Penarth Group, Triassic, Northern Ireland

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Mitchell, W I (ed.). 2004. The geology of Northern Ireland-our natural foundation. Geological Survey of Northern Ireland, Belfast.

W I Mitchell

Penarth Group

Distribution of Permian, Triassic and Jurassic rocks in northeast Northern Ireland. (P947841)
Detailed lithostratigraphy of the Late Triassic (Rhaetian) and Jurassic rocks. (P947940)
Description of the main lithological divisions (P947847)
Detailed geological map of the Waterloo section near Larne. (P947848)
Fossil 12 Late Triassic (Rhaetian) bivalve (12) Rhaetavicula contorta from the Penarth Group at Waterloo, Co. Antrim. (P948078)
The Waterloo foreshore near Larne showing the Collin Glen Formation (Mercia Mudstone Group) in foreground overlain by the Penarth Group. (P948009)

Regional uplift and erosion in the post-Early Jurassic and pre-Cretaceous confines the Penarth Group to rare exposures at the margins of the Antrim Plateau (P947841). Correlation has been made with similar sequences in England (P947940). The Penarth Group at Larne rests disconformably on rocks of the Mercia Mudstone Group (P948009) and is conformably succeeded by the Waterloo Mudstone Formation. In the Magilligan borehole the contact between the Penarth Group and Waterloo Mudstone Formation is a non-sequence. The thickness of the Penarth Group varies from 25.4 m [1] in the Magilligan borehole to 20 m in the Larne No. 1 borehole [2].

In the Larne No. 1 borehole the Westbury Formation (P947940) consists of about 8.5m of black and dark grey shaley mudstone with silty laminae and a thin sandstone.[2] Identical rocks in the faulted lower part of the Waterloo section [3] (P947848) (P947847) contain diagnostic bivalves including Rhaetavicula contorta (P948078) Fossil 12. The base of this section is a thin bed of sandstone and includes fish teeth and scales that occur through the rock sequence. In the Larne No. 1 borehole the succeeding rocks of the Lilstock Formation (P947940) commence with the Cotham Member which consists of about 5 m of fossiliferous brown and grey mudstones with a distinctive striped appearance caused by siltstone laminae. Above the Cotham Member, and below the base of the succeeding Pre-planorbis beds, the Langport Member comprises about 7 m of dark grey calcareous micaceous mudstone with bivalves, laminae of siltstone with desiccation cracks and cross laminations and thin beds of micritic limestone.

The top of the Mercia Mudstone Group and most of the Penarth Group were exposed in the gorge of The Lynn [C 709 278], near Limavady [1]. The section at Cloghfin exposes rocks, with the exception of Jurassic strata, between the top of the Mercia Mudstone Group and the Glenarm Chalk Member of the Late Cretaceous Ulster White Limestone Formation.


  1. 1.0 1.1 Bazley, R A B, Brandon, A, and Arthurs, J W. 1997. Geology of the country around Limavady and Londonderry. Geological Survey of Northern Ireland Technical Report GSNI/97/1.
  2. 2.0 2.1 Manning, P I, and Wilson, H E. 1975. The stratigraphy of the Larne Borehole, County Antrim. Bulletin of the Geological Survey of Great Britain, 50, 1–50.
  3. 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.