|Derraghadoan Pit, Dungannon||Tyrone|
|General view of west wall of Derraghadoan Pit, Dungannon, Co.Tyrone.|
|Site Type: ||Pit|
|Site Status: ||PASSI|
|District: ||Dungannon District Council|
|Grid Reference: ||H793644|
|Rock Age: ||Carboniferous (Brigantian, Visean)|
|Rock Name: ||Leitrim Group, Rossmore Mudstone Formation|
|Rock Type: ||Mudstone, Siltstone|
|Fossil Groups: ||Bivalve, Brachiopod, Coral, Gastropod, Goniatite, Nautiloid, Polyzoan|
|Other interest: ||No data, Marine sediments|
Summary of site:
Rocks of Carboniferous age dominate the geology of Ireland so understanding major changes within the period is important. For the first 30 million years of the Carboniferous Period (from around 362 to 332 million years ago), the rocks are marine in origin, dominated by limestones and shales, generally richly endowed with tropical marine fossils. This major sub-period of the Carboniferous is called the Dinantian. At its close there was a major change in events and land and fresh water deposits became the norm in the succeeding Namurian. The Rossmore Mudstone Formation straddles the divide between the two, shedding some light on the transition in this area.
One problem with this formation is its lack of surface exposure, so this pit just north of Dungannon, exposing over 12m of rock, is important. The succession here is monotonous and consists of dark grey to black shaley mudstones with three thin but conspicuous beds of siltstone. There is a single ironstone bed and oval concretions (small cemented patches of mudstone) are scattered throughout.
It reflects a time of stability with marine muds slowly accumulating in a fairly deep offshore hollow on the continental shelf of a supercontinent called Laurentia. Conditions were tropical as Laurentia crept slowly northwards towards the equator. At long intervals thin silt deposits in shallow water became unstable and slid downslope, stirring as they swept into the deep basin to settle as thin sheets on the mud. Life was sparse on this seabed, restricted to a few sea snails and bivalve molluscs. The corpses of some swimming animals, such as nautiloids and goniatites (extinct spiral-shelled relatives of octopuses and squid), occasionally sank into the muds.
There are more sandstones towards the top of the Rossmore Mudstone, although they are not seen here, indicating an increasing proximity to the coal forest deltas that would soon invade the region.
The importance of this site lies in its exposure of the Rossmore Mudstone Formation which is now the best surviving. It also shows the last major marine phase of the Dinantian. For these reasons it should be preserved but management of the site to maintain an exposed face following cessation of working will need careful planning.