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.

Brandes, C. 2018 (October): Ice-marginal forced regressive deltas in glacial lake basins: geomorphology, facies variability and large-scale depositional architecture.This study presents a synthesis of the geomorphology, facies variability and depositional architecture of ice-marginal deltas affected by rapid lake-level change. The integration of digital elevation models, outcrop, borehole, groundpenetrating radar and high-resolution shear-wave seismic data allows for a comprehensive analysis of these delta systems and provides information about the distinct types of deltaic facies and geometries generated under different lake-level trends. The exposed delta sediments record mainly the phase of maximum lake level and subsequent lake drainage. The stair-stepped profiles of the delta systems reflect the progressive basinward lobe deposition during forced regression when the lakes successively drained. Depending on the rate and magnitude of lake-level fall, fanshaped, lobate or more digitate tongue-like delta morphologies developed. Deposits of the stair-stepped transgressive delta bodies are buried, downlapped and onlapped by the younger forced regressive deposits. The delta styles comprise both Gilbert-type deltas and shoal-water deltas. The sedimentary facies of the steep Gilberttype delta foresets include a wide range of gravity-flow deposits. Delta deposits of the forced-regressive phase are commonly dominated by coarse-grained debrisflow deposits, indicating strong upslope erosion and cannibalization of older delta deposits. Deposits of supercritical turbidity currents are particularly common in sand-rich Gilbert-type deltas that formed during slow rises in lake level and during highstands. Foreset beds consist typically of laterally and vertically stacked deposits of antidunes and cyclic steps. The trigger mechanisms for these supercritical turbidity currents were both hyperpycnal meltwater flows and slope-failure events. Shoal-water deltas formed at low water depths during both low rates of lake-level rise and forced regression. Deposition occurred from tractional flows. Transgressive mouthbars form laterally extensive sand-rich delta bodies with a digitate, multi-tongue morphology. In contrast, forced regressive gravelly shoal-water deltas show a high dispersion of flow directions and form laterally overlapping delta lobes. Deformation structures in the forced-regressive ice-marginal deltas are mainly extensional features, including normal faults, small graben or half-graben structures and shear-deformation bands, which are related to gravitational delta tectonics, postglacial faulting during glacial-isostatic adjustment, and crestal collapse above salt domes. A neotectonic component cannot be ruled out in some cases. facies and the larger-scale depositional architecture. The results are compared with other marine and lacustrine delta examples from the literature, and provide information about the distinct types

In this paper, we will present the stratigraphic evolution, internal facies architecture and geomorphology of the Middle Pleistocene Emme delta, controlled by rapid high-amplitude lakelevel change.The Emme delta was deposited on the northern margin of glacial Lake Weser, located in north-west Germany. Rates of lake-level rise were probably 450 mm year À 1 and rates of lake-level fall 30^50 m within a few days or weeks, due to the opening of lake outlets.We use digital elevation models, sedimentology and shear wave seismics to improve earlier reconstructions and investigate the in£uence of rapid base-level change on delta development. Shear wave seismic data resolve architectural elements in the range of metres and bridge the common gap between outcrop and conventional compression wave seismic data.The radial delta complex is about 2 km long, 1.8 km wide and up to 70 m thick, overlying a concave, up to131 steep dipping ramp surface. It consists of vertically and laterally stacked delta lobes, caused by lobe switching during base-level change. During the lakelevel rise, vertically stacked (Gilbert-type) delta systems formed.The decrease in thickness and lateral extent indicates a rapid upslope shift of depocentres. A high rate and magnitude of lake-level fall (50 m) promoted the development of a single incised valley and the deposition of forced regressive coarse-grained delta lobes in front of the valley.The incised valley was ¢lled during decreasing rates of lake-level fall and low base-level, because the alluvial gradient was larger than the emergent lake pro¢le. Attached sand-rich forced regressive aprons formed during lower magnitudes of lake-level falls in the range of 30^35 m.Valley incision occurred, but was limited to the uppermost portion of the delta, controlled by the steep slope.The incised valley related to the ¢nal lake drainage is associated with long-wavelength (60^90 m) bedforms at the downslope end, attributed to the formation of standing waves as a result of a hydraulic jump. Estimated palaeo£ow depth during standing wave formation was $9^14 m and £ow velocity was10^12 m s À 1 . Because subsidence, waves or tides did not play a major role, the Emme delta can be used as an analogue-based predictive stratigraphical and sedimentological model for steep glacigenic deltas controlled by rapid base-level change and can help to understand better the facies distribution and three-dimensional geometry of these depositional systems.

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