|Cappagh Quarry, Co.Tyrone, viewed from the north-east side of Cappagh Mountain, is the only quarry working the Barrack Hill Andesite Member (Gortfinbar Conglomerate Formation, Fintona Group) south of the Tempo-Sixmilecross Fault..|
|Site Type: ||Quarry (working)|
|Site Status: ||PASSI|
|District: ||Dungannon District Council|
|Grid Reference: ||H697674|
|Rock Age: ||Devonian (Middle Devonian, Upper Devonian)|
|Rock Name: ||Barrack Hill Andesite Member, Fintona Group, Gortfinbar Conglomerate Formation, Greenhill Andesite Member|
|Rock Type: ||Andesite, Conglomerate, Sand, Sandstone, Trachyte|
|Minerals: ||Calcite, Chlorite|
|Other interest: ||flow unit, flow-banding, magmatic roll, vesicles, Extrusion|
Summary of site:
South of the Tempo-Sixmilecross Fault on the Fintona Block there is an extensive area of red to purple sandstones and conglomerates forming a distinctive part of the Fintona Group. They are unfossiliferous, unlike the rocks north of the fault, leaving their age in doubt.
The rocks are divided into three formations and it is the enormously thick (estimated at around 2,500 m) middle division, the Gortfinbar Conglomerate Formation, that is the focus of interest here. Cappagh Quarry on Barrack Hill exposes extensive areas of trachyte and trachy-andesite volcanic lavas. There are many small outcrops further up the hill and, in all, a thickness of 260m for the lavas is estimated. These lavas and their other associated volcanic rocks form the base of the Gortfinbar Conglomerate Formation and provide its pebbles, cobbles and boulders. Nowhere is the contact between the lava pile and conglomerates seen but its field relationships, and the fact that the conglomerate clasts are composed of the identical trachy-andesite lava, leave little doubt that they were contemporaneous.
The composition of the basalts is suitable for radiometric dating, using the potassium-argon decay series. The unstable radioactive minerals are, in effect, a clock ticking at constant speed from the time of their formation; the minerals therefore decay to new forms at a constant rate. Using this principle, the minerals in the lavas have been analysed and have yielded two distinct groups of dates. The oldest, at around 375 million years, fixes the age of the lavas, almost exactly on the boundary between the middle and late Devonian. The second group is 100 million years later and these dates were derived exclusively from reddened lavas. This presents a problem because all the lavas were formed at around the same time. The anomaly is almost certainly explained by the action of hot mineral-saturated groundwater circulating into parts of the volcanic mass in early Permian times, during the Variscan period of mountain building. This formed new, unstable minerals minerals and effectively ‘reset the clock’ to that date.
The lavas are interesting in their own right as rare examples of volcanicity in the middle to late Devonian. They are flow banded, which means that elongated crystals become aligned in layers as the lava flows, and they also include many trapped bubbles (vesicles) that were later lined or completely filled with minerals.
A smaller outcrop, 3.5km north west of Ballygawley and higher in the formation, exposes similar andesitic lavas that have been named the Greenhill Andesite Member.
The absolute dating provided by the Barrack Hill Andesite Member fixes the time frame for all three formations south of the Tempo-Sixmilecross Fault. It is therefore key to the interpretation of over 250km˛ of terrain and should be preserved for further reference and study.