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Water-escape structure is considered to be a product of fluidization. The exact mechanism of formation of this structure has, however, not been adequately addressed. In the present study, laboratory experiments were carried out to observe the effect of cyclic loading on water-saturated sediment in a close system, with an objective to decipher the specific mechanism of formation of water-escape structure. Present study reveals that an optimum permeability contrast between the successive sedimentary layers of a fining upward succession is the prerequisite for formation of water-escape structures. The onset of fluidization comes into existence when at a critical minor principal stress condition during shearing, the underlying coarse-grained layer attains drained contractive state and the overlying finer-grained layer retains the undrained dilative state. In the contractive layer the sediments tend to become more compact through rearrangements of the particles and displacement of the interstitial fluid. The pore fluid is squeezed out and the sand structure suffers volume contraction. As the upwelling fluid is obstructed by the dilative layer, water-filled cavities are formed at the interface. On progressive shaking these water-filled cavities keep on enlarging and finally the expelled water ruptures through the overlying sand layer resulting water-escape structure. This study also depicts that no influx of external fluid is essential in fluidization of sediments, as was believed earlier. • Experimental study was carried out on the effect of cyclic loading on water-saturated sediment in a close system. • Adequate permeability contrast between successive layers is the prerequisite for formation of water-escape structures. • On shaking, the lower layer attains drained contractive state and the overlying one retains undrained dilative state. • In the underlying contractive layer, the pore fluid is squeezed out and the sand structure suffers volume contraction. • The upwelling fluid is obstructed by the overlying dilative layer. • Water-filled cavities are formed at the interface and keep on enlarging on progressive shaking. • The expelled water finally ruptures through the overlying sand layer resulting water-escape structure. [ABSTRACT FROM AUTHOR]

The recognition of seismically induced soft-sediment deformation structures (SSDS) in sedimentary successions characterized by different facies, and hence by different rheology, is challenging. This is the case for high porosity and high permeability aeolian facies interbedded with muddy wet interdune deposits and alluvial conglomerates and sandstones. Several types of SSDS have been studied in two exposures of the Upper Pliocene (2.9–2.6 Ma) sediments of a fault-bounded intracontinental aeolian dune field in the Teruel Basin (Iberian Chain, eastern Spain). Among SSDS, load and fluid-escape structures, apart from several animal tracks, have been recognized. Those structures show an irregular distribution through the studied stratigraphic sections, being scarce in homogenous aeolian sands and frequent in water-related facies. A detailed study of the distribution and geometry of SSDS and their relationships with respect to the stratigraphic architecture and facies has allowed a critical discrimination of trigger mechanisms, i.e. biological or physical overloading vs. earthquakes. The seismically induced structures are concentrated into seven deformed beds, showing an uneven lateral distribution and geometry closely controlled by the hosting sedimentary facies and their rheology. These seismites resulted from liquefaction during moderate earthquakes (estimated magnitude from 5.0 to 6.8). The most probable seismogenic source was the Sierra del Pobo normal fault zone, located 2 km to the East. Results show how an appropriate recognition of sedimentary facies is crucial to understand the lateral variability of seismites in sedimentary environments characterized by sharp facies changes. [ABSTRACT FROM AUTHOR]

A river terrace of the River Gauja (Latvia), built of Weichselian glaciolacustrine deposits, contains a large number of erratic boulders from the Fennoscandian Shield. These erratic boulders include several types of granites and granodiorites. Some of the granodiorites are so strongly weathered that they fall apart into mm-sized grains of individual minerals when it is attempted to take them out of the host sediment. This strongly weathered nature makes them physically comparable to unconsolidated sand. A consequence is that they may be subjected to soft-sediment deformation. The erratic granodiorite boulder under study here is the first described to show such a soft-sediment deformation structure inside: it has been intruded by a water/sediment mixture composed of the underlying silty/sandy material that must have been liquefied, possibly due to pressure exerted by the overburden, or – more likely – by the weight of the concentration of gravel of which the granodiorite boulder forms a part. The overpressurized water/sediment mixture tried to escape upwards through, among a few other places, the strongly weathered granodiorite, forming an escape structure. [ABSTRACT FROM AUTHOR]