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Deposition of Salt—Continued


When the water level in the evaporite basin falls below the surface of previously deposited halite, halite is exposed to vadose processes. Halite is dissolved by rainwater and dew and reprecipitates from shallow ground water as cements and displacive crystals and capillary crusts. Dissolution forms microkarst pits several feet deep. Siliciclastic mudstone is transported across the dry flat by eolian and sheetwash processes and is concentrated because of halite dissolution. Repeated dissolution and precipitation of halite in this low-accommodation environment creates chaotic mixtures of halite and mudstone (Handford, 1982; Rosen, 1989).

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Halite dissolution in the depositional environment as a result of exposure. (a) Photomicrograph of displacive halite formed by ground-water precipitation in sandstone, then dissolved by fresh water that flooded the surface. Dissolution of overlying halite has formed an insoluble residue. Gruy Federal Rex White, 1,879 ft below datum. (b) Karst pit dissolved in halite in the vadose environment during exposure is filled with mudstone and displacive halite. Core from DOE Stone and Webster G. Friemel, 2,531.7 ft below datum.

In contrast to these shallow-water environments in which halite thickness is limited by syndepositional dissolution, syndepositional dissolution is suppressed in high-accommodation settings. In water more than a few feet deep, both precipitation and dissolution on the basin floor are limited. The dense, stratified brine reduces communication between the surface of the water mass, where evaporation and dilution occur, and sediments on the brine pool floor. Halite precipitates at the brine-air interface where it forms floating crystals and rafts of crystals (Hovorka, 1994). These crystals founder and form cumulates of millimeter-diameter crystals on the floor of the water body. When less saline water floods a deep-water basin, it floats on top of the dense brine already present and does not dissolve previously precipitated halite on the floor of the water body. Gypsum that precipitates at the surface as floodwater evaporates falls to the basin floor as another cumulate layer, forming finely laminated, fine-grained halite with anhydrite laminae. Castile halite units exhibit this deep-water texture and document the rapid accumulation of evaporites when accommodation is not limited by water depth (Hovorka, 1990).

The effects of dissolution in the depositional environment cannot be directly observed in the Midland Basin because core is not available through these evaporites. Based on an understanding of these processes in the adjacent Palo Duro and Delaware Basins, however, it is reasonable to infer that synsedimentary dissolution influenced salt quality in the Midland Basin. Observed updip increases in the amount of mudstone-halite and mudstone beds is related to decreased accommodation and increased exposure in these settings. Gradual thinning of salt beds toward the basin margins is also most likely related to accommodation by a synsedimentary dissolution mechanism. Increased salt-bed thickness as well as relatively high salt purity are expected in areas of high accommodation such as the Delaware Basin.


Base-of-Cycle Dissolution