6.4 Marine Evaporites
faulting of underlying rocks, extension or convergence
of the sedimentary pile containing salts, or overthrusting of salt-bearing strata by the fronts of orogenic
belts, largely modify the nontectonic processes. Here,
some general rules controlling nontectonic salt flow
and a few examples of tectonic overprint are briefly
discussed.
Salt domes and other salt structures are commonly
explained as the result ofbuoyancy Where lower-density salt is buried under strata ofhigher density, the salt
tends to flow upward through the overlying sediment
(buoyancy halokinesis). In this case, the burial depth of
the salt should be at least 900 to 1200 m; otherwise the
overlying rocks do not reach the high density required
for this process (see below). However, apart from
buoyancy, overburden anomalies (difJerentialloading
halokinesis) may cause salt sinks and salt dome
growth. This mechanism does not direcdy depend on
the density of the overlying sedimentary rocks and
therefore ac counts for the initiation of salt structures at
shallow burial depth (as litde as 100 m) as observed in
several salt provinces. The combined effect of both
mechanisms, buoyancyand overburden anomalies, can
be summarized for the most important type of salt deposits, i.e. rock salt, as follows.
Mechanics of Salt Flow
Rock salt behaves like a Newtonian or viscoelastic
fluid. To understand this bevavior an approach similar
to that applied to groundwater systems is used. The
fluid starts to flow if a certain hydraulic (pressure )
gradient within the fluid exists. Because of its
viscoelastic nature, salt flow takes place only if the
differential pressure or stress exceeds the yield point
of the salt body. Salt flow always occurs in the direction of the maximum hydraulic gradient, i.e., from the
area ofthe highest hydraulic head to that ofthe lowermost head (Fig. 6.15a).
A hydraulic gradient is defined by a difference in hydraulic
head between two points within the salt layer. The hydraulic
head of a certain point within the fluid is the sum of its gravity potential, G, at its elevation, z, above an arbitrary datum
line, and the fluid pressure head, p, at this point, i.e., the
height of fluid column in a manometer adjusted at this point.
A tilted salt layer can flow upward, ifthe decrease in pressure, ~p (e.g., PS-PA in Fig. 6.l5a), is greater than the increase in gravitational potential, ~G, defined by the difference in elevation between two points and the density ofthe
fluid (e.g., ~G = zsDs - ZADS). This occurs when the average
rock density, DR, is greater than that ofthe salt, Ds; otherwise
the salt flows downhilI (tTom C to B in Fig. 6.l5a). Hence,
the total overburden load does not playa direct role in this
model; solely the pressure gradients and gravitational potential are important. In addition, the yield point of the
viscoelastic salt body must be overcome in order to initiate
flow.
279
If a horizontal salt layer is buried under horizontal and
laterally uniform younger sediments, the hydraulic
gradient within the salt equals the gradient ofthe gravity potential, because the overburden weight is the
same everywhere. In this case no salt flow occurs.
Howeve:-, ifthe salt surface is locally elevated (elevation z in Fig. 6.15c), a gradient in the hydraulic head of
the salt layer is built up. Salt flows toward the irregularity, ifthe density ofthe overlying rocks in the range
of z exceeds that of salt, and it flows away from the
irregularity for cap rocks oflower densities. Sirnilarly,
a local increase in overburden pressure (differential
loading), for example by a prograding delta lobe, can
cause a significant hydraulic gradient in the salt layer
and thus force the salt to flow away from the delta lobe
Fig. 6.15b). Other sedimentary load anomalies include
reefs, sandy shoals, desert dunes, etc .. All of these can
trigger salt flow in the subsurface.
Evolution of Salt Structures
Salt structures are highly variable dependent on their
position within a basin, the original thickness of salt
deposits, and the history of subsidence and sediment
accumulation. Here, solely the evolution and shape of
some idealized salt structures can be demonstrated.
The very common salt domes are typically one to
several km in diameter and have steeply dipping or
even overhanging sides that may extend several km
downward. They usually exhibit the following stages
in their dcvelopment (Fig. 6.15c-e):
(1) Initiation and salt pillow formation (cf. Fig.
6.16a,b).
(2) Erosional truncation of overlying rocks.
(3) Extrusion of salt domes or pillars through erosional holes in the sedimentary cover (piercement
salt domes, diapirs).
(4) Collapse and burial.
Much salt is lost during the stage of extrusion through
erosion and dissolution on land or below the sea (cf.
Sect. 9.2). Once the original salt layer next to the dome
is completely evacuated, the upward movement of salt
ceases. Then the salt structure can reach a comparatively stable configuration, regardless of whether or
not the dome is buried under an increasing sediment
cover. However, lateral spreading or subsolution of
salt may continue. The internal structure of many salt
domes is characterized by tight folds and highly deformed salt bodies of different nature (Fig. 6.15e).
