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Earth Science Conservation Review
| Portballintrae | Antrim |
| Site number: | 7 | File Number: | C94 |
| ESCC: | IS | ||
| Locality Type: | Coastal section | Status: | ASSI |
| Grid Reference: | C923423 Approximate | ||
| Google maps: | 55.21956,-6.54972 | ||
| County: | Antrim | Council area: | Causeway Coast & Glens Borough Council |
| Period: | Quaternary | ||
| Stages: | Late Midlandian, Pleistocene | ||
| Site Description | |
|---|---|
| Highlights: | |
The sequence demonstrates a transition from glacigenic diamict into shallow water, wave-influenced marine sands which are overlain by late glacial shoreline gravels. Facies evidence of this type have a low preservation potential due to erosion during isostatic uplift. It demonstrates that the environmental signature of the deglacial to late glacial transition was one of rapid local environmental changes driven by rapid rates of isostatic rebound, differential tectonics and eustatic changes. The marine sands contain a wide range of wave-influenced sedimentary structures which have not previously been described from glacigenic sequences in the UK (McCabe, Carter and Haynes, 1984). | |
| Introduction: | |
The area surrounding Portballintrae is a gently undulating to scoured glacial landscape rising to 70-80m above sea level. The most complete exposures occur at the western side of the small horseshoe-shaped bay (350m x 400m) developed along a N-S fault exploited by marine erosion prior to the last glaciation. The almost landlocked bay increased the preservation potential of this sequence in what is otherwise a 'high energy' coastal setting (Carter, 1988). Striated rock and erosional marks on the floor and periphery of the bay clearly pre-date the glacigenic deposits which fill the structural low. Ice directional indicators show that the last ice sheet moved northwards from inland centers of ice dispersion onto the continental shelf. Stephens (1963) documented red marine clay from this site and Stephens, Hannon and Creighton (1975) interpreted part or all of the sequence as glacial till. A more recent assessment of the sequence indicates diamict formation at tidewater followed by shallow marine deposition and shoreface gravels during rapid isostatic uplife (McCabe, Carter and Haynes, 1994). | |
| Description: | |
Lithofacies associations: Three major lithofacies associations (massive diamict, rhythmically-bedded muds and sands, gravels and sands) have been recognised on the basis of sediment geometry, stratigraphic position, facies contacts and primary sedimentary structures. Massive diamict: Massive, muddy diamict with dispersed clasts and contorted pods of mud overlies striated s-forms at the base of the section. Stacked beds of diamict are separated by either abrupt textural discontinuities or sandy washes. Fabrics are random and the clasts are mainly basalt (85-95%). Contacts with the overlying rhythmically-bedded unit are sharp to gradational or brecciated. Rhythmically-bedded sands/muds: This tabular unit consists of fourteen rhythmically bedded packages and flat lamination. Each package consists of up to four lithofacies which occur in the same stratigraphic position and can be traced laterally without any appreciable change in grain size, internal organisation, thickness or structure. Sand and silt streaked muds (SSM) occur in the basal parts of each package and consist of delicately interwoven laminae with gradations between low-angle cross-lamination and more parallel, drape lamination. Sandy stringers swell into linsen structures. The overlying mud-streaked sand (MSS) consist of thin bundles of interwoven laminae with discontinuous partings of red mud or small scale, cross laminated sand with concave-up mud flasers draping low amplitude ripples. The top of the MSS is marked by erosive ripples and a prominent train of symmetrical, sharp- crested ripples (RS). Ripples are form discordant with internal structural diversity such as chevron upbuilding, off-shooting draping foresets (shoreward dipping) and bundled upbuilding. The dominant ripple train (RS) is overlain by a laminated mud drape (LMD). Finely laminated drapes show complex structures including wavy lamination, lenticular sand-lensed mud, knitted undulations and cross-bedded laminae with both form discordant and concordant relationships. The foraminifera in the sands/muds are dominated by Elphidium clavatum and Cassidulina obtusa and the assemblage is of high dominance and low diversity. Exclusively temperate to warm species are absent. At site 2 small scale hummocky cross-stratified sands occur immediately above the diamict and grade into the rhythmically bedded sands and muds. Gravel and sand: The rhythmites are truncated by a sharp planar erosion surface or a gravel lag. Overlying gravel and sand infill cross-cutting scour features. Gravelly infills are characterised by weakly defined to amalgamated junctions and abrupt textural changes between sand and gravel dominated beds. Gravelly beds are massive with rounding poorly-developed and an absence of current parallel alignments. | |
| Importance: | |
The facies sequence, especially the presence of wave-influenced, shallow marine sands between diamict and overlying marine gravels, is the most important geological feature of the site. It demonstrates that shallow marine deposition occurred following ice retreat during a phase of high relative sea level. Within the context of the northwestern sector of the last British ice sheet it is the only site known to show progression from glaciomarine, to shallow marine and wave-influenced facies and eventually shoreface gravels. At all known sites there is a marked erosion surface between the glacigenic diamicts and the overlying late- glacial shoreline facies. This site therefore demonstrates the persistence of high relative sea levels into the late-glacial. The site is of local and national importance because it raises questions regarding the magnitude, timing and patterns of crustal deformation during the last deglacial phase. Sites of this importance may prove critical in the analysis of future ice sheet reconstructions in northwestern Britain. | |
