H. W. van Gent

3D seismic study of complex intra-salt deformation: An example from the Upper Permian Zechstein 3 stringer, western Dutch offshore

Abstract 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.

Strain and Temperature an Space and Time

A basin may be called “complex” if its temporal evolution was complicated by several tectonic or thermal events such as stretching and inversion or if it is composed of a variety of regionally distributed sub-basins with differing histories. Aulacogens or typically inverted “failed rifts” are examples of the first type. The second type is frequently encountered in intraplate settings located above continental crust. This may be because the continental crust is rather inhomogeneous as it results from long-term accretion of terrains, manifested by different thicknesses and materials.