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Evaporite Mineral Species in Mancos Shale and Salt Efflorescence, Upper Colorado River Basin¹

ABSTRACT

Evaporite mineral species associated with Late Cretaceous marine sedimentary formations in a selected region of the Upper Colorado River Basin were identified by x-ray diffraction analysis. Mancos Shale, a major contributor of dissolved mineral salts to the Colorado River and several of its tributaries in the Upper Basin, contains abundant gypsum as the only detectable evaporite mineral. Salt efflorescence associated with the Mancos Shale, however, contains variable quantities of Ca, Na, and Mg sulfate evaporite species, including gypsum (CaSO₄ · 2H₂O), epsomite (Mg₂SO₄ · 10H₂O), hexahydrite (MgSO₄ · 6H₂O), pentahydrite (MgSO₄ · 5H₂O), starkeyite (leonhardtite) (MgSO₄ · 4H₂O), kieserite (MgSO₄ · H₂O), loewite [Na₄Mg₂(SO₄)₄ · 5H₂O], bloedite [Na₂Mg(SO₄)₂ · 4H₂O],mirabilite (NaSO₄ · 10H₂O), and thenardite [Na₂SO₄). The hydrated sulfates of Na and Mg, as very labile salts, are subject to change in response to relatively minor changes in temperature and moisture status. Dominant evaporite mineral species associated with a few localized seeps and springs emanating from the Late Cretaceous Mount Garfield formation of the Mesaverde group, overlying the Mancos Shale, include soda (Na₂CO₃ · 10H₂O), trona (Na₂CO₃ · NaHCO₃ · 2H₂O), nahcolite (NaHCO₃), and halite (NaCl). Origin of the evaporite mineral species and their significance in relation to salt loading of the Colorado River are discussed.

¹ Contribution from the Department of Land, Air and Water Resources, Univ. of California, Davis CA 95616. The research leading to this report was supported by the Office of Water Research and Technology, USDI, under the Matching Grant program of Public Law 95-467, as amended, and by the University of California Water Resources Center, as part of Office of Water Research and Technology Project no. B-170-COLO and B-201-CAL and Water Resources Center Project UCAL-WRC-W-562.

² Professor of Soil Science, Post Graduate Research Associate, and Professor of Water Science, respectively.

Received for publication August 10, 1981. Accepted for publication January 18, 1982.