Clastic sedimentation processes in lakes of Northern Patagonia: implications for paleoclimatic and paleoearthquake reconstructions

EMMANUEL CHAPRON; DANIEL ARIZTEGUI; SANDOR MULSOW; GUSTAVO VILLAROSA; MARIO PINO; VALERIA OUTES; O. MAGAND; ETIENNE JUVIGNIE

San Carlos de Bariloche, Río Negro, Argentina

Congreso; IV Congreso Latinoamericano de Sedimentología y XI Reunión Argentina de Sedimentología.; 2006

ASOCIACIÓN ARGENTINA DE SEDIMENTOLOGÍA

The recent sedimentation processes in contrasting lake basins at both sides of the Andes in Northern Patagonia are documented by high-resolution seismic reflection profiling (3.5 kHz pinger) and short gravity cores at selected sites. In Lago Puyehue (40.5 °S, Chile) the correlation of seismic data and multidisciplinary analysis of sediment cores from two coring sites (PU-I and PU-II) allow recognizing two main sedimentary environments in this oligotrophic lake: (i) a proximal one including the delta of the main tributary, a deep flat basin with a ponded geometry and limited acoustic penetration and (ii) a distal one composed of several sub-basins with a draping pattern and a better acoustic penetration. Based on these seismic facies and sediment cores well-dated by radionuclides and the identification of the 1960 Chilean earthquake (Mw 9.5) and of tephra layers (Rininahue 1907, Cordon Caulle 1921-22 and 1960), we argue that clastic sedimentation results from the development of homopycnal flows at the end of the winter season in this monomictic lake. While distal clastic environments (PU-II site) are dominated by biogenic production and appear to have been little affected by the 1960 Chilean earthquake and nearby volcanoes, this may not be the case for more proximal environments (PU-I site) which also experience sporadic hyperpycnal flows during major flood events, such as the one induced in 1960 after the earthquakes and the induced Puyehue-Cordon-Caulle eruption. At the Argentinean side of the Andes at 41°S, similar available acoustic facies are recognized in large and small lakes and suggest similar sedimentation processes. The distal basin of Lago Nahuel Huapi offshore Bariloche is characterized by a good seismic penetration, a draping pattern and a large recent mass wasting deposit (MWD) producing a megaturbidite in the deep basin. The proximal basin of proglacial Lago Frias facing the Rio Frias delta has a limited acoustic penetration (ca. 3 m) and shows a ponded geometry affected by frequent MWDs. The regional correlation of short cores is provided by 137Cs dating in lakes Puyehue and Frias and by the identification of Cordon Caulle 1921-22 and 1960 tephras in lakes Puyehue and Nahuel Huapi and in their catchment areas. This event stratigraphy allows correlation of the formation of striking sedimentary events in these basins with the consequences of the May-June 1960 earthquakes and the induced Cordon Caulle eruption along the Liquiñe-Ofqui Fault Zone: a mega hyperpycnal flood deposit in Lago Puyehue, a large MWD and a megaturbidite in Lago Nahuel Huapi but only a thin unusual organic rich layer in Lago Frias. 137Cs dating in lakes Puyehue and Frias and by the identification of Cordon Caulle 1921-22 and 1960 tephras in lakes Puyehue and Nahuel Huapi and in their catchment areas. This event stratigraphy allows correlation of the formation of striking sedimentary events in these basins with the consequences of the May-June 1960 earthquakes and the induced Cordon Caulle eruption along the Liquiñe-Ofqui Fault Zone: a mega hyperpycnal flood deposit in Lago Puyehue, a large MWD and a megaturbidite in Lago Nahuel Huapi but only a thin unusual organic rich layer in Lago Frias.