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Earth Science Conservation Review
| Carboniferous Subarea-4; Fintona Block | Fermanagh, Tyrone |
| Site number: | 1177 | ||
| Locality Type: | Various | ||
| Grid Reference: | H4461 Approximate | ||
| Google maps: | 54.49598,-7.32092 | ||
| County: | Fermanagh, Tyrone | Council area: | Fermanagh & Omagh District Council |
| Period: | Carboniferous | ||
| Stages: | Asbian, Brigantian, Namurian, Pendelian, Visean, Westphalian | ||
| Lithostrat: | Ballinamallard Mudstone Formation, Ballyreagh Conglomerate Formation, Drumlish Conglomerate Formation, Greenan Sandstone Formation, Topped Mountain Sandstone Formation, Tullanaglare Mudstone Formation | ||
| Site Description | |
|---|---|
| Description: | |
INTRODUCTION Subarea-4 of the Carboniferous outcrop in N. Ireland is divided into 2 parts. Purple-red conglomerates, sandstones and mudstones that crop out in the Pomeroy-Fintona-Irvinestown area have been traditionally assigned to the Lower Old Red Sandstone. The principal outcrop, which covers an area of about 700km2, has been termed the Fintona Block and the rocks of Lower Old Red Sandstone facies, collectively defined as the Fintona Group (House et al., 1977). Palaeontological evidence for the age of the Fintona Group was first reported by Harper and Hartley (1938) who found two fragmentary fossil fish, one of them resembling Pteraspis rostrata, at a locality 2km north of Lisbellaw. The Fintona Block was formerly divided into two parts by the Tempo-Sixmilecross Fault (Wilson, 1953). Detailed mapping by the Geological Survey of Northern Ireland has provided new stratigraphical and structural data and the occurrence of large areas of Carboniferous rocks in the Fintona Block was first recognised by Mitchell and Owens (1990). On the basis of the new palaeontological and stratigraphical information 4 segments were recognised in the Fintona Block each bounded by major faults. From north to south the segments are: 1: Milltown Segment | Carboniferous (late Vis‚an-early | Silesian and late Silesian) age. 2: Tedd Cross Roads Segment | Devonian (Frasnian-early | Famennian) age. 3: Irvinestown Segment | Devonian (Frasnian-Famennian) age. 4: Tempo-Lisbellaw Segment | Carboniferous (late Vis‚an-early | Silesian) age. As a result of this work Mitchell and Owens revised the nomenclature of lithostratigraphical units recognised in the Fintona Block as follows. CARBONIFEROUS MILLTOWN SEGMENT | TEMPO-LISBELLAW SEGMENT Slievebane Group | Kilskeery Group ------------------------------|---------------------------------- A. Late Silesian |A. Late Visean-early Silesian - Drumlish Conglomerate Fmn.| - Ballinamallard Mudstone Fmn. - Tullanaglare Mudstone Fmn.| - Ballyreagh Conglomerate Fmn. | - Topped Mountain Sandstone Fmn. B. Late Vis‚an-early Silesian | - Greenan Sandstone Fmn. | DEVONIAN The Devonian rocks in the Tedd Cross Roads and Irvinestown segments are assigned to the Tedd Formation and Shanmullagh Formation respectively. All Devonian rocks, including those in the outcrop south of the Tempo-Sixmilecross Fault, are assigned to the Fintona Group and will not be discussed here. Thus the description of the Carboniferous rocks in the Fintona Block only includes the strata that occur in the Milltown and Tempo- Lisbellaw segments. TEMPO-LISBELLAW SEGMENT The Tempo-Lisbellaw Segment occupies the southern part of the Fintona Block north of the Tempo-Sixmilecross Fault. The rocks within the segment form a roughly triangular outcrop bounded by faults, which are to the south, the Tempo-Sixmilecross Fault, to the west the Lower Lough Erne Fault and to the north the Killadeas-Seskinore Fault (Mitchell & Owens, 1990). All the rocks that occur within the Tempo-Lisbellaw Segment are Carboniferous and range in age from the late Vis‚an to early Namurian. They are divided into 3 formations that are defined as the Kilskeery Group and have been dated on the basis of miospore assemblages. Neither the stratigraphical base nor top of the Kilskeery Group is preserved, being everywhere excised by the bounding faults. I - KILSKEERY GROUP The Kilskeery Group is composed exclusively of red-bed sedimentary rocks of Carboniferous age that crop out only in the Tempo-Lisbellaw Segment. Formerly referred to the Fintona Group by all previous geologists (GSNI, 1982; Simon, 1984) these rocks were considered to be typical of the Lower Old Red Sandstone facies and, on the basis of an identifiction of a fossil fish fragment as Pteraspis cf. rostrata, were assigned a Lower Devonian age (Harper & Hartley, 1938). A re-examination of this simple, identifiable, broken specimen by Blieck (1991) confirmed its pteraspid