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Earth Science Conservation Review
| Coalpit Bay – Lower Palaeozoic | Down |
| Site number: | 1350 | ||
| Locality Type: | Coastal Section, Foreshore | Status: | ASSI |
| Grid Reference: | J594788 | ||
| Google maps: | 54.63262,-5.53146 | ||
| County: | Down | Council area: | Ards & North Down Borough Council |
| Period: | Ordovician, Silurian | ||
| Stages: | Caradoc, Ashgill, Llandovery | ||
| Site Description | |
|---|---|
| Highlights: | |
Peach and Horne (1899) divided the area of Lower Palaeozoic rocks forming the Southern Uplands of Scotland into Northern, Central and Southern Belts. In the many strike-parallel fault-bounded tracts of the Northern Belt and the northern parts of the Central Belt the greywacke successions which compose the bulk of the Southern Uplands - Down - Longford Terrane rest conformably on the Moffat Shale Group (Floyd 1996). The Moffat Shales consist of a thin, condensed, pelagic succession of grey or black, richly fossiliferous, graptolitic shales and mudstones ranging in age from Caradoc to Upper Llandovery. In the mid-nineteenth century the Moffat ‘Series’ provided Charles Lapworth (1878) with the material for his pioneer studies of graptolite morphology and biostratigraphy, in turn the essential basis for the establishment of the Ordovician System. The classic Lapworth outcrops of the Moffat Shales, particularly Dob’s Linn and Craigmichen, are in the upper Ettrick Valley region of Selkirkshire, a few kilometres NE of the town of Moffat. The stratigraphy and structure of the Scottish Southern Uplands continue WSW along strike across the North Channel into Ireland and are particularly well developed on the County Down coast. The most complete, most accessible and historically important outcrop of the Moffat Shale Group in Ireland is at Coalpit Bay, Donaghadee. Even though the succession is less completely exposed than at the type localities in the Ettrick Valley, some 160km ENE along strike, the several constituent formations established by Lapworth in the Moffat outcrops are readily recognised at this Irish locality, where they are composed of the same lithologies and contain the same rich graptolite faunas. The Glenkiln and Hartfell Formations, which compose the Ordovician part of the succession, are very irregularly folded and have been displaced by numerous faults. Although the shales are fossiliferous, the faunas have not been collected and studied in quite the same detail as at Moffat. Yet graptolites collected from Coalpit Bay and described by Swanston and Lapworth in the nineteenth century were figured in the classic monograph on “British Graptolites” by Elles and Wood (1918). Swanston (1875, 1877) collected some 45 graptolite species, identified by Lapworth (1877). Further collecting since (Clark 1902, Anderson 1962, Griffith and Wilson 1982) has established the presence of at least 70 graptolite species and nearly all the graptolite biozones between gracilis, at the base of the Caradoc and sedgwickii in the Upper Llandovery. The graptolite identifications imply that the stratigraphic equivalents of all four formations of the Moffat Shale Group, ie Glenkiln Shale, Lower Hartfell Shale, Upper Hartfell Shale and Birkhill Shale, are present, at least in part, and this is confirmed by lithological correlation. The total stratigraphic thickness of the Moffat Shale Group at Coalpit Bay, as logged for this report, is some 60 metres. Some 50 thin light grey clay bands are interbedded with the siltstones, mudstones and shales of the Moffat Shale Group. These were tentatively described as ash falls by Anderson (1962) and later positively identified as bentonites by Cameron and Anderson (1980). Since 1980 many more bentonite bands have been recognised throughout the Southern Uplands, each representing a fall of fine-grained volcanic material spewed out from a neighbouring volcanic arc. The geochemistry of the bentonites is a potential aid to stratigraphic correlation (Huff et al. 1991) and provides evidence about the palaeogeographic setting of the Southern Uplands - Down - Longford Terrane on the northwestern margin of the Iapetus Ocean. The coarse greywacke sandstones of the Millisle Formation (Anderson 1962, Barnes et al. 1987), cropping out along the coast for a kilometre to the south of Colapit Bay, display many spectacular sole markings, of textbook quality, on their upturned southern surfaces. Indeed this is probably the best locality in Ireland at which to demonstrate and study turbidite sole markings. | |
| Introduction: | |
