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).
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.