| Site Description |
|---|
 | | Exposure of Cretaceous basal conglomerate at junction of Ulster White Limestone Formation and Sherwood Sandstone Group, Murlough Bay. |
|
| Highlights: |
|---|
| The Cretaceous basal conglomerate at Murlough Bay is unique in Northern Ireland in containing macrofossil evidence for horizons in the Lower Jurassic much younger than any now known to be preserved onshore. Hiatuses within the conglomerate and between it and the overlying Ulster White Limestone indicate a complex history of erosion and deposition. |
| Introduction: |
|---|
| Murlough Bay (grid ref. 34/2142) lies on the north Antrim coast, between Fair Head to the west and Torr Head to the east, and approximately 10 km east of the town of Ballycastle. The pre-Cretaceous geology of the area is complex, comprising mostly Dalradian metasediments but with a small patch of red sandstones and conglomerates, rich in quartz pebbles from the underlying Dalradian, assigned to the Sherwood Sandstone Group of the Triassic. The Cretaceous outcrop extends from a fault on the western side of Murlough Bay eastwards to the slopes above Torr Head. The main cliff face on the upper slopes of Murlough Bay is developed in typical Ulster White Limestone and the underlying basal conglomerate is well exposed and fairly accessible over a distance of several hundred metres. On the western side of Murlough Bay the basal conglomerate rests upon Triassic pebbly sandstones but to the east, towards Torr Head, it rests directly on Dalradian metasediments. |
| The most detailed account of the basal conglomerate and its fossil fauna at Murlough Bay is that of Wilson and Robbie (1966) but it has been mentioned or discussed on numerous other occasions. Its age and relationship to the more typical 'greensand' facies of the basins to north and south of the Highland Border Ridge has been discussed by Reid (1957, 1959, 1962, 1971) and other aspects of its distribution and environment of deposition have been discussed by several different authors (Hancock 1961, Hume 1897, Tate 1865). One of the most significant aspects of the basal conglomerate at Murlough Bay is the occurrence of fossil material derived from strata not known to occur now anywhere in Ireland. These fossils, although rare, have been the subject of a number of publications in their own right (Anderson 1957, Hartley 1933, Savage 1963, Versey 1958). |
| Description: |
|---|
| The basal conglomerate at Murlough Bay comprises abundant quartz pebbles, typically up to about 5 cm across, together with fragments of other Dalradian lithologies and occasional fragments of Triassic sandstone. Reid (1971) also reported clasts of a fine-grained sandstone "resembling those of the Cenomanian Yellow Beds" (=Islandmagee Siltstones). The quartz pebbles are indistinguishable from those in the underlying Triassic pebbly sandstones. The matrix is a soft, glauconitic, sandy chalk. Reid (1971) reported that in places the conglomerate consisted of two or more layers, with eroded tops and different contents and matrices. In places large (> 10 cm across) clasts of reworked Cretaceous conglomerate can be recognised within the main mass of the basal conglomerate; they can be distinguished by their more brownish colour, particularly towards the margins, and the green staining of their surfaces. The contact of the basal conglomerate with the underlying Triassic sandstones is very uneven; in places glauconitic sands are piped down for several centimetres into the Triassic sandstone below. The upper surface of the basal conglomerate is sharply delimited from sandy glauconitic Ulster White Limestone above; stromatolitic laminations are commonly developed on the upper surface of the conglomerate. |
