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Earth Science Conservation Review
| Craigbardahessiagh and Bardahessiagh River - Lower Palaeozoic | Tyrone |
| Site number: | 29 | File Number: | H77 |
| ESCC: | EO | ||
| Locality Type: | Quarry (disused), Stream section | Status: | ASSI |
| Grid Reference: | H72057385,H72557360,H725733 Corners of site, Not Entered | ||
| Google maps: | 54.60774,-6.88573 | ||
| County: | Tyrone | Council area: | Mid Ulster District Council |
| Period: | Silurian, Ordovician | ||
| Stages: | Ashgill, Caradoc, Cautleyan, Hirnantian, Llandovery, Longvillian, Marshbrookian, Rawtheyan | ||
| Lithostrat: | Bardahessiagh Formation, Killey Bridge Formation, Tirnaskea Formation | ||
| Site Description | |
|---|---|
| Highlights: | |
A highly fossiliferous sequence of Ordovician shelf or platformal sediments lies on the southern flank of Craigbardahessiagh. Although poorly exposed, very diverse shelly faunas have been collected here which are of considerable importance in palaeogeographic reconstructions for the Ordovician. The sequence also records a long continuous subsidence history for this platformal area from the Caradoc to Llandovery series. | |
| Introduction: | |
The only occurrence of fossiliferous Lower Palaeozoic platformal sediments in Northern Ireland is a small inlier to the east of Pomeroy, Co. Tyrone. The northern section of the inlier, around Craigbardahessiagh, contains Ordovician rocks ranging from the Caradoc to Hirnantian stage, and these are comformably overlain by Lower Silurian (Llandovery) deposits. Rich Ordovician shelly faunas from Pomeroy were first described by Portlock (1837, 1843), and subsequently there have been numerous publications dealing with the various fossil groups, but little work on the structure and stratigraphy of the area. Exposure at the present time is limited to a stream section through the upper part of the sequence. However, the site also includes an area of farmland where older rocks are known to subcrop, and an historically important quarry site (now infilled) from which large collections of Caradocian fossils were made in the last century (Tunnicliff, 1980). | |
| Description: | |
The Palaeozoic sediments in this area have an average dip of 42- 45 deg. SSW. The oldest are of Middle Ordovician age, and comprise the Bardahessiagh Formation of Fearnsides et al. 1907. This formation has an estimated thickness of 160m (Mitchell, 1977), its base unconformably overlying granodiorites and basic intrusives of the Tyrone Igneous Complex which form the summit of Craigbardahessiagh. Most of the Bardahessiagh Formation is very poorly known, although evidence from drift blocks and small temporary exposures suggests that it consists predominantly of fine-grained muddy sandstones, with minor fine conglomerate horizons towards its base. Palaeontological evidence indicates that no part is older than mid Caradoc. In 1992 the upper part of the formation was temporarily exposed by trenching, which enabled the recognition of three members within the formation (not yet formally established). The Quarry Sandstone Member (QSM) roughly corresponds to the Bardahessiagh Formation of previous authors, with the 'flagstone quarry' site, which formerly exposed around 4m of medium-grained micaceous sandstone, taken as the type locality for the member. 4-6m of sandstone (unseen) lie between the top-most quarried horizons and the sandstones at the base of the trenched sequence. The top 2.5m of the QSM consists of poorly-bedded, fine calcareous sandstone. These are usually micaceous, and have an increasing proportion of silt and mud grade material towards the top. Primary sedimentary structures are largely absent, but bioturbated horizons are common. Lenses and impersistant beds of richly fossiliferous `rottenstone' occur, often with the fossil material contained in them normally graded. The mud-rich top of the QSM grades into the Well Field Mudstone Member (WFM). This member is approximately 3m thick and consists