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Earth Science Conservation Review
| Tullybrick | Londonderry |
| Site number: | 587 | ||
| ESCC: | EO | ||
| Locality Type: | Stream section | Status: | ESCR |
| Grid Reference: | H732904 Centroid | ||
| Google maps: | 54.75666,-6.863 | ||
| County: | Londonderry | Council area: | Mid Ulster District Council |
| Period: | Ordovician | ||
| Lithostrat: | Tyrone Volcanic Group | ||
| Site Description | |
|---|---|
| Highlights: | |
This site is located in an unnamed tributary to Tullybrick Burn where there are numerous accessible streambed outcrops of pale coloured sericite tuffs and grey phyllite. These rocks belong to the Tyrone Volcanic Group (Ordovician). There is evidence of tectonic deformation and development of pervasive cataclastic (mylonitic) texture. | |
| Introduction: | |
The Tullybrick site is in a stream section in the townland of Tullybrick south of the Moyola Valley and close to the Six Towns. Streambed outcrops extend for a distance of about 600m upstream from the Bridge at [27320 39040]. Access to the site is from the Tullybrick Road via the bridge parapet. On Portlock's early geological map of the area (1843), the rocks in the vicinity of Tullybrick were represented as "mica schists" and were not distinguished from the metamorphic rocks of the Sperrin Mountains. On the first edition of the one-inch to the mile scale geological map (Sheet 26, Draperstown, Geological Survey of Ireland, 1882) the rocks at Tullybrick were shown as "chloritic, Talcose & Micaceous Schists of Lower Silurian age". In the memoir, which accompanied the map, Nolan (1884) referred to "shining thin-bedded schists ..." in the nearby Altihaskey Glen. Hartley (1933) published the first detailed lithological map and description of the central Tyrone Ordovician volcanicplutonic terrane and incorporated the rocks at Tullybrick in a division which he referred to as the "Phyllites and Chloritic Schists of Tullybrick". Hartley considered that the schists at Tullybrick were part of the volcanic tuff and lava (volcanic) series into which they grade. On the second edition (1:50,000 scale) of Sheet 26(Draperstown) (Geological Survey of Northern Ireland, 1995) the stratigraphy of the volcanic and plutonic rocks of Tyrone and south Londonderry was revised and the rocks at Tullybrick are included within the Tyrone Volcanic Group. The Tyrone Volcanic Group forms part of the Tyrone-Girvan Sub-Terrane of the Midland Valley Terrane as designated by Bluck et al. (1992). This terrane comprises ophiolitic and arc-volcanic type geological sequences which formed along the Laurentian foreland at the northern margin of the Iapetus Ocean during closure of the Iapetus Ocean during Ordovician times (Hutton et al., 1985). | |
| Description: | |
In the lower stretch of the section, directly upstream from the bridge, there are exposures of fine-grained pale-buff and pink siliceous schist and phyllite. These have a well-developed foliation (84º/038ºN) and a conspicuous mylonitic texture with a strongly developed horizontal mineral stretching lineation. These rocks contain conspicuous sulphides in the form of scattered randomly orientated euhedral pyrite cubes. Further upstream, lithologies alternate between grey and green chloritic and sericitic schist and grey silky phyllitic rocks which once more locally display well developed mylonitic texture and horizontal mineral stretching lineation. At the top of the section there is an isolated exposure of a pale green granular rock with conspicuous plagioclase phenocrysts. In thin section (JTP 13) this rock has a fine-grained grey green chloritic matrix with a spaced cleavage delineated by lenses of green-brown chlorite. This lithology probably represents metamorphosed basic lava. | |
| Importance: | |
Tullybrick is a site of national importance. It contains exposures of a distinctive suite of strongly deformed Ordovician tuffs and basic lava in the footwall of the Omagh Fault. The presence of strong horizontal mineral-stretching lineations and a well-developed mylonitic texture indicates a cataclastic deformation related to transform faulting. It is likely that deformation is related to faulting and terrane accretion in the Tyrone-Girvan sub-terrane and is distinct from the later deformation phase which resulted in southeasterly directed thrusting along the Omagh Fault (Alsop & Hutton, 1993) which produced the main deformation in the Tyrone Volcanic Group. | |
