Unravelling megaturbidite deposition: Evidence for turbidite stacking/amalgamation and seiche influence during the 1601 CE earthquake at Lake Lucerne, Switzerland

Megaturbidites are commonly used to reconstruct the seismic history (palaeoseismology) of areas where large earthquakes occur. However, the depositional mechanisms and sedimentary characteristics of these deposits are not yet fully understood. This study unravels the sequence of sediment deposition that occurred in Lake Lucerne (Vitznau Basin) following the 1601 ce earthquake in central Switzerland. During this event, slope failures were triggered, generating mass flows and turbidity currents that led to the formation of mass-transport deposits and a megaturbidite. These deposits are sampled in 28 sediment cores, which are examined with X-ray computed tomography scans (medical and μCT), grain-size analysis and natural remanent magnetisation. This suite of analyses allows a detailed reconstruction of turbidite stacking and amalgamation in the centre of the basin, followed by settling of finer sediments influenced by a lake seiche. Initial deposition of mass-transport deposits is followed by sandy turbidites reaching the depocentre. Some of these turbidite sands can be linked to their source areas, and evidence is found of some turbidites being overridden by mass flows in the peripheral parts of the megaturbidite deposit. Hereafter, sedimentation becomes controlled by seiche-induced currents, which rework fine sediments upon deposition, leading to subtle grain-size variations at the base of the seiche-influenced sub-unit and a ponded geometry of the megaturbidite. As the seiche movement dampens, a relatively muddy, homogeneous sub-unit is deposited that drapes the basin plain. Overall, this study provides the first highly detailed sedimentological analysis of megaturbidite deposition in a lake, demonstrating the distinct sedimentological imprint of lake seiching and turbidite amalgamation/stacking. This will improve the recognition and interpretation of earthquake-induced megaturbidites in other lake records or isolated basins, and demonstrates the value of using (μ)CT scans in combination with traditional sedimentological parameters to reconstruct the depositional processes of megaturbidites