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Earth Science Conservation Review
| Sruhanleanantawey | Londonderry |
| Site number: | 588 | ||
| ESCC: | EO | ||
| Locality Type: | Stream section | Status: | ASSI |
| Grid Reference: | H788886 Centroid | ||
| Google maps: | 54.73964,-6.77653 | ||
| County: | Londonderry | Council area: | Mid Ulster District Council |
| Period: | Ordovician | ||
| Stages: | Arenig, Llanvirn | ||
| Lithostrat: | Tyrone Volcanic Group | ||
| Site Description | |
|---|---|
| Highlights: | |
This site consists of outcrops in the bed and banks of Sruhanleanantawey Burn and consists of black shale interbedded with mixed volcanic and volcaniclastic rocks (tuff, phyllite and chert) all belonging to the Tyrone Volcanic Group. The black shale contains a fragmentary but determinable graptolite fauna. So far, this is the only locality in the Tyrone Volcanic Group to have yielded a diagnostic fauna. | |
| Introduction: | |
Sruhanleanantawey Burn rises on Glenarudda Mountain at a height of about 500m above sea level. The stream flows northwards to become a tributary of the White Water which in turn is part of the Moyola catchment. Various lithologies of the Tyrone Volcanic Group are exposed in the bed and banks of Sruhanleanantawey Burn above the 380m contour. Most significantly, isolated outcrops of chert, black siltstone and mudstone are interbedded, apparently conformably, with the volcanic sequence. The black mudstones contain a fragmentary but identifiable graptolite fauna indicating an Arenig to Llanvirn age for the mudstone and by implication the Tyrone Volcanic Group at this locality. On Portlock's early geological map of the area, (1843) all rocks in the vicinity of Sruhanleanantawey were classified together as "metamorphic rocks of hornblendic type". There was no specific reference to the black graptolitic shales on either the map or in the text. On the first edition of the one-inch to the mile scale geological map (Sheet 27, Cookstown, Geological Survey of Ireland, 1880) rocks on the northern slope of Glenarudda Mountain were subdivided into "pyroxenic rocks" and granitoid porphyry (elvanite) with no indication of any other lithologies. The memoir which accompanied the map (Egan, 1881) contained no specific reference to the site. Cole (1887) identified a volcanic series including andesites and tuff within the hornblendic and pyroxenic rock as shown on the Geological Survey map but did not specifically describe the graptolitic sediments at Sruhanleanantawey. Hartley (1933) published the first detailed lithological map and description of the central Tyrone Ordovician volcanicplutonic terrane. He included the rocks on the north slope of Glenarudda Hill within a broad belt of tuffs and andesites which he referred to as the Tyrone Igneous Series, drawing a lithological analogy with the Highland Border rocks of Scotland. Hartley (1936) later discovered cleaved and pyritized graptolitic shales in Sruhanleanantawey Burn and, on the basis of the presence of Dicranograptus, proposed an Upper Llandeilo to Lower Caradoc (Ordovician) age for these strata. On the second edition (1:50,000 scale) of Sheet 27, Cookstown, Geological Survey of Northern Ireland, 1983), the northern slopes of Glenarudda Mountain were mapped as undifferentiated volcanic rocks of Upper Ordovician age. Other lithologies present include quartz porphyry and metadiorite (Cameron et al., 1997). In 1984, Hutton resampled Hartley's original graptolite locality and recovered a more extensive fauna that included fragments of Tetragraptus serra (Brongniart) and Sigmagraptus s.l. This led to a reappraisal of the age of the graptolitic beds, and by inference the Tyrone Volcanic Group. On the basis of the new graptolite fauna, Hutton et al. (1992) proposed an Arenig to Llanvirn age (Lower Ordovician) for the Tyrone Volcanic Group as opposed to the Upper Llandeilo-Lower Caradoc age originally proposed by Hartley (1936). 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: | |
A variety of contrasting lithologies can be seen in a traverse up Sruhanleanantawey Burn from the confluence with Sruhanoam Burn [3788 3888]. The lower section of the Burn contains isolated outcrops mainly of reddish and pink weathered, platey and locally sheared porphyry with euhedral quartz, plagioclase feldspar and less commonly greenish mica phenocrysts in a pinkish fine-grained dacitic matrix. Further upstream, the outcrops alternate between dacite porphyry and greenish grey tuffs and lava with conspicuous hornblende and feldspar phenocrysts. In this section it is often difficult to distinguish between tuff and schistose lava. Contacts between lavas, tuffs and porphyry are not exposed although they can be traced to within a metre or