Sinking of a fragmented anhydrite layer in rock salt

Abstract

Abstract Two-dimensional numerical models are used to study the gravity-driven sinking of a dense boudinaged anhydrite layer within rock salt. We analyze models with an infinite array of regularly spaced blocks, two- and three-block clusters, and irregularly spaced blocks. We show that the mass of the blocks, their shapes, and spatial arrangement are the dominating factors that control sinking velocity. Variation of the block separation distance can modify the velocity by many orders of magnitude. For the model with the regularly spaced cuboidal blocks, we derive a new approximate formula for their terminal velocity. The solution works best, when the block separation distance to block width is smaller than 1 and the block height to block separation distance is larger than 1. The horizontally aligned and regularly spaced blocks sink vertically with a constant velocity. For clustered blocks, the velocity depends on the number of blocks in the cluster and their arrangement and it is larger than for the case of the regularly aligned blocks. The blocks can exhibit swinging or rotational motions and can change their spatial arrangement, which in turn affects their sinking velocity. Additionally, we show examples of complex structures that develop during block sinking through a stratified salt in the case of two-block clusters. In the case of an irregularly spaced array, the blocks sink faster (up to few orders of magnitude) than in the regularly spaced arrangement and the mean sinking velocity generally increases with the increasing irregularity of the blocks positions. The blocks often tend to cluster initiating a locally fast downward flow that generates backflow, which can lead to the upward motion of non-clustered blocks. The irregular spacing differentiates sinking velocities of individual blocks and promotes their vertical separation distance. We show that the mechanical interaction between the blocks can explain low sinking rates of anhydrite blocks observed in Zechstein basin.