A salt dome is a type of structural dome formed when a thick bed of evaporite
minerals found at depth intrudes vertically into surrounding rock strata,
forming a diapir. It is important in petroleum geology because salt
structures are impermeable and can lead to the formation of a stratigraphic
trap. Formation
The formation of a salt dome begins with the deposition of salt in a restricted
marine basin. Because the flow of salt-rich seawater into the basin is not
balanced by outflow, much to all water lost from the basin is via evaporation,
resulting in the precipitation and deposition of salt evaporites. The rate
of sedimentation of salt is significantly larger than the rate of
sedimentation of clastics, but it is recognised that a single evaporation
event is rarely enough to produce the vast quantities of salt needed to form a
layer thick enough for salt diapirs to be formed. This indicates that a
sustained period of episodic flooding and evaporation of the basin must occur,
as can be seen from the example of the Mediterranean Messinian salinity crisis.
At the present day, evaporite deposits can be seen accumulating in basins that
merely have restricted access but do not completely dry out; they provide an
analogue to some deposits recognised in the geologic record, such as the
Garabogazköl basin in Turkmenistan. Over time, the layer of salt is covered
with deposited sediment, becoming buried under an increasingly large overburden.
The overlying sediment will undergo compaction, causing an increase in
density and therefore a decrease in buoyancy. Unlike clastics, pressure has
a significantly smaller effect on the density of salt due to its crystal
structure and this eventually leads to it becoming more buoyant than the
sediment above it. The ductility of salt initially allows it to plastically
deform and flow laterally, decoupling the overlying sediment from the
underlying sediment. Since the salt has a larger buoyancy than the sediment
above - and if a significant faulting event affects the lower surface of the
salt - the salt can begin to flow vertically, forming a salt pillow. The
vertical growth of these salt pillows creates pressure on the upward surface,
causing extension and faulting.. Eventually, over millions of years, the
salt will pierce and break through the overlying sediment, first as a
dome-shaped and then a mushroom-shaped - fully formed salt diapir. If the rising
salt diapir breaches the surface, it can become a flowing salt glacier. In cross
section, these large domes may be anywhere from 1 to 10 kilometres across,
and extend as deep as 6.5 kilometres. Occurrences
Major occurrences of salt domes are found along the Gulf Coast of the USA in
Texas and Louisiana. One example of an island formed by a salt dome is Avery
Island in Louisiana. At present ocean levels it is no longer surrounded by the
sea but it is surrounded by bayous on all sides.
Another example of an emergent salt dome is at Onion Creek, Utah / Fisher Towers
near Moab, Utah, U.S. These two images show a Cretaceous age salt body that has
risen as a ridge through several hundred meters of overburden, predominantly
sandstone. As the salt body rose, the overburden formed an anticline which
fractured and eroded to expose the salt body.
The term "salt dome" is also sometimes inaccurately used to refer to
dome-shaped silos used to store rock salt for melting snow on highways. These
domes are actually called monolithic domes and are used to store a variety of
bulk goods. Commercial uses
The rock salt that is found in salt domes is mostly impermeable. As the salt
moves up towards the surface, it can penetrate and/or bend strata of existing
rock with it. As these strata are penetrated, they are generally bent
slightly upwards at the point of contact with the dome, and can form pockets
where petroleum and natural gas can collect between impermeable strata of
rock and the salt. The strata immediately above the dome that are not
penetrated are pushed upward, creating a dome-like reservoir above the salt where
petroleum can also gather. These oil pools can eventually be extracted, and
indeed form a major source of the petroleum produced along the coast of
the Gulf of Mexico. The caprock above the salt domes is
sometimes the site of deposits of native sulfur, which is recovered by the Frasch
process. Other uses include storing oil, natural
gas, hydrogen gas, or even hazardous waste in large caverns formed after salt
mining, as well as excavating the domes themselves for uses in everything from
table salt to the granular material used to prevent roadways from icing over.
See also Plasticity
Salt tectonics Strategic Petroleum Reserve
Underground hydrogen storage Structural trap
References