| Interpretation: | |
The stacked beds of diamict with sandy inclusions and high percentages of muds are typical of resedimentation around unstable aprons which accumulate near ice margins in response to a substantial hydraulic head. Overlying rhythmically bedded sands and muds contain a wide range of diagnostic structures described from shallow marine settings (eg. Rineck and Singh, 1965; De Raaf et al., 1977). Each rhythmically bedded package (SSM - MSS - RS - LMD) consists essentially of a structured microsequence which can be related to increasing wave energy/agitation during the course of a storm. The mud drapes which cap the dominant ripple trains indicate an abrupt decrease in energy levels as the storms waned. The small scale hummocky cross stratified sands also reflect storm wave reworking (basal flat lamination), depositional and erosional oscillatory flows (hummocks), reduced storm activity (undulatory laminae and accretion of rolling-grain ripples) and fair weather reworking (mud cap). The gravelly lags and truncation of the rhythmites is a result of marine erosion during a fall in relative sea level during isostatic uplift. This general phase may also be marked by the numerous, late-glacial raised marine features along the north coast. Gravel emplacement by erosion of onshore drift bluffs and periodic off-shore directed sediment pulses probably occurred during storm set-up. The three major facies associations are indicative of tidewater sedimentation (diamict) followed by shallow water, marine conditions during uplift and finally mass emplacement of lower beach face gravels. The sequence indicates rapid deglaciation of the shelf, tidewater sedimentation and deceleration within the tectonic/isostatic/eustatic system. Regional comparisons indicate that the sequence formed around 17ka B.P. though this is in conflict with offshore records of lowered sea level (eg. Peacock et al., 1992). Apparent differences in relative sea levels could be a function of either extremely rapid environmental changes or differential tectonics with forebulge development on the outer shelf creating shallow water and a deep water moat, peripheral to the ice margin on the inner shelf. | |
| Conclusions: | |
The Portballintrae sequence consists of glaciomarine diamict, shallow marine sands and muds overlain by shoreface gravels. It demonstrates high relative sea level during ice retreat along the north coast of Ireland and is a critical deglacial marker horizon. The sedimentary sequence is unique in northwest Britain. Nationally it is important because of its sedimentological characteristics and the quesitons it raises for ice sheet reconstruction, isostasy and crustal deformation. | |
| Notes: | |
The boundary of the site's scientific interest occurs between the beach and the roads at the cliff top. | |
| Keywords | |
|---|---|
| Rocks: | Diamict, Gravel, Sand |
| Fossil Groups: | Foraminifera |
| Fossil List: | Cassidulina obtusa, Elphidium clavatum |
| Relations: | Beach deposit, Glaciomarine deposit, Late-glacial deposit |
| Measurements | |||||
|---|---|---|---|---|---|
| Length: | 130 m | Width: | Height: | up to 10 m | |
| Depth: | Area: | ||||
| Access | |
|---|---|
| Management: | |
Critical sites under moderate marine erosion can be expected to provide more scientific data/exposure with time. However, building of sea walls or groynes may well stop or change the local patterns of marine erosion. The result would be degradation of a site of national importance. At present the site can withstand pressure from geological sampling. There are no problems of access and usual geological safety standards should be observed. Notifiable operations include dumping on the cliff slopes, engineering works and use of heavy vehicles for any slope modifications. | |
| Threats: | Increased marine erosion due to building of sea walls or groynes. |
| Uses | |
| Uses: | The site is currently being eroded and trimmed by the sea. This maintains the site for future scientific and educational purposes. |
| References (6) | |||
|---|---|---|---|
| Carter, R.W.G., 1988. Coastal Environments., pp.1-617 Academic Press, London. | |||
| DeRaaf, J.F.M., Boersma, J.R. & Van Gelder, A., 1977. Wave generated structures and sequences from a shallow marine succession, Lower Carboniferous, Co. Cork, Ireland. Sedimentology, vol. 24, pp.451-483. | |||
| McCabe, A.M., Carter, R.W.G. & Haynes, J., 1994. A shallow marine emergent sequence from the northwestern sector of the last British ice sheet, Portballintrae, Northern Ireland. Marine Geology, vol. 117, pp.19-34. | |||
| Reineck, H.E. & Singh, I.B., 1975. Depositional Sedimentary Environments., pp.439 Springer, New York. | |||
| Stephens, N., 1963. Late glacial sea levels in north-east Ireland. Irish Geography, vol. 4, pp.345-359. | |||
| Stephens, N., Creighton, J.R. & Hannon, M.A., 1975. The Late Pleistocene period in north-eastern Ireland - An Assessment. Irish Geography, vol. 8, pp.1-23. | |||
| Map(s): | See the 1:50,000 O.S. Sheet 5 (Ballycastle). | ||
| Record type: | Environment Serv. GIS, ESCR report | ||
| Entered by: | Crowther, P.R. | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Portballintrae, County Antrim, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=7. Accessed on 2026-11-08. |
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