Dissolved Mineral Salts Derived from Mancos Shale

The Mancos shale, a sedimentary Upper Cretaceous marine formation exposed over extensive areas in the Upper Colorado River Basin, is recognized as a major contributor to the dissolved mineral salt load in the Colorado River system. It contains gypsum and alkaline earth carbonates, and its clay mineralogy is mica, kaolin, smectite, and interstratified mica‐vermiculite. Selected surface samples of partially weathered Mancos shale were also found to contain from 281 to 345 g/kg clay (< 2 µ m diam) and to exhibit cation exchange capacities (CEC) ranging from 13.25 to 19.96 cmol kg −1 . Water suspension extracts from partially weathered shale (1:5 and 1:10 shale/water ratios) were saturated with respect to gypsum [electrical conductivity (EC) values up to 3.16 dS m −1 and soluble Ca and SO 4 up to 25.90 and 40.95 mmol (± L −1 , respectively]. Additional Na, K, and Mg released by water indicated sources of soluble ions other than gypsum. Water extracts from unweathered shale yielded much higher quantities of Na (and higher Mg in most cases) than did comparable extracts from partially weathered shale. Barium chloride released larger quantities of Mg from both partially weathered and unweathered shale and larger quantities of Na from unweathered shale than did water at comparable dilutions. The data substantiate the important role of the cation exchange complex in retention and release of soluble ions in Mancos shale. Sodium and magnesium were preferentially adsorbed by phyllosilicates within the shale and Ca was precipitated as calcite and gypsum following emergence after deposition in the Cretaceous sea. This cycle is essentially reversed in the present environment. Dispersed gypsum and alkaline earth carbonates provide soluble Ca to displace adsorbed Na and Mg that add to the dissolved mineral salt load of the Colorado River system.