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Earth Science Conservation Review
| Portnaloub | Antrim |
| Site number: | 118 | ||
| ESCC: | EF | ||
| Locality Type: | Foreshore | Status: | ASSI |
| Grid Reference: | D167431 Centroid | ||
| Google maps: | 55.22158,-6.1662 | ||
| County: | Antrim | Council area: | Causeway Coast & Glens Borough Council |
| Period: | Carboniferous | ||
| Stages: | Visean | ||
| Lithostrat: | Murlough Bay Coal Group | ||
| Site Description | |
|---|---|
| Highlights: | |
The foreshore at Portnaloub exposes sediments deposited in a lacustrine oil-shale environment. Fragmentary fish remains are abundant in the upper part of the sequence whilst lower down articulated fish are preserved in three dimensions within ironstone nodules. | |
| Introduction: | |
To the west of Fair Head the foreshore is obscured for some distance by massive blocks of Tertiary dolerite and of Carboniferous sandstone fallen from the cliffs above. However, at Portnaloub gently dipping rock ledges expose several metres of dark, organic-rich Carboniferous mudstone. Little has been published on this part of the Carboniferous succession other than brief accounts by Wright (1924) and Wilson and Robbie (1966); the latter gives a measured section of the exposed succession (Wilson and Robbie 1966, p. 94). The fossil fish fauna of this part of the succession appears to have gone largely un-noticed although collophane nodules with fish scales are mentioned, as are the ostracod-strewn bedding planes. | |
| Description: | |
Between high- and low-water at Portnaloub (Grid ref. 34/167431) there is a large exposure of tough grey-black shale dipping at a few degrees to the south. This sequence, approximately 7.5 metres thick, lies at the top of the Murlough Bay Coal Group. The lowest shales are exposed in offshore rocks and have been baked and contorted by dolerite intrusions. The two metres of shales exposed immediately above low-water are dark grey-black in colour and contain several irregular bands of ironstone, with ironstone nodules sparsely distributed through this unit. Most of these nodules have nuclei of fish coprolites but some contain plant remains or even articulated fish preserved in three dimensions. These dark shales are overlain by lighter shales, siltstones and sandy shales totalling about four metres in thickness. These lighter-coloured sediments contain sparse drifted plant debris and, towards the top, an increasing abundance of phosphatised fish coprolites, scales and spines. Fragmentary crustacean remains also occur while some horizons are completely covered by monospecific accumulations of ostracods. A further 1.5 metres of grey shale with ironstone may be exposed above this if not obscured by beach shingle, but the overlying Portnaloub Coal is always concealed. The remains of a shaft for the Portnaloub Mine can still be seen just above high-water. A similar succession was recorded in the Cross Borehole, described in Wilson and Robbie (1966, p. 92). They record fish debris among which they identified acanthodians, palaeoniscids (including Rhadinichthys sp.) and Megalichthys. | |
| Importance: | |
The Murlough Bay Coal Group is the only occurrence of an early Carboniferous oil-shale facies in Ireland and the exposure at Portnaloub is by far the clearest, much of its outcrop being obscured by the massive block scree below Fair Head. It is one of only a very few localities in Northern Ireland to have yielded articulated fish remains and the only one with three-dimensional preservation in nodules. | |
| Interpretation: | |
Thin coals and seat earths are developed only towards the top and base of the Murlough Bay Coal Group, which instead is dominated by dark grey oil-shales such as those exposed at Portnaloub. These shales were deposited in a freshwater lacustrine environment in which high rates of sedimentation in a stratified water column created anoxic bottom waters which facilitated the exceptional preservation of the fish fauna. The observed sequence indicates that sedimentation rates decreased and oxygenation increased upwards through the succession, with intact vertebrate preservation lower in the sequence being replaced by a disarticulated and phosphatic mode of preservation higher in the sequence. The diminishing sedimentation rate also resulted in an increase in the concentration of fish debris. Shallowing at the top of the sequence finally culminated in emergence and the development of a more typical fluvio-deltaic coal-swamp environment. The extent of the lake is unknown but the relatively thick sequence of oil-shales suggest that it was fairly long-lived and extensive. Direct comparison can be drawn with the well-known Lower Carboniferous oil-shale sequence in the Midland Valley of Scotland, where fish faunas also are preserved in ironstone nodules. Several of the well-preserved fish from the lower part of the sequence have been examined by Dr. Mike Coates at University College London. His conclusions (M. Coates, pers. comm.) are that all are actinopterygian and at least some may be assigned to 'Elonichthys' robisoni (Hibbert), though Coates notes that 'Elonichthys' is a notorious para- or polyphyletic 'dustbin taxon'. Preservation generally is poor and much of the material is specifically indeterminate. However, 'Elonichthys' robisoni is the most common actinopterygian in the Lower Carboniferous of the Scottish successions, which further highlights the links between the Ballycastle and Scottish Lower Carboniferous. | |
| Conclusions: | |
The oil shales of the Murlough Bay Coal Group were deposited in a lacustrine environment in which increasing oxygenation through time is reflected in increasingly fragmentary preservation up through the succession. | |
| 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 119 - Sillees River, Glenasheevar, Key Site 120 - Tircrevan Burn, and Key Site 121 - Tullyconnell. | |
| Keywords | |
|---|---|
| Rocks: | Ironstone, Oil-shale, Shale, Siltstone |
| Fossil Groups: | Crustacea, Fish, Plant |
| Relations: | Lacustrine sediments |
| Measurements | |||||
|---|---|---|---|---|---|
| Length: | Width: | Height: | ~7.5m | ||
| Depth: | Area: | ||||
| Access | |
|---|---|
| Management: | |
As an intertidal foreshore exposure the site requires no specific management and hence no management recommendations are made here. | |
| Uses | |
| Uses: | The site is a foreshore exposure between high and low water mark. Coal has been mined from associated sediments slightly higher in the succession but this activity ceased many decades ago and is unlikely to be revived. |
| References (3) | |||
|---|---|---|---|
| Mitchell, W.I., 1995. The Carboniferous System in Northern Ireland - Earth Science Conservation Review (Unpublished Report) 6 vols, 430 pp, Department of the Environment. | |||
| Wilson, H.E. & Robbie, J.A., 1966. Geology of the country around Ballycastle. Memoirs of the Geological Survey Northern Ireland. | |||
| Wright, W.B., 1924. The Geology of the Ballycastle Coalfield. Memoirs of the Geological Survey of Ireland. | |||
| 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. Portnaloub, County Antrim, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=118. Accessed on 2026-11-08. |
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