affinity and this places an uppermost Silurian (Pridoli) to Middle Devonian (Givetian) age range on the specimen. The age of the sandstone from which the pteraspid fragment was recovered was subsequently determined as late Vis‚an to early Namurian by Mitchell and Owens (1990) on the basis of a sparse, but diagnostic, miospore assemblage contained in flakes of green mudstone in the sandstone. In order to unravel the rather confused picture of Devonian and Carboniferous lithostratigraphical names that were applied to these rocks, Mitchell and Owens erected a new nomenclature for the Tempo-Lisbellaw Segment. Three formations were recognised; the basal Topped Mountain Sandstone Formation (formerly referred to the Shanmaghery Sandstone by the GSNI, 1982), the Ballyreagh Conglomerate Formation (formerly referred to the Gortfinbar Conglomerate by the GSNI, 1982) and at the top the Ballinamallard Mudstone Formation (GSNI, 1982). The three formations were then included, as the constituent units, in the Kilskeery Group. The terms Shanmaghery Sandstone and Gortfinbar Conglomerate formations are restricted to rocks of probable middle-upper Devonian age that crop out in the area of the Fintona Block south of the Tempo-Sixmilecross Fault, and are referred to the Fintona Group. The thickness of the Kilskeery Group is estimated to be ~2200m although there is poor control on the thickness of the Topped Mountain Sandstone and, more particularly, the Ballinamallard Mudstone Formation. (i) TOPPED MOUNTAIN SANDSTONE FORMATION The outcrop of the Topped Mountain Sandstone Formation is confined to the southeastern margin of the Tempo-Lisbellaw Segment. It is truncated on its western and southeastern sides by the Lower Lough Erne and Tempo-Sixmilecross faults respectively and to the NW is overlain conformably and gradationally by the Ballyreagh Conglomerate Formation. At its SW end the formation outcrop is ~4km wide but this narrows gradually to a width of ~2km to the NE over a distance of 17km before the outcrop is terminated, in the vicinity of Barr Church (H39 56), by a NW-SE trending fault. The formation consists of almost 1000m of cross-laminated, fine- to coarse-grained, grey feldspathic sandstones, stained red-brown when weathered, interbedded with greyish-red and green mudstones with pedogenic carbonate nodules and mud-flake conglomerates. Mitchell and Owens (1990, p.412) selected a stratotype area for this formation covering the summit of Topped Mountain (H311 457) and the spur trending NNE from the summit cairn. Those authors also nominated a stratotype area for the succeeding Ballyreagh Conglomerate Formation on the south and east flanks of Ballyreagh Mountain (H31 49). During the reconnaisance survey conducted for this project it became apparent very quickly that Largy Quarry (H298 475), although located within the outcrop, and near the top of the Topped Mountain Formation, exhibits an interbedded sequence of sandstones and conglomerates that are quite typical of these two formations. It is thus proposed to nominate Largy Quarry as the principal site covering both of these formations. Within the Topped Mountain Sandstone Formation are two thin units that were named by the GSNI (1982) as the Coolcran Conglomerate Member and the Glen Calcareous Member. The age of the Topped Mountain Formation is based on the occurrence of miospore assemblages. The oldest recorded assemblage was obtained from the same locality where Harper and Hartley recorded the pteraspid fragment. This occurs in the townland of Lisreagh (H2987 4290), 1.9km NNW of Lisbellaw. Small clasts of green mudstone contained a limited miospore assemblage of late Vis‚an - early Namurian age including: Bascaudaspora canipa Densosporites cf. brevispinosus Lycospora pusilla A more diverse and more diagnostic miospore assemblage was also recovered from the Topped Mountain Sandstone at the NE end of the formation outcrop (H396 570), 6km SW of Fintona. Pale grey-green siltstones with laminae of dark green mudstone containing the following miospores occur ~130m below the top of the formation. This association suggests an assignment to the VF or NC Miospore Zones, equivalent to the Brigantian and Pendleian stages. Bascaudaspora canipa Densosporites sp. Lycospora pusilla Planisporites sp. Punctatisporites nitidus Schulzospora campyloptera S. elongata Tripartites vetustus For site specific information see; Key Site 243 - Largy Quarry. (1) COOLCRAN CONGLOMERATE MEMBER The Coolcran Conglomerate Member occurs ~200m below the top of the Topped Mountain Sandstone Formation and consists of ~40m of interbedded pebbly sandstones, sandstones and conglomerates. The importance of the stratotype