Wind sedimentation tunnel experiments on the origins of aeolian strata

Abstract

ABSTRACTThe origins and sedimentary features of grainfall‐, avalanche‐, and ripple‐produced strata have been studied experimentally in a wind sedimentation tunnel. Rate of deposition, wind velocity and wind duration have been shown to control specific sedimentary features of these types of strata.Grainfall‐produced strata were deposited on a horizontal surface, and surfaces sloping up to the angle of initial yield for dry sand (about 34°). Thickness of a grainfall‐produced stratum depended upon rate of deposition and duration of a specific wind event. Grainfall‐produced strata were both non‐graded and graded. Graded strata were produced by changes in wind velocity which controlled size of sand in transport and flying distances of individual grains. Distinctive features of grainfall‐produced strata are: (a) gradual thinning, or tapering downwind (e.g. down the simulated slipface and across the simulated interdune; (b) extreme variability of thickness from less than 1 mm (wind gusts of a few seconds) to 10 cm or more (sustained gusts).Aeolian avalanche‐produced strata were formed when grainfall‐produced strata steepened above the angle of initial yield and sheared downslope. A rapid transition in sedimentary features from top to bottom of the slipface characterized avalanche‐produced strata of the slump degeneration type in dry sand derived from grainfall deposition. Fadeout laminae formed near the top of the simulated slipface and about 1 m farther down the slipface were flame structures and drag folds. Near the base of the slipface, the avalanche truncated and then overrode grainfall‐produced deposits. Distinctive features of avalanche‐produced strata for a 2.5 m long slipface are the deformation structures, a thickness of 1 or 2 cm, sandflow toes, and steep dip (34°). Each avalanche‐produced stratum was roughly tabular in cross‐section parallel to wind direction, with gradual pinchout upslope.Aeolian ripple‐produced strata were deposited on horizontal surfaces, and surfaces sloping to as much as 28°. Thickness of a ripple‐produced stratum depended upon rate of deposition, morphology of the ripple, and rate of ripple migration. A maximum thickness of several centimetres was observed for a single ripple‐produced stratum. Shape and attitude of ripple foresets was controlled by ripple morphology. Distinctive features of aeolian ripple‐produced strata are: (a) presence of ripple foresets; (b) abrupt changes in thickness of a stratum or pinchout over downwind distances of a few centimetres; (c) low average foreset‐to‐diastem angle (10–15°); (d) low ripple‐climb angle (<10°).