Mullynaskeagh
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Geology & landscape
Mining / Industrial: Quarry · Geological Period: Carboniferous · Rock Type: Sandstone, Shale
Landscape processes: Glacial
Description
Mullynaskeagh Upper Palaeozoic palaeontology A disused clay pit at Mullynaskeagh exposes the thickest development of the Sheena Shale Member of the Lower Carboniferous Leitrim Group some 330 million years old. Its rich and diverse fossil assemblage, particularly of goniatites, is unmatched by any other site of similar age in Northern Ireland. Exposure of the Doagh series, of which Mullynaskeagh forms a part, is generally poor on account of the prevalence, in this part of the succession, of easily weathered mudstones, the presence of a variable thickness of glacial drift, and the widespread planting of conifers over a substantial area of the outlier. Mullyneaskeagh represents an important and easily accessible exposure of part of this rock series. The Mullynaskeagh clay pit exposes one of the clearest sections through any part of the Leitrim Group in this area. About 18.3 metres of shales are exposed in the quarry, dipping gently to the south, assigned to the Sheena Shale Member. The shales in the quarry are rather weathered but typically are grey to dark blue-grey in colour. The site has yielded a diverse fossil fauna. The goniatites together with gastropods and the bivalves Nucula and Nuculopsis occur in some abundance while corals and brachiopods occur patchily. The Sheena Shale is presumed to have been deposited on top of a drowned delta represented by the underlying Glenade Sandstone. The composition of the fauna at Mullynaskeagh indicates a sea floor within a sheltered basin, oxygen poor conditions and a water depth probably greater than 100 metres. The limited occurrence of small corals and brachiopods at this site lends further support to this setting. It suggests that only occasionally did the sea floor became sufficiently well-oxygenated for groups such as corals and brachiopods to be come established. Preservation of the mineral aragonite from which the shells are comprised together with the internal moulds of the goniatites also indicates a poorly oxygenated benthic environment. That even the smallest specimens are preserved in exquisite detail, indicates negligible post-mortem transport, further supporting the idea of this these rocks forming in a sheltered basin setting. https://www.daera-ni.gov.uk/publications/mullynaskeagh-assi