The Stampede tephra: a middle Pleistocene marker bed in glacial and eolian deposits of central Alaska

Abstract

A deformed block of silt incorporated in a push moraine of the Lignite Creek drift of the Nenana River valley in central Alaska contains the 10 cm thick Stampede tephra. The silt block and tephra are folded, sheared, and cut by multiple fault planes and appear to have been entrained by a glacier and deformed by glaciotectonic processes while frozen. The Stampede tephra is also found in a paleosol preserved within a sequence of eolian sediments near the Tanana River some 175 km to the northeast, allowing direct tephrochronologic correlations between the depositional record of Quaternary glaciations in the Alaska Range and eolian sediments in unglaciated central Alaska. The Lignite Creek drift predates deposits of the late Wisconsin Riley Creek and penultimate Healy glaciations but postdates the Stampede tephra, indicating that it dates to the middle Quaternary, and is not part of the Tertiary Nenana Gravel Formation.