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Earth Science Conservation Review
| Cloghany pothole, Beighy Townland | Fermanagh |
| Site number: | 193 | File Number: | H13 |
| ESCC: | EO | ||
| Locality Type: | Pothole | Status: | ASSI |
| Grid Reference: | H161302 Centroid | ||
| Google maps: | 54.22094,-7.75316 | ||
| County: | Fermanagh | Council area: | Fermanagh & Omagh District Council |
| Period: | Carboniferous | ||
| Stages: | Asbian, Visean | ||
| Lithostrat: | Cloghany Limestone Member, Dartry Limestone Formation | ||
| Site Description | |
|---|---|
| Highlights: | |
This lithostratigraphical unit only occurs in Subarea-1 in Co. Fermanagh and was first recognised on, and is confined to, the north and east sides of Cuilcagh Mountain. It occurs just below or at the top of the Dartry Limestone Formation and differs from the underlying cherty limestones in lacking chert and consisting of fine foraminiferal grainstone. | |
| Description: | |
The Cloghany Limestone Member was identified as a new lithostratigraphical unit by the GSNI (1991) during the remapping of the Derrygonnelly-Marble Arch area of Co. Fermanagh. Depending on the presence or absence of the succeeding Carn Limestone Member of the Dartry Formation the Cloghany Member either occurs just below or at the top of this Formation. At its type locality the member attains a maximum thickness of 1.3m of which the lowest 0.3m are formed of fissile blue-grey shales and a thin nodular bioclastic limestone which rest directly on the bedded cherty limestones of the typical Dartry Limestone Formation. The top surface of this shale is deeply burrowed. The dominant and most distinctive part of the member is a limestone bed between 0.66 and 1m thick, which succeeds the basal shales and consists of pale to medium grey, coarsely crinoidal, fine-grained, foraminiferal packstone-grainstone. At the stratotype this limestone contains abundant foraminifera and algae including the following forms: Archaediscus stage angulatus A. ex gr. karveri A. krestovnikovi A. ex gr. stilus A. varsanofievae Aoujgalia sp. Brunsia sp. Chantonia sp. Endothyra sp. Eostaffella sp. Nodosarchaediscus sp. Tetrataxis sp. Valvullinella sp. Vissariotaxis cf. compressa At Polliniska (H166 289), 1.25km south-southeast of Cloghany the member is 1.15m thick and the top surface of the limestone bed forms the lip of the pothole into which the east-flowing river disappears. A minimum thickness of 0.97m is recorded to the north of the members' outcrop at the pothole (H156 309) 240m northwest of Lagacurragh. On the north side of Cuilcagh Mountain, because of strike faulting between the Dartry Formation and succeeding Meenymore and Glenade Sandstone formations, the Cloghany Limestone Member is exposed only in the Pollasumera River at two localities (H129 323). It consists of yellow to fawn-grey biomicrosparites and is overlain disconformably by algal laminated carbonates and shales of the Meenymore Formation. On the east flank of Cuilcagh the outcrop width of the Cloghany Limestone varies from a few metres in the north to more than 500m in the area of the stratotype pothole. This extreme outcrop width occurs entirely on the top surface of the main limestone bed which caps a long karstified dip surface that is inclined westwards at about 6 deg. Exposures of the overlying Carn Member are thus confined to the western, downslope end of this limestone surface in a narrow zone bordering the margin of the higher land underlain by thick glacial till and which largely conceals the Meenymore Formation. | |
| Interpretation: | |
Overlying the top bed of cherty limestone of the Dartry Formation are strata which consist essentially of alternating marine limestones and shales. The Cloghany Member, at the base of this sequence, consists of the lowest shale, and the lowest, and thickest, limestone bed of this sequence. This shale bed, at the base of the Cloghany Member, is identical to the shales of the succeeding Carn Member but the limestone in the Cloghany Member is quite different from the fine-grained limestones in the Carn Member, consisting of a relatively coarse-grained grainstone- packstone. Within this shale-limestone sequence many of the limestones are bioturbated but deep vertical burrows only occur at one level, in the top of the lowest shale bed which belongs to the Cloghany Member and underlies the thick grainstone bed. This suggests a period of reduced or non-deposition of sediment during a phase of sea-level shallowing when various burrowing animals were able to colonise the mud (later shale) substrate, for the first and only time. Coarse-grained, lime-mud or detrital mud-free, limestones are usually associated with strong currents in shallow, high energy environments and there are no indications in this limestone, in the Cloghany Member, to suggest otherwise. | |
| Conclusions: | |
The distribution of the Cloghany Limestone Member around Cuilcagh Mountain has proved to be crucial in defining the top of the Dartry Limestone Formation there and in formulating a palaeogeographic reconstruction for this area during part of the late-Asbian. | |
| Notes: | |
For information on other sites in Subarea-1 see; Key Site 1174 - Subarea-1; Derrygonnelly-Marble Arch-Cuilcagh. | |
| Keywords | |
|---|---|
| Rocks: | Limestone, Shale |
| Fossil Groups: | Foraminifera |
| Fossil List: | Archaediscus stage angulatus |
| Relations: | Marine sediments |
| Map(s): | See 1:50,000 O.S. Sheet 26 (Lough Allen); GSNI (1991) 1:50,000 Sheet 44, 56 and 43 (Derrygonnelly & Marble Arch). | ||
| Record type: | ESCR report | ||
| Entered by: | Doughty, P.J. | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Cloghany pothole, Beighy Townland, County Fermanagh, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=193. Accessed on 2026-11-06. |
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