Deep channel incisions in the Rupelian clays filled with the middle Burdigalian sands (northern Belgium and southern Netherlands)

This study provides evidence for the presence of channel incisions at the base of Lower Miocene glauconitic sands deposited in the southern North Sea Basin. A cross-boundary correlation panel between boreholes in Belgium and the Netherlands shows that the glauconitic sands of the Lower to Middle Miocene Berchem Formation strongly vary in thickness from less than 15 m to almost 70 m. The palynological analyses of borehole samples show that the main thickness change can be attributed to the presence/absence of the middle Burdigalian sands (dinocyst biozone NSM3) in the basal section of the Berchem Formation. These sands can be interpreted as infill of channels cut in the Rupelian clays of the Boom Formation. Indeed, also along a seismic line nearby, individual channels incised up to 45 m deep into the Boom Formation can be observed at the base of the Berchem Formation. The results of this study raise the question of whether other local middle Burdigalian sediment bodies in the region may also represent channel infills. Because the base of the Berchem Formation coincides with a major hiatus spanning the late Oligocene to the earliest Miocene, the channel incisions at this base could be related to the eustatic sea-level fall that occurred at the Oligocene–Miocene transition. During a middle Burdigalian sea-level rise, the sea transgressed into the channels, widening and filling them with sediments. Compared to other transgressed paleo-valley systems, which generally comprise at least in part fluvial or estuarine sediments, those in the base of the Berchem Formation are entirely composed of shallow marine, glauconite-rich sediments. By late Burdigalian times, the channels were filled, and sediment thicknesses became much more uniform across the region.