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Mullynaskeagh Clay PitFermanagh
Mullynaskeagh Clay Pit, west of Derrygonelly, Co.Fermanagh, August 1997.
Summary Full report
Site Type: Pit
Site Status: PASSI
District:
Grid Reference: H069531
Rocks
Rock Age: Carboniferous (Brigantian)
Rock Name: Bellavally Formation, Leitrim Group, Sheena Shale Member
Rock Type: Limestone, Shale
Interest
Minerals: Pyrite
Fossil Groups: Ammonite, Arthropod, Bivalve, Brachiopod, Coral, Gastropod, Goniatite, Nautiloid
Other interest: No data, Marine sediments

Summary of site:

The Mullynaskea Clay Pit is beside a track in the Conagher Forest about a 1.5 km walk from the car park on the road between Garrison and Derrygonnelly. About 18 m of dark blue to grey shales are exposed, part of the Carboniferous Bellavally Formation. All the shales in the quarry belong to a subdivision of the Bellavally, the Sheena Shale Member. Within the shales there are nodules and many thin nodular limestones and about 10 m above the base there is a prominent pale brown-weathering limestone over 10 cm thick

The site is noted for its fossils which occur largely above the brown-weathering limestone. The fauna is rich and unusual. There are 15 cephalopod species (free-swimming, related to the octopus and nautilus), 8 gastropod species (sea snails), 7 brachiopod species, 6 species of bivalve molluscs, a few corals and some ostracods. There could be many more. The wealth of goniatites and gastropods is noteworthy, as is the way that nearly all the fossils are preserved. They are generally uncrushed with shells replaced by iron pyrites (commonly known as fool's gold) which gives them a very attractive appearance when fresh but creates serious conservation problems unless they are kept in dry (low humidity) conditions; otherwise the pyrite dissociates into iron and sulphur, a grey acidic powder, destroying the fossil. The species present and their mode of preservation add significantly to our understanding of the environment in which the Sheena Shale Member was formed. The cephalopods in their coiled shells lived and fed in the waters above the sea bed, probably well above, near the surface but the presence of a variety of sea snails in combination with thin shelled bivalve molluscs would tend to indicate a sea bed in fairly deep water with little light, much fine organic debris and low oxygen levels. Add to this the pyrite preservation, a feature of oxygen starved environments and the preservation of fine detail on delicate fossils and a picture emerges of a deep ocean bed with feeble currents but still with some light penetration (light does not penetrate below 150 m), blanketed in black organic mud (sea snails and thin-shelled bivalve molluscs feed on fine organic particles) with low levels of oxygen (probably removed by the decay of organic matter). Corals and brachiopods would have found these conditions close to intolerable and it is significant that the specimens collected were stunted and limited to certain levels in the succession tending to suggest that conditions improved somewhat for brief periods. One of the cephalopods, the goniatite Goniatites falcatus, lived for only a very limited time interval and indicates the Brigantian time stage around 335 million years ago. At that time this sea bed was a marginal ocean shelf on the coast of the continent of Laurentia, then astride the equator and drifting imperceptibly into the northern hemisphere. The shales are heavily weather and would benefit from a refreshing of the faces. The spoil generated could then be left on site as a resource for educational collecting. As a potential Area of Special Scientific Interest, collecting from the quarry face should be firmly regulated. Examples of much of the fauna collected here can be seen in the collections of the Ulster Museum.


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