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Devonian - Fintona Block south of Tempo-Sixmilecross FaultFermanagh, Tyrone
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Summary Full report
Site Type: Various
Site Status:
District: Fermanagh District Council, Omagh District Council
Grid Reference: H3548, H5768
Rocks
Rock Age: Devonian (Middle Devonian, Upper Devonian)
Rock Name: Barrack Hill Andesite Member, Fintona Group, Gortfinbar Conglomerate Formation, Greenhill Andesite Member, Raveagh Sandstone Formation, Shanmaghery Sandstone Formation
Rock Type: Andesite, Conglomerate, Mudstone, Sand, Sandstone, Siltstone
Interest
Other interest: cross-bedding, dessication cracks, fault, ripple marks, Extrusion, alluvial fan

Summary of site:

The rocks of the Fintona Block have proved problematic ever since their first mapping in the nineteenth century. Segments originally supposed to be Devonian (although there was never evidence to support the date) have since been shunted into the Carboniferous as microscopic spores have been revealed and studied. Then, when Devonian spores were first found, they placed the rocks of the Irvinestown sector into the later part of the period only for them to be relegated to the earliest Devonian on reinterpretation. The Tempo-Sixmilecross Fault is a major crustal fracture crossing the block. It starts 2km north of Pomeroy and extends west south west to Seskinore, then veers south west to the south of Fintona continues along this line through Carry Bridge then north of Slieve Rushen where it finally loses its identity in a fault complex. The rocks to the south of the fault form a belt 10km across attributed to the Devonian Fintona Group.

The finds of Devonian spores north of the fault offered hope of similar finds in this huge area of roughly 275kmē to the south. In the event, none was found but andesites (volcanic rocks) in the middle division of the rocks, the Gortfinbar Conglomerate Formation, contain radioactive minerals, particularly the isotope potassium 40. These minerals yielded potassium-argon decay series dates of around 375 million years, on the boundary between the middle and late Devonian. Since the volcanoes were active during the period of conglomerate formation, this date is the first scientific indication of their true age. The rocks in this area are divided into three formations, the Shanmaghery Sandstone Formation at the base, the Gortfinbar Conglomerate Formation (containing the andesite) in the middle and the Raveagh Sandstone Formation on top. The Shanmaghery Sandstone formation sits unconformably (with a time break of around 50 million years) on the early Silurian rocks below. It is estimated to be around 600m thick, although this is more an educated guess than a measured sequence, largely due to poor rock exposure. It consists of purple-brown to red, lime-rich sandstones, siltstones and mudstones. The sandstones are coarse near the base and grain size decreases towards the top. Evidence of shallow water deposition is plentiful, particularly in the cross lamination and ripple marks. Polygonal desiccation cracks in the mudstone show that the sediments dried out completely in the protracted droughts between rare flash floods. There is no transition into the overlying Gortfinbar Conglomerate Formation. The conglomerates appear abruptly across a single bedding plane. They are coarse grained, with individual clasts reaching 40cm across; all the clasts are volcanic in origin, being derived from lavas of the Greenhill and Barrack Hill rhyolites and andesites that flowed over the sediments. The beds are coarsest near the lavas. There are some sandstones in the sequence that show cross bedding, suggesting shallow water flowing from the east. The formation is enormously thick, estimated at about 2,500m but exposure is so limited that this figure is open to question. The Shanmaghery Formation becomes progressively finer grained towards the top and grades into the Raveagh Sandstone Formation. The latter is silty and muddy and red-brown in colour. Laminations predominate, many with ripples, and desiccation cracks are again a feature. Rock exposure is so poor throughout the area that it has proved impossible to select stratotype sections to properly characterise the three formations, but the andesites are of extreme importance because they have provided the only dating evidence and should therefore be protected.


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