Age of a Widespread Layer of Volcanic Ash in the Southwestern Yukon Territory

Abstract

Radiocarbon dates pertaining to a widespread layer of volcanic ash in the southwestern Yukon Territory are here reported. The volcanic ash generally occurs in lacustrine sediments and in peat and loess deposited during the Little Ice Age and thus affords a valuable marker horizon for correlating these deposits. ... Bostock ... constructed an isopach map showing two coalescing fans of ash with a combined area of about 129,000 sq. mi. and a maximum thickness of about 300 ft. near the international boundary about 10 mi. south of the White River. Both Bostock ... and Capps ... suggested that there was probably the source of the ash. Moffit and Knopf ... reported that a sample of this ash collected in the White River Basin, Alaska was an andesitic pumice. Berger ... described ash from the Tepee Lake area, southern Yukon Territory, and concluded that it was of dacitic composition. ... A microscopic analysis of an ash sample collected near the southeast shore of Kluane Lake, southwestern Yukon Territory, in 1963 showed it to be composed of whole and broken euhedral crystals of plagioclase (An 35 to An 50), hornblende, biotite, and a trace of magnetite. The glass sherds have a refractive index of approximately 1.510, suggesting a dacitic composition .... Knopf reported a composition slightly more calcic than Ab 1, An 1, or essentially labradorite, whereas the two analyses from the Yukon Territory show the plagioclase to be andesine. The ash layer, 1 inch thick, was found in a peat bog on the timbered rocky knob separating the Slims and Kaskawulsh rivers, southwestern Yukon Territory, approximately 100 yd. north of and about 40 ft. above the Little Ice Age terminal moraine of the Kaskawulsh Glacier. In an excavation the top of the l-in. thick ash layer in this locality was 13 in. below the surface of the bog. Samples of peat, 0.5 in. thick, were collected from positions immediately above and below the ash for radiocarbon dating. The sample Y-1363 from just above the ash yielded an age of 1460 ±70 years B.P. and the sample Y-1364 from just below the ash was dated 1390 ±70 years B.P., indicating that the time of the ash fall was around 1425 ±50 years ago. Although the lower sample provides a younger date, the difference is not significant in view of the statistical error of ±70 years. ... Near the southeast shore of Kluane Lake, Yukon Territory, the ash occurs near the base of a deposit of loess 4 ft. thick. The centre part of a tree buried there in the growth position immediately above the ash was dated at 870 ±100 years B.P. (sample Y-1365) and gives a minimum age for the ash. ...