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Earth Science Conservation Review
| River Blackwater, Benburb | Armagh, Tyrone |
| Site number: | 261 | File Number: | H85 |
| ESCC: | EO | ||
| Locality Type: | River bank, River bed | Status: | ASSI |
| Grid Reference: | H802518,H819519 End(s) of linear site, Not Entered | ||
| Google maps: | 54.40892,-6.76484 | ||
| County: | Armagh, Tyrone | Council area: | Armagh City, Banbridge & Craigavon Borough Council |
| Period: | Carboniferous | ||
| Stages: | Visean | ||
| Lithostrat: | Benburb Mudstone Member, Blackstokes Limestone Formation, Blackwater Limestone Formation, Carrickaness Sandstone Formation, Crow Hill Congl & Sst Member, Drumflugh Limestone Member, Glenview Limestone Member, Gorestown Mudstone Member, Island Sandstone Me | ||
| Site Description | |
|---|---|
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| Highlights: | |
About 230m of strata belonging to the Tyrone Group are almost completely exposed in the valley of the River Blackwater at Benburb and can be divided into 4 lithostratigraphical formations which contain fossils of Asbian and Brigantian age. | |
| Description: | |
The deeply incised valley of the River Blackwater at Benburb extends from Milltown in the west to Benburb Bridge in the east. Rock is exposed in natural cliff sections at many points on both sides of the valley but quarrying operations, in particular on the south (Armagh) side of the valley, at and between Blackstokes Bridge (H8125 5185) and Benburb Bridge, reveal more extensive sections in the Maydown Limestone and Blackstokes Limestone formations, in particular. Degradation and partial concealment of these quarries by new vegetation is prevalent but this problem has been counter balanced by recent renovation of the old Ulster Canal, near Milltown, in the process clearing some exposures. (i) MAYDOWN LIMESTONE FORMATION The Maydown Limestone Formation comprises about 126m of dark grey calcareous shales, silty shales, siltstones, silty limestones and thick crinoidal limestones that represent the lowest beds in the Benburb section. The Maydown Formation is superbly exposed at its type section in Maydown Quarry (H8175 5190) and in the cliff section (H817 519) to the west of the quarry. The base of the formation is concealed by drift but is considered to extend up to the Benburb Fault, 40m west of Benburb Bridge. The top of the formation is marked by the abrupt change from the thick limestones of the Maydown Formation to mudstones of the overlying Gorestown Mudstone Member at the base of the succeeding Blackstokes Formation. This is exposed in the abandoned quarry (H8115 5180), 130m SW of Blackstokes Bridge. The sequence becomes progressively more carbonate-rich towards the top and dark silty shales that dominate the lower part of the section, at the east end of Maydown Quarry, are strongly reminiscent of the Benbulben Shale Formation. These are succeeded gradually by a section, in the west end and centre of the quarry, comprising very fossiliferous nodular and argillaceous limestones and blue-grey calcareous silty mudstones in equal proportions (Photograph 102). Within this section, very similar to the Glencar Limestone Formation, are 8m of beds comprising parallel and cross-laminated, fine-grained sandstones, with plant debris at the base, succeeded by virtually unlaminated calcareous sandstones with a capping of hard silty limestone. These are interbedded with grey calcareous shales. The highest beds in the formation are virtually shale-free consisting of dark grey, crinoidal packstones with a somewhat reduced macrofauna, similar to the Dartry Limestone Formation. Fossils are abundant throughout the formation and include bivalves, bryozoa, echinoid and crinoidal debris and corals and brachiopods including the following forms: SOLITARY CORALS: Caninophyllum monense, Palaeosmilia murchisoni, Siphonophyllia benburbensis. COMPOUND CORALS: Lithostrotion aranea, L. portlocki, Siphonodendron martini, S. pauciradiale. BRACHIOPODS: Antiquatonia sp., Dielasma hastatum, Echinoconchus punctatus, E. subelegans, Gigantoproductus sp. maximus gp., Linoprotonia sp., Orthotetoid, Productus sp., Pustula sp., Schizophoria resupinata, Spirifer bisulcatus. This fauna is of Asbian age and the Maydown Limestone is the most likely horizon for the type material of the solitary coral Siphonophyllia benburbensis (Lewis, 1927, p.378). (1) CROW HILL CONGLOMERATE AND SANDSTONE MEMBER This member occurs in the upper part of the Maydown Formation, intercalated in the limestones, and comprises a diverse suite of sediments with a maximum exposed thickness of 5.5m, including cobble-conglomerates, coarse-grained sandstones, micaceous silt- stones and a bioclastic limestone. Its stratigraphical position is at least 