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Earth Science Conservation Review
| Tircrevan Burn - Palaeontology | Londonderry |
| Site number: | 120 | ||
| ESCC: | EO | ||
| Locality Type: | Stream section | Status: | ASSI |
| Grid Reference: | C701326 Centroid | ||
| Google maps: | 55.13609,-6.90085 | ||
| County: | Londonderry | Council area: | Causeway Coast & Glens Borough Council |
| Period: | Cretaceous | ||
| Stages: | Cenomanian | ||
| Lithostrat: | Cretaceous Basal Conglomerate, Hibernian Greensands Formation | ||
| Site Description | |
|---|---|
| Highlights: | |
Tircrevan Burn is the only site on the Londonderry Shelf to expose the Cretaceous basal conglomerate in direct unconformable contact with Lower Lias clays. The 'bone bed' at the base of the local Cretaceous succession is rich in reworked vertebrate remains, though these have yet to be investigated. | |
| Introduction: | |
Tircrevan Burn (Grid ref C/701326) is the largest of several minor streams descending the north side of Binevenagh, 10 km north of Limavady, Co. Derry. In the course of its descent it is incised into massive lavas of the Upper Basalt Formation and the Upper Cretaceous and Lower Jurassic rocks which underlie them. Little has been written about this locality. It was described briefly by Portlock (1843) and, more recently, by Hancock (1961) and by Reid (1971). Bazley, Brandon and Arthurs (1997) briefly mentioned the palaeontological significance of the Cretaceous basal conglomerate. The site was included on the general map and ribbon diagram of Fletcher (1977) although he otherwise does not specifically mention the locality. The most recent account of the site is the ESCR Report on Mesozoic stratigraphy (see site 'Tircrevan Burn - Mesozoic stratigraphy'). | |
| Description: | |
The course of Tircrevan Burn down the scarp of Binevenagh is very steep, descending some 300 metres over a distance of little more than 2 kilometres. Landslips are a major feature of the area, with large masses of competent lithologies, the Antrim Basalts and the Ulster White Limestone, moving downslope on the less competent mudstones of the Lower Lias. Blockfalls from the basalt cliffs also have contributed to substantial scree development lower on the slopes. Consequently the succession is much disturbed and useful exposures are confined to what are best interpreted as very large slumped blocks rather than in situ strata. One such section, some 10 metres in total thickness, was described by Portlock (1843; pp. 108-109) although he did not cite thicknesses of individual units. Nonetheless he commented upon the presence of reworked Lower Lias fossils in the basal Cretaceous 'Greensand', notably the bivalve Gryphaea arcuata (=incurva), alongside typical Cretaceous fossils including the teeth of the chimaeroid Oxyrhina. Hancock (1961) briefly described the basal part of the Cretaceous succession at Tircrevan Burn, recording a brown-coloured bone bed 2 to 3 inches (5-8 cm) overlying the Lias clay and succeeded by 30 inches (75 cm) of sandy glauconitic chalk. The bone bed he described as being "largely made up of reworked Lias with a little quartz and glauconite", concluding that the 'bones' were rolled fish teeth from the Lias. He stated that this lower division had yielded no macrofossils other than derived Liassic fossils in the basal 'bone bed'. Reid (1971) mentioned the "undated basal bed at Tircrevan Burn" as possibly being a correlative of pre-early Senonian deposits identified in the Murlough Bay area. Bazley et al. (1997) describe the basal unit of the Hibernian Greensand as comprising fossiliferous, calcareous brown glauconitic sandstones resting on a basal 'bone bed' containing rolled small teeth and bones together with reworked Gryphaea arcuata and crinoid debris derived from the underlying Lower Lias. They also noted diminutive gastropods and bivalves, typically 1-3 mm across, which they considered could be either derived from the Lower Lias or could be of Cretaceous age. A visit to the site in September 1997 revealed the jumbled and chaotic nature of the succession exposed in the Burn. Several large slumped masses of Ulster White Limestone are exposed in the banks of the Tircrevan Burn about 200 metres downstream of its confluence with a smaller stream near the corner of the Forestry plantation. About 20 metres downstream of an obvious chalk 'portal', on the