Bryce (1852, pp.42-43), in a description of the slates of County Down, noted “a few imperfect fossils, which seem to make them referable to the Lower Silurian Group” but gave no specific locality. In his “Manual of Geology” Jukes (1862, p.434) recorded “ a band of anthracite” in County Cavan, which “may also be traced in County Down, as if striking from similar anthracite bands in the south of Scotland. It is associated with black shales containing an assemblage of graptolites, resembling those of the Scottish beds.” It seems very likely that this description refers, at least in part, to the outcrop at Coalpit Bay, where the black Birkhill mudstones are very fossiliferous and bear a superficial resemblance to a very hard coal. Murchison, in the fourth edition of ‘Siluria’ (1867, p.183) incorporated Jukes’ information about the anthracite band and described it as “a fuel of better quality (in short, a real coal) as well as of larger dimensions than any similar substance hitherto noticed in the Silurian rocks”. It may well be that neither Jukes nor Murchison ever visited Coalpit Bay and that they were simply misinformed about the identification of the “anthracite”. The earliest specific reference to Coalpit Bay appears to be by J.L.Warren in the Survey Memoir of 1871, p.25: “…Coal Pit Bay, where a band of black carbonaceous slates full of graptolite remains has been observed. Immediately S. of the slates the beds are intersected by a dyke composed of a pale grey finely crystalline calcareous rock, about six feet wide, branching into three as it goes seaward. South of the dyke another band of graptolite shales occurs, over which are gray grits, becoming massive as we go south, with layers of gray slates”. In the same Memoir (Hull, Warren and Leonard 1871, pp. 16-17), W.H. Baily lists 11 graptolites, including Rastrites peregrinus, from the black slates at Coalpit Bay. Swanston (1875) recorded “the finding of twelve species of graptolitidae, and three crustaceans” at Coalpit Bay. Swanston sent his graptolite specimens to Charles Lapworth and quoted Lapworth’s letter in reply, summarising results and conclusions: “The beds of black shale, containing this peculiar grouping of forms, must be of middle Silurian age, perhaps Lower Llandovery. They form, in all probability, merely the western continuation of one of the black shale bands so common in the Silurians of the South of Scotland. They answer exactly to some of the central beds of the Birkhill shales, or the highest subdivisions of the Moffat group…” Two years of intensive collecting by Swanston followed, enabling him to present a second paper to the Belfast Naturalists’ Field club in 1877. This lists 80 graptolites, 3 brachiopods, 3 crustaceans and 1 plant from 8 localities in the Lower Palaeozoic rocks of County Down. The collections were sent to Lapworth who recorded and described some 45 graptolite species from Coalpit Bay (Lapworth 1877). Swanston described a stratigraphic succession and correctly recognised the Glenkiln Shales, the Lower and Upper Hartfell “Series”, the Lower and Upper Birkhill Shales in the Moffat Shale succession at Coalpit Bay. Each formation was identified on a lithological basis supported by the identification of diagnostic graptolites. In his accompanying paper on the graptolites Lapworth (1877) noted (p. 126) that “The Coalpit Bay Division of the County Down Silurians has yielded all the Graptolites of the Birkhill Shales with the exception… of the Rastrites maximus zone” and predicted, correctly and with remarkable clarity, “that should the black shales be detected in the future in the southern districts of the County Down, the maximus zone will probably be found”. Lapworth also confirmed the presence of the Hartfell Shales, though without proof of the Dicellograptus anceps Zone, and the presence of the upper fossiliferous portion of the Glenkiln Shales. Swanston’s careful and detailed cross-sections of the strata at Coalpit Bay are preserved in the collection of the Ulster Museum. Clarke (1902, p.499) recorded his discovery of Rastrites maximus at Tieveshilly, exactly in accord with Lapworth’s prediction, and the presence of Dicellograptus anceps “in a pocket of black shale in greenish mudstone at Coalpit Bay”. The Elles and Wood (1918) Monograph of British Graptolites illustrates specimens of at least seven different graptolite species from Coalpit Bay, including specimens of Dimorphograptus confertus var. Swanstoni (Lapworth). In a PhD study of the Lower Palaeozoic rocks of the Ards Peninsula, Anderson (1962) described the Moffat Group stratigraphic succession of Glenkiln, Hartfell and Birkhill Formations at Coalpit Bay and provided a map and cross-section showing their distribution and structure. The map was later published in a field guide to the Ards Peninsula (Anderson, 1978) and is reproduced again in this report as Figure 1. Some 14 graptolite faunas were collected from the bay in support of the stratigraphic correlation. A number of inarticulate brachiopods were also collected from the Lower Hartfell Shales. Coalpit Bay lies close to the southeastern corner of Sheet 29 of the Geological Survey of Northern Ireland. The sheet was resurveyed in the years 1958-1960 and the results recorded in the Sheet Memoir (Griffith and Wilson 1982). Graptolites were collected from eight localities in the bay and identified by Drs. Strachan and Rushton. Rushton’s account of the graptolite palaeontology is the most comprehensive