Laminar flow modelling of initial salt deformation

Abstract

Although salt structures can assume complex geometries, their gross regional distribution in a salt province is often observed to follow a more simple and regular pattern that can be mathematically described in terms of an initial horizontal salt flow caused by loading variations in the overburden. Analytical studies and scaled physical models have been carried out to investigate the initiation of salt deformation driven by differential load gradients. The theory of laminar flow within a uniform horizontal layer of salt subjected to three load gradients is here developed using basic hydrodynamic principles. The exertions of a linear load gradient and a 2-D load gradients are seen to initiate a sinusoidal deformation of the salt while the exertion of a 3-D radial negative load gradient induces a series of circular salt swells with amplitudes decaying away from the location of minimum load. Scaled physical models were built to investigate the initial deformations of a viscous horizontal layer subjected to the three differential loading regimes, one representing a prograding geological template, a hemispherical basin and a negative radial load gradient. Dry, fine quartz sand was used to simulate the overburden while viscous asphalt at room temperature simulated the salt. All models were carried out in glass or perspex walled containers to enable the visual observation of deformations. The deformations of the top layer of asphalt were observed to match the predictions of the analytical relations to a remarkable degree. For a given load gradient, the shape, magnitude and wavelength of these salt deformations are seen to depend on the density of the overburden, viscosity and thickness of the salt, and on the time needed for the deformation to develop. These basic analytical considerations are successfully used to explain the spatial distribution of salt structures observed in nature. The linearity, trend and spacing of a set of parallel salt walls found in Northwest Germany and a set of orthogonal salt ridges in Northwest Libya are also explained by linear load gradients generated by progradational overburdens.