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Earth Science Conservation Review
| St. John's Point | Down |
| Site number: | 1190 | ||
| ESCC: | EF | ||
| Locality Type: | Coastal section | Status: | ASSI |
| Grid Reference: | J527333 Centroid | ||
| Google maps: | 54.22629,-5.65853 | ||
| County: | Down | Council area: | Newry, Mourne & Down District Council |
| Period: | Tertiary | ||
| Stages: | Eocene, Palaeocene | ||
| Site Description | |
|---|---|
| Highlights: | |
A dyke swarm of Tertiary age intruded into Lower Palaeozoic sandstones and shales in the Killough-Ardglass area. The dykes are principally basaltic in composition and have a number of petrological and chemical differences from the adjacent Mourne dyke swarm, and also from the Antrim dykes. | |
| Introduction: | |
Tertiary dykes belonging to the Killough-Ardglass swarm are exposed in coastal sections, mainly between Guns Island (J598408) NW of Ardglass and Ringsallin Point (J465353) SE of Ballykinler. The greatest concentration of dykes occurs between the coastguard station at Killough (J540353) and the lighthouse at St. John's Point (J527333). The dykes occur as single, multiple and composite intrusions and a number are highly porphyritic with labradorite phenocrysts. The dykes are basaltic with a preponderance of olivine-bearing rocks with the predominant direction of intrusion NW-SE. A comprehensive petrographical, geochemical and field study was made by Tomkeieff and Marshall (1940) detailing the occurrence and classification of the dykes and estimating the degree of crustal stretch involved in their intrusion. The nature and composition of the feldspar phenocrysts was recorded initially by Hutchinson and Campbell Smith (1912) and further commented on by Tsuboi (1923) and Campbell Smith (1928). The country rock consists of Lower Palaeozoic sediments, mainly greywackes and mudstones generally highly folded and faulted. Lamprophyre dykes of the Ards Peninsula swarm intersect the Tertiary swarm on its fringes around Killard Point (J613435) some 3km north of Gun's Island. | |
| Description: | |
The coastline from Killard Point (J613435) west to Dundrum Bay provides a near continuous section across the Killough-Ardglass dyke swarm. In thickness the dykes range from a few cm to 8m but are rarely greater than 3m in width. They are intruded into Lower Palaeozoic sandstones and mudstones which have been extensively folded and faulted, and the dykes are clearly later than the tectonic events which effected the country rocks. The majority of the swarm follows a NW-SE trend with a small component trending in a NE-SW direction. Tomkeieff and Marshall (1940) presented information on the density of dyke intrusion, their aggregate thickness and the lithologies present for 12 strips, each one mile wide, between Killard Point and Dundrum Bay. The intrusion density reaches a maximum around St. John's Point with a maximum crustal extension estimated at 2.15%. (i) Petrology of the Dykes The dykes occur as single, multiple and composite intrusions and are all basaltic in composition. They have been classified by Tomkeieff and Marshall as olivine-basalt, olivine-dolerite, spilite and trachybasalt. The olivine basalts and dolerites constitute the bulk of the dykes with olivine consistently present along with plagioclase feldspar, augite, iron-ore and subsidiary chlorite, apatite, analcite and calcite. The plagioclase is generally labradorite and the composition of the plagioclase phenocrysts was commented on by Hutchinson and Campbell Smith (1912) and Tsuboi (1923). The predominant texture is sub-ophitic. Olivine occasionally occurs as large phenocrysts in olivine-rich dykes such as dyke 34 (Tomkeieff & Marshall, op. cit.). A number of the composite dykes (e.g. No. 53, St. John's Point) consist of aphyric and porphyritic components, with large phenocrysts (up to 2-3cm across) of oligoclase-albite composition. (ii) Geochemistry of the dykes Major element analyses for five dyke samples of olivine-dolerite and trachybasalt are presented by Tomkeieff and Marshall (op. cit.), along with normative analyses. The composition of the olivine-dolerite is described as corresponding to an alkali- enriched plateau-magma type (broadly similar to the Skye Main Lava Series with more alkalis than the Lower or Upper Basalts in Antrim) while the trachybasalt has higher alkalis and silica and more closely resembles the intermediate lava compositions found in Antrim. All normative analyses quoted are qz- or hy-normative. | |
