Abstract

Sediment cores recovered from alpine and subalpine lakes up to 250 km apart in northern British Columbia contain five previously unrecognized tephras. Two black phonolitic tephras, each 5–10 mm thick, occur within 2–4 cm of each other in basal sediments from seven lakes in the Finlay River – Dease Lake area. The upper and lower Finlay tephras are slightly older than 10 220 – 10 560 cal year B.P. and likely originate from two closely spaced eruptions of one or two large volcanoes in the northern Cordilleran volcanic province. The Finlay tephras occur at the transition between deglacial sediments and organic-rich postglacial mud in the lake cores and, therefore, closely delimit the termination of the Fraser Glaciation in northern British Columbia. Sediments in Bob Quinn Lake, which lies on the east edge of the northern Coast Mountains, contain two black tephras that differ in age and composition from the Finlay tephras. The lower Bob Quinn tephra is 3–4 mm thick, basaltic in composition, and is derived from an eruption in the Iskut River volcanic field about 9400 cal years ago. The upper Bob Quinn tephra is 12 mm thick, trachytic in composition, and probably 7000–8000 cal years old. A fifth tephra occurs as a cryptotephra near the top of two cores from the Finlay River area and is correlated to the east lobe of the White River tephra (ca. 1150 cal year B.P.). Although present throughout southern Yukon, the White River tephra has not previously been documented this far south in British Columbia. The tephras are valuable new isochrons for future paleoenvironmental studies in northern British Columbia.

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