 | | Vertically disposed, hard calcareous siltstones and very silty limestone at base of Benbulben Shale Formation, in E bank of stream. |
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| Site Type: | Stream section | | Site Status: | ASSI | | Council area: | Mid Ulster District Council | | Grid Reference: | H52224883,H52584844 | | Bing maps: | 54.38561,-7.19653 | | Google maps: | 54.38561,-7.19653 | | Rocks |
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| Rock Age: | Carboniferous (Visean) | | Rock Name: | Benbulben Shale Formation, Bundoran Shale Formation, Dartry Limestone Formation, Glencar Limestone Formation, Mullaghmore Sandstone, Tyrone Group | | Rock Type: | Limestone, Mudstone, Sand, Sandstone, Shale | | Interest |
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| Fossil Groups: | Alga, Brachiopod, Coral, Foraminifera | | Other interest: | Deltaic sediments, Marine sediments |
| Summary of site: |
| A unique combination of events has preserved the longest time-span seen in one section anywhere in the early Carboniferous rocks of Northern Ireland. This exceptional evidence is exposed in a little over 0.5km of streambed along the Fardross Forest River Walk, where rocks from the Arundian to the Asbian stages of the Visean record events over a period of 7 million years. In this small area around 290 million years ago violent movement along a major north-east/south-west trending crustal fracture, the Clogher Valley Fault, cut through this series of rocks and upended them into an almost vertical position where the Fardross stream now flows across them. Rarely is such a thickness of rocks preserved in such a confined space. | | At the bottom of the section, hard against the Clogher Valley Fault, a few metres of crushed and heavily veined limestone are preserved. This is the very top of the Bundoran Shale Formation; despite its condition, samples have yielded a microscopic fauna of foraminifera, suggesting a middle Arundian age. | | Lying against the Bundoran Shale at an angle of 70 degrees is the base of the Mullaghmore Sandstone Formation, originally on top of the Bundoran Shale. It consists of 150m of pale brown to red sandstones interbedded with dark grey mudstones and siltstones. The sandstones are lime rich and in part recognisably crinoidal. Lenses and pockets of coal occur near the base. A fauna of foraminifera was found in a sandy limestone high in the formation and the mudstones above have a rich marine fauna of bivalve molluscs, brachiopods, crinoids and a scattering of sea urchin spines. Taken together, they indicate a middle Arundian age. | | Above a 34cm bed of sandstone that marks the top of the Mullaghmore Sandstone, dark grey-brown siltstones (some very thick with muddy limestones of variable thickness) predominate for the next 140m. This is the Benbulben Shale Formation. A rock sample taken from near the base yielded altered foraminifera sufficiently well preserved to hint at a late Arundian age; this was confirmed by the prolific macrofauna of brachiopods and corals further up the succession, but the appearance of Gigantoproductus brachiopod species nearer the top suggests that the upper beds could be of Asbian age. No clear evidence of the Holkerian (the stage between the Arundian and Asbian) is seen at Fardross but, judging by these faunas, it should be present somewhere above the mid-point in the formation. | | The contact between the Benbulben Shale and the overlying Glencar Limestone Formation is currently not exposed, so its nature (normally a brief gradation from mudstone to limestone) is not seen. When rocks next appear in the stream they are typical Glencar Limestone lithologies interbedded with dark grey shales and they persist for the next 100m. There is a limited fauna of corals and brachiopods in the first 30m but the upper section yields a rich coral/brachiopod fauna with a microfauna typical of the Asbian. | | The junction with the final formation of the succession, the Dartry Limestone, is seen in the streambed but is best exposed in an old quarry forming part of the bank. There is a marked increase of silt in these cherty limestones, which could also be called limey siltstones at times, which have a tendency to weather into thin sheets. Fossils occur sparsely and the fauna as a whole is species poor. Sponge spicules and clotted bacterial crusts are found with around 20 species of brachiopods and six of corals. This fauna is consistent with an Asbian age. In total around 275m of Dartry Limestone is present - a considerable development, but by no means the entire thickness. The rest of the succession is not exposed at Fardross. | | Fardross relates about 7 million years of the continuous history of an extensive ocean basin which lasted from middle Arundian to late Asbian times. The basin was on the southern periphery of a huge supercontinent called Laurussia which had just crossed the equator. It consisted of modern day North America, Greenland and Europe (including western Russia and Scandinavia) and it was creeping imperceptibly, a few millimetres a year, to the north. | | About 344 million years ago (middle Arundian) the tropical floor of the basin was covered with carbonate sediments on to which a delta rapidly encroached, fed by sands and silts from nearby land (Mullaghmore Sandstone). On several occasions plants colonized its surface but by 342 million years ago the intermittent incursions of the sea gave way to the flooding of the entire delta surface as the ocean floor subsided suddenly and the shoreline receded to the north. On the floor of this basin, on top of the delta sands, a rich variety of animals thrived; their shells mixed with the plentiful silts and muds pouring into the sea along the now distant shoreline (Benbulben Shale). About 339 million years ago (early Asbian), the silt and mud supply dramatically declined and organic lime muds and shell sands dominated sedimentation, although there were still periodic influxes of mud (Glencar Limestone). At this point the basin deepened to the limits of light penetration, as indicated by the absence of algae and sea floor foraminifera and the presence of sponges and cyanobacteria. In these circumstances, faunas became impoverished and populations small, with many shells transported into the area on lime mud and silt bearing currents (Dartry Limestone). This basin persisted through to late Asbian times but there are no rocks of this age exposed in the Fardross section. | | The Fardross stream section is a geologist’s dream. Five formations of early Carboniferous sediments are well exposed over little more than 0.5km and their relationships to each other and the evolution of the ocean basin in which they were formed tell a clear story. An added bonus is the idyllic setting in a public forest with excellent footpaths and an absence of development pressures. This is an ideal Area of Special Scientific Interest. |
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