Microfacies, microtaphonomic traits and foraminiferal assemblages from Upper Jurassic oolitic–coral limestones: stratigraphic Xuctuations in a shallowing-upward sequence (French Jura, Middle Oxfordian)
Résumé
The characterization and distribution of the microfacies and the microfossil assemblages of a Middle Oxfordian section from Jura Mountains composed by thick oolitic–coral limestones is analyzed. Six microfacies types(mainly grainstones) are differentiated mainly composed by ooids, intraclasts and bioclasts. Foraminiferal assemblages are dominated by agglutinated forms. Benthic microbial communities and sessile foraminifera are the main components of the encrustations. The whole set of microfossil assemblages is typical of shallow subtidal environments rich in “algae” (Cayeuxia, “Solenopora”, Thaumatoporella, Bacinella, Girvanella and Terquemella) and foraminifera such as Nautiloculina oolithica, Redmondoides lugeoni, Ammobaculites coprolitiformis, Troglotella incrustans and Rectocyclammina. The increasing upward record of debris of algae and Nautiloculina, and the decrease of serpulids, bryozoans, nodosariids and ophthalmidiids indicate a shallowing- upward trend. The stratigraphic distribution of microfacies and microfossil assemblages lead to diVerentiate two main successive phases. The Wrst is a deeper subtidal environment in an open shelf, while the second is a shallow subtidal environment with evolution from winnowed to more restricted conditions. Microfabrics of radial to concentric ooids upwards in the section correspond to higher energy environments related to an oolitic shoal. This study shows how a very detailed analysis of microfacies, which integrates oolitic features, microfossil assemblages and microtaphonomy is potentially a useful tool for interpreting hydrodynamism and sequence evolution in marine carbonate shallow environments.