Changes in distal sedimentation regime of the Rhone delta system controlled by subaquatic channels (Lake Geneva, Switzerland/France)
Introduction
Submarine channel or canyon systems on continental margins serve as pathways for turbidity currents and other gravity flows transporting sediments to depositional areas in the deep basins (Kolla 2007). In large clastic lakes, the subaqueous flow of incoming rivers may develop meandering channels or canyons on their submerged delta slopes and distal fan lobes that are similar to their marine counterparts (Mulder and Chapron 2011). The activity of subaquatic channels with regard to gravity flows is often controlled by the position of the river mouth, which can shift regularly similar to avulsion in the river systems (Reitz et al. 2010). The river mouth position in turn is influenced by external factors that control the course of the river bed, such as regional tilting, changes of the drainage systems due to earthquakes, sediment deposition and avulsion during major floods (Wells and Dorr 1987), as well as continuous sediment accumulation (Migeon et al. 2006). Human impact, such as river embankments, river channelizing and river deviations, can also drastically change the pathways and the regime of the sublacustrine turbidity currents and thus influence the entire sublacustrine delta system (Anselmetti, F.S., et al., 2007, Loizeau, J.-L. and Dominik, J., 2000, Syvitski, J.P.M., et al., 2009, Wirth, S.B., et al., 2011). Understanding the timing and causes of shifts in subaquatic delta-related channel systems is important because they control the depositional patterns in the more distal areas of these environments. In the specific framework of paleoflood reconstruction (Gilli, A., et al., 2003, Glur, L., et al., 2013, Mulder, T. and Chapron, E., 2011, Mulder, T., et al., 2001a, Mulder, T., et al., 2001b, Wirth, S.B., et al., 2013) and paleoseismic reconstruction (Goldfinger, C., et al., 2012, Nelson, C.H., et al., 2012), for instance, the influence of the river/delta/canyon system evolution on the spatial variability of event-related deposits in the basin needs to be constrained as a prerequisite to the interpretation of any clastic sedimentary record.
In this study, we present a synthesis of seismic reflection and sediment core data from the distal parts of the Rhone delta system in Lake Geneva with the aim to reconstruct the evolution of its sedimentation regime during the last 1500 years.
Section snippets
Regional and sedimentological setting
Lake Geneva is the largest perialpine lake in Western Europe with a surface area of 580 km2 and a volume of 89 km3 of freshwater. Its basin was formed by glacial erosion during the Pleistocene (Wildi and Pugin 1998). The two main rivers feeding Lake Geneva are the Rhone and the Dranse River (Fig. 1). Using geochemical and petrophysical analysis the Rhone and Dranse turbidites can be distinguished as the drainage areas present different outcropping rocks. Dranse turbidites are characterized by
Historical Rhone River corrections
The Rhone River upstream of Lake Geneva is presently heavily impacted by human activities and settlements, mainly due to dams and embankment constructions (Vischer 2003), but also by water intake for agricultural and industrial purposes. The various measures of the so-called “Rhone corrections” led to a widespread loss of natural swamp areas in the Rhone valley. The natural River course before the diverse corrections was characterized by the alternation between a meandering system and single
Methods
Several seismic reflection, sediment coring and multibeam campaigns were conducted with the boat “La Licorne” of the Institute Forel, University of Geneva.
Seismic reflection data from the main basin of Lake Geneva were acquired with a 3.5 kHz pinger source in April 2010 and with a Sodera Mini GI airgun, with a central frequency of 330 Hz, together with GPS navigation (positioning error of 3–5 m) in 2002 (Fig. 1). The pinger data, which have a theoretical vertical resolution of 0.1 m, were processed
Seismic reflection in Rhone delta system
Airgun seismic reflection data (Dupuy, D., 2006, Zingg, O., et al., 2003) from the Rhone delta system image the present channels of the Rhone delta and reveal two buried paleochannels, ‘CI’ and ‘CII’ (Figs. 2A and 2B). The paleochannels CI and CII are located near the present courses of channels C5 and C8, respectively (Fig. 2B). Because of low seismic penetration on the sublacustrine delta slope due to gas blanking, it is difficult to correlate seismic units in the deep basin with those
Discussion: what is controlling the Rhone fan sedimentation?
The presented seismic and lithological datasets reveal several changes in the spatial deposition pattern, sedimentation rate and turbidite frequency, which occurred in the distal Rhone delta basin during the last ca. 1500 years. General sedimentation rate trends may be linked to the climatic influence on Alpine denudation rates and sediment transport during the Little Ice Age (around 1280 to 1860 cal AD; Holzhauser, H., et al., 2005, Holzhauser, H. and Zumbühl, H.J., 1999, Stuiver, M. and Quay,
Conclusions
Based on reflection seismic and sediment core data, we observe several shifts in the frequency and spatial distribution of turbidites in Lake Geneva, and we interpret this as a shift of the main subaquatic Rhone channel system through which river-borne underflows are assumed to be routed to the deep basin. The shift to a system with the main active channels C3/C4 located in the northern part of the delta is dated at 1480 ± 20 cal AD and is likely related, directly or indirectly, to the centennial
Acknowledgments
This work was funded by the Swiss National Science Foundation (Research grants 200021-121666 and 200020-146889 and R'Equip-146889). Katrina Kremer is currently funded as Postdoc by the Swiss National Science Foundation (Research grant 133481). We thank Flavio Anselmetti, Fred Arlaud, Philippe Arpagaus, Yves Cauderay, Robert Hofmann, Reto Seifert, and Chadia Volery for their help during seismic reflection and coring campaigns in the ‘Grand Lac’. Thanks to Adrian Gilli for access to the
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- 1
Present address: Geological Institute, ETH Zurich, Sonneggstrasse 5, 8092, Zurich, Switzerland.
- 2
Present address: Institute of Physical Chemistry Rocasolano (IQFR-CSIC), 28003 Madrid, Spain.