COMPLEMENTARY USE OF SEQUENCE STRATIGRAPH Y AND OF BIOSTRATIGRAPHY FOR THE STRATIGRAPHY OF THE LOWER CRETACEOUS - EXAMPLE OF THE URGONIAN CARBONATE PLATFORM OF THE JURA AND THE NORTHERN SUB-ALPINE CHAINS

Through a critical analysis of the different ammonite biozones present ly applied or proposed, the authors illustrate the inherent constraint s and limits of these biozones, which still remain the basis of the Lo wer Cretaceous stratigraphy. Some contradictions and inaccuracies whic h appear in the orbitolinid successions recently used for the correlat ions within the Urgonian limestones are also noted. A biostratigraphic al orbitolinid scale is proposed for the first time for the Upper Haut erivian-Bedoulian period. This scale is demonstrated in various outcro ps which provide orbitolinids and ammonites from the same levels, and based on two lineages of ubiquitous orbitolinids: Praedictyorbitolina/ Dictyorbitolina (Upper Hauterivian-Lower Barremian) and Valserina/Palo rbitolina (Upper Hauterivian-Bedoulian), the evolutionary steps of whi ch are closely related to the ammonite biozones. Using both the sequen ce stratigraphy concepts and methods [as defined by Vail er ni., 1977] and this new biostratigraphical scale proves to be the best way to ge t reliable platform/basin correlations. As a result, the history of th e Urgonian carbonate platform of the Jura and the northern Subalpine C hains can be summarized in the following steps: - originating in the n orthern Jura at the end of the lower Hauterivian; - prograding basinwa rd from the Jura (Switzerland) to the southern Vercors (S-E France) du ring the Upper Hauterivian and the Lower Barremian, concurrently with a progressive emersion landward; - aggrading during the Upper Barremia n and the Lower Bedoulian. While the rudist limestone get their maximu m extension basinward, marine deposits subdue back some part of the pr eviously exposed area; - disappearing during the Middle Bedoulian: syn chronized throughout the platform, this event seems to originate from the subaerial exposure of the whole carbonate area. In terms of sequen ce stratigraphy, it results from a -forced regression-marked by a type 1 sequence boundary.