Nelia W. Dunbar
Submitted to William Cassidy Retirement Symposium
Dust bands, some of which were early recognized as volcanic ash layers, are found interlayered with ice at many meteorite stranding surfaces. Investigation at Allan Hill, and associated blue ice fields, have shown that virtually all the "dust bands", which range from thin, faint laminae to distinct bands up to 50 cm thick, are composed of primary, unabraded volcanic ash fragments ranging in size from <2 to 250 µm. The layers represent true time-stratigraphic markers in the ice and dip from near-horizontal to vertical, reflecting the geometry of local ice flow. The distribution of tephra layers at Allan Hills is controlled by ice flow, which, in turn, is controlled by bedrock topography. In areas of rapid ice flow, the ash layers are strongly thinned, but in areas of stagnant ice, typically associated with high concentrations of meteorites, the layers are thickened and contorted. Although the only volcanic ash layers that have been successfully dated are found in meteorite-free blue ice areas, some insights into timing of meteorite deposition has nevertheless be gained. An individual ash layer has been correlated over a distance of ~100 km, based on petrographic and geochemical criteria, and is found at the Main Allan Hills, Far Western, and Reckling Moraine icefields, suggesting that the ice at these three localities is all of the same age. Understanding of blue ice/volcanic ash sections has lead to the acceptance of the potential for "horizontal ice cores" as suggested by Cassidy and Whillans (1990).