Fluidization and Associated Soft-sediment Deformation in Eolian Sandstones

Abstract

On the southern shore of the Moray Firth, Scotland, the foreshore and cliffs east of Hopeman Harbor display a wide variety of soft-sediment deformation structures formed in unconsolidated Late Permian eolian sands. These include flows of water-saturated sand containing rip-up clasts that overturned the underlying dune sand; dune bedding that is now vertical; subvertical pipes and swirls of partly dilated sand; sand dikes; widespread partial to complete homogenization of dune sand; and a vertical escape structure some 20 m (66 ft) high. The driving force behind the deformation is believed to be widespread heavy rain over the northern edge of the Grampian highlands, causing slumping of the southward-migrating dune sands and possibly slight local northward sliding subparallel to the regional top Devonian erosion surface; this could have induced major increases in internal hydrodynamic pressure. Because the pores in the dune sands were filled with air prior to flooding, some of the vertical deformation structures may have been formed by the upward escape of air through rain-dampened dune sand driven by the hydrodynamic increase in water pressure. Probable coeval deformation, but of a different style, has been seen in cores recovered from oil and gas fields of the central and southern North Sea.