Kilometers

FIGURE 5 Map showing the distribution of eolian sand and loess (silt) and paleowind directions in Venezuela and Colombia. (Redrawn from Iriondo, 1997.)

10,000 B.P.) as is seen in the sediments of Lake Valencia (Bradbury et al. 1981).

12.5. EOLIAN RECORDS IN THE DESERTS OF THE SOUTHWESTERN UNITED STATES AND NORTHERN MÉXICO

Dune fields are widespread over the desert regions of the southwestern United States and northern México, although these features are generally smaller than those found in the Great Plains to the east (Fig. 6). The largest dune fields, mostly stabilized, are found on the Colorado Plateau in northeastern Arizona and northwestern New Mexico (Hack, 1941; Wells, et al. 1990). Elsewhere in this mostly arid region, some dune fields are active, such as the Gran Desierto in México, most of the Algodones and parts of the Kelso dunes in California, and White Sands in New Mexico.

Detailed studies on the Colorado Plateau show that these dune fields have a long and complex history. Wells et al. (1990) conducted stratigraphic and soil ge-omorphic studies in the Chaco dune field of northwestern New Mexico and identified three main periods of eolian sand activity. Radiocarbon ages indicate that while the oldest eolian sand unit in the area is of late Pleistocene age (-16,000-12,000 14C years B.P.), the other two units are both of late Holocene age, ca. 56002800 14C years B.P. and less than -1900 14C years B.P. These ages agree with late Holocene luminescence ages of dunes elsewhere on the Colorado Plateau in northeastern Arizona (Stokes and Breed, 1993). Results of both studies indicate that, in contrast to earlier concepts, the mid-Holocene Altithermal warm period was

FIGURE 6 Map showing the distribution of eolian sand (stippled) in desert regions of northern México and the southwestern United States, modern sand transport directions, and late Holocene paleo-winds from stabilized dunes. Data compiled mainly from Hack (1941), Jennings (1967), Hunt (1977), Lancaster et al. (1987), Wells et al. (1990), Lancaster (1994), Muhs and Holliday (1995), and Muhs et al. (1995); modern wind data are compiled by the authors.

FIGURE 6 Map showing the distribution of eolian sand (stippled) in desert regions of northern México and the southwestern United States, modern sand transport directions, and late Holocene paleo-winds from stabilized dunes. Data compiled mainly from Hack (1941), Jennings (1967), Hunt (1977), Lancaster et al. (1987), Wells et al. (1990), Lancaster (1994), Muhs and Holliday (1995), and Muhs et al. (1995); modern wind data are compiled by the authors.

not a particularly important period for eolian sand activity. Wells et al. (1990) point out that late Holocene eo-lian sand is volumetrically far more important than mid-Holocene eolian sand.

Although they are of smaller extent than elsewhere in North America, the dunes of the Mojave and Sono-ran Desert regions of southeastern California, Arizona, and northern México have received the most attention for studies of geomorphology, sediment transport, and origins (Lancaster et al., 1987; Tchakerian, 1991; Lancaster, 1994; Zimbelman et al., 1995; Muhs et al., 1995; Lancaster and Tchakerian, 1996), as well as geochronol-ogy (Clarke, 1994; Wintle et al., 1994; Rendell et al., 1994; Clarke et al., 1996; Rendell and Sheffer, 1996; Clarke and Rendell, 1998). Despite the evidence that Great Plains dunes seem to record relatively arid periods (see discussion later), a general concept that is emerging from studies in the Mojave and Sonoran Deserts is that the degree of activity of many dunes is only indirectly a function of climate. Many of the dune fields in southeastern California seem to be supply-limited systems, and therefore, the degree of dune activity may not be related to overall moisture balance per se, but rather to the effect climate has on supplying new sediment from fluvial or lacustrine sources (Lancaster, 1994; Lancaster and Tchakerian, 1996; Muhs et al., 1995; Clarke and Rendell, 1998). In an overall summary of luminescence dating of Mojave Desert dunes and sand sheets, Clarke and Rendell (1998) showed that ages range from as old as -23,000 cal. years B.P. to the late Holocene, and are similar to ages of nearby pluvial lake high stands, whether of late Pleistocene or Holocene age. They suggested that the main effect of climate change on dune activity is to bring about conditions whereby new supplies of sediment become available to dune fields, such as from pluvial lake beaches or alluvial sources. An arid climatic regime with a minimal vegetation cover almost certainly plays some role in degree of dune activity, however. For example, Muhs et al. (1995) showed that while sediments of the Algodones dunes had a pluvial lake source in the past, the region is sufficiently arid now that the dunes are active, despite no new supplies of pluvial lake beach sand.

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