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Earth Science Conservation Review
| Middletown Burn, Culvacullion Townland | Tyrone |
| Site number: | 283 | File Number: | H48 |
| ESCC: | EO | ||
| Locality Type: | Stream section | Status: | ESCR |
| Grid Reference: | H4987 1 km square | ||
| Google maps: | 54.72906,-7.23941 | ||
| County: | Tyrone | Council area: | Fermanagh & Omagh District Council |
| Period: | Carboniferous | ||
| Stages: | Tournasian | ||
| Lithostrat: | Omagh Sandstone Group | ||
| Site Description | |
|---|---|
| Highlights: | |
Fossiliferous exposures of the Omagh Sandstone Group occur above the basal unconformity. | |
| Description: | |
Middletown Burn is located on the south side of Slievebeg and Slievemore about 2km north of Gortin. The stream is one of a group of tributaries of the Owenkillew River draining the SW slopes of the Sperrin Mountains. In the stream located above (NE) of the Culvacullion Road meta- morphic rocks are exposed about 350m upstream of the roadbridge (H4915 8787). The basal contact of the Carboniferous sequence is concealed. Succeeding strata are inclined southwards at ~18 deg. and consist of thick beds of grey-green sandstones (0.25-0.3m) and thin purple-red mudstones which pass up into thin bedded sandstones and finally grey mudstones and siltstones with flaser- bedded sandstone laminae. Strata exposed between these beds and a point 100m upstream of the road bridge are predominantly grey to dark grey blocky and laminated mudstones, micaceous siltstones and thin to flaser bedded pale grey-fawn medium to fine-grained sandstones. Occasional thin beds (0.05m) of laminated stromatolitic limestone occur in the mudstones. Desiccation cracks are common in most lithologies and on the top surface of the algal (cyanobacterial) limestones. Load casts are a feature of the base of sandstones overlying mudstones. The apparent absence of a basal conglomerate was noted by Sheridan (1972, p.22) who commented that rocks of Upper Old Red Sandstone facies, i.e. red-bed sandstones and conglomerates were poorly represented or lacking in the Newtownstewart Outlier. Two samples of grey, block and laminated mudstone from exposures at the SW end of the Middletown Burn section, 100m NE of the roadbridge contained moderately abundant and moderately preserved miospores including the following species: Acanthotriletes macrogaleatus, Anaplanisporites sp., Aurorospora sp., Calamospora sp., Convolutispora permixta, Crassispora erychera, Dictyotriletes trivialis, Endoculeospora gradzinskii, Knoxisporites literatus, Kraeuselisporites hibernicus, Lophozonotriletes tuberosus, cf. Lycospora sp., Punctatisporites spp., Rugospora polyptycha, Schopfites claviger, S. delicatus, Spelaeotriletes balteatus, S. crustatus, S. resolutus, Umbonatisporites distinctus, U. abstrusus, Velamisporites magnus, V. perinatus and Verrucosisporites irregularis. Based on the composition of the miospore assemblages obtained from Middletown Burn and other localities in the Newtownstewart Outlier the samples have been assigned to the Carboniferous (Tournaisian) spore zones described by Higgs et al. (1988). With the exception of S. claviger (CM Biozone) the index taxa of these zones are not common in these assemblages and the samples from Middletown Burn are presumed to be not older than CM Biozone. In some of the assemblages are very rare specimens of Lycospora sp. or cf. Lycospora sp. The appearance of Lycospora marks the base of the Pu Biozone and in the Newtownstewart Outlier may indicate that the samples are from close to the CM-Pu zonal boundary. | |
| Interpretation: | |
