Salt Tectonics: An Advanced Examination of Rock Salt
Susan Bratek
(http://www.enerkon.com/Images/saltcave.png)
Abstract
The study of salt structures is very important and can help to determine what kind of
deformation occurred in a certain area. The properties of salt are very unique. There are
various forces acting on salt that contribute to its ability to flow, as well as forces that
restrict salt’s flow. There are many types of structures including diapirs, sheets, pillows,
and glaciers. Diapirs can take different forms and can evolve through each form during
its lifespan. Salt sheets can also form from different types of advance. Different
structures form in areas of extension and shortening, thus allowing geologists to
determine what kind of stress the area is under. Louisiana and the Dead Sea Basin show
great examples of various types of salt structures. Salt is very important economically
also, as it creates hydrocarbon traps, which allow natural oil and gas to collect.
Understanding salt and how it can deform the surrounding area is essential to finding and
being able to use these natural resources.
Introduction
Salt tectonics is a very significant area of study for geologists around the world. It can
greatly influence the structures that are around it. Understanding salt tectonics is an
important factor in oil and gas exploration. It can create hydrocarbon traps for oil and
gas, and knowing how, where, and why these form can be very profitable. Salt diapirs
are important for providing sources for salt and sulfur. Salt structures can be found as
layers, pillows, diapirs, and sheets, and are located all over the world. Regional
extension or shortening plays a big part in what type of formation the salt will be found
as. The regions of Louisiana and the Dead Sea Basin contain salt and the settings there
are examined for how salt affects those areas.
Salt and Its Significance
Rock salt is a crystalline aggregate of the mineral halite (NaCl). Salt is weak, flows like
a fluid at the surface and subsurface, and is less dense than most carbonates and all
moderately to fully compacted siliciclastic rocks. Slightly impure salt has a density of
2200 kg/m³ (Hudec and Jackson, 2007). A density inversion is present, causing the salt
to rise. It is easy for the salt to rise as it flows very easily in its solid state. Salt is very
unstable and can lose potential energy by overturning. Because of this, basins containing
salt are much more susceptible to deformation than basins without salt. Dry rock salt
deforms by dislocation creep, which is slip that occurs within the crystalline lattice. Wet
salt is much weaker, and deforms by diffusion creep, which is the deformation of crystals
by movement along grain boundaries (Hudec and Jackson, 2007). Due to salt’s
weakness, its tectonism is related to regional deformation.
The most important forces acting on salt are differential loading, displacement of a salt
body relative to another, and a thermal gradient. On the other hand, strength of
overburden and boundary drag inhibits the flow of salt (Hudec and Jackson, 2007).
Many things can be involved in salt tectonics. Regional extension, regional shortening,
and even deformation caused by gravity are just some. Interest in salt tectonics comes
from the oil and gas industries. It creates hydrocarbon traps, and salt is a seal to fluid
migration.
Salt is also a good conductor of heat, raising the thermal maturity of rocks above salt
structures, and cooling those below it. Salt can change the temperature distribution and
life of a sedimentary basin. Evolving salt domes may affect the maturation level of
source rocks near the domes. Salt domes exposed at the surface deplete the heat from
underneath and from the side of the dome. In the Gulf of Mexico, pre- and early Tertiary
salt diapirs and sheets may have slowed the maturation of subsalt source rocks in deeper
regions of the basin (Mello et al., 1995). At room temperature, the thermal conductivity
of salt is usually three times greater than any other porous sediment. The geometry of
these structures also adds to these effects. Organic matter maturation is thought to follow
temperature-dependent chemical reactions. Any temperature differences created by salt
can affect the maturation of the source rock next to it (Mello et al., 1995). The shape of
the salt structure is key to influencing the movement of heat in a sedimentary basin.
Common shapes are layers, pillows, domes, and mushroom shaped domes (Fig. 1). The
presence of salt produces a ‘cooling’ effect when compared to a basin that does not
contain salt. This effect is dependent on the thickness of the salt. The efficiency of salt
to absorb or release heat increases the closer the structure is to the surface (Mello et al.,
1995). Therefore, salt controls the maturation of source rocks near salt domes and sheets.
Fig. 1. Diagram showing various salt structures. Structural maturity and size increase
toward the background. (a) Structures rising from line sources. (b) Structures rising from
point sources (Hudec and Jackson, 2007).
