PLEISTOCENE STRATIGRAPHY AND PALEOENVIRONMENTS OF THE WHITE RIVER, SOUTHWEST YUKON TERRITORY
Exposures examined along the White River with both glacial and non-glacial middle to late Pleistocene sediment constrain the glacial limits and reconstructed paleoenvironments in southwest Yukon. These sections contain two glacial limits beyond the marine oxygen isotope stage (MIS) 2 extent. The non-glacial deposits contain up to 30 m of loess, lacustrine silt and clay, peat and gravel with tephra beds, paleosols, pollen, plant and insect macrofossils and mammal fossils. Organics with finite and non-finite 14C ages, Dawson tephra (25 300 14C yr BP), Woodchopper Creek tephra (77.8 ± 4.1 ka to 124 ± 10 ka) and newly sampled tephras provide a chronologic framework for these deposits.
A thick, likely MIS 6 aged till forms the lowest unit. Moraine-dammed glaciolacustrine deposits above this till were deposited during the transition into MIS 5. Intercalated gravel and sand beds with Woodchopper Creek tephra suggest this lake drained before ~77 ka. MIS 5 age peat units containing beaver-chewed wood indicate a white spruce forest with a climate as warm or warmer than present developed, probably after MIS 5e. This contrasts with overlying organic-rich pond sediments with abundant birch but no conifers. The presence of the informally named Donjek tephra, found elsewhere in association with Woodchopper Creek tephra, below these pond deposits suggests that an open birch tundra existed before MIS 4. Glaciation during MIS 4 deposited till and locally glaciotectonized the underlying sediments. Late MIS 3 to early MIS 2 deposits are identified by Dawson tephra and finite-aged (30.9 ± 0.3 and 39.8 ± 0.6 ka 14C BP) steppe-bison and woolly mammoth bones. The environment at this time was characterized by open, steppe-tundra vegetation (sage, grasses and forbs). The White River sections allow dating of the maximum and penultimate limits of the St. Elias lobe of the northern Cordilleran Ice Sheet and reconstructions of environmental change for much of the past 190 ka.