Glacially folded outwash near lago llanquihue, southern lake district, chile

Abstract

Folded outwash occurs in four distinct clusters in an arcuate arrangement just west of the terminal Llanquihue moraines deposited by the Lago Llanquihue piedmont ice lobe at the last glacial maximum. These clusters are physically connected along the eastern side to the Llanquihue terminal moraines, and along the western side to the Llanquihue outwash plain. Each cluster consists of three to eleven elongated ridges. The maximum height of individual ridges varies from cluster to cluster beween 18 and 28 m; the maximum length of individual ridges is between 93 and 1074 m. The average orientation of the ridges ranges over a 60° sector relative to former ice‐flow direction. The folded out‐wash sediments are cut by two distinct internal fault systems with only a faint surface expression below the Holocene top soil.The folded outwash ridges are interpreted as a push moraine system produced by the same mechanical forces that act in a critically tapered wedge. The folded sediment is a sandy gravel with an angle of friction on the order of ϕ = 40°. Interpretations of structural data and of mechanical comparisons point to a basal thrust plane in a sand unit with ϕ between 24° and 30° and with a pore water pressure index of l = 0.7.It is very unlikely that the observed and analyzed features wereformed under permafrost conditions.