Formation of Salt Structures
The driving force of salt structure formation is differential loading. There are three types
of loading which can affect salt flow. Gravitational loading is a combination of the
weight of rocks above the salt and the gravitational forces in the salt. Salt is forced to
move from its source layer into a diapir by loading. Displacement loading is the forced
displacement of one boundary of rock relative to another (Fig. 2). This type of loading is
seen in basins with preexisting salt structures. Thermal loading is any change in volume
that is caused by changes in temperature. Hot salt expands and becomes buoyant. This
starts convection within the salt layers (Hudec and Jackson, 2007). In Louisiana coastal
plains, salt domes are vertical bodies. Buoyancy produces upward flow of salt, while
dissolution of salt domes produces downward convection. In the case where salt
structures form under water, these structures and sea floor relief have the potential to
drive long-term convection across the seafloor (Wilson and Ruppel, 2007).
Fig. 2. Effects of displacement loading on salt structures. (a) Shortening. (b) Extension
(Hudec and Jackson, 2007).
The resisting forces of salt are the strength of overburden and boundary friction (Hudec
and Jackson, 2007). See Fig. 3. At the brittle-ductile transition, rocks increase in shear
and frictional strength. Thick sedimentary roofs make it harder for the salt to distort than
thin ones. Therefore, the salt has a harder time trying to pierce and move up through the
overlying rock. Boundary drag is another resistance, where there is drag along the top
and bottom of the salt layer surfaces. If driving forces can overcome resisting forces,
then salt will flow (Hudec and Jackson, 2007). Buoyancy was once considered the main
driving force in salt movement, and while it still affects the salt, it is not seen anymore as
an initiating force.
Fig. 3. Illustration of forces resisting salt flow. Diapir formation causes deformation of
the overlying roof. Salt is sheared near the edges of salt bodies during flow (Hudec and
Jackson, 2007).
Types of Diapirs
Jackson and Talbot, 1991 state that salt diapirs are masses of salt that have flowed
ductilely and appear to have discordantly pierced or intruded the overburden. They can
be found in the north German Plain, western Iran, the Gulf Coast region of the United
States and Mexico, around the Caspian Sea, the Mediterranean Sea, the Red Sea, west
central Africa, and the Canadian Arctic (Twiss and Moore, 2007). Diapirism is a process
that forms salt domes, metamorphic gneiss domes, and igneous plutons (Hudec and
Jackson, 2007). Diapirs begin as anticlinal or domal uplifts and evolve into walls,
columns, bulbs, or mushroom shapes. They are dome-shaped structures where the rocks
in the core have risen ductilely and broken through the overlying rock. One way these
structures move is by buoyancy, with low density rock deeper in the Earth rising up
through high density rock on top of it. When the salt moves up through the overlying
rock, these sediments become bent upward.
Salt domes can be active, passive, reactive, or dormant (Yin and Groshong Jr., 2006).
See Fig. 4.
Fig. 4. Different types of diapirs. A salt diaper can go through all of these stages (Hudec
and Jackson, 2007).
For a diapir to form, the salt must move up into the place previously taken by the
overburden. Therefore that rock must be displaced or removed (Hudec and Jackson,
2007). The overburden rock can be deformed in many ways. Extension is one way. The
overburden can be extended, letting a reactive diapir move up between the separated fault
blocks. Also, parts of the overburden can be lifted or rotated, allowing the diapir to push
its way through them, called active diapirism. The active stage is most dynamic and
complex in salt dome growth history (Yin and Groshong Jr., 2006). Another way is
erosion. The roof could be eroded completely down, allowing the diapir to rise up. And
lastly, salt may be positioned into its overburden in the hanging wall of a thrust fault. All
of these stages can occur during the life of a single salt structure (Yin and Groshong Jr.,
2006).
In the case where the overburden is a fluid, diapirs can rise through ductile thinning of
the roof. This is very rare, so most diapirs rise up through brittle overburdens. Passive
diapirsm occurs when a diapir completely breaks through its overburden and is exposed
at the surface (Fig. 5). Sediments then accumulate around its exposed surface. Many of
the tallest salt domes and walls have been passive diapirs (Hudec and Jackson, 2007).
Fig. 5. The top section shows a reactive diapir. In the bottom section, the diapir evolved
to form a passive diapir. The inward dipping normal faults shows an extensional origin
(Hudec and Jackson, 2007).
Reactive diapirism occurs during extension, where salt moves up and fills the hole
created by the thinning sediments and separation of fault blocks. This thins and weakens
the salt’s roof. It can then turn into an active diapir if the diapir can break through the
roof. This is due to the buoyancy of the salt. Active diapirs break through pretty fast and
pierce the surface, turning into a passive diapir (Fig. 6). Asymmetric forms of this kind
of reactive diapir that a have one dominant fold are called salt rollers.
Fig.6. (a) Reactive diapir in the Gulf of Mexico. (b) Previously passive diapir in the Gulf
of Mexico, now buried. (c) Previously active diapir in the Lower Congo Basin, Gabon.
(d) Allochthonous salt sheet in the Gulf of Mexico (Hudec and Jackson, 2007).
During shortening, the overburden may collapse. Diapir enlargement is where salt can
rise up through the roof by a combination of normal faulting, erosion, and active
diapirism. When preexisting salt structures are shortened, a teardrop shape diapir is
produced. This upper part of the structure can become separated from its original salt
source. This part continues to move upward, however the lower part remains as an
autochthonous salt pedestal (Hudec and Jackson, 2007). See Fig. 7 and 8.
Fig. 7. Formation of a teardrop diapir. Shortening causes the feeder to become
completely pinched off (Hudec and Jackson, 2007).
Fig. 8. Seismic section of a teardrop diapir in the northern Lower Congo Basin, Gabon
(Hudec and Jackson, 2007).
Salt Sheets
A salt sheet is made up of salt lying over younger rocks, and its source is a layer that has
a width greater than its thickness (Hudec and Jackson, 2007). Salt sheets that have an
extrusive advance are caused by salt coming from its source faster than sedimentation,
erosion, and dissolution can control it. This kind of salt extrusion is an intense passive
diapir. Another type of salt sheet is an open-toed sheet. This is when salt is somewhat
buried by a roof, but extrudes a small part (the toe). This slows the flow of salt. The
thrust advance sheet is when the sheet, along with the roof, move forward along a thrust
fault at the edge of the salt sheet. One other type that is very rare is the salt wing. These
break through from the back of the diapir into a preexisting salt layer that joins the diapir
(Fig. 9).
Fig. 9. Four different types of salt-sheet advance (Hudec and Jackson, 2007).
Life Span of a Salt Structure
Salt structures change shape as they evolve. What can occur when salt diapirs pierce the
surface relies on the environment into which they are exposed. The maximum height of
the dome is dependent on the speed the salt moves up and the rate at which it breaks
down by gravity spreading and climatic erosion. In regions where salt dissolution is
slow, such as in the Zagros Mountains, a rounded dome will rise in a few thousand years
to about 400m above its salt source (Talbot, 2005). The amount of salt lost to dissolution
is dependent on the shape of the salt flow forming the structure (Brewer and Kenyon,
1996). The exposed salt cannot support its own weight and the bottom parts of the dome
spread out in a sheet of allochthonous salt over the area. Younger salt will take a rounded
shape of a viscous fountain. Salt fountains can maintain the same structure shape where
tectonic and climatic erosion breaks down the salt as fast as it rises. Over time salt
withdrawal will close the floor and roof of the salt source. Salt that is left over in the
source layer will hold up the overburden in a turtleback structure between diapirs (Talbot,
2005). After time, the tops of the domes are dissolved away and what is left is caprock
(Twiss and Moore, 2007). Erosion takes place and eventually only an empty void lined
by soils above a breccia chimney points to the site of a former salt extrusion (Talbot,
2005).
Extension vs. Shortening
Salt tectonics in a region of extension can be found in active rift basins along with the
outer shelf and upper slope of passive margins. In an extensional region, diapirs rise in
the areas where overburden thickness is minor (Ferrer et al., 2008). Here will be found
reactive diapir rise and extensional diapir fall. Salt thickness is very important on what
structure will form in this setting. Also very important is the amount of salt available. If
there is not enough to continue the growth, the diapir will fall. If salt is in abundance, the
diapir will grow passively or reactively.
Inverted rift basins, at convergent plate boundaries, and at the downdip toes of passive
margins is where shortened salt structures can be found (Hudec and Jackson, 2007).
Shortening causes the overburden to thicken, and will slow the growth of diapirs because
the roof rock becomes stronger. Strike slip faults have little influence on salt, but salt
may flow if tensional or compressive stress is found. Salt structures will form at bends or
stepovers of strike slip faults. Also, since a releasing bend forms a pull-apart basin, a
preexisting diapir can become bigger, and move up or fall (Hudec and Jackson, 2007).
This all depends on the amount of salt available.
