Eolian Records In Alaska And Northwestern Canada

In eastern Washington, detailed studies by A.J. Busacca and colleagues (Busacca, 1991; Busacca et al., 1992; McDonald and Busacca, 1992; Busacca and McDonald, 1994; Berger and Busacca, 1995; Richardson et al., 1997) have added tremendously to our knowledge of loess history in this region. Loess in eastern Washington and adjacent parts of Idaho and Oregon may cover as much as 60,000 km2 (McDonald and Busacca, 1992). It is as thick as 75 m (Ringe, 1970), and Busacca (1991) estimates that loess deposition may have begun as early as 2 Ma (million years ago). There are dozens of paleosols within eastern Washington loess, indicating many periods of nondeposition and surface stability between times of loess deposition. The loess itself is thought to be derived primarily from fine-grained slackwater sediments, which in turn are derived from cataclysmic floods of proglacial Lake Missoula (McDonald and Busacca, 1992).

Loess is areally the most extensive surficial deposit in Alaska and adjacent parts of the Yukon Territory in Canada and is found over most parts of this region where rugged mountains, such as the Alaska Range and the Brooks Range, are absent (Fig. 18). As with South America and the North American midcontinent, the classical concept has been that most loess deposition in Alaska took place during glacial periods. However, extensive Holocene loess is found in Alaska (Pewe, 1975; Beget, 1990, 1996; Muhs et al., 1997c), and, at localities near Fairbanks (Eva Creek and Fox), Holocene deposits are almost as thick or thicker than the best candidates for deposits of last-glacial age (Fig. 19). Because glaciers still exist in Alaska and northwestern Canada, it is not surprising that abundant glaciogenic silt is available in many of the region's major river systems, and contemporary loess deposition is well documented (Pewe, 1951, 1975; Nickling, 1978).

Eolian sand is also extensive in Alaska (Fig. 18), but

FIGURE 15 Map showing the distribution of mostly late Holocene eolian sand (stippled) in the northern Great Plains of southern Canada and the northern United States, modern sand-moving winds, and late Holocene paleowinds based on dune orientations. Data compiled from Running (1996), Wolfe (1997), Muhs et al. (1997b), and Muhs and Wolfe (1999).

FIGURE 15 Map showing the distribution of mostly late Holocene eolian sand (stippled) in the northern Great Plains of southern Canada and the northern United States, modern sand-moving winds, and late Holocene paleowinds based on dune orientations. Data compiled from Running (1996), Wolfe (1997), Muhs et al. (1997b), and Muhs and Wolfe (1999).

with few exceptions is largely stabilized by boreal forest or tundra vegetation. Geomorphic and stratigraph-ic evidence suggests that most eolian sand was derived from glaciofluvial sediments and was deposited originally in sand sheets rather than dunes, at least in part because of limited sand supply (Lea and Waythomas, 1990; Lea, 1996). Later dune building is thought to have occurred mainly as a result of reworking of previously deposited sheet sand. It is not clear at all localities, however, whether most of this dune building took place during latest Pleistocene, early Holocene, or mid-Holocene time (Lea and Waythomas, 1990). Available radiocarbon ages suggest that much eolian sand deposition, whether of sand sheets or dunes, took place in the interval from ca. 18,000 to ca. 10,000 14C B.P. However, Holocene eolian sand deposition is also docu-

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