Abstract

The stratal types composing aeolian dunes preserve a record of the transport and sorting of grains and are categorized into: 1) grainflow strata, 2) grainfall laminae, and 3) wind-ripple laminae. The arrangement of these deposits in the cross beds of a formative dune is largely unexplored. Here, field results from White Sands Dune Field, New Mexico, USA, are used to test the hypothesis that dune height controls the arrangement, abundance, and geometry of cross-stratification types. Grainflow thicknesses and deposit widths were measured on wind-scoured stoss-side exposures of seven crescentic dunes with heights ranging from 1.7 m to 11.2 m. Dozens of grainflow thickness measurements were taken along transverse-oriented strata normal to the crest on each dune. The results show that grainflow thickness averages from 1 cm to 4 cm. These data show a positive trend between mean grainflow thickness and dune height but only for the grainflow thicknesses measured at the bases of dunes. The tallest dune (11.2 m) produced many thick grainflow packages of 10 cm to 30 cm in which individual grainflow strata were indistinguishable from each other. This amalgamation was also found to be characteristic of larger dunes—the product of a lack of grainfall deposits separating individual grainflows. These differences in grainflow strata at the bases of dune lee slopes are linked to the temporary storage of sediment along the upper parts of lee slopes. In taller dunes with longer lee slopes, amalgamated grainflows which require multiple avalanche events and take longer time to reach the base transport temporarily stored sediment at upper parts of the slope. This allows time for wind ripples to rework accumulations near the base, where grainfall deposition is also limited. Shorter dunes lack this temporary storage mechanism, as individual grainflows can move across the entire lee slope in a single event, and grainfall accumulates across the entire lee slope. These stratigraphic measurements and process-based understanding will be useful in estimating original dune height in ancient cross-strata and will lead to a better interpretation of aeolian stratigraphy.