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Earth Science Conservation Review
| Derrygonnelly road cutting | Fermanagh |
| Site number: | 179 | File Number: | H15 |
| ESCC: | EO | ||
| Locality Type: | Roadside section | Status: | local interest |
| Grid Reference: | H123512 Centroid | ||
| Google maps: | 54.4097,-7.81055 | ||
| County: | Fermanagh | Council area: | Fermanagh & Omagh District Council |
| Period: | Carboniferous | ||
| Stages: | Arundian, Visean | ||
| Lithostrat: | Benbulben Shale Formation, Mullaghmore Sandstone, Tyrone Group | ||
| Site Description | |
|---|---|
| Highlights: | |
This is the only known exposure in Subarea-1 of the conformable transition between the Mullaghmore Sandstone Formation and succeeding Benbulben Shale Formation. Microfauna in the top of the former formation indicate a mid- to late-Arundian age. | |
| Description: | |
Located some 600m south-southeast of the outskirts of Derrygonnelly on the east side of the B61 road to Enniskillen is a unique section exposing the conformable transition between the Mullaghmore and Benbulben formations. The section exposes 1-1.5m of rock beneath about 3m of till and is some 50-60m long. The lowest beds, at the north end, consist of brown-grey, fine- to medium-grained feldspathic sandstones in beds 20-25cm thick. These are succeeded by grey bioclastic calcareous sandstones and shaley mudstones. The top bed of the Mullaghmore Formation, beneath the Benbulben Shale comprises 0.25m of medium to coarse- grained oncolitic, bioclastic grainstone/packstone which contains a diverse microbiota dominated by primitive archaediscids. The assemblages includes the following taxa. Aoujgalia sp. Aphralysia sp. cf. Asphaltina sp. Biseriella sp. Coelosporella sp. Endothyra sp. Girvanella sp. Glomodiscus sp. Koninckopora sp. K. inflata K. minuta Mediocris sp. Nannopora fragillissima Paraarchaediscus sp. Planoarchaediscus sp. Pseudoammodiscus sp. P. volgensis Tetrataxis sp. T. medius Uralodiscus sp. Viseidiscus sp. The association of Glomodiscus and Uralodiscus with Paraarchaediscus indicates a mid- to late-Arundian age for the top of the Mullaghmore Sandstone. The lowest beds of the Benbulben Shale Formation succeeding this limestone consist of 0.6m of dark grey unfossiliferous earthy shales interbedded with thin silty sandstones and are exposed in the extreme south of the section dipping southwest at 10 deg. | |
| Interpretation: | |
The transition from the Mullaghmore Sandstone to the Benbulben Shale was induced by a reduction in the amount and coarseness of clastic sediment and an increase in salinity resulting from a further regional marine transgression and drowning of the former delta top. Interbedded bioclastic oncolitic sandstones with unfossiliferous rippled sandstones testify to the existence of shallow, turbulent water with alternating hypersaline and brackish conditions. The lowest beds of the Benbulben Shale are unfossiliferous and may represent a delta slope environment, now located some distance from shallow nearshore environments that migrated northwards rapidly in response to the marine transgression. | |
| Conclusions: | |
The roadside section near Derrygonnelly is unique in Subarea-1 in exposing the conformable contact between the Mullaghmore Sandstone and Benbulben Shale and in reinforcing the Arundian age of the former formation. | |
| Notes: | |
For information on other sites in Subarea-1 see; Key Site 1174 - Subarea-1; Derrygonnelly-Marble Arch-Cuilcagh. | |
| Keywords | |
|---|---|
| Rocks: | Mudstone, Sand, Sandstone, Shale |
| Fossil Groups: | Foraminifera |
| Fossil List: | Glomodiscus sp. , Paraarchaediscus sp., Uralodiscus sp. |
| Relations: | Marine sediments |
| Access | |
|---|---|
| Approach: | 0.6km SSE of the outskirts of Derrygonnelly on the east side of the B61 road to Enniskillen. |
| References (3) | |||
|---|---|---|---|
| Buckman, J.O., 1992. Palaeoenvironment of a Lower Carboniferous sandstone succession, northwest Ireland: ichnological and sedimentological studies. Special Publications of the Geological Society of London, numb. 62 In Basins on the Atlantic Seaboard: Petroleum Sedimentology and Basin Evolution., pp.217-241. | |||
| Higgs, K., 1984. Stratigraphic palynology of the Carboniferous rocks in Northwest Ireland. Bulletin of the Geological Survey of Ireland, vol. 3, pp.171-201. | |||
| Oswald, D.H., 1955. The Carboniferous Rocks between the Ox Mountains and Donegal Bay. Journal of the Geological Society of London, vol. 111, pp.167-186. | |||
| Map(s): | See 1:50,000 O.S. Sheet 17 (Lower Lough Erne). | ||
| Record type: | ESCR report | ||
| Entered by: | Doughty, P.J. | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Derrygonnelly road cutting, County Fermanagh, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=179. Accessed on 2026-11-06. |
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