section is considerably enhanced by the occurrence there of a thin mudstone bed that has yielded a miospore assemblage diagnostic of the late Vis‚an. For stratotype section site specific information see; Key Site 244 - Glen-Coolcran townlands stream. (2) GLEN CALCAREOUS MEMBER The outcrop of this member is confined to two adjacent stream sections (H335 510, H339 510), some 350m apart, in Glen Townland, 2.1km SW of Brougher Mountain. It occurs ~40m below the top of the formation and consists of some 20m of pale and medium grey calcareous siltstones with thin, fine-grained sandstones and mudstones. This member is unusual, in the Topped Mountain Formation, in having a consistently grey colour when all other lithologies are either purple-red or are tinged with purple and red. Lack of exposure prevented this member being traced beyond the stratotype area and no miospores were recovered, despite sampling several different mudstone horizons. (ii) BALLYREAGH CONGLOMERATE FORMATION Within the Tempo-Lisbellaw Segment the outcrop of the Ballyreagh Conglomerate Formation parallels the trace of the underlying Topped Mountain Formation, on its NW side. The outcrop width of the conglomerate decreases gradually to the NE and the formation thickness is ~350m. Lithologies typical of this formation occur in Largy Quarry (H299 476), interbedded with sandstones, at a stratigraphical horizon near to the top of the Topped Mountain Formation. It appears that the lower and upper boundaries of the Ballyreagh Conglomerate Formation are gradational. At the base, the sandstone/siltstone dominated sequence of the Topped Mountain Formation becomes interbedded with pebble layers and conglomerates, and at the top of the Ballyreagh Conglomerate, there is a gradual increase in the proportion of sandstones, siltstones and ultimately mudstones that characterise the Ballinamallard Formation. No palynological evidence for the age of the conglomerate is available. (iii) BALLINAMALLARD MUDSTONE FORMATION The highest unit of the Kilskeery Group consists of ~1000m of greyish-red mudstones and siltstones, with subordinate sandstones, of the Ballinamallard Mudstone Formation. The formation outcrop occupies the NW part of the Tempo-Lisbellaw Segment and exposures are relatively scarce in this low-lying land located NW of the ridge of high ground between Largy Mountain, Ballyreagh Mountain and Brougher Mountain. The GSNI (1982) recognised 3 separate sandstone members within the Ballinamallard Formation; the Coa Sandstone Member at the base, the Errington Sandstone Member in the middle and near the top, the Corkill Sandstone Member. The recognition of these members is based on an increase in the proportion of sandstone beds, although mudstones still occur as part of their succession. The stratotype sections for the Ballinamallard Formation and the three sandstone Members are as follows: -Ballinamallard Formation- The Ballinamallard River (H268 529), 600m ENE of Ballinamallard upstream of the disused railway bridge, Co. Fermanagh. -Coa Sandstone Member- West of Breagho Bridge (H266 476), 5km south of Ballinamallard. -Errington Sandstone Member- At Errington (H3125 5340), 5km east of Ballinamallard. -Corkill Sandstone Member- The Ballinamallard River SW of Corkill (H293 546), Co. Tyrone, 3.5km NE of Ballinamallard. The COA MEMBER consists of ~150m of hard, grey to reddish-brown sandstones with thin mudstones with cross-laminations and desiccation cracks. The ERRINGTON MEMBER is ~190m thick and consists of relatively soft fine-grained sandstone with thick beds of greyish-red mudstone. Sandstone beds are dominant in the upper and lower parts of the member, while the middle 60m of strata consist mainly of mudstones. From the basal beds of the Errington Member at Ardgart (H2535 4810) a sparse miospore assemblage was recovered from a 0.01m bed of green mudstone. Miospores recovered include the following forms indicating a post-Tournaisian age. Discernisporites sp. Lycosporapusilla Punctatisporites sp. Cyclogranisporites sp. Granulatisporites sp. Knoxisporites rotatus The CORKILL MEMBER consists of ~55m of interbedded sandstones and mudstones, in equal proportions and has gradational lower and upper contacts. The highest beds of the Ballinamallard Formation have been excised by the Killadeas-Seskinore Fault and the complete age range of this formation and the Kilskeery Group, into the Namurian, is not known. For site specific information see; Key Site 246 - Makenny Townland stream, near Hollow House. | |
| Notes: | |
