A CAUTIONARY TALE OF TWO TEPHRAS: THE SCIENTIFIC METHOD EKEING OUT ERRORS IN THE LATE CENOZOIC TEPHROCHRONOLOGICAL RECORD OF EASTERN BERINGIA

The two tephra beds of concern are the Sheep Creek tephra (SCt) at Eva Creek, Fairbanks, Alaska, ~ 190 ka, and a tephra bed at Ash Bend on the Stewart River, Yukon. They were considered to be equivalent given their petrographic features and glass composition. Both consist mostly of inflated pumice and hornblende, contain ferrian ilmenites, and have a very similar major-element glass composition of an adakitic nature, suggesting a source vent in the Wrangell volcanic field (WVF). Small, but reproducible, differences in the trace-element composition of the glass were recognized and taken to indicate a compositionally-zoned magma body that was (partially) emptied during one or several closely-spaced eruptions. A glass fission-track (ft) age of ~170 ka on Dominion Creek tephra (DCt), 30 cm above the supposed SCt in the Klondike goldfields, supported this correlation, which inferred that the Reid glacial drift in Yukon was of MIS 8 age or older, and the buried forest bed just below SCt belonged to MIS 7.

A provisional OSL age on sediments close to the supposed SCt in the Klondike of ~ 80 ka by R. Roberts (University of Wollongong, Australia) forced a re-examination of the ft data. A coarser sample of DCt was recovered and found to contain 15% contaminant glass shards, which were avoided in the recount, to give a revised glass-ft age of 82±9 ka – close to the U-series date on the SC-like tephra at Canyon Creek, Alaska. This new understanding explains why Old Crow tephra (OCt), ~ 140 ka, has never been found in close association with the supposed SCt in the Klondike – unlike the Fairbanks region, where OCt occurs 3 m above SCt.

Petrographic, chemical, and age data now support the following eruptive history of the SC-like tephras: 1) SCt-F (Fairbanks) erupted from vent in WVF ~ 190 ka with fallout directed to NW, 2) SCt-C (Canyon Creek) erupted from same vent ~ 80 ka with fallout directed to N, 3) SCt-K (Klondike), ~ 80 ka, is co-magmatic with SCt-C and probably erupted shortly after it with fallout directed to NE, 4) Ash Bend tephra, ~ 1 m above SCt-K, is co-magmatic with SCt-K, but several thousand years younger based on its tundra-like environmental setting. Hence, the buried forest bed just below SCt-K in the Klondike is of MIS 5 age, and the younger, minimum age of the Reid glacial drift at Ash Bend now permits an MIS 6 age, although work by others supports an MIS 8 age.