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Most of the information on subsurface evaporitic structures comes from 3D seismic data. However, this data only provide limited information about the internal structure of the evaporites, which is known from salt mines and salt diapir outcrops. Brittle intra-salt layers (carbonate, anhydrite, clay) of at least 10 m thickness form good reflectors in evaporites, but the structure and dynamics of such ‘stringers’ in the salt movement are poorly understood. In this study, we investigate the intra-salt Zechstein 3 (Z3) stringer from 3D seismic data in an area offshore the Netherlands. Observations show complex deformation including boudinage, folding and stacking. Reflections from thin and steep stringer parts are strongly reduced, and we present different structural models and tests of these. We compare our observations to structural models from salt mines and analogue/numerical models of intra-salt deformation. A smoothed representation of the upper surface of the stringer fragments follows the shape of Top Salt, but smaller-scale stringer geometries strongly differ from this and imply boudinage. The imaged disharmonic patterns of constrictional folds provide evidence for the complexity of the intra-salt, in agreement with observations in salt mines. This may be explained by interaction of the layered salt rheology, complex three-dimensional salt flow, different phases and styles of basement tectonics and movement of the overburden.

Classically, the North-Sea Chalk is assumed to have deposited under quiet, homogeneous pelagic conditions with local re-deposition in slumps and slides. However, recently the observation of highly discontinuous reflection patterns on 2D and 3D seismic data from the NW European Chalk Group initiated a revision of some general ideas of chalk deposition, assuming that long-lived, contour-parallel bottom currents exert a primary influence on the development of intra-chalk channels, drifts and mounds. In this study, an alternative explanation is suggested for the formation of many of the significant intra-chalk seismic and stratal discontinuities by interpreting these as being caused by gravity-driven processes developing in response to intense syndepositional tectonics. Submarine mass-transport systems identified in the study area include large-scale slumps, slides, debris flows and turbidites. The latter occur in sinuous channel systems flanked by large master overbanks, with the channel fill exhibiting well-developed secondary banks and overbanks on the respective outer bends of the inner channel thalweg. This first documentation of channelised density-flow deposits in the North-Sea Chalk has important consequences for the interpretation and prediction of re-deposited chalk units, emphasising at the same time the strength of vigorous 3D seismic discontinuity detection for subsurface sedimentary-systems analysis.