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Rockfield (Mitton's) QuarryFermanagh
Rockfield (Milton's) Quarry, Co. Fermanagh; N face exposes well-bedded cherty limestone towards top of Dartry Limestone Formation (broad band of pale limestone is Ballagh Limestone Members).
Summary Full report
Site Type: Quarry (working)
Site Status: PASSI
District: Fermanagh District Council
Grid Reference: H405310
Rocks
Rock Age: Carboniferous (Visean)
Rock Name: Dartry Limestone Formation, Leitrim Group, Meenymore Formation, Tyrone Group
Rock Type: Limestone, Shale
Interest
Fossil Groups: Bivalve, Brachiopod, Coral, Crinoid, Foraminifera, Gastropod, Goniatite, Nautiloid, Trilobite
Other interest: No data, Marine sediments

Summary of site:

This large, disused quarry, over 300 m long and up to 100m wide, is important because it is one of the few places where the junction between the Dartry Limestone Formation and the Meenymore Formation can be seen. Unfortunately the outcrop of the upper part of Dartry Limestone is high on the quarry face and only the last few metres below the junction with the Meenymore Formation are accessible.

The Dartry Limestone here is muddy and fine-grained with sponge spicules and the chert nodules typical of the formation but a bed around 7 m thick with its base 20 m below the top of the formation is recognisably different. This pale weathering limestone is a grainstone (consisting of carbonate grains cemented by clear calcite) typically found in shallow, agitated water where mud and silt is constantly winnowed away. Yet here it is sandwiched between deep water muddy limestones with an intervening black iron-rich shale lying directly on top of it. The last 60 cm of the Dartry Limestone are distinctive; they contain no chert and are much disturbed by sediment-feeding invertebrates. Under the microscope this rock can be seen to be a packstone (carbonate grains supported by a muddy matrix). At the top of the quarry the first 2 m of the Meenymore Formation rest directly on the Dartry Limestone. The rocks, mostly mudstones and fine-grained, tidal carbonate mudstones, have a sprinkling of crinoid skeletal plates for the first metre or so. Other disconnected outcrops are blocky mudstones at one point and silty mudstones at another. The formation is highly fossiliferous with a fauna including sea snails, nautiloids, small solitary corals, crinoids, brachiopods, bivalve molluscs and, rarely, trilobites. It has always been recognised that the Dartry Limestone was deposited in deep water that shallowed rapidly before the vast tidal flats of the Meenymore Formation smothered the area. However, the presence of shallow-water, pure carbonates in typical Dartry deep water rocks presents a problem, particularly when a black anaerobic shale that could only have formed at depth, lies on top of them. They do however resemble another grainstone in the Dartry of the immediate area, the Ballagh Limestone Member. The explanation is probably the same for both occurrences, that they formed in shallow water some distance from the area, became unstable and then slid under gravity to settle in deep water. The length of the rock succession in this abandoned quarry combined with the junction of the Dartry Limestone and Meenymore Formations makes this a site of considerable interest and the occurrence of the Ballagh Limestone Member, though inaccessible, add to importance.


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