Case Studies of Parasequence-Stepped Ammonite Turnovers in Transgressive Systems Tracts (Jurassic, Vocontian Trough, France)

Abstract

The Mesozoic series of the Vocontian Trough is made up of a subpelagic, bed-scale limestone-marl alternation, which oscillates in a complex way from marl- to limestone-dominated. Sixth to second order cycle may be evidenced in the set of superimposed cyclicities. Platform-to-basin high-resolution correlations allow identification of third-order depositional sequences of sequence stratigraphy as a modulation in the pattern of the roughly 100-k.y.-muted, four to five bed parasequences. So, the authors can describe in detail ammonite turnovers versus systems tracts in basinal deposits. Usually, most of the change occurs in transgressive systems tracts; very few at sequence boundaries. In the several tens of Mesozoic sequences studied, transgressive systems tracts are of variable duration, spanning from one to five parasequences. The occurrence of cold ammonites during third-order sea-level rises, together with the marl-shift in the pattern of parasequences, suggests that coolings are coincident with transgressions, not with lowstands. If correct, most of the stress that forces the evolution of ammonites could be due to short-lived amplifications of the orbitally-forced high-frequency ( glacio-eustatic) oscillation during tectonically controlled third-order rises in sea level.