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Tircar Stream, FivemiletownTyrone
PHOTO TO BE ADDED Tircar Stream section, Fivemiletown, Co. Tyrone.
Summary Full report
Site Type: Stream section
Site Status: PASSI
District: Dungannon District Council
Grid Reference: H48804677, H48564728
Rocks
Rock Age: Carboniferous (Asbian, Visean)
Rock Name: Alderwood Mudstone Member, Leitrim Group, Meenymore Formation
Rock Type: Limestone, Mudstone, Sand, Sandstone
Interest
Fossil Groups: Arthropod, Brachiopod, Conodont, Coral, Crinoid, Goniatite, Nautiloid
Other interest: No data, Marine sediments

Summary of site:

Towards the end of the early Carboniferous period, around 335 million years ago, the ocean shelf that had been steadily accumulating the crushed shells and other lime carbonate remains of the marine fauna, finally shallowed to the point where it came under tidal and supratidal influence. This, the final phase of fully marine conditions, had deposited what was to become the Dartry Limestone Formation. Onto this vast tidal flat (120 km long, stretching from what is now Draperstown into County Sligo and extending 40 km out from the shore), tidal, intertidal, salt flat and even lake sediments were deposited which were to become the Meenymore Formation.

In the Tircar stream bed, part of the Meenymore succession is exposed which forms a recognisable subdivision, the Alderwood Mudstone Member. The rocks are close to a major zone of disruption, the Clogher Valley Fault, which has created steep dips of up to 40 degrees to the north west. The base of the member, a mudstone, lies on gritty, brown Meenymore sandstone. It is succeeded by a variety of tidal and intertidal limestones, themselves in part torn up by wave action and redeposited. Nodules of black chert are also frequent and there are shaley horizons with pyrite (iron sulphide) present suggesting hostile, anoxic conditions at times. The succession continues to the north with fine carbonates and more mudstones, some bearing fossils. The rocks are increasingly dolomitized as the Clogher Valley Fault is approached. Fossils occur throughout the member, particularly brachiopods, small solitary corals, ostracods, serpulid worms and a few goniatites but it is the microfauna, particularly the conodonts, that have proved most useful suggesting an Asbian age, the penultimate stage of the Lower Carboniferous. Since the full report was written a trilobite, Paladin bakewellensis, has been found in the member and it is typical of the final stage of the Lower Carboniferous, the Brigantian. From this it seems likely that the Meenymore Formation commenced in the Asbian and extended into the Brigantian. Altogether, over 200 m of the Alderwood Mudstone Member are seen in the Tircar stream section, the best and longest section available and consequently important.


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