The structural geology of boulby (potash) mine, Cleveland, United Kingdom

Abstract

This is the first description of the structural geology of the Upper Permian potash and salt of the third Zechstein Cycle exposed in Boulby Mine. The near seam lithologies are described briefly and the associated veins, vein networks, shear zones and slides are introduced. It is argued that the Boulby Potash migrated from lower levels to those at which it is now found to fill fractures which developed in a grey anhydrite shale produced by the reduction and wetting of the lower parts of the Carnallitic (Rotten) Marl. Continued introduction of chlorides of Na and K into vein networks dispersed the shale as clasts of ever decreasing size in a sub-horizontal zone of gneissose sylvinite which, when it reached a significant thickness, began to flow in an extraordinary manner. Hundreds of asymmetric sub-horizontal lobes of gneissose sylvinite root to and repeat the first formed layer in structures which developed to various degrees in different parts of the mine. These are shown to represent small-scale movement cells (e.g. 15 × 30 to 100m in axial section) in which the movements integrate within a pattern of larger-scale movement cells ( 15 × 200 to 400 m). Where the sylvinite was thickest these movement cells progressed to a stage which involved the complete circulation of a compound layer of sylvinite and grey shale. Continued circulation almost homogenised this compound layer to sylvinite in which the grey shale has been disrupted and dispersed to small aggregates and individual clay particles included within the chloride grains. The circulation cells in the sylvinite are rectilinear rolls close to where thin but extensive sub-horizontal lobes of Boulby Halite moved through and over them late in the structural history. Away from such salt lobes, the small-scale circulation cells are concentric within the boundaries of larger cells which have polygonal planforms. After rejecting explanations invoking near surface slumping for the small-scale circulation cells it is argued that they developed near their maximum depth of burial in late Jurassic or Cretaceous times. The multiwavelength circulation pattern in the Boulby Potash is interpreted as due to thermal convection influenced by primary gravitational instabilities on a larger scale. The sub-horizontal lobes of Boulby Halite are assigned to a final Laramide transcurrent reactivation of faults inherited from the basement during earlier phases of movement.