PROGLACIAL SUBAQUATIC OUTWASH FAN AND DELTA SEDIMENTS IN PUSH MORAINES - INDICATORS OF SUBGLACIAL MELTWATER ACTIVITY

A significant proportion of the glaciofluvial sediments in the Rehburg Stage (Saalian glaciation) push moraines in Germany appears to be subaquatic in origin. They form subaquatic fans, deltas and lacustrine deposits, composed mainly of reworked autochthonous sediments, with only minor amounts of erratic (mainly Scandinavian) material. Subaquatic sediments occur in three positions with respect to the push moraines: (1) pre-tectonic and early syn-tectonic sediments, incorporated in the push moraine and overlying preglacial sediments, (2) syn-tectonic sediments overlying angular unconformities and filling depressions between anticlinal ridges on top of the push moraine, and (3) post-tectonic sediments on the distal push moraine slopes. This indicates that lakes existed prior to and during the advance of the Scandinavian ice sheet onto the North German lowlands, and also after formation of the push moraines and prior to their overriding. The topsets are commonly overlain by a deformation till produced by subglacial shearing of these sediments. The Rehburg push moraines closely follow the northern margin of the Hercynian highlands in Germany. Blocking of the ice-marginal drainage by advancing ice lobes produced vast lakes between the ice sheet and the northern slopes of these highlands. Both the sedimentology and the petrology of the sediments in the Rehburg push moraines suggest that these subaquatic sediments are products of subglacial erosion beneath ice lobes or ice streams. Since the pre-tectonic lakes formed at the ice margin, the subaquatic fans and deltas were constructed from the mouths of subglacial tunnels. Thus it is proposed that these sediments are eskerine in origin. A subglacial provenance is supported by the occurrence in some of these sediments of layers and lenses of a pure and uniform petrology, intercalated with strata of a more mixed petrology. These are interpreted as the products of meltwater tunnelling in the saturated sediments beneath the ice lobes which produced the push moraines.