Sediment-petrologic characteristics of Saalian and Weichselian deposits in the Hummling region, NW Germany

Abstract

In the lowlands of northwest Germany, Saalian (fluvio-) glacial plateaus and ice-pushed hills are surrounded by flat and low-lying terrain consisting of Weichselian fluvial to Aeolian sands. The present work refers to the Ems-Hase fluvial basin and the adjacent part of the fluvioglacial Hümmling plateau. On the basis of heavy-mineral analyses and other data it was found that: (i) The Weichselian deposits are significantly richer in garnet and alterite than the Saalian meltwater sands which flank and underlie them in the Ems-Hase fluvial basin. It is thought that Weichselian fluvial deposition was preceded by a period of erosion related to the low sea level of the last ice age. During that phase, the ancient riversystem extended its course in upstream direction and cut down into the headwater portion of its drainage basin. As a result, the subsequent infilling of the valleys was primarily by material derived from pre-Cenozoic rocks with the share of the Saalian substrate being subordinate only. (ii) There is no significant difference in unstable-species content between fluvial and aeolian sands but for the occurrence of traces of glaucophane only in the second type. The latter feature suggests that, during the Late Weichselian Pleniglacial, deflation from the then dry part of the North Sea floor contributed to the deposition of the windborne coversands in the study area. Yet, the effect of the long-distance aeolian transport must have been slight only, and the buildup of the coversands resulted mainly from the local reworking by wind of the fluvial-sand substrate. (iii) Mineralogically, there is a distinct contrast between the fluvial sands from the northern part and those from the southern part of the study area. The pertinent heavy-mineral spectra, supplemented with previously published analyses, reveal downstream trends in the composition of the Weichselian fluvial sand. These trends reflect changes in the sediment supply from Cretaceous and older rocks in the south to thick Cenozoic beds in the north. The northern strata include shallow marine deposits of Tertiary and Pleistocene (mainly Holsteinian) age and material laid down by the Late Tertiary to Elsterian north German riversystem.