THE ROLE OF LAKES IN MORAINE FORMATION, CHILEAN LAKE DISTRICT

The nested moraines of the Chilean Lake District have been used to establish a glacial chronology for southernmost South America. This paper focuses on non‐climatic controls which may have modulated the climatic signal. It presents a model for formation of the moraines around Lagos Puyehue and Rupanco where there is a nested complex dating from the last glaciation. These moraines can be divided into two types on the basis of their form, position and constituent materials: rampart moraines are broad amalgamated moraine complexes whilst ridge moraines are narrow single ridges usually located around lakeshores. Both types have lateral moraines with low up‐glacier longitudinal gradients. Sections in the moraines show they are largely composed of stratified glaciofluvial sediments overthrust on their proximal flanks by clay‐rich diamicts containing reworked glaciofluvial material. Despite their different characteristics, a single model explains the features of both moraine types and their location around the down‐glacier ends of the lakes. Moraine formation depends crucially on the presence of a layer of water‐saturated, fine, impermeable sediment in the lake basins which allows the glacier lobes to advance with negligible surface gradients, probably on a deforming bed. Although the formation of moraines requires a climatically triggered advance, their precise position is not dictated by climatic factors but by contrasts in sediment permeability and grain‐size.