Diapirs and Structural feautures By M.P. Billings

1. Diapirs
And
Related Structural
Features
PRESENTED BY
RATHINAVEL .K
33820104
I M.Sc Applied Geology,
University Of Madras (Guindy Campus)
Chennai -25.

2. Contents
• Introduction
• Evoporite diapirs
(shape, composition, internal structure)
• Shalesheath
• rock glaciers
• origin of diapirs