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Killuney StreamArmagh
Killuney Stream, Armagh City, Co. Armagh.
Summary Full report
Site Type: Stream section
Site Status: PASSI
District: Armagh District Council
Grid Reference: H8946
Rocks
Rock Age: Carboniferous (Tournasian)
Rock Name: Annaclare Group, Killuney Conglomerate Formation, Retreat Siltstone Formation
Rock Type: Conglomerate, Greywacke, Sand, Sandstone, Siltstone
Interest
Fossil Groups: Bivalve, Gastropod, Microfossil, Microfossils
Other interest: dessication cracks, No Data

Summary of site:

A section in the streambed at Killuney records the earliest incursions of the marine invasion of this area at the start of the Carboniferous period during the Courceyan stage, around 360 million years ago. The Tyrone Group is made up of 6 formations and this stream contains the first two, the Killuney Conglomerate Formation and the Retreat Siltstone Formation. Both are defined from this stream making it the stratotype, the fundamental comparator, for both.

The lowest beds of the Killuney Conglomerate Formation lie above the unconformity with the underlying Ordovician rocks. The fist beds are breccias, angular blocks up to 40 cm across of Ordovician greywacke (a sandstone) and siltstone in a matrix of chips and flakes of the same siltstone. They are grey in colour with purple-brown staining. They continue downstream and are eventually overlain by purple red conglomerates and sandstones forming a series of repeated cycles. Each commences with a conglomerate (with pebbles of Carboniferous sandstone) grading into a pebbly sandstone, then sandstone and finally a 10 cm sequence of laminated purple siltstones and mudstones. The cycles become progressively finer towards the top so that the last few lose their conglomerates. The total thickness of the formation is estimated at 80 to 100 m. The junction with the overlying Retreat Siltstone Formation is not seen in the stream section but its recognition is made easy by a change in colour from the purple-reds to greys and greens. The lowest beds seen in the stream are flaggy sandstones, with purple-pink mottling, passing upwards into coarse-grained, lime-rich sandstones in beds 7 to 20 cm thick, followed by gritty green sandstones and, less commonly, of a purple-red colour. After an unexposed interval, a thickness of 2.4 m of unfossiliferous limestones, sandstones, siltstones and mudstones appear topped by a sandy limestone containing laminated bacterial films. Elsewhere, in other sections, fossils and desiccation cracks are found in the formation. The top of the formation, at its junction with the Ballynahone Micrite Formation, is not seen here. It is usual in a stratotype to define its base but although this is the best exposure of the formation, neither base nor top is seen. The total thickness is estimated at 75 to 90 m. In Ireland in the early Carboniferous the shoreline moved progressively from south to north and the sequence at Killuney follows a rational sequence starting with terrestrial breccias stained red by general weathering, probably in a hot, dry environment. The following cycles show a progression to brackish (the bacterial films suggest deposition in hot brine) and stream sediments which become more dominant in the Retreat Siltstones until finally the first encroachment of the sea laid down the limestones. Despite the limestones, it seems unlikely that there were ever open sea conditions in this area while both these formations were being laid down. The area was probably featureless lowland with distant hills to the south east and north. The climate was equatorial as the supercontinent of Laurentia (now parts of North America, Greenland and the British Isles) crept imperceptibly northwards across the equator of the time. Stratotypes are essential to our understanding of events through geological time so this stream, with two, must be protected and preserved. The present condition of this very confined and overgrown distributary shouts for attention, particularly to prevent pollution from stock braking through from adjacent fields and to stop effluent draining from the farm on the south bank of the stream. Local development could also present a threat as housing spreads inexorably around the city of Armagh.


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