yet published, providing modern lists of some 70 species from the eight localities. On the basis of this work and the previous records, Rushton recognised the presence or doubtful presence of the Ordovician Zones: gracilis and/or peltifer, wilsoni, complanatus and anceps. In the Silurian “a complete succession of graptolite zones from acuminatus (?) to possibly the base of sedgwickii is postulated” (Griffith and Wilson 1982) p.18. Despite the earlier work by Swanston and Lapworth and this almost complete Moffat Shale graptolite stratigraphy the Survey found that “any attempt at assembling a stratigraphy for the area is fraught with difficulties. Every exposure of graptolitic mudstone is intensely faulted so that nowhere is it possible to establish a succession and the best that has been possible are to identify, from the graptolitic faunas, the occurrence of certain Glenkiln and Hartfell zones. Consequently the following synthesis contains many imponderables and is tentative in the extreme”. The Survey Memoir introduced the new term Coalpit Bay Mudstone Formation for the Moffat Shale Group at Coalpit Bay but did not attempt to recognise or map the distribution of the Glenkiln, Hartfell and Birkhill Formations. Their section and map is reproduced here as Figure 2. (On that map graptolite Localities 4 and 5, with their wilsoni and gracilis or peltifer faunas, appear to occur within the outcrop of the easily identified Upper Hartfell Barren Mudstones (cf. Fig. 1) and are therefore probably incorrectly located – see below). In their paper on Silurian metabentonites in County Down, Cameron and Anderson (1980) provided a partial lithological log of the upper part of the Birkhill Formation at Coalpit Bay. Stratigraphic columns representing the range of the Moffat Shale Group at Coalpit Bay have appeared in several papers and textbooks (e.g. Barnes, Anderson and McCurry, 1987, fig. 5, Cocks, Holland and Rickards, 1992, fig. 10 (right)). | |
| Description: | |
At high tide, Coalpit Bay, also known locally as Recreation Grounds Bay, can be seen to lie between two well-defined headlands composed of greywacke (Figs. 1 & 2). The northern headland is at the southern end of Donaghadee Commons and is occupied by a car park for users of the adjacent tennis courts and bowling green. The southern headland is some 120 metres south of the point where the access road to the car park bends sharply north (left) on reaching the coast. Between the two headlands, two small promontories or peninsulas of rock, each a few tens of metres across, project eastward into the 200 metre-wide bay. These small peninsulas expose much of the Moffat Shale succession and to facilitate description are referred to in this report as the ‘Northern Peninsula’ and ‘Southern Peninsula’. Apart from the two peninsulas, the floor of the bay is largely covered by shingle and boulders, with a few small rock outcrops. Immediately east of the coastal path on the northern edge of the southern headland there is an arcuate rock face, some 3m high and about 15m long, which resembles a very small quarry. This is the “Pit” outcrop, which gives the bay its historic name. The junction between the Birkhill Shale Formation, at the top of the Moffat Shale Group, and the overlying turbidites of the Coalpit Bay Formation (Anderson 1962, 1979) is well exposed in the face. The grey-black siltstones of the Birkhill Shales Formation which floor the pit bear some resemblance to bedded coal seams and this presumably is the basis of Jukes’ (1962) and Murchison’s (1867) error. The Shales in the floor of the pit split readily along bedding and yield many beautifully preserved graptolites. At low tide, Coalpit Bay drains to a 100 metre-wide expanse of rock outcrop, boulders and shingle and the outline topography described above is less easy to recognise. Lithostratigraphy and Structure The overall geological structure of the rocks in the Bay is essentially anticlinal, as illustrated in the cross section on Figure 1. The greywackes on the northern headland strike approximately east-west and young or face northward and those south of, and stratigraphically above, above the Birkhill Shales on the southern headland dip and face south. In detail the structure is much more complicated. The two greywacke formations forming the headlands are of distinctly different lithologies and probably do not correlate. The two peninsulas within the Bay each have separate and subordinate anticlinal structures, with that of the southern peninsula much complicated by faulting and interformational décollement. The Ordovician strata are entirely confined to the Southern Peninsula, folded about the hinge of an anticline, which can be traced from low water mark near the northeastern extremity of the peninsula on a trend 075o to 255o, parallel to and about 10 metres south of the northern limit of the outcrop. The anticline plunges westward at between 20 o and 30o. The northern limb is truncated by faulting and erosion so that the succession is best established