ESCR - The Jurassic System in N. Ireland

Earth Science Conservation Review

Summary Full report
The Jurassic System in N. Ireland - Introduction	Antrim, Down, Londonderry, Tyrone

Site number:	1193
Locality Type:	Various
Grid Reference:
Google maps:	,
County:	Antrim, Down, Londonderry, Tyrone
Period:	Jurassic
Stages:	Aalenian, Bajocian, Bathonian, Callovian, Hettangian, Kimmeridgian, Lower Jurassic, Middle Jurassic, Oxfordian, Pliensbachian, Portlandian, Sinemurian, Toarcian, Upper Jurassic
Lithostrat:	Tircrevan Sandstone Member, Waterloo Mudstone Formation

Site Description
Introduction:
The Jurassic System is named from the development of rocks of this age in the Jura Mountains of France and Switzerland. Here rocks previously called `Calcaires de Jura' were first recognised as a limestone formation separate from the German Muschelkalk by Alexander von Humboldt in 1795. In Britain the term Jurassic was in use as synonymous with the Oolite Series but excluded the Lias. The stages in the Jurassic are shown in the following table: TABLE TO BE ADDED
Description:
CORRELATION OF JURASSIC STRATA (i) Macrofossils Ammonites have long been regarded as the prime biostratigraphic tools for time correlation of the Jurassic rocks. The group has wide geographical distribution, the species are of limited stratigraphic range because of their rapid evolutionary rates, they are largely but not entirely independent of facies, and are common and easily identified (Cope et al., 1980). In spite of the need for some caution (Kennedy & Cobban, 1977) they remain the principal zonation fossils, although the need wherever possible to use macro- and micro-fossil assemblages must be emphasised. The ammonite zonation of the Lias is shown in the following table: TABLE TO BE ADDED Brachiopods have also been used in the British Jurassic as guide fossils but two features militate against their use as zone fossils with an accuracy in any way approaching that of ammonites. Firstly, they belong to the benthos and thus are essentially sessile in habit unlike most ammonites. As a result of this it follows that many brachiopods are entirely provincial in their distribution. Secondly, brachiopod taxonomic entities, whether genera or species, are correspondingly more long-lived or less rapidly evolving than ammonites. This means that the chronological resolution that can be achieved with them is much less precise than with ammonites. Gastropods have been one of the most neglected groups of British Jurassic fossils and consequently have been little used in correlation. Even so, when used within one basin and major facies of deposition, they may have value in the local correlation of rocks. Similarly, because of their relationships to substrates in the majority of cases and their relatively slow rate of evolution bivalves have been little used in the correlation of British Jurassic rocks. (ii) Microfossils The great majority of marine ostracods live on or near the bottom or in some cases in the upper few millimetres of the bottom oozes through which they root and burrow after food. The nature of the bottom sediments is, therefore, of extreme importance in determining the species of ostracod which will be found in any given place. Ostracods are very far from ideal in effecting time correlations of rocks as one can never be sure if the stratigraphic range of a particular Jurassic ostracod at any one place may be a time controlled phenomenon or an ecologically controlled one. Ostracods should not be used for correlation if only one species is present, an assemblage of ostracods and other fauna is essential. Coccoliths have a considerable potential in the correlation of Jurassic rocks especially those of pelagic facies. Barnard and Hay (1974) recognised a series of 21 overlapping coccolith zones and these can be compared with the equivalent 65 ammonite zones. Further testing and possible modification of the coccolith zonation is necessary before great reliance can be put on it. Riley and Sarjeant (1972) believed that dinoflagellates will ultimately enable a stratigraphical correlation equally as accurate as, and more widely applicable than, the ammonites. The Jurassic has been divided into 12 biostratigraphic units based on the contained microplankton. The stratigraphic usefulness of dinoflagellates is much restricted by the long ranges of many dinoflagellate species. Ranges of particular microplankton may also be discordant between different areas. DISTRIBUTION OF JURASSIC STRATA IN NORTHERN IRELAND From the Larne No. 1 borehole, dark mudstones below 58m contained Psiloceras intermedium, P. leptoptychum and P. johnstoni which indicate the Caloceras johnstoni Subzone (Manning & Wilson, 1975). Other Hettangian strata are known from Barney's Point, Island Magee. Sinemurian strata are known from Island Magee, Portrush, White Park Bay and the Port More borehole (Ivimey-Cook, 1975). The Port More borehole revealed horizons up to the Pleinsbachian Acanthopleuroceras valdani Subzone. Higher horizons are not known in situ anywhere, but fossils derived from the youngest Pleinsbachian Pleuroceras spinatum Zone have been found in the drift and in the basal Cretaceous conglomerate (Wilson & Manning, 1978). Jurassic strata are limited to Co. Antrim and north Co. Londonderry in N. Ireland. They are present, where not removed by pre- or early Cretaceous erosion, beneath a large part of the Antrim Plateau. They crop out around the fringes of the Antrim Plateau, from Collin Glen to Carnlough. Isolated exposures are at White Park Bay, Portrush and from Magilligan to Tircreven Burn. Lias rocks are also known from the Port More, Magilligan, Mire House and Larne boreholes. LITHOSTRATIGRAPHY OF THE JURASSIC A formal stratigraphy of the Jurassic rocks of N. Ireland has only recently been established (see table below). The Lias strata are designated the Waterloo Mudstone Formation (GSNI, 1994) and in Co. Londonderry the Tircreven Sandstone Member has been defined (GSNI, 1981): TABLE TO BE ADDED The base of the Jurassic is defined by the occurrence of Psiloceras planorbis which in N. Ireland occurs a few metres above the base of the Waterloo Mudstone Formation. The Jurassic sequence was 51.5m thick in the Larne No. 1 borehole and 248.9m thick in the Port More borehole. In general the Jurassic consists of grey mudstones with beds of limestone and siltstone although in Co. Londonderry it contains a prominent sandstone member. Lower Jurassic sediments are also known from the East Irish Sea Basin and the Solway Firth Basin (Jackson et al, 1995). JURASSIC PALAEOGEOGRAPHY The interplay of regional or global sea-level changes and continuing crustal extension related to rifting in the North Atlantic and Tethyan megarifts dominated the palaeogeographical evolution of Britain during the Jurassic (Bradshaw et al., 1992). The sea-level rise that commenced in the Rhaetian continued into the Jurassic to usher in a new phase of predominantly marine sedimentation. The climate was warm and humid, the seas generally shallow. Land areas became well vegetated. The major northward transgression initiated during the Rhaetian continued in Hettangian times. The maximum extent of the Lower Liassic transgression probably occurred during the Early Pleinsbachian when the structural highs were at least partially flooded. The extent of land remaining, including the Irish landmass, is largely conjectural. During the rest of the Jurassic, of which no sediments are preserved, because they were removed by early to middle Cretaceous erosion, Ireland probably existed as a landmass with the development of alluvial plains.
Notes:
SITE RATING A notation used to categorise each site is based on a 3, 2 and 1 star system for International, National and Local interest sites respectively. For site specific information on Jurassic sites see Key Site 16 - Waterloo and Key Site 1205 - Portrush - Jurassic.

