The origin of the Tertiary lignite deposits in the Lower Rhine region, Germany

Abstract

The Miocene lignite deposits of the Lower Rhine region amount to 55,000 million t. The Main seam is up to 100 m thick, the Upper seam is locally up to 40 m thick. The two seams occur in a complementary manner: the Main seam reaches its maximum thickness in areas where the Upper seam is replaced by clastics, and vice versa. The Main seam is found approximately in the centre of a subsiding basin, whereas the Upper seam is deposited near its margin. These observations suggested an investigation of subsidence, initial peat thickness, peat compaction and the factors governing the distribution of marine and continental sediments. Interpretation of the findings greatly depends on the length of time needed for the forming of the peat. Chronostratigraphic sections of the Lower Rhine Tertiary indicate that the peat of the Main seam may have been accumulated within a period of about 9 Ma and the Upper seam in 2 Ma. It can be argued from an initial peat thickness of up to 270 m that subsidence and peat growth rate averaged 0.03 mm/a or less. This seems to be reasonable as far as the distribution of contemporary clastics is concerned, but it would indicate an extremely low peat growth rate compared to observations from other regions. During the accumulation of the Main seam peat, two concurrent subsidence patterns can be recognized. One may have been initiated by the North Sea rift system. The other seems to be related to Variscan fold structures below the Tertiary. Above Variscan synclines, the surface subsided more than above anticlines, leading to an increased thickness of peat and clastics above synclines. Information from a great number of boreholes in the region, earthquake activity, and the recent tectonic stress field seem to indicate that the Variscan structures have been continuously deformed in the 35 Ma since the early Oligocene. Fluvial sedimentation shifted during the accumulation of the Main seam towards the rim of the subsiding basin. This phenomenon greatly improved the possibility of uninterrupted peat growth in the basin centre. The accumulation of the Main seam ceased when a new river system was established. Compaction of the Main seam peat and the beginning of pronounced block-tilting were important factors which lead to the peat of the Upper seam being deposited only near the southwestern margin of the former basin.