by following the stratigraphy upward to the west along the trace of the anticline and then southward across the southern limb (See Fig. 1). Numerous minor faults and folds complicate stratigraphic study and render thickness determinations difficult and somewhat insecure, particularly in the lower part of the succession (see Fig. 3). The Glenkiln Shale, the oldest stratigraphical formation, forms a wedge about 25m long by 8m across in the hinge of the anticline at the northeast end of its exposed trace. The formation consists of 9 metres of hard dark grey mudstone and fine siltstone, grey-black shale, in beds a few millimetres thick, and some beds of light grey chert individually ranging up to 0.3m thick. The chert beds, which appear characteristic of this formation at Coalpit Bay, have been tectonically pulled apart into very roughly square or rectangular blocks enclosed in the more ductile, darker grey mudstone and shale. The base of the formation is not exposed and the entire Glenkiln outcrop is covered at high tide. Toward its top the Glenkiln Shale is cleaved and shaly and there is a sharply defined structural discontinuity between the Glenkiln and the succeeding lower Hartfell Shale. The junction is well exposed at the hinge of the anticline in the base of a small scarp at the easternmost end of the small high-water-mark island shown on the current 1: 2500 Ordnance Survey Sheet. At this point the overlying Hartfell appears to lie with strong angular unconformity on the Glenkiln. This is best understood by recognising that Glenkiln and Lower Hartfell at this point are of contrasting rheologies. During the intense Caledonian tectonic deformation the two formations, which demonstrate a normal stratigraphic relationship at many localities in southern Scotland, have separated along a décollement on the formation boundary Resting on the Glenkiln Shale to the north, west and south, the Lower Hartfell Shale consists of 15 metres of grey-black flaggy mudstones, well bedded on a 3 to 5 centimetre scale. The mudstone is hard, silicified and darker grey in colour than the mudstones of the Glenkiln Formation. Unlike the Glenkiln, the flaggy Hartfell mudstone beds are internally laminated throughout. 5.5 metres above the base of the formation there is a 15 to 40 cm thick bed of medium grey, fine-grained, buff weathering limestone. The limestone bed can be traced, with difficulty, across numerous small faults around the main anticline and several sub-parallel minor folds. In the grey black mudstone above the limestone bed there are a number of discontinuous seams of grey or green metabentonite, individually ranging up to 20 centimetres thick. 15 metres above its base of the Lower Hartfell Shale passes up into a much lighter grey and less well bedded mudstone or fine siltstone. The two lithologies are interlayered on a half-metre to metre scale in a transition which is 3.4 metres thick. Above the transition the light grey mudstone dominates. This lighter grey mudstone and siltstone is clearly the Upper Hartfell Shale of the Moffat succession, described by Lapworth as the “Zone of barren mudstones”. Throughout its 22-metre thickness the Upper Hartfell Shale is very weakly bedded and roughly cleaved. Again the junction between the well-bedded Lower and much more massive Upper Hartfell is one of considerable rheological contrast and its outcrop is typically characterised by minor faults and metre-scale discordant folds. Some 5 metres above the base the Upper Hartfell barren mudstones there is a 5 metre thick unit which includes three bands of black mudstone, 1m, 0.3m and 1.4m thick. Similar bands of black mudstone occurring within the barren mudstones in the Moffat outcrops have yielded an anceps Zone graptolite fauna but at Coalpit Bay only fragmental smears of sheared graptolite material have been found to date, although Clark’s (1902) discovery of a specimen of Dicellograptus anceps may well be from one of these three black mudstone bands. Above the bands of black mudstone a further 12.5 metres of uniform grey mudstone crop out stratigraphically beneath a sharp junction with continuous grey-black mudstones representing the base of the Birkhill Shale. The junction between the Upper Hartfell Shale and Birkhill Shale coincides almost exactly with the outcrop of a prominent, roughly concordant, 2-metre thick, dark red lamprophyre dyke. The dyke is a single sheet extending for 30 metres along strike at the western, high water, end of its outcrop. Eastward it offsets, transform fashion, across a series of sinistral strike-slip faults and divides into as many as four thinner, generally bedding-parallel, lamprophyre sheets. The dyke is one of the “Younger Series” (Reynolds 1931), late Caledonian, lamprophyres of which at least 150 crop out in the Ards Peninsula. A four-metre thickness of black shale interlayered with more massive black mudstones crop out south of the dyke before the outcrop fails beneath shingle cover. These shales and mudstones are clearly of the Birkhill