HR: 08:05h
AN: V21C-01    [Abstracts]
TI: Tephrochronolgical Studies of Late Neogene Sediments in Interior Alaska and the Yukon Territory
AU: * Westgate, J A
EM: westgate@geology.utoronto.ca
AF: University of Toronto, Dept. of Geology, Toronto, ON M5S 3B1 Canada
AU: Preece, S J
EM: sjpreece@rogers.com
AF: University of Toronto, Dept. of Geology, Toronto, ON M5S 3B1 Canada
AU: Froese, D G
EM: duane.froese@ualberta.ca
AF: University of Alberta, Dept. of Earth and Atmospheric Sciences, Edmonton, AB T6G 2E3 Canada
AU: Schweger, C E
EM: charles.schweger@ualberta.ca
AF: University of Alberta, Dept. of Anthropology, Edmonton, AB T6G 2H4 Canada
AB: Our tephra studies of Late Neogene sediments in interior Alaska and Yukon are motivated by the need to provide a reliable time-stratigraphic framework for on-going palaeoenvironmental projects. Key sites are located in the Fairbanks, Chicken (Alaska) and Klondike (Yukon) goldfields, Old Crow Basin (Yukon), and the numerous bluffs along the Yukon River in Canada and eastern Alaska. Tephra beds are characterized by their field setting, petrography, geochemical composition of glass (majors and traces) and mineral phases (especially FeTi oxides), palaeomagnetic properties, and age (determined mostly by glass-fission-track methods). Two compositional groups are recognized. Type I beds have abundant bubble-wall glass shards and a small crop of crystals with pyroxene $>$ hornblende. Its glass has a rhyolitic to dacitic composition with relatively high FeO$_{t}$, Cs, Hf and low Al$_{2}$O$_{3}$, CaO, and Sr. REE profiles have a well-developed Eu anomaly with La/Yb $<$ 13. Volcanics with this chemical signature are common throughout the Aleutian Alaska Peninsula arc (AAPA), which is, therefore, the presumed source of the type I distal beds. In contrast, type II beds have more abundant crystals (hornblende $>$ $>$ pyroxene) and the rhyolitic glass is mainly in the form of highly inflated pumice with high Al$_{2}$O$_{3}$, CaO, and Sr. REE profiles are steep with low heavy REE content along with a very weakly developed Eu anomaly, if present. The type II beds are unusual and have many of the characteristics of adakites, known to occur at Mount Drum and Mount Churchill in the Wrangell volcanic field (WVF), and at Hayes volcano at the northeastern end of the Alaska Peninsula arc. It is likely, therefore, that the source vents for the type II beds in interior Alaska and Yukon are located in or near the WVF. Twenty-five distinctive tephra beds have been recognized in the Gold Hill Loess at Fairbanks and a comparable number have been discovered in the Klondike goldfields, although few beds are common to both regions. Tephra beds related to large-magnitude explosive eruptions with inferred widespread distributions, given their location, thickness, and presumed source, respectively, include, from the WVF: White River Ash (1-2 ka), Sheep Creek tephra in Alaska (190 ka), Gold Run (700 ka), SP (870 ka), WP (1.0 Ma), Paradise Hill (1.5 Ma), Fort Selkirk (1.5 Ma), Little Timber (2.3 Ma), Lost Chicken (2.8 Ma), and Quartz Creek (3.0 Ma). Corresponding units from the AAPA include: Dawson tephra (24 ka), VT (80 ka), Old Crow (140 ka), Ester (800 ka), Mosquito Gulch (1.5 Ma), PA (2.0 Ma), and Dago Hill (3.2 Ma). Application of the tephrochronological method to the Late Neogene sediments of eastern Beringia has placed several important palaeoenvironmental events into a precise chronologic context. (1) Preglacial vegetation of {\it Pinus} and {\it Picea}, with rare {\it Abies}, {\it Larix}, {\it Alnus}, {\it Betula}, and {\it Corylus} existed in eastern Beringia as late as 2.8 Ma; (2) loess deposition in interior Alaska began $\sim$ 3.0 Ma; (3) permafrost was established in the area by 3.0 Ma; (4) the first continental glacier invaded Yukon between 3.0 to 2.6 Ma; and (5) the characteristic interglacial boreal forest, dominated by {\it Picea}, {\it Abies}, {\it Betula}, and {\it Alnus}, was established by 2.3 Ma.
DE: 8404 Ash deposits
DE: 9604 Cenozoic
DE: 1749 Volcanology, geochemistry, and petrology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting