AN OUTRAGEOUS HYPOTHESIS (?): MEGADUNE LAYERING CAN RECORD DEFORMATION AND WASTAGE OF THE ANTARCTIC ICE SHEET

Megadunes form strange zebra –like ice patterns, unique to higher parts of the Antarctic Plateau. Individual megadune bands are 10 - 100 km long, 1 - 2 km wide, and are separated by bands of glazed ice in rhythmic cycles numbering in the 10s to 100s. These pairs crop out as 3 - 4 m high topographic steps with tread width to riser height ratios of about 1000:1. The flat or tread surfaces are covered by 0.5 - 5 m-high, sharp-edged, elongate ice dunes or sastrugi (Russian for “cut with a blade”) with long axes parallel to katabatic winds. Standard interpretations ascribe megadunes to wave processes of katabatic winds but precise origin mechanisms remain elusive. They are generally viewed as currently forming or evolving ice features, mere meteorological curiosities of the ice surface. Accordingly, the larger scientific community focuses only limited attention on them. Here we modify that view with photogeologic and structural geologic evidence from a high resolution (125 m/pixel) MODIS mosaic image of Antarctica suggesting that megadunes may be outcrop lines of cryptic layers that mark the abandoned top of an older ice sheet. Their zebra patterns are truncated by an angular unconformity at the base of some firn deposits, appear as wind-swept surfaces between migrating ice dunes, and are locally offset by arrays of kink bands or minor faults. They form great lines and sweeping curves across the plateau somewhat independent of the trends of modern katabatic winds. Their pattern changes along the Lake Vostok disturbed zone and shows collapse and/or rotation at the heads of outflow regions of the Wilkes sub-sea basin and the head of one arm of Lambert Glacier. Ground penetrating radar shows the steps migrated upwind in antidune fashion to produce previously unrecognized climbing anti-dune ripple pseudo-bedding or layering, analogous to well-known pseudo-bedding of normal climbing dune ripples. We propose that megadunes represent a transitional style of deposition marking an abrupt change to the modern ice dome and ice divide style. Etched flats of those abandoned anti-dune layers now form a roughened base for younger sastrugi development. Younger warping of the abandoned and etched surface records subsequent wastage of the ice sheet while tilted and reoriented megadune layers allow a start at structural geologic mapping of the ice itself.