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Sand Sheet and Protodune Development and Preservation in Aeolian Systems


A short and dramatic geomorphic and stratigraphic spatial transition occurs at the upwind margins of aeolian dune fields. The geomorphology progresses from flat-lying sand sheets with strong spatial grain sorting to protodunes to dunes over a few hundred meters. A commiserate change in stratification type occurs from near flat-lying wind ripple strata, to low angle wind ripple cross-stratification, to high angle dune cross stratification. This study closely documents and analyzes this transition within modern and ancient aeolian environments. A ground penetrating radar (GPR) survey conducted at White Sands Dune Field, New Mexico, documented stratification changes occurring at White Sands' upwind margin. Additionally, a data collection campaign around Canyonlands National Park in Utah relied on Giga-Pan photogrammetry to document the occurrence and frequency of stratification within the rock record similar to that documented at White Sands. The field campaign focused on the upwind margin of the Permian White Rim sandstone, which served as an example of a wet aeolian system, and the dune field interior and downwind margin of the Permian Cedar Mesa Sandstone, which, in specific localities, served as an example of a dry aeolian system. This study resulted in four key observations. (1) Within modern environments dune strata scour depth often exceeds average protodune height. (2) A significant coarse-grained and larger size fraction exists within flat-lying sand-sheet strata and is present to a lesser extent as coarse grained lag within low angle protodune strata. (3) Sand-sheet, protodune, and dune stratification occur in a predictable vertical succession bounded by surfaces marking dune field deflation. (4) Sand sheet and protodune stratification occur at a much lower frequency within the rock record for dry than wet aeolian systems. These observations agree with previous studies of sand sheet strata and more precisely recognize some low-angle aeolian stratification as protodune strata. Understanding the autogenic sediment transport dynamics operating at a dune field's upwind margin helps illustrate the necessity of sand sheet and protodune strata development for dune field origination and growth and also explains the relative absence of sand-sheet and protodune strata within the rock record.