MIDDLE PLEISTOCENE LOESS AND TEPHRA DEPOSITS IN THE FAIRBANKS REGION

Reports of middle Pleistocene (~780-125 ka) loess deposits and their associated paleoecological and permafrost records are limited and fragmentary in and around Fairbanks, Alaska. The most complete middle Pleistocene deposits to date have been reported from sites along the Yukon River in Alaska—Chester Bluff and the Palisades—with some limited exposure at Gold Hill near Fairbanks. Here we report initial findings from a new site near Ester, Alaska, that indicate the presence of extensive middle Pleistocene loess deposition. The site, informally known as Largent Mine, comprises a series of placer mining cuts that expose a complex of interbedded loess, paleosols and volcanic ash (tephra), often heavily cryoturbated. Tephras collected at the site include classic late-middle Pleistocene markers such as Boneyard, Old Crow, Halfway House, VT, and DAB tephras. This suggests the preservation of a relatively complete late MIS 6 to MIS 5 record. New correlations to previously unpublished data from Birch Creek, Alaska, and several other published localities throughout Beringia, as distant as the Klondike, Yukon, indicate the presence of older middle Pleistocene loess as well. This includes the first discovery of the 80-pup tephra outside of the Klondike. Previously thought to be closer in age to Old Crow, tephra associations between Largent Mine and Birch Creek show that this tephra is older than ~200 ka, potentially quite a bit older. Below 80-pup, at the loess-gravel boundary, are organic deposits with large wood fragments and the Alyeska Pipeline tephra (APt). This tephra is an important regional marker that does not have an independent age determination but is found below the Chester Bluff tephra (CBt; ~360 ka) and is likely younger than ~500 ka. Overall, the cross-correlations to other localities, combined with the tephra within the site, suggest that Largent Mine contains an important middle Pleistocene record. This site provides a unique snapshot into the middle Pleistocene around Fairbanks, and helps build and clarify the tephrostratigraphic framework for Beringia.