- Upper Devonian) and the presumed Lower Devonian Cross Slieve Group of NE Antrim. In addition there are stark contrasts between the sediment types, facies, provenance and origins of the Cross Slieve Group and the restricted suite of sedimentary rocks now assigned to the Tedd and Shanmullagh formations, in particular, of the Fintona Group.
The Fintona Block is divided into two parts by the Tempo- Sixmilecross Fault (Wilson, 1953) and the rocks in each part will be described separately.
(i) North of the Tempo-Sixmilecross Fault-Irvinestown/Tedd Cross Roads Segments
The rocks in these segments occupy the northern part of the Fintona Block, north of the Tempo-Sixmilecross and Killadeas- Seskinore faults and south of the Castle Archdale Fault - Omagh Fault complex. In the southwest adjacent to Lower Lough Erne, the Devonian rocks are faulted against the Lower Carboniferous Tyrone Group and in the northeast, near Carrickmore, rest unconformably on the Tyrone Volcanic Group.
All of the rocks in the Tedd Cross Roads Segment are assigned to the Tedd Formation, and in the Irvinestown Segment to the Shanmullagh Formation. They consist exclusively of red-beds in which there are no conglomerates and very few examples of coarse- grained sandstones. The bulk of the rocks consist of fine-grained fluvial sandstones and siltstones, playa lake mudstones with abundant pedogenic (soil) horizons. Beds of green and dark green mudstones frequently contain miospores of Upper Devonian age and in particular the Frasnian and Lower Famennian Stages. Because of the gross similarity of red-bed lithologies throughout the outcrop of the Shanmullagh Formation and the apparent absence of widespread marker horizons the overall thickness of these rocks is not known. It is however evident that they were deposited in the axial region of the late Devonian sedimentary basin in this part of Northern Ireland in a location far removed from sediment source areas.
(ii) South of the Tempo-Sixmilecross Fault
These rocks of the Fintona Group are bounded on their north side by this fault and are located at least 10km south of the supposed position of the southwesterly extension, in Ireland, of the Highland Boundary Fault. They are divided into three formations and consist of about 3500m of red-beds and volcanic rocks although exposure over much of the outcrop is poor and the possible repetitive effects of concealed faulting should not be discounted.
The basal conglomerate of the lowest formation, the Shanmaghery Sandstone is exposed south of Pomeroy resting unconformably on Llandovery (early Silurian) rocks of that eponymous inlier. It is a thin conglomerate suggesting that the local provenance of the clasts had a subdued topography. Contrast this with the very thick, and consistently very coarse, alluvial fan conglomerates found at the base of, and frequently throughout, older Devonian sequences in NE Antrim and Scotland.
Succeeding volcanoclastic conglomerates of the Gortfinbar Conglomerate Formation were apparently derived largely by the erosion of different centres of contemporaneous volcanic lavas that erupted in the east of the outcrop around Cappagh. Two separate phases of volcanism occur within the formation but importantly, there is no evidence of the development of contemporaneous and diuturnal fault scarps that would have provided a significant supply of clasts from pre-existing non-volcanic rocks.
At the top of the sequence is the Raveagh Sandstone Formation which consists entirely of sandstones but is very poorly exposed.
| Enlander, I., Dempster, M. & Doughty, P., 2024. The Devonian System in N. Ireland - Introduction, County Antrim, Fermanagh, Tyrone, site summary. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/summary.php?item=1147. Accessed on 2024-12-26 |
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