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Selected Properties, Distribution, Source, and Age of Eolian Deposits and Soils of Southwest Colorado

USDA-SCS, Soil Survey Office, 2423 Mancos Road, Cortez, CO 81321

W. D. Nettleton

USDA-SCS, NSSL, Lincoln, NE

G. A. Bowman

USDA-SCS, State Office; Denver, CO

V. L. Clay

Virginia City, NV

* Corresponding author.

ABSTRACT

Reddish-hued soils that formed in eolian deposits are located on relatively stable landforms throughout most of the Colorado Plateau geologic province of Southwest Colorado. Because the particle size control sections of these soils change gradually from fine-loamy to fine-silty to fine in a distance of about 35 km, information about the distribution and depositional patterns was needed to more accurately map the soils of the area. Decreases in the amounts of fine sand and coarser (0.1–2.0 mm) and increases in the amount of total silt along southwest to northeast transects demonstrated downwind sorting. Field observations indicated that north and east aspects have deeper soils and thicker sola. This is presumed to be a windward/leeward effect. Analyses of particle size distribution, very fine sand and clay minerology, carbonate content, color, and other soil characteristics in and around the soil survey area indicate that the sources of the eolian material are west and south of the area and that there are small increases in the degree of weathering in the soils from the southwest to the northeast. These small increases in weathering appear to be a result of an increase in precipitation with the increase in elevation, an increase in effectiveness of the precipitation as the temperatures change from mesic to frigid, and as a result of an increase in silt content of the eolian deposits from the southwest to northeast. The most recent episode of major eolian deposition in the area began about 15 970 ± 155 yr B.P. according to radiocarbon dating. The degree of weathering in the soils suggests that most of the eolian material was deposited in late Pleistocene time and that additions during Holocene time did not greatly affect the genesis of the soils.

Contributions from USDA-SCS, Cortez, CO, Durango, CO, Montrose, CO, Norwood, CO; NSSL, Lincoln, NE, state offices, Denver, CO, and Salt Lake City, UT; U.S. Dep. of Interior, BLM, Moab, UT, and Dolores Archeological Project. Dolores, CO.

Received for publication March 19, 1987.