Benburb - Milltown

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Armagh CountyTyrone County

Climate exposure

This is about exposure to a nationwide pressure this habitat type carries — not a verdict on this site's own management.

Habitats

Fast flowing river and stream

Geology & landscape

Geological Feature: Gorge / Ravine, Rock Outcrop · Mining / Industrial: Quarry · Geological Period: Carboniferous, Ordovician, Silurian, Triassic · Rock Type: Conglomerate, Limestone, Mudstone, Sandstone, Shale

Landscape processes: Coastal, Fluvial

Description

Benburb-Milltown Carboniferous stratigraphy, Triassic stratigraphy Benburb-Milltown is of special scientific interest because of the exposures present allowing the areas geology to be described and understood. The River Blackwater gorge between Benburb and Milltown hosts a series of disused quarries, together with natural cliff exposures. Collectively these exposures represent over 230m of Lower Carboniferous rock strata belonging to the Tyrone Group that have been classified into four formations and a series of subdivisions known as Members. In addition, exposures within the River Blackwater provide outcrop of the younger Triassic rocks in this area comprising two named geological formations. All the formations and members mentioned in this account have their stratotypes, or reference localities, in this area. The lowest and hence oldest division present is the Maydown Limestone Formation, 126m of dark grey lime-rich shales, siltstones, silty limestones and crinoidal lim,estones 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 nearby exposures 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 or crinoids animals related to star fish. The species gathered here, particularly the solitary corals, have restricted time ranges and so indicate an Asbian age for these rocks, around 337 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 Maydown Limestone Formation a sequence of conglomerate and sandstone around 5.5m thick has been recognised and named the Crow Hill Conglomerate and Sandstone Member. The conglomerate is at the base of this section and contains frequent 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 2m 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 20m thick in total. A grey mudstone with thin limestones, the Gorestown Mudstone Member, is the lowest 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 consists 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 Gigantoprodllctus 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, with approximately 60m 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 Formation is around 26m thick and has been divided into 6 members, all fossiliferous. From the base these are 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 Blackwater 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 the Blackwater Formation. The boundary between the two is poorly marked by fossils. Many Brigantian fossil forms first appear in the late Asbian so are not reliable indicators. The best marker available is the primitive plant genus 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. The history of palaeontological studies in the area is also notable. It is 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 after Benburb. In addition, in the 19th century Portlock, McCoy and Davis described several species of fossils, notably corals, echinoids, bryozoa and fish, based on specimens from Benburb. The Benburb-Milltown area hosts the type localities for a series of younger rocks within the site. These outcrop along the River Blackwater upstream of the Milltown area and date from the Early Triassic period, around 245 million years ago. These rocks are known as the Milltown Conglomerate Formation and the Derrycreevy Sandstone Formation and while no fossil remains have been recovered, it is assumed, by analogy with modern environments, that the original sediments were deposited in desert conditions. The type locality of the Milltown Conglomerate Formation forms the north bank and river bed of the Blackwater opposite the mill factory at Milltown. Here 12 m of conglomerate containing small pebbles can be seen in beds up to 50 cm thick separated by partings of coarse-grained red and purple sandstone. The dominant rock in the pebbles is white vein quartz but fragments of Carboniferous, Ordovician and Silurian rocks have also been recognised. Between the mill factory and the weir on the north bank of the Blackwater, 800 m to the north west, is the type locality of the Derrycreevy Sandstone Formation. It sits on a foundation of Milltown Conglomerate that rapidly gives way to 150 m of brick-red, fine-grained sandstone which forms the base of the Derrycreevy Sandstone. The top of the formation is not seen but rare examples of large scale cross bedding up to 1 m thick occur which probably originated as aeolian deposits in the form of low sand dunes. The age of these rocks is difficult to determine with the absence of any fossils but the most recent appraisal places them in the Triassic period as part of the Sherwood Sandstone Group. This provides information on the extent of depositional basins and range of palaeo-environments within the Sherwood Group, a geological unit of importance for hydrogeology and potentially for hydrocarbon exploration. The wider area is of considerable importance for the buildings and other structures associated with past industries. With many of the outcrops associated with these historical industries, the relationship between the areas geology, development of rock outcrop and industrial archaeology is very notable. https://www.daera-ni.gov.uk/publications/benburb-milltown-assi

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