|Plaister Quarry, Aughnacloy||Tyrone|
|Plaistor Quarry, Aughnacloy, Co. Tyrone, exposes Maydown Limestone Formation.|
|Site Type: ||Quarry (disused)|
|Site Status: ||PASSI|
|District: ||Dungannon District Council|
|Grid Reference: ||H678539|
|Rock Age: ||Carboniferous (Visean)|
|Rock Name: ||Maydown Limestone Formation, Tyrone Group|
|Rock Type: ||Crinoidal limestone, Limestone|
|Fossil Groups: ||Bivalve, Brachiopod, Coral, Gastropod, Polyzoan, Trilobite|
|Other interest: ||No data, Marine sediments|
Summary of site:
The rocks in this quarry were shown on the 1970 geological map of the area to belong to the Carnteel Formation, immediately below the Dartry Limestone, formed towards the end of the early Carboniferous period about 340 million years ago. A complete revision of all the Carboniferous rocks of Northern Ireland has subsequently revealed the rocks to belong to the Maydown Limestone Formation, a fossil-rich equivalent of the Dartry Limestone and of the same age.
The rocks exposed in the quarry are limestones and silty shales totalling about 8 m in thickness. The limestone beds are 10 to 50 cm thick, blue-grey in colour and close observation shows them to consist of cemented crinoid debris with a little mud. The bedding surfaces are wavy and the limestones are separated by dark brown siltstones (often enclosing thin, lumpy limestones), up to 20 cm thick.
All these beds contain fossils and the importance of the quarry lies in the large number of species of corals and brachiopods found here. One solitary and seven colonial coral species have been collected and 14 brachiopod species. Taken together the fauna indicates the Asbian stage but one of the brachiopods, Gigantoproductus tulensis, has a very limited time range, restricted to the early part of the stage, confirming an age of almost exactly 339 million years.
The fauna lived on a shallow, much winnowed, tropical sea bed when the area was on the fringe of a continent called Laurentia, creeping northwards across the equator.
This unusual quarry with its prolific fauna yielding a precise date is important and should be protected from adverse development.