Slipfaceless ‘whaleback’ dunes in a polar desert, Victoria Valley, Antarctica: Insights from ground penetrating radar

Abstract

The Lower Victoria Valley is one of the McMurdo Dry Valleys, Antarctica, and has the largest concentration of aeolian sand dunes on the continent. Aeolian bedforms include elongate slipfaceless dunes which have been called ‘whaleback’ dunes. These dunes are composed of coarse sand and capped by granule ripples. Ground penetrating radar (GPR) profiles across one of these dunes reveal low-angle inclined sigmoid/tangential and convex reflections that are interpreted as strata within the dune. These dipping strata record the migration of the whaleback dune and show that they are not sand mantles as previously described but are actively migrating, long-wavelength, low-amplitude bedforms resembling zibar. An 800 m profile along the axis of the dune reveals low-angle dips from west to east showing that the dune has accreted towards the east. GPR profiles collected at 100 m intervals transverse to the dune axis reveal sets of cross-stratification dipping towards the south at the eastern end of the dune and towards the north at its western end. These apparent dips are resolved to show that the dune has migrated from west to east, driven by westerly foehn winds and katabatic winds blowing from the Polar ice cap, whilst at the same time it has been building obliquely towards the south. Accretion towards the north at the western end of the dune is attributed to reworking of the dune sand by easterly winds blowing inland from McMurdo Sound. An unconformity within the dune suggests that the wind regime has varied in the past. Beneath the unconformity the direction of dune migration was from west to east, and eastward extension of the dune continues at the present day. However, this is combined with accretion towards the north and west at the western end of the dune which is not recorded in the older sediments beneath the unconformity. Sand wedge structures that may have taken hundreds to thousands of years to form are found within the dune, and suggest that arid conditions suitable for dune construction have persisted in Victoria Valley for a considerable time.