Shales Formation, on both lithological and palaeontological evidence. Several very thin (c. 2mm), light grey, metabentonite beds occur within this four-metre succession. The structural nature of the junction between the Upper Hartfell Shale and Birkhill Shale is difficult to determine because of the presence of the dyke. A collection of graptolites from the shales 0.6m south of the dyke indicates the lower part of the cyphus Biozone (R.B. Rickards pers.com.) and this implies that the three lowest Birkhill biozones, acuminatus, atavus and acinaces, are missing or extremely attenuated at this outcrop. The simplest and most likely explanation is that the formational (and systemic) boundary is once again the site of shearing and décollement, which has effectively removed the lowest part of the Birkhill Shale. The atavus and acuminatus Biozones are both recognised in faunas collected from the Birkhill Shales of the Northern Peninsula. Southward across a 7 metre exposure gap the Birkhill Shale re-emerges from beneath the shingle cover and continues in the form of a flat, relatively smooth, platform for some 50 metres southward across the east-west strike of the beds to a 3 metre high bluff or cliff. The cliff is formed by the thick, erosion-resistant greywacke sandstones of the succeeding Coalpit Bay Greywacke Formation (Anderson 1962), one of the many quartz-rich greywacke formations of the Llandovery-age Gala Group. This low, flat Birkhill Shale outcrop has only been completely exposed since about 1990: previously it was almost completely covered by shingle. At the northern edge there are several open folds of 2 to 3 metres wavelength, plunging gently WSW. From the most southerly anticlinal hinge the beds of the Birkhill Shale formation dip and young southward in a regular fashion, the dip decreasing gradually from 50o to 35o to the SSE. Figure 4, a log of the Birkhill Shale Formation stratigraphy on a traverse 040o - 220o along the eastern side of the platform, records a stratigraphic thickness of 17.4 metres in this partial transect of the formation. The lithology is predominantly of thin black shales alternating with hard black mudstones and fie-grained grey siltstones on a centimetre scale, the mudstones becoming grey-black and slightly silty toward the top. 0.56 metres of laminated siltstones and shales occur immediately beneath the lowest turbidite bed of the succeeding Coalpit Bay Greywacke Formation. Within the 17.4-metre thickness, there are some 46 bentonite beds. The bentonites consist of soft pale grey or pale green clay. The clay mineralogy described by Cameron and Anderson (1980) clearly indicates a volcanic origin. The individual layers are thus interpreted as ash falls from an active volcanic arc (See Huff et al. 1991 for discussion). The bentonites are generally uniform thickness beds at outcrop scale but even between adjacent outcrops there are apparent changes in thickness, which make the correlation of individual beds difficult or impossible. The thickness of the individual bentonites in the Birkhill Shale at Coalpit Bay varies from about five to 110 millimetres and the total thickness of bentonite in the 17.4 metre logged section of Birkhill Shale is at least 1.50 metres. The stratigraphic value of the thickness determination is marred by the presence of a 2-metre thick strike-slip duplex occurring between 3 and 5 metres beneath the junction with the overlying turbidites. The duplex consists of imbricated, lenticular, fault defined slices of shale, mudstone and at least five bentonite beds, individual slices about a metre long and 0.3 m thick. The sense of overlap indicates sinistral strike-slip. Though small in scale, the duplex, best seen at the eastern end of its outcrop, is of demonstration quality. Clearly some repetition of beds occurs within the duplex but there is no obvious repetition in the section above and below. The probable stratigraphic effect is therefore to increase the apparent thickness of the Birkhill Shale by about a metre. This 17.4 metre measured section of Birkhill Shale at Coalpit Bay spans only from the cyphus Biozone to the top of the unit. There is no clear indication of how much of the formation is missing due to lack of outcrop or to tectonic excision at its junction with the upper Hartfell Shale beneath. Even allowing for the repetition a few centimetres of beds in the small duplex it is clear that 17.4 metres is likely to underestimate significantly the true thickness of the unit. In the type Moffat area the Birkhill Shale is about 40 metres thick (Floyd 1996), including 1.6 metres at the base assigned to the topmost Ordovician Glyptograptus persculptus Biozone (Williams 1986). The Northern Peninsula at Coalpit Bay appears to consist entirely of Birkhill Shale folded into a small antiform, which is directly reflected in the hump-backed topography of the outcrop. Although the outcrop is quite large in area, it exposes a stratigraphic thickness of no more than fifteen metres of hard, silicified, black mudstones