In the lower half of the Devonian Prairie Evaporite Formation, halite units 2–10 cm thick alternate with anhydrite units, most of which are less than 1 cm thick. Such halite-anhydrite alternations are thought to result from seasonal changes in the basin waters during deposition and are called seasonal layers.
Halite occurs as opaque “chevron” grains with abundant liquid inclusions and as transparent grains without liquid inclusions. The chevron grains are conspicuously elongated perpendicular to bedding and contain a distinctive chevron structure caused by zones of liquid inclusions. They started growth as cubes, which were oriented with (110) or (111) approximately parallel to bedding; subsequently growth took place on only two or three upward-directed faces. These grains are considered to be primary and grown on the Devonian sea floor. Similar chevron grains were produced artificially during competitive growth into a salt solution.
Inclusion-free halite grains tend to have irregular shape and no elongation perpendicular to bedding. They are larger than associated chevron grains and locally appear to have replaced them. Their bromide content is higher than that of associated chevron grains.
Bromide analyses of successive chevron grains above an anhydrite layer show a rapid increase to a maximum value and then a steady decrease as the overlying anhydrite layer is approached. This variation of bromide is most satisfactorily explained by influxes of sea water or brine. Following the deposition of anhydrite, the influxes increased markedly in salinity and then decreased before deposition of the overlying anhydrite layer.