east side of a small pool and boulder cascade, a small patch of Lias clay was exposed abutting against a tilted block of glauconitic chalk dipping southwards at about 35 degrees. The precise nature of the unconformity with the Lower Lias beneath could not be seen and the main exposure was somewhat obscured by algae, lichens and bryophytes. However, at least 3 metres of bioturbated and glauconitic calcareous sandstone could be seen and was found to be fairly rich in rolled vertebrate remains. Although the presence of a 'bone bed' has been highlighted in most previous accounts of the section, no details of the contained vertebrate fauna are available other than the mention by Portlock (1843) of the teeth of Oxyrhina, a Cretaceous chimaeroid. Furthermore, although a diverse Lias fauna from the site is held in the collections of the Geological Survey at Keyworth, this does not include any vertebrate material from the site. Dr. A. Brandon (BGS, Keyworth) recalled that the basal 'bone bed' was rich in fish teeth (A. Brandon pers. comm., March 1999). The only specimen from Tircrevan Burn in the collections of the Ulster Museum is a pair of robust crustacean chelae from creamy glauconitic sandstones from near the base of the Cretaceous succession. Tircrevan Burn appears now to be the only site on the western flank of the Antrim Plateau to expose the basal beds of the Cretaceous succession in Northern Ireland. Both Hancock (1961) and Reid (1971) described a succession on Keady Mountain in which typical hard Ulster White Limestone passes downwards into glauconitic sand, which is underlain by a conglomeratic basal bed containing numerous specimens of the belemnite Actinocamax verus. This conglomerate was said by Hancock (1961) to rest on the green, secondarily reduced, top of the predominantly red Mercia Mudstone Group beneath. However, only the upper part of the Ulster White Limestone is currently exposed in the now disused quarry on Keady Mountain and there are no other significant exposures elsewhere. Further south, on Benbradagh, exposures of the Ulster White Limestone are more extensive but none extend down into the basal beds. Comments by Portlock (1843) that "on the face of Benbradagh next Dungiven, it is from 12 to 25 feet of visible thickness" implies that at that time too the basal beds of the Ulster White Limestone were not exposed there. A full measured section through the entire Jurassic and Cretaceous succession in Tircrevan Burn is contained in the ESCR report by Legg (1997) (see site 'Tircrevan Burn - Mesozoic stratigraphy'). | |
| Importance: | |
Tircrevan Burn is of national importance as the only site in Northern Ireland at which a Cretaceous basal conglomerate contains abundant reworked macrovertebrate remains. | |
| Interpretation: | |
Conglomerates containing reworked Jurassic fossils are a significant element of the Cretaceous succession in Northern Ireland. They are most strikingly developed across the Highland Border Ridge, exemplified by the magnificent exposures at Murlough Bay, but similar conglomerates are also found at the base of the succession elsewhere. Tircrevan Burn is located close to the eastern margin of the Londonderry Shelf, a Cretaceous 'high' analogous to the Highland Border Ridge. The Ulster White Limestone Formation in this area is highly condensed by comparison with the succession in the North Antrim Basin to the east, with several members absent altogether (Fletcher 1977, fig. 8). According to Fletcher (1977, fig. 8) the base of the Cretaceous succession, in which the 'bone bed' occurs, is represented by the Glauconitic Sands Member of the Hibernian Greensands Formation, considered to be of Cenomanian age. The composition of the reworked fauna within the basal conglomerate differs significantly from that at Murlough Bay. The commonest identified fossils are Gryphaea arcuata, a thick-shelled bivalve resistant to prolonged erosion, and isocrinid columnals. Similarly, much of the vertebrate material comprises rolled fish teeth, which also are more resistant to erosion than other parts of the skeleton. These suggest a fairly prolonged period of erosion prior to deposition of the main Cretaceous succession. All of the derived material appears to be local and derived from the Lower Lias clays beneath the unconformity or is of unspecified Cretaceous age. Indeed, there is a significant faunal similarity between