12m below the top of the formation since it does not occur in the quarry 130m SE of Blackstokes Bridge, where that thickness of limestone is exposed beneath the Blackstokes Fmn. The Crow Hill Member type section occurs on the steep south bank of the valley, 60m west of Maydown Quarry. The conglomerate horizon (Photograph 104) marking the base of the member here type locality is 3.6m thick. At its base the conglomerate overlies a bed of dark grey shale, with Siphonodendron colonies, with no apparent erosion. Clasts in the conglomerate consist mainly of rounded blocks of grey fossiliferous limestone and rolled colonies of the corals L. aranea, S. martini and S. pauciradiale, up to 0.25m in diameter, set in a calcareous matrix of quartz grains, vein-quartz pebbles, rounded chips of limestone and many shell fragments. A lens of coarse-grained sandstone 0.08m thick and 1m long, with small pebbles of vein-quartz, occurs in the middle of the conglomerate. Pieces of S. benburbensis and large fragments of Psammodus teeth are common in the matrix. Conformably overlying the conglomerate in the type section are about 2m of white and pale grey, cross-laminated, coarse-grained, calcareous, quartz sandstones with abundant fragments of S. martini and brachiopods and complete, though abraded, small specimens of the solitary coral P. murchisoni. To the west in the Benburb section the member consists of finer grained sandstones and siltstones and a variably thick bed of bioclastic limestone. (ii) BLACKSTOKES LIMESTONE FORMATION Although the rocks of this formation are not completely exposed in the Benburb section, the 2 comprising members are estimated to be ~20m thick. At the base, the Gorestown Mudstone Member consists of grey mudstones, with subordinate limestone beds, and the overlying Rookwood Limestone Member consists of bituminous limestones with shale partings. Two sections covering this formation are known; on the Armagh bank of the River Blackwater, in an abandoned quarry (H812 518) and on the Tyrone bank, in the cliff behind the disused Benburb Corn Mill (H812 520). (1) GORESTOWN MUDSTONE MEMBER At the type section in the disused quarry (H8115 5181) 130m SW of Blackstokes Bridge, 5.1m of tough, blocky-bedded mudstones with two thin limestone beds conformably overlie the top limestone bed of the Maydown Formation. All the strata on the south face of the quarry have been baked and indurated by the intrusion of three Tertiary dykes. The deep blue-black and dark grey-green colours of the limestones and mudstones respectively are certainly secondary, but the richly fossiliferous nature of the sediments is still evident despite the contact metamorphism. The mudstones contain an abundance of bivalves, brachiopods, bryozoa, crinoid ossicles, echinoderm plates, ostracods, solitary corals, sponge spicules and trilobites. Of the two limestone beds the lower one consists of 0.2m of hard, grey unfossiliferous, pyritous micrite, while the upper bed comprises 0.5m of very fossiliferous argillaceous biomicrite. The brachiopod assemblage in the latter bed includes: Alitaria panderi, Echinoconchus cf. vetusta, Leptagonia sp., Pleuropugnoides sp., Productina cf. margaritacea, Schizophoria sp. This is indicative of reef facies limestone. A complete specimen of the trilobite Cummingella carringtonensis was also found. In the cliff section on the Tyrone bank of the river the Gorestown Mudstone Member is similarly incompletely exposed but at least 5.8m of alternating limestones and mudstones are present, separated from the overlying Rookwood Member by a 6m gap. These sediments have not been intruded by Tertiary dykes and the grey-brown mudstones contain abundant brachiopods including: Antiquatonia sp., Buxtonia sp., Chonetipustula sp., Dielasma sp., Echinoconchus sp., Eomarginifera cambriensis, Lingula sp., Linoprotonia sp., Megachonetes sp., Orthotetoid The lowest limestone bed in the section is ~2m thick and contains a few large specimens of Gigantoproductus sp., bivalves, crinoid ossicles and ostracods. The uppermost limestone is ~0.7m thick and contains a rich fauna dominated by brachiopods and bivalves. (2) ROOKWOOD LIMESTONE MEMBER At both major exposures of this member, ~8m of limestones and thin shales are exposed however the maximum thickness is probably nearer 11m. The type section is at the NW end of the same disused quarry (H8115 5181) in which the type section of the Gorestown Mudstone is located. Individual limestone beds are up to 0.75m thick and consist of dark grey-brown, bituminous argillaceous biomicrite in which the faunal content is reduced to a few large specimens of Gigantoproductus, ostracods, bivalves with worm tracks in the interbedded shales. The shales are usually <0.01m thick, but thicker examples yield a fauna of fish teeth