During the Pleistocene, northern Germany was affected by three major glaciations of the Elsterian, Saalian and Weichselian glacial periods (Fig. 1A). From the Saalian glaciation two major ice ad-vances, the Drenthe and Warthe are known (Ehlers et al., 2004, 2011). Saalian ice-marginal deposits have been studied at four sites in northern Germany (Fig. 1A), where large ice-dammed lakes formed at the southwestern margin of the Scandinavian ice sheet due to the blockage of river drainage path-ways to the north (Thome, 1983; Klostermann, 1992a; Van der Wateren, 1994; Eissmann, 2002; Win-semann et al., 2009, 2011). Ice-marginal glacilacustrine deposits consist of coarse-grained deltas and subaqueous fans, which bear evidence of fluctuating lake-levels (e.g., Winsemann et al., 2009, 2011).
During deglaciation the lakes catastrophically drained due to the opening of lake outlets, which caused high-magnitude outburst floods (Winsemann et al., 2011). The drainage pathway of the glacial lake-outburst floods has been reconstructed by Meinsen et al. (2011). In the most proximal areas the drain-age pathway is characterised by the occurrence of deep plunge pools, channels, streamlined hills and large V-shaped megaflutes. In the Lower Rhine Embayment highly dissected push moraines indicate the flood pathway towards the west into the North Sea.
Subaqueous ice-contact fan deposits
Subaqueous ice-contact fans (also known as grounding line fans) are deposited where sediment-laden meltwater is released as an efflux jet from en- or subglacial conduits into a standing water body, and are known from both glacimarine (Powell, 1990) and glacilacustrine (Russell & Arnott, 2003; Russell et al., 2007; Hornung et al., 2007; Winsemann et al., 2009) environments. Depositional processes by meltwater flows released at the grounding line of the glacier can be explained with the plane-wall jet flow model, with the basin floor acting as the basal boundary of the flow (Powell, 1990; Gorrell &
Shaw, 1991; Russell & Arnott, 2003; Russell et al., 2007). Typically, jet flows and their deposits can be subdivided into three zones: (i) zone of flow establishment (ZFE), (ii) zone of flow transition (ZFT) and (iii) zone of established flow (ZEF; Bates, 1953; Powell, 1990; Russell & Arnott, 2003). Within initially supercritical jets, a hydraulic jump associated with rapid deceleration occurs in the ZFT, while entrainment of ambient water will be highest within the supercritical jet region (Rajaratnam & Subra-manyam, 1986; Russell & Arnott, 2003).
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Fig. 1 (previous page): A) Overview map of the study area, showing the maximum extent of the Older Saalian Drenthe and Younger Saalian Warthe ice advances (modified after Ehlers et al., 2011). White stars indicate the locations of the studied outcrops (B: Buschhaus; P: Porta; R: Reichswald; W: Wollin). B) Hill-shaded relief model of the Buschhaus fan (dashed black line), showing the location of the outcrop. The digital elevation mo-del (DEM) is based on data from the Landesamt für Geoinformation und Landentwicklung Niedersachsen (LGLN). C) Hill-shaded relief model of the Porta fan (dashed black line), showing the location of the outcrop.
The DEM is based on data from the Bezirksregierung Köln. D) 2D cross-section of the Buschhaus fan.
The Porta subaqueous fan and delta complex (Fig. 1A, C) was deposited on a flat lake-bottom surface at the margin of the retreating Saalian Drenthe ice sheet. The 600 m wide Porta Westfalica pass at the northwestern margin of the fan acted as a bedrock feeder channel (Winsemann et al., 2009). The northernmost fan complex is up to 55 m thick and has a radial shape with a diameter of approximately 6.5 km. The sedimentary facies and large-scale architecture of the fan complex have been studied in detail by Hornung et al. (2007) and Winsemann et al. (2009) and interpreted as deposited by subcriti-cal and supercritisubcriti-cal meltwater jets. Gravel-rich incipient fan deposits are unconformably overlying lake-bottom deposits and are overlain by sand-rich fan deposits (Fig. 1E). New outcrop walls in sand pits (Fig. 1C) provide a three-dimensional, but non-continuous, exposure of the jet-efflux deposits with a length of ~300 m approximately parallel to the main palaeoflow direction and ~350 m across the main palaeoflow direction, which was towards the SSW. The stacking pattern displays a rapid downflow evolution from highly scoured and planar stratified gravel to low-angle cross-stratified sand to trough cross-cross-stratified and climbing-ripple cross-laminated sand.
