Stratigraphic Context of Old Crow Tephra, Holitna Lowland, Interior Southwest Alaska

Abstract

A thick deposit of Old Crow tephra was discovered in a bluff exposure along the middle Holitna River near the Kulukbuk Hills (61°20′N latitude, 157°10′W longitude) in interior southwest Alaska. This locality is the southwesternmost-known deposit of Old Crow tephra in Alaska. Thickness and grain-size data from this site support a source volcano in the eastern Aleutian arc. Pleistocene stratigraphic sequences in the lowland are dominated by upward-fining eolian sand-sheet deposits and loess separated by organic silt. These deposits record at least two episodes of regional glaciation and an intervening nonglacial period (marine oxygen isotope stage 3, stage 5, or both). Old Crow tephra crops out near the top of the lower upward-fining eolian unit, indicating that the ash erupted near the end of an interval of periglacial eolian sedimentation. The sequence of eolian deposits that contain Old Crow tephra probably accumulated during the latter part of marine oxygen isotope stage 6, whereas the overlying eolian sequence formed during the last glaciation (stage 2). This stratigraphic position is consistent with other stratigraphic contexts for the tephra and with fission-track and thermoluminescence ages of ca. 140,000 ± 10,000 yr B.P.