3D SOFT-SEDIMENT DEFORMATION STRUCTURES - EVIDENCE FOR QUATERNARY SEISMICITY IN THE MADRID BASIN, SPAIN

Three-dimensional patterns of soft-sediment deformation, affecting mid dle Pleistocene fluvial deposits in the Lower Manzanares Valley (SW Ma drid City), are reported. They promote a small-scale hummocky topograp hy, defined by different-sized domes, mud volcanoes, wavy ridges and f issures on an ancient flood plain surface. Related channel fill deposi ts are pervasively dewatered, indicating liquefaction. The sedimentolo gical context reveals that the deformation of both floodplain surface and channel-fill deposits, were contemporaneous (singular deformed lev el) and generated at, or near, the ground surface; this rules out the possibility of sediment-loading and indicates a possible earthquake-in duced origin.