The influence of overpressure and focused fluid flow on subaquatic slope stability in a formerly glaciated basin: Lake Villarrica (South-Central Chile)

Abstract

Pore fluid pressure is a key parameter for subaquatic slope stability and has been put forward to explain the development of many submarine landslides, especially when occurring on very gentle slopes or in areas with high sedimentation rate. Due to the sparse availability of in-situ pore pressure data, a profound understanding and quantification of excess pore pressure development and its influence on subaquatic landslide processes is still missing. In this study, we use glacigenic Lake Villarrica (Chile) as a model basin, in which we document in-situ excess pore pressure, focused fluid escape features and subaquatic slope failures at relatively shallow subsurface depth (<10m). A subaquatic slope was characterized in great detail by a dense network of high-resolution seismic-reflection profiles, multibeam bathymetry and sediment cores. In-situ undrained shear strength and formation pore fluid pressures were documented using free-fall piezocone penetrometer (CPTU) and subsequent pore pressure dissipation tests. We show four independent lines of evidence for overpressure of variable magnitude within the sedimentary slopes: i) pockmarks and other fluid escape features on seismic profiles and bathymetric maps, ii) estimation of ambient in-situ pore pressure via CPTU dissipation tests, iii) underconsolidation and downward decrease in in-situ undrained shear strength within units of uniform lithology, and iv) hydrofractured glacio-lacustrine sediments. Locally, overpressure reaches about 80–90% of the hydrostatic vertical effective stress. We hypothesize that glacier-proximal sediments are strongly underconsolidated and overpressured due to rapid deposition and/or subglacial meltwater pumping. Postglacial consolidation of a thick glacier-proximal unit leads to the upward expulsion of excess pore water. A permeability barrier is formed by the overlying glacio-lacustrine clays and fine silts, focusing these migrating fluids to topographic highs. There, very high overpressure ratios facilitate the development of hydro-fractures, repeated fluid escape and pockmarks on the lake bottom. Overpressure may decrease slope stability in Lake Villarrica in different manners: by focused fluid escape weakening sediments, by generating underconsolidated sequences of low undrained shear strength, and by facilitating static failure. Based on high-quality in-situ data, this study confirms that high overpressure ratios in shallow sediments may be a typical feature at formerly-glaciated marine and lacustrine environments and highlights the major role of pore fluid overpressure for preconditioning subaquatic slopes to failure.