PHYSICAL AND CHEMICAL INDICATORS OF WEATHERING IN SOILS OF THE MANCOS SHALE, GUNNISON GORGE NATIONAL CONSERVATION AREA, CO, USA

The Mancos Fm., a Cretaceous black shale, comprises much of the bedrock underlying the Gunnison Gorge National Conservation Area (NCA) in western CO, and it contributes sediment and dissolved weathering products to the upper Colorado River. Runoff from unirrigated Mancos shale has a high salt content and detectable concentrations of Co, Fe, Mn, Ni, and V, associated with suspended solids, and dissolved Se and Zn. A better understanding of the chemistry and patterns of weathering in the Mancos Fm. may assist efforts to mitigate this contribution to the Colorado River.

A mass balance approach is used to evaluate the weathering profile within the Mancos soil and saprolite. It combines chemical analysis of the weathered zone with measurements of rock and soil density. Density measurements are needed to interpret gains and losses of chemical elements because they measure changes in porosity and grain density brought about by weathering and metasomatism.

Analyses of chemistry, mineralogy, and bulk and particle density were performed on samples from 12 soil pits in the Elephant Skin and Candy Lane areas of the NCA, on slopes of varying aspects and angles. The pits were 3–6 m deep and allowed sample collection from the ground surface through the weathered section into the underlying less-weathered shale.

Bulk and particle densities generally increase with depth, ranging from 0.7 to 2.3 g/cc. The lowest densities, 0.7 – 0.9 g/cc, correspond to semi-horizontal, cm-thick, veins of Fe-banding. Chemical analysis indicates these bands have been weathered more intensely than the surrounding soil and shale. Within the soil profiles, Fe, Mn, and Na generally increase with depth, indicating these elements are mobile in the upper, more weathered zone. Se tends to increase with depth, from approx 1.8 to 2.5 ppm, but its concentrations are variable. The low-density Fe bands are characterized by high concentrations of Fe, Mn, low concentrations of Se, and have higher Al/Si and Na/Si ratios than the surrounding soils. These physical and chemical characteristics suggest dissolution of the original minerals, including feldspars, leaching of Se, and deposition of clays, Fe and Mn minerals, and salts from the weathering solution. Analyses will continue to interpret chemical changes within the soil profile as a function of density and mineralogy.