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.

No abstract

No abstract

No abstract

No abstract

No abstract

During the maximum advance of the Early Saalian Scandinavian Ice Sheet ice-dammed lakes developed within valleys of the Northwest German Mountain ranges. The blocking of the River Weser valley led to the formation of glacial Lake Rinteln where most of the sediment was deposited by meltwater. Two subaqueous fan complexes have been identified, which formed lakeward of retreating ice-lobes during an overall lake-level rise. The sediment transport on the fans has been dominated by sustained gravity-flows, mainly cohesionless debris flows or high-and low-density turbidity currents, reflecting discharge of semi-continuous meltwater flows. Intercalations of surge-type high-and lowdensity turbidites increase towards the mid-and lower-fan slopes, indicating more ice-distal and periodic deposition. Individual fan bodies commonly have a coarse-grained proximal core of steeply dipping gravel, overlain by gently to steeply dipping mid-to outer fan deposits. During glacier retreat commonly fine-grained sediments, rich in ice-rafted debris, were deposited on the ice-distal and ice-proximal slopes of the abandoned fans. Climbing-ripple cross-laminated sand may onlap coarse-grained upper fan gravel and in some cases overtop the older fan deposits. Phases of glacier still-stands are characterised by fan systems that display an upward flattening of fan clinoforms and minor vertical facies changes. The position of ice marginal fans was controlled by the combination of bedrock topography and water depth. At the eastern lake margin topographic highs served as pinning points for the retreating glacier and facilitated ice margin stabilisation. A strong lake-level fall probably triggered a major drainage event that tapped previously unconnected reservoirs of englacial and subglacial meltwater.

No abstract