The stratigraphic reinterpretation of the palynologically analysed Miocene succession of the Wijshagen borehole along the southern margin of the North Sea Basin allowed an age assessment - late Burdigalian to early Serravalian - for the Genk Sand Member of the Bolderberg Formation. The depositional environment varied during Burdigalian to Serravalian times from continental (peat formation) to open marine (glauconitic sands), respectively from south to north in the Roer Valley Rift System. The study area of the Wijshagen borehole is located in the central part of the Roer Valley Rift System between these extreme environments. During the Burdigalian, the glauconitic fine clayey sands of the Houthalen Sand Member were deposited in the study area. From the late Burdigalian onwards, the glauconite content decreased and lignite content increased as a result of high influx of clastic material in the Roer Valley Rift System, and marked the start of the deposition of the Genk Sand Member. The Genk Sand Member shows an overall coarsening-upwards trend, which is consistent with the gradual infill of the available accommodation space in the Roer Valley Rift System by northwest-prograding clastic delta sequences. Dinoflagellate cyst analyses indicate that the Genk Sand Member was largely deposited in a marginal marine environment with only short pulses of continental input. These pulses of continental input increase in a southerly or landward direction where they led to the development of thick lignite seams.