Methodology for reconstructing wind direction, wind speed and duration of wind events from aeolian cross‐strata

Abstract

A methodology for reconstructing wind direction, speed, and event duration from aeolian dune cross‐strata was developed from analysis of crescentic dunes at White Sands, New Mexico, during wind events. Dune lee faces were surveyed, lee‐face deposits mapped, deposition rates measured, grain size sampled by stratification type, and winds characterized from meteorological and field data. The spatial distribution of lee‐face stratification styles is a function of the incidence angle formed between the wind and the brinkline, with secondary controls by wind speed and dune sinuosity and height. Sets of wind‐ripple strata form at incidence angles of 25°–40°, grainfall/grainflow foresets over wind‐ripple bottomsets at 40°–70°, and grainflow/grainfall foresets at 70°–90°. Erosional reactivation surfaces form at incidence angles up to 15°; bypass surfaces up to 25°. The total sediment load is fractionated within lee‐face stratification types. Wind speed can be reconstructed from relationships between grain size, transport mode, shear velocity and grain‐settling velocity. Where the full range of grain transport modes occurs and grain size is limited by shear stress, the shear velocity and grain‐size range in each transport mode can be estimated by assuming the coarse fraction in grainflow strata traveled in creep, and the coarse fraction in grainfall traveled in saltation. The minimum duration of a wind event can be estimated using measures of shear velocity, dune height and dune forward migration. Method limitations arise with source‐area control on grain size, extremes in wind events, and severe truncation of sets of cross‐strata.