EOLIAN INFLUENCES ON SOILS OF THE GRAND VALLEY, WESTERN COLORADO

Soils formed on Colorado River terraces and tributary fans and terraces in the Grand Valley of western Colorado are strongly influenced by eolian inputs. These eolian-influenced soils blanket alluvial landforms and can be subdivided into 2 categories (red calcic and gray gypsic soils) based on differences in soil color, mineralogy, and chemistry.

Red calcic eolian soils are concentrated in the western half of the Grand Valley. The soils veneer gently sloping terraces and alluvial fan remnants underlain by brown and gray sands and gravels derived from Cretaceous rocks of the Bookcliffs. In contrast to the underlying alluvium, the eolian soils are red in color, sandy, and contain up to 40% carbonate. Red calcic soils in the Grand Valley are similar morphologically and chemically to red upland soils located upwind (SW) of the Grand Valley on the Uncompahgre Plateau (CO) and Canyonlands (UT). Both the Grand Valley and upland eolian soils are red, sandy, calcic, and contain low concentrations of Mg, K, V, and Zn.

Gray gypsic eolian soils are concentrated in the eastern half of the Grand Valley. The soils also veneer alluvial terraces and fan remnants and overlie sands and gravels of the Colorado and Gunnison Rivers as well as alluvium of tributaries that drain Grand Mesa. The presence of gypsic horizons containing up to 1500 ppm of soil sulfate in settings associated with sulfate-poor alluvial parent materials strongly suggest that eolian inputs provide sulfate to these soils. A probable source for the sulfate are extensive Cretaceous Mancos shale badlands that are located upwind (SE) of the gypsic soils in the Grand Valley.

Results of soil mapping and mineralogic and chemical analyses show that NE-directed winds originating from Canyonlands and the Uncompahgre Plateau and NW-directed winds from the Delta-Montrose region converge in the Grand Valley region. These winds deposit calcic and sulfate-rich eolian materials on stable landforms such as terraces and alluvial fan remnants. Recognition of these eolian veneers is important for understanding soil chemistry and how it relates to vegetation communities as well as for correlating geomorphic surfaces and evaluating their ages.