Sruhanleanantawey Burn

Also within

Londonderry County Sperrin Scenic Area

Habitats

Fast flowing river and stream

Geology & landscape

Geological Period: Ordovician · Rock Type: Chert, Mudstone, Shale, Siltstone

Landscape processes: Volcanic / Igneous

Description

Sruhanleanantawey Burn Ordovician stratigraphy, Caledonian igneous Sruhanleanantawey Burn has been designated an ASSI because of its' important geology. The stream section exposes a sequence of sedimentary, volcanic and volcani clastic rocks of Ordovician age, some 460 million years old. The sequence is part of the Tyrone Volcanic Group and fragments of fossil organisms called graptolites have been found in shale at the site. These fossils have allowed more accurate dating of the rocks to the Arenig-Llanvirn stages of the Lower Ordovician. The Tyrone Volcanic Group is comprised of rocks that record a phase during the closing of the Iapetus Ocean and the available evidence suggests they formed in an island arc setting. Isolated outcrops of pink-red weathered quartz porphyry, an igneous rock, occur in the lower section of the burn. The porphyry is locally sheared and contains euhedral quartz, plagioclase feldspar and less commonly greenish mica phenocrysts in a pinkish fine-grained dacitic matrix. Moving upstream, the outcrops alternate between the dacite porphyry and greenish grey tuffs and lava with clear hornblende and feldspar phenocrysts. Contacts between the lavas, tuffs and porphyry are not exposed at the site, although they can be traced to within a metre or so. The porphyry does not show any chilling near its' margins. The upper part of Sruhanleanantawey Burn exposes pale grey, very fine-grained slatey, phyllitic rocks that dip steeply downstream. These rocks show faint sedimentary bedding. Further upstream, phyllites are interbedded with coarse tuffs and dark grey coarse-grained crystal tuff, the former with a well-developed bedding/cleavage intersection lineation. This part of the succession also contains chert and light grey thin-bedded tuffaceous siltstone. The top of the Sruhanleanantawey Burn section contains 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 and Sigmagraptus s.l. 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. Sruhanleanantawey Burn contains exposures of volcanic and volcani clastic rocks which formed close to or in an aqueous marine environment in an island arc setting, during the Ordovician as the Iapetus Ocean was closing. This arc formed as the ancient ocean crust was being subducted in a north westerly direction under the ancient continent of Laurentia. https://www.daera-ni.gov.uk/publications/sruhanleanantawey-burn-assi

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