of mainly unfossiliferous grey micaceous mudstone, which typically weathers into small shaley flakes. In the middle of the WFM a cross-bedded siltstone, 5cm to 15cm thick, makes a prominent marker horizon. The top 4cm of the member is more silty and massive, containing fine sandy stringers which reveal high levels of bioturbation. A band of richly fossiliferous decalcified nodules occurs approximately 3cm below the top of the member. Overlying the WFM is a 70cm thick unit consisting of highly micaceous sandstones and siltstones, the Mitchell Siltstone Member (MSM). The internal structure is complex and highly variable, although three consistent units can be distinguished over short distances. The lower unit consists of very micaceous flaggy sandstone, with a fossiliferous lower bed containing rip- up clasts of grey mudstone, and having an erosive base. The middle unit is characterised by its very high fossil content. It is composed of distinctive micaceous siltstones which typically weather to a very soft and friable orange-brown 'rottenstone'. Much of the unit is cross-bedded, and lenses of large fossil debris and mud clasts are concentrated in the bottom of cross sets towards the base of the unit. Rare drift blocks which Mitchell (1977) referred to as 'junction beds' are thought to have been derived from this unit. The upper unit consists of very micaceous flaggy sandstone with less abundant fossils. No in situ material was exposed above the MSM, but it is probable that at the type locality, about 8m of grey fissile mudstone like that of the WFM is present, before the Caradoc sequence is truncated by younger Ashgill shales thrust over it from the south. The lowest Ashgill sediments present in this area are slightly calcarous grey shales of the Killey Bridge Formation (KBF). A vertical thickness of approximately 230m is sporadically exposed in the bed and banks of the Bardahessiagh River. On faunal evidence, the upper part of the KBF, of Rawthyan age, is represented at the top of the sequence, but most of the KBF here is of Cautleyan age. Sparsely distributed fossils are present through the section, but occur most abundantly in a brachiopod- rich grey sandstone bed a few metres below the top of the formation (Mitchell, 1977). The KBF is conformably overlain by darker, carbonaceous, mudstones of the Tirnaskea Formation (TF). It has a very much more restricted fauna of Hirnantian aspect, and in this area is about 8m thick. A conformable Ordovician-Silurian boundary section can be examined here, the lowest Silurian sediments being grey-blue shales with a sparse lower Llandovery graptolite fauna. The Ashgill-Llandovery sequence is repeated within the site area on either side of a NNW-SSE trending normal fault with a downthrow of 120m to the west. Faunas: The Bardahessiagh Formation (BF) contains very rich Middle Ordovician (Caradocian) shelf or platformal shelly faunas dominated by brachiopods. In the QSM, brachiopods are accompanied by trilobites, bivalves, gastropods, bryozoans, corals, and cephalopods (in order of increasing rarity). The brachiopod- trilobite faunas are characteristic of relatively shallow water inner shelf/platformal benthic assemblages. In terms of their biogeographic associations, the BF faunas are most similar to those of eastern north America and the Girvan district of Scotland. The QSM correlates best with the lower Ardwell Group at Girvan (Mitchell, 1977; J.K. Ingham, pers. comm.), and with the middle part of the Mohawk series in the Appalachians. The top 2m of the QSM shows a marked faunal change. As well as all of the inner platformal elements, which occur in the under- lying sandstones, deeper water elements such as the trilobite Tretaspis, and the brachiopod Sericoidea, appear. The WFM is largely unfossiliferous, although the thin siltstone unit towards the middle of the member contains abundant small brachiopods and a distinctive deep water fauna of small trilobites (eg. Yumenaspis, Ampyxina, and Robergiella). The top 30cm