ESCR - Dunseverick Slip

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Summary Full report
Dunseverick Slip	Antrim

Site number:	73	File Number:	C94
ESCC:	EF
Locality Type:	Coastal section	Status:	ASSI
Grid Reference:	C99994455 Centroid
Google maps:	55.23783,-6.42957
County:	Antrim	Council area:	Causeway Coast & Glens Borough Council
Period:	Tertiary
Stages:	Eocene, Palaeocene
Lithostrat:	Antrim Lava Group, Interbasaltic Formation, Tholeiitic Middle Basalt Member

Site Description
Foreshore east of Dunseverick Slip; outlier of tholeitic andesite in foreground rests on tholeitic basalt; rock armouring rests on tholeitic andesite nearer the slip.
Highlights:
A discrete flow unit of unusual composition and texture occurs among the tholeiitic basalts of the Causeway Coast.
Introduction:
Among the three basalt formations which comprise the Antrim Lava Group, intermediate and late differentiates have been recorded from the Lower Basalt Formation by several authorities; see the works of Patterson (1951), Walker (1960), and the Geological Survey of Northern Ireland publications. Among the Causeway tholeiites no such late differentiate had been identified until Francis (1978) discovered an unusual textured flow at Dunseverick Slip. A detailed account of this flow unit is given by Francis et al (1986).
Description:
A flow unit some 2 m thick is enclosed within a normal tholeiitic basalt which is probably number five in the sequence of flows within the Causeway Tholeiite Member of the Interbasaltic Formation. The rock type is a tholeiitic andesite with a fasciculate/spherulitic texture showing spherulitic growths of plagioclase feldspar up to 30 cm in diameter. It is exposed below HWM Foreshore alongside Dunseverick Slip; tholeitic andesite occurs in the lower part of the rock platform. immediately west of the slipway at Dunseverick, and as outliers further west on the dip slope of the underlying tholeiitic basalt. This lower contact is planar and shows no special features; the upper contact was exposed at HWM west of the slip and provided good evidence of a liquid-liquid interface between the andesite and overlying basalt flow units. Unfortunately this exposure was destroyed or buried under rock armouring during construction of a new slipway. Material from this upper contact is kept in the Geology Dept. Queen's University, Belfast [following closure of the Dept in 2001, the rock collections were transferred to the Ulster Museum, Belfast]. Further down the shore and immediately west of the slipway, this upper contact is still accessible. Rock platform near low water, close to slipway; upper contact of tholeitic andesite (coarse-grained) exposed showing remnants of overlying basalt (fine-grained).
Importance:
The only example known of a late differentiate within the Causeway tholeiite flows.
Interpretation:
Rapid and copious outpouring of the Causeway tholeiite magma would allow little time for crystal fractionation processes to produce differentiated magma. This could account for the rare occurrence of this rock type. Such a volume of differentiated magma with a lower solidification temperature than the host melt which brought it to the surface, would be in a superheated condition and almost free of crystal nucleii. Subsequent crystallization of the andesitic lava would be considerably delayed; the host lava would have to cool down first. Slow cooling would produce few crystal nucleii and growth from these under supercooled conditions would result in the skeletal and dendritic forms of plagioclase, pyroxene and iron ores.
Conclusions:
This site is worthy of conservation despite the depradations of the civil engineering works which improved the local fishing facilities. The rock type is unique to Northern Ireland both by texture and composition.
Notes:
Further dumping and rock armouring will have to be controlled; it may be possible to exhume the upper contact of this unusual flow unit which yielded the most interesting material without detriment to the fishing interests.

Keywords
Rocks:	Andesite, Tholeiite
Structures:	flow unit, spherulitic
Relations:	Extrusion
NonGeol:	A fishing boat slipway is adjacent to site. Some d

Measurements
Length:		Width:		Height:
Depth:	2 m	Area:

Access
Approach:	Access is made via the minor road linking Dunseverick village with Dunseverick slip. A National Trust carpark at the slip offers access to the coastal path between Benbane Head and White Park Bay.
Threats:	Further dumping of refuse and rock armouring.

References (6)
Francis, P.E., 1978. The Geology of the Causeway Coast, North Antrim (Hons. thesis). (Thesis) Queen's University of Belfast.
Francis, P.E., Lyle, P. & Preston, J., 1986. A tholeiitic andesite flow unit among the Causeway Basalts of North Antrim in Northern Ireland Geological Magazine, vol. 123, pp.105-112.
Geological Survey of N.Ireland, 1970. Memoir of the Belfast and Lagan Valley regions GSNI.
Geological Survey of N.Ireland, 1982. Memoir of the Carrickfergus and Bangor regions GSNI.
Patterson, E.M., 1951. An occurrence of Quartz-trachyte among the Tertiary basalt lavas of North-East Ireland Proceedings of the Royal Irish Academy, vol. 53 pt. B, pp.265-287.
Walker, G.P.L., 1960. An occurrence of mugearite in Antrim Geological Magazine, vol. 97, pp.62-64.

Map(s):	For site location see the 1:50,000 O.S. Sheet 5 (Ballycastle) Discoverer Series. For relevant geology see Fig. 2, Francis et al (1986).
Record type:	Environment Serv. GIS, ESCR report, J. Preston l
Entered by:	Hamilton, M.J.
Last updated:	6 Jun 2005

Enlander, I., Dempster, M. & Doughty, P., 2026. Dunseverick Slip, County Antrim, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=73. Accessed on 2026-11-06.

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