Bathymetric zonation of Jurassic trace fossils from the coast of Yorkshire, England

Abstract

Four independent lines of evidence suggest that the distribution of the Jurassic trace fossils of Yorkshire is governed by sea depth. In the succession as a whole, a cyclic repetition of ichnogenera exists. Extensive bioturbation is confined to the latest beds of each progressively-shallowing cycle, suggesting it is essentially a very shallow water phenomenon. Annelid burrows (e.g., Chondrites, Cylindrites, Teichichnus) always precede Crustacean burrows (e.g., Rhizocorallium, Thalassinoides) and thus indicate slightly deeper water. Crustacean burrows abound in rocks formed under both open sea and lagoonal conditions, and their producers appear to have been euryhaline and tolerant of wide E h ranges in the surrounding sediment. Lateral variation within individual beds of the Bajocian Scarborough Limestone Series shows a passage from coarse, littoral, calcareous grits with Thalassinoides and Rhizocorallium in the north at Blea Wyke to finer, argillaceous limestones with Asterosoma and Teichichnus in the south at Hundale Point, suggestive of deeper water. Gross stratigraphic thinning from predominantly cross-bedded sandstone and massive grifs, with sandy shales containing abundant bysally-attached pernids and oysters (104 ft. at Blea Wyke), to fine black shales with cephalopod Teloceras blagdeni in the south (7 ft. at Gristhorpe Bay); thinning in all units with marked ecological change in 15 miles indicates an appreciable bathymetric gradient. The absence from bioturbated horizobs of any molluscan fauna except for comminuted shell debris, in an otherwise highly fossiliferous series suggests very shallow, littoral conditions. That conditions were frequently intertidal is indicated by abundant lug-worm burrows ( Arenicolites statheri) and parallel Thalassinoides and Rhizocorallium, oriented in response to tidal flux. The majority of the trace fossils is figured from the British Jurassic for the first time: Teichichnus and Asterosoma have not previously been recorded from the Middle Jurassic.