This paper presents a detailed analysis of the high-resolution facies architecture of the Middle Pleistocene Porta subaqueous ice-contact fan and delta complex, deposited on the northern margin of glacial Lake Weser (Northwest Germany). A total of 10 sand and gravel pits and more than 100 wells were examined to document the complex facies architecture. The field study was supplemented with a ground-penetrating radar survey and a shear-wave seismic survey. All collected sedimentological and geophysical data were integrated into a high-resolution three-dimensional geological model for reconstructing the spatial distribution of facies associations. The Porta subaqueous fan and delta complex consist of three fan bodies deposited on a flat lake-bottom surface at the margin of a retreating ice lobe. The northernmost fan complex is up to 55 m thick, 6AE2 km wide and 6AE5 km long. The incipient fan deposition is characterized by high-energy flows of a plane-wall jet. Very coarse-grained, highly scoured jet-efflux deposits with an elongate plan shape indicate a high Froude number, probably >5. These jet-efflux sediments are deposited in front of a large 3AE2 km long, up to 1AE2 km wide, and up to 25 m deep flute-like scour, indicating the most proximal erosion and bypass area of the jet that widens and deepens with distance downstream to the region of maximum turbulence (approximately five times the conduit diameter). Evidence for subsequent flow splitting is given by the presence of two marginal gravel fan lobes, deposited in front of 1AE3 to 2AE5 km long flute-like scours, that are 0AE8 to 1 km wide and 7 to 20 m deep. In response to continued aggradation, small jets developed at the periphery of these bar-like deposits and filled in the low areas adjacent to the original superelevated regions, locally raising the depositional surface and characterized by large-scale trough cross-stratified sand and pebbly sand. The incision of an up to 1AE2 km wide and up to 35 m deep channel into the evolving fan is attributed to a catastrophic drainage event, probably related to a lake outburst and lake-level fall in the range of 40 to 60 m. At the mouth of this channel, highly scoured jet-efflux deposits formed under hydraulic-jump conditions during flow expansion. Subsequently, Gilbert-type deltas formed on the truncated fan margin, recording a second lake-level drop in the range of 30 to 40 m. These catastrophic lake-level falls were probably caused by rapid ice-lobe retreat controlled by the convex-up bottom topography of the ice valley.

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J. 2014 (January): Climate control on the evolution of Late Pleistocene alluvial-fan and aeolian sand-sheet systems in NW Germany.The Late Pleistocene was characterized by rapid climate oscillations with alternation of warm and cold periods that lasted up to several thousand years. Although much work has been carried out on the palaeoclimate reconstruction, a direct correlation of ice-core, marine and terrestrial records is still difficult. Here we present new data from late Middle Pleniglacial to Lateglacial alluvial-fan and aeolian sand-sheet deposits in northwestern Germany. Records of Late Pleniglacial alluvial fans in central Europe are very rare, and OSL dating is used to determine the timing of fan aggradation. In contrast to fluvial systems that commonly show a delay between climate change and incision/aggradation, the small alluvial-fan systems of the Senne area responded rapidly to climatic changes and therefore act as important terrestrial climate archives for this time span. The onset of alluvial-fan deposition correlates with the climate change from warm to cold at the end of MIS 3 (29.3Ϯ3.2 ka). Strong fan progradation started at 24.4Ϯ2.8 ka and may be related to a period of higher humidity. The vertical stacking pattern of sedimentary facies and channel styles indicate a subsequrent overall decrease in water and sediment supply, with less sustained discharges and more sporadic runoffs from the catchment area, corresponding to an increasing aridity in central Europe during the Late Pleniglacial. Major phases of channel incision and fan aggradation may have been controlled by millennial-scale Dansgaard-Oeschger cycles. The incision of channel systems is attributed to unstable climate phases at cold-warm (dry-wet) or warm-cold (wet-dry) transitions. The alluvial-fan deposits are bounded by an erosion surface and are overlain by aeolian sand-sheets that were periodically affected by flash-floods. This unconformity might be correlated with the Beuningen Gravel Bed, which is an important marker horizon in deposits of the Late Pleniglacial resulting from deflation under polar desert conditions. The deposition of aeolian sand-sheet systems (19.6Ϯ2.1 to 13.1Ϯ1.5 ka) indicates a rapid increase in aridity at the end of the Late Pleniglacial. Intercalated flash-floods deposits and palaeosols (Finow type) point to temporarily wet conditions during the Lateglacial. The formation of an ephemeral channel network probably marks the warm-cold transition from the Allerød to the Younger Dryas.