MILLTOWN SEGMENT Within this segment are 3 lithostratigraphical formations. Sites representing the stratotype sections of the Tullanaglare and Drumlish formations occur within this segment. The stratotype of the Greenan Sandstone Formation is however located to the north of the Milltown Segment on the south side of Greenan Mountain. (i) GREENAN SANDSTONE FORMATION The outcrop of the Greenan Sandstone Formation occurs at the very northern edge of the Fintona Block, immediately south of the faults bordering its northern margin, namely the Castle Archdale Fault and the Omagh Thrust. The most westerly exposures of the formation occur at the junction of the Crockraver and Castle Archdale faults and the outcrop extend for over 25km to the NE, the final exposure being located in the Glencurry Burn, NE of Omagh, between the Tirquin Road bridge (H492 747) and the A505, 850m to the south at H495 738. The formation was initially defined by Simon (1984) who considered it to rest unconformably on conglomerates, presumed to be of Devonian age, of the Drumlish Formation and to young northwards towards the Castle Archdale Fault. Though undated, Simon noted a similarity with rocks of Upper Old Red Sandstone facies, and probable late Devonian or early Carboniferous age, in the Midland Valley of Scotland. The recent resurvey of the Kesh-Omagh area by the Geological Survey of Northern Ireland identified the unconformity between the basal breccia of the Greenan Sandstone Formation and metamorphic basement in the stratotype section (Key Site 247) on the south side of Greenan Mountain, north of the Castle Archdale Fault. Though the breccia can be traced for only 0.8km to the SW of the stratotype, sandstones, which form the main part of the formation, crop out along the entire length of the Milltown Segment, south of the Castle Archdale Fault. The basal breccia is ~30m thick and the remainder of the formation consists of at least 500m of sandstones with minor siltstones and mudstones. Miospores recovered from the Greenan Sandstone Formation are of late Vis‚an to early Silesian age and this formation is unequivocally older than the Drumlish Conglomerate Formation. For site specific information see; Key Site 247 - Greenan Mountain stream. II - SLIEVEBANE GROUP The Slievebane Group is composed mainly of clastic sedimentary rocks of Westphalian age, that crop out in the Milltown Segment. New biostratigraphical evidence (Mitchell & Owens, 1990) demonstrated that Simon's (1984) tentative correlation of the Drumlish Formation with sequences in other parts of the Fintona Block was incorrect, as was his interpretation of the stratigraph- ical relationship between the Greenan and Drumlish formations. The Slievebane Group consists of about 1100m of predominantly conglomeratic and arenaceous sediments. Simon (1984) recognised the Drumlish Conglomerate Formation and concluded that these strata were tightly folded into a tight NE-SW trending anticline and overlain unconformably by the Greenan Sandstone Formation. However it is now apparent that all strata of the Slievebane Group in the Milltown Segment young southwards and that the steep inclination and overturning of these strata at its northern edge occurred during the Variscan Orogeny (early Permian) in response to repeated southerly-directed thrusting on the Omagh Fault when these rocks were located in the footwall of that, and other faults. Northeast of the Milltown Segment, the outcrop of the Slievebane Group is confined to very small fault-bounded lenticles caught up in the Omagh Fault zone, SW of Omagh. Two formations are recognised: the basal Tullanaglare Mudstone Formation (Mitchell & Owens, 1990) overlain by the Drumlish Conglomerate Formation (Simon, 1984). Although the outcrop of the former formation is limited to a few localities, it contains diagnostic miospore assemblages. In contrast the conglomerates of the latter formation are very well exposed, in particular forming a virtually drift-free ridge up to 500m wide between Raw Lough and Tullanaguiggy, but they contain very few lithologies suitable for palynological analysis and yield limited miospore assemblages. Neither the base nor the top of the sequence of rocks comprising the Slievebane Group is exposed, being everywhere excised by faults. (ii) TULLANAGLARE MUDSTONE FORMATION The outcrop of the Tullanaglare Mudstone Formation is confined to the Milltown Segment and has a strike length of 10km. Within the outcrop exposures are very rare. However the occurrence of fresh float is considered to indicate the persistence of this formation, in particular as the dominantly grey lithologies are "sandwiched" between red sandstones of the Greenan Formation, to the north, and grey-purple conglomerates of the Drumlish Formation, to the south. The existence of this formation was recognised