ABSTRACT: Onset of Mid-Pliocene permafrost and the pre-glacial to glacial transition in the Klondike Area, Yukon Territory.

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Abstract

Recent stratigraphic, geochronologic, and palynologic investigations of the Klondike goldfields, west-central Yukon, reveal a rich record of Plio-Pleistocene deposits interbedded with over 30 tephras recording much of the last 4 Ma (Froese et al. in press, Schweger et al. 1999, Preece and Westgate 1999; Fig.1, 2). Surprisingly, these tephras are largely distinct from those found in the Fairbanks area, with only 3 known correlatives (NT, Sheep Creek and Old Crow). The Klondikedeposits are of particular interest since they include sediments that immediately precede and follow the onset of glaciation in the region, and provide an important dataset relative to otherPliocene records in northwestern North America. Since work on tephras and the younger stratigraphic record (mid-late Pleistocene) is still in progress (palynology, regional correlation and fission track dating), aspects of this dataset will most certainly be refined in the near future. This presentation focuses on the mid-Pliocene record of permafrost, and the earliest evidence of glaciation in the Klondike area. Early-mid Pliocene pre-glacial sedimentation is recorded by the lower White Channel gravel that is well exposed in thick terrace sequences throughout the goldfields. Polarity of the unit is mixed (reverse-normal-reverse-normal) from multiple sites, and pollen is characterized by Pinus (up to 40%) and Picea (up to 30%) (Schweger et al. 1999, in prep). An upper WhiteChannel gravel unit marks latest aggradation, and includes ice-wedge casts characterized by a dominant normal polarity. At one location, Quartz Creek, an ice-wedge cast infill includes theQuartz Creek tephra which has an 40Ar-39Ar age on hornblende of 2.64 +/- 0.24 Ma (Kunk 1995), which agrees with the minimum age of the L-shaped spectrum of 2.71 Ma- the step least likely to contain extraneous argon. This age is consistent with a normal polarity (Gauss >2.6 Ma). Slightly earlier evidence of permafrost is found at Jackson Hill in which a reverse magnetization was found immediately overlying an ice-wedge cast. Presumably, this reverse represents a sub-chron within the Gauss (Kaena or Mammoth?). However, some caution should be reserved since this reverse magnetization was not found associated with the other ice-wedge casts in the area (all normally magnetized). Pollen from the upper White Channel and collected from within Quartz Creek tephra, shows a decrease in pinus abundance (~10%) and likely represents the latest age of a taxonomically richer boreal forest that existed in Yukon-Alaska prior to the onset of glaciation (Schweger et al. 1999, in prep; J. White pers. com 1999). The first evidence of glaciation in the Klondike area is recorded by the Klondike gravel, an extensive glaciofluvial outwash derived from the first Cordilleran ice sheet advance in the region (Fig.2). The lower portion of the Klondike gravel interbeds with the White Channel in the Klondike valley and is also magnetically normal, indicating an equivalent magnetostratigraphic unit. A lower terrace (incised 100 m) in the upper White Channel/Klondike gravel terrace is overlain by approximately 15 m of loess and reworked loess, called the Midnight Dome loess (MDL). The MDL contains multiple tephras (at least six), and at least three interglacial organichorizons (Schweger et al. 1999). The deposits have a reverse-normal-reverse-normal polarity sequence assigned to the late Matuyama chron, including Jaramillo sub-chron (1.07-0.99 Ma), and early Brunhes chron (<0.78 Ma). The basal tephra has been correlated as the NT tephra from Gold Hill Loess at Fairbanks (Westgate et al. 1990, Preece et al. 1999). At Gold Hill, NT occurs 4 m above PA tephra (2.02 ą0.14 Ma) and 4 m below AT (~1.0 Ma- Jaramillo sub-chron),suggesting an age of about 1.5 Ma for the base of the MDL, consistent with the pre-Jaramillo reversed polarity (Fig.3).Based on the close association of the Klondike gravel with the first evidence of permafrost in the upper White Channel gravel and Quartz Creek tephra (2.64 +/- 0.24 Ma) we think the first glaciation also occurred within the Gauss sub-chron (>2.6 Ma), suggesting large ice volumes in the northern Cordillera by this time. This is consistent with age estimates for the onset of major glaciation (ice-rafting) in the North Pacific (Krissek et al. 1995). However, what is perhaps more difficult to understand is the association of permafrost, indicated by ice-wedge casts (perhaps by 3.1 Ma), during what has been considered the mid-Pliocene warm interval (Dowsett et al. 1994).

Bibliography of Canadian Geomorphology