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Earth Science Conservation Review
| Keenaght Water, Cullion Townland | Londonderry |
| Site number: | 306 | File Number: | H89 |
| ESCC: | EO | ||
| Locality Type: | Stream section | Status: | ESCR |
| Grid Reference: | H8190 1 km square | ||
| Google maps: | 54.75187,-6.74199 | ||
| County: | Londonderry | Council area: | Mid Ulster District Council |
| Period: | Carboniferous | ||
| Stages: | Asbian, Visean | ||
| Lithostrat: | Armagh Group, Desertmartin Limestone Formation, Leitrim Group, Meenymore Formation | ||
| Site Description | |
|---|---|
| Highlights: | |
The two parts of the Cullion section expose rocks near the top of the Desertmartin Limestone Formation and near the base of the succeeding Meenymore Formation. For the first time, the outcrop of the Meenymore Formation is extended beyond Co. Fermanagh, and the area of Slieve Beagh, and this record represents the most easterly location in Northern Ireland. | |
| Description: | |
The outcrop of late Vis‚an rocks in the Keenaght Water in Cullion Townland is divided into two parts. The northern part (H8133 9096) is located 210m SW of Redfern's Bridge, and exposes the Desertmartin Limestone Formation, while the southern section (H8112 9054) occurs a further 475m to the SW and exposes the Meenymore Formation. (i) DESERTMARTIN LIMESTONE FORMATION The northern part of the Cullion section exposes about 2m of strata which are thought to occur in the upper part of the Desertmartin Formation. They consist mainly of coarse clastic sediments with the exception of the top two beds. Conglomerates and pebbly sandstones dominate the lower part of the section. They are unfossiliferous and the clasts consist almost exclusively of vein quartz. At the top of the section are two beds of limestone. Both are quartz-rich and contain variably sized grains and angular pebbles of vein quartz. They are fossiliferous and contain the following: CORALS: Lithostrotion portlocki. BRACHIOPODS: Gigantoproductus spp., G. sp. cf. semiglobosus, Linoprotonia hemisphaerica, and Spirifer bisulcatus. The significant element of the macrofauna is G. cf. semiglobosus whose range is currently restricted to the late Asbian. (ii) MEENYMORE FORMATION Exposures of lithologies of the Meenymore Formation occur between 350m and 500m SW of the exposures of the Desertmartin Limestone between H8122 9063 and H8110 9052. Yellow-brown medium-grained sandstones and grey unfossiliferous mudstones occur at both ends of this section whilst in the middle part is an exposure of some 5m of dark grey laminated mudstones with a profuse fauna of brachiopods, in particular well-preserved conjoined spiriferids and Rhipidomella, small solitary corals, bivalves and orthocone nautiloids. Bioturbation structures, solid and septarian carbonate nodules abound and most bedding surfaces have a light sprinkling of carbonaceous fragments. Thin beds of micritic limestone occur near the base of the mudstones. | |
| Interpretation: | |
Throughout the outcrop of the Desertmartin Limestone Formation there is a significant content of isolated quartz grains and pebbles and of thin sandstone horizons in the limestone sequence. Because of the paucity of exposures the full lithological variation of this formation cannot be determined but grainstones do constitute a major part of the sequence. Such carbonate sediments originate in shallow water, often in turbulent conditions that winnow out fine carbonate or detrital mud. The occurrence of exotic clasts of vein quartz indicates a near-shore location and a source of coarse debris. The sandstones and conglomerates that occur at Cullion are representative of a shoreline sedimentary environment that is in direct contact with the source of such coarse detritus, presumably a fluvial origin. The thickness of these coarse clastic sediments is unknown but it seems likely that they are not the basal beds of the Desertmartin Formation and are interbedded within the limestone sequence. The sandstone-shale-mudstone lithologies that are intermittently exposed in the southern part of the Cullion section are quite typical of the Meenymore Formation. Marine bands, containing a prolific, if somewhat restricted, fauna are common in all parts of the formation outcrop further south and west in N. Ireland. The outcrop of the Meenymore Formation in the Cullion outlier is the most easterly occurrence of this formation in N. Ireland. It is located nearly 45km NE of the nearest outcrop, on Slieve Beagh in Co. Tyrone, and over 80km ENE of the major outcrop of this formation, in the Derrygonnelly-Cuilcagh Mountains-Lisnaskea area of Co. Fermanagh (Carboniferous Subareas 1 and 2). In a regional context this exposure of the Meenymore Formation is highly significant and extends considerably the known areal distribution of the late Vis‚an tidal and sabkha flats on which these sediments accumulated. Based on the present outcrop pattern of this formation, a conservative estimate of the areal extent of these flats is in the order of 4-5000km2, extending from near the coast in Co. Sligo, covering all of the Fermanagh Highlands and the Clogher Valley and Slieve Beagh, south of the Fintona Block, and reaching the Desertmartin-Slieve Gallion area in Co. Londonderry. Because it is now known that lithologies of the Meenymore Formation do not occur in the last Asbian sequence of the Cookstown area (Carboniferous Subarea-7) south of the Cullion outlier it is assumed that the width of the tidal flats, in this area, had narrowed to about 10km, compared to a width of 40-50km in Co. Fermanagh. In Co. Fermanagh the Meenymore Formation is regarded as the richest source of hyrocarbons in that Carboniferous sequence. Their occurrence in Co. Londonderry is quite unexpected and if their outcrop were to extend beneath the Antrim Plateau, then the hydrocarbon potential of deeper parts of the Phanerozoic sequence beneath Co. Antrim, which is concealed by the basalt lavas, is enhanced considerably. | |
| Conclusions: | |
The outcrop of the Desertmartin Limestone Formation in the Desertmartin area represents the most northerly occurrence of Asbian limestones in N. Ireland. The occurrence of lithologies of the Meenymore Formation in Cullion Townland is unique in Co. Londonderry and was not known until this project was instigated. The occurrence of these rocks has important implications for the hydrocarbon prospectivity of NE Ireland. | |
| Notes: | |
For information on other sites in Subarea-12 see Key Site 1185 - Subarea-12; Draperstown-Glenshane Pass-Limavady. | |
| Keywords | |
|---|---|
| Rocks: | Conglomerate, Limestone, Mudstone, Sand, Sandstone, Septarian nodule |
| Fossil Groups: | Bivalve, Brachiopod, Coral, Nautiloid |
| Fossil List: | Gigantoproductus sp. , Gigantoproductus sp. cf. semiglobosus , Linoprotonia hemisphaerica , Lithostrotion portlocki , Spirifer bisulcatus |
| Relations: | Fluvial sediments, Marine sediments |
| Paleoenv: | sabkha, tidal flat |
| Measurements | |||||
|---|---|---|---|---|---|
| Length: | 150 m+ | Width: | Height: | ||
| Depth: | Area: | ||||
| Access | |
|---|---|
| Approach: | The section begins 210m SW of Redfern's Bridge, ~5km SE of Draperstown. |
| Map(s): | GSNI (1983) 1:50,000 Solid Geology of Sheet 27 (Cookstown). | ||
| Record type: | ESCR report | ||
| Entered by: | Doughty, P.J. | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Keenaght Water, Cullion Townland, County Londonderry, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=306. Accessed on 2026-11-06. |
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