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Petrographic studies of marine evaporites reveal that the initial assemblages of evaporite minerals were generated in 2 distinctly different ways. Some grew or accumulated at the sediment surface where they were evidently precipitated from an overlying brine; others were emplaced within a preexisting, unconsolidated, host sediment, and were formed in response to conditions that prevailed beneath the sediment surface within the interstitial waters of the sediment itself. The latter mode of origin characterizes supratidal sabkha-facies evaporites, which have been found to be an essential constituent of many marginal evaporite complexes. In the sabkha environment there is no overlying body of brine and the evaporite minerals are emplaced within the sediment from marine-deriv d groundwater.
This same mode of genesis of evaporite minerals can be demonstrated for some of the anhydrite, halite, and potash deposits in several "basin-facies" evaporites. The possibility arises therefore that these units record phases of emergence and, as such, suggest that the whole evaporite assemblage was deposited in an environment of shoal water and emergent banks, and not beneath a deep body of brine as previously envisaged. In view of the evident relief within some of the basins, as inferred for example from the thickness of the carbonate reefs relative to the floor on which the evaporites were deposited, such an interpretation requires that profound changes in water level may have occurred within the basins.
The evaporites of the Middle Devonian Elk Point basin are reviewed in terms of the various modes of evaporite-mineral genesis, as a background to discussion on the carbonate-evaporite relations within the complex.
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