|Craigbardahessiagh and Bardahessiagh River||Tyrone|
|Site Type: ||Quarry (disused), Stream section|
|Site Status: ||ASSI|
|District: ||Cookstown District Council|
|Grid Reference: ||H72057385, H72557360, H725733|
|Rock Age: ||Silurian, Ordovician (Ashgill, Caradoc, Cautleyan, Hirnantian, Llandovery, Longvillian, Marshbrookian, Rawtheyan)|
|Rock Name: ||Bardahessiagh Formation, Killey Bridge Formation, Tirnaskea Formation|
|Rock Type: ||Mudstone, Sand, Sandstone, Shale, Siltstone|
|Fossil Groups: ||Bivalve, Brachiopod, Coral, Gastropod, Graptolite, Polyzoan, Trilobite|
|Other interest: ||Caledonian, platform subsidence, sub-terrane, thrust, turbidite, No Data, shelf|
Summary of site:
Between 450 and 440 million years ago (late in the Ordovician period), the northern half of Ireland was part of the continent of Laurentia. This large continent straddled the equator at the time and was drifting imperceptibly northwards. In the shallow seas on its southern margin the rocks now found at Craigbardahessiagh and in the banks of the Bardahessiagh River were formed. They provide vitally important evidence of the geography of the time and the site is also internationally important because it contains one of the richest faunas of Caradocian age anywhere in the world. Over 50 species new to science have been described from the area so far (with more to follow).
The earliest rocks seen belong to the Bardahessiagh Formation (160m thick), which has been divided into three members. The lowest member lies unconformably on coarse-grained igneous rocks of the Tyrone Igneous Complex. It is a silty sandstone with discontinuous beds and lenses which are rich in fossils. Towards the top, the proportion of mud-sized particles increases and here the sediment has been disturbed by contemporaneous grazing and burrowing by animals.
The second member is only 3m thick and consists of grey, silty mudstone. The top 4cm are more silty and again show considerable biological disturbance. Within this topmost layer there are nodules which are full of fossils, preserved as natural moulds formed by a process of decalcification.
The third member commences with mica-rich sandstones and siltstones within which is a remarkable and distinctive horizon, packed with an enormous number and diversity of fossils - mostly new to science. A loose block found by an amateur geologist was the first sign of this horizon’s existence. Subsequent trenching by a team from the Ulster Museum exposed a section from which many fossils were collected, enabling its exact position in the sequence to be precisely determined.
These upper beds of the Bardahessiagh Formation (known as the ‘Junction Beds’) continue upwards and are truncated by a thrust fault. All these rocks belong to a time division called the Caradoc epoch.
The next Ordovician beds seen in the area are 4 million years younger, part of the Ashgill epoch (the last of the period). These rocks - the Killey Bridge Formation - are lime-rich grey shales with infrequent fossils which are, however, sufficient to determine their age. The formation is 230m thick.
The Killey Bridge Formation passes upwards into dark-coloured, carbonaceous mudstones with sparse fossils indicating, again, the topmost division of the Ordovician (the Ashgill), finally grading into blue-grey shales about 8m thick, containing graptolites. These fossils are ideal for dating rocks and here they indicate a Llandovery age, the first epoch of the succeeding Silurian period. This boundary between two periods can be seen elsewhere locally, separated from this section by faulting.
The fossils, combined with the sediments, tell a clear story of changes in the ocean on the southern margin of Laurentia during the last third (about 15 million years) of the Ordovician period. The sea began to encroach on a shore which exposed the roots of the Tyrone Igneous Complex. At first this ocean margin was shallow and the sea bed thronged with life. Further subsidence drove the shoreline northwards but at Craigbardahessiagh the sea formed a deeper shelf with a deeper water fauna. Yet further deepening occurred and the inshore sediments with their fauna and loose shells began to slide in large masses downslope mingling with deeper sediments before finally coming to rest on the deep ocean floor. The exceptionally rich fossil bed belongs to this phase of development. The Killey Bridge sediments are also of a similar deep water environment and the graptolitic shales of the Tirnaskea Formation indicate even deeper water in the area at the end of the Ordovician and the start of the Silurian period.
The fossils, as well as indicating their environment and age, also show similarities and differences with other faunas living at the same time. They can therefore assist in the reconstruction of ancient geographies. At Bardahessiagh the faunas are similar to those in the rocks of the eastern United States Appalachian Mountains region and to those over the North Channel around Girvan in Scotland. They are very different from faunas found further south in Ireland and western Britain which lived on the shelf of an entirely different continent called Avalonia, almost 2000km to the south across the Iapetus Ocean. It would be the start of the Devonian period before these two continents collided.
Craigbardahessiagh and the Bardahessiagh River outcrops uniquely record a series of environmental events in the late Ordovician period with equally unique faunas that betray affinities with North America and Scotland. The localities in this part of Tyrone are of exceptional national and international importance, although some key horizons have only been located by trenching.
At present the land is poor quality grazing and fencing of a few pockets to provide protection for certain outcrops threatened by the passage of livestock would be desirable. The research interest of the area and its further potential are so high that excavation of artificial outcrops could be justified.