Changing ideas on the identity and stratigraphic significance of the Sheep Creek tephra beds in Alaska and the Yukon Territory, northwestern North America

Abstract

Sheep Creek tephra (SCt) consists of inflated, rhyolitic pumice with abundant plagioclase and hornblende, lesser amounts of ferrian ilmenite and magnetite, and trace quantities of basaltic hornblende, augite, quartz, apatite, and zircon. It has an adakitic composition; the glass has low Y, Yb, and high La/Yb and Sr/Y values. Previously, compositional differences between samples were known to exist and thought to be due to derivation from a compositionally zoned magma involving several closely spaced eruptions. SCt was recognized as a useful stratigraphic marker across central Alaska and western Yukon with an age of ∼190 ka. New OSL dates on a SCt occurrence in western Yukon are ∼80 ka and force a reassessment of the earlier work. SCt is now known to be made up of several stratigraphic units: SCt-F (Fairbanks, Alaska; ∼200 ka), SCt-CC (Canyon Creek, Alaska), and in the western Yukon, SCt-C, SCt-K (∼80 ka), and SCt-A. Beds C, K, and A must be similar in age because of their close stratigraphic association at Ash Bend in Yukon. Compositional and temporal controls, including Sr isotopes, suggest Mount Drum in the Wrangell volcanic field as the source of the SCt beds. Revision of the age of the SCt beds in western Yukon means that the Reid Glaciation of Yukon is likely of MIS 6 age and probably correlates with the Delta Glaciation of Alaska. Further, the buried forest bed just below SCt-K in the Klondike goldfields must be of MIS 5 age, not MIS 7, as previously thought.