Fluidization of Buried Mass-Wasting Deposits in Lake Sediments And
Total Page：16
File Type：pdf, Size：1020Kb
Sedimentary Geology 213 (2009) 121-135 Contents lists available at ScienceDirect Sedimentary Geology Sedimentary Geology ELSEVIER journal homepage: www.elsevier.com /locate/sedgeo Fluidization of buried mass-wasting deposits in lake sediments and its relevance for paleoseismology: Results from a reflection seismic study of lakes Villarrica and Calafquén (South-Central Chile) Jasper Moernauta *, Mare De Batista, Katrien Heirman a, Maarten Van Daele a, Mario Pino b, Robert Brümmer b, Roberto Urrutiac a Renard Centre of Marine Geology (RCMG), Ghent University, Krijgslaan 281 (S8), 9000 Ghent, Belgium b Instituto de Geociencias, Universidad Austral de Chile, Casilla 567, Valdivia, Chile c Centro EULA, Universidad de Concepción, Casilla ISO-C, Concepción, Chile ARTICLE INFO ABSTRACT Article history: A dense grid of very-high resolution seismic profiles on Lake Villarrica provides a quasi-3D view on Received 5 May 2008 intercalated lenses of low-amplitude reflections, which are connected by acoustic wipe-out patches and Received in revised form 2 December 2008 fractures to an underlying voluminous mass-wasting deposit. The lenses are interpreted as being created by Accepted 12 December 2008 earthquake-triggered liquefaction in this buried mass-wasting deposit and subsequent sediment fluidization and extrusion at the paleo-lake bottom. These sediment volcanoes are mapped in detail. They have a rather Keywords: uniform circular geometry and show a linear relationship between apparent width and maximum thickness Fluidization structures Lake sediments on a seismic section. The largest sediment volcanoes are up to 80 m wide and 1.9 m thick. Their slope angles Reflection seismic profiling designate a syn- to post-depositional sagging of most sediment volcanoes. Sediment volcano detection and South-Central Chile mapping from nearby Lake Calafquén further strengthen the revealed geometrical relationships. Locally, Paleoseismology some of the sediment/fluid escape structures extend to a higher position in the stratigraphy, which points to a polyphase escape process associated with multiple multi-century spaced strong earthquakes. Thickness and morphology of the source layer seem to exert a dominant control in the production of sediment/fluid extrusions. This study shows that reflection seismic profiling allowed recognizing 4 different seismic events in the studied stratigraphie interval, which are evidenced by mass-wasting deposits and/or fluidization features. © 2008 Elsevier B.V. All rights reserved. 1. Introduction outcrop availability often hamper the possibilities of studying the spatial and geometrical aspects o f this sediment extrusion process in Fluidization structures, such as sand blows, are a common effect of more detail. However, successful attempts have been made for severe seismic shaking in continental settings and their use in visualizing sand-blow craters, vents and source layers using ground- paleoseismological research is widely acknowledged (Audemard and penetrating radar (GPR) techniques (Liu and Li, 2001 ; Al-Shukri et al., De Santis, 1991 ; Obermeier, 1996; Hibsch et al., 1997 ; Green et al., 2004; 2006; Maurya et al., 2006). Obermeier et al., 2005; Mörner, 2005; Castilla and Audemard, 2007). Lacustrine sedimentary sequences are generally uniform over The processes behind liquefaction/fluidization of soft sediments are longer distances than those in continental settings. Fluidization studied w ith laboratory experiments, by means of which various kinds structures in lacustrine deposits are, therefore, often used for of soft-sediment deformation structures have been produced (Nichols paleoseismic analyses (e.g. Beck et al., 1996; Rodríguez-Pascua et al., et al., 1994; Owen, 1996; M oretii et al., 1999 and references therein). 2000; Singh and Jain, 2007; etc.). However, point observations (cores) Comprehensive mapping and measuring of fluidization structures in or 2D logging of available lacustrine outcrops often still remain the field allowed to define empirical relationships between quantita insufficient to accurately study the geometry and spatial distribution tive earthquake parameters, such as magnitude and/or epicenter of earthquake-induced sediment blows. Reflection seismic techniques location, and sand-blow dimensions (Castilla and Audemard, 2007 and of appropriately high resolution can be applied in order to overcome references therein). Unfortunately, erosional processes, strong lateral these typical limitations and to provide a (pseudo) 3D image of the variability of sediment layers, anthropogenic disturbance and limited fluidization features. Even though most lacustrine paleoseismological studies using reflection seismics focus