| Fossils generally are rare in the basal conglomerate, though locally may be common, and include a mixture of both Jurassic and Cretaceous taxa. Symes et al. (1888) were the first to mention the Lias fossils while Hume (1897) mentioned Lias nodules from the same locality, although neither publication gave further details. Hartley (1933) cited a number of fossils from the conglomerate at Murlough Bay which he described as being "sparingly fossiliferous". Of these the most significant was a fragment of a dactylioceratid ammonite, identified by L.F. Spath as allied to the Dactylioceras crassum group and hence of approximately mid-Toarcian, Bifrons Zone age. Savage (1963) also reported the discovery of a fragment of an indeterminate Dactylioceras of Lower Toarcian age from the conglomerate while Versey (1958) recorded a specimen of Pleuroceras transiens, an Upper Pliensbachian form confined to a restricted stratigraphic range around the Margaritatus-Spinatum Zone boundary. Wilson and Robbie (1966) also recorded Pleuroceras cf. solare, typical of the Upper Pliensbachian, Spinatum Zone, but also Tragophylloceras ibex and Beaniceras cf. rotundum, forms diagnostic of the Ibex Zone of the Lower Pliensbachian. |
| Further evidence for significant reworking of Middle and Upper Lias (Upper Pliensbachian-Toarcian) sediments in this area is provided by additional specimens in the collections of the Ulster Museum. BELUM K25225 is a fragment of Pleuroceras spinatum, indicating the Spinatum Zone of the late Upper Pliensbachian, while BELUM K12580 is a phosphatised fragment of a depressed whorl of an indeterminate dactylioceratid, indicative of a mid-Toarcian, perhaps Bifrons Zone, age. |
| A moderately diverse fauna of Cretaceous age also has been recorded from the conglomerate, with some undoubtedly being derived from earlier Cretaceous deposits. |
| JURASSIC TAXA |
| Beaniceras cf. rotundum S.S.Buckman, Tragophylloceras ibex (Quenstedt), Pleuroceras transiens (Frentzen), Pleuroceras cf. solare (Phillips), Dactylioceras sp. [crassum group], Amberleya (Eucyclus) cf. subimbricata (d'Orbigny). |
| CRETACEOUS TAXA |
| Orbirhynchia cf. dispansa Pettitt, Orbirhynchia cf. multicostata Pettitt, Orbirhynchia cf. orbigyni Pettitt, Orbirhynchia juv. cf. reedensis (Etheridge), Orbirhynchia cf. wiestii (Quenstedt), Cretirhynchia aff. plicatilis (J.Sowerby), cf. Concinnithyris obesa (J. de C. Sowerby), Gibbithyris subrotunda (J.Sowerby), cf. Gibbithyris hibernica (Davidson), Neoliothyrina sp., cf. Rectithyris depressa (Lamarck), Terebratulina striata Wahl., Terebratulina striatula (Mantell), indeterminate terebratulids [probably derived], Atreta nilssoni (Hag.), Chlamys (? Aequipecten) sp., Exogyra columba (Lamarck), Gryphaeostrea canaliculata (J.Sowerby), Inoceramus sp., Lima (Plagiostoma) sp., Lithophaga sp. [crypt], Ostrea vindinnensis (d'Orbigny), Pachythaerus sp., Plicatula sp., Pycnodonte cf. vesicularis (Lamarck), Spondylus sp., Paraesa cf. faba (J. de C.Sowerby), Neithea quinquecostata (J.Sowerby), Lewesiceras cf. sharpei Spath, indeterminate belemnites, Trochosmilia sp., Cidarid spines, Ptychodus decurrens Agassiz, ?Lamna sp., indeterminate elasmobranch teeth. |
| Importance: |
|---|
| The basal conglomerate beneath the Ulster White Limestone Formation at Murlough Bay is unique in Ireland in preserving macrofossil evidence for the former existence of Lower Jurassic strata from horizons substantially younger than any now known to be preserved onshore in Northern Ireland. The source of this material, the reason for its preservation on an apparent late Cretaceous 'high', and the absence of similar material in other Cretaceous basal conglomerates in Northern Ireland, remains enigmatic. |
| Interpretation: |
|---|
| Reid (1971) recognised three facies in the sediments of the Upper Hibernian Greensands and Ulster White Limestone; greensands and glauconitic chalks, conglomerates, and typical Ulster White Limestone. He observed that from the south-east as far as Carnlough the earliest Cretaceous sediments deposited were greensands, but from Garron Point westwards to sections east of Ballycastle, on the Highland Border Ridge zone, these beds are replaced by thin conglomerates. The Murlough Bay basal conglomerate clearly represents a prolonged period of greatly reduced sedimentation and erosion of pre-Cretaceous rocks on a sea-floor 'high'. The presence of reworked clasts of an earlier 'basal conglomerate', and the occurrence of the ammonite Lewesiceras cf. sharpei, also demonstrates that the conglomerate does not represent a single depositional event but represents the culmination of several episodes of deposition and erosion perhaps spanning part of the Turonian and extending through the Coniacian and into the early Santonian. |
