JACQUIN, T., and P. R. VAIL, Rice University, Houston, TX

ABSTRACT: Relative Sea Level Changes and Carbonate Factories of the Urgonian Platform, Hauterivian-Aptian, Vercors, France

Long term second-order and short term third-order relative sea level changes control the development of the Urgonian platform in the French western Alps. At a second-order scale (Hauterivian-Aptian) the Urgonian platform develops over the drowned late Berriasian-middle Valanginian platform. During the regressive phase (Hauterivian-early Barremian), the platform progrades tens of kilometers toward the southeast. During the transgressive phase (late Barremian-early Aptian), the platform aggrades, backsteps, and then drowns. Subsidence analysis indicates this second-order cycle is controlled by changes in rates of subsidence or possibly tectono-eustasy. In the basin, bioclastic turbidites develop only when the Urgonian carbonate platform is building. The platform sheds little material wh n it backsteps and drowns.

At a third-order scale, two types of carbonate factories alternate within the Urgonian platform: highstand factories that are able to keep up with sea level rises, and lowstand factories that build out the carbonate platform by synsedimentary reworking, redeposition, and progradation. The Urgonian facies (a lagoonal mudstone with large-sized benthic foraminifers, algae, and rudistids) is a product of highstand factories. The facies changes laterally into platform-rim buildups with corals and stromatoporids, and rapidly thins basinward by sediment starvation. Highstand shedding is very limited. Both the bioclastic turbidites and the large seismic-scale clinoforms visible in the outcrops of the southern Vercors are the products of the lowstand factories. These lowstand turbidites and sh al-water prograding grainstones onlap against mappable surfaces that extend from the basin to the platform. They consist of three distinct units: (1) a lower basinal sheet of platform-derived massive grainstones, (2) a thick apron of fine-grained turbidites and mudstones with proximal channels, some of which are filled with coarse grainstones, and (3) a prograding unit pinching out at the offlap break of the previous highstand in the area of the stromatoporids. The prograding grainstones are matrix-poor cross-stratified beds in the shallow water areas and facies change to mudstones in the basin. The lack of boundstones in the proximal parts of the lowstand prograding intervals, together with the lack of well-developed early cementation, ooids, and aggregates suggest that these lowstand f ctories were temperate-water systems. They contrast with the Urgonian highstand factories that were particularly well equipped to keep up with sea level rises such as modern tropical carbonates.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.