Late quaternary sediments in Lake Zürich, Switzerland

Abstract

/ Lake ZSrich occupies a glacially overdeepened perialpine trough in the northern Middlelands of Switzerland. A total of 154.4 m of Quaternary sediments and 47.3 m of Tertiary Molasse bedrock has been cored from the deepest part of the lake, some 10 kn~ south of the city of Zfirich. Some 16.8 m of gravels and sands directly overlying the bedrock include basal till and probably earliest ~Ubglacial fluvial and lacustrine deposits. These are overlain by 98.6 m of fine-grained, glacial-aged sediments comprising completely deformed proglacial and/or subglacial lacustrine muds, separated by l~ur basal mud tills. The lack of interglacial sediments, fossils, and Other datable material, and the presence of severe sediment deformation and unknown amounts of erosion prevent the establishment of an exact chronostratigraphy for sediments older than the upper mud till. Above it some 8.6 m of lacustrine muds were deposited, folded, faulted, and tilted during the final opening of the lake at about 17,500-17,000 years ago. Superimposed are 30.4 m of final Wfirm and post-glacial sediments comprising (from oldest): cyclic proglacial mud, thick-bedded and laminated mud, a complex transition zone, laminated carbonate, laminated marl, and diatom-calcite varves. These sediments reflect changing catchment and lacustrine conditions including: glacial proximity, catchment stability, lake inflow characteristics, thermal structure, chemistry, and bed stability. Average sedimentation rates ranged from 11 cm yr-1 immediately after glacier withdrawal, to as low as 0.4 mm yr-1 as the environment stabilized.The lack of coarse outwash deposits separating the fine-grained glaciolacustrine sediments from a corresponding underlying basal till suggests that deglaciation of the deep northern basin of Lake ZSrich was by stagnation-zone retreat rather than by retreat of an active ice-front.

A subaquatic moraine complex in overdeepened Lake Thun (Switzerland) unravelling the deglaciation history of the Aare Glacier

“…If we consider U5 separately, apply the assumption above and take into the calculation ~100 ± 5 m of bottom set sediments deposited in less than 1.2 ± 1.6 ka, we obtain a sedimentation rate of >8.3 ± 1.3 cm/yr during SMC formation. This seems a reasonable estimate for a rapidly disintegrating Aare Glacier, as the sedimentation rate in Lake Zurich immediately after glacial withdrawal ~17 ka ago was calculated to be ~11 cm/yr (Lister et al, 1984). However, better time constraints could possibly result in a sedimentation rate even twice as high.…”

“…The base of U3 is interpreted as the onset of the last glaciation with an erosive base removing partially underlying U2, causing a non-uniform thickness distribution and undulating top of U2, and emplacing a glacial till (Lister et al, 1984). The relatively smooth and continuous top of U3, with a pronounced horizontal character at the deepest section of the basin, therefore indicates terminal deposits that have not been overprinted by a subsequent glacial readvance with deforming ice contact at the base.…”

“…1) A multi-cycle concept, where U1 and U2 belong to independent pre-LGM glacial cycles, as it is documented for the Aare valley at nearby drill site Thalgut (Schlüchter, 1989a;1989b; see Fig. 10 for location) or known from the drill site at Lake Zurich (Lister et al, 1984) with four till sequences that are all topped by glacio-lacustrine sediments deposited in a lacustrine (subglacial) environment. Also Lake Annecy and Lake Le Bourget, France, host remnants of pre-LGM sediments (Van Rensbergen et al, 1998;van Rensbergen et al, 1999).…”

Erosion under climate and human pressures: An alpine lake sediment perspective

“…Those lakes were on the other hand intensively studied through geophysical (seismic) investigations revealing active depocenters filled by several tens of meters of Holocene sediments brought by some of the main rivers draining the Alps as well as giant mass-wasting deposits (Baster et al, 2003;Fanetti et al, 2008;Finckh et al, 1984;Fiore et al, 2011;Girardclos et al, 2005;Hsu and Kelts, 1970). When sediment cores spanning the whole Holocene were retrieved from coring sites that were not directly exposed to deposition of the main rivers, studied proxies were in general not related to erosion (Anad on et al, 2006) or not representative of the whole lake catchment area (Girardclos et al, 2005;Lauterbach et al, 2012;Lister et al, 1983;Moscariello et al, 1998). One can only regret this state of research, especially since we know that perialpine lakes are one of the world hotspots in the 1970's where fundamental limnogeology (Hsü and Kelts, 1984) and sedimentology (Hsü, 1989) concepts and investigation tools were designed and tested.…”

Holocene climatic fluctuations and positioning of the Southern Hemisphere westerlies in Tierra del Fuego (54° S), Patagonia

“…The seismic stratigraphy is characterised by a thick transparent chaotic unit (unit EA), followed by a series of transparent subunits separated by almost equally spaced continuous medium‐ to high‐amplitude reflections (unit EB) and topped by intercalations of thinly spaced, high‐amplitude internal reflections with low‐amplitude to transparent intervals (unit EC). Unit EA has been interpreted to represent glacially derived sediments, while overlying unit EB is interpreted as fining upward sequences of proglacial turbidites, following a Swiss approach for investigation of perialpine lakes (Lister et al , 1984). Unit EC contrasts sharply and represents a major environmental change with the onset of a pelagic‐style sedimentation intercalated with sequences of downslope mass‐flow events.…”