Jurassic, Northern Ireland
|Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.|
W I Mitchell
The break up of Pangea in the Late Triassic brought an end to the protracted period of predominantly continental conditions that had influenced the climate of Ireland since the end of the Carboniferous . The gradual spread of the Late Triassic marine transgression resulted from a global rise in sea level and by the Early Jurassic much of Ireland and Britain was covered by the sea (P947852).
Early Jurassic rocks in Northern Ireland, consisting mostly of grey calcareous mudstone and thin nodular limestone, only crop out around the margins of the Antrim Plateau (P947841). The incompetent mudstone below precipitous cliffs of chalk and basalt has caused landslips, so exposures of Jurassic rocks are rare, of limited extent and commonly occur within slumped blocks.
The maximum thickness of Jurassic rocks recorded is 248 m in the Port More borehole and about 125 m in the Mire House borehole (P947841). The strata range in age from Hettangian to Pliensbachian and contain ammonites representing most of the zones between the planorbis and ibex biozones (P947941). In the former borehole the absence of Hettangian and mid-Sinemurian strata in an otherwise almost complete sequence is caused by intrusion of Palaeogene dolerite sills. Although younger Jurassic deposits (post-ibex Zone) are not known, the occurrence of derived ammonites of the spinatum Biozone in the basal Cretaceous conglomerate and as erratics in the glacial till of north Co. Antrim suggests their presence offshore beneath the North Channel and around Rathlin Island. It is likely that younger Jurassic rocks were once present in Northern Ireland but were removed by erosion in pre-Cretaceous times.
A single formation, the Waterloo Mudstone Formation, based on the Waterloo section at Larne (P947848), (P947847), is recognised consisting mostly of fossiliferous medium to dark grey calcareous mudstone with laminae of silty mudstone and pale grey siltstone and thin beds of nodular limestone. In the Port More borehole the formation is again represented by grey calcareous mudstone with thin limestone beds becoming common in the raricostatum and lower part of the jamesoni biozones (P947941). In the upper part of the jamesoni Biozone in the borehole, siltstone beds and ironstone nodules are common. Strata of the succeeding ibex Biozone consist of dark grey micaceous mudstone. Jurassic strata in the Tircrevan Burn section [C 703 320] north of Limavady are about 52m thick and include the 13 m thick Tircrevan Sandstone Member . The lower contact of the sandstone is a non-sequence with invertebrate burrows excavated into the underlying grey mudstone  but elsewhere, as at the Larne stratotype it appears conformable. The succeeding 6m of strata consist of grey and brown, fine-grained, micaceous and bioturbated silty sandstone that contains plant fragments and grades up through calcareous sandstone to sandy limestone with bivalves, gastropods and crinoid ossicles in the upper 7 m. The upper contact is concealed. Because mudstones of the Waterloo Mudstone Formation readily absorb water, internal cohesion is lowered and at some localities they appear at surface as superficial mudflows, for example at Minnis North [D 338 135] in east Co. Antrim where the coast road is frequently inundated (P947841); (P948067).
The Waterloo Mudstone Formation contains a rich macrofauna, with ammonites proving the standard zonal sequence. The formation outcrop is divided into two geographically separate areas that are composed of rocks of different ages (P947941). The earliest Jurassic rocks, of Hettangian and early Sinemurian age, crop out in southeast Co. Antrim, particularly at Waterloo [D 409 037] near Larne (P947848), (P947847) and are recorded in boreholes in east Co. Tyrone. At Waterloo, above the exposed conformable Triassic-Jurassic boundary the lowest 25m of the Waterloo Mudstone Formation contains abundant bivalves such as Gryphaea and Plagiostoma (P948079) Fossils 1 and 2 and rare ammonites, including the zonal taxon Psiloceras planorbis (P948079) Fossil 3. In higher strata, specimens of the index taxa of the liasicus, angulata (P948079) Fossils 4 and 5 and bucklandi biozones  are rare. North of Larne, some 10m of vertically bedded grey mudstone and thin limestone of the early Sinemurian bucklandi Biozone are exposed 300 m north of Garron Point (P947841) [D 301 245].
The sequence of Jurassic rocks in the Port More borehole ranges in age from the late Sinemurian to Pliensbachian . Fossiliferous mudstone exposed at White Park Bay [D 015 440] is often concealed by landslip and sand dunes (P948010) and (P947853). At Oweynamuck the fauna includes bivalves, brachiopods and ammonites of the valdani Subzone of the ibex Biozone. The remaining exposures belong to the raricostatum Biozone and in particular the macdonnelli Subzone (P947941). In north Co. Londonderry, mudstones below the Tircrevan Sandstone Member yields Schlotheimia of the late Hettangian angulata Biozone while those above the sandstone contain Arnioceras semicostatum (P948079) Fossil 6 and Euagassiceras indicating the sauzeanum Subzone of the semicostatum Biozone.
One of the classic geological localities in the British Isles is at Portrush in north Co. Antrim (P947841). Here, fossiliferous Jurassic mudstone (‘Portrush Rock’) containing the ammonite Paltechioceras (P948079) Fossil 7 exposed on the east side of the main promontory has been hornfelsed by a Palaeogene dolerite intrusion, the Portrush Sill.
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- Ivimey-Cook, H C. 1975. The stratigraphy of the Rhaetic and Lower Jurassic in East Antrim. Bulletin of the Geological Survey of Great Britain, 50, 51–69.
- Wilson, H E, and Manning, P I. 1978. Geology of the Causeway Coast. Memoir of the Geological Survey of Northern Ireland, Sheet 7 (Northern Ireland).