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Supratidal flats of the Colorado River delta are sites of active evaporite formation. Major evaporite minerals are gypsum and halite. Flooding of the flats by Gulf waters several times per year causes the Ometepec Lagoon to be covered by a standing body of water not deeper than 2 ft. After the initial stage of gypsum crystallization, halite forms until evaporation reduces the standing brine to a ground-water brine. The dominant growth mechanism of halite is a competitive upward growth of cubes which nucleate on the brine-pan floors. The upward advancing edges of the cubes produce chevron grains, an internal inverted V-structure caused by rapid alternations of fluid inclusion-rich and inclusion-free zones. Entrapment of brine inclusions represents a rapid growth stage, whe eas the inclusion-free zones represent slow-growth periods. Very rapid fluctuations in growth rates seem necessary during crystallization of chevron halite. The very shallow depths of Salina Ometepec brines are extremely susceptible to rapid changes in growth-rate-control factors such as brine temperature, concentration, and rate of evaporation. These factors in turn are dependent on air temperature, humidity, wind, etc. If the assumptions are made that (1) inclusions are dependent on growth rate, and (2) growth rate is controlled by various combinations of the above factors, it is logical to conclude that chevron halite is a characteristic feature of halites produced in environments of rapidly fluctuating growth rates. Brine depths of these environments would be expected to be very shal ow, as it is difficult to envision the necessary rapid fluctuations in a deep body of brine.
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