ESTIMATES OF THE AMOUNT AND RATE OF SEA-LEVEL CHANGE ACROSS THE RHAETIAN-HETTANGIAN AND PLIENSBACHIAN-TOARCIAN BOUNDARIES (LATEST TRIASSIC TO EARLY JURASSIC)

An attempt is made to determine the amount and rate of sea-level chang e for two major events in the latest Triassic to early Jurassic, both of which are associated with mass extinctions, using the best availabl e facies evidence and time scale. The amount of marine deepening acros s the Pliensbachian-Toarcian boundary, based on analysis of the excell ent Yorkshire coast section, is likely to have been a few tens of metr es, at a rate of between 1 cm 1.2 ka(-1) and 1 cm 0.4 ka(-1) The chang e across the Rhaetian-Hettangian (Triassic-Jurassic) boundary was diff erent in that the sea-level rise was immediately preceded by a sea-lev el fall, effectively a 'regression-transgression' coupler. The earlies t Jurassic sea-level rise is likely to have been much more rapid than that across the Pliensbachian-Toarcian boundary, at a rate of at least 1 cm 0.2 ka(-1). Whereas the speed of Pliensbachian-Toarcian change i s consistent with the growth of oceanic ridges, the appreciably more r apid Triassic-Jurassic change is suggestive of a different tectonoeust atic mechanism involving stress-induced changes in plate density.