and shales. The rock does not split readily and graptolites are relatively difficult to extract. Three faunas have been recently been collected, identified and attributed to the acuminatus and atavus Biozones (R.B. Rickards pers.com.), thus representing a lower part of the Birkhill Shale Formation which is either absent or not yet recognised in the more southerly outcrop. At the Pit outcrop and on the low cliff to the east sandy turbidites of the Coalpit Bay Greywacke Formation rest directly and apparently conformably on the Birkhill Shale Formation. The lowest turbidite bed is some 65 cm thick, contains 20cm long rip-up clasts of shale and includes an extremely unusual layer of light grey-white sandstone just above its base. The light colour of this five-centimetre band contrasts with the normal dark grey of the clay-rich greywacke. The succeeding 20 metres consist of massive beds of quartz-rich greywacke sandstone, including one bed that is 4 metres thick, alternating with thin beds of slate. The beds dip and young to the south but it is possible to observe rough sole markings on the lower (under) surfaces of some of the turbidites. The shale or slate interbeds are 10 to 40cm thick. The lithology is thus quite typical of the Gala Group that stratigraphically succeeds the Moffat Shale Group in the various tracts in the northern part of the Central Belt of the Scottish Southern Uplands. In otherwise continuous outcrop 20 metres south of the Pit and directly opposite the 16th of 17 houses in the row of coastal bungalows on the south side of Coalpit Bay, a narrow east-west gully separates the Coalpit Bay Greywacke Formation on its north side from somewhat less thickly bedded north-younging sandy turbidites on its southern side. On the north side of the gully, the beds young south and verge north on the slaty cleavage developed in the shale interbeds. The beds on the south side of the gully dip steeply north and verge south and are at the stratigraphic top of the 1200m thick Millisle Sandstone Formation which is exposed on the coast to the south. The Millisle Formation has yet to yield fossils but it is most unlikely to occur stratigraphically below the Moffat Shale Group. It follows that the gully marks the trace of a fault, the Southern Coalpit Bay Fault of Anderson (1962), which, despite a lack of any obvious deformation or fault rock at outcrop, has a stratigraphic displacement of over 1000 metres. When traced eastward this faulted northern boundary of the Millisle Sandstones is clearly displaced northward by a sinistral strike-slip fault with a slip of at least 300 metres. The various seaweed-covered low-tide outcrops and islands of rock east of the Moffat Shales at Coalpit Bay consist of steeply overturned, north younging sandstones of the Millisle Sandstone Formation. Southward of Coalpit Bay, and thus beyond the designated boundaries of the Key Site, the Millisle Sandstones are commonly overturned, dipping moderately or steeply to the south but younging northward. In excellent coastal outcrop, the upturned bases of the sandstones show a variety of well-developed, photogenic sole-markings, including flute casts, groove casts, load casts and tool markings. Particularly excellent flute casts occur on an overturned bed forming a low-tide promontory some 455m south of the Pit (J595784). A large, open, upright, south-verging fold pair crops out 250 metres south of the pit at Galloway’s Bridge (J594786). The upward facing anticline is very well exposed but the hinge of the syncline immediately to the south is less obvious. Robby’s Point, the headland on the north side of Coalpit Bay, is again composed of Gala Group turbidites, again predominantly dipping steeply southward but younging or facing north. These rocks, of the Donaghadee Sandstones Formation (Anderson 1962), are subtly different in lithology from both the Coalpit Bay and Millisle Formations and do not obviously correlate with either. Their relationship to the Moffat Shale Group is not exposed and the turbidites and their interbedded shales have yet to yield graptolites. Biostratigraphy and Graptolite Palaeontology The Moffat Shale Group (Webb et al. 1993), originally termed the Moffat ‘Series’ by Lapworth (1878) consists of four formations in the type area: Glenkiln Shale Formation (20m+), Lower Hartfell Shale Formation (22m), Upper Hartfell Shale Formation (28m) and Birkhill Shale Formation (40m). Figure 5, with graptolite biozones compiled from Floyd 1996, Fig. 2 and Cocks, Holland and Rickards 1992, Fig. 7, outlines the biostratigraphy as a basis for comparison with the present state of knowledge of graptolite faunas at Coalpit Bay. The Glenkiln and Lower Hartfell Shale clearly correspond to Unit 6 of Swanston’s subdivision of the rocks at Coalpit Bay (Swanston 1877, p.110). From this unit he obtained a fauna including the zone fossil Climacograptus peltifer. Using Swanston’s collections, Lapworth (1877, p.126) noted the presence of the upper, fossiliferous part of the Glenkiln shale at Coalpit