The sub-basaltic surface in northeast Ireland and its significance for interpreting the Tertiary history of the region

Abstract

In northeast Ireland a striking unconformity exists between late Cretaceous, marine limestones (Ulster White Limestone Formation) and Paleocene, subaerially emplaced, basalt flows (Antrim Lava Group) above. The well-cemented nature of the Ulster White Limestone has been attributed to thermal or loading effects associated with the Antrim Lava Group. However, the geomorphology of the palaeokarst beneath can be used to demonstrate that both cementation and joint emplacement in the limestone significantly preceded burial beneath the basalts. A thin, irregularly developed palaeosol, dominated by red-brown or grey clay above a relatively thin basal lag of flint debris, is present on the palaeokarst surface. It has been interpreted previously as a Terra Rossa, derived from the accumulation of insoluble residues remaining after limestone dissolution. However, the colour, mineralogy and geochemistry of the clay, and the low proportion of flints to clay, indicate instead that the clay fraction was derived from ash falls, usually basaltic but occasionally rhyolitic, prior to the first lava flows. In the long term the survival of the Permian and Mesozoic sediments in northeastern Ireland cannot be ascribed solely to the supposed ‘carapace’ effect of the Tertiary basalts, with continued basin subsidence from Mesozoic into Tertiary times being of greater significance. However, it did play a role in the shorter term. Variation in the preserved thickness of limestone beneath the basalts at any one site was controlled by three factors; original thickness of the limestone, its elevation in Paleocene times, and the time elapsed between subaerial emergence and burial by the advancing lava flows. Preservation of significant thicknesses of Ulster White Limestone in the western part of the Antrim Plateau, where previous work has indicated that the onset of volcanism was later than in the east, suggests that limestone uplift preceded lava emplacement by perhaps only 10 4 -10 5 years and was due to local thermal doming on a scale of only a few tens of kilometres. Tertiary subsidence must also be invoked for the Kingscourt half-graben further to the south, where friable, autobrecciated basalts overlie a spectacular palaeokarst surface developed on Permian gypsum.