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Ashfield Mountain Bar stream, Slieve BeaghTyrone
Stream section in Ashfield Mountain Bar, Slieve Beagh, Co. Tyrone.
Summary Full report
Site Type: Stream section
Site Status: PASSI
District: Dungannon District Council
Grid Reference: H543475
Rocks
Rock Age: Carboniferous (Visean)
Rock Name: Leitrim Group, Meenymore Formation
Rock Type: Limestone, Mudstone, Sand, Sandstone
Interest
Fossil Groups: Bivalve, Brachiopod, Conodont, Coral, Crinoid, Foraminifera, Goniatite, Polyzoan
Other interest: cross-bedding, Deltaic sediments, Fluvial sediments, Marine sediments

Summary of site:

This stream section exposes about 40m of strata of the Meenymore Formation, broken by three gaps. The section is interesting because it shows both marine and deltaic sediments that are quite different from other outcrops of the formation.

The earliest bed seen is an iron-rich mudstone which is overlain by five beds of fine granular limestone containing occasional fossils of goniatites, solitary corals and crinoid ossicles, capped by a limy mudstone where goniatites are more prolific. There is then a break of 7m above which coarse grained limestones occur containing microscopic algae and foraminifera. There is then a further break to a sequence of 5m of blue/grey mudstones with a rich coral and brachiopod fauna, including overturned coral colonies, bivalves, bryozoa, echinoids and crinoids. An alien block of granular limestone was collected from these beds and found to contain microscopic conodonts, both shallow and deep water forms. There is then a further break to 1.5m of channelled fine sands (typical of a river and delta environment) interbedded with silts and muds. The Meenymore Formation was deposited during the Carboniferous period when its geography was equatorial. Elsewhere the formation shows evidence of extensive salt flats, but at Ashfield Mountain Bar the lower beds suggest deeper water conditions (the iron-rich mudstones and claystones) with shallow water faunas being swept in from time to time. The sparse, broken and tumbled nature of the fossils is strong supporting evidence, reinforced by the mixing of deep and shallow water conodonts. This deeper water environment was followed by shallowing and finally the area was inundated by a delta. The Meenymore Formation was laid down in a tropical climate during the Asbian stage of the Carboniferous period, at a time when the area was within a few degrees of the equator. It was a period of extreme aridity, with rocks reflecting widespread salt flat conditions at or near sea level. The section at Ashfield Mountain Bar is unusual for this time because it contains marine rocks formed in moderately deep water (iron-rich mudstones and clay stones) containing material swept in from shallower waters nearby (eroded and overturned fossils, a mixture of shallow and deep water species). Later beds of limestone contain a rich fossil fauna suggesting that animals lived on, and in, that seabed. The final rocks of this section were deposited by rivers and deltas (sandstones interbedded with silts and muds, showing channel structures) which finally overwhelmed the area. The rocks in this stream section are important because they show that conditions in Meenymore times differed considerably from place to place. All fossil time indicators, largely goniatites and conodonts, confirm the Asbian age of the rocks.


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