Gravel “megaripples” in the Puna of Argentina are the most extreme aeolian megaripples on Earth and are useful analogs for aeolian processes on Mars. Field observations, supplemented by experimental and numerical constraints on wind characteristics and aeolian transport, reveal their conditions of formation and growth to be an aeolian geomorphology “perfect storm.”
The bedforms are formed on a substrate of weakly indurated ignimbrite, aeolian deflation of which yields a bimodal lag of lithics and pumice clasts onto an undulating surface. Under normal wind conditions in this region, the lithics are organized into bedforms on local upslopes and “highs” through creep induced by the impact of saltating sand and pumice. The gravel bedforms grow through “shadowing” and trap sand and silt that is gradually kinetically sieved down to “lift” the gravel mantle upwards to form the megaripples. These observations connote that the largest features are not ripples in the sense of migrating bedforms, but rather nucleation sites of wind-transported sediment. Strong control by bedrock topography means that the largest bedform wavelengths are not a result of particle trajectories, and this complicates their comparison with other ripples and may require a new classification.
Of relevance to Mars, the Puna megaripples are morphologically and contextually similar to small ripple-like transverse aeolian ridges (TARs). Moreover, the Puna gravels have similar equivalent weight (mg) to those composing granule ripples at Meridiani Planum, and their local origin may have implications for the origin of sediment in martian aeolian bedforms. Finally, the stable yet dynamic character of the Puna megaripples could help reconcile current models of TARs with periodic bedrock ridges that may be produced by aeolian erosion.