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Earth Science Conservation Review
| Cultra - Craigavad - Carboniferous | Down |
| Site number: | 291 | File Number: | J48 |
| ESCC: | EF | ||
| Locality Type: | Coastal section | Status: | ASSI |
| Grid Reference: | J408805,J425816 End(s) of linear site, Not Entered | ||
| Google maps: | 54.6534,-5.81844 | ||
| County: | Down | Council area: | Ards & North Down Borough Council |
| Period: | Carboniferous | ||
| Stages: | Tournasian | ||
| Lithostrat: | Ballycultra Formation, Craigavad Sandstone Formation, Holywood Group | ||
| Site Description | |
|---|---|
| Highlights: | |
Dinantian rocks are exposed near Cultra, on the south side of Belfast Lough. The strata, amounting to ~280m in the exposed part belong to the Holywood Group and are divided into two formations, the Craigavad Sandstone Formation and the succeeding Ballycultra Formation. Miospore assemblages from the latter formation are indicative of a late Tournaisian (CM Biozone) age. The strata are presumed to rest unconformably on Lower Palaeozoic (Ordovician) rocks and record the progressive changes induced by a marine transgression from an early terrestrial environment (Upper Old Red Sandstone facies) through brackish/hypersaline environments to a late marine influence in the Ballycultra Formation. | |
| Introduction: | |
Lower Carboniferous rocks occur in well exposed shore sections near Cultra and in some less perfectly exposed stream sections near Cultra and Holywood. They are thought to lie unconformably on Ordovician rocks and to be similarly overlain by Permian beds. At the base of the Holywood Group is the Craigavad Sandstone Formation, c.140m of rocks of the 'Upper Old Red Sandstone' facies which is overlain by the Ballycultra Formation, of 'Cementstone' facies, also c.140m thick and which was subdivided by Griffith and Wilson (1982) into 3 informal divisions. Because of the position of this Lower Carboniferous outcrop on the north side of the Lower Palaeozoic Down-Longford massif and their age and lithological composition, these rocks have been equated with the Cementstone Group and rocks of Upper Old Red Sandstone facies in the Midland Valley of Scotland. In Co. Down the occurrence of this outcrop is unique and in N. Ireland, rocks of the same age, range of lithologies and comparable palaeogeographical position are only found in the Armagh area. Location: The principal exposure of Lower Carboniferous rocks in north Co. Down occurs on the SE shore of Belfast Lough between Craigavad (J426 816) and Cultra (J408 805). Bibliographic Review: The Carboniferous rocks at Cultra have attracted attention from the earliest days of Irish geology. Because the beds are closely associated with exposures of Permian strata, the literature on these rocks is confusing with, on occasion, Carboniferous strata being assigned to the Permian, and vice versa. The first account of these rocks at Cultra was given by Griffith (1843, p.45) in which he concluded that they were on the basis of their fauna of Carboniferous age, and not Permian as he had previously suspected. The Geological Survey of Ireland (1867-69) recognised both Carboniferous and Permian rocks in the shore section at Cultra and identified their unconformable relationship but did not describe the basal arenaceous rocks that are now assigned to the Craigavad Formation. Despite the independent assertions by Wright (1872) and Anderson (1873) who concluded that all the rocks at Cultra were Carboniferous, this matter was resolved in 1904 by Lamplugh who eventually persuaded Wright that all the fossils collected by him were indeed from the Carboniferous outcrop. It is quite clear from the literature however that the first geologist to recognise both Permian and Carboniferous rocks at Cultra and to support that conclusion by the identification of index taxa for the Permian System, and the Magnesian Limestone in particular, was E.W. Binney (1855). Further research on these Carboniferous rocks was thereafter concerned with determining their true age with some authors preferring an early Vis‚an age (Turner, 1952; Griffith & Wilson, 1982) and others assigning them to the Tournaisian (Freshney, 1961). The most recent study of miospore assemblages from the middle section of the Ballycultra Formation by Clayton (1986) indicates an assignment to the CM