Field Trip Reports 2014Field_Trip_Reports.html

April 30th - Coalpit Bay, Donaghadee, County Down


The Southern Uplands – Down - Longford Terrane is arguably the most distinctive of the suspect terranes which compose the British and Irish Caledonides. The terrane is dominated throughout by well bedded greywackes. These greywackes only rarely contain fossils but at some 30 localities in Ireland and at an even greater number in the Southern Uplands of Scotland the thick greywacke successions rest on a thin (<150m thick) succession of black shales and mudstones which are commonly rich in graptolites. 

These graptolites were studied by an English schoolteacher, Charles Lapworth, working in the area of Moffat in the central part of the Southern Uplands in the middle years of the 19thcentury. Lapworth’s brilliant work ultimately allowed him to establish the Ordovician System and the graptolite zonation for much of both the Ordovician and Silurian Systems. Now, almost two centuries later, the graptolites of the Moffat Shales continue to be key to understanding the stratigraphy and structure of  the Southern Uplands – Down - Longford Terrane.


Lapworth established a lithological and biostratigraphic succession common to a number of Southern Upland localities and particularly well exposed at Dob’s Linn and at Craigmichen Scar. He recognised five members in three formations straddling the Ordovician/Silurian systemic boundary.


1- Upper Birkhill Shale (Succeeded conformably by thick Gala Group greywacke)

2- Lower Birkhill Shale

3- Upper Hartfell Shale (commonly known as the “Barren Mudstones” because of its almost complete lack of graptolites)

4- Lower Hartfell Shale (Rather flaggy grey siltstones and black shales)

5- Glenkiln Shale (Contains thick cherty mudstones interbedded with grey-black shale.)


For further detail and description see Chapter 4 of the Regional Guide to the geology of Northern Ireland.


All five of these members are well exposed at Coalpit Bay, one kilometre south of Donaghadee, where the total thickness of the Moffat Shale Group, much disturbed by folding and faulting, is about 110 metres and spans some 18 graptolite zones. Indeed Coalpit Bay offers easily the best and most complete exposure of the Moffat Shale in Ireland.


Charles Lapworth visited Coalpit Bay about 1875, apparently encouraged by William Swanston, then President of the Belfast Naturalists’ Field Club. The two men published a 40-page joint paper “On the Silurian rocks of the CountyDown” in the Proceedings of the Field Club for 1877 and Lapworth honoured his host by giving one of the new graptolite species his name.


About 30 members of the BNFC and the Belfast Geologists’ Society were present at Coalpit Bay on a cool, grey and ultimately wet Wednesday evening for this, the first meeting of the BNFC’s summer excursion programme.  All of the five Moffat Shale Group members listed above were demonstrated and examined and numerous graptolites were collected, despite the fact that the rocks of Coalpit Bay are dangerously slippery when wet.


As explained above the Moffat Shale Group outcrops in the Southern Uplands, Down and Armagh provide the vital clues to understanding the stratigraphy, age and tectonic structure of the Southern Uplands-Down-Longford Terrane.  The recent Tellus survey with its airborne resistivity/electomagnetics has traced the Moffat Shale across the two Northern Irish counties and facilitated the discovery of further outcrops.


Dr Bernard Anderson

 

Graptolite

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