Devonian, Northern Ireland

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Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.

W I Mitchell

Position of the continents in the Early Devonian (c. 400Ma (1)). (P947811)
Distribution of Devonian and putative Devonian rocks in Northern Ireland. (P947812)
Geological setting of the Fintona Group. (P947813)
Subdivisions and age of the Fintona Group in Northern Ireland. (P947922)

In Ireland and Britain there is evidence that fault-controlled uplift of the Caledonian mountains during the last stages of the Caledonian Orogeny in Early Devonian times was accompanied by rapid subsidence of non-marine sedimentary basins (P947811). Reconstructions envisage a mountain range of Himalayan proportions extending from western Ireland to Scotland, composed largely of metamorphic rocks located to the north of those basins [1]. From its southern margin, which was bounded by the active Highland Boundary Fault, river systems, comparable in size to the present-day Ganges-Brahmaputra system of the Indian subcontinent, transported sediment southwards. The conglomerate and coarse-grained sandstone was deposited on alluvial fans banked up against the mountain front with finer sediment deposited to the south on alluvial floodplains and in lakes in the axial region of the basins. This reconstruction is applicable to western Ireland and Scotland [2] but is only partly supported in Northern Ireland.

Because of the absence of macrofossils in rocks of ‘Lower’ and ‘Upper Old Red Sandstone’ facies in Northern Ireland, their assignment to the Devonian was based on lithological comparisons with successions in the Midland Valley of Scotland. When those rocks were finally dated to the Early Devonian and Late Devonian-early Carboniferous respectively, the ages were applied indiscriminately to all ‘Devonian’ rocks in Northern Ireland (P947812). In 1938, the discovery of a fish (‘pteraspid’) fragment near Lisbellaw [3] was heralded as an affirmation of the long suspected Early Devonian age for the ‘Old Red Sandstone’ facies rocks. However, samples of green mudstone from the western half of the Fintona Block yielded Early Devonian to late Carboniferous miospore assemblages [4]. They demonstrated a Carboniferous age for most of the conglomerates in the Fintona Block and prompted a revision of Devonian palaeogeography. Nevertheless, there was still no unequivocal evidence of Early Devonian strata in the Fintona Block and even the celebrated fish locality subsequently proved to be early Carboniferous (late Viséan) [5]. Intriguingly, a reassessment of the fish fragment identified it as an indeterminate pteraspid of Early Devonian affinity and ranging from uppermost Silurian (Prídolí) to Middle Devonian (Givetian) [6]. The specimen was probably reworked either from the Shanmullagh Formation (P947813) or from Devonian rocks long since eroded or now concealed.

Based on palynological data, the Shanmullagh Formation was originally assigned a Late Devonian (Frasnian-Famennian) age [7][8] although there was still uncertainty as the palynomorph assemblages lacked Late Devonian zonal taxa. A further review [9] of the palynology now indicates an age ranging from the latest Lockhovian to early late Emsian (P947922).

Stratigraphy
Fintona Group
Fintona Block
Tedd Cross Roads Segment
Irvinestown Segment
Composition and age of the palynomorph assemblages
South of the Tempo-Sixmilecross Fault
Cross Slieve Group and Red Arch Formation, Northeast Co. Antrim

References

  1. Simon, J B, and Bluck, B J. 1982. Palaeodrainage of the southern margin of the Caledonian mountain chain in the northern British Isles. Transactions of the Royal Society of Edinburgh, 73, 11–15.
  2. Graham, J R, Richardson, J B, and Clayton, G. 1983. Age and significance of the Old Red Sandstone around Clew Bay, NW Ireland. Transactions of the Royal Society of Edinburgh, 73, 245–49.
  3. Harper, J C, and Hartley, J J. 1938. The Silurian Inlier of Lisbellaw, County Fermanagh, with a note on the age of the Fintona beds. Proceedings of the Royal Irish Academy, 45B, 73–87.
  4. Mitchell, W I, and Owens, B. 1990. The geology of the western part of the Fintona Block, Northern Ireland-evolution of Carboniferous basins. Geological Magazine, 127, 407–26.
  5. Mitchell, W. I. and Owens, B. 1990. The geology of the western part of the Fintona Block, Northern Ireland-evolution of Carboniferous basins. Geological Magazine, 127, 407-26.
  6. Blieck, A. 1991. Reappraisal of the heterostracans (Agnathan vertebrates) of Northern Ireland. Irish Journal of Earth Sciences, 11, 65–69.
  7. Geological Survey of Northern Ireland, 1996. Kesh, Northern Ireland Sheet 32. Solid Geology. 1:50 000. (Keyworth, Nottingham: British Geological Survey).
  8. Geological Survey of Northern Ireland, 1996. Omagh, Northern Ireland Sheet 33. Solid Geology. 1:50 000. (Keyworth, Nottingham: British Geological Survey).
  9. Stephenson, M H, and Mitchell, W I. 2002. Definitive new palynological evidence for the early Devonian age of the Fintona Group, Northern Ireland. Irish Journal of Earth Sciences, 20, 41–52.