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Earth Science Conservation Review
| Introduction to Clay-with-Flints of Northern Ireland |
| Site number: | 1300 | ||
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| Google maps: | , | ||
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| Period: | Cretaceous, Tertiary | ||
| Site Description | |
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| Introduction: | |
The Clay-with-Flints is a distinctive lithostratigraphical unit that occurs in Northern Ireland between the white chalk of the Ulster White Limestone Formation and black basalt lava flows of the Antrim Lava Group. Within the 10 million year (Ma) time gap between the youngest representatives of the Late Cretaceous chalk (late-early Maastrichtian) and the earliest Palaeogene basalt lavas (c. 59Ma) it is the only deposit present. Exposures of the Clay-with-Flints are mainly confined to the margins of the Antrim Plateau, primarily in Counties Antrim and Londonderry, but with minor occurrences in north County Armagh (Figure 1). As the name suggests, lithologically, the Clay-with-Flints consists of two main components, namely flints and a matrix of clay. However, it is the multicoloured nature of the clay that gives the horizon its distinctive appearance, and this can vary from buff to orange to bluishgrey and finally deep reddish brown. Traditionally both components of the Claywith-Flints were regarded as being derived by subaerial solution weathering of the underlying chalk. The Ulster White Limestone Formation (Figure 2) is the only stratigraphical horizon in Northern Ireland that contains flints of the type that are found in the Clay-with-Flints and is therefore regarded as the unique source of debris. The origin of the clay fraction is, however, more controversial. The recurring view (Smith & McAlister, 1995) that the clay fraction was a lateritic soil that accumulated autochthonously and represented a mix of the finest insoluble residue derived by solution of the chalk, with contributions from an aeolian component and illuvium from the overlying basalt lavas, has been disproved. There is now incontrovertible evidence that the clay fraction was derived predominantly from a volcanic source. The conspicuous nature of the Clay-with-Flints, contrasting as it does with the white limestone below and the black basalt lava flows above, has attracted comment from generations of geologists (Portlock, 1843; Lamplugh et al., 1904; Cleland, 1938; Smith & McAllister, 1995). One of the earliest, and without doubt one of the most perceptive, comments on the origins of the Clay-with-Flints, largely ignored by later generations of geologists, was made by Portlock (1843, p. 117) who offered the opinion that, ‘…..clays, and flinty ochreous conglomerates, which appear to indicate a deposit of volcanic mud, prior to any more decided basaltic flow;…..’. In contrast, Cleland concluded that the red colouration of the clay component in the Clay-with-Flints indicated a source in the Late Triassic Mercia Mudstone Group. However, as a rule, most previous accounts attributed the formation of the Clay-with-Flints to the authochthonous accumulation of insoluble material released by dissolution of the chalk. Indeed this mechanism of autochthonous accumulation has also been applied to account for the origin of the Clay-with-Flints in southern England. Once again, this hypothesis had already been discredited over a century ago by Jukes-Browne (1906) who had demonstrated convincingly that the insoluble clay content of the chalk was insufficient to account either for the thickness of the Clay-with-Flints or for the relative proportion, often more than 50%, of clay to flints in the deposit. He had calculated a figure of between 0.88-1.54% by weight of insoluble residue in the chalk of southern England and thus concluded that dissolution of the chalk would always yield a much greater thickness of flints than clay residue. Calculation of the purity of the Ulster White Limestone Formation has shown an insoluble residue of less than 0.5%. Given that the chalk in Northern Ireland has a much greater proportion of flints to clay than the chalk from southern England, such a small clay fraction when released by dissolution of the chalk, could not account for the amount of clay in the Clay-with-Flints in Northern Ireland. The present thickness of Cretaceous chalk is a reflection of tectonism and weathering prior to the accumulation of the Clay-with-Flints. Some indication of the thickness of eroded chalk is provided by the occurrence of paramoudra, the largest barrel-shaped flint nodules in the Cretaceous chalk in Northern Ireland, in the Clay-with-Flints at Cloghfin [J484 942], on the coast of southeast County Antrim at the southern end of Island Magee. There, the deposit rests on the Glenarm Chalk Member of the Ulster White Limestone Formation, but since paramoudra are restricted to the uppermost Ballymagarry Chalk Member (Figure 2) at least 38m of chalk post-dating the Glenarm Chalk Member has been removed to release them for incorporation into the Clay-with-Flints. That figure would rise to over 70m if Maastrichtian chalk had been deposited in the Larne-Lough Neagh basin, although there is no evidence of chalk younger than the late Campanian (Portrush Chalk Member). Despite the erosion of a variable thickness of chalk (Figure 3), the Clay-with-Flints accumulated on a flat or gently undulose land surface. At many exposures, the presence of grikes up to 5m deep illustrate the effects of solution of the chalk and while usually infilled by the Clay-with-Flints, the intervening clints may also be buried by 1-2m of the deposit. | |
