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

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 (CaSO 4 · 2H 2 O), epsomite (Mg 2 SO 4 · 10H 2 O), hexahydrite (MgSO 4 · 6H 2 O), pentahydrite (MgSO 4 · 5H 2 O), starkeyite (leonhardtite) (MgSO 4 · 4H 2 O), kieserite (MgSO 4 · H 2 O), loewite [Na 4 Mg 2 (SO 4 ) 4 · 5H 2 O], bloedite [Na 2 Mg(SO 4 ) 2 · 4H 2 O],mirabilite (NaSO 4 · 10H 2 O), and thenardite [Na 2 SO 4 ). 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 (Na 2 CO 3 · 10H 2 O), trona (Na 2 CO 3 · NaHCO 3 · 2H 2 O), nahcolite (NaHCO 3 ), and halite (NaCl). Origin of the evaporite mineral species and their significance in relation to salt loading of the Colorado River are discussed.