| Site Description |
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| Highlights: |
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| At the northern margin of the Fintona Block, in Co. Fermanagh, the Tedd Formation is exposed only in the Tedd Cross Roads Segment. This segment is one of four in the area north of the Tempo-Sixmilecross Fault (TSF) and is bounded by faults on all sides. The rocks of the Tedd Formation are the oldest Devonian rocks that occur north of the TSF and have been dated by the occurrence of sparse miospore assemblages at one locality in the segment. All the rocks are red-beds. |
| Introduction: |
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| In Counties Tyrone and Fermanagh the outcrop of Devonian rocks is confined to the Fintona Block which is divided into two parts by the Tempo-Sixmilecross Fault. North of this fault the Devonian rocks are assigned to two formations, namely the Tedd Formation of the Tedd Cross Roads Segment and the Shanmullagh Formation of the Irvinestown Segment. Neither the base nor top of the Tedd Formation, nor the top of the Shanmullagh Formation are exposed, being everywhere excised by faults. In the extreme northeast of the Irvinestown Segment, west of Carrickmore, the Devonian rocks rest unconformably on the Tyrone Volcanic Group and although no miospores have been found in these basal beds they should equate with the Tedd Formation. |
| Location: The Tedd Cross Roads Segment is a fault-bounded lenticle some 2km long and with a maximum width of 600m, located about 3-4km north- northeast of Irvinestown, in Co. Fermanagh. |
| Bibliographic Review: Prior to 1990 all the rocks of the Fintona Block were believed to be Devonian and the fault-bounded segments were unrecognised. Mitchell and Owens (1990) recognised the existence of large areas of Carboniferous rocks, of various ages, within the northern part of the Fintona Block, and identified the faulted margins of the Tedd Cross Roads Segment but could not date the rocks therein. Although Mitchell (1992) suggested a Silesian (Westphalian) age for the Tedd Formation on the basis of sedimentological evidence this is now known to be incorrect and the rocks are Devonian. |
| General Geology: The Tedd Formation consists of about 500m of fining upwards red- beds comprising pebbly and coarse-grained sandstones in the lower part of the sequence, becoming thin, very fine-grained sandstones and siltstones and blocky, red-brown mudstones in the top 200m of the formation. Way-up evidence from structures such as desiccation cracks and ripple marks shows that the succession youngs to the south. Because of the high inclination of strata in much of the segment the landscape in that area is dominated by linear ridges expressing the strike of the beds and demonstrating only a thin till cover on bedrock. Despite an intensive search only one horizon of green mudstones was found in the Tedd Formation. |
| Description: |
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| This occurrence of green mudstones within the Tedd Formation is unique in the Tedd Cross Roads Segment. For site specific information see; |
| Key Site 315 - Tedd Cross Roads, Cloghfin. |
| Interpretation: |
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| The rapid disappearance upwards in the Tedd Formation of pebbly and coarse-grained sandstones is a good indication of the low- lying nature and distant location of the source area of this detritus. This conclusion is further supported by the tenuous nature of the basal conglomerate that overlies basement rocks at Carrickmore and for up to 10km to the west. The dominant lithologies in the top 200m of the formation are blocky and laminated mudstones that are clearly of lacustrine origin and were deposited in temporary playa lakes on an alluvial plain. |
| The locality that includes the green mudstone horizon is part of a sequence of rocks that represent a transitional facies between the dominantly sheetflood conditions in the lower part of the sequence and the lacustrine conditions in the upper part. Because of the absence of continuous sections in the Tedd Formation it is not possible to define channel bedforms in the sandstones although it would seem reasonable to assume that this type of flow may have existed in the lower part of the sequence. |
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| For general information and reference list on the Devonian System in N. Ireland see; |
| Key Site 1147 - The Devonian System in N. Ireland - Introduction. |
| References |
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| Blieck, A. 1991: Reappraisal of the heterostracans (agnathan vertebrates ) of Northern Ireland. Irish Journal of Earth Sciences, vol. 11 pt. 1, pp.65-69 |
| Clayton, G., Higgs, K. and Keegan, J. B. 1974: Late Devonian and Early Carboniferous occurrences of the spore genus Emphanisporites McGregor in Southern Ireland. Pollen et Spores, vol. 19 pt. 3, pp.415-425 |
| Harper, J. C. and Hartley, J. J. 1938: The Silurian inlier of Lisbellaw, Co Fermanagh, with a note on the age of the Fintona Beds. Proceedings of the Royal Irish Academy, vol. B45, pp.73-87 |
| Higgs, K., Clayton, G. and Keegan, J. B. 1988: Stratigraphic and systematic palynology of the Tournaisian rocks of Ireland. Special Papers of the Geological Survey of Ireland, numb. 7, pp.1-93 |
| Mitchell, W. I. and Owens, B. 1990: The geology of the western part of the Fintona Block: evolution of Carboniferous basins. Geological Magazine, vol. 127 pt. 5, pp.407-426 |
| Simon, J. B. 1984: Sedimentation and tectonic setting of the lower Old Red Sandstone of the Fintona and Curlew Mountain districts. Irish Journal of Earth Sciences, vol. 6, pp.213-228 |
| Map(s): | GSNI 1:50,000 Sheet 34 (Pomeroy); / GSNI 1:50,000 Sheet 45 (Enniskillen); / GSNI 1:50,000 Sheet 46 (Clogher); / O.S. 1:50,000 Sheet 17 (Lower Lough Erne). |