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Carganamuck QuarryArmagh
SW end of Carganamuck Quarry, near Armagh City, Co. Armagh.
Summary Full report
Site Type: Quarry (working)
Site Status: PASSI
District: Armagh District Council
Grid Reference: H878498
Rocks
Rock Age: Carboniferous (Brigantian, Visean)
Rock Name: Armagh Group
Rock Type: Limestone
Interest
Fossil Groups: Alga, Brachiopod, Coral, Foraminifera
Other interest: palaeokarst, Marine sediments

Summary of site:

Carganamuck Quarry is an exceptional exposure of Brigantian rocks which are rarely seen in Northern Ireland. It yields beautifully preserved fossils in great variety and numbers, exhibits one of the finest palaeokarst surfaces in Europe, and has additional interest in the form of dykes and glacial features.

The youngest stage of the Viséan epoch of the Carboniferous is called the Brigantian. This stage usually marks the end of the dominant limestone sedimentation before the encroachment of a series of massive deltas that deposited the ‘Millstone Grit’ (coarse sandstones of the Namurian epoch). In Northern Ireland, there are only two places where large areas of shallow water limestones were formed at this time. One is at Castle Espie, a site now extensively flooded, while the other is around Carganamuck where there are a few small outcrops and a truly remarkable quarry. The lower part of the quarry succession exposes around 25m of regularly bedded limestones with purple and red shale partings. The limestone consists of closely packed grains of calcite, the spaces between being filled with lime mud; some grains have bacterial coatings. The limestone beds are 30-50cm thick and much disturbed by burrowing, while the shales are less than 10cm thick. The bases of the limestone beds and the shales yield the richest collecting with more than 20 species of coral and a further 20 species of brachiopods (including 14 species of large and sturdy examples of the genus Gigantoproductus). Under the microscope over 60 species of foraminifera have been recognised. The combination of corals and brachiopods found here amply confirms the early Brigantian age of all these beds. These limestones terminate at an irregular surface where they are heavily weathered along their joints and bedding planes and above this surface is an irregular bed between 0.5 and 2m thick defined by red and yellow clays above and below around a grey crumbly core. This surface and the clays form the palaeokarst (a limestone surface exposed during a lengthy period when sea level fell and atmospheric acids etched it into limestone pavements on which soil- forming vegetation became established). Above the palaeokarst are more beds of clean limestones consisting of carbonate grains, originally with only water between them, the spaces now being filled with clear calcite. There are suggestions of weak palaeokarsts in these rocks too. There are few fossils at this upper level and none to suggest an age later than the early Brigantian. The top of the sequence is not seen but is thought to be an angular unconformity with the Triassic Sherwood Sandstone Group above. Red sandstones can be clearly seen on the north side of the Callan River, around 150m away. The story in the quarry is of the last days of the great Carboniferous limestone equatorial sea, at a time when conditions were changing rapidly. Sea level fell for long periods, exposing the limestones to the weather, and plants colonised the low-lying coasts (creating landscapes rather like the Florida keys today. The sea returned and retreated intermittently and, although there is no clear evidence on site, deltas like those around Ballycastle at that time almost certainly buried the limestones, only to be completely stripped sometime in the later Carboniferous or Permian. Dolerite dykes (presumed to be Tertiary in age) can be seen at the west end of the quarry. Where the glacial deposits which mask the limestones have been cleared away, there are fine glacially polished surfaces with glacial striations (directional scratches caused by rocks in the moving ice).


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