of the member seems to have an even deeper water fauna, with the appearance of the trilobites Porterfieldia, Kainella, and Cryptolithus. Cryptolithus tesselatus Green, is a biostrati- graphically significant trilobite which permits a direct correlation of this horizon and the overlying MSM, with the Craighead Limestone of Girvan and the lower part of the Martinsburg Shale in the Appalachians. Sparse graptolites also occur in the top Well Field Mudstones, but no age-diagnostic species have been recognised. The KBF has a sparse fauna of brachiopods, trilobites, and molluscs, which is very different from that of the BF. It consists of a mixture of faunal elements (genera) which during the Middle Ordovician were restricted to different faunal provinces. Species of the trinucleid trilobites Tretaspis and Cryptolithus allow the correlation of the upper KBF with the lower Drummuck Group of the Girvan district (J.K. Ingham, pers. comm.), and therefore establishes a Rawthyan (late Ashgill) age for these beds. The fauna of the Tirnaskea Formation is very restricted, consisting of just two brachiopod species, two trilobites, a few molluscs, and graptolites. The basal Silurian shales above contain only graptolites. | |
| Importance: | |
National Geology: This is the only site in Northern Ireland with a middle Ordovician shelf/platformal setting. It clearly establishes the Scoto-Appalachian affinities of the province to the north of the Iapetus suture, and reveals a relatively continuous and long subsidence history for this area through the Upper Ordovician. The site is historically important and the faunas have been intensively studied. International Geology: The site includes the type localities of more than 50 species (mostly mid Caradoc from the BF). The recently excavated sequence at the top of the BF contains one of the most diverse Caradoc faunas known, and will yield significant information on benthic depth communities and the evolution of deeper water shelly communities. | |
| Interpretation: | |
The Bardahessiagh Formation was deposited as a transgressive sequence of shelf or platformal sediments, over an eroded surface of the Tyrone Igneous Complex, during the mid-Caradoc. Whilst the Quarry Sandstone Member records mainly inner platformal sedimentation and faunas, the overlying WFM indicates that substantial deeping (subsidence) was occurring, allowing the development of outer platformal facies and faunas. There is abundant evidence of downslope sediment mass-transport throughout the formation. The most remarkable example of this is the MSM, which was deposited by turbidity currents. Although laid down in an outer platformal facies, the very diverse fauna includes inner platformal benthic communities mixed together with a range of deeper water communities which were picked up by turbidity currents as they rushed downslope. It is likely that sedimentation continued through the late Caradoc and early Ashgill, but no rocks of these ages occur in this area because of late Caledonian thrusting. Upper Ordovician sediments occurring here were deposited through the Cautleyan, Rawthyan, and Hirnantian stages. The Killey Bridge Formation was deposited in a relatively deep water outer shelf/platformal setting, again with some input from upslope via turbidity currents. Globally, the Hirnantian stage is associated with a fall in relative sea level due to the late Ordovician glaciation. In this area, however, deepening was controlled by tectonic subsidence, so resulting in Hirnantian environments and faunas more usually seen in continential slope settings. Water depth at this time was probably in excess of 400m, and there is no evidence of eustatic shallowing. The overlying graptolitic shale facies of the Llandovery Little River Group are certainly indicative of very deep water slope facies deposition. Faunas of the BF are most similar to those of the Scoto- Appalachian faunal province, and have low similarity with coeval faunas in southern Ireland or the Anglo-Welsh area. There are strong palaeontological and stratigraphic reasons for considering that the Pomeroy and Girvan districts share a unique tectonic history through the Ordovican, and together constitute a discrete sub-terrane within the present-day patchwork of crustal fragments which were swept together during closure of the Iapetus Ocean. | |
