Description et biozonation des sections de crinoïdes saccocomidés du Jurassique supérieur (Oxfordien-Tithonien) du domaine téthysien occidental

Abstract

The study of several hundreds of thin sections, from dozens of outcrops in three different palaeogeographic domains: the External Rif Chain (Morocco), the Vocontian Basin (Southeastern France) and the Northern Tunisia, has helped better to characterize the stratigraphic distribution of the pelagic crinoids, roveacrinid saccocomids. The saccocomids, appearing in the Middle Oxfordian, are thriving noticeably in the three studied basins at the base of the Early Tithonian substage (Hybonotum ammonite zone) and dominate the pelagic microfauna up to the relative disappearance of the group at the top of the Late Tithonian, at the limit of the A2 and A3 subzones of calpionellids. Careful examination of the respective skeletal elements of this fossil group in thin sections lead to define 47 types of skeletal sections, divided into 6 different groups. These skeletal sections are described and named on the base of their geometric forms. Most of these sections have typical geometric shapes and occur in coeval stratigraphic levels in the three investigated basins. Several of these sections are characteristic of a specific stratigraphic interval, either by their geometric shape or by their relative frequency. The stratigraphic distribution of the investigated skeletal sections, previously considered as of limited stratigraphical value, allowed to define 7 association and relative frequency biozones for the Late Oxfordian-Late Tithonian interval. These biozones are correlated with the ammonite zones.