Palaeoceanographic and palaeoclimatic reorganization around the Middle–Late Jurassic transition

A Middle to Upper Jurassic succession of submarine hardgrounds overlain by nodular limestones is exposed in the Jura mountains and in the Helvetic of the Swiss Alps. These sediments were accumulated along the northern shelf of the east–west trending Tethys seaway. Submarine hardgrounds and nodular limestones were also formed on the Briançonnais High, today outcropping in the middle Penninic nappe pile of the Alps. Hardgrounds record strong and persistent current activity along the northern Tethys shelf and on the Briançonnais High during the Callovian and Early Oxfordian. The transition from hardgrounds to nodular limestones corresponds to a major reorganization of Tethys oceanography. The change occurred in Plicatilis ammonite Zone (Middle Oxfordian). Carbon isotope stratigraphy, calibrated against an ammonite-dated reference section in the French Subalpine Basin, serves as a correlation tool between ammonite-dated sections and successions with poor biostratigraphic resolution. Correlation demonstrates that the end of hardground formation was synchronous over wide parts of the northern Tethys. The change in shelf sedimentation coincides with a positive carbon isotope excursion with an amplitude of 1.5‰. The change in C-isotope stratigraphy indicates that observed reorganization of current patterns along the northern Tethys shelf was coupled with global change in oceanography and climate. We propose that the change in northern Tethys sedimentation was caused by opening of new seaways at a time of progressive collapse of Pangaea.