AGE OF SHEEP CREEK TEPHRA (PLEISTOCENE) IN CENTRAL ALASKA FROM THERMOLUMINESCENCE DATING OF BRACKETING LOESS

The age of the Sheep Creek tephra (SCt), a widespread marker ash bed i n eastern Alaska and western Yukon Territory, has been ambiguous and c ontroversial, We have obtained three reliable thermoluminescence age e stimates from bracketing loess near Fairbanks that imply a deposition age of about 190,000 +/- 20,000 yr for SCt. Three of six loess samples near and closely bracketing the SCt beds near Fairbanks yielded young er age estimates (similar to 117,000 and similar to 135,000 yr), most likely (based on field aspects) because of reworking and contamination by translocated grains. The new, reliable age assignment of 190,000 y r confirms independent stratigraphic evidence of a pre-last interglaci ation age, and stratigraphic evidence from one site (Upper Eva Creek) that SCt is older than the more-widespread 140,000-yr-old Old Crow tep hra. The SCt age also has implications for regional correlations of gl acial and nonglacial deposits. In particular, it supports the stratigr aphic and geomorphic interpretation that the Delta Glaciation in the e ast-central Alaska Range and the Reid Glaciation in western Yukon Terr itory are older than the last interglaciation (isotope substage 5e). ( C) 1996 University of Washington.