The structures in the Jurassic and Cretaceous Rocks on the Dorset Coast between White Nothe and Mupe Bay

Abstract

The structures associated with the steep, northward inclined Portland and Purbeck Beds, and the nearly vertical or overturned Wealden Beds and Chalk, exposed on the Dorset coast between White Nothe and Mupe Bay, form the subject of an important paper by W. J. Arkell (1938). Arkell thought that the structures occurred on the steep north limb of a major monocline which was formed by lateral pressures from the south. In the present paper the minor structures in the Purbeck Beds are described in detail and it is shown that they can be related to the development of a fold formed by the increasing northward dip of the underlying massive Portland Beds. The direction of overturning of the minor folds indicates movement of the beds down the dip slope and not upward drag as postulated by Arkell. The formation of the sets of shear planes and faults recognised by Arkell in the Chalk is considered, and it is suggested that they can be divided into two genetic groups. The first group of structures formed during the closing of the syncline, while the second group brought about its modification by thrust displacements on southward inclined thrust planes and late faults. Arkell's conclusions concerning the causes of the reduction in the thickness of the incompetent Wealden Beds are summarised briefly, but by the construction of cross-sections of the main structure, it is demonstrated that the missing portions of the Purbeck and Wealden successions could have been displaced only by thrust movements on southward inclined major faults. A stage-by-stage synthesis of the development of the structures is presented and it is suggested that they were formed as the result of the accommodation of the blanket of largely unconsolidated sedimentary beds above a major thrust fault in the basement. At first a broad flexure developed and this became most pronounced in the highest division, the Chalk, because of the buckling and northward sliding of these beds off the rising land to the south. As the magnitude of the fault increased, successively higher beds fractured and eventually the fault extended to the surface and brought about the displacement and modification of the structures formed during the earlier phase of folding.