DEVELOPMENT OF PROTODUNES AND LOW-ANGLE CROSS-STRATIFICATION IN AEOLIAN SYSTEMS (Invited Presentation)

Protodunes are low-relief, slipfaceless migrating hummocks of sand thought to represent fundamental, emergent bedforms that develop from a flat bed of sand and evolve into sand dunes. Here we present a database that shows the correlation between protodune wavelength and grain size and show the dynamics of protodunes and the generation of low-angle cross-stratification from protodune migration at White Sands Dune Field, New Mexico. We used time-series aerial photos from 1996, 2003, 2005 and 2012 and digital elevation models from 2007, 2008, 2009, 2010, 2015 and 2016 to map protodune migration. We used ground penetrating radar (GPR) to image cross-stratification within the protodunes. Protodunes at White Sands Dune Field in New Mexico are found at the upwind margin of the field, on dune stoss slopes, and in interdune areas. Protodunes at the upwind margin of the dune field vary in wavelength and amplitude due to variations in grain size. This variation in protodune size, in turn, gives rise to different sized dunes at the upwind margin of the field and different downwind dune-field pattern characteristics. Wind ripples cover protodunes in all areas of the field, but the protodunes at the upwind margin are covered by coarse-grained ripples. A progression of ripple patterns occurs over protodunes in which ripples coarsen in wavelength and grain size toward the protodune crest and then decrease in wavelength and grain size toward the troughs. Ripple migration across the protodunes appears to the primary mode by which the protodunes migrate. Trenching and GPR data show low-angle cross-stratification generated by wind ripples migrating down the protodune lee slope. Internal bounding surfaces within the protodunes likely arise from lateral migration and lee slope reactivation in response to the complex wind regime and dune-modified secondary flow within the dune field at White Sands. Understanding the morphology, distribution, and genesis of protodunes in dune fields provides a basis to evaluate the significance of protodune strata in the rock record.