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Summary Full report
Site Type: Subcrop
Site Status: PASSI
Grid Reference: H844747
Rock Age: Permian (Upper Permian)
Rock Name: Magnesian Limestone
Rock Type: Dolomite
Fossil Groups: Arthropod, Bivalve, Coral, Foraminifera, Gastropod, Polyzoan
Other interest: No data, No Data

Summary of site:

Although a major borehole at Larne drilled through more than 1.260 m of concealed Permian rocks, outcrops of this period are uncommon and restricted to the north Down area, Armagh and east Tyrone. Only at Cultra, where a small patch is exposed on the foreshore, is there good exposure.

The dominant Permian rock of English outcrops is a creamy-yellow dolomitic sediment, the Magnesian Limestone, much favoured as a building stone as, for example, in York Minster. It has been found at 2 Northern Ireland sites, at the previously mentioned shore locality at Cultra and around the village of Grange about 2 km south east of Cookstown. There is no exposure here but excavations around Tullyconnell Hill, 1 km east of Grange, has turned up blocks of the rock. Of the 2 occurrences, the Tullyconnell rocks have proved to have the richer and better preserved fossil fauna. Fortunately 5 boreholes have been sunk in this area and have proved a thickness of Magnesian Limestone ranging between 18 and 23 m. The rock is described as a medium to fine-grained, lime-rich, dolomitic limestone (dolomite is a calcium magnesium carbonate mineral), predominantly yellow to cream in colour with tinges of grey and pink in places. It is oolitic in part (ooliths are chemically deposited spheres less than 2 mm in diameter) and porous. Isolated quartz pebbles are scattered throughout. The fauna consists of 25 species; 1 coral, 3 bryozoans, 7 bivalve molluscs, 7 sea snails, 2 ostracods and 5 foraminifera. There was also one doubtful, simple plant. During late Permian times, about 250 million years ago, an extensive sea, the Zechstein Sea, occupied northern Europe and the North Sea Basin. On its western side there was a substantial coastal indentation with many islands. Its southern shore extended from the area of the Wash on the English east coast to the north of Ireland; the northern shore was aligned with the Midland Valley of Scotland. This narrowing western extension of the Zechstein is called the Bakevillia Sea (Bakevillia is the dominant bivalve mollusc genus) and it was in this sea, festooned with islands, that the rocks found in Northern Ireland were formed. At this time the Irish area was equatorial with baking temperatures causing high rates of evaporation, creating a sea with elevated salinity, a hostile environment for sea creatures. It is suggested that throughout the period of Magnesian Limestone formation, in what is now the Grange area, the increasing salinity of the Bakevillia Sea was the major influence on the marine fauna creating stunted growth and the progressive elimination of species until only those with the highest salinity tolerances survived. This is almost certainly the reason for the total absence of echinoderms (brittle starfish, starfish, crinoids, sea urchins etc), cephalopods (early, mostly coiled, relatives of Nautilus, octopus and squids) and brachiopods (lamp shells). The bivalve Bakevillia was the most persistent of all. A major claim to fame of the Magnesian Limestone at Tullyconnell is its geographic position, the most westerly outcrop of Permian marine rocks in Europe. It gives a unique picture of late Permian marine transgressions at their extreme western limits. It also provides vital information on faunal diversity and the relationship with salinity levels. The Magnesian Limestone occurs immediately below ground level at Tullyconnell and it may be worth excavating for outcrop as future research requirements demand. In any case this is a site of outstanding national and international importance which must achieve Area of Special Scientific Interest (ASSI) status.

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