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Earth Science Conservation Review
| Derraghadoan Pit, Dungannon | Tyrone |
| Site number: | 281 | File Number: | H76 |
| ESCC: | EA | ||
| Locality Type: | Pit | Status: | ESCR |
| Grid Reference: | H793644 Centroid | ||
| Google maps: | 54.52222,-6.77532 | ||
| County: | Tyrone | Council area: | Mid Ulster District Council |
| Period: | Carboniferous | ||
| Stages: | Brigantian, Visean | ||
| Lithostrat: | Leitrim Group, Rossmore Mudstone Formation | ||
| Site Description | |
|---|---|
| Highlights: | |
Derraghadoan Pit is one of the few remaining exposures of the Rossmore Mudstone Formation in the Dungannon-Coalisland area and displays lithologies typical of the formation, but virtually no fossils. | |
| Description: | |
Derraghadoan Pit is located on the west side of the A29 Dungannon-Cookstown road about 1.5km north of Dungannon. It comprises an upper working face, adjacent to the road, and a lower temporarily abandoned level, in which the rock is slightly weathered and bedding characteristics are more readily visible. The NW face of the quarry is some 10-12m high and is partially divided into two benches. All the rock consists of virtually horizontal black to dark grey calcareous shaley mudstones and massive mudstones that contain abundant flattened ovoid calcareous nodules. Very few fossils were found in the rocks. Bedding in this face is emphasised by the occurrence of beds of very hard and calcareous siltstone. Three such beds in the upper part of this face are 0.08, 0.1 and 0.05m thick from the lowest to highest. At the very base of the upper bench is a lenticular bed of very hard, dark grey, coarse-grained ironstone between 0.18 and 0.35m thick. | |
| Interpretation: | |
The environment in which the Rossmore Mudstones were deposited was clearly unaffected by either tectonic episodes of significance, major variations in water depth or fluctuations in current activity. Continuous deposition of fine mud occurred in a marine environment interrupted only infrequently by introduction of silt or sand debris. This is usually concentrated in thin beds which, in all probability originated from turbidity flow. The apparent absence of an erosive base and the continuous bed form of these siltstones and sandstones suggests possible sheet flow in a distal location on a very gently sloping or flat seafloor. Interpretation of the sedimentological and faunal information from borehole records seems to indicate a cyclical deepening of the basin gradually, above the top of the limestones shallowing once again below the base of the coal-bearing Namurian strata. The effects on benthic organisms are reflected in the relatively abundant assemblages, in the lower part of the succession. These include solitary corals, brachiopods, in particular gigantoproductids, and bryozoa. In higher beds these disappear and the dominant forms are of mixed planktonic and benthic origin including nautiloids and goniatites, with gastropods and bivalves as the dominant bottom dwelling forms. The increasing proximity of detrital source areas and possible shallowing of the sea is indicated by the prevalence of sandstones in the upper part of the formation. In NW Ireland Leitrim Group strata of Brigantian-early Pendleian age belong to the Carraun and Dergvone shale formations. These are lithologically very reminiscent of the Rossmore Mudstone Formation and show an identical change in the faunal assemblages that characterise the succession at various levels. The likelihood exists that much of N. Ireland at this time was affected by relatively tranquil tectonic conditions. A marine transgression that eventually drowned the remaining land area in NE Co. Antrim greatly expanded the basin margins and land areas, that provided detritus to the basin, lay some distance to the north and ? NE. | |
| Conclusions: | |
Derraghadoan Pit is probably the best, if not the only, exposure of the Rossmore Mudstone Formation in Subarea-7 in County Tyrone. | |
| Notes: | |
For information on other sites in Subarea-7 see Key Site 1180 - Subarea-7; Dungannon-Coalisland-Cookstown. | |
| Keywords | |
|---|---|
| Rocks: | Mudstone, Siltstone |
| Fossil Groups: | Bivalve, Brachiopod, Coral, Gastropod, Goniatite, Nautiloid, Polyzoan |
| Relations: | Marine sediments |
| Measurements | |||||
|---|---|---|---|---|---|
| Length: | Width: | Height: | 10-12 m | ||
| Depth: | Area: | ||||
| Access | |
|---|---|
| Approach: | Derraghadoan Pit is located on the west side of the A29 Dungannon-Cookstown road about 1.5km north of Dungannon. |
| Map(s): | See 1:50,000 O.S. Sheet 19 (Armagh). | ||
| Record type: | ESCR report | ||
| Entered by: | Doughty, P.J. | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Derraghadoan Pit, Dungannon, County Tyrone, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=281. Accessed on 2026-11-06. |
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