| Conclusions: | |
The unique Ordovician stratigraphic and palaeontological record preserved here reveals a long history of shelf/platformal subsidence from the mid-Caradoc into the Llandovery. Strong initial biographic affinities with the Scoto-Appalachian province gradually become diluted as the Iapetus narrowed and migration routes were created. The fine gradation of depth association benthic communities ranging from inner platform associations to the upper slope facies is remarkably well exhibited in this small area, and the extremely diverse fossil assemblages are of international importance. Unfortunately, a lack of exposure means that the Caradoc rocks cannot presently be examined in the field, and there is still considerable scope for research on these Lower Palaeozoic rocks of the Pomeroy Inlier. | |
| Notes: | |
The full range of ESCR site relating to the Lower Palaeozoic are as follows: Outer Ards Stratigraphic, Palaeontological and Structural sites Coalpit Bay – Lower Palaeozoic (record no. 1350) Helen’s Bay – Grey Point – Lower Palaeozoic (record no. 1351) Kearney Point – Knockinelder Bay – Lower Palaeozoic (record no. 1352) Orlock Bridge – Lower Palaeozoic (record no. 1353) Tieveshilly – Carrstown – Lower Palaeozoic (record no. 1354) Orlock Bridge - Structural (record no. 652) Whiskin Rocks - Structural (record no. 653) White House Port, Cloghy - Structural (record no. 654) Knockinelder Bay and Kearney Point - Structural (record no. 655) Millin Bay - Structural (record no. 656) Pomeroy area Craigbardahessiagh and Bardahessiagh River – Palaeontology (record no. 29) Lime Hill Farm – Palaeontology (record no. 33) Little River and Slate Quarry Bridge – Palaeontology (record no. 31) | |
| Keywords | |
|---|---|
| Rocks: | Mudstone, Sand, Sandstone, Shale, Siltstone |
| Fossil Groups: | Bivalve, Brachiopod, Coral, Gastropod, Graptolite, Polyzoan, Trilobite |
| Fossil List: | Cryptolithus tesselatus Green |
| Structures: | Caledonian, platform subsidence, sub-terrane, thrust, turbidite |
| Paleoenv: | shelf |
| Access | |
|---|---|
| Approach: | ~3km NE of Pomeroy. |
| Management: | |
Within the site area, three core areas of particular importance have been identified. Core area A is known locally as the Well Field. The site of the 'flagstone quarry' is located in the north east corner, and this may be regarded as the type locality for most, if not all, fossil species described by Portlock (1843) and subsequent workers from the 'Bardahessiagh Formation' (as known prior to 1992). The section exposed by trenching in 1992 runs for 35m south from the quarry site. This will be defined as the type locality for the WFM and MSM of the Bardahessiagh Formation, as well as the type locality for the many new, but still undescribed, species collected in 1992. At present there is no surface exposure of the Bardahessiagh Formation. Whilst the lithology of the WFM and MSM precludes the possibility of creating any permenant exposure (the sequence trenched was deeply disturbed by late glacial cryoturbation), it would be highly desirable to create a new representative exposure in the QSM within the site area, perhaps in the field directly north of the Well Field. Core areas B and C consist of the late Ordovician - basal Silurian sections along the Bardahessiagh River. They include the river bed, bank exposures, and 5m of farmland on either bank. Current exposure is patchy, and could certainly be improved by digging back the stream banks in places where they have slumped and become overgrown. Livestock access to the stream is the major contributor to bank degradation and consequent loss of geological information. Fencing some exposures might be desirable. Occassional geological sampling will not prove a threat to the site's scientific interest, but regular collecting would rapidly degrade the existing exposures. | |
