Reinvestigation of the invertebrate fauna of the Boom Clay Formation and the Ruisbroek Sand Member (Oligocene, Rupelian) of Belgium, with the description of a new lithostratigraphic unit: the Sint Niklaas Phosphorite Bed

The Rupelian stage, defined in Belgium, is recognized now as a global chronostratigraphic unit. Rupelian deposits in Belgium have been studied extensively in the past. More recently in-depth studies have been carried out on its stratigraphy, sedimentology, micropalaeontology and vertebrate fossils. Gastropods, however, have not been revised since the work of Glibert (1957). Most invertebrate data predate the stratigraphic synthesis of Vandenberghe (1978), that enabled identification of different members and sedimentary cycles in the Boom Clay Formation. Other parts of the Rupelian sequence (such as the Ruisbroek Sand Member, Kerniel Sand Member and the Sint Niklaas Phosphorite Bed) have only been recognized or adequately dated since Glibert (1957) in the mid-1980s and their malacofauna is not yet described. Here some preliminary results of numerous recent samplings of Rupelian deposits in Belgium are presented and analyzed. Special focus is given to the Boom Clay Formation and the Sint Niklaas Phosphorite Bed. It is concluded that the molluscan fauna of the Boom Clay Formation can be used to delimit its various members and levels. The composition and diversity of the fauna could possibly be linked to successive sea level changes. The Sint Niklaas Phosphorite Bed 2.5 to 3 m below the base of the Boom Clay Formation is formally introduced as a new stratigraphic unit within the Ruisbroek Sand Member.