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Abstract

The typical salt cycle in the Paradox Member of the Hermosa Formation (Middle Pennsyl-vanian) in southeastern Utah consists of basal penesaline beds (dolomitic limestone, dolomite, anhydrite) and overlying bedded rock salt with or without intercalated potash salt beds. Each cycle is thought to represent an episode of seawater recharge followed by progressive evaporation and salt precipitation. The clay minerals identified in drill core through the penesaline interval of a single cycle are discrete chlorite (clinochlore), several mixed-layer clinochlore/trioctahedral smectites (including corrensite), talc, and illite. The illite contains negligible expandable interlayers. The progression from discrete clinochlore to corrensite parallels increasing salinity through the cycle; carbonate lithologies characteristically contain chlorite/ smectites having as much as 15% expandable layers, whereas chlorite/smectites in the sulfate rocks typically contain 20–50% expandable layers. Based on data from Recent saline lakes and chemical considerations the transformation to the chlorite/smectites took place during burial diagenesis of precursor, authigenic, Mg-rich smectites that formed in response to evaporite brine compositions.

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