Abstract

Clastic sedimentation processes in Lake Puyehue (40.5°S; 72.5°W, Chile), are described based on correlation of high-resolution seismic profiles and multidisciplinary analysis of short sediment cores from two contrasting coring sites (PU-I and PU-II). Two main sedimentary environments are recognized in this oligotrophic lake: (i) a proximal one, which includes the delta of the main tributary (the Golgol River) and is a deep flat basin with a ponded geometry and limited acoustic penetration, and (ii) a distal one that is composed of several sub-basins highlighting a drape geometry and better acoustic penetration. Based on the correlation of seismic data and sediment cores dated by radionuclides and the identification of tephra layers (Rininahue1907, Cordon Caulle 1921-22 and 1960), we argue that clastic sedimentation results essentially from the development of homopycnal flows at the end of the winter season in this monomictic lake. Whereas distal clastic environments (PU-II coring site) are dominated by biogenic production and appear to have been little affected by the 1960 Chilean earthquake (Mw 9.5) and nearby volcanic eruptions, this may not be the case for more proximal clastic environments (PU-I coring site) which also experience sporadic hyperpycnal flows during major flood events. In May-June 1960, for example, as several earthquake-triggered landslides dammed the course of the Golgol River and ca. 7 × 106 m3 of volcaniclastic sediments fell over the drainage basin of the lake during the earthquake-induced Puyehue-Cordon-Caulle eruption, successive outbursts in the Golgol valley induced a mega-flood in Lake Puyehue. This hyperpycnal flow reworked a mixture of volcaniclastic and regolith-derived sediments from the catchment, as well as lacustrine sediments surrounding the Golgol delta (during the rising limb of the flood). These recently reworked sediments are strikingly enriched in excess 210Pb activity suggesting that positive anomalies in the activity of this isotope can be used to determine the source area of clastic sediments. © 2007 Elsevier B.V. All rights reserved.