Oblique Migration of Megaripples Under an Obtuse Bimodal Wind Regime in Qaidam Basin, China

Abstract

Megaripple crestlines are commonly treated as transverse indicators of wind or sand-transport direction. However, Martian observations show that some large ripples, bright-toned megaripples, and small Transverse Aeolian Ridges (TARs) can migrate obliquely or longitudinally, a phenomenon not previously documented on Earth. Here we document terrestrial megaripples in the Lenghu area of the Qaidam Basin, China, that are similar in scale to some Martian large ripples and TARs, and comparable in cross-sectional form to TARs. These bedforms have nearly symmetrical cross-sections and comfparable grain-size distributions between the two slopes. Their crestlines are oriented about 65° oblique to the resultant drift direction (RDD; 129.1°), and they occur under an obtuse bimodal wind regime. Excavated trenches show bidirectional cross-stratification: the maximum-dip direction of the lower deposits is not parallel to the cross-section orientation but aligns with the RDD. In contrast, the upper deposits align with the transport direction of dyed sand, forming a 63.5° angle with the crestlines. Consequently, dyed-sand transport directions indicate both cross-crest and along-crest movement. These results show that the Lenghu megaripples migrate obliquely, consistent with a process analogue of oblique or linear dune dynamics under bimodal winds, and suggest that megaripple crestlines and stratification should be used cautiously as simple transverse wind indicators.