Non-annual cycles of varve-like sedimentation in Walensee, Switzerland*


  • *

    Contribution No 129 of the Laboratory of Experimental Geology.

  • Versuchsanstalt für Wasserbau, Hydrologie und Glaziologie (VAW), ETH.


Dating of recent varve-like sediments from the perialpine Lake of Walenstadt (=Walensee) indicates that the number of laminae deposited in the 165 years between 1811 and 1976 ranges from 300 to 360 depending on sample location. Direct evidence that up to five graded laminae may be deposited during one year was found in 1912 by an engineer of the Swiss Federal Hydrological Agency who was using sediment traps to determine the annual accumulation rate. These layers are considered to be deposits of continuous-fed turbidity currents generated by hyperpycnal inflow during river-flood stages. Current measurements revealed that these underflows can occur sporadically throughout the year, but are especially common during the snow-melting season and after heavy rainfalls. Currents with bottom-velocities up to 50 cm s−1 were detected during summertime when the lake is thermally stratified, indicating that such stratification is no obstacle to the generation of turbidity underflows. The laminated sediments of the Walensee do not represent deposition of annual cycles and these non-annual varve-like sediments seem to be less regularly rhythmic than the annual varves of Lake Zurich.