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McNally's Burn, BroughdergTyrone
McNally's Burn, Broughderg, Co. Tyrone.
Summary Full report
Site Type: Stream section
Site Status: PASSI
Grid Reference: C671878
Rock Age: Ordovician, Precambrian (Arenig, Dalradian, Llanvirn)
Rock Name: Broughderg Schists, Mullaghcarn Formation, Southern Highlands Group, Tyrone Volcanic Group
Rock Type: Graphitic schist, Pelitic rock, Phyllite, Psammite, Schist, Tuff
Other interest: No data, No Data

Summary of site:

In the stream north of McNally's Bridge, the finest exposures of the thrust fault contact between the Dalradian, Mullaghcarn Formation, to the north and the Ordovician, Tyrone Volcanic Group, to the south, can be seen. Better known as the Omagh Fault, this is one of Northern Ireland's major structural features, following a general north east/south west trend for almost 50 km.

The first exposures, 200 m upstream from the bridge, are in both banks of the stream where greyish-green, silvery schists with phyllites (originally volcanic dusts called tuffs) can be seen. White quartz veins are common here, a feature of early stage metamorphism. This is the first appearance of the Ordovician rocks of the Tyrone Volcanic Group. They continue upstream into a zone of increasing structural complexity with small scale, northerly inclined, thrust faults in overlapping slices (imbricate structure) with minor folds and drag folds overturned to the south east, ˘pushed÷ from the north west during thrusting. On an inclined fault plane (a line of fracture along which there has been displacement) the overhanging wall is called the hangingwall and the block beneath, the footwall. In the waterfall, the footwall of the Omagh Fault is exposed and consists of hard and brittle, banded, green chloritic tuffs containing the mineral epidote. These are still the Ordovician volcanic rocks and show considerable evidence of high strain with stretching and finely banded pulverized rocks called mylonites the result of intense crushing as rocks ground past each other in the fault plane. A little further upstream, where the streambed is less steep, Dalradian rocks of the Mullaghcarn Formation can be seen in the hangingwall of the fault. In the intermittent exposures, greyish-green chloritic phyllites (originally shales or mudstones) and quartz albite psammites (originally sandstone) are the typical rocks. Near the fault they are considerably deformed and have a platy structure but further upstream, away from the fault, they show a more normal, equal-grained, structure. The Dalradian rocks were formed around 600 million years ago near the South Pole on the bed of deep oceanic basins fringing a rifting supercontinent. The Ordovician rocks were formed about 130 million years later in massive volcanic eruptions near the coast of a continental fragment detached from same supercontinent. 5 million years later, around 465 million years ago, both sets of rocks were severely deformed in a continental collision in the vicinity of a volcanic island arc where regional metamorphism transformed the original sediments and volcanic tuffs into something near their present appearance. This vast collision buried both to a great depth where not only was their mineralogy changed but also their structure. The Dalradian rocks were severely deformed and a gigantic fold, over 40 km across, was created which overturned to the south east. The rocks in the hangingwall are on the southern, overturned limb of this fold. At least three phases of deformation and cleavage can be recognised in the Mullaghcarn Formation. Lineation on the same trend can also be seen in the volcanics of the footwall. It was during this period of massive stress that the Dalradians were thrust over the Ordovician volcanics along a considerable front, just one phase in the creation of a great mountain range. On the scale of metamorphic alteration the degree of change of both these rock groups was relatively low, falling within the greenschist facies but the Dalradians were slightly more affected than the Ordovician volcanics. This site along the plane of the Omagh Thrust shows features of thrust faulting, associated deformation in the immediate vicinity of the fault plane and the relationships between two terranes (areas with well-defined margins with different stratigraphical and structural histories), the Grampian to the north, the Tyrone-Girvan to the south. The Northern Ireland rocks are an extension of these classic terranes, originally defined in Scotland and consequently the site has international significance. The gold-bearing shear zones of the southern Sperrin Mountains are thought to result from the final stages of activity along the Omagh Fault, thus adding economic significance. Access is currently fairly easy but the outcrops are limited, full of geological interest and consequently in need of careful management to ensure its survival. Control of sampling is therefore an issue.

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