so. There is no apparent chill texture visible in the porphyry in these marginal areas. The upper reaches of the Sruhanleanantawey Burn were mapped and logged in detail by the Geological Survey of Northern Ireland during the resurvey of the Cookstown sheet in 1970 (Cameron et al., 1997). Here pale grey, very fine-grained slatey phyllitic rocks dip steeply downstream. These rocks have a faintly visible sedimentary bedding (Thin Section N12960). Further upstream phyllites are interbedded with coarse tuffs and dark grey coarse-grained crystal tuff (NI2964), the former with a well-developed bedding/cleavage intersection lineation. This part of the succession also contains chert (NI2963)* and light grey thin-bedded tuffaceous siltstone (NI2965). At the top of the Sruhanleanantawey Burn section, there are restricted outcrops of black to dark greyish blue graphitic siltstone and mudstone. These beds contain a sparse graptolite fauna that includes fragments of Tetragraptus serra (Brongniart) and Sigmagraptus s.l. (Hutton et al., 1992). These rocks have a poorly developed cleavage roughly normal to bedding. The mudstone is locally pyritiferous with discrete cubes throughout the rock and pyrite coatings on many of the cleavage surfaces. | |
| Importance: | |
Sruhanleanantawey Burn is a site of national and arguably international importance. It contains a datable graptolite fauna in mudstone and siltstone conformably interbedded with the Tyrone Volcanic Group. | |
| Interpretation: | |
The uppermost reaches of Sruhanleanantawey Burn contain a mixed succession of schistose lava and tuff interbedded with thin siltstone, mudstone and chert. These are volcanic and volcaniclastic rocks which formed close to or in an aqueous environment. These rocks are typical of the Tyrone Volcanic Group that has been interpreted as island-arc complex. This arc formed as a result of progressive closure of the Iapetus Ocean which once separated the North American continental plate (Laurentia) from southern Britain (Avalonia). Arc magmatism was triggered by the north-westerly directed subduction of Iapetus oceanic crust along the Laurentian continental margin. The graptolite fauna from graphitic mudstone interbedded with volcanic tuff and lavas at the top of the section indicates a Lower Ordovician (Arenig to Llanvirn) for the Tyrone Volcanic Group. The quartz feldspar porphyry exposed in the lower reaches of the Sruhanleanantawey Burn forms part of a series of sill-like bodies in the Slieve Gallion area. Although not demonstrable at this site, there is clear evidence elsewhere around Slieve Gallion that the porphyry post-dates both the volcanics and the main granodiorite and therefore cannot be regarded merely as a marginal facies of the Slieve Gallion Granite (Cameron et al., 1997). | |
| Conclusions: | |
Sruhanleanantawey Burn is considered to be a site of national and international importance. It is a valuable educational and research resource containing the only datable graptolite fauna so far recorded in the Tyrone Volcanic Group. | |
| 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 587 - Tullybrick 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 | |
|---|---|
| Rocks: | Chert, Mudstone, Porphyry |
| Fossil Groups: | Graptolite |
| Structures: | Caledonian |
| Access | |
|---|---|
| Management: | |
This site encompasses a number of stream bed and bank outcrops in an area of unfenced rough pasture. At the time of the inspection visit, there was no apparent threat to the site from development. The most important part of this site is the section of mudstones and associated sediments including the graptolite locality at the top of the Burn. This is also potentially the most vulnerable part of the site. Mudstone forms a relatively minor proportion of the succession overall and outcrop is limited. The present day outcrop is largely the result of excavations by Hutton between 1984 and 1989. This is now becoming overgrown. Further large-scale excavations should be discouraged or at least closely controlled. A sampling and hammering ban should also be considered for this part of the site. 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 in the future. | |
| Threats: | A sampling and hammering ban should be introduced. |
| Uses | |
| Uses: | Rough pasture |
| Map(s): | GEOLOGICAL SURVEY OF IRELAND, 1880. 1:63,360 Geological Sheet 27 (Cookstown). (Dublin, Ordnance Survey for the Geological Survey of Ireland). | ||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Last updated: | 04 MAR 2001 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Sruhanleanantawey, County Londonderry, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=588. Accessed on 2026-11-08. |
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