The Triassic-Jurassic boundary in Great Britain

Abstract

SummaryIt was recently proposed that the base of the Jurassic system be taken at the base of the Blue Lias in the Watchet area of Somerset thus including the underlying White Lias in the Triassic System. The mapped base of the Blue Lias however is a diachronous horizon ranging from pre-planorbis Beds age in Somerset to Schlotheimia angulata Zone age in Warwickshire. No certain chronological correlation is possible between the base of the Blue Lias in the Watchet area and any limestone or other horizon in the pre-planorbis Beds elsewhere nor is there even a palaeontological separation associated with this horizon. This paper reiterates established Geological Survey practice of taking the Triassic-Jurassic boundary in Great Britain at the top of the Cotham Beds and thus including the White Lias limestones in the Lower Lias. The White Lias limestones are regarded as a local shallow-water facies of the pre-planorbis Beds since they both contain similar non-ammonitiferous marine macrofossil assemblages;the Cotham Beds provide palaeontological separation since they divide theseassemblages from the different marine macrofossil assemblages found in the Lower Rhaetic Westbury Beds and also contain the boundary between the Rhaetipollis and Heliosporites miospore zones. The lacustrine or lagoonal Cotham Beds are similar in lithology to the Tea Green Marl and also include reddened beds of mudstone like those of the Keuper Marl; they are therefore better contained in the continental Triassic system rather than the marine Jurassic system.