Biozone and a late Tournaisian age. General Geology: The Type Area and sole outcrop of the Holywood Group is in the Cultra area of north Co. Down. The group and formation names were proposed by Griffith and Wilson (1982, p.23) but formal nomination of stratotype sections/localities was not attempted. However since virtually all outcrops of the group occur on the foreshore between the western bounding fault at Grey House (J4080 8038) and the bounding fault on the shore at Craigavad (J4255 8150), just over 2km to the NE, it seems reasonable to nominate this section as the Type Area for the Holywood Group and the Craigavad Sandstone and Ballycultra formations. Although the basal contact between the Carboniferous Holywood Group and Ordovician rocks of the Down-Longford massif is everywhere apparently faulted, their original relationship was clearly unconformable and the lowest beds of the Craigavad Sandstone Formation were deposited directly on folded and cleaved Lower Palaeozoic rocks. The transitional strata occurring between the Craigavad and Ballycultra formations consist of interbedded shales and sandstones that combine the red colours so typical of the older sandstone formation and the grey-green colours that typify the shale-limestone-evaporite sedimentary rocks of the succeeding Ballycultra Formation. Whilst thin sandstones that are more typical of the Craigavad Formation occur in the Ballycultra Formation, beds of replacement evaporites and laminated stromatolitic limestones are confined solely to the latter and the formation base is taken at the base of a prominent 1m thick deep greyish red, micaceous silty shale bed at Sections 1 and 2. The age, thickness and nature of Carboniferous beds that lie beneath the southern part of Belfast Lough is not known but it has been suggested that rocks of Upper Carboniferous age may be present there (Griffith & Wilson, 1982, p.33). The Holywood Group is in turn overlain unconformably by Permian rocks including the fossiliferous Upper Permian Magnesian Limestone, on the shore in Ballycultra Townland. | |
| Description: | |
(i) CRAIGAVAD SANDSTONE FORMATION All the rocks included in this formation are characterised by having an intense red colouration and consisting mainly of sandstones. The lowest beds are poorly exposed massive, coarse- grained and micaceous pebbly sandstones with pebbles consisting of vein quartz and Lower Palaeozoic greywacke. Throughout the formation beds of calcareous sandstone become thinner towards the top and frequently contain isolated nodules or more rarely, beds, between 15-30cm thick, of pedogenic carbonate. The formation is generally unfossiliferous although some of the higher shaley beds on the foreshore contain a sparse fauna of bivalves and fish debris and a 0.45m shale located 1.05m below the top contains abundant ostracods and annelid trails. (ii) BALLYCULTA FORMATION These beds are well exposed in a series of faulted sections on the foreshore at Cultra but it has not been possible to reconstruct a complete succession and there remain two major portions of the formation for which there are no exposed representatives or for which the strata are too disrupted by faulting, to allow a reasonable succession to be drawn up. The exposed parts of the formation are divisible into 3 divisions as follows (from the top): Division C 21m, concealed strata ~24m, Division B 43m, concealed and disrupted strata ~20m, Division A 32m, Craigavad Sandstone Formation. The Ballycultra Formation consists of a series of thin-bedded dark grey and greenish grey shales and mudstones, greenish grey siltstones and micaceous sandstones, thin olive-grey, compact, non-laminated dolomitic micrites and olive-grey, laminated carbonates of probable algal origin associated with thin pink beds of nodular sparry calcite, which replaces gypsum or anhydrite. Deposition was cyclical and all lithologies are calcareous. Red colouration, due to depositional factors and which is so characteristic of the Craigavad Sandstone Formation, is common in the lowest part of Division A and is not present above the lowest part of Division B, greenish grey and dark grey colour becoming predominant. The red, pink and orange colours seen towards the top of Division C are due to secondary staining from the overlying Permian rocks. All lithologies, except the evaporites and associated laminated