| Reid (1962) described in some detail a similar basal conglomerate on the east side of Red Bay, some 15 km SSE of Murlough Bay. This conglomerate has a superficial similarity to that at Murlough Bay and also contains derived fossils, though only of Senonian and Cenomanian age. However, here it grades upwards into glauconitic chalk whereas at Murlough Bay, and elsewhere on the Highland Border Ridge zone, there is a truncating erosion surface seen between the top of the conglomerate and the overlying glauconitic chalk. Reid (1971) considered the conglomerates of the Red Bay and Garron Point areas to represent material swept southwards from the Ridge zone proper. |
| The precise age of the basal conglomerate at Murlough Bay has been much discussed. Reid (1957) concluded, from the occurrence of the hexactinellid sponge Rhizopterion cribrosum, that part at least of the conglomerates can be assigned to the middle or upper Senonian (Santonian or Campanian). However, in a subsequent paper (Reid 1971) he concluded that there was no evidence for an age later than early Senonian (= Coniacian) and that the presence of Lewesiceras indicated an age no greater than Turonian. The succeeding Ulster White Limestone varies in age across the Highland Border Ridge from the U.socialis Zone to the B.mucronata Zone (mid-Santonian to late Campanian), as was clearly depicted by Fletcher (1977, fig. 4). |
| The abundance of quartz pebbles can be directly related to the unconformable relationship between the Cretaceous strata and the Triassic or Dalradian rocks beneath and is, in itself, unremarkable. However, the presence of a moderately diverse fauna of reworked Jurassic fossils representing strata not known anywhere in Ireland is unique. Ammonites indicative of the Ibex, Spinatum and Bifrons Zones have been recovered, spanning strata from the Lower Pliensbachian to the mid-Toarcian. The preponderance of ammonites in this derived Jurassic fauna almost certainly is an artefact of collecting, since they are amongst the most easily recognised fossils, even in a worn fragmentary state. The presence of these derived fossils at Murlough Bay from horizons so much higher in the Lower Jurassic than any now known to occur in Northern Ireland is intriguing. Their presence is still more enigmatic when it is considered that the basal conglomerate here rests on a basement high underlain by unfossiliferous Triassic or Dalradian rocks and that the nearest known outcrops of Lower Jurassic strata lie more than 15 km to west or east. This contrasts starkly with other occurrences of Cretaceous 'basal conglomerate' which have yielded derived Lower Jurassic fossils, for instance at Tircrevan Burn and at White Park Bay. At these sites invariably there is a close stratigraphic correspondence between the derived fauna and the Lower Jurassic sediments beneath the unconformity. |
| The source of the derived Lower Jurassic material at Murlough Bay remains unknown. Strata of the appropriate age are well developed in various parts of the Hebrides Trough (Hesselbo et al. 1998) while substantial thicknesses of Mesozoic sediment are known to be present in other offshore basins to the northwest of Ireland. It is possible that uplift of one of these sedimentary basins during Cretaceous times may have contributed sediment to the Murlough Bay area, with subsequent basin inversion creating the present situation with thin onshore sequences and thick offshore ones. Blocks of Middle Lias shales were reported in considerable abundance from temporary excavations at Ballycastle in the last century, with further material obtained from Ballintoy (Langtry 1874, Gray 1870, Tate 1870). Such a local concentration of material does not favour derivation from a distant source, such as the Hebrides, and suggests that rocks of Middle and Upper Lias age may underlie the Cretaceous strata of Rathlin Island and outcrop beneath the North Channel and Rathlin Sound (Wilson and Robbie 1966). However, until such time as Lower Jurassic sediments of the appropriate age are discovered offshore, if indeed they still exist there, the derived Jurassic fossils at Murlough Bay must remain an enigma. |