and scales, rare brachiopods, clisiophylloid and zaphrentoid corals, gastropods and ostracods. (iii) CARRICKANESS SANDSTONE FORMATION This formation consists of at least 60m of sandstones, siltstones and mudstones, which are unfossiliferous except for plant remains and rare bivalves. Several small sections of sandstone attributed to this formation crop out on the steep, wooded south bank of the old Ulster Canal between Milltown House (H8042 5155) and Blackstokes Bridge quarry. Many of the early authors on Irish geology mentioned the occurrence of coal seams near Benburb and Egan (1872) reported, "a few thin beds of coal 3-4 inches in thickness", together with thin beds of ironstone. The sandstones are white or pale grey, fine- to medium-grained quartz sands, cross-laminated with rippled surfaces. Heavy iron- staining imparts a red to orange colouration at many localities. Interbedded with the sandstones are bioturbated and carbonaceous rippled and cross-laminated siltstones. Dark grey to fawn blocky mudstones, with ironstone nodules up to 0.1m long, contain well preserved plant remains. In the quarry (H8055 5167) on the north bank of the River Blackwater, NE of Milltown House, the contact between the top of the sandstone and the basal member of the Blackwater Formation is exposed. This formation is also exposed on the south bank of the Ulster Canal, 300m ESE of Milltown Mill. (iv) BLACKWATER LIMESTONE FORMATION The Blackwater Limestone Formation is about 26m thick and can be divided into 6 members, which are from the base: Tullymore Limestone Member, Glenview Limestone Member, Drumflugh Limestone Member, Benburb Mudstone Member, Island Sandstone Member and the Outlet Limestone Member. A continuous section through the 3 lower members is exposed both on the north bank of the River Blackwater and on the south bank of the canal, ENE of Milltown House. The Benburb Member is exposed in isolation from the other members on both banks of the River Blackwater and the Island and Outlet members are exposed together in the Mill Race at Milltown, just before it enters the River Blackwater. (1) TULLYMORE LIMESTONE MEMBER The type section of these beds is on the Tyrone bank of the River Blackwater in the disused quarry (H8055 5167) 190m NNE of Milltown House, where, at most, 4.06m of thickly bedded, grey to fawn- tinged, medium-grained packstones with thin, discontinuous layers of grey-green micrite rest with marked discontinuity on sandstones of the Carrickaness Formation. Most of the macrofauna are fragmentary but partings of dark grey shale contain broken bryozoa and small brachiopods and the following corals: Dibunophyllum bourtonense, Koninckophyllum sp., Siphonodendron martini, S. pauciradiale. Limestone beds at the base and top of the member contain scattered quartz grains, ooids and cyanobacterial films suggesting proximity to, or deposition in, a very shallow water environment. The following microbiota occur in the lowest and uppermost beds: Basal bed ALGAE: calcisphaeres, Girvanella sp., Koninckopora inflata, stacheiinids. Basal bed FORAMINIFERA: lituotubellid, Neobrunsia sp., Palaeotextularia sp., Paraarchaediscus sp., P. stilus., Pseudoammodiscus sp. Top bed ALGAE: calcisphaeres, Donozella sp., globochaete, stacheiinids. Top bed FORAMINIFERA: Brunsia pseudopulchra, Earlandia sp., Endothyra sp., E. bowmani, E. excellens, E. ex. gr. phrissa, Paraarchaediscus sp. stage angulatus, Plectogyranopsis convexa, Pseudoammodiscus sp. On the south bank of the Ulster Canal (H8068 5156) the member is reduced in thickness to 1.7m and both the upper and lower junctions are clearly exposed. (2) GLENVIEW LIMESTONE MEMBER This member is the most distinctive lithology in the Blackwater Formation. At its type section on the Tyrone bank of the River Blackwater in the disused quarry (H8055 5167), it consists of 2.79m of white-weathering, orange-brown, virtually unbedded and shale-free, medium-grained packstones, conformably overlying the Tullymore Limestone Member. Fossil debris consists mainly of shell fragments and rare pieces of the coral S. martini near the base. The basal bed contain a poor microbiota including the following foraminifera: Archaediscus sp., Earlandia sp., Endothyra ex. gr. phrissa, Paraarchaediscus stilus stage angulatus, Priscella sp. The presence of angulatus stage Archaediscus indicates a post- Holkerian age. The absence of Koninckopora is clearly fortuitous since it does occur at the top of the member and its absence in the basal bed cannot be taken as indicating a Brigantian age. In the type section, the top bed consists of 0.53m of pale orange, medium grainstone which is characterised by stromatolite-like colonies of Chaetetes depressus up to 0.15m high. The microbiota in the top bed of this member, sampled on the south