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Rocks and Minerals

lowered, or they can be left unaf-
fected while surrounding strata
subside relatively. Uplifted stra-
ta have the structural features of
domes or anticlines; character-
istically they are domed over or
around (or both) the core (includ-
ing cap and sheath if present)
and dip down into the surround-
ing synclines. The domed strata
are generally broken by faults
that radiate out from the salt on
circular domes but that may be
more linear on elongate domes
or anticlines with one fault or set
of faults predominant. Lowered Jashak-Salt-Dome - Iran Sun World Travel Agency
strata develop into synclines,
and a circular depression called forces, tectonic forces, or some rock such as sandstone, and if
a rim syncline may encircle or combination of these forces, at buried by sandstone they would
nearly encircle the domal up- the same time or with one force be gravitationally unstable. All of
lift. Unaffected strata develop following the other. Whatever them are deposited by normal
into highs surrounded by low the precise circumstance, de- processes of sedimentation and
areas. These highs, called rem- velopment of diapiric structures are widespread in sedimentary
nant highs or turtleback highs, requires a rock that flows. strata.
do not have as much vertical Although rock flow is difficult Study of models and natu-
relief as the salt domes among to visualize because of slow ral salt structures have led to a
which they are interspersed. rates of movement, its results reconstruction of the sequence
Present-day structure of strata
around salt domes may not in can be clearly seen: stone- of events in the development
work that sags, mine and tun-
of salt domes (shown in Figure
every instance coincide with the
present-day position of the salt. nel openings that flow shut, and 2). First, thick salt is deposited
and buried by denser sedimen-
This offset relationship suggests glaciers of rock salt that move
that late uplift of the salt dome down mountainsides with all the tary strata. The salt and over-
shifted its centre compared with features of glaciers of ice. Given lying strata become unstable
early uplift. very long periods of time and the and salt begins to flow from an
relatively high temperature and undeformed bed to a rounded
Origin Of Salt Domes pressure due to depth of burial, salt pillow. Flow continues into
In general, salt structures as- considerable movement of a the centre of the pillow, dom-
sociated with folds have been relatively plastic material such ing the overlying strata; at the
linked with the same forces that as salt can result. A movement same time the area from which
caused the folding. Salt struc- of one millimetre (0.039 inch) a the salt flowed subsides, form-
tures in areas without any ap- year, for example, over a period ing a rim syncline. The strata
parent folding, however, puzzled of 1,000,000 years would pro- overlying the salt, because they
early geologists and gave rise to duce a net movement of 1,000 are literally spread apart, are
a bewildering series of hypoth- m. The most common rocks that subject to tension, and fractures
eses. It is now generally agreed flow are halite, sylvite, gypsum, (faults) develop. Eventually, the
that salt structures (and diapiric and high-pressure shale. These salt breaks through the centre
or piercement structures) devel- rocks also have densities that of the domed area, giving rise to
op as the result of gravitational are lower than consolidated a plug-shaped salt mass in the
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