Abstract The detailed analysis of folding in rocks was in part pioneered by John Ramsay, and resulted in a range of techniques and criteria to define folds. Although folding of unlithified or ‘soft’ sediments is typically assumed to produce similar geometries to those in ‘hard rocks’, there has to date been little detailed analysis of such folds. The aim of this paper is therefore to investigate folds developed during soft-sediment deformation (SSD) by applying techniques established for the analysis of tectonic folds during hard-rock deformation (HRD). We use the Late Pleistocene Lisan Formation exposed around the Dead Sea as our case study, as the laminated lake sediments record intricacies of fold detail generated during seismically triggered slumping of mass transport deposits (MTDs) towards the depocentre of the basin. While it is frequently assumed that folds created during SSD are chaotic and form disharmonic structures, we provide analyses that show harmonic fold trains may form during slumping, although larger upright folds cannot be traced for significant distances and are more typically disharmonic. Our analysis also reveals a range of fold styles, with more competent detrital-rich layers displaying buckles (Class 1B), as well as upright Class 1A folds marked by thickened limbs. Class 1A buckle folds are generally considered to be created by flattening that overprints folds with an original Class 1B geometry. As thickened fold limbs are truncated by overlying erosive surfaces, the vertical flattening is considered to have occurred during the slump event. Different fold shapes may partially reflect variable flattening, depending on the original orientation of upright or recumbent folds, together with continued downslope-directed simple-shear deformation that modifies the fold geometry. Analysis of fold wavelength, amplitude and bed thickness allows us to plot strain contour maps, and indicates that beds defining slump folds display viscosity contrasts in the range of 50–250, which are similar to values estimated from folds created during HRD in metamorphic rocks. A range of refold patterns, similar to those established by John Ramsay in metamorphic rocks, are observed within slumps, and are truncated by the overlying sediments, indicating that they formed during a single progressive slump event rather than distinct ‘episodes’ of superimposed deformation. This study confirms that techniques developed for the analysis of folds created during HRD are equally applicable to those formed during SSD, and that resulting folds are generally indistinguishable from one another. Extreme caution should therefore be exercised when interpreting the origin of folds in the rock record where the palaeogeographical and tectonic contexts become increasingly uncertain, thereby leading to potential misidentification of folds created during SSD.

Abstract Up until the late 1980s geology contributed very little to the study of tsunamis because most were generated by earthquakes which were mainly the domain of seismologists. In 1987–88 however, sediments deposited as tsunamis flooded land were discovered. Subsequently they began to be widely used to identify prehistorical tsunami events, providing a longer-term record than previously available from historical accounts. The sediments offered an opportunity to better define tsunami frequency that could underpin improved risk assessment. When over 2200 people died from a catastrophic tsunami in Papua New Guinea (PNG) in 1998, and a submarine landslide was controversially proven to be the mechanism, marine geologists provided the leadership that led to the identification of this previously unrecognized danger. The catastrophic tsunami in the Indian Ocean in 2004 confirmed the critical importance of sedimentological research in understanding tsunamis. In 2011, the Japan earthquake and tsunami further confirmed the importance of both sediments in tsunami hazard mitigation and the dangers from seabed sediment failures in tsunami generation. Here we recount the history of geological involvement in tsunami science and its importance in advancing understanding of the extent, magnitude and nature of the hazard from tsunamis.

Soft-sediment deformation structures crop out in the Lower Cretaceous succession of the Gubbio anticline in the Umbria-Marche Apennines of Italy. The deformation interval is ~13 m thick and occurs between the upper Hauterivian–lower Aptian Maiolica Formation and the Aptian Marne a Fucoidi Formation. It can be observed along the anticline for a distance of 12 km. Different types of deformation structures are distributed in several outcrops, with detachment extensional structures prevailing in the southeast sector. Imbricated slides, slump structures, and chaotic layers are distributed vertically and longitudinally in the middle and/or lower part of the deformed sediments. In the northwest sector of the anticline, compressional duplex structures can be considered the lower section of a large sediment failure. Geometrical and kinematic analysis of the fold axis trends and sliding surfaces have led to infer a single, large gravitational event possibly Albian in age. The synsedimentary deformation could be activated by several internal trigger mechanisms induced by external regional tectonic events such as earthquakes. An orthogonal system of calcite veins crossing the limestone layers represents the primary pathway for fluid-driven breaching of joint seals. These fluids can be related to the significant increase in the total organic carbon in the Hauterivian–Aptian layer of the Maiolica and Marne a Fucoidi Formations. This suggests the possibility that the limestone layer, sandwiched and sealed between clay of the organic-rich black shales, could have favored a pore pressure increase approaching lithostatic stress. With a thin overburden, lithostatic stress is more easily reached at low hydrostatic pressure. This slump sheet occurrence suggests the existence of a local paleoslope dipping toward the north-northwest, where the mass involved in the deformation is distributed over an estimated area of 60 km 2 for a volume of 0.8 km 3 of displaced sediments. The restoration and rotation of the slump fold hinges to the Early Cretaceous direction, in line with available paleomagnetic data, have shown that the strike of the slope corresponds to the main trend of the oldest Jurassic extensional lineaments and is linked to transform faults of the westernmost Tethys rifting systems.