Miocene Glacial Stratigraphy and Landscape Evolution of the Western Asgard Range, Antarctica

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Abstract
40Ar/39Ar dated in-situ volcanic ashfall deposits indicate that the surficial stratigraphy of the western Asgard Range in the Dry Valleys region extends back at least to 15.0 Ma. The preservation of Miocene and Pliocene colluvium and drift on valley slopes shows that major bedrock landforms are relict and that little slope evolution has occurred during the last 15.0 Ma. Major bedrock landforms can be compared directly with buttes, mesas, box canyons, and escarpments of platform deserts. Although glaciers played a role in shaping the present topography of the western Asgard Range, the main landforms were probably cut by progressive scarp retreat, propagation of embayments with theater-shaped heads, and isolation of buttes and mesas prior to 15.0 Ma. We also recognize a phase of wet-based alpine glacier expansion in theater-headed embayments prior to 15.0 Ma ago, along with a late-Miocene phase of northeast-flowing ice-sheet overriding between 14.8/ 15.2 Ma and 13.6 Ma. However, there is no evidence for notable glacier expansion during the last 13.6 Ma. Instead, in-situ ashfall deposits indicate persistent hyper-arid, cold-desert conditions with only minor advance (less than 2.5 km) of rock glaciers and glacierets at valley heads. Our results are consistent with Pliocene stability rather than meltdown of the adjacent East Antarctic Ice Sheet.

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