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Origin of the Salina Salt of Michigan
Louis F. Dellwig
The purpose of this study was to determine the manner of deposition of the Salina salt, including the approximate temperature of deposition and the origin of the dolomite-anhydrite laminae.
Specimens were obtained from drill cores from the Michigan basin and from the Detroit mine of the International Salt Company. The bulk of the data was obtained through thin section study and this was supplemented by the examination of polished slabs, water-insoluble residues, and sections of core from which the salt had been dissolved.
Pyramidal shaped hopper crystals of halite, comparable to those crystals produced in the manufacture of grainer salt and those formed in the manufacture of solar salt, were identified in the Salina salt. The pyramidal forms were outlined by liquid inclusions, and from these inclusions the temperatures of formation were determined as being between 32.0 and 48.4° C.
The salt was deposited in layers or bands of three distinct types: (1) cloudy layers of inclusion-rich pyramidal shaped hopper crystals of halite, (2) clear layers of inclusion-free halite, and (3) laminae of anhydrite and dolomite. These units are in part obscured in the recrystallized salt.
The alternation of bands of clear and cloudy halite was the result of temperature changes probably related to the seasons. The salt crystals, initially formed on the surface, grew as pyramidal shaped hoppers. Until the brine became saturated, settling hopper crystals were dissolved. After the saturation point had been reached pyramidal shaped hopper crystals accumulated on the bottom of the basin to form a layer of cloudy salt. With subsequent cooling the brine passed into the supersaturated state and settling hopper crystals provided nuclei for clear overgrowths. These crystals which grew on the bottom of the basin are inclusion-free and form the layers of clear salt. The return of higher temperatures followed again by lower temperatures would cause a repetition of this sequence. Thi cycle could have been interrupted at any time by the addition of brine to the basin or by periods of unusual climatic conditions.
The deposition of the anhydrite-dolomite laminae may be considered to have been the result of the influx of normal sea water. The addition of brine to the basin as a surface layer would have permitted the maintenance of life in this brine until the concentration reached the point at which mass extermination took place. Subsequent decomposition of the remains caused reducing conditions to develop in the basin and iron precipitated as pyrite. Indicative of this influx is the association with the laminae of carbonaceous material, pyrite, and a petroliferous odor and the lack of a recognizable relationship between the clear or cloudy salt and the anhydrite-dolomite laminae. Iron associated with the halite layers is in the form of hematite, indicating that oxidizing conditions were normal or the basin.
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