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Earth Science Conservation Review
| Sandy Braes - Palaeogene | Antrim |
| Site number: | 517 | ||
| ESCC: | ED | ||
| Locality Type: | Inland exposure, Quarry (disused) | Status: | ASSI |
| Grid Reference: | J206958 Centroid | ||
| Google maps: | 54.796,-6.12488 | ||
| County: | Antrim | Council area: | Antrim & Newtownabbey Borough Council |
| Period: | Tertiary | ||
| Stages: | Eocene, Palaeocene | ||
| Lithostrat: | Antrim Lava Group, Interbasaltic Formation, Lower Basalt Formation, Tardree Rhyolite Complex | ||
| Site Description | |
|---|---|
| Highlights: | |
Sandy Braes represents an important component of the Tardree Rhyolite Complex, part of the Interbasaltic Formation of the Antrim Lava Group. The Sandy Braes area comprises a major vent complex of a range of obsidian and rhyolite tuffs and agglomerates bounded by Lower Basalts, rhyolites, rhyolite tuffs and agglomerates. | |
| Introduction: | |
The Sandy Braes vent (J206958) occurs at the northeast end of the Tardree Rhyolite Complex in mid-Antrim, some 10km northeast of the town of Antrim. The vent itself is around 500m wide at its maximum and consists of a range of rhyolitic and obsidian tuffs and agglomerates in steep contact with Lower Basalts, rhyolite lava flows and a range of acid tuffs and agglomerates. Detailed descriptions of the rock types and field relationships are given by Old (1975) who includes the complex in the Interbasaltic Formation of the Antrim Lava Group. Sheet 28 (Antrim) of the Geological Survey of Northern Ireland (GSNI) shows the occurrence of the various vent rocks within the Tardree Complex. The acid rocks here are seen as the culmination of fractionation trends seen elsewhere in the Lower Basalt Formation (LBF) in mid-Antrim (Lyle 1980, 1985) and the geochemistry and age have been commented on by Meighan (1979) and Meighan et al. (1984). | |
| Description: | |
The outcrops around the Sandy Braes area are described in detail by Old (1975), see Fig. 2 p. 25 or the simplified solid geology map of the area which is included on sheet 28 (Antrim) of the Geological Survey of Northern Ireland. The southeast side of the Tardree Complex, which includes the Sandy Braes locality, is believed to be bounded by a major NE-SW fault, the Browndod Fault. The northern boundary is obscured by drift but the nearest outcrops of basalt in this direction are considered LBF in age. To the southeast of the complex the acid rocks are in contact with lavas of the Upper Basalt Formation, with basalts seen over-lying rhyolite in places. A detailed description of the lithologies and field relationships of the Tardree Complex is given by Old (1975) with description of the perlitic obsidian vent at Sandy Braes given by Cameron & Sabine (1969) and Sabine (1970). Borehole data obtained since 1970 have allowed the structure of the Sandy Braes vent to be determined in greater detail than previously. Principal rock types recognised are: (a) Massive columnar dome-forming rhyolite of Tardree Mountain; (b) Rubbly rhyolite flows; (c) Platy rhyolite flows; (d) Widely distributed and varied pyroclastics, including the vent complex recognised at Sandy Braes. The distribution of the main rock types at Sandy Braes is shown in Fig. 3 from Old (1975). Cameron and Sabine (1969) recognised that the agglomerate at Sandy Braes has a welded matrix enclosing blocks and fiamme of welded obsidian tuff. Sequences of obsidian and de-vitrified, spherulitic welded tuffs, rhyolite and rhyolite agglomerate encountered in boreholes and which dipped generally at steep angles, are interpreted as large rafts of earlier ash flows through which the vent passed. They occur most frequently on the eastern side of the vent. Fig. 4 from Old (1975) shows a section across the Sandy Braes vent including the position of the Sandy Braes quarry. Rocks similar to those forming these rafts crop out around the vent and probably represent the ash flows through which the vent was drilled. Rhyolites in contact with the vent margin to the west and south are exposed in a number of places. This rhyolite is pink/grey in colour and most exposures show a steeply dipping platy pointing, generally with a NNW trend. Dykes: Old (1975) describes two multiple dykes revealed during pitting operations at Sandy Braes. A number of outcrops of these dykes can still be seen in the remains of the pits. Old suggests these dykes may prove that basalt magma approached the surface during the formation of the Tardree complex, although no extrusive basalt has been recognised within the acid volcanics. In both dykes a central core of fresh ophitic dolerite passes outwards to decomposed, vertically banded chilled margins with some fresh dolerite suggesting multiple intrusion. The outer margins of the dykes are formed of fresh pitch-stone porphyry but the dykes are not thought to be composite as the two parts of the intrusion appear quite separate. GEOCHEMISTRY The geochemistry of the Sandy Braes, porphyritic obsidian was studied in detail by Meighan (1979) and Meighan et al. (1984) who concluded that the general field relationships of the acid volcanics favour an essentially basaltic differentiation origin, in that the eruptions were uniquely associated with the major interval of basaltic quiescence marked by the Interbasaltic Formation. The Sandy Braes porphyritic obsidian lies on trends passing from olivine tholeiite through intermediate compositions (Lyle 1980, Lyle & Thompson 1983). Sr isotope data however indicate there is a crustal component in the composition of the Sandy Braes porphyritic obsidian and in general terms they can be considered as a contaminated basaltic differentiate. | |