bank of the Ulster Canal (H8055 5157) contains an impoverished fauna of foraminifera including Paraarchaediscus, Palaeotextularia and Koninckopora inflata, indicating Asbian age. (3) DRUMFLUGH LIMESTONE MEMBER The type section of this member is on the south bank of the disused Ulster Canal (H8055 5157) 110m ENE of Milltown House, where 3.35m of strata consist of dark grey calcareous shales, up to 0.12m thick, alternating with pale grey flaggy and rubbly bedded bioturbated, fine-grained packstones. Bioturbation effects are well preserved on the base of the lowest limestone bed as burrows up to 0.06m deep, excavated into a 0.12m thick shale horizon which overlies the Glenview Member (Photograph 106). The other (higher) limestone beds, which are not so deeply burrowed, have thin impersistent partings of bituminous shale. Specimens of the brachiopod Gigantoproductus edelbergensis occur at the base of the lowest limestone bed, with specimens of Siphonodendron pauci- radiale and the trilobite Metaphillipsia sp. Exposures in the stream flowing northwards through Milltown village are richly fossiliferous containing the corals Siphonodendron junceum, S. pauciradiale and 'Caninia' juddi and a brachiopod fauna including Pugilis pugilis, a species which is not known below the late Asbian, and extends into the Brigantian. The Asbian-Brigantian stage boundary was believed by Mitchell and Mitchell (1982) to occur in the Blackwater Limestone Formation between the Tullymore and Drumflugh Limestone members. Based on the presence of the alga Koninckopora, which does not occur in Brigantian and younger strata, in the top bed of the Glenview Member, all of these strata are Asbian. Some elements of the macrofauna recorded in the Drumflugh Member, and listed above, for example 'C.' juddi, G. edelbergensis and P. pugilis were formerly regarded as Brigantian indicators but are now known to range down into the late Asbian. Early Brigantian foraminifera are non- diagnostic and the extinction of Koninckopora, which is recorded virtually on a global scale, is a more satisfactory indicator. The basal and top beds of the Drumflugh Member contain the following microbiota: Basal bed ALGAE: calcisphaeres, Donezella sp., globochaete, stacheiinids. Basal bed FORAMINIFERA: Archaediscus cf. karreri, Brunsia sp., Earlandia sp., Endostaffella sp., Endothyra bowmani, E. excelleus, cf. Howchinia sp., Palaeonubecularia sp., Paraarchaediscus stilus, Tetrataxis sp, Valvullinella sp. Top bed ALGAE: Asphaltina sp, calcisphaeres, Spumisalebra sp., stacheiinids, Ungardella sp. Top bed FORAMINIFERA: Earlandia sp., Endothyra sp., E. ex. gr. phrissa, Howchinia sp., Paraarchaediscus sp., Priscella sp., Pseudoammodiscus sp., Tetrataxis sp., Valvullinella sp. In both of these assemblages the presence of Howchinia suggests an age no older than late Asbian. More significant however is the persistent absence, in both of these assemblages and in a limestone sample from the succeeding Benburb Mudstone Member, of Koninckopora. This, and the absence of Brigantian indicators makes an earliest Brigantian age most likely and the base of this stage thus coincides with the base of the Drumflugh Limestone Member. (4) BENBURB LIMESTONE MEMBER The type section occurs at the western end of the most complete section of the Blackwater Formation on the south bank of the Ulster Canal, 90m ENE of Milltown House, in a north-flowing rivulet (H8050 5158). Here the member consists of 1.4m of highly fossiliferous, dark grey to brown crinoidal mudstones with a 0.19m limestone bed, comprising medium-grained packstone with fenestral cavities and peloids, in the middle of the type section. Bivalves, including Posidonia becheri, brachiopods such as Antiquatonia?, Echinoconchus?, Linoprotonia sp., Pustula?, Schizophoria sp., chonetoids, ostracods and bryozoa are abundant together with a few specimens of Lingula sp., fish teeth and scales. An impoverished microbiota in this limestone bed includes the algae Nostocites sp., Shartmophycus fusus, cf. Ungardella sp. and calcisphaeres and the foraminifera Howchinia sp., Planoarchaediscus sp. and Tetrataxis sp. (5) ISLAND SANDSTONE MEMBER Both this member and the overlying Outlet Member are exposed only in the overgrown extension of the Mill Race (H8041 5165) 100m north of Milltown House, on the south bank of the narrow, curved island. Numerous loose blocks of both members are present on the north bank of the island at the edge of the River Blackwater. About 1m of pale purple, fine-grained, unfossiliferous, calcareous sandstone and thinly laminated siltstones comprise this member. (6) OUTLET LIMESTONE MEMBER The highest Carboniferous beds exposed in the Benburb section consist of ~1.3m of pale yellow and fawn coloured, thinly laminated, dolomitic, algal limestone with algal nodules, flattened worm tracks, scattered crinoid ossicles and fragments of bivalves, brachiopods, bryozoa, ostracods, plant debris and fish scales and teeth. Thin beds of purple mudstone with productoid fragments, pale pink and yellow-tinged, dolomitic, algal limestone and green, red and white, soft muddy sandstone are also found but all the exposures are below the level of the river and are rarely accessible. Trilobites found in the pink algal limestones include Archaegonus (A.) cf. aequalis and A. (A.) cf. antecedens. These species are typical of the crenistria Zone (P1a) in Germany but a slightly younger age seems most likely. | |
