Tephrochronology of late Cenozoic loess at Fairbanks, central Alaska

Abstract

Nineteen distinctive, distal tephra beds from the Gold Hill Loess in the Fairbanks region, central Alaska, are divided into two groups based on their color, mineralogy, glass-shard morphology, and the major- and trace-element composition of their glass shards. This dichotomy in tephra composition is believed to reflect the two source regions, namely, the eastern Aleutian arc–Alaska Peninsula region and the Wrangell volcanic field. Despite the fragmentary nature of the preserved loess record, which contains many local unconformities, these tephra beds can be used to link the numerous loess exposures in the Fairbanks region in their proper time sequence, establishing thereby a comprehensive, reliable, time-stratigraphic framework. Because these distal tephra beds are related to large-magnitude volcanic eruptions, given the long distance to the nearest possible source vent, they will also enable correlation of the late Cenozoic sedimentary sequence at Fairbanks to those of adjacent regions of Alaska and the Yukon. Tephra beds with bubble-wall or chunky glass shards have been dated by the isothermal plateau fission-track method. Results demonstrate that the Gold Hill Loess spans a long interval of time from late Pliocene time to the last interglacial, about 125 ka. However, the age of the basal part of this formation is not well constrained due to the apparent absence of tephra beds. Paleomagnetic studies suggest that loess deposition in the Fairbanks region began about 3 Ma.