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Dunes of Cactus Plain, western Arizona

Origin of the Parker-Cactus Plain Dune Field

For the past 5 million years, the Colorado River has transported abundant sediment southward from the Colorado Plateau through the Basin and Range Province to the Gulf of California. Along that route, sand rich in quartz grains was deposited by the river to form local beaches, bars, and in wider valleys, broad floodplains. Reworking of sands by strong westerly winds associated with low-pressure systems from the Pacific transported the grains eastward.  

Colorado River & environs east of Parker, Arizona, ca. 1940s (B&W) and recent.
Colorado River & environs east of Parker, Arizona, ca. 1947-1953 (B&W) and 2017. 

Southeast of Parker, on the Arizona side of the lower Colorado River, an extensive dune complex comprising the Parker and Cactus Plain dune fields covers at least 180 sq. miles. Using geochemical fingerprinting Jim Zimbelman and Steve Williams (2002) sourced the mostly rounded, quartz-dominated sands to the nearby Colorado River. Immediately south of Parker, the modern floodplain of the Colorado River widens dramatically – an area now covered by irrigated agricultural fields. River sediment, including lots of quartz-rich sand, spread across this floodplain before it was tamed by upstream dams and irrigation. At times in the Quaternary and the Pliocene when the river was transporting and depositing more sediment, this floodplain was even more extensive.

There is a strong SW-NE orientation for larger curvilinear dunes in the western part of the Cactus Plain, resulting from prevailing SW winds associated with fall-winter-spring low pressure systems blowing sand from the Colorado River floodplain. Farther east the dune forms are more complex, and include W-E linear dunes and low, crescent-shaped forms. We mapped the central and western part of the Plain a few years ago - DGM-108 mainly, but also a bit covered by DGM-107 and DGM-111.

Zimbelman and Williams state that the dunes of the Cactus Plain are stabilized by vegetation. The attached image from Google Earth shows WSW-ENE oriented dunes with many little speckles, which are bushes and clumps of grass that partially stabilize the dunes; the larger dark dots are palo verde or mesquite trees, mostly in interdune areas.

Google Earth image showing partially vegetated dunes
Google Earth image showing lightly vegetated dunes.

There is certainly plenty of sand still moving around on the dunes, however. Note that in several places the dunes cause the small tributary wash in the middle of the image to divert to the E to get around the end of a dune. There is dynamic interaction between the dune sand migrating ENE and flow events of the tributary that occasionally trim the end of the dune. Also, several dunes have encroached onto the floodplain of the larger wash in the NE part of the image, where the ends of the dunes form prongs over the light gray floodplain deposits and locally bleed into the adjacent small channels. Small-scale dunes superimposed on the larger dune forms are very common throughout the parts of the Cactus Plain that I have visited (see ground photos for examples). Clear evidence of continuing sand movement abounds.

Acknowledgments.  Debra Block (US Geological Survey, Flagstaff, AZ) graciously provided the historical mosaic of the Parker – Cactus Plain area.

Zimbelman, J.R. and Williams, S.H., 2002, ‘Geochemical indicators of separate sources for eolian sands in the eastern Mojave Desert, California, and western Arizona’. Geological Society of American Bulleting, v. 114, p. 490-496.

Posted by Phil Pearthree 2/7/2018