The relation of salt dome creation to the thickness of
the prirnary salt deposit is indicated in Fig. 6.16a,b
displaying the post-depositional processes above a salt
wedge. '!'hin salts can only lead to low-relief salt pillows. On the tops of both salt pillows and larger salt
domes or high-relief salt anticlines, extensional fault
faulting of underlying rocks, extension or convergence
of the sedimentary pile containing salts, or overthrusting of salt-bearing strata by the fronts of orogenic
belts, largely modify the nontectonic processes. Here,
some general rules controlling nontectonic salt flow
and a few examples of tectonic overprint are briefly
discussed.
Salt domes and other salt structures are commonly
explained as the result ofbuoyancy Where lower-density salt is buried under strata ofhigher density, the salt
tends to flow upward through the overlying sediment
(buoyancy halokinesis). In this case, the burial depth of
the salt should be at least 900 to 1200 m; otherwise the
overlying rocks do not reach the high density required
for this process (see below). However, apart from
buoyancy, overburden anomalies (difJerentialloading
halokinesis) may cause salt sinks and salt dome
growth. This mechanism does not direcdy depend on
the density of the overlying sedimentary rocks and
therefore ac counts for the initiation of salt structures at
shallow burial depth (as litde as 100 m) as observed in
several salt provinces. The combined effect of both
mechanisms, buoyancyand overburden anomalies, can
be summarized for the most important type of salt deposits, i.e. rock salt, as follows.
Mechanics of Salt Flow
Rock salt behaves like a Newtonian or viscoelastic
fluid. To understand this bevavior an approach similar
to that applied to groundwater systems is used. The
fluid starts to flow if a certain hydraulic (pressure )
gradient within the fluid exists. Because of its
viscoelastic nature, salt flow takes place only if the
differential pressure or stress exceeds the yield point
of the salt body. Salt flow always occurs in the direction of the maximum hydraulic gradient, i.e., from the
area ofthe highest hydraulic head to that ofthe lowermost head (Fig. 6.15a).
A hydraulic gradient is defined by a difference in hydraulic
head between two points within the salt layer. The hydraulic
head of a certain point within the fluid is the sum of its gravity potential, G, at its elevation, z, above an arbitrary datum
line, and the fluid pressure head, p, at this point, i.e., the
height of fluid column in a manometer adjusted at this point.
A tilted salt layer can flow upward, ifthe decrease in pressure, ~p (e.g., PS-PA in Fig. 6.l5a), is greater than the increase in gravitational potential, ~G, defined by the difference in elevation between two points and the density ofthe
fluid (e.g., ~G = zsDs - ZADS). This occurs when the average
rock density, DR, is greater than that ofthe salt, Ds; otherwise
the salt flows downhilI (tTom C to B in Fig. 6.l5a). Hence,
the total overburden load does not playa direct role in this
model; solely the pressure gradients and gravitational potential are important. In addition, the yield point of the
viscoelastic salt body must be overcome in order to initiate
flow.
279
If a horizontal salt layer is buried under horizontal and
laterally uniform younger sediments, the hydraulic
gradient within the salt equals the gradient ofthe gravity potential, because the overburden weight is the
same everywhere. In this case no salt flow occurs.
Howeve:-, ifthe salt surface is locally elevated (elevation z in Fig. 6.15c), a gradient in the hydraulic head of
the salt layer is built up. Salt flows toward the irregularity, ifthe density ofthe overlying rocks in the range
of z exceeds that of salt, and it flows away from the
irregularity for cap rocks oflower densities. Sirnilarly,
a local increase in overburden pressure (differential
loading), for example by a prograding delta lobe, can
cause a significant hydraulic gradient in the salt layer
and thus force the salt to flow away from the delta lobe
Fig. 6.15b). Other sedimentary load anomalies include
reefs, sandy shoals, desert dunes, etc .. All of these can
trigger salt flow in the subsurface.
Evolution of Salt Structures
Salt structures are highly variable dependent on their
position within a basin, the original thickness of salt
deposits, and the history of subsidence and sediment
accumulation. Here, solely the evolution and shape of
some idealized salt structures can be demonstrated.
The very common salt domes are typically one to
several km in diameter and have steeply dipping or
even overhanging sides that may extend several km
downward. They usually exhibit the following stages
in their dcvelopment (Fig. 6.15c-e):
(1) Initiation and salt pillow formation (cf. Fig.
6.16a,b).
(2) Erosional truncation of overlying rocks.
(3) Extrusion of salt domes or pillars through erosional holes in the sedimentary cover (piercement
salt domes, diapirs).
(4) Collapse and burial.
Much salt is lost during the stage of extrusion through
erosion and dissolution on land or below the sea (cf.
Sect. 9.2). Once the original salt layer next to the dome
is completely evacuated, the upward movement of salt
ceases. Then the salt structure can reach a comparatively stable configuration, regardless of whether or
not the dome is buried under an increasing sediment
cover. However, lateral spreading or subsolution of
salt may continue. The internal structure of many salt
domes is characterized by tight folds and highly deformed salt bodies of different nature (Fig. 6.15e).
The relation of salt dome creation to the thickness of
the prirnary salt deposit is indicated in Fig. 6.16a,b
displaying the post-depositional processes above a salt
wedge. '!'hin salts can only lead to low-relief salt pillows. On the tops of both salt pillows and larger salt
domes or high-relief salt anticlines, extensional fault