Keywords
Rocks:	Limestone, Mudstone
Fossil Groups:	Ammonite, Brachiopod, Crustacea, Gastropod, Microfossil, Microfossils
Fossil List:	Psiloceras planorbis (J. de C. Sowerby)
Relations:	Marine sediments
Paleoenv:	alluvial plain, shelf

References (9)
Barnard, T. & Hay, W.W., 1974. On Jurassic coccoliths. A tentative zonation of the Jurassic of southern England and northern France. Eclog. Geol. Helvet., vol. 67, pp.563-585.
Bradshaw, M.J., Cope, J.C.W., Cripps, D.W., Donovan, D.T., Howarth, M.K., Rawson, P.F., West, I.M. & Wimbledon, W.A., 1992. Memoirs of the Geological Society of London, numb. 13 In Atlas of Palaeogeography and Lithofacies.
Cope, J.C.W., Getty, T.A., Howarth, M.K., Morton, N. & Torrens, H.S., 1980. A correlation of Jurassic rocks in the British Isles. Geological Society of London Special Report, numb. 14.
Ivimey-Cook, H.C., 1975. The stratigraphy of the Rhaetic and Lower Jurassic in East Antrim. Bulletins of the Geological Survey of Great Britain, numb. 50, pp.51-70.
Jackson, D.I., Jackson, A.A., Evans, D., Wingfield, R.T.R., Barnes, R.P. & Arthur, M.J., 1995. United Kingdom Offshore Regional Report; the geology of the Irish Sea. HMSO for the British Geological Survey., London.
Kennedy, W.J. & Cobban, W.A., 1977. The role of ammonites in biostratigraphy. In Concepts and methods of Biostratigraphy., pp.309-320.
Manning, P.I. & Wilson, H.E., 1975. The stratigraphy of the Larne Borehole, County Antrim. Bulletins of the Geological Survey of Great Britain, numb. 50, pp.1-50.
Riley, L.A. & Sarjeant, W.A.S., 1972. Survey of the stratigraphical distribution of dinoflagellates, acritarchs and tasmanitids in the Jurassic. Geophytology, vol. 2, pp.1-40.
Wilson, H.E. & Manning, P.I., 1978. Geology of the Causeway coast. Memoir for one-inch geological sheet 7. Memoirs of the Geological Survey Northern Ireland, HMSO, Belfast.

Map(s):	GSNI (1981) 1:50,000 Geological Sheet 12 (Limavady) GSNI (1994) 1:50,000 Geological Sheet 21 (Larne).
Record type:	ESCR report
Entered by:	E M Porter

Enlander, I., Dempster, M. & Doughty, P., 2026. The Jurassic System in N. Ireland - Introduction, County Antrim, Down, Londonderry, Tyrone, site details. [In] Earth Science Conservation Review.
https://www.habitas.org.uk/escr/site.php?item=1193. Accessed on 2026-11-07.