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Mountfield Old QuarryTyrone
Outcrop of the Omagh Fault in Mountfield Old Quarry, Co. Tyrone.
Summary Full report
Site Type: Quarry (disused)
Site Status: PASSI
District:
Grid Reference: H535787
Rocks
Rock Age: Ordovician, Precambrian (Arenig, Dalradian, Llanvirn)
Rock Name: Mullaghcarn Formation, Southern Highlands Group, Tyrone Volcanic Group
Rock Type: Mylonite, Pelitic rock, Psammite, Semi-pelitic rock
Interest
Other interest: fault, No Data

Summary of site:

The Omagh Fault is one of Northern Ireland's most prominent and important structural features. It runs for a distance of 55 km from just south west of Draperstown in the north east, towards the north east corner of Lower Lough Erne in the south west and this site is the finest exposure of the fault to be seen anywhere along its entire length.

It is a thrust fault inclined at 40 degrees to the north west. In faults of this kind the forces operating are compressive so the rocks are thrust up the incline of the fault plane over the rocks below. If the forces involved are extremely powerful, as they were in this case, old rocks can be pushed over much younger rocks, sometimes for considerable distances. In describing inclined faults the rocks above the inclined fracture form the hangingwall, those below, the footwall. At the top of the north west face in this abandoned quarry there is a zone more than 3 m thick consisting of finely banded, pale green to silvery grey pulverised rock. The banded appearance marks this rock as a mylonites resulting from extremely powerful forces operating on rigid rocks in major fault zones. The rocks of the hangingwall are the ancient Dalradians of the Mullaghcarn Formation, about 600 million years old; those in the footwall, Ordovician lavas and volcanic dusts of the Tyrone Volcanic Group, erupted about 470 million years ago. Unfortunately the ground behind the north west quarry face is heavily forested so the transition from mylonites to adjacent, uncrushed Mullaghmore Formation can not be seen but metamorphosed basalts and tuff (volcanic dust) in the footwall are well exposed. A thrust of the magnitude seen along the Omagh Fault involved the enormous forces of the jostling large plates that cover the earth's surface. Evidence for the stresses involved can be seen in the deformation structures in the rocks on both sides of the fault. The older Dalradians show three phases of deformation and the Tyrone Volcanic Group only one. During this continental closure, called the Grampian Orogeny, the first period of activity created minor structures in the Dalradians that are now barely discernible. It was the second phase that had major effects creating the giant folds overturned to the south, such as the Sperrin Fold. The third phase promoted the thrust to the south creating the Omagh Fault and the deformation affecting the Tyrone Volcanics. The fault was therefore created about 465 million years ago but there appears to have been some minor movement along its western extremity (the Castle Archdale Fault) into late Carboniferous times, around 310 million years ago. This site provides access to superb outcrops of the thrust zone of the Omagh Fault only seen elsewhere but less clearly, at McNally's Burn (site 141). The fault is one of the boundaries between two very different geological areas, called terranes (large areas defined structurally on all sides and enclosing very different rocks), defined in Scotland but extending across the north of Ireland, the Central Highland Terrane to the north and the Midland Valley Terrane to the south. The controversy surrounding all aspects of their relationship is one of the major outstanding issues in our understanding of this early period in the evolution of early northern Britain and places such localities as this in key research positions. Access to the quarry is currently straightforward and there is no dumping. Its scientific importance could be further enhanced if exposures could be created to show rocks of the Mullaghcarn Formation as and when forestry activity presents an opportunity. Future threats could result from the reopening of the quarry or from landfill but with careful planning and management both might be accommodated with possible enhancement of the geology. This is one of Northern Ireland's sites of supreme importance that must be preserved for posterity.


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