Recognising surface versus sub-surface deformation of soft-sediments: Consequences and considerations for palaeoseismic studies

Soft-sediment deformation structures associated with slumps and mass transport deposits (MTDs) are generally considered to form at the surface when unlithified sediment moves downslope under the influence of gravity. Where stratigraphic sequences contain several deformed horizons, the question arises as to whether repeated slope failure at the sediment surface has systematically built-up multiple MTDs in the stratigraphic record in a ‘sequential failure model’. Alternatively, a single failure event may concurrently create surficial and sub-surface deformed ‘intrastratal’ horizons at different stratigraphic levels in a ‘synchronous failure model’. The implications of these differing models are important as sub-surface deformation can be significantly younger than the depositional age of beds it affects thereby weakening age-depth correlations used to estimate the timing of palaeo-earthquakes. In order to investigate the potential for sub-surface deformation, we examine the late Pleistocene Lisan Formation exposed around the Dead Sea Basin that contains numerous MTDs and gravity-driven fold and thrust systems. Surficial deformation is recognised by identifying irregular erosive surfaces above MTDs that are overlain by sedimentary caps deposited out of suspension following the failure event. Such surficial deformation is also characterised by thickened sedimentary successions that create ‘growth’ sequences. Conversely, sub-surface intrastratal deformation is typified by detachment-bound folds and thrusts that are marked by repetitions of stratigraphy across the upper detachment surface, fluidised sediment that intrudes upwards into the overlying sequence, together with abrupt truncations of older faults developed in overburden above the detachment. MTDs created at the surface form relatively competent horizons when subsequently buried as they are internally disrupted and lack ‘layer-cake’ geometries, while repeated seismicity can lead to dewatering and compaction resulting in ‘seismic strengthening’. Later sub-surface deformation may therefore be focussed adjacent to earlier MTDs that influence the mechanical stratigraphy, leading to secondary failures and complications when attempting to ‘balance’ extension and contraction that may be of different ages. Sub-surface deformation is localised along discrete detachments that carry the overlying sequence downslope as relatively intact slides, affecting what appear to be ‘undeformed’ beds between individual MTDs. As sub-surface deformation does not directly correlate with sedimentary caps, the rates of movement on deeper detachments remain unconstrained and may be significantly slower than surficial deformation resulting in downslope creep of the sediment pile.