Flood, Seismic or Volcanic Deposits? New Insights from X-Ray Computed Tomography

Wednesday, 17 December 2014: 1:40 PM
Maarten E Van Daele1, Jasper Moernaut2, Flor Vermassen1, Mateu Llurba1, Nore Praet1, Michael Markus Strupler3,4, Flavio Anselmetti5, Veerle Cnudde6, Peter J Haeussler7, Mario Pino2, Roberto Urrutia8 and Marc A O De Batist1, (1)Ghent University, Renard Centre of Marine Geology, Ghent, Belgium, (2)Universidad Austral de Chile, Escuela de Geología, Instituto de Ciencias Ambientales & Evolutivas, Valdivia, Chile, (3)Universiteit Gent, Gent, Belgium, (4)ETH Zurich, Geological Insitute, Sediment Dynamics, Zurich, Switzerland, (5)University of Bern, Bern, Switzerland, (6)Ghent University, Sedimentary Geology and Engineering Geology (SGIG)-UGCT, Department of Geology and Soil Science, Ghent, Belgium, (7)USGS Alaska Science Center, Anchorage, AK, United States, (8)Universidad de Concepcion, Concepcion, Chile
Event deposits, such as e.g. turbidites incorporated in marine or lacustrine sediment sequences, may be caused by a wide range of possible triggering processes: failure of underwater slopes – either spontaneous or in response to earthquake shaking, hyperpycnal flows and floods, volcanic processes, etc. Determining the exact triggering process remains, however, a major challenge. Especially when studying the event deposits on sediment cores, which typically have diameters of only a few cm, only a small spatial window is available to analyze diagnostic textural and facies characteristics. We have performed X-ray CT scans on sediment cores from Chilean, Alaskan and Swiss lakes. Even when using relatively low-resolution CT scans (0.6 mm voxel size), many sedimentary structures and fabrics that are not visible by eye, are revealed. For example, the CT scans allow to distinguish tephra layers that are deposited by fall-out, from those that reached the basin by river transport or mud flows and from tephra layers that have been reworked and re-deposited by turbidity currents. The 3D data generated by the CT scans also allow to examine relative orientations of sedimentary structures (e.g. convolute lamination) and fabrics (e.g. imbricated mud clasts), which can be used to reconstruct flow directions. Such relative flow directions allow to determine whether a deposit (e.g. a turbidite) had one or several source areas, the latter being typical for seismically triggered turbidites. When the sediment core can be oriented (e.g. using geomagnetic properties), absolute flow directions can be reconstructed. X-ray CT scanning, at different resolution, is thus becoming an increasingly important tool for discriminating the exact origin of EDs, as it can help determining whether e.g. an ash layer was deposited as fall out from an ash cloud or fluvially washed into the lake, or whether a turbidite was triggered by an earthquake or a flood.