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School of Earth Sciences, University of Birmingham, Birmingham B15 2TT, UK (e-mail: A.Hallam{at}bham.ac.uk)
An attempt is made to determine the amount and rate of sea-level change for two major events in the latest Triassic to early Jurassic, both of which are associated with mass extinctions, using the best available facies evidence and time scale. The amount of marine deepening across the Pliensbachian—Toarcian boundary, based on analysis of the excellent Yorkshire coast section, is likely to have been a few tens of metres, at a rate of between 1cm 1.2 ka – 1 and 1cm 0.4 ka – 1. The change across the Rhaetian—Hettangian (Triassic—Jurassic) boundary was different in that the sea-level rise was immediately preceded by a sea-level fall, effectively a 'regression-transgression' couplet. The earliest 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 is consistent with the growth of oceanic ridges, the appreciably more rapid Triassic—Jurassic change is suggestive of a different tectonoeustatic mechanism involving stress-induced changes in plate density.
Keywords: Early Jurassic, Late Triassic, transgressions, regressions.
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