Louisiana
One area where many salt structures can be found is beneath the outer shelf and upper
slope of offshore central Louisiana. The salt bodies found here can be broken up into
three groups: reactive, active, and passive diapirs (Rowan, 1995). It was found that most
of the diapirism in this area was formed by regional extension. Reactive diapirs are rare
in this study area. Reactive diapirism is where salt rises up and fills the space created by
the thinning sediments and separation of fault blocks, due to extension. An example that
was studied is triangular in profile, elongated east-west, and overlain by extensional
graben. It is surrounded by normal faults and is located on a salt sheet (Fig. 10).
Fig. 10. Seismic profile of a reactive diapir. The salt body is overlain by a crestal graben
and faults that get older down the salt flanks (Rowan, 1995).
There are many areas where raised overburden is found due to salt bodies rising and
pushing upwards. This is indicative of active diapirism. However, it is hard to determine
if these are really active diapirs because there is no piercement. Probable examples of
these are found in a deep trough in eastern Ewing Bank and Green Canyon (Rowan,
1995). See Fig. 11.
Fig. 11. Map showing distribution of different types of salt structures (Rowan, 1995).
These structures are isolated, cylindrical, deformed, and located over deep salt rollers.
They may have originally been reactive diapirs which evolved into active diapirs, just
showing how a single salt structure can change shape over time (Fig. 12).
Fig. 12. Seismic profile of an active diapir (Rowan, 1995).
Passive diapirs are found more abundantly than the other two forms in this study area.
Passive salt structures are commonly asymmetric (Fig. 13). These diapirs can be found
on shallow salt sheets. Most of the salt bodies studied in this area formed as passive
diapirs. Again, passive diapirsm occurs when a diapir completely breaks through its
overburden and is exposed at the surface.
Fig. 13. Seismic profile of an asymmetric, passive diapir (Rowan, 1995).
While it seems that there are not as many reactive and active diapirs in this area of study,
it is hard to tell if the passive diapirs present now had evolved from reactive or active
diapirs. A reason why there are not many reactive diapirs could be due to the deposition
rate. It has been suggested that reactive diapirs only grow when deposition is slow
relative to extension (Rowan, 1995). The high rates of deposition offshore of Louisiana
could explain the lack of reactive diapirs.
Dead Sea Basin
There are many salt structures that have formed in the Dead Sea basin. En echelon salt
ridges, large salt diapirs, normal faults, salt walls, and rollovers are some of them. Smit
et al. (2008) has found that all of these structures have formed due to sinistral strike-slip
shear. See Fig. 14.
Fig. 14. Structure of the Dead Sea Basin. (a) Location of the Dead Sea Basin in the
frame of the Dead Sea transform system. (b) Main faults in the Dead Sea Basin (Smit et
al., 2008).
The Dead Sea has a water level that is 400 m below the Mediterranean Sea level. It is a
350 m deep hyper-saline lake (Smit el al., 2008). The presence of a thick salt layer in the
Pliocene Sedom Formation is what causes the deformation in this region. It forms many
diapirs in this area. One diapir, the Lisan diapir, contains salt which rises to a depth of
150 m (Smit et al., 2008). This structure forms the Lisan peninsula which divides the
north and south sub-basins. The Lisan diapir most likely formed from an anticline of an
en echelon fold train (Smit et al., 2008). In the southern basin, there is the Sedom diapir
which rises to the surface and outlines a 200 m high ridge (Fig. 15). This diapir can be
found above the Sedom fault, which is a strike-slip fault along the western part of the
basin. This zone shows an area of extension.
Fig. 15. Sections of a map of the Dead Sea Basin showing the salt diapirs (Smit et al.,
2008).
Salt walls are located north of the Sedom diapir. Here, the salt has constructed a wall
against the fault. This took on its shape during normal faulting. There are also many
elongated salt ridges that can be found in the northern basin. These ridges have an
oblique trend, which makes them en echelon ridges (Fig. 16) (Smit et al., 2008).
Fig. 16. (a) En echelon folding above a salt layer. (b) Three types of en echelon folds
(Smit et al., 2008).
Rollovers can also be found at the southern part of the salt layer. These structures are
connected with listric normal faults. This region shows deformation due to extension.
Conclusion
There are many different kinds of salt structures and many different ways in which they
have formed. Salt has unique properties, such as its density and ability to flow like a
fluid, which significantly influences deformation in the regions where it is found. Also,
whether the region has experienced extension or shortening will determine what kinds of
structures will form. Louisiana and the Dead Sea Basin show great examples of salt
tectonics. Finally, salt is very significant in that it can form hydrocarbon traps for oil and
gas to form. This is very important in the exploration for these natural resources,
especially with the economy of the United States recently.
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