23-25 September 2013
GEOMAR East shore
Europe/Berlin timezone
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Earthquake-triggered onshore mass movements cause widespread deformation of basin-plain sediments and multidirectional, stacked turbidites and debrites (Aysén fjord, Chilean Patagonia)

Presented by Dr. Maarten VAN DAELE on 23 Sep 2013 from 17:15 to 19:15
Type: Poster presentation
Session: Poster session
Track: Poster presentations

Content

The 2007 Mw 6.2 earthquake in Aysén fjord caused not only onshore mass movements, but also widespread basin-plain deformation and density-flows. Deformed basin-plain deposits are mapped using multibeam bathymetry and seismic-reflection (sparker) profiling. To better understand the facies successions in, and the intercalation of, the (distal) density-flow deposits, we also studied the 2007 deposits in 22 short sediment cores by combining grain-size analysis with X-ray computed tomography (CT) scanning. Deformed basin-plain deposits can be divided in frontally emergent and confined deposits, with both a deep and shallow basal shear surface. All deformed basin-plain deposits with a deep basal shear surface are induced by the weight and impact of a slope-adjacent mass-flow wedge. The frontally emergent –most mobile– basin-plain deformation is triggered by mass flows originating from onshore mass movements (i.e. debris flows, rock slides and avalanches) propagating into the fjord. The encountered facies in the sediment cores correspond to classical divisions of debrites and turbidites. The deposits consist of a succession of several sub-deposits deposited under different flow directions and can be interpreted as stacked turbidites. We used orientations of i) folds, ii) imbricated mud clasts, iii) back- and foresets of climbing ripples and iv) asymmetric convolute lamination, to determine relative flow directions at the location of the cores. By assigning the basal flow of the stacked debrites and turbidites to the closest principal mass-flow, the absolute flow directions of the sub-deposits were determined, which, in combination with multibeam basin-floor morphology, allowed reconstruction of the 2007 density-flow successions in Aysén fjord. We conclude that X-ray CT scans provide crucial information for reconstructing paleoflows and can be a useful tool in marine and lacustrine sedimentology and paleoseismology. The multidirectionality of sub-deposits in turbidites is, next to differences in mineralogy, a new criterion to identify stacked turbidites. These multidirectional, stacked turbidites are an indication of simultaneous triggering of density flows and can therefore in most cases be attributed to earthquakes, ruling out other triggers, such as floods, storms or other sediment failures. Similar event deposits (with both basin-plain deformation and distal turbidites) are encountered in the sedimentary infill of the fjord. We hypothesize that during the Holocene three to four similar events have struck Aysén fjord.

Place

Location: GEOMAR East shore
Address: Wischhofstr. 1-3 / D-24148 Kiel
Room: Lithothek

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