| Importance: | |
The locality represents an important type section for explosive acid volcanicity within the Antrim Lava Group. While rhyolite flows are noted from elsewhere in Antrim, the Sandy Braes vent is the only confirmed example of a rhyolite vent in the mid-Antrim area. The Tardree Rhyolite complex provides important information on the volcanic environment existing towards the end of LBF times and during the Interbasaltic period of quiescence. The geochemical characteristics show similarities to their analogues elsewhere in the British Tertiary Volcanic Province and as such play an important part in the overall elucidation of acid magma genesis and fractionation in the Tertiary Igneous provinces of Britain and Ireland. | |
| Interpretation: | |
The rocks exposed at Sandy Braes are an integral component of the Tardree Rhyolite Complex, considered by consensus to be part of the Interbasaltic Formation. Old (1975) reviews the evidence for its stratigraphical position within the Antrim Lava Group and presents new field and borehole evidence that concludes that it can be shown conclusively in the field that basalts overlie the rhyolites and several recent boreholes have proved the existence of basalt underlying the rhyolite. Meighan et al. (1984) concluded that the general field relationships of the acid volcanics favour an essentially basaltic differentiation origin since they were uniquely associated with the major interval of volcanic quiescence at the end of the first cycle (Lyle & Thompson, 1983) represented by the Interbasaltic Formation. Cameron & Sabine (1969) recognised that the agglomerates of Sandy Braes occur in a vent and the detailed structure of the vent was clarified and detailed by Old (1975) with the recognition of several ash flows and a number of tuffisite and multiple dykes. The sequences of obsidian, welded tuffs and agglomerates and rhyolites found in boreholes and exposures in the Sandy Braes area are interpreted as rafts of earlier ash flows through which the Sandy Braes vent passed. | |
| Conclusions: | |
The locality represents an accessible example of explosive acid volcanicity within the Interbasaltic Formation of the Antrim Lava Group in mid-Antrim. The rock types found and their relationships within the overall vent structure around Sandy Braes are the only occurrence in northeast Ireland of rhyolitic ash flows, tuffs and agglomerates. | |
| Notes: | |
This site is also important for Mineralogy and Metallogenesis. See Key Site 537 - Sandy Braes. | |
| Keywords | |
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| Rocks: | Agglomerate, Breccia, Breccia, Volcanic, Obsidian, Rhyolite |
| Structures: | volcanic vent |
| Relations: | Extrusion |
| Access | |
|---|---|
| Approach: | Access to the quarry is by single lane track from the Kells to Doagh Road. |
| Management: | |
The quarry shows good exposures of the obsidian agglomerate on the quarry faces, with abundant loose material available for collection. The quarry faces however are in poor condition, crumbling and overgrown and the quarry floor is being used for uncontrolled dumping of various domestic waste materials. A major clean-up of the quarry faces and flows would be required to restore the site and to maximise its value as a type locality for Tertiary magmatism in northeast Ireland. Across the area of the vent (See map Fig. 3 from Old 1975) there are signs of trenches and pits from previous assessments of the quarry materials. | |
| Uses | |
| Uses: | The area around the quarry is used for rough grazing, mainly sheep. The quarry is currently disused but is still under licence for mineral extraction of perlite materials. |
| References (7) | |||
|---|---|---|---|
| Cameron, I.B. & Sabine, P.A., 1969. The Tertiary welded-tuff vent agglomerate and associated rocks at Sandy Braes, Co. Antrim Report of the Institute of Geological Sciences, numb. 69/6. | |||
| Lyle, P., 1980. A petrological and geochemical study of the Tertiary basaltic rocks of northeast Ireland. Irish Journal of Earth Sciences, vol. 2, pp.137-152. | |||
| Lyle, P., 1985. The petrogenesis of the Tertiary basaltic and intermediate lavas of northeast Ireland Scottish Journal of Geology, vol. 21, pp.71-84. | |||
| Lyle, P. & Thompson, S.J., 1983. The classification and chemistry of the Tertiary intermediate lavas of northeast Ireland Scottish Journal of Geology, vol. 19, pp.17-27. | |||
| Meighan, I.G., 1979. The acid igneous rocks of the British Tertiary Province. Bulletins of the Geological Survey of Great Britain, numb. 70, pp.10-22. | |||
| Meighan, I.G., Gibson, D. & Hood, D.N., 1984. Some aspects of Tertiary acid magmatism in NE Ireland Mineralogical Magazine, vol. 48, pp.351-363. | |||
| Sabine, P.A., 1970. Perlitic obsidian at Sandy Braes, Co. Antrim, its devitrification and volumetric relationships Report of the Institute of Geological Sciences, numb. 70/11. | |||
| Map(s): | Sheet 28, Antrim, 1:50,000 G.S.N.I. | ||
| Record type: | ESCR report | ||
| Entered by: | E M Porter | ||
| Last updated: | 23 Dec 2003 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Sandy Braes - Palaeogene, County Antrim, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=517. Accessed on 2026-11-08. |
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