| Site Description |
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 | | Cushendun Bay, Co. Antrim. |
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| Highlights: |
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| Outcrops at the northern end of Cushendun Bay display much of the lithological variation within the Glendun Formation. The folded and cleaved metasedimentary rocks are cut by quartz-feldspar porphyry veins related to the nearby Cushendun Granite and locally there is evidence of extensive pneumatolytic alteration of the host rocks in the vicinity of pegmatite veins. |
| Introduction: |
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| The site is located between high and low water at the northern end of Cushendun Bay in the vicinity of Rock Port. Part of the site, south of Rock Port, falls within a National Trust Conservation Area. Site access is by walking along the beach and dunes from the car park at [D248 329]. |
| On the first edition of the one inch to one mile geological map of the area (Cushendall, Sheet 14 Geological Survey of Ireland, 1886), the Dalradian rocks in Cushendun Bay and at Rock Port were simply classified as "metamorphic rocks". |
| McCallien (1931) and Bailey & McCallien (1934) produced the first detailed lithostratigraphic maps of the Dalradian of north-east Antrim. They based their structural interpretation of North-east Antrim on the premise that they were viewing the south-westerly continuation into Ireland of recumbent fold structures already identified on the Cowal Peninsula, SW Scotland (Gunn et al., 1897). Bailey & McCallien adopted the lithostratigraphic nomenclature which was currently in use in Scotland and included the rocks at Cushendall within the Ben Ledi Grits, the youngest of the Upper Dalradian Formations in the area. |
| A more formal approach to lithostratigraphic terminology for the North-east Antrim Dalradian was adopted for the first edition of the Solid Geology Map of Northern Ireland (Geological Survey of Northern Ireland, 1977). For the first time, local lithostratigraphic names were applied and the rocks at this site were referred to as the Glendun Schists, see table below. |
| The Glendun Schists were subsequently redefined by Arthurs (1976) who produced a major revision of the Dalradian geology of north-east Antrim and defined the lithostratigraphic framework. This work was largely based on reconnaissance mapping and further detailed work is required in this area. Nevertheless, it has been incorporated in the second edition of the Solid Geology Map of Northern Ireland (Geological Survey of Northern Ireland, 1997). |
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| TABLE TO BE ADDED Dalradian stratigraphy and ESCR sites in North-east Co. Antrim. |
| Description: |
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| The outcrops at the northern end of Cushendun Bay and in the vicinity of Rock Port [D252 325] consist mainly of grey and greenish grey quartz schist (psammite) and albite-chlorite- biotite schist locally with tourmaline (Glendun Formation). Albite is particularly conspicuous in the more biotite and chlorite rich bands within the succession and occurs as pink euhedral porphyroblasts which resist weathering on wind and wave-polished rock surfaces. |
Both north and south of Rock Port, there are outcrops of untectonized pinkish brown porphyry sheet intrusions
 | | Quartz-feldspar porphyry sheet cutting Glendun Formation, Rock Port, Cushendun. |
containing quartz and euhedral feldspar (oligoclase) phenocrysts with visible compositional zoning. This porphyry appears to be closely related to a late-Caledonian ("Younger Granite") boss (Cushendun Granite) which lies just over 1km to the west of the site. Outcrops of porphyry to the north and south of Rock Port contain partially rounded and sub-angular xenoliths of schist. The porphyry is closely associated with locally intense quartz- feldspar pegmatite veining. Reynolds (1942) described these veins as consisting of albite, like that of the albite schists, associated with quartz and described relics of replaced schist remaining within the pegmatites. It is also apparent there has been some limited potassic metasomatism in the vicinity of the main porphyry intrusions. |
The wave polished outcrops south of the pier at Rock Port display details of the structural deformation of the metasediments. In addition to bedding and an early, co-planar S1, a number of tight isoclinal and disharmonic folds have a well developed cleavage (S2) (050°/30°SE) which in places is axial planar to the tight folds. S2 is the regionally dominant fabric throughout north-east Antrim and is related to the regional scale overfold (Altmore Fold is equivalent to the Cowal Anticline). The main S2 is in turn crenulated locally by a patchily developed S3.
 | | Folding in quartz-biotite schist (Glendun Formation), Cushendun Bay. |
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The northern limit of this site is marked by outcrops of massive, dark green hornblende-rich metabasite on the northern side of the rocky inlet and north of the pier at Rock Port.
 | | Metabasite intrusion, Rock Port, Cushendun. |
Similar metabasites (epidiorites) occur throughout north-east Antrim, and represent metamorphosed dolerite intrusions. Mineralogically the metabasites are variable but for the most part consist of +/- quartz together with hornblende, albite, ilmenite, variable amounts of epidote, zoisite, actinolite, biotite and some apatite and garnet (Wilson et al., 1966). |
| Importance: |
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| This is a site of national and potentially international importance. It is one of the principal localities cited by Reynolds (1942) in the scientific debate about the origin of the albite schists and their relationship to tectonism during the Caledonian Orogeny. |
| At the height of this debate Reynolds argued that albite schists formed as a result of a complex sequence of metasomatic events which introduced sodium, potassium and possibly iron and magnesium into the Dalradian sediments at the early stages of deformation. This contradicted the opinion expressed by Bailey & McCallien (1934) that the albite schists merely reflected the compositional variation within the sedimentary protolith and were the consequence of regional metamorphism of albite-rich mud rocks. |
| Interpretation: |
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| During the Upper Dalradian ensialic marine basins developed along the unstable margin of the Laurentian continent. These basins were then infilled with predominantly siliciclastic turbidite facies sediments derived as a result of erosion of the nearby continental landmass. |
| During the Caledonian Orogeny, the Dalradian rocks were intensely deformed. In north-east Antrim this deformation was complex and polyphase resulting in folding and formation of up to four recognisable fold phases and related cleavages (Arthurs, 1976; Goldring, 1961). Structurally, the rocks at Cushendun Bay now lie in the steeply downturned nose of the Altmore Anticline (regarded as the south-westerly continuation into Ireland of the Cowal Anticline in SW Scotland. (Gunn et al., 1897; Arthurs, 1976), in a structural setting possibly analogous to the Highland Boundary Steep Belt in Scotland. |
| Deformation was accompanied by comparatively low-grade regional metamorphism. The strata at Cushendun Bay contain mineral assemblages indicative of pressure and temperature conditions within the quartz-albite-muscovite-chlorite sub-facies of the Greenschist Facies. |
| Outcrops at Cushendun and Rock Port testify to the metasomatic effects of intrusion of the Cushendun Granite. This largely undeformed ("Younger Granite") intrusion led to extensive metasomatism and pneumatolytic alteration in the surrounding metasedimentary rocks. It is worth noting however that although the porphyry veins contain albite similar to albite in the albite schist, the albite in the albite schists pre-dates the emplacement of the granite porphyry. |
| Conclusions: |
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| Cushendun Bay is a site of national and international significance. The site was cited in the scientific literature during the debate surrounding the origin of albite schists in the British Dalradian. The site also offers an opportunity to study wave-polished outcrops of strata representative of the Glendun Formation, late Caledonian granite porphyry intrusions and metasomatism. |
| Notes: |
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For site specific information and references on other Dalradian sites in North-east Antrim see the following sites: |
| Key Site 155 - Benvan Key Site 156 - Escort Port Key Site 157 - Torr Head Key Site 158 - Altmore Burn Key Site 159 - Leckpatrick Point Key Site 160 - Carnaneigh Key Site 161 - Loughan Bay Key Site 162 - Loughareema Key Site 163 - Knocknacarry Bridge |
| For site specific information and references on other Dalradian sites in the Sperrin Mountains, Londonderry and Tyrone see the following sites: |
| For Central and Southern Sperrin Mountains: |
| Key Site 130 - Butterlope Glen Key Site 131 - Craig Key Site 132 - Mullaghcarbatagh Key Site 133 - Henry's Bridge, Glensass Burn Key Site 134 - Oughtboy Burn Key Site 135 - Garvagh Burn Key Site 136 - Barnes Gap Key Site 137 - Barnes Burn Key Site 138 - Golan Burn Key Site 139 - Glendarragh Burn, Glenlark Key Site 140 - Mountfield Old Quarry Key Site 141 - McNally's Burn, Broughderg |
| For North Sperrin Mountains and Londonderry: |
| Key Site 142 - Letterbrat Quarry Key Site 143 - Banagher Glen Quarry Key Site 144 - Strabane Quarry Key Site 145 - Glenmornan River, Artigarvan Key Site 146 - Balix Hill Key Site 147 - Drain Quarry Key Site 148 - Kildoag Quarry Key Site 149 - Loughermore Mountain Key Site 150 - Burntollet and Tamnymore Wood Key Site 151 - Dog Leap, Limavady Key Site 152 - Kittybane Quarry Key Site 153 - Prehen Quarry Key Site 154 - Creevagh Hill Old Quarry |
| For general information on the Dalradian rocks of Northern Ireland see the following site: |
| Key Site 124 - The Dalradian Rocks of Northern Ireland |
| For general and site specific information on the Pre-Dalradian rocks of Northern Ireland see the following site: |
| Key Site 123 - The Pre-Dalradian Rocks of Northern Ireland |
| For general information and an extensive reference list on the Precambrian of Northern Ireland see the following site: |
| Key Site 122 - An Introduction to the Precambrian of Northern Ireland |
| This site is also important for Structural Geology: See Key Site 651 - Cushendun Bay; and Caledonian Igneous Geology: See Key Site 594 - Cushendun. |
| References |
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| Arthurs, J.W. 1976a: The geology and metalliferous mineral potential of the Sperrin Mountains area Geological Survey of Northern Ireland Special Report |
| Bailey, E.B. and McCallien, W.J. 1934: The metamorphic rocks of North-east Antrim Transactions of the Royal Society of Edinburgh: Earth Sciences, vol. 58, pp.163-177 |
| Gunn, W., Clough, C.T. and Hill, J.B. 1897: The Geology of Cowal Memoir of the Geological Survey of Scotland |
| McCallien, W.J. 1931: A contribution to the correlation of the Dalradian rocks of Scotland Geological Magazine, vol. 68, pp.153-175 |
| Reynolds, D.L. 1942: The albite schists of Antrim and their relationship to Caledonian orogenesis Proceedings of the Royal Irish Academy, vol. 48 pt. B, pp.43-66 |
| Wilson, H. E. and Robbie, J. A. 1966: Geology of the country around Ballycastle. Memoirs of the Geological Survey Northern Ireland, |
| Map(s): | GSI (1884). 1:63,360 Geological Sheet 14 (Cushendall), Dublin, OS for GSI / GSNI (1977). 1:250,000 Northern Ireland, Solid Geology (1st edition), Southampton, OS for IGS / GSNI (1997). 1:250,000 Northern Ireland, Solid Geology (2nd edition), BGS, Keyworth, Nottingham |