carbonates, are commonly fossiliferous and contain abundant plant fragments, ostracods, bivalves, algae and serpulid reefs. Rare specimens of rhynchonelloid brachiopods and the crustacean 'Estheria' are recorded in Division B, in a nodule at the base of Bed 63, an olive-grey micrite. The brachiopods are associated with abundant large fragments of fish in such a way as to suggest that the fish skeleton served as an anchoring point in an otherwise muddy environment. The composition of the ostracod faunas in the Ballycultra Formation indicates a clear correlation with part of the Lower Border Group of the Northumberland Trough. The lack of clearly diagnostic macrofossils makes the dating of the Lower Border Group difficult but they have been generally regarded as being of late Tournaisian or early Vis‚an age and thus this age was applied to the Holywood Group by Griffith and Wilson (1982). More recently however Clayton (1986) recovered three well- preserved miospore assemblages from Division B of the Ballycultra Formation. These are assigned to the Schopfites claviger- Auroraspora macra (CM) Biozone, indicative of the late Tournaisian. According to Riley (1993) the Tournaisian-Vis‚an junction does not in fact coincide with the base of the Chadian Stage, as previously correlated but the early part of the Chadian is now apparently of late Tournaisian age and thus correlates with the upper part of the CM Biozone. Strictly speaking the miospore assemblages fall within the upper Tournaisian but may be of late Courceyan or early Chadian age. The three miospore assemblages contain the following taxa: Acanthotriletes hacquebardii, Anaplanisporites baccatus, Auroraspora macra, Baculatisporites fusticulus, Colatisporites decorus, C. denticulatus, Convolutispora circumvallata, C. mellita, Crassispora trychera, Cristatisporites menendezii, Cyclogranisporites commodus, Dictyotriletes cf. cheveriensis, Discernisporites micromanifestus, Grandispora echinata, Knoxisporites literatus, K. triangularis, Pertriletes perinatus, Prolycospora rugulosa, Pulvinospora scolecophora, Punctatisporites minutus, P. planus, Raistrickia corynoges, Retusotriletes incohatus, Rugospora corporata, R. minuta, R. polyptycha, Schopfites claviger, Spelaeotriletes pretiosus, Umbonatisporites distinctus, and Verrucosisporites nitidus. The absence of specimens of Lycospora pusilla the index taxon for the Pu Biozone, which succeeds the CM Biozone and is usually equated with the base of the Vis‚an Series is significant and allows the assemblages to be assigned with confidence to the CM Biozone. | |
| Interpretation: | |
The Holywood Group of north County Down comprises a lower sand-dominant unit about 140m thick, the Craigavad Sandstone Formation of 'Upper Old Red Sandtone' facies, overlain by the Ballycultra Formation of so-called 'Cementstone' facies, also about 140m thick. Rocks belonging to the latter facies, and of the same age, are known in many parts of Northern Ireland in the Clogher Valley, County Armagh, the Kesh-Omagh area, Draperstown, and in Scotland in the Lower Carboniferous of the Midland Valley. The arenaceous sediments of the Craigavad Formation were deposited in a non-marine environment by high-energy rivers, the detritus being eroded rapidly from a nearby land area, composed presumably of the Lower Palaeozoic rocks that are exposed in County Down, south of the Cultra-Craigavad area. Persistent conditions of oxidation resulted in the red colouration of all the rocks and the frequency of ripple marks in the sandstones was caused by deposition in a shallow water turbulent environment. Periodic emergence of the sediment surface and exposure to high rates of evaporation and pedogenesis (soil formation) resulted in the formation of incomplete calcrete profiles and isolated nodules of micritic carbonate. Towards the top of the formation sandstones become gradually thinner and are interbedded with red silty shales, a few micritic limestones but no evaporites. Throughout the section covering the top of the Craigavad Sandstone and Section A of the succeeding Ballycultra Formation the indications are of an increasingly brackish water environment with a sparse restricted fauna and the introduction of reducing conditions as evidenced by the grey and green colours of the rocks. Higher in the formation the common occurrence of ripple marks, desiccation cracks, evaporite beds of calcitised gypsum or anhydrite, micritic dolomites and algal stromatolites, together with a sparse bivalve-ostracod-annelid fauna is indicative of a hypersaline tidal-flat environment under arid/semi-arid climatic conditions. The influx of fresh water from the adjacent land area probably neutralised the hypersalinity, caused by the evaporation of shallow water, and made the environment less hostile for colonisation by bivalves, ostracods and serpulid reefs. Fully marine conditions, although hinted at by the very rare occurrence of brachiopods must have lain well outside the Cultra area. | |
