Pauline Cornard, Gerald Degenhart, Peter Tropper, Jasper Moernaut, Michael Strasser

A new application of micro-CT on consolidated sediment-gravity flow deposits is presented with the objectives to characterize its internal sedimentary three-dimensional (3D) structures. The proposed concept is applied to three types of deep-marine sandstones, sampled in study areas showing different compositional properties: the Upper Cretaceous Gosau Group (Austria), the Eocene Hecho Group (Spain) and the Oligocene Annot Formation (France). From micro-CT data, the particle-size distribution is reconstructed in 3D, permitting a better visualization of the sedimentary structures showing distinct density contrast (grains versus matrix), most of the time, non-visible in 2D. Particle-size distribution show similar trend than grain-size computed from thin-section image analysis, testifying of the reliability of the micro-CT computed particle-size distribution. Because of the limitation of micro-CT to distinguish mineral phases, micro-CT data are complemented by micro-XRF and thin-section petrographic analysis. By separating particles based on their CT-density, it is possible to isolate the coarsest fraction underlying the sedimentary structures from the fine-grained matrix. Although some sedimentary structures do not appear visible from micro-XRF or thin-section analysis, due to too homogeneous mineral composition or grain size in the original flow or similar composition between matrix and grains, micro-CT techniques highlight internal sedimentary structures by isolating the coarsest fraction. This study brings into light (i) the potential of micro-CT and particle-size distribution in the analyses of sedimentary structures from outcrop data and related interpreted processes (sediment transport) and (ii) the importance to consider the mineralogical composition and the degree of grain sorting before to reach any interpretation of the processes at the origin of structureless deposits. Considering the importance of the sedimentary-structure visualization towards a reliable interpretation of processes at the origin of the deposits, micro-CT technique is a new and reliable tool to assess the physical properties of consolidated sandstones and read their internal sedimentary 3D structures.