Dawson tephra: a prominent stratigraphic marker of Late Wisconsinan age in west-central Yukon, Canada

Abstract

Dawson tephra is the most prominent tephra bed in the Quaternary deposits of the Klondike and Sixtymile areas of the Yukon. It occurs in organic-rich, reworked, Late Wisconsinan loess deposits that occupy valley-bottom sites and has a maximum thickness of 30 cm, although most values are in the range of 15–30 cm. The tephra consists of thin, bubble-wall shards that have a distinctive, rhyolitic composition. Orthopyroxene, plagioclase, magnetite, and ilmenite are relatively abundant in the small crystal component, which also contains minor amounts of clinopyroxene, apatite, and zircon. Overall, the composition of Dawson tephra bears a close resemblance to that of the well-known Old Crow tephra, and, like that tephra, demonstrates a source in the Aleutian – Alaska Peninsula region. Given that the nearest vent in the Alaska Peninsula is 700 km distant from the Klondike, it follows that this explosive eruption must have been of great magnitude, distributing tephra over southern and central Alaska and the Yukon. The age of this eruption is not precisely known, but it did occur during the interval 22–12 ka.