Widespread relict dunefields of Noctis Labyrinthus, Mars: Evidence of bedform formation on unusually steep slopes

Evidence for relic dunes and dunefields is valuable for its palaeoclimatic implications of a planetary surface, but bedforms are rarely clearly preserved in planform, such that they might be observable by orbital sensors. Uniformly-spaced patterning of the landscape, covering ~3000 km 2 , on the slopes of very large valleys in eastern Noctis Labyrinthus, Mars, is here interpreted as a series of palaeodunefields, with a number of very unusual characteristics. Occurring at elevations sometimes in excess of 7000 m, these are candidates for the highest elevation aeolian bedforms yet observed on Mars. Using data from the Mars Reconnaissance Orbiter's CTX and HiRISE cameras, we describe the attributes of these dunes and set them in geomorphological context of a remarkable landscape. Their morphology and distribution are consistent with aeolian bedforms, and their scale and orientation suggest that they can be classified as transverse climbing dunes. Unlike most climbing dunes, however, which typically form isolated bedforms, the large number of repeating bedforms dunes here suggests that these might be better termed as part of a climbing dunefield. The palaeodunes are found on slopes averaging 17°, and are present even on underlying slopes up to the angle of repose of sand on Mars, at about 32–34°. Consideration of the geometry of the present-day expression of relative relief (approximately 4–10 m), and models of potential past geometries of dunes on steep slopes, raises the possibility that these dunes may never have had leeside slip faces. Their antiquity is indicated by extensive cratering of the indurated surfaces, and crater-counting statistics suggest a Late Amazonian age of around 5 Ma. Their distribution, primarily on east-facing slopes, implies formation under a westward sediment flux, with deposition presumably occurring due to topographically-induced slowdown of winds on the large (sometimes >5000 m relief), very smooth-sided valley walls. • Extensive palaeo-dunefields identified on slopes of canyons in Noctis Labyrinthus, Mars • Palaeodunes occupy unusual geomorphological setting • Palaeodunes occur on underlying slopes up to the angle of repose of sand (~32–34°) • Indurated dunes retain topographic expression, but may have lacked avalanching lee slopes