| Threats: | Over-sampling; livestock access to the stream. |
| Uses | |
| Uses: | Pasturing. |
| Educ. Level: | Undergraduate / Postgraduate |
| References (22) | |||
|---|---|---|---|
| Baily, W.H., 1878. Explanatory memoir to accompany sheet 34 of the maps of the Geological Survey of Ireland. Memoirs of the Geological Survey of Great Britain, Dublin & London. | |||
| Blake, J.F., 1882. A Monograph of the British fossil Cephalopoda, Pt. 1, Introduction and Silurian species. Palaeontographical Society Monographs, pp.248, plates 31. | |||
| Cocks, L.R.M., Holland, C.H., Rickards, R.B. & Strachen, I., 1971. A correlation of Silurian rocks in the British Isles. Journal of the Geological Society of London, vol. 127, pp.103-136. | |||
| Cripps, A.P., 1988. A new species of stem-group Chordate from the Upper Ordovician of Northern Ireland. Palaeontology, vol. 31, pp.1053-1078. | |||
| Elles, G.L., 1942. The type of Monograptus sedgwickii (Portlock). Geological Magazine, vol. 79, pp.31-32. | |||
| Evans, D.H., 1993. The cephalopod fauna of the Killey Bridge Formation (Ordovician, Ashgill), Pomeroy, Co. Tyrone . Irish Journal of Earth Sciences, vol. 12, pp.155-189. | |||
| Fearnsides, W.G., Elles, G.L. & Smith, B., 1907. The Lower Palaeozoic rocks of Pomeroy. Proceedings of the Royal Irish Academy, vol. 26(B), pp.97-128. | |||
| Harper, J.C., 1948. The Ordovician and Silurian rocks of Ireland. Proceedings of the Liverpool Geological Society, vol. 20, pp.48-67. | |||
| Hutt, J.E., 1968. A redescription of the Llandoverian monograptid 'Graptolithus' tenuis Portlock, 1843. Geological Magazine, vol. 105, pp.251-255. | |||
| Jaekel, O.N., 1899. Stammersgeschichte der Pelmatozoen. 1, thecoidea and cystoidea., pp.442, plates 18 Julius Springer, Berlin. | |||
| Larcom, T., 1837. Ordnance Survey of the county of Londonderry. Vol. 1, Notices i., pp.3-6, plates 3. | |||
| McCoy, F., 1846. A synopsis of the Silurian fossils of Ireland., pp.72, plates 5, Dublin. | |||
| Mitchell, W.I., 1977. The Ordovician Brachiopoda from Pomeroy, Co. Tyrone. Palaeontographical Society Monographs, pp.138, plates 28. | |||
| Palmer, D. & Rickards, R.B., 1981. Graptolites., pp.182 Boydell Press, Woodridge. | |||
| Portlock, J.E., 1843. Report on the Geology of the County of Londonderry and of parts of Tyrone and Fermanagh. HMSO, Dublin. | |||
| Reed, F.R.C., 1920-1921. A monograph of the British Ordovician and Silurian Bellerphonetacea. Palaeontographical Society Monographs, pp.92, plates 13. | |||
| Reed, F.R.C., 1952. Revision of certain Ordovician fossils from County Tyrone. Proceedings of the Royal Irish Academy, vol. 55,section B, pp.31-136. | |||
| Tripp, R.P., 1957. The trilobite Encrinurus multisegmentatus (Portlock) and allied Middle and Upper Ordovician species. Palaeontology, vol. 1, pp.60-72. | |||
| Tunnicliff, S.P., 1978. Types of the Ordovician trilobites Celtencrinurus multisegmentatus (Portlock) and Cryptolithus latus (Portlock). Palaeontology, vol. 21, pp.455-458. | |||
| Tunnicliff, S.P., 1980. A catalogue of the Lower Palaeozoic fossils in the collection of Major J. E. Portlock., pp.111 Ulster Museum, Belfast. | |||
| Tunnicliff, S.P., 1982. A revision of late Ordovician bivalves from Pomeroy, Co. Tyrone, Ireland. Palaeontology, vol. 25, pp.43-88. | |||
| Tunnicliff, S.P., 1983. The oldest known nowakiid (Tentaculitoidae). Palaeontology, vol. 26, pp.851-854. | |||
| Map(s): | See the 1:50,000 O.S. Sheet 13 (The Sperrins). | ||
| Record type: | Environment Serv. GIS, ESCR report | ||
| Entered by: | Crowther, P.R. | ||
| Last updated: | 12 Jun 2004 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Craigbardahessiagh and Bardahessiagh River - Lower Palaeozoic, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=29. Accessed on 2026-11-06. |
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