Turbidites and Varves in Lake Brienz (Switzerland): Deposition of Clastic Detritus by Density Currents

Lake Brienz is a 14 km long and 261 m deep oligotrophic valley lake which lies in the front ranges of the Swiss Alps. Sedimentation is entirely clastic and is dominated by two rivers which enter the lake at opposite ends. The sediment load is transported and deposited in the lake by overflows, interflows and underflows (low- and high-density turbidity currents) depending on the density difference between river and lake water. Whereas high-density turbidity currents, which deposit up to 150 cm-thick graded sand layers, occur only once or twice per century after catastrophic flooding, low-density turbidity currents occur annually during periods of high discharge and deposit centimetre-thick faintly graded sand layers. Fine-grained sediment supplied by overflows and interflows rains down continuously during summer thermal stratification to form the dark-grey summer half-couplet of a varve; at turnover in the autumn the remaining sediment trapped at the thermocline settles out and forms the light-grey winter layer. Turbidites grade distally into thin dark-grey layers indistinguishable from the dark-grey summer half-couplet. Turbidites on the basin plain can be correlated with varves on the slopes. Therefore, in Lake Brienz the formation of varves and turbidites is genetically related and depends on the existence of over-and interflows, turbidity currents and seasonal thermal stratification.