|Cole Bridge Stream, Fivemiletown||Tyrone|
|PHOTO TO BE ADDED Cole Bridge Stream, 4.25km N of Fivemiletown. Co. Tyrone.|
|Site Type: ||Stream section|
|Site Status: ||PASSI|
|District: ||Dungannon District Council|
|Grid Reference: ||H4452|
|Rock Age: ||Carboniferous (Tournasian)|
|Rock Name: ||Ballyness Formation, Clogher Valley Formation, Tyrone Group|
|Rock Type: ||Mudstone, Sand, Sandstone, Shale, Siltstone|
|Fossil Groups: ||Brachiopod, Coral, Miospore|
|Other interest: ||No data, Marine sediments|
Summary of site:
The progressive invasion of a landscape carved into Devonian rocks by the northerly advancing sea in the early Carboniferous period is a major event in the geological history of Northern Ireland. The section in the Cole Bridge Stream and its immediate area records one of the earliest transitions from terrestrial to marine conditions: its date is confirmed by fossils.
The contact between the Devonian rocks and the earliest Carboniferous rocks (the Ballyness Formation) is not seen in this area but the earliest rocks of the formation in this section are red sandstones and conglomerates, grading upwards into red to purple sandstones with beds of pink, pale green and fawn pebbly sandstones and fine grained sandstones (some lime rich) and siltstones. The red colouration of the older rocks is lost above these beds. Following a break in exposure, glimpses of grey, lime-rich sandstones and siltstones are seen and then 2.5m of grey, fine to medium grained sandstone with thin films of green to grey silty mudstones and a single lenticular lamination, around 1cm thick, of lime silt and green shale. This lamination is the first indication of the onset of marine conditions. About 50cm below the top of this section, microscopic miospores were found in a mudstone and, while none was specific in age terms, together they imply an early Carboniferous age, thought originally to be Tournaisian (the absolute base of the Carboniferous) but now considered more likely to be of the Chadian stage of the Visean (still very early in the period).
After a break of about 1m in exposure, pale grey, fine lime muds, either on or close to a tidal shore, appear with dark sandy silts and shales. These are the basal beds of the Clogher Valley Formation and the shales contain a rich flora of miospores. Over 30 species have been identified. Marine conditions are confirmed by three brachiopod species and a single coral. Taken together, these fossils also indicate a Chadian age, placing a fairly accurate time of 348 million years on the first marine incursion of the Carboniferous sea.
The sequence of events read from these deposits starts with terrestrial upland desert conditions on the fringe of an alluvial fan, with an ocean to the south. The sea advanced northwards fairly rapidly at the start of Clogher Valley Formation times, overwhelming low lying land. This area of what is now modern Co. Tyrone was then part of a northerly drifting supercontinent, named Laurentia by geologists, some 15º south of the equator.
The contact between the two formations and the presence of extensive spore floras associated with it make this a site of scientific importance that should be protected.