| Description: | |
Lithological composition of the Clay-with-Flints The Clay-with-Flints typically has a soft, highly weathered, clay matrix consisting of an intergrowth of fine-grained quartz, clay minerals, disseminated opaque oxides and haematite. Slight variations in the optical density of the clays may reflect primary compositional differences, while the distribution of opaque oxides occasionally highlights a primary fabric resembling a eutaxitic (banded) texture. While many of the flint clasts are broken into sharp fragments, some still retain their original shape. This is evident at a shoreface exposure [D119 416] about 250m northwest of Ballycastle harbour (Site 7-Locality 1), where there is no clay matrix and the deposit consists of self-supporting, whole and fragmented, flints and a matrix of coarse flint chips. At this site, the typical Clay-with-Flints deposit is also exposed however, in the cliff face a further 250m to the northwest. At many localities, the internal physical integrity of whole flints is relatively unchanged from when they developed in the underlying chalk. At Soldierstown Quarry while the flints in the lower part of the channel fill still retain their original shape regardless of their size. However, internally they have been completely shattered into fine shards indicative either of repeated, hard, physical contact between the nodules whilst being transported or due to compaction pressures. Thus, while the outer surface of the nodules is still intact, they show a pattern of closely spaced crazed fractures reflecting the internal shattering of the nodule. The colour of the Clay-with-Flints reflects those of the component clay minerals and ranges from grey and dark bluish grey to white, pale fawn, purplish to deep reddish brown. Flints usually retain the dull grey and greyish brown hues, with a white weathering crust, which predominate in the chalk. However, at some localities the flints are concentrically banded in shades of brown, pink, red and purple due to haematite production and the effect of high temperatures. Textures and sedimentary structures that are typical of air-fall ash deposits and lahars are also evident in the Clay-with-Flints. At Devlin’s Quarry [Site 3; H866 875], the typical soft clay deposit is believed to represent an unsilicified extension of the Donald’s Hill Ignimbrite Formation. In Gibson’s Quarry [Site 2; H980 546], west of Portadown, the upper layer of the deposit is an air-fall accretionary lapilli-rich ash almost devoid of flint clasts. Sediment flow in the Clay-with-Flints There now seems to be indisputable evidence that a significant degree of cementation and joint emplacement in the Ulster White Limestone Formation preceded burial beneath the basalt lavas of the Antrim Lava Group (Simms, 2000). During the several million years when the chalk bedrock in Northern Ireland was exposed to weathering, large quantities of flint were released by dissolution. In many places this process also created a spectacular palaeokarst terrain of pinnacles and dolines with a relief of more than 15m (Photographs 1A and 1B). The resulting landscape will thus have been carpeted with flint nodules, many of them intact but with some broken examples and, more than likely, an amount of wind blown dust. At almost every exposure of the Clay-with-Flints around the margin of the Antrim Plateau in Northern Ireland there is evidence of a volcaniclastic origin for this deposit. With the exception of the Donald’s Hill Ignimbrite Formation (DHIF), the effect of intense subaerial weathering on the unsilicified Clay-with-Flints has destroyed many of the internal textures. Nevertheless, visible evidence of deposition from an air-fall ash, in particular the presence of varying concentrations of lapilli in the clay, is still discernible at most localities of the Clay-with-Flints. Additionally, at many localities, not only is there evidence of a volcaniclastic origin for the clay matrix, there are sedimentary structures indicating sediment flow, principally defined by the dispersal of flint clasts of profoundly differing dimensions within the mud matrix (Photographs 2A, B and C). The flints very often form a clast supported framework with the remaining voids filled by the clay matrix. Particularly elongate flints often show a tendency to be arranged either vertically or at a steep angle to the margins of the deposit. More rarely large flints may be found at the top of the deposit indicating that the matrix density was sufficient to ‘float’ such large clasts during flow. Only very rarely is it possible to recognise an imbrication in the flint clasts and even then it may affect only the more elongate examples. While it is often evident that the proportion of flint clasts in the clay matrix decreases gradually in higher parts of the deposit, there is not a concomitant decrease in their size and indeed, the largest flints are often found in the upper parts of the deposit. In conclusion, the available evidence appears to indicate that the original land surface of the Ulster White Limestone Formation was covered