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Golan Burn, GortinTyrone
Golan Burn, Gortin, Co. Tyrone.
Summary Full report
Site Type: Stream section
Site Status: PASSI
District:
Grid Reference: H523872
Rocks
Rock Age: Carboniferous, Precambrian (Courcyean, Dalradian)
Rock Name: Glenelly Formation, Golan Burn Member, Owenkillew Sandstone Group
Rock Type: Limestone, Psammite, Schist
Interest
Other interest: No data, No Data

Summary of site:

The ancient Dalradian metamorphic (altered) rocks of the Glenelly Formation are all very similar in appearance, which often makes their various divisions difficult to recognise in areas where rock exposure is limited. This site is important because a highly characteristic subdivision of the Glenelly Formation, the Golan Burn Member, is exposed here. It is not only the best exposure of the member but also its stratotype (see glossary).

In the Golan Burn, immediately upstream from the bridge, rocks typical of the Glenelly Formation appear in outcrop. They are mostly pink to grey, quartz mica schists with thin, quartzite flags and rare lenses of grey, fine-grained limestone. About 100m upstream, a bedded siltstone appears above an unconformity (a time break in the rock record) forming a thin ‘skin’ on the surface of the Glenelly rocks for a further 100m. This siltstone is the base of the Carboniferous rocks of the area, part of the Owenkillew Sandstone Formation (better seen further west). Still further upstream, the typical metamorphic rocks of the Glenelly Formation re-emerge in the stream, but about 300m upstream of the bridge, at the waterfall, a 10m thickness of recognisably different rocks appears. These are impure, metamorphosed limestones with silvery schists and altered sandstones incorporating lime-rich veins and lenses. This sequence is the stratotype of the Golan Bridge Member. It used to be thought of as a local variation of the Clogherny Member but it is now known to rest on top of it. Part of the problem is the thin skim of Carboniferous rocks that obscures the general picture. The original rocks, before they were metamorphosed, were sediments washed into an extensive, coastal marine basin about 600 million years ago. They settled on the shallow flanks but instability, probably created by subsidence, triggered detachment of large masses of these deposits, which slid in coherent clouds into deep water, to settle on the ocean floor. The limestones of the Golan Bridge Member are shallow water deposits showing that water depth at the margin of the basin was limited for protracted periods. All the Dalradian rocks were metamorphosed when the region was involved in a massive continental collision about 465 million years ago. The enormous forces involved overturned the rocks of this area of the Sperrins in an overfold, so the entire sequence is upside down. The degree of metamorphism of rocks is established from the new minerals formed during the process and in the Golan Bridge Member: chlorite, biotite, muscovite, albite-oligoclase and tourmaline form an assemblage indicating a low degree of alteration. Stratotypes are the essential building blocks of geological history and this section should therefore be preserved for future reference and research. There are no immediate threats to its survival.


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