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River Blackwater, BenburbArmagh, Tyrone
Maydown Quarry adjacent to River Blackwater at Benburb, Co. Armagh, exposes the type section of the Maydown Limestone Formation, Tyrone Group.
Summary Full report
Site Type: River bank, River bed
Site Status: PASSI
District: Armagh District Council, Dungannon District Council
Grid Reference: H802518, H819519
Rocks
Rock Age: Carboniferous (Visean)
Rock Name: Benburb Mudstone Member, Blackstokes Limestone Formation, Blackwater Limestone Formation, Carrickaness Sandstone Formation, Crow Hill Congl & Sst Member, Drumflugh Limestone Member, Glenview Limestone Member, Gorestown Mudstone Member, Island Sandstone Me
Rock Type: Conglomerate, Ironstone, Limestone, Mudstone, Sand, Sandstone
Interest
Fossil Groups: Bivalve, Brachiopod, Coral, Crinoid, Echinoderm, Fish, Foraminifera, Gastropod, Microfossil, Microfossils, Plant, Polyzoan, Sponge, Trilobite
Other interest: cross-bedding, ripple marks, Marine sediments

Summary of site:

The gorge at Benburb and the line of quarries on the south side of the River Blackwater extending west from Benburb Bridge into the gorge, expose over 230 m of Lower Carboniferous strata of four formations. Exposure is good allowing the formations to be subdivided into members.

The lowest division seen in the area is the Maydown Limestone Formation, 126 m of dark grey lime-rich shales, siltstones, silty limestones and crinoidal limestones (limestones composed almost entirely of crinoid ossicles, circular skeletal plates). With the exception of the base, the entire thickness can be seen in Maydown Quarry and the cliffs to the west. The formation has a rich fossil fauna including corals, lamp shells (brachiopods), bivalve molluscs, moss animals (bryozoa), sea urchins (echinoids) and stone lilies (animals related to star fish). The species gathered here, particularly the solitary corals, have limited time ranges that indicate an Asbian age, around 340 million years ago. There are also remains of land plants in a series of sandstones 8 m thick within the section. About 12 m below the top of the formation a conglomerate and sandstone around 5.5 m thick has been recognised and named the Crow Hill Conglomerate and Sandstone Member. The conglomerate is at the base with cobbles, which are actually rolled fragments of colonial coral colonies of the genera Lithostrotion and Siphonodendron, set in a variable matrix. A lens of coarse grained sandstone, containing pieces of the giant coral Siphonophyllia and fragments of shark teeth, occurs in the middle of the conglomerate. Passing upwards the conglomerate gives way to 2 m of pale grey to white coarse-grained sandstones containing plentiful fragments of corals. The Blackstokes Limestone Formation follows the Maydown and consists of two members estimated to be about 20 m thick in total. A grey mudstone with thin limestones, the Gorestown Mudstone, is the first member and contains an abundance of fossils including bivalve molluscs, brachiopods, bryozoa, crinoids, sea urchin fragments, ostracods, solitary corals, sponge spicules and trilobites. The section on the northern, Tyrone, side of the river is slightly different with more limestones. The second member, the Rookwood Limestone Member, is thought to be about 11 m of limestones with shale partings. The limestones are dark in colour and fine-grained. Fossils are infrequent and limited in variety. Large brachiopods of the genus Gigantoproductus are conspicuous and occur with bivalve molluscs and ostracods. There are trace fossils in the shales in the form of feeding tracks of an unknown animal. The shales have also yielded fish remains (teeth and scales), brachiopods, sea snails and solitary corals. The Carrickaness Sandstone Formation follows, approximately 60 m of sandstones, siltstones and mudstones almost devoid of fossils. The few found are restricted to plant remains and bivalve molluscs. The outcrop is fragmentary and best seen on the wooded bank of the Ulster Canal between Milltown House and in Blackstokes Bridge Quarry. The sandstones are pale grey to white with ripple marks and cross laminations, suggesting shallow water conditions. Early descriptions mention thin coal seams in this formation. The junction of the Carrickaness Sandstone with the overlying Blackwater Limestone Formation can be seen in the north bank of the river and the south bank of the Ulster Canal. The Blackwater Limestone is around 26 m thick and has been divided into 6 members, all fossiliferous, including microfossils; from the base the Tullymore Limestone Member, the Glenview Limestone Member, the Drumflugh Limestone Member, the Benburb Mudstone Member, the Island Sandstone Member and the Outlet Limestone Member. All the fossils up to the Backwater Limestone Formation in this area indicate an Asbian age, the penultimate stage of the Lower Carboniferous but there is good reason to believe that the final stage, the Brigantian, commences somewhere in this formation. The boundary between the two is ill-marked by fossils; many Brigantian forms first appear in the late Asbian so are not reliable indicators. The best marker available is a minute primitive plant genus called Koninckopora which becomes extinct world-wide at the end of the Asbian. It occurs in the Tullymore and Glenview Limestone members but is absent from then onwards. On this evidence and that of less precise Brigantian indicators the base of the Brigantian is tentatively placed at the base of the Drumflugh Limestone Member. Although no direct correlation can be made with the much thicker rocks of equivalent age in County Fermanagh the overall pattern of sedimentation is similar. For example, the Carrickaness Sandstone Formation at Benburb may well equate with the Glenade Sandstone Formation of Fermanagh. In the science of stratigraphy, every rock formation and member is defined from an observable rock sequence, usually its best or most typical occurrence. Such sites are designated type sections or stratotypes and they are the key building blocks of the geological history of any area. All the formations and members mentioned in this account have their stratotypes in this area which is consequently an essential part of our natural heritage and scientific culture requiring designated site status and protection. It is also believed that the giant solitary coral Siphonophyllia benburbensis, a spectacular and famous fossil, was first collected in this area in the 1930s by H P Lewis and the species was named by him from Benburb, the nearest village.


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