The stratigraphic position of the lower Miocene Kiel Sand Member of the Berchem Formation in the Antwerp area (northern Belgium) is not well constrained and its depositional environments are poorly known. Due to a spatial limited decalcification front, the Kiel Sand Member is completely decalcified in southern Antwerp and gradually becomes fossiliferous to the north-east of the city. The stratigraphy and palaeontology of the fossiliferous sediments in three temporary exposures are presented. The dinoflagellate cyst analysis of fossiliferous horizons shows the relative progress of a transgression in the southern North Sea Basin during the early–middle Burdigalian, that probably initiated in the late Aquitanian. The Kiel Sand Member contains an important mollusc fauna, with several species reported for the first time from this member. The taphonomy and fauna of the shell beds indicate a shallow marine, high energetic depositional environment, strongly influenced by storms, currents, waves and a rather low sedimentation rate. The climate was warm-temperate to subtropical. In all studied sections, the Kiel Sand Member could be clearly distinguished from the Antwerpen Sand Member: similarities and differences are discussed. Moving to the north of Antwerp, the erosive base of the Antwerpen Sand Member cuts deeper into the Kiel Sand Member. The Early Miocene Unconformity (EMU) is suggested at this contact.

The enigmatic megamouth shark is a rare occurrence in both Recent oceans and in the fossil record. A temporary construction site in the city of Antwerp (NW Belgium) gave the opportunity to study the poorly known early Miocene Kiel Sand Member of the Berchem Formation. In this Burdigalian deposit, a previously unknown fossiliferous horizon with Glycymeris and Cyrtodaria was sampled which contained a single Megachasma tooth. This specimen, attributed to Megachasma aff. pelagios by its resemblance with extant M. pelagios, is the first in situ record from un-reworked strata in Belgium. Only around a dozen fossil Megachasma teeth have been reported from Europe. It is concluded that a circumglobal megamouth population existed during the Burdigalian. Some evolutionary remarks are made.