The Lonfearn Member, Lealt Shale Formation, (Middle Jurassic) of the Inner Hebrides, Scotland

Abstract

The Lonfearn Member, Lealt Shale Formation, is a distinctive unit within the Great Estuarine Group in the Middle Jurassic of the Inner Hebrides, Scotland. Its dominant lithology of fine-grained, partly laminated mudstone marks it out as a time of minimum input of coarse clastic material into the Hebridean Basin, when consistent facies extended over some 90 km between north Skye and the Isle of Eigg. Interbedded with the mudstones are thin (decimetre) shelly and partly ferruginous and oolitic limestones, the combination giving a unique facies association in the British Bathonian. The predominantly low-salinity biota is dominated by the bivalve Neomiodon, ostracods and spinicaudatans. Episodes of inferred higher salinity aid in correlating sections from the type locality in North Trotternish, Skye to other localities, principally Strathaird in south Skye and the Isle of Eigg. We describe the principal outcrops of the Lonfearn Member and the remarkable facies continuity within the Great Estuarine Group indicating tectonic quiescence within the basin and hinterland at this time. Supplementary material: The detailed measured stratigraphical logs discussed in this paper are available at https://doi.org/10.6084/m9.figshare.c.4093991