Mercia Mudstone Group, Triassic, Northern Ireland

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

W I Mitchell

Mercia Mudstone Group

Comparative lithostratigraphy of the Triassic rocks in the Port More and Lame No.2 boreholes (for location of boreholes see Figure 9.2). (P947846)

Distribution of Permian, Triassic and Jurassic rocks in northeast Northern Ireland. (P947841)

Litho- and chronostratigraphy of the Permian, Triassic and Jurassic rocks. (P947937)

Fossils 5-11 Mid- to Late Triassic (Anisian-Carnian) miospores from the Mercia Mudstone Group in the Larne No. 2 borehole: 5 Guttulapollenites hannonicus, 6 Verrucosisporites thuringiacus, 7 Angustisulcites gorpii, 8 Perotriletes minor, 9 Tsugaepollenites oriens, 10 Echinitosporites iliacoides, 11 Retisulcites perforatus. (P948077)

The six formations of the Mercia Mudstone Group (P947846) were originally defined in the Port More borehole and overall consist of calcareous, reddish to brown mudstone and thin, laminated, micaceous siltstone that weather to a brick red colour . Sandstone is common only in the basal Lagavarra Formation, the transitional unit from the underlying Sherwood Sandstone Group. In higher formations sandstone is restricted to beds up to 2.5 m thick, in the past called "skerries", such as the Suitcase Sandstone and Coolmaghra Skerry, which are typically hard, fine-grained, pale greenish white to buff and often dolomitic.

The maximum thickness of about 1030 m attained by the Group in Northern Ireland is based on the combined sections recorded in the Larne No. 1 and No. 2 boreholes. At Larne (P947846) the Group includes 400 m of halite beds that are only 40 m thick at Carrickfergus (P947841). Elsewhere the maximum thickness of the Group in boreholes is 620 m at Port More , , 492 m at Ballymacilroy, 315 m at Langford Lodge and 122 m at Killary Glebe (P947841). The presence of nodular anhydrite, gypsum and pseudomorphs after halite is evidence of an evaporitic depositional environment. Sedimentary structures in the lowest three formations and in the topmost Collin Glen Formation include rhythmic laminations, load casts, flame structures, oscillation ripples and desiccation cracks. In contrast, these structures are rare in the massive mudstones of the Knocksoghey and Port More formations. Although the main outcrop of this Group is on the north side of the Lagan Valley at Belfast, exposures also occur on the east coast of Co. Antrim as far north as Glenariff and, in Co. Londonderry, north and east of Limavady .

At the top of the Mercia Mudstone Group is the Collin Glen Formation (formerly the ‘Tea Green Marls’). Alternating red and green mudstone about 1m thick at the base are succeeded by up to 10 m of pale greyish green silty mudstone with thin beds of autobrecciated micrite. At Waterloo (P947841), the top 50m of mudstone of the Mercia Mudstone Group, including part of the Knocksoghey Formation and the Port More and Collin Glen formations, has been metasomatised and altered to a green colour with abundant pea-sized pseudo-pisoliths and pseudo-ooliths. The Mercia Mudstone Group is succeeded abruptly by grey and black mudstones of the Penarth Group.

The age of the Mercia Mudstone Group is determined from miospore assemblages recorded from deep boreholes (P947937). The assemblages (P948077) Fossils 5–11, of early Mid-Triassic (Anisian) age, from the Larne No. 2 borehole occur in the Lagavarra, Craiganee and Glenstaghey formations. The top beds of the Glenstaghey Formation and the Knocksoghey Formation also contain miospores (P948077) Fossils 10 and 11 of Ladinian (late Mid-Triassic) to Carnian (early Late Triassic) age. The Collin Glen Formation contains rare fossils including fish debris and the crustacean Euestheria.