| Interpretation: | |
The 230m of Tyrone Group strata that crop out in the Benburb section are of Asbian and early Brigantian age. In terms of their correlation with late Vis‚an strata in west Fermanagh, they equate with rocks ranging from the top of the Benbulben Shale Formation to the basal part of the Carraun Shale Formation. The thickness of the strata at Benburb is only about 1/3 of the thickness of strata of the same age in Fermanagh. Nevertheless there are unmistakable similarities between the two areas in their changing patterns of sedimentation. The gradual upward disappearance of the shale fraction in the Maydown Formation at Benburb can be matched in the sedimentary change between the Benbulben Shale, Glencar Limestone and Dartry Limestone formations. No sedimentary equivalents of the same age as the Meenymore Formation occur at Benburb but it is possible that the clastic facies of the Crow Hill Conglomerate and Sandstone Member may be representative of the same regressive period in west Fermanagh when the largely intertidal and supratidal sediments of the Meenymore Formation were deposited. The possible contemporaneous deposition of the Carrickaness Sandstone Formation at Benburb and the Glenade Sandstone Formation in Fermanagh (inter alia) has already been discussed in Subarea-5, under the section describing the Carrickaness Sandstone Formation. At Benburb, the top of the sandstone is succeeded by predominantly marine sediments (Blackwater Formation) of late Asbian and Brigantian age and in this respect mirrors precisely the age of the Bellavally Formation, which succeeds the Glenade Sandstone in Fermanagh. Sedimentologically however these two formations are quite different. | |
| Conclusions: | |
The Lower Carboniferous sequence exposed at Benburb provides one of the most complete and highly fossiliferous successions of strata in south Co. Tyrone and Co. Armagh. It is divided into four formations and constituent members, all of which have type sections in this section. | |
| Notes: | |
For information on other sites in Subarea-6 see Key Site 1179 - Subarea-6; Armagh-Benburb. | |
| Keywords | |
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| Rocks: | Conglomerate, Ironstone, Limestone, Mudstone, Sand, Sandstone |
| Fossil Groups: | Bivalve, Brachiopod, Coral, Crinoid, Echinoderm, Fish, Foraminifera, Gastropod, Microfossil, Microfossils, Plant, Polyzoan, Sponge, Trilobite |
| Structures: | cross-bedding, ripple marks |
| Relations: | Marine sediments |
| Measurements | |||||
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| Length: | ~230 m | Width: | Height: | ||
| Depth: | Area: | ||||
| Access | |
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| Approach: | The deeply incised valley of the River Blackwater at Benburb extends from Milltown, in the west to Benburb Bridge, in the east. |
| References (6) | |||
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| Davis, J.W., 1883. On the fossil fishes of the Carboniferous Limestone Series of Great Britain. Scientific Transactions of the Royal Dublin Society, vol. 1, pp.327-600. | |||
| Egan, F.W., 1873. Explanatory memoir to accompany Sheet 47 of the maps of the Geological Survey of Ireland, including the country around Armagh. Memoirs of the Geological Survey of Ireland, Dublin. | |||
| Lewis, H.P., 1927. Caninia cylindrica Scouler and other large Caninias from the Carboniferous Limestone of Ireland. Scientific Proceedings of the Royal Dublin Society, vol. 18, pp.373-382. | |||
| McCoy, F., 1844. A synopsis of the characteristic Carboniferous Limestone fossils of Ireland., pp.1-207 University Press, Dublin. | |||
| Portlock, J.E., 1843. Report on the Geology of the County of Londonderry and of parts of Tyrone and Fermanagh. HMSO, Dublin. | |||
| 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. | |||
| Record type: | ESCR report | ||
| Entered by: | Doughty, P.J. | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. River Blackwater, Benburb, County Armagh, Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=261. Accessed on 2026-11-08. |
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