Eustatic cycles in the Jurassic

Abstract

Stage-by-stage analysis of the shallow marine Jurassic sequences in northwest Europe reveals a number of widespread, synchronous cycles of deepening and shallowing water, independent of local tectonics and facies development. The more important cycles are shown to correlate with phases of world-wide transgression and regression and hence are attributed to eustatic changes of sea level. The most important times of sea-level rise were the Early Hettangian, Early to Mid-Toarcian, Early Bajocian, Late Bathonian to Early Callovian and Mid-Oxfordian, and the most important times of sea level fall in the Late Toarcian to Aalenian, Early Bathonian, Late Callovian and Late Tithonian/Volgian. Until late in the Oxfordian or early in the Kimmeridgian, the secular trend was towards rise in sea level (positive eustasy), with the amount of episodic falls failing to match the intervening rises; in the Tithonian this trend was checked or reversed. Of five eustatic models discussed, the one most favoured by the evidence involves a rapid rise of sea level followed by a longer phase of stillstand and then rapid fall. These changes are tentatively related to episodes of uplift and subsidence of oceanic ridges. The influence of eustasy on marine faunas is briefly discussed with reference to two important macroinvertebrate groups. The ammonites were a stenotopic group highly susceptible to extinction, especially at times of regression, when the development of endemic faunas was also favoured. Times of transgression were marked by migratory spread and evolutionary radiation. The bivalves were relatively eurytopic and little affected by short-term changes of sea level, but times of low sea level in the Early and Late Jurassic correlate with increased endemism.