Because there is no detailed information available on the Carboniferous sequence that occurs in the Newtownstewart Outlier the true thickness of these strata (Omagh Sandstone Group) is not known. Earlier estimates suggested a thickness of ~1200-1500m. Given a maximum outcrop width of ~4km for the outlier, an average southerly dip of 20 deg. and an absence of strike faulting then this estimate may not be unreasonable. As such the thickness of these largely non-marine sediments of late Tournaisian age is far in excess of the thickness of contemporaneous sequences elsewhere in N. Ireland and cannot be explained in terms of regional subsidence associated with progressively northward back-stepping fault scarps and a marine transgression. Indeed the incidence of marine sediments anywhere in this sequence is very difficult to prove and, at present, it is considered more likely that the sediments were deposited in a complex lacustrine system during a period of maximum, fault-bounded extensional subsidence within an asymmetric half-graben basin. Measurement of planar cross- laminations at a few localities seems to indicate sediment flow to the north although much more data are required. Thick sequences of late Tournaisian non-marine sediments are virtually unknown onshore in Ireland and are not recorded offshore to the west of Ireland. They are however very well represented in the Maritimes Basin in Atlantic Canada where comparable facies associations are well documented and very thick sequences comprise up to 3000m of Upper Devonian-Tournaisian clastic rocks (Hamblin, 1992; Hamblin & Rust, 1989) in both onshore and offshore locations. It could also be argued that because of the clear similarities between the sediments in the Newtownstewart Outlier and the Horton Group in NE Canada, which consists of the basal non-marine clastics of the Maritimes Basin, that it might be more appropriate to apply this name to the Irish rocks rather than consign them to the Omagh Sandstone Group. | |
| Conclusions: | |
The Newtownstewart Outlier contains a thick sequence of non- marine clastic sedimentary rocks that are unique in Northern Ireland. The absence of a basal unit of sandy and conglomeratic Upper Old Red Sandstone facies, and of sediments showing some indication of a marine influence, is unusual. Miospore assemblages determine an age of deposition in the late Tournaisian, probably straddling the CM-Pu zonal boundary. | |
| Notes: | |
For more information on Subarea-8 see Key Site 1181 - Subarea-8; Newtownstewart Outlier. | |
| Keywords | |
|---|---|
| Rocks: | Algal limestone, Limestone, Mudstone, Sand, Sandstone |
| Fossil Groups: | Miospore, Miospores |
| Fossil List: | Lycospora sp. , Schopfites claviger |
| Structures: | cross-bedding, dessication cracks |
| Relations: | Deltaic sediments, Lacustrine sediments |
| Measurements | |||||
|---|---|---|---|---|---|
| Length: | 250 m | Width: | Height: | ||
| Depth: | Area: | ||||
| Access | |
|---|---|
| Approach: | Middletown Burn is located on the south side of Slievebeg and Slievemore about 2km north of Gortin. |
| References (5) | |||
|---|---|---|---|
| Hamblin, A.P., 1992. Half-graben lacustrine sedimentary rocks of the Lower Carboniferous Strathlorne Formation, Horton Group, Cape Breton Island, Nova Scotia, Canada. Sedimentology, vol. 39, pp.263-284. | |||
| Hamblin, A.P. & Rust, B.R., 1989. Tecton-sedimentary analysis of alternate-polarity half-graben basin-fill successions: Late Devonian-Early Carboniferous Horton Group, Cape Breton Island, Nova Scotia. Basin Research, vol. 2, pp.239-255. | |||
| Higgs, K., Clayton, G. & 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. | |||
| Sheridan, D.J.R., 1972. Upper Old Red Sandstone and Lower Carboniferous of the Slieve Beagh Syncline and its setting in the northwest Carboniferous basin, Ireland. Special Papers of the Geological Survey of Ireland, vol. 2, pp.1-120. | |||
| Simpson, I.M., 1955. The Lower Carboniferous Stratigraphy of the Omagh Syncline, Northern Ireland. Quarterly Journal of the Geological Society of London, vol. 110, pp.391-408. | |||
| Record type: | ESCR report | ||
| Entered by: | Doughty, P.J. | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Middletown Burn, Culvacullion Townland, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=283. Accessed on 2026-11-06. |
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