PLEISTOCENE STRATIGRAPHY AND PALEOENVIRONMENTS OF THE WHITE RIVER, YUKON

Exposures examined along the White River in southwest Yukon Territory contain thick accumulations of glacial and non-glacial sediment from the middle to late Pleistocene. The White River sections are located beyond the limit of marine oxygen isotope (MIS) stage 2 glaciation and contain deposits from MIS 4 and MIS 6 glaciations. The non-glacial deposits contain up to ten metres of re-worked loess, lacustrine silt and clay, peat and gravel with tephra beds, paleosols, plant and insect macrofossils and large mammal fossils. The presence of organics with finite and non-finite ages, and Dawson (25 300 ¹⁴C yr BP) and Woodchopper Creek (between 77.8 ± 4.1 ka and 131 ± 11 ka) tephras provide a chronologic framework for these deposits. These data combined with pollen and macrofossil analysis are used to reconstruct the glacial and non-glacial history of the White River area. A thick, likely MIS 6 aged till forms the bottom unit in all the sections. Above this, two MIS 5 age (sensu lato) peat units containing beaver-chewed wood represent a shallow depression with mostly birch shrubs growing near a shoreline within a white spruce forest in a climate as warm or warmer than present. This contrasts with another MIS 5 aged sequence of organic-rich pond sediments containing abundant birch but no conifers, suggesting that an open birch tundra existed at some point before MIS 4 glacial advance. Till deposited during MIS 4 is observed at several sections. Late MIS 3 to early MIS 2 deposits above this till are identified by finite-aged (30.9 ± 0.3 and 39.8 ± 0.6 ka ¹⁴C BP) bison and mammoth bones and Dawson tephra. During this time the environment was treeless, dry and dominated by Artemisia and grasses. The presence of interstratified glacial and organic non-glacial deposits allow the reconstruction of environmental change along the White River for much of the past 190 000 years.