by a layer of flint nodules with minor interstitial clay. It also now seems indisputable that much, if not all, of the clay matrix is volcaniclastic in origin. In the aftermath of that long period of time during which the dissolution of the chalk land surface occurred, if the prevailing volcanic environment had then delivered the clay fraction solely as an ash-fall deposit, such a mechanism cannot explain the apparently random distribution of flint clasts in the clay matrix that is evident at so many localities of the Clay-with-Flints. Based on evidence from many localities, it would appear that the distribution of flint clasts can only have been produced as result of their incorporation and thorough mixing, by flow, with the impetus for that flow initially being provided by pyroclastic surges and lahars resulting from a major volcanic eruption of andesitic or perhaps rhyodacitic composition, prior to the production of the basalt lavas of the Antrim Lava Group. The Donald’s Hill locality provides most of the important evidence for such an origin with other localities offering additional support for this theory. The Clay-with-Flints in Soldierstown Quarry At Soldierstown Quarry [J157 634] 3km NNE of Moira Co. Antrim, the lithology typical of the Clay-with-Flints is found at two separate stratigraphical levels. This unique occurrence provides an insight into aspects of the volcanic evolution of Northern Ireland immediately prior to, but also during the early stages of, the eruption of the basaltic lava flows. Unfortunately, this locality has been partially restored for development and the critical quarry section is now concealed behind soil screens. Nevertheless, the geological features that characterise the quarry are still preserved, although they are not visible today, but photographs exist of that section and analyses are available from various parts of the deposit. The features that were formerly visible in the quarry are described in this report as Site 11, in the event that future use of the site might impinge in a more destructive way on their existence. Soldierstown Quarry is quite typical of all other sections in Northern Ireland, where the chalk-basalt lava contact is exposed, in possessing the Clay-with-Flints horizon. In addition to this expected occurrence, the remarkable feature of this quarry is the presence, entirely within the Antrim Lava Group, of lithologies typical of the Claywith-Flints occupying a channel excavated into the top of the lowest basalt lava flow and then overlain by the succeeding flow (Photographs 3A, B). It may be inferred that both basalt lava flows emanated from an eruptive centre located to the north of the quarry. It is also evident that a significant period of time lapsed between the eruption of the two lava flows during which the first flow cooled and solidified, allowing development of the channel and its infill of at least 10m of sediment typical of the Clay-with-Flints. The source of the fluidised sediment fill of the channel lay to the south of the quarry where there were no basalt lava flows and the chalk land surface was covered by a vegetated regolith of flints and volcanic clay from the first eruptions that pre-dated the basaltic volcanism. Petrogenesis of the Clay-with-Flints In 1999, Mitchell et al. recognised the occurrence of a 1 m thick silicified welded ashflow tuff, the DHIF, at the western margin of the Antrim Plateau in Northern Ireland. As with the Clay-with-Flints elsewhere in Northern Ireland, the DHIF is located stratigraphically between chalk of the Ulster White Limestone Formation and basalt lava flows of the Antrim Lava Group. The DHIF and the Clay-with-Flints are thus laterally equivalent. Originally the DHIF was deposited from a pyroclastic flow which was the product of an episode of voluminous ‘silicic’ volcanism that preceded the outpouring of flood basaltic lavas. Unlike the Clay-with-Flints, the DHIF is extremely hard and siliceous and has not been altered to the same degree by the extreme subaerial weathering. The DHIF is the upper part of the Clay-with-Flints which was altered by secondary silicification as a result of fluids percolating downwards from the overlying basalt lava flows. The effect was to preserve minute details of banding (eutaxitic texture) in the matrix defined by deformed glass shards, fiammé and pumice, the presence of lithic fragments mostly of flint and of undeformed crystalline lithic and glass fragments. Whole-rock geochemical analysis of the DHIF yielded a result of 98% SiO2. The DHIF is characterised by the presence of deep reddish purple and pink hematized flint clasts. At Devlin’s Quarry (Site 3), located 40km SSE from Donald’s Hill, the Clay-with-Flints still contains an abundance of highly coloured hematized flints and thus closely resembles, and is believed to represent an unsilicified extension of, the DHIF. Geochemistry of the Clay-with-Flints A brief resumé of the history of geochemical investigations into the Clay-with-Flints is provided by Simms (2000). He reiterates the conclusion that geochemically the Clay-with-Flints is profoundly dissimilar to chalk of the Ulster White Limestone Formation and that the clay matrix was not derived from its weathering. Based on a combination of new data and previous research, Simms thus concluded that the Claywith-Flints was instead derived from moderately weathered and reworked