by Mitchell and Owens (1990) who also recovered diagnostic miospore assemblages of Langsettian and Duckmantian (Westphalian A and B) age. For site specific information see; Key Site 249 - Parkhill Lough stream. (iii) DRUMLISH CONGLOMERATE FORMATION The southern part of the Milltown Segment is occupied by the outcrop of the Drumlish Conglomerate Formation. Its outcrop extends from near Lough Bresk in the SW (H20 60) for about 22km to near Clanabogan (H40 69), SW of Omagh. The base of the formation, resting on the thick calcrete member at the top of the Tullanaglare Formation is exposed at 3 localities. Everywhere, the top of the Drumlish Formation is excised by the Cloghfin Fault and the strata are juxtaposed against Devonian fine-grained red-beds. The existence of the Drumlish Formation was recognised by Simon (1984) who proposed a tentative correlation with sandstones and conglomerates, south of the Tempo-Sixmilecross Fault, that are undoubtedly of Devonian age and are included in the Fintona Group. This correlation was shown to be incorrect by Mitchell and Owens (1990) who determined its Carboniferous age, its position within the Phanerozoic sequence of the Fintona Block and the fact that its closest correlatives occur in NE Canada and not in the British Isles. The Drumlish Formation comprises about 1000m of volcaniclastic conglomerates and coarse-grained sandstones that were deposited by braided streams in the proximal part of an alluvial fan (Simon, 1984). Sedimentary structures such as cross-laminations, erosion surfaces, steep-sided channels and lenticular bed forms are common. Evidence of overall upward fining is seen in the gradual decrease in clast size, increase in the proportion of sand matrix and the upward transition from clast-supported conglomerates to matrix-supported, pebbly coarse sandstones. Clasts consist of vein quartz, metamorphic rocks including quartzite and semi-pelite and granite. Clasts of fresh, vesicular volcanic rocks, up to 0.5m diameter, are very common in the conglomerate, often in a matrix composed largely of angular and rounded fragments of weathered rhyolite. The size and relative abundance of these volcanic clasts, in the lower part of the Drumlish Formation, compared with metamorphic lithologies suggests that they originated in a source area close to the basin margin, in which successive lava flows replenished the supply of fresh debris to the alluvial fans. The age of the Drumlish Conglomerate Formation is not known precisely. Sparse miospore assemblages were recovered from two horizons in the conglomerate. The lower horizon, which occurs about 30m above the base of the formation, consists of 1-7mm of green-blue, gritty clay and yields Crassispora kosankei and ? Laevigatosporites medius which indicate a Silesian age. Miospores, recovered from a film of green, muddy and sandy siltstone coating the base of a thick conglomerate bed 280m above the base of the formation include Laevigatosporites desmoinesensis, indicating a late Namurian-Westphalian age. For site specific information see; Key Site 250 - Straduff Quarry. ------------------------------------------------------------------ For more information on the Carboniferous System in N. Ireland see; Key Site 1171 - The Carboniferous System in N. Ireland - Introduction. Key Site 1172 - Carboniferous Biostratigraphy - Overview. Key Site 1173 - Carboniferous Palaeogeography of N. Ireland. | |
| Keywords | |
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| Rocks: | Conglomerate, Mudstone, Sand, Sandstone |
| Fossil Groups: | Miospore |
| Structures: | dessication cracks, fault, ripple marks |
| References (6) | |||
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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. | |||
| Harper, J.C. & 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. | |||
| House, M.R., Richardson, J.B., Chaloner, W.G., Allen, J.R.L., Holland, C.H. & Westol, T.S., 1977. A correlation of Devonian rocks in the British Isles. Geological Society of London Special Report, numb. 7, pp.1-110. | |||
| Mitchell, W.I. & 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. | |||
| Wilson, H.E., 1953. The petrography of the Old Red Sandstone rocks of the north of Ireland. Proceedings of the Royal Irish Academy, vol. 55, pp.283-320. | |||
| Map(s): | GSNI (1982) 1:50,000 Solid Geology Of Sheet 45 (Enniskillen); O.S. 1:50,000 Sheet 18 (Enniskillen). | ||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Carboniferous Subarea-4; Fintona Block, County Fermanagh, Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=1177. Accessed on 2026-11-05. |
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