The Oligocene/Miocene boundary in the ON-Mol-1 and Weelde boreholes along the southern margin of the North Sea Basin, Belgium

1. Introduction During the late Oligocene and Neogene, glauconitic sands were deposited in shallow marine conditions across the southern North Sea Basin (Fig. 1). The deposition of these glauconitic sands was not continuous, but interrupted by several hiatuses. One of the most important and widespread hiatuses in the southern North Sea Basin coincides with the Oligocene/Miocene boundary, in the Netherlands often referred to as Savian phase (in Wong et al., 2001; Verbeek et al., 2002; Munsterman & Brinkhuis, 2004). In the northern Campine Block, a structural entity along the southern margin of the North Sea Basin, this hiatus coincides with the boundary between the Voort and Berchem Formations (Figs 2 and 3). Here, age information on the late Oligocene Voort Formation was mainly based on biostratigraphic and radiometric studies on cored sections of the ON-Mol-1 (DOV ON-Mol-1) and Weelde boreholes (DOV kb8d8e-B161; cf. Van Simaeys, 2004; Van Simaeys et al. 2004; 2005; De Man & Van Simaeys, 2004; De Man et al., 2010). The age information on the early to middle Miocene Berchem Formation was then again based on biostratigraphic studies in its outcropping areas near and within the city of Antwerp and in several boreholes across the northern Campine Block (Louwye, 2005 and references therein), not including the ON-Mol-1 and Weelde boreholes. As the Voort and Berchem Formations were studied separately, the boundary between them or the Oligocene/Miocene hiatus was not yet biostratigra