Evaporites are precipitated in shallow seawater
that is evaporating. Usually "evaporite" refers to gypsum
(hydrated calcium sulfate or CaSO4•2H2O) or other
types of gypsum, such as anhydrite (CaSO4), gypsite, selenite,
and others. All are very soluble in water and are rapidly excavated by
running water. Some limestone caves have gypsum formations, such as gypsum
flowers and selenite needles, which are rarely found in gypsum caves themselves.
1. Gypsum Plain
The gypsum of the Permian Castile Formation in West Texas contains numerous
caves in almost as high a density as the Ellenburger Group in the Llano
Region. The Gypsum Plain has very low relief, with little outlet for infiltrating
groundwater. Accordingly, most of the caves begin small and become smaller.
A few are moderately long (e.g., Crystal Cave and Wiggley Cave, Culberson
County) where groundwater can apparently exit to a deeper zone or to a
surface valley. A few caves appear to result from artesian flow from the
underlying sandstone. Even though the Castile is up to 530 m thick, the
lower portion is predominantly anhydrite, and only about the upper 30 m
has hydrated to form gypsum. Hydration is partly responsible for the irregular
jointing of the gypsum; regional fracture systems have caused some regular
jointing. Salt beds at depth are being removed by dissolution and result
in broad subsidence sinkholes on the surface (e.g., Wink Sink, Winkler
County). The Gypsum Plain extends north into New Mexico near Carlsbad.
2. Northwest Texas
Permian-age gypsum beds up to 7.6 m thick are interbedded with shale and
dolomite. Erosion through the Blaine Formation provides an outlet for groundwater
that infiltrates its upland exposures. Where the distance from sink to
outlet is long, caves are also long if not blocked by collapse or cut by
erosion (e.g., River Styx Cave, King County). Some caves form as cut-offs
across the bends of surface drainages (e.g., Aspermont Bat Cave, Stonewall
County). Artesian salt springs also rise through the gypsum, the result
of deep solution of salt beds in the evaporite sequence (e.g., Estelline
Salt Spring, Hall County; Salt Spring Cave, King County). The Northwest
Texas region extends into western Oklahoma.
3. Kirschberg Gypsum
Gypsum caves are known from the Kirschberg gypsum beds in the middle of
the Edwards Group in Kimble, Gillespie, Mason, and Menard counties on the
Central Edwards Plateau. The Kirschberg is no more than 6 m thick, and
its caves are not extensive. Regional dissolution of the gypsum has resulted
in obvious slumping of overlying limestone beds.