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Devonian - Shanmullagh Formation; Miospore localitiesTyrone
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Summary Full report
Site Type: Pit, Quarry (disused)
Site Status:
District: Omagh District Council
Grid Reference: H2860, H3464
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: No data, No Data, lacustrine

Summary of site:

From the earliest nineteenth-century mapping of Ireland, the Fintona Block has presented a problem. Its geological setting and rock types show strong affinities with British Devonian rocks of continental origin grouped under the general banner of Old Red Sandstone. The total absence of fossils, however, always left this inference open to challenge. Things appeared to improve in 1938 with a single find of fish fragments of Devonian age in the area, only to deflate when the entire sector of rocks in which they were found was reallocated to the Carboniferous period in 1990 following new and conclusive research. The fish remains are now thought to be derived but at least they suggest that Devonian rocks are present somewhere nearby.

This account describes more recent work in two areas near Irvinestown, in the Shanmullagh Formation where green mudstones were sampled and examined for microscopic spores. They were found at 47 localities but only 22 yielded identifiable specimens. Although it was not known whether a date for the entire formation could be extrapolated from these limited outcrops, it was hoped that the situation could at least be clarified. The assemblage of spores showed quite clearly that the mudstones were of Devonian age and, although their significance was not fully understood, they were believed to show a date late in the period, somewhere around 370 million years ago. Since the full report for this site was written, that opinion has been revised and the spores are now known to indicate an early Devonian age, between 400 and 390 million years ago. The time stages involved are the late Lochkovian, the entire Pragian and the earliest Emsian. The mudstones were formed in a shallow desert lake, mostly washed in by rare flash flood events and to a lesser extent by blown dust. They are rare beds in a succession of red-brown, desert sandstones and mudstones normally baked dry under the desert sun. Few land plants had evolved at this time but there must have been sparse growth somewhere in the immediate vicinity. In world geographic terms the area was on the southern edge of the supercontinent of Laurentia-Baltica (including most of modern North America, Greenland, Ireland, Britain and a substantial part of Scandinavia and the Baltic fringe) following the collision of the two. It spanned the equator at the time, with a newly formed range of giant mountains, similar to the modern Himalayas, dominating its eastern rim. The Shanmullagh area was then about 20 south, roughly the latitude of northern Australia at the present day. One of the sites was partly overgrown at the time it was sampled; the other, a small farm quarry, could easily share the same fate. Since the flora here is the only conclusive proof of the Devonian age of these rocks, this is a vital locality and should be designated and protected.


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