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German domes, when relative
age of the internal layers can
be deciphered, older material is
generally in the centre of the salt
mass and younger at the edges.
Study of halite grains in some
Gulf Coast salt domes indicates
a complex pattern of orienta-
tion that varies both vertically
and horizontally in the domes.
Mineral grains in the centre of a
Caspian salt dome are vertical;
those at its edge are horizontal.

Cap rock
Cap rock is a cap of lime-
stone–anhydrite, characteris-
tically 100 metres (328 feet)
thick but ranging from 0 to 300
m. In many cases, particularly Figure (1)
on Gulf Coast salt domes, the
cap can be divided into three
zones, more or less horizontally,
namely, an upper calcite zone,
a middle transitional zone char-
acterized by the presence of
gypsum and sulfur, and a lower
anhydrite zone. These zones
are irregular and generally are
gradational with each other, al-
though in some instances the
contact between gypsum and
anhydrite is quite abrupt. Cap
rock is generally believed to de-
velop from solution of salt from
the top of the salt core; this
leaves a residue of insoluble
anhydrite that is later altered to Land of Turquoise Domes
gypsum, calcite, and sulfur. Pre- completely encase the salt (like and the stratification (bedding
sumably, solution takes place in a sheath), or it may be limited to planes) of the shale is distorted.
the circulating (shallow) water the lower portions of the salt. It Fossils in the shale are older
zone; deeply buried domes with is most common on the deeper than in surrounding sediments,
cap rock must have been shal- portions of salt domes whose indicating that the shale came
low at some former time and tops are near the surface or on from an older, and therefore
subsequently buried. deeply buried salt domes. The deeper, layer.

Shale sheath is a feature that fluid pressure within the shale The strata around salt cores
is common to many Gulf Coast is significantly greater than that can be affected in three ways:
salt domes. In shape, it may within the surrounding rocks, they can be uplifted, they can be
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