the reworked material in the basal bone bed and that of the underlying metre or so of the Lower Lias. Legg (1997) describes the uppermost two units of the Lower Lias as being very fossiliferous, with both Gryphaea arcuata and crinoid columnals mentioned. The richness of the reworked fauna may well be a direct reflection of the fossiliferous nature of the underlying sediments rather than a consequence of some long-term concentrative mechanism. This local derivation of material is fairly typical of basal Cretaceous conglomerates in Northern Ireland although the conspicuous presence of vertebrate material is rather atypical and may indicate that the Lower Lias beneath is fairly rich in vertebrate remains, itself perhaps due to reduced levels of sedimentation in early Jurassic times. However, many of these basal Cretaceous units have not been sampled in any detail and, as has been demonstrated recently for the Hibernian Greensands at Minnis North, vertebrate remains may be significantly more abundant in these pre-Ulster White Limestone units than has hitherto been suspected. | |
| Conclusions: | |
Tircrevan Burn is the only locality on the Londonderry Shelf which exposes the basal conglomerate of the local Cretaceous succession directly overlying Lower Lias clays. It contains abundant reworked early Jurassic fossils, including a relative abundance of vertebrate material which has yet to be fully documented. All of the derived fauna appears to be local in origin, contrasting with the obviously 'exotic' origin for much of the reworked early Jurassic material in the basal conglomerate at Murlough Bay. | |
| Notes: | |
For general information and references on the Palaeontology of Northern Ireland see Key Site 109 - Palaeontological sites of N. Ireland - Introduction For other site specific information and references on the Palaeontology of Northern Ireland see Key Site 110 - Ballycastle Pellet Chalk, Key Site 111 - Ballypalady Plant Bed, Key Site 112 - Carrick Lough, Bunnahone Lough and the Sillees River, Key Site 113 - Faragandoo, Key Site 114 - Minnis North Mudflow, Key Site 115 - Moyola River, Key Site 116 - Mullynaskeagh Clay Pit, Key Site 117 - Murlough Bay, Key Site 118 - Portnaloub, Key Site 119 - Sillees River, Glenasheevar, and Key Site 121 - Tullyconnell. | |
| Keywords | |
|---|---|
| Rocks: | Bone-bed, Conglomerate, Glauconitic sandstone, Sand, Sandstone |
| Fossil Groups: | Bivalve, Crinoid, Fish |
| Fossil List: | Gryphaea arcuata |
| Paleoenv: | sea-floor 'high' |
| Access | |
|---|---|
| Management: | |
The site lies within a wooded and deeply incised stream valley prone to erosive undercutting of the banks, mass movement and obscurement by vegetation. The extent of the critical exposure is, therefore, quite likely to vary with these factors. There is no easy solution to maintaining the exposure in a clear state, although it may be necessary to clear it artificially at some future date. | |
| Threats: | Collapse of stream banks; obscurement by vegetation. |
| Uses | |
| Uses: | The steepness and instability of the stream gully around the site render it unsuitable for agriculture and forestry. |
| References (6) | |||
|---|---|---|---|
| Bazley, R.A.B., Brandon, A. & Arthurs, J.W., 1997. Geology of the Country around Limavady and Londonderry GSNI Technical Report 97/1. | |||
| Fletcher, T.P., 1977. Lithostratigraphy of the Chalk (Ulster White Limestone Formation) in Northern Ireland. Report of the Institute of Geological Sciences, vol. 77/14. | |||
| Hancock, J.M., 1961. The Cretaceous System in Northern Ireland. Quarterly Journal of the Geological Society of London, vol. 117, pp.11-36. | |||
| Legg, I., 1997. Mesozoic stratigraphy in Northern Ireland (Unpublished Report) Earth Science Conservation Review. | |||
| Portlock, J.E., 1843. Report on the Geology of the County of Londonderry and of parts of Tyrone and Fermanagh. HMSO, Dublin. | |||
| Reid, R.E.H., 1971. The Cretaceous rocks of north-eastern Ireland. Irish Naturalists Journal, vol. 1, pp.105-129. | |||
| Record type: | ESCR report | Recorded by: | Michael J. Simms, Department of Geology, Ulster Mu |
| Entered by: | E M Porter | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Tircrevan Burn - Palaeontology, County Londonderry, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=120. Accessed on 2026-11-07. |
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