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Devonian - Shanmullagh Formation; Stratotype localitiesTyrone
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Summary Full report
Site Type: Cliff, Quarry (disused)
Site Status:
District: Fermanagh District Council, Omagh District Council
Grid Reference: H255598
Rocks
Rock Age: Devonian (Famennian, Frasnian)
Rock Name: Fintona Group, Shanmullagh Formation (Form Blm)
Rock Type: Mudstone, Sand, Sandstone, Siltstone
Interest
Fossil Groups: Miospore, Miospores
Other interest: dessication cracks, ripple marks, No Data, alluvial plain, braided stream, desert

Summary of site:

A name for a rock formation is normally associated with a site showing its most important and diagnostic characteristics. It also usually defines its base and sometimes its top. Such sites are called stratotypes and they become the reference for that particular formation. This account deals with the stratotype for the Shanmullagh Formation and the Devonian rocks of part of the Fintona Block and they present unusual problems.

The formation covers an area of around 300km˛, stretching east from a north-south line off the eastern shore of Lower Lough Erne in the west to about 3km south of Omagh in the north and beyond Sixmilecross in the south. Yet despite this vast expanse, nowhere have rocks been seen that could be described as distinctive. Everywhere they occur, they consist of purplish to red-brown sandstones, siltstones and mudstones, with the occasional fossil soil horizon. Conglomerates and coarse sandstones are entirely absent. Furthermore, the block is defined by faults on three sides and the sediments encroach on to the Tyrone Igneous Complex to the east, making it impossible to define a base or top. In these circumstances, all that can be done is to select a site showing the typical lithologies of the formation and another two with green mudstones that have yielded the microscopic plant spores used for dating. The stratotype is the grid referenced wall of the disused quarry in Shanmullagh townland. All these rocks were formed on a low, flat alluvial plain in a desert environment prone to short but intense periods of flooding. Sands and silts were carried far out into the basin by these events, spilling out of their dry season channels in sheet floods covering huge areas. Ripple marks attest to the shallow nature of the brief periods of standing water and the long arid periods are marked by polygonal desiccation cracks when only wind blown dust would add to the surface. The flaked, dried mud would be carried off in the next flood to settle in the next generation of beds. Some exceptional floods reached the centre of the basin where they carved channels in the sands and muds up to 2m deep and 7m wide but none contains anything coarser than baked mud flakes. When the full account of these localities was written, it was believed that the sparse evidence supported a late Devonian age for the formation. Subsequent work has clearly established their early Devonian age, sometime around 390-400 million years ago. For a discussion of the spore dating and the geography of the time, see site record ‘Devonian – Shanmullagh Formation; Miospore localities’. There are no immediate threats to the stratotype but disused quarries are always at risk.


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