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3. Dating the Reid Glaciation
Figure. 1. Glacial limits map of Yukon showing
the preserved portion of Reid glacial deposits
beyond the limit of the McConnell-age deposits.
Location of the study area is also indicated by
the arrow.
Brent Ward
Simon Fraser University
Jeffrey Bond
Yukon Geological Survey
John Gosse
Dalhousie University
Over the course of the last 3 million years, North America, including Yukon, has been glaciated numerous times. In Yukon, some of these older glacial deposits are preserved beyond the extent of the last glaciation (Fig. 1). Immediately beyond the limit of the McConnell (last) glaciation are glacial deposits assigned to what is called the Reid glaciation. The age of this glaciation, while it is understood to pre-date the McConnell glaciation, remains enigmatic. The subdued nature of Reid landforms, when compared to the Late Wisconsinan McConnell landforms, indicate an older age (Bostock, 1966; Hughes et al. 1969[l1]). Initially, the Reid was considered to be Early Wisconsinan (oxygen isotope stage 4) based on overlying non-finite radiocarbon ages (Hughes et al. 1969). It was then considered to be Illinoian (oxygen isotope stage 6) because of an interglacial paleosol developed on deposits of Reid age (Hughes, 1987[l2]). Studies relating to the Sheep Creek tephra, which occurs above deposits of Reid age along the Stewart River at Ash Bend (Hughes et al. 1987, Westgate et al. 2001), pushed its age back even further. The Sheep Creek tephra occurs 2 to 3 m below the Old Crow tephra (dated at 140 Ka) in the Fairbanks area (Preece et al., 1999), where a thermoluminescence age of 190 Ka has been obtained (Berger et al., 1996). These data, and paleoecological interpretation of the sediments associated with Sheep Creek tephra at Ash Bend, led Westgate et al. (2001) to propose an oxygen isotope stage (OIS) 8 or older age for the Reid glaciation. Correlation to OIS 8 appeared to be confirmed by an Ar-Ar age of 311 Ka on basalt underlying Reid outwash in the Ft. Selkirk area (Huscroft et al., 2004). Unfortunately, recent work indicates there are more than one Sheep Creek tephra; the tephra at the Ash bend site is considerably younger than the one dated in the Fairbanks area (Duane Froese, personal communication, 2005). This allows the possibility that the Reid Glaciation is either OIS 6 or 8.
Figure. 2. Brent Ward, Karin Fecova and
John Gosse sampling the surface of a large
Reid erratic west of Aishihik Lake.
Cosmogenic exposure dating is a relatively new method that can provide an accurate age of the Reid Glaciation. It is based on the accumulation of isotopes, in this case Be10, in the upper 1-2 m of the earth's surface due to bombardment by cosmic radiation. The main advantage of this method is that the actual glacial deposits, usually large boulders at the surface, yield absolute age dates, rather than the relative age dates obtained by stratigraphically related tephras or basalts. The Reid Glaciation will be dated by sampling large boulders in the Aishihik Lake map area (Fig. 2). This area was chosen because of the well preserved Reid surfaces mapped by Owen Hughes (Hughes, 1990) and the presence of granitic bedrock, conducive for Be10 cosmogenic dating.
Determining the age of the Reid Glaciation will also assist in our understanding of glacier extents and correlation. The apparent lack of an extensive OIS 6 glaciation in central Yukon is problematic in terms of correlation to both the classical Midwestern North American glacial stratigraphy and the oxygen isotope curve; based on these records there should have been more extensive ice in stage 6 compared to stage 2 and 8. If the Reid is OIS 8 then some explanation for this dichotomy in glacier extent must be proposed. Thus, the age of the Reid has significant implications to climate modeling as well as correlations to other areas affected by the Cordilleran Ice Sheet and adjacent areas in Alaska.
References:
Berger, G.W., Pewe, T.L., Westgate, J.A. and Preece, S.J., 1996. Age of Sheep Creek Tephra (Pleistocene) in central Alaska from thermoluminescence dating of bracketing loess. Quaternary Research vol. 45, p. 263-270.
Bostock, H.S., 1966. Notes on glaciation in central Yukon Territory. Geological Survey of Canada Paper 65-36, 18 p.
Hughes, O.L., Campbell, R.B., Muller, J. and Wheeler, J.D., 1969. Glacial limits and flow patterns, Yukon Territory south of 65° N latitude. Geological Survey of Canada, Paper 68-34, 9 p.
Hughes, O.L. 1987. Quaternary Geology. In: Guidebook to Quaternary Research in Yukon, S.R. Morison and C.A.S. Smith (eds.), XII INQUA Congress, Ottawa, Canada, National Research Council of Canada, Ottawa, p. 12-16.
Hughes, O.L., Harington, C.R., Schweger, C.E. and Matthews, J.V., Jr., 1987. Stop 15: Ash Bend Section. In: Guidebook to Quaternary Research in Yukon, S.R. Morison and C.A.S. Smith (eds.), XII INQUA Congress, Ottawa, Canada, National Research Council of Canada, Ottawa, p. 50-53.
Huscroft, C.A., Ward, B.C., Barendregt, R.W., Jackson, L.E., Jr. and Opdyke, N.D., 2004. Pleistocene volcanic damming of Yukon River and the maximum age of the Reid Glaciation, west central Yukon. Canadian Journal of Earth Sciences vol. 41, p. 151-164.
Preece, S.J., Westgate, J.A., Alloway, B.V. and Milner, M.W., 2000. Characterization, identity, distribution and source of late Cenozoic tephra beds in the Klondike district, Yukon. Canadian Journal of Earth Sciences vol. 37, p. 983-996.
Westgate, J.A., Preece, S.J., Froese, D.G., Walter, R.C., Sandhu, A. and Schweger, C.E., 2001. Dating Early and Middle (Reid) Pleistocene glaciations in Central Yukon by tephrochronology. Quaternary Research vol. 56, p. 335-348.