| Importance: | |
The Killough-Ardglass dyke swarm presents clear and generally accessible examples of basaltic minor intrusions. The importance of the swarm lies in the obvious geochemical differences when compared to the nearby Mourne swarm and the numerically more important Antrim swarm to the north. The significance of the several regional dyke swarms which can be identified to the overall petrogenetic scheme of the Tertiary igneous activity in NE Ireland has yet to be fully explored and these occurrences represent an important component of the evidence. | |
| Interpretation: | |
The Killough-Ardglass dyke swarm is one of several swarms of Tertiary age that can be recognized in the eastern part of Northern Ireland. Other examples include the swarms found around the northern and southern margins of the Antrim Plateau and the dykes associated with the Mournes. The limited geochemical evidence currently available (Tomkeieff & Marshall, 1940) indicates some differences between these dykes and the dykes and lavas of Antrim. In addition, while evidence exists for a degree of differentiation within the Killough-Ardglass dykes, there is not the range of basic to acid rock types found in the adjacent Mourne swarm. The compositions within the Killough-Ardglass swarm described as "trachybasalt" bear chemical and normative similarities to some of the intermediate composition lavas found towards the top of the Lower Basalt succession in Antrim (Lyle & Thompson, 1983). Since the south Down examples are all qz- or hy-normative they can be considered as part of a transitional suite showing tholeiitic affinities (Lyle, 1980) and the "trachybasalt" members can be re-defined as tholeiitic basaltic andesite or tholeiitic andesite using the revised nomenclature proposed by Lyle and Thompson, 1983. | |
| Conclusions: | |
The section of coast shows clearly the mechanisms of multiple and composite minor intrusions of basaltic rock into a structurally complex sedimentary sequence. | |
| Keywords | |
|---|---|
| Rocks: | Basalt, Dolerite, Spilite, Trachybasalt |
| Structures: | dyke, phenocrysts |
| Relations: | Intrusion |
| Access | |
|---|---|
| Approach: | Access by foot is open from the end of the Ballykinler/ Killough Road. |
| Restrictions: | There is restricted vehicle access to the lighthouse at St. John's Point. |
| Management: | |
Conditions underfoot are often difficult due to the generally steep dip of the sediments. As many of the best exposures are below the High Water Mark there are additional difficulties from water and seaweed cover and great care is necessary. There are no maintenance activities envisaged for the localities and sampling is likely to be limited to specific research projects. | |
| Uses | |
| Uses: | The main exposures are on the foreshore, usually between the High and Low Water Marks. Adjacent land use is mixed agricultural. A public footpath follows the coast to the east. There is an ancient monument site, St. John's Church near the termination of the road from Killough and Ballykinler. |
| References (4) | |||
|---|---|---|---|
| Hutchinson, A. & Campbell Smith, W., 1912. On sericite from North Wales and on penninite and labradorite from Ireland. Mineralogical Magazine, vol. 16, pp.264-264. | |||
| Smith, W.Campbell, 1928. The optical orientation of labradorite from County Down (Ireland) determined by the Fedorov method. Mineralogical Magazine, vol. 21, pp.543-543. | |||
| Tomkeieff, S.I. & Marshall, C.E., 1940. The Killough - Ardglass dyke swarm. Quarterly Journal of the Geological Society of London, vol. 96, pp.321-338. | |||
| Tsuboi, S., 1923. Optical dispersion of three intermediate plagioclases. Mineralogical Magazine, vol. 20, pp.93-93. | |||
| Map(s): | See 1:50,000 O.S. Sheet 21 Strangford Lough. | ||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. St. John's Point, County Down, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=1190. Accessed on 2026-11-08. |
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