Buschhaus subaqueous fan
The Buschhaus subaqueous fan (Fig. 1A, B) was deposited in front of a morainal ridge during the glacial retreat of the Saalian Drenthe ice sheet (Duphorn et al., 1974; Look, 1984; Lang et al., 2012b).
The fan has a radial diameter of approximately 2.3 km and is up to 50 m thick. Borehole data indicate that gravel-rich deposits of the incipient fan are unconformably overlain by sand-rich fan deposits (Fig. 1D). The subaqueous fan deposits were studied in a sand pit, where a ~25 m thick succession is exposed. The upper ~12 m of the exposed succession display a vertically and laterally highly variable stack of bedforms, including low-angle cross-stratification, inclined-parallel stratification, planar, trough and sigmoidal cross-stratification and climbing-ripple cross-lamination. Beds dip very gently (1-5°) to the south and unconformably overlie large-scale trough cross-stratified gravel of the basal fan deposits. The outcrop walls provide a three-dimensional exposure of the upper section, with walls of
~80 m length parallel to the main palaeoflow direction and ~60 m across the main palaeoflow direc-tion, which was towards the SSW.
Wollin subaqueous fan
The Wollin subaqueous fan was deposited in an ice-marginal setting during the retreat of the Saalian Warthe ice sheet (Ziermann, 1987; Lippstreu, 1995). The sand-rich subaqueous fan deposits have a total thickness of ~35 m and are interbedded with fine-grained glacilacustrine deposits (Ziermann, 1987). The subaqueous fan deposits were studied in a sand pit near Wollin (Fig. 1A), where ~10 m thick deposits are exposed, overlain by a glacilacustrine delta and till. The exposed subaqueous fan deposits consist mainly of sinusoidal stratified, well-sorted, fine-grained sand with intercalated low-angle cross-stratified and trough cross-stratified sand. The outcrop wall provided a ~60 m long section oblique to the main palaeoflow direction, which was towards the SSW.
Fig. 2: Hill-shaded relief model of the Reichswald push-moraine ridge, showing the location of the outcrop. The DEM is based on data from the Bezirksregierung Köln.
Glacial lake-outburst flood deposits
Glacial lake-outburst flood deposits were studied at the Reichswald push-moraine ridge, which forms part of a large northwest-southeast trending system of push moraines in the Lower Rhine Embayment and the Netherlands (Klostermann, 1992a, b; Busschers et al., 2008; Skupin & Zandstra, 2010; Figs.
1A, 2). The push-moraine complex of the Lower Rhine Embayment was probably dissected by Middle Pleistocene (Saalian Drenthe) glacial lake-outburst floods (Meinsen et al., 2011; Fig. 2). In the lee of the Reichswald push moraine ridge sand- and gravel-rich sediments, which were partly deposited by the glacial lake-outburst floods, form a gently southwestward dipping ramp. Superimposed dry chan-nels on top and in the lee of the push-moraine ridge can still be observed in the modern topography.
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The studied outcrop section is located within one of these large dry channels (Fig. 2). The channel is slightly sinuous and widens downflow from 1 km in the north to 2.8 km in the southwest. The channel floor lies 10-15 m deeper than the surrounding surface. The studied ~ 15 m thick succession is ex-posed in a sand pit, which is located at the inner side of a channel bend (Fig. 2). The outcrop walls provided discontinuous sections oblique to main the palaeoflow direction, which was towards the southwest.