Stratigraphic evolution and preservation of aeolian dune and damp/wet interdune strata: an example fro the Triassic Helsby Sandstone Formation, Cheshire Basin, UK

摘要

New and previously published models of wet aeolian system evolution form a spectrum of types that may be explained in terms of aeolian dune dynamics, rate of water table rise and/or periodicity of interdune flooding. This is illustrated with an example from the Mid-Triassic (Anisian) Helsby Sandstone Formation, Cheshire, UK. Lenses of damp and wet interdune strata exhibit an intertonguing, transitional relationship with the toe-sets of overlying aeolian dune units. This signifies dune migration that was ontemporaneous with water table-controlled accumulation in adjacent interdunes. Downwind changes in the geometry and facies of the interdune units indicate periodic expansion and contraction of the interdunes in response to changes in the elevation of the groundwater table and episodic flooding, during which accumulation of dune strata continued relatively uninterrupted. This contrasts with other models for accumulation in wet aeolian systems where interdune flooding is associated with a cessation in aeolian bedform climbing and the formation of a bypass or erosional supersurface. Architectural panels document the detailed stratgraphy in orientations both parallel and perpendicular to aeolian transport direction, enabling a quantitative three-dimensional reconstruction of genetically related aeolian dune and interdune elements. Sets of aeolian dune strata are composed of grainflow and translatent wind-ripple strata and are divided by a hierarchy of bounding surfaces originating from oblique migration of superimposed dunes over slipfaceless, sinuous-crested parent bedforms, together with lee-slope reactivation under non-equilibrium flow conditions. Silty-mudstone and sandstone interdune units are characterized by wind ripple-, wavy- and subaqueous wave ripple-laminae, desiccation cracks, mud flakes, raindrop imprints, load casts, flutes, intraformational rip-up clasts and vertebrate and invertebrate footprint impressions and trackways. These units result from accumulation on a substrate that varied from dry-through damp- to wet-surface conditions. Interdune ponds were flooded by either fluvial incursions or rises in groundwater table and were periodically subject to gradual desiccation and reflooding. Red silty-mudstone beds of subaqueous origin pass laterally into horizontally laminated wind-ripple beds indicating a progressive transition from wet-through damp- to dry-surface conditions within a single interdune.