Geomorphic and Stratigraphic Signatures of Protodunes

Protodunes are precursors to the development of sand dunes and, all else being equal, protodune stratification should occur in the stratigraphic record associated with dune stratification. However, the internal stratification of these little studied bedforms has remained largely unknown and the process which dictate how protodune stratification accumulates and is preserved in the rock record are poorly understood. Here we show that protodune migration generates low-angle stratification that is transitional to angle-of-repose dune stratification, document the occurrence of low-angle stratification in the rock record, and develop a model to explain the distribution of lowangle, protodune stratification in the stratigraphic record. A ground-penetrating radar (GPR) survey conducted across a train of protodunes that give way downwind to angleof-repose dunes at the upwind margin at White Sands Dune Field, New Mexico shows a spatial transition in dip angles of GPR reflectors from flat-lying to 15° before transitioning to dunes with lee slope angles > 20°. Correlation between individual radar reflections and successive time series topographic datasets establishes that the low-angle reflectors are generated by the migration of the protodunes. Surface grain size fined downwind across the transect from 1.01 mm to less than 0.55 mm. The highest abundance of fine grains coincided with the development of the first angle-of-repose slipface at the most downwind transect. In Paleozoic eolian sequences, 15° dune cross- bedding matches observations of protodunes stratification at White Sands Dune Field’s upwind margin. Where present, low-angle stratification consistently occurred at the base of erg growth sequences. The presence or absence of protodunes stratification likely arises due to variable conditions for accumulation at the initiation of the sequence and from variable scour from overriding dunes. From these observations we define a model of the development of low-angle protodune stratification within eolian successions based on process-scale dynamics