Bay, categorically stating “all of its characteristic fossils have already been collected”. Anderson (1962) collected three faunas from the same “Unit 6” shales and siltstones immediately north of the Barren Mudstone outcrop on the Southern Peninsula (Localities 1, 2 and 3 on Fig 1). The faunas included specimens of Climacograptus peltifer and were regarded as evidence of the presence of both Glenkiln and Hartfell Formations. Rushton (in Griffith and Wilson 1982, p.10) believed the fauna of locality 5 on the memoir map of Coalpit Bay (reproduced here as Figure 2) indicated the gracilis and/or peltifer Biozones. He also described (p. 10) the C. wilsoni Biozone as doubtfully present just north of the dyke at locality 4. (The lithology of the sediments alongside the dyke and for some 20m to the north is clearly that of the Upper Hartfell Barren Mudstones and it thus seems that localities 4 and 5 on the Survey’s Coalpit Bay map and section are misplaced, probably by about 20 to 25 metres.) Although Swanston and Lapworth recognised the presence of the black fossil-bearing shale seam within the grey Upper Hartfell Barren Mudstones, a seam that at Moffat commonly carries Dicellograptus anceps, they found no direct palaeontological evidence of the D. anceps Biozone (Lapworth 1877, p. 126). Clark (1902, p.499) claimed to have found D. anceps “in a pocket of black slate in greenish mudstone at Coalpit Bay. This description fits the black shale band in the Barren Mudstones and so supports Lapwoth’s lithological correlation. Anderson (1962) collected fragmental Climacograptids, Dicellograptids and Orthograptids from the same black band (Locality 4 on Fig. 1), a fauna that he suggested was probably of Hartfell age. Rushton (in Griffith and Wilson 1982, p.11) notes that the Dicellograptus forchammeri recorded by Swanston (1877, p. 111) “is referable to Dicellograptus complanatus and represents the upper Hartfell ‘D. complanatus Band”. It thus seems that the Ordovician shales at Coalpit Bay carry some evidence of all but two (clingani and linearis) of the seven biozones between gracilis and anceps. Lapworth (1877, p.125) stated that Coalpit Bay “has yielded all the Graptolites of the Birkhill shales, with the exception … of the Rastrites maximus zone”. Rushton (in Griffith and Wilson 1982, pp. 20-23 and Table 1) essentially supports this view. Rushton described the graptolite faunas of the Birkhill Shale Formation (Coalpit Bay Mudstones of the 1982 Survey Memoir) in considerable detail, concluding that (p. 20) “all the zones from vesiculosus to convolutus are present and it is possible that parts of the acuminatus and sedgwickii zones may also be represented”. However, as noted earlier, these do not crop in a continuous simple succession. Evidence of the acuminatus zone has not been found south of the outcrop of Upper Hartfell Barren Mudstones. At the time of writing it appears that the lower parts of the Birkhill Shale Formation crop out only on the northern peninsula. Three graptolite faunas from the silicified shales and mudstones of the northern peninsula have been assigned to the acuminatus and atavus Biozones by R.B. Rickards (pers. com.). It thus appears that the rocks exposed on the northern peninsula equate to at least in part of the stratigraphic sequence tectonically excised from between the Upper Hartfell Barren Mudstones and the Birkhill Shale in the outcrop on the southern peninsula. Thus, although the stratigraphic succession has been fragmented by faulting, representative exposure of all the formations and members of the Moffat Shale Group is thus available at Coalpit Bay, exactly as claimed by Swanston and Lapworth. | |
| Importance: | |
National Geology: In the Northern belt and in the northern tracts of the Central belt of the Southern Uplands - Down – Longford Terrane greywacke (turbidite) successions are seen to rest on a sequence of fine grained, deep water sediments. These pelagic sediments, the “Moffat Shales”, provided Lapworth with the graptolite material for his classic zonation and interpretation of the Upper Ordovician and Lower Silurian rocks. The Moffat Shales are now interpreted as the pelagic sediments either in the closing Iapetus Ocean (McKerrow, Leggett and Eales 1977) or in a back-arc basin on its northern margin (Stone, Floyd, Barnes and Lintern 1987), beyond the limits of greywacke sedimentation. Coalpit Bay provides the most complete, accessible and historically famous outcrop of the Moffat Shales in Ireland. The succeeding turbidites have sole markings and other sedimentary structures of demonstration quality, particularly well exposed on the coast for one kilometre south of the bay. International Geology: The Southern Uplands - Down – Longford Terrane now figures in many internationally used textbooks as a model accretionary prism produced by the offscraping of oceanic sediments from a subducting slab of Lower Palaeozoic oceanic crust (Mitchell and McKerrow 1975, McKerrow, Leggett and Eales 1977, Leggett 1987, McKerrow 1987). The unresolved ‘Southern Uplands Controversy’, between proponents of this model and those favouring the interpretation of the same rocks as a rotated thrust stack of sediments and lavas accumulated in a back-arc basin (Stone et al. 1987), is also widely quoted in the international literature on geotectonics. The field evidence for these models came largely from the Scottish outcrop of the terrane and particularly from the stratigraphic relations exposed around Moffat. Coalpit Bay provides an Irish counterpart of the critical evidence. The historic and beautifully preserved graptolite faunas are part of the evidence and clearly very important in their own right. | |
| Interpretation: | |
Excellent coastal outcrop at Coalpit Bay exposes, at least in part, all the formations and members of the Moffat Shale Group ranging in age from the peltifer Biozone to the base of the sedgwickii Biozone. Greywacke sandstones (turbidites) of the Gala Group succeed the Moffat Shales. | |
| Conclusions: | |
A remarkable representation of the Moffat Shale Group providing important information supporting the accretionary prism model for development of the Lower Palaeozoic rocks of Northern Ireland and on the timing of the Iapetus closure | |
| Notes: | |
TO BE SOURCED AND ENTERED Figure 1. Geological map and cross section of Coalpit Bay copied from Anderson 1978. Figure 2. Geological map and cross section of Coalpit Bay copied from the NI Geological Survey Memoir (Griffith & Wilson 1982, Fig. 3) Figure 3. Lithostratigraphy of the Moffat Shale Group on the southern peninsula, Coalpit Bay Figure 4. Lithostratigraphy of the Birkhill Shales in a partial transect 30m northeast of the ‘pit’ at Coalpit Bay. Figure 5. Age relations between the Moffat Shale formations as exposed at Coalpit Bay Figure 6. A photocopy of the Ordnance survey 1: 2500 sheet showing the Coalpit Bay Key Site is also attached. The area of the Moffat Shale Group outcrop, i.e. the area of prime interest, is outlined in red. The coastal outcrop of the Millisle Sandstones immediately south of Coalpit Bay has many excellent examples of turbidite sole markings and this is outlined in blue. The full range of ESCR site relating to the Lower Palaeozoic are as follows: Outer Ards Stratigraphic, Palaeontological and Structural sites Coalpit Bay – Lower Palaeozoic (record no. 1350) Helen’s Bay – Grey Point – Lower Palaeozoic (record no. 1351) Kearney Point – Knockinelder Bay – Lower Palaeozoic (record no. 1352) Orlock Bridge – Lower Palaeozoic (record no. 1353) Tieveshilly – Carrstown – Lower Palaeozoic (record no. 1354) Orlock Bridge - Structural (record no. 652) Whiskin Rocks - Structural (record no. 653) White House Port, Cloghy - Structural (record no. 654) Knockinelder Bay and Kearney Point - Structural (record no. 655) Millin Bay - Structural (record no. 656) Pomeroy area Craigbardahessiagh and Bardahessiagh River – Palaeontology (record no. 29) Lime Hill Farm – Palaeontology (record no. 33) Little River and Slate Quarry Bridge – Palaeontology (record no. 31) | |
| Keywords | |
|---|---|
| Rocks: | Turbidite, Greywacke, Shale, Bentonite |
| Fossil Groups: | Graptolite |
| Access | |
|---|---|
| Approach: | Public access south of Donaghadee |
| Restrictions: | Public access |
| Management: | |
Coalpit Bay is on the edge of a recreational area on the southern outskirts of the small town of Donaghadee. Much of the key outcrop is intertidal and the remainder is a very few metres above high water mark. A much used coastal footpath separates the outcrop from ‘The Commons’, which is a coastal park with open access, the public Recreation Grounds, consisting of putting green, bowling green and large, open car park, and a row of coastal bungalows | |
| Threats: | The only significant threat to the Coalpit Bay outcrop would appear to be through the possible construction of coastal works such as sewage outfalls, boating slipways, piers or promenades. Plans for such projects clearly need to be monitored to ensure that they do not affect the important parts of the outcrop. For aesthetic reasons the old, long disused, sewage outfall between northern and southern peninsulas should be removed. The fossiliferous shales are so abundant that collection and examination by geological parties is unlikely to cause significant damage. The leaders of such parties might be instructed to make sure that hammers are not used to deface or damage the superb sedimentary structures in the greywackes south of the bay. Maps and information leaflets might be made available to such visitors at the building that serves the sporting facilities in the adjoining recreation grounds. |
| Record type: | ESCR report | ||
| Entered by: | I J Enlander | ||
| Last updated: | 1 April 2019 | ||
| Enlander, I., Dempster, M. & Doughty, P., 2026. Coalpit Bay – Lower Palaeozoic, County Down, site details. [In] Earth Science Conservation Review. https://www.habitas.org.uk/escr/site.php?item=1350. Accessed on 2026-11-06. |
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