| Conclusions: | |
The gradual submergence of the margins of the Down-Longford massif during Lower Carboniferous times is recorded in the outliers of Tournaisian and Vis‚an rocks that occur in Co. Armagh, but more particularly in Co. Down. The sequence of strata of the Holywood Group that occur in the Cultra area of north Co. Down were deposited on the north side of the Lower Palaeozoic massif and correlate with Tournaisian rocks in Co. Armagh and at the southern margin of the Midland Valley of Scotland. The coastal exposures of the Ballycultra Formation, in particular west of the yacht club slipway at Cultra, are unique in north Co. Down, but in addition they cannot be matched anywhere else in N. Ireland in terms of exposures of rocks of the same age. The exposures are particularly important in preserving the evaporite replacement beds and associated laminated stromatolitic limestones that characterise the Ballycultra Formation. The Cultra section is also unique in N. Ireland in that it exposes the unconformity between these Carboniferous rocks and the succeeding Permian strata. | |
| Notes: | |
For information on other sites in Subarea-10 see Key Site 1183 - Subarea-10; North County Down. | |
| Keywords | |
|---|---|
| Minerals: | Anhydrite, Gypsum |
| Rocks: | Algal limestone, Dolomite, Evaporite, Limestone, Mudstone, Sand, Sandstone, Shale, Siltstone |
| Fossil Groups: | Alga, Annelid, Bivalve, Brachiopod, Coral, Crustacea, Fish, Miospore, Plant |
| Fossil List: | Auroraspora macra |
| Structures: | dessication cracks, ripple marks |
| Relations: | Fluvial sediments, Marine sediments |
| Access | |
|---|---|
| Approach: | The SE shore of Belfast Lough between Craigavad and Cultra. |
| References (10) | |||
|---|---|---|---|
| Anderson, J., 1873. On the Geological Formations of County Down. Proceedings of the Belfast Natural History and Philosophical Society, vol. 1871-72, pp.41-49. | |||
| Binney, E.W., 1855. On the Permian Beds of north-west England. Memoirs and Proceedings of the Manchester Literary and Philosophical Society, vol. 12, pp.209-69. | |||
| Clayton, G., 1986. Late Tournaisian miospores from the Ballycultra Formation at Cultra County Down, Northern Ireland. Irish Journal of Earth Sciences, vol. 8, pp.73-79. | |||
| Freshney, E.C., 1986. The Cementstone Group of the West Midland Valley of Scotland (Ph.D. thesis). (Thesis) University of Glasgow. | |||
| Griffith, A.E. & Wilson, H.E., 1982. Geology of the country around Carrickfergus and Bangor. Memoir of the Geological Survey of Northern Ireland, Sheet 29 (N. Ireland). | |||
| Griffith, R., 1843. On the lower portion of the Carboniferous Limestone Series of Ireland. Reports of the British Association for the Advancement of Science, vol. 1843, pp.45-46. | |||
| Lamplugh, G., Kilroe, J.R., M’Henry, A., Seymour, H.J., Wright, W.B. & Muff, H.B., 1904. The Geology of the Country Around Belfast. Memoirs of the Geological Survey of Ireland. | |||
| Riley, N.J., 1993. Dinantian (Lower Carboniferous) biostratigraphy and chronostratigraphy in the British Isles. Journal of the Geological Society of London, vol. 150, pp.427-446. | |||
| Turner, J.S., 1952. The Lower Carboniferous Rocks of Ireland. Liverpool & Manchester Geological Journal, vol. 1, pp.113-147. | |||
| Wright, J., 1872. Geology of Cultra, Co. Down. Report of the Belfast Naturalists' Field Club, vol. 1871-72, pp.33-37. | |||
| Record type: | ESCR report | ||
| Entered by: | Doughty, P.J. | ||
| Last updated: | 12 Jun 2004 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Cultra - Craigavad - Carboniferous, County Down, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=291. Accessed on 2026-11-06. |
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