mainly on the recognition of basin-wide * Corresponding author. Tel: +32 9 2644637; fax: +32 9 2644967. slope-failure events (Schnellmann et al., 2002; Karlin et al., 2004; E-mail address: [email protected] (J. Moemaut). Strasser et al., 2006), some examples o f liquefaction-related structures 0037-0738/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.sedgeo.2008.12.002 122 ]. Moemaut et al. / Sedimentary Geology 213 (2009) 121-135 have also been identified on high-resolution seismic records. In the present study, we will illustrate the application of closely- Structures caused by vertical sediment mobilization with minimal spaced very-high-resolution reflection seismic profiles for the analysis horizontal displacement have been interpreted in various lacustrine of fluidization structures in the lacustrine infill of Lake Villarrica in sedimentary records, and have in most cases been attributed to a South-Central Chile. We w ill further discuss and document the origin, seismic origin (Shilts and Clague, 1992; Shilts et al., 1992; Hofmann geometry and spatial distribution of these structures as well as their et al., 2006). Diapir-like structures, which were possibly earthquake- paleoseismological implications, and we will compare them with triggered, have also been observed on subbottom echosounder similar structures in the subsurface o f nearby Lake Calafquén. profiles (Clague et al., 1989). In Lake Puyehue (South-Central Chile, 40.7°S), Moernaut et al. (2007) interpreted vertical fluid-escape 2. Study area structures, which were attributed to the dewatering of a buried mass-wasting deposit. Temporal coincidence between the top of these Lake Villarrica and Lake Calafquén (resp. 39.25°S; 214 masi and structures and basin-wide mass-wasting deposits points towards a 39.55°S; 204 masi) are large glacigenic lakes (ca. 20x10/5 km) common genesis, most likely a strong earthquake. It has not been located at the foot of the Cordillera de Ios Andes. The lake basins established yet whether such indications of fluid-escape events in originated from glacial valley overdeepening and the formation of lacustrine sedimentary records can effectively be used as trustworthy large frontal moraine ridges during the Late Quaternary glaciations paleoseismological tools. (Laugenie, 1982). The landscape around the lakes is dominated by ^Trancura River É^Lake Villarrica Villarrica Pu con Villarrica Volcano Lahar pathway Lake Pullinque Main river Lake Pellaifa Moraine Fig. 1. Morphological setting of Lake Villarrica and Lake Calafquén. Lake surrounding topography derived from SRTM data. Slope shader illumination from the North. Moraine locations (“Llanquihue” moraine belt) based on Laugenie (1982). Lahar pathways derived from Laugenie (1982) and satellite pictures. Lake Villarrica: Bathymetric contours every 10 m. Deepest part: 167 m below lake level (based on SHOA, 1987). Lake Calafquén: Bathymetric contours every 20 m, derived from Volland et al. (2007) combined with our seismic dataset. Deepest part: 212 m below lake level. Inset figure: location of the study area in South-Central Chile. ]. Moernaut et al. / Sedimentary Geology 213 (2009) 121-135 123 10km Lake Villarrica 10km 0________ 1 2km Calculated bathymetry SW part of Lake Villarrica Pinger survey lines Sparker survey lines Lake Calafquén Fig. 2. A) Seismic survey lines on Lake Villarrica. B) Seismic survey lines on Lake Calafquén. C) Bathymetry for the SW part of Lake Villarrica calculated from reflection seismic data. Isobaths every 5 m. Villarrica Volcano, one of South America's most active volcanoes has a surface of about 2650 km2 and comprises Villarrica Volcano (W itter et al., 2004). Destructive lahars have frequently occurred (2847 masi), Quetrupillán Volcano (2360 masi) and Sollipulli during the past decades and sometimes propagated into the eastern Volcano (2282 masi). Its main tributary is the Trancura River, the part of the lakes. The western part of Lake Villarrica is morpholo course and alluvial plain of which have been strongly influenced by gically protected from laharic inflow by the presence of the moraine lava flows and lahars from the Villarrica Volcano. Toltén River, which ridges. Morphologically, Lake Villarrica consists of a single, deep cross-cuts the moraine ridges, constitutes the outflow of the lake central basin (up to 167 m depth), and a shallower area with more towards the Pacific Ocean. morphological variability in the SW part of the lake (Figs. 1 and 2C). A The landscape of South-Central Chile is strongly impacted by small rocky island (Allaquillén Island) marks the transition from the recurrent, large megathrust earthquakes, such as the giant AD shallower area to the central basin. Lake Villarrica's catchment area 1960 Valdivia earthquake (Mw: 9.5), which originate at the interface