Sills, minor igneous intrusions, Palaeogene, Northern Ireland

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

M R Cooper and T P Johnston

Sills

Fair Head [D 180 438], the spectacular cliff formed by an 82 m thick Palaeogene dolerite sill intruding coal-bearing Carboniferous rocks. 6km ENE of Ballycastle, Co. Antrim. (P948041)

Columnar jointed dolerite of the Binnagapple Sill intruding sandstone and coal seams of the Carboniferous Ballycastle Group, 4 km ENE of Ballycastle, Co. Antrim. (P948042)

During the main phase of volcanic activity in the early Palaeogene, some rising magma failed to reach the surface and was injected sideways into the sedimentary strata to form sills.

The Scrabo Sill near Newtownards is the main component of a complex of linked olivine dolerite and gabbro sills intruding the Triassic Sherwood Sandstone Group . Other sills in this complex are exposed at Ballyrainey and Dundonald while the Ballyalton borehole penetrated a dolerite sill 160 m thick . At the south quarry, on the east face of Scrabo Hill, a vent agglomerate containing blocks of Triassic sandstone in a greenish yellow tuffaceous matrix is closely associated with the dolerite sill and feeder dykes. This is indicative of an early phase of explosive activity pre-dating emplacement of the Scrabo Sill .

The 82 m thick Fair Head Sill (P948041) forms the prominent headland east of Ballycastle in the Carboniferous coalfield of northeast Co. Antrim and is the largest component in a complex of olivine dolerite sills . It dips gently to the south and thins to the southeast, where it cuts Carboniferous, Triassic and Cretaceous rocks. On the east side of Fair Head, the Binnagapple Sill intrudes the coal-bearing Murlough Shale Formation (P948042). This sill, which occurs some 13 m below the Fair Head Sill, consists of 15 m of columnar-jointed olivine dolerite and forms the prominent cliffs to the south and west of Fair Head, reappearing on the shore at Farragandoo [D 167 431].

The Portrush Sill, which forms Ramore Head [C 852 415] and extends at least 2 km offshore to form the islands of the Skerries, also underlies much of Portrush. It is 45 m thick and intrudes and has hornfelsed the fossiliferous Jurassic Waterloo Mudstone Formation (see Jurassic article) for at least 8 m from the intrusion contact. The sill has a fine-grained chilled margin, and a middle consisting of massive, coarse dolerite with dark gabbroic ‘clots’ of olivine ± augite or feldspar-rich segregations. The dolerite also contains mudstone xenoliths and is cut by late-stage veins of pyroxene and plagioclase . The Ramore Head locality was central to the scientific debate in the 18th Century between 'Neptunists' and 'Vulcanists' who expressed opposing views about the origin of volcanic rocks. Neptunists believed that basalt was laid down under water and quoted the occurrence of ammonites in the hornfelsed Jurassic mudstone at Portrush in support of their argument. Although the Vulcanists, who favoured a volcanic origin for basalt, quickly demonstrated that the fossiliferous rock was marine shale, the controversy reached an international audience and, for a while, focussed the attentions of the scientific community on this locality in Northern Ireland.

In Co. Fermanagh, the olivine-dolerite Garrison Sill intrudes early Carboniferous strata in the core of a Variscan periclinal fold and outcrops over an area of about 15 km² some 5 km ENE of Garrison . The sill is 20–30 m thick on the northern limb of the fold and 5–15 m on the southern limb. The upper and lower margins of the sill are exposed in the disused quarry at Slisgarrow (H014 516). Here, contact metamorphism has resulted in a calcsilicate mineral assemblage in impure limestone of the Carboniferous Meenymore Formation (see Carboniferous article). Minerals present include grossularite garnet, diopside, wollastonite and plagioclase .