volcanic ash. However, within the broad category of the Clay-with-Flints, he recognised two main variants which were defined by the clay fraction having either a red or a grey colour. From the geochemical results Simms concluded that, of the two main colour variants found within the Clay-with-Flints, the red clay originated from basaltic ash falls and grey clay from the weathering of rhyolitic ash. However, more recent geochemical data have revealed a much more complex system than previously envisaged. Given the volcaniclastic origin of the Clay-with-Flints, when compared with the composition of fresh volcanic rocks, it appears that the alkalis have been leached out and/or redistributed, whilst silica (SiO2) has been enriched due primarily to flint incorporation. With regards to silica levels there is a range of values from 53 to 94wt%. These encompass the range of volcanic lithologies from basaltic andesite to rhyolite, but more importantly they suggest that silica, in the form of flint, has added to geochemical values greater than around 75wt%. Such artificially increased silica values would thus have resulted in the effective dilution of other elements present, which would also have undergone redistribution due to alteration processes. One example of this process is demonstrated by the alkali (Na20 and K20) content which is universally low, suggesting extensive leaching. Previous interpretations of the nature of the parent magma responsible for the clay component of the Clay-with-Flints, based on silica values, appeared to indicate a volcanic source of rhyolitic composition. Based on geochemical data the following conclusions can be drawn: 1) Geochemical data from samples distributed throughout the outcrop of the Clay-with-Flints confirm that there is nothing inconsistent with a volcaniclastic origin for the clay fraction of the deposit. 2) Using immobile element ratios, which are unaffected by weathering and the addition of silica, the new geochemical data show that, initially, the parental magma was less evolved than previously envisaged based on enhanced silica values and was of andesitic, or more likely rhyodacitic, composition (Figure 4). 3) As volcanism proceeded the products became progressively less evolved, with higher parts of the Clay-with-Flints being basaltic andesite in composition (Figure 4). Biostratigraphy of the Clay-with-Flints The Palaeogene age of the Clay-with-Flints is determined from low diversity assemblages of pteridophyte spores and bisaccate plant pollen such as Liquidambar and Milfordia which have a Palaeogene range base. At Belshaw’s Quarry (Site 1) the basal clay of the deposit also yields abundant reworked Late Cretaceous dinoflagellate cysts (Figure 5, Fossils 1-4). In this assemblage, key taxa such as Cannosphaeropsis utinensis, Neoeurysphaeridium glabrum and Odontochitina operculata, of late Campanian-early Maastrichtian age (Belemnella lanceolata Zone), are associated with the long ranging form Palaeoperidinium pyrophorum. The black colour of the dinoflagellate cysts is caused by secondary heating, in excess of c.200°C, of organic material in close proximity to a dolerite dyke that intrudes the chalk, Clay-with-Flints and basalt lava flows. The organic residues also contain wood and plant tissue, rare insect debris (largely wing material) and other biogenic material from the chalk such as phosphatic nodules, fish scales and teeth. At Tircrevan Burn (Site 6), the Claywith-Flints is succeeded by at least 40cm of soft, greyish blue, carbonaceous clays and dark grey to black lignitic clay and lignite which are overlain by basalt lava. Organic residues contain plant tissue, wood fragments, long-ranging Cretaceous and Palaeogene spores and pollen including Caryapollenites simplex, Triatriopollenites, Tricolporopollenites and Trivestibulopollenites betuloides (Figure 5, Fossil 5) of Palaeogene aspect. The range base of Carya pollen is in the Paleocene. Based on an age of 59Ma (early Thanetian) for the first basalt lava, the lignitic beds are either late Danian or Selandian in age and were deposited in low-lying, swamp conditions with a cool temperate climate (Fossils 1-5 here). | |
| Notes: | |
The full series of Clay-with-Flints records is as follows: Record no. 1300 Introduction to Clay-with-Flints of Northern Ireland Record no. 1301 Belshaw’s Quarry – Clay-with-Flints Record no. 1302 Gibson’s Quarry, Portadown – Clay-with-Flints Record no. 1303 Devlin’ Quarry, Moneymore – Clay-with-Flints Record no. 1304 Glenshane Pass – Clay-with-Flints Record no. 1305 Donald’s Hill – Clay-with-Flints Record no. 1306 Tircrevan Burn – Clay-with-Flints Record no. 1307 Ballycastle – Clay-with-Flints Record no. 1308 Demesne Quarry, Glenarm – Clay-with-Flints Record no. 1309 Magheramorne Quarry, Larne – Clay-with-Flints Record no. 1310 Bellevue, Belfast, Co Antrim – Clay-with-Flints Record no. 1311 Soldierstown Quarry, Moira – Clay-with-Flints | |
| Keywords | |
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| Rocks: | Chalk, Clay, Flint, Basalt |
| Record type: | ESCR report | ||
| Entered by: | I J Enlander | ||
| Last updated: | 28 Mar 2019 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Introduction to Clay-with-Flints of Northern Ireland, County , site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=1300. Accessed on 2026-11-06. |
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