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Dating Early and Middle (Reid) Pleistocene Glaciations in Central Yukon by Tephrochronology

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Dating Early and Middle (Reid) Pleistocene Glaciations in Central Yukon by Tephrochronology Quaternary Research 56, 335–348 (2001) doi:10.1006/qres.2001.2274, available online at http://www.idealibrary.com on Dating Early and Middle (Reid) Pleistocene Glaciations in Central Yukon by Tephrochronology John A. Westgate and Shari J. Preece Physical Sciences Division, University of Toronto at Scarborough, 1265 Military Trail, Scarborough, Ontario M1C 1A4, Canada E-mail: [email protected] Duane G. Froese Department of Geography, University of Calgary, Calgary, Alberta T2N 1N4, Canada Robert C. Walter and Amanjit S. Sandhu Physical Sciences Division, University of Toronto at Scarborough, 1265 Military Trail, Scarborough, Ontario M1C 1A4, Canada and Charles E. Schweger Department of Anthropology, University of Alberta, Edmonton, Alberta T6G 2E3, Canada Received March 5, 2001 Cenozoic. Bostock (1966) recognized four advances of this ice The late Cenozoic deposits of central Yukon contain numerous sheet across the region, each presumably flowing westward from distal tephra beds, derived from vents in the Wrangell Mountains the Selwyn Mountains and northwestward from the Cassiar and Aleutian arc–Alaska Peninsula region. We use a few of these Mountains, as did the last glacier advance. He called them, in or- tephra beds to gain a better understanding on the timing of ex- der of decreasing age, the Nansen, Klaza, Reid, and McConnell tensive Pleistocene glaciations that affected this area. Exposures at advances, each being less extensive than its predecessor (Fig. 1). Fort Selkirk show that the Cordilleran Ice Sheet advanced close to The westernmost part of central Yukon remained unglaciated. the outer limit of glaciation about 1.5 myr ago. At the Midnight It has been suggested that the progressive restriction in the ex- Dome Terrace, near Dawson City, exposed outwash gravel, aeolian tent of the Cordilleran Ice Sheet in central Yukon was a re- sand, and loess, related to valley glaciers in the adjacent Ogilvie Mountains, are of the same age. Reid glacial deposits at Ash Bend sponse to increased elevation of the St. Elias Mountains and on the Stewart River are older than oxygen isotope stage (OIS) 6 Alaska Range during the Pleistocene, thereby denying access to and likely of OIS 8 age, that is, about 250,000 yr B.P. Supporting central Yukon of warm, moist Pacific air masses (Armentrout, evidence for this chronology comes from major peaks in the rates 1983; Tarnocai and Schweger, 1991; Burn, 1994). Hughes et al. of terrigeneous sediment input into the Gulf of Alaska at 1.5 and (1969) mapped in detail the limits and ice-flow directions of the 0.25 myr B.P. C 2001 University of Washington. Reid and McConnell advances in central and southern Yukon Key Words: glass-fission-track dating; Yukon; late Cenozoic; and grouped deposits of the older glacier advances into a single Reid Glaciation; basalt flows; magnetostratigraphy; interglacial pre-Reid unit because the absence of a glacial topography— beds; Mosquito Gulch tephra; Midnight Dome tephra; Sheep Creek the consequence of mass-wasting processes acting over a long tephra; Fort Selkirk tephra. period of time—prevented an accurate definition of their res- pective outer limits. Pre-Reid glacial deposits are also readily differentiated from younger glacial drift sheets by their rela- INTRODUCTION tively thick Luvisolic paleosols with well-developed argillic and rubified Bt horizons, features that formed when more temperate Most of central Yukon was repeatedly covered by the north- climates prevailed (Smith et al., 1986; Tarnocai and Schweger, ernmost part of the Cordilleran Ice Sheet during the late 1991). More recent studies provide evidence on the number of Supplementary data (Tables D1 and D2) for this article are available on pre-Reid glaciations in central Yukon. Glaciofluvial terraces IDEAL (http://www.idealibrary.com). along the lower Klondike River (Fig. 1) point to three pre-Reid 335 0033-5894/01 $35.00 Copyright C 2001 by the University of Washington. All rights of reproduction in any form reserved. 336 WESTGATE ET AL. FIG. 1. Location of the Fort Selkirk, Klondike, and Ash Bend areas (insets) in the Yukon Territory in relation to the distribution of glacial drift sheets (modified after Hughes et al., 1989). glaciations (Duk-Rodkin, 1996). Two pre-Reid glaciations are 1982; Hughes, 1986, Hughes et al., 1989; Jackson et al., 1996), documented at Fort Selkirk (Fig. 1; Jackson et al., 1996); four although radiocarbon dates do provide good chronological con- pre-Reid tills separated by paleosols are present at Little trol for the late Pleistocene glaciation (Hughes 1986; Matthews Bear River, Mackenzie Mountains (Tarnocai and Schweger, et al., 1990). Basaltic flows and distal tephra beds, which can be 1991; Duk-Rodkin et al., 1996); and as many as seven are dated by several radioisotopic methods, are locally intercalated noted by Duk-Rodkin (1999), based on stratigraphic sections with the late Cenozoic deposits of central Yukon and offer an in the Tintina Trench north of Dawson. In the latter work, opportunity for an improved glacial chronology, but this poten- Duk-Rodkin (1999) presented a more accurate and detailed map tial has not been fully exploited as yet. Recent application of of the distribution of the same three glacial drift sheets identi- paleomagnetic methods has offered new and important insights fied by Hughes et al. (1969). According to Froese et al. (2000), into the antiquity of the glacial record in central Yukon (Froese the oldest pre-Reid Cordilleran glaciation is probably of late et al., 2000), but, given the fragmentary preservation of conti- Pliocene age. nental sedimentary deposits, these methods in themselves do not Few numerical age determinations constrain the timing of the offer a unique or precise chronological solution. For example, older late Cenozoic glaciations in central Yukon (Naeser et al., the data gathered by Froese et al. (2000) permitted two scenarios DATING PLEISTOCENE GLACIATIONS BY TEPHROCHRONOLOGY 337 zone just above the Fort Selkirk tephra belongs to the Jaramillo Subchron (1.07–0.99 Ma; Shackleton et al., 1990). We build on the work of Jackson et al. (1996) and present a coherent chronology for the volcanics in the Fort Selkirk area, which, in turn, permits precise definition of the age of associated glacial deposits. Sections on the right bank of the Yukon River, immediately downstream of Fort Selkirk (Fig. 1), expose glacial deposits that are sandwiched between an upper and a lower sequence of basalt flows. Till is overlain by a fining-upward succession of stratified sediments consisting of poorly sorted gravel, sand, Fort Selkirk tephra, and silt (Table 1, Fig. 3). A parsimonious interpretation of these sediments would be that they represent a single glacier advance (till) and retreat (outwash sediments and loess) cycle. The upper basalt flows are continuously exposed between localities 1 and 2 (Fig. 1) but exposures of the lower basalt flows along this part of the Yukon River are restricted to FIG. 2. Location of the two major volcanic zones (black) responsible for the Fort Selkirk area (localities 2–5; Fig. 1). deposition of silicic tephra across Alaska and the Yukon Territory: Aleutian arc– Alaska Peninsula (AAAP) and Wrangell volcanic field (WVF). The three study Fort Selkirk tephra is a prominent, 20-cm-thick bed in the sites are indicated by filled squares: Midnight Dome Terrace (MDT), Ash Bend uppermost part of the stratified sediments (Fig. 3A and 3F). It (AB), and Fort Selkirk (FS). has a silty loam texture with abundant chunky and platy glass shards as well as pumice fragments. Conspicuous minerals in- on the age of deposits in the Klondike goldfields. The full power clude feldspar, hypersthene, biotite, hornblende, magnetite and of magnetostratigraphic information in glaciated terrains is re- ilmenite; minor minerals include apatite and a pink zircon. The alized only when it is calibrated with numerical ages. rhyolitic composition of the glass and its highly hydrated nature Central Yukon lies within the sphere of influence of volcanoes readily distinguish it from other tephra beds described in this in the Wrangell Mountains and the more distant Aleutian arc– study (Table 2, Fig. 4). A relatively low abundance of rare earth Alaska Peninsula (Fig. 2), so that its late Cenozoic sediments elements (REE) in its glass together with a steep REE profile and contain many distal tephra beds (Preece et al., 2000; Westgate weak Eu anomaly indicate a type-II tephra bed (Preece et al., et al., 2000). We have used a few of these tephra beds to gain an 1999) derived from the Wrangell volcanic field—an assignment improved understanding of the age of two Cordilleran glacia- that is corroborated by the Yb–Sr plot (Fig. 5). tions as well as one centered in the Ogilvie Mountains, just to Potassium-argon ages on the basalt flows (Table3) and fission- the north of the Klondike goldfields. The key geological expo- track ages based on the glass shards in Fort Selkirk tephra sures are located at Fort Selkirk on the Yukon River, Midnight Dome Terrace near the confluence of the Klondike and Yukon Rivers, and Ash Bend on the Stewart River (Fig. 1). Our approach TABLE 1 relies on the tephrochronological method, augmented by glass- Lithostratigraphy at Site 1, Fort Selkirk, Yukon Territory fission-track, K-Ar, and TL (thermoluminescense) ages, which Description Thickness (m) are supported at some localities by paleomagnetic information. Basalt lava flows; vesicular at base; numerous individual 30.00+ flows; basal part of upper basalt flow series FORT SELKIRK AREA Basaltic pillow lava (Fig. 3E) 0.50 Basaltic lapilli and ash; welded 0.12 Valley-filling lavas are extensive in the Fort Selkirk Basaltic lapilli with scoria fragments up to 2 cm; loose 0.40 area, which lies close to the confluence of the Yukon and Pelly Basaltic lapilli with some pillow lava and hyaloclastites; 0.50 Rivers (Jackson, 1989; Jackson et al., 1990, 1996).
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