Identifying soft-sediment deformation in rocks

The correct identification of ‘sedimentary’ folds and fabrics created during gravity-driven deformation of unlithified successions from those ‘tectonic’ structures formed during regional deformation is essential when interpreting geological histories preserved within the rock record. This topic has become increasingly relevant over the past 40 years as improved seismic resolution and coverage have led to the realisation that significant portions of unlithified sediments along the continental margins undergo gravity-driven deformation to create mass transport deposits (MTD's). The late-Pleistocene Lisan Formation, exposed in the Dead Sea Basin, was chosen as a case study because it remains poorly lithified, and structures developed within it are unequivocally related to ‘soft-sediment’ deformation (SSD) created when the succession underwent downslope-directed movement. This work tests various assertions previously used to deduce if structures were formed in unlithified sediments or during ‘hard-rock’ deformation (HRD) associated with subsequent tectonism. Within the Lisan Formation, we describe veins developed along fractures, and cleavage forming axial-planar to folds, that are structures previously assumed to be restricted to HRD. In addition, truncated folds, incorporation of deformed fragile fragments into overlying sediment, and cross-cutting clastic dykes are all indicative of SSD. The key diagnostic feature in establishing SSD is the sedimentary infill of irregular erosive surfaces that truncate underlying structures. Although compaction and diagenesis have not played a significant role in the case study, caution should be exercised when examining structures preserved in the rock record as folds and fabrics originally created by SSD may be considerably enhanced and altered where significant overburden exists.