| Interpretation: | |
The rocks at Tullybrick are deformed volcanic tuffs and lavas which form part of the Tyrone Volcanic Group. The Tyrone Volcanic Group and the closely related Tyrone Plutonic Complex Ophiolite are important components of the Caledonian-Appalachian Orogen in Ireland. The Plutonic Complex represents a fragment of ancient oceanic crust obducted onto the continental margin of Laurentia at the northern margin of the Iapetus Ocean sometime during the Arenig-Llanvirn (Ordovician) (Hutton et al., 1985). The rocks at this site (Tyrone Volcanic Group) were originally erupted as lavas and explosive pyroclastic flows in a volcanic island arc environment. Subsequent deformation and low-grade (Greenschist Facies) metamorphism during the Caledonian Orogeny resulted in the obliteration and modification of most of the original depositional and textural structures. The presence of strong mylonitic textures in the rocks at Tullybrick and in the surrounding area is evidence of intense cataclastic deformation. These textures are accompanied by a very strong horizontal stretching lineation which indicates that, despite proximity to the Omagh Fault, which separates the Tyrone Volcanic Group from the Dalradian (Southern Highland Group), the rocks at Tullybrick were deformed as a result of transform movements most probably related to accretion of the Tyrone-Girvan sub-terrane. | |
| Conclusions: | |
Tullybrick is considered to be a site of national importance which affords an opportunity to study a distinctive suite of volcanic rocks within the Tyrone Volcanic Group. The presence of strong cataclastic deformation and the location of this site in the footwall of the Omagh Fault make it of particular interest to structural geologists studying the evolution of the Tyrone-Girvan Sub Terrane and the tectonic history of the Omagh Fault. | |
| Notes: | |
For information and references on sites within the Tyrone Basic Plutonic Complex see the following sites: Key Site 580 - Black Rock Key Site 581 - Scalp Hill For information and references on other sites within the Tyrone Volcanic Group see the following sites: Key Site 582 - Mveela More Key Site 583 - Creggan Key Site 584 - Copney Hill Key Site 585 - Beaghbeg Key Site 586 - Bonnety Bush, Altihaskey Key Site 588 - Stuhanleanantawey Burn For information and references on Granitoid sites within the Caledonian Complexes of N. Ireland see the following sites: Key Site 589 - Cashel Rock Key Site 590 - Craigballyharky and Craigbardahessiagh Key Site 591 - Craiganawork, Pomeroy Key Site 592 - Tintagh, Slieve Gallion Key Site 593 - Loughmacrory Key Site 594 - Cushendun For general information and references on the Caledonian Igneous Complexes of N. Ireland, and for directions to other regional sites, see the following site: Key Site 595 - Introduction to the Caledonian Igneous Complexes of N. Ireland | |
| Keywords | |
|---|---|
| Minerals: | Chlorite, Sericite |
| Rocks: | Mylonite, Schist |
| Structures: | Caledonian, fault |
| Relations: | Extrusion |
| Access | |
|---|---|
| Approach: | Access to the site is from the Tullybrick Road via the bridge parapet. |
| Management: | |
The entire site is located in a steep-sided valley fenced from adjoining pasture. No direct threat to the integrity of the site was evident at the time of inspection. In future, the main foreseeable threat is likely to come from dumping of fill materials from the surrounding fields. There is sufficient outcrop at this site so that hammering or sampling restrictions are unnecessary. The rocks of the Tyrone Volcanic Group are considered to be prospective for a number of minerals including base metals and gold. Licensed mineral exploration has previously been carried out throughout central Tyrone. The area which includes this site is likely to attract further exploration activity sometime in the future. | |
| Map(s): | GEOLOGICAL SURVEY OF IRELAND, 1882. 1:63,360 Geological Sheet 26 (Draperstown). (Dublin, Ordnance Survey for the Geological Survey of Ireland). GEOLOGICAL SURVEY OF NORTHERN IRELAND, 1995. Draperstown. Northern Ireland Sheet 26. Solid and Drift Geology. 1:50,000. (Keyworth, Nottingham: British Geological Survey). | ||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Last updated: | 04 MAR 2001 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Tullybrick, County Londonderry, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=587. Accessed on 2026-11-07. |
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