Lateral transport of suevite inferred from 3D shape-fabric analysis; evidence from the Ries impact crater, Germany

The transport mechanism of suevite particles during impact cratering is poorly understood and was studied at the 15 Ma Ries crater in southern Germany. Two emplacement modes of suevite deposits are generally discussed: (1) fallback of plume material into the crater and its periphery upon collapse of an ejecta plume and (2) horizontal transport of ejected material, akin to emplacement of pyroclastic deposits erupting from volcanic centers. In order to differentiate between the two emplacement modes of suevite deposition, we analyzed the shape fabrics of suevite components from two localities outside the Ries crater by fitting shape-fabric ellipsoids to measured shape-fabric ellipses and by applying high-resolution, X-ray-computed tomography to analyze the three-dimensional shape and orientation of the suevite particles. We show that the preferred orientation of long axes of elongated particles is disposed either radially or concentrically with respect to the crater center. Our observations indicate that suevite material was not only derived from an ejecta plume but was transported by lateral flow under viscous conditions upon fallback. This flow regime resembles that known from pyroclastic flows.