TEPHRA-FILLED TROUGHS IN PATTERNED GROUND: A PROXY FOR THE DISTRIBUTION OF SUBSURFACE ICE

Contraction-crack polygons occur at the surface of unconsolidated debris in the western Dry Valleys region of Antarctica. In plan view, V-shaped troughs circumscribe active polygons. The concentration of subsurface ice is a primary control for the cross-sectional shape of polygon troughs. Deep troughs, in excess of 3 m deep and 5 m wide, form in sediment saturated with ice (for example, stagnant debris-covered glaciers). Shallow troughs, ~1 m deep and of variable width, form in sediment undersaturated with ice (for example, ice-cemented sand-and-gravel deposits). Trough depth is thus a proxy for the concentration of underlying ice. A proxy for paleo-ice concentration in surface deposits comes from examining the configuration of tephra that occupies ancient polygon troughs. Ashfall faithfully represents the cross-sectional shape of troughs, just as the configuration of recent snowfall conforms to the shape of modern polygon troughs. Cryoturbation beyond that associated with thermal contraction is minimal in the hyper-arid, cold desert climate of the western Dry Valleys region. Here, we describe the morphology of dated ashfall that occupies relict polygon troughs in the western Asgard Range and in the Quartermain Mountains. The results suggest that ice-cored drifts were of greater areal extent during mid-to-late Miocene time than they are today.