Response of a proglacial delta to rapid high-amplitude lake-level change: an integration of outcrop data and high-resolution shear wave seismics

Abstract

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.

Sedimentary imprints of former ice-sheet margins: Insights from an end-Ordovician archive (SW Libya)

“…In front of temperate ice-sheet margins, fluvioglacial meltwater flows are primarily seasonally driven (seasonal melt and elevated discharges) but are also characterised by recurrent, lowerfrequency and high-magnitude flow events, typically linked with outbreak flows from ice-dammed lakes. Therefore sedimentary structures related to high-magnitude flow events within any glacial record indicate an ice-marginal to proglacial context (Maizels, 1997;Russell and Knudsen, 2002;Johnsen and Brennand, 2004;Marren, 2005;Carrivick and Russell, 2007;Thieler et al, 2007;Duller et al, 2008;Jones and Fielding, 2008;Winsemann et al, 2011;Carling, 2013;Lang and Winsemann, 2013). Deposits related to end-Ordovician glacial outburst events (like Icelandic jökulhlaups) have been inferred from recurrent assemblages associating megabeds and climbing-dune cross-stratification (Le Heron et al, 2006CDCS of Ghienne et al, 2010;Girard et al, 2012a,b;Deschamps et al, 2013).…”

“…Most often stoss-depositional structures are generated, with preservation of the topographies of formative bedforms. CDCS are recognised in flood-dominated depositional systems (Mutti et al, 2003) but are poorly represented in the Quaternary glacial record, where they only occur as transient bedforms in spatially restricted locations of outwash depositional systems (e.g., Gorell and Shaw, 1991;Russell et al, 2007;Winsemann et al, 2011). CDCS are however ubiquitous in the end-Ordovician glacial record Girard et al, 2012aGirard et al, , 2012b.…”

“…They frequently involve conglomeratic sediments but have also been identified in sandier successions linked to glacially-related deltas and sandar (Johnsen and Brennand, 2004;Duller et al, 2008;Winsemann et al, 2011;Carling, 2013;Lang and Winsemann, 2013) or subglacial conduits (Brennand and Shaw, 1996).…”

Fluvial responses to the Weichselian ice sheet advances and retreats: implications for understanding river paleohydrology and pattern changes in Central Poland

“…The upper segment of the lithotype B1 (large-scale and low angle crossbeds Sl with sigmoidal laminae) reflects lateral aggradation of humpback dunes (Kostaschuk and Villard 1996;Mohrig and Smith 1996;Kostaschuk 2000;Winsemann et al 2011). The morphology of these bedforms was changeable due to a variable rate of suspended sediment concentration (Kostaschuk and Villard 1996;Mohrig and Smith 1996;Kostaschuk 2000;Lang and Winsemann 2013).…”

Structural glaciology of Isunguata Sermia, West Greenland

“…Marginal lakes play an important role within the glaciological environment due to their influence on both landform development (Winsemann, Brandes, & Polom, 2011) and ice flow dynamics (Diolaiuti et al, 2006).…”