BIOSEDIMENTATION IN MIXED CARBONATE-SILICICLASTICS: USING COMPONENT ANALYSIS AND MICROTAPHOFACIES TO ANALYZE DEPOSITIONAL ENVIRONMENTS OF THE NORTH ALPINE FORELAND BASIN

During the Early Miocene, the North Alpine Foreland Basin was flooded by a shallow marine sea. The resulting marine deposits are dominated by siliciclastics generated by the erosion of the advancing Alpine front in the south, riverine sources to the north and subsequent sediments transport both perpendicular and parallel to the basin axis. The presence of localized areas of sediments with increased carbonate content is dictated by the proliferation of specific benthic skeletal components. These areas are distributed along the northern margin of the basin where the Molasse Sea transgressed upon various basement rocks and show differential composition depending on divergent topographic sea floor configurations, energy conditions and siliciclastic input.

This study is an ongoing investigation of the Randengrobkalk in Southwestern Germany in order to show how applying standard microfacies techniques, microtaphofacies analysis and following diagenetic pathways can lead to an interpretation of facies patterns within carbonate-siliciclastic transitions. The microfacies analysis include component identification, fabric analysis, multivariate treatment of quantified data, and facies designations. Microtaphofacies analysis reveals differential preservation of the carbonate components including fragmentation, abrasion and bioerosion and encrustation. Diagenetic features follow the preservation of calcitic and dissolution of aragonitic shell fabrics as well as the subsequent cement generations.

Carbonates dominated sediments consist of rudstones, grainstones and packstones dominated by calcitic and aragonitic shelled bivalves, gastropods, bryozoans and balanid barnacles with rare coralline algae and small benthic foraminifera. Non-biogenic components consist of quartz, Jurassic extraclasts and glauconite. Preservation of components range from well–preserved to highly fragmented and rounded components leading to a preservation bias of thick shelled, compact shell material. Aragonitic bivalves and gastropods are present as moulds or replaced by calcite. Lateral and vertical transitions within component diversity, taphonomic features and sedimentary fabric are interpreted to represent a change from a proximal to distal environments.