| Conclusions: |
|---|
| Basal conglomerates, of late Cretaceous age, are well exposed beneath cliffs of Ulster White Limestone at Murlough Bay. Although dominated by reworked quartz pebbles from the Dalradian and Triassic rocks beneath and only poorly fossiliferous, this basal conglomerate contains a mixture of autochthonous and derived elements indicating the former existence nearby of Lower Jurassic horizons much younger than any now surviving in Northern Ireland. In this respect it is unique among the Cretaceous basal conglomerates of Northern Ireland, which otherwise contain elements derived only from immediately preceding Cretaceous horizons or immediately underlying Jurassic sediments. |
| Notes: |
|---|
For general information and references on the Palaeontology of Northern Ireland see Key Site 109 - Palaeontological sites of N. Ireland - Introduction |
| For other site specific information and references on the Palaeontology of Northern Ireland see Key Site 110 - Ballycastle Pellet Chalk, Key Site 111 - Ballypalady Plant Bed, Key Site 112 - Carrick Lough, Bunnahone Lough and the Sillees River, Key Site 113 - Faragandoo, Key Site 114 - Minnis North Mudflow, Key Site 115 - Moyola River, Key Site 116 - Mullynaskeagh Clay Pit, Key Site 118 - Portnaloub, Key Site 119 - Sillees River, Glenasheevar, Key Site 120 - Tircrevan Burn, and Key Site 121 - Tullyconnell. |
| References |
|---|
| Anderson, F.W. 1957: Report of the Palaeontological Department. Summary of Progress of the Geological Survey for 1956 , pp.50-52 |
| Fletcher, T. P. 1977: Lithostratigraphy of the Chalk (Ulster White Limestone Formation) in Northern Ireland. Report of the Institute of Geological Sciences, vol. 77/14 |
| Gray, W. 1870: Seventh Annual Report of the Belfast Naturalists' Field Club , pp.49-50 |
| Hancock, J.M. 1961: The Cretaceous System in Northern Ireland. Quarterly Journal of the Geological Society of London, vol. 117, pp.11-36 |
| Hartley, J.J. 1933: Notes on fossils recently obtained from the 'Chloritic' conglomerate of Murlough Bay, Co. Antrim Irish Naturalists Journal, vol. 4, pp.238-240 |
| Hesselbo, S.P., Oates, M.J. and Jenkyns, H.C. 1998: The lower Lias Group of the Hebrides Basin Scottish Journal of Geology, vol. 34, pp.23-60 |
| Hume, W.F. 1897: The Cretaceous strata of County Antrim. Quarterly Journal of the Geological Society of London, vol. 53, pp.540-606 |
| Langtry, G. 1874: On the occurrence of the Middle Lias at Ballycastle British Association Report,, pp.88 |
| Reid, R.E.H. 1957: Remarks on the Upper Cretaceous Hexactinellida of County Antrim Irish Naturalists Journal, vol. 12, pp.236-243 |
| Reid, R.E.H. 1959: Age of the Cretaceous basal conglomerate at Murlough Bay, Co. Antrim Geological Magazine, vol. 96, pp.86-87 |
| Reid, R.E.H 1962: The Cretaceous succession in the area between Red Bay and Garron Point, County Antrim Irish Naturalists Journal, vol. 14, pp.73-77 |
| Savage, R.J.G. 1963: Upper Lias ammonite from Cretaceous conglomerate of Murlough Bay Irish Naturalists Journal, vol. 14, pp.179-180 |
| Symes, R.G., Egan, F.W. & McHenry, A. 1888: Explanatory Memoir to accompany Sheets 7 and 8. Memoirs of the Geological Survey of Ireland |
| Tate, R. 1865: On the correlations of the Cretaceous formations of the NE of Ireland. Quarterly Journal of the Geological Society of London, vol. 21, pp.15-44 |
| Tate, R. 1870: Note on the Middle Lias in the NE of Ireland. Quarterly Journal of the Geological Society of London, vol. 23, pp.15-44 |
| Versey, H.C. 1958: Derived ammonites in basal Cretaceous conglomerate Geological Magazine, vol. 95, pp.440 |
| Wilson, H. E. and Robbie, J. A. 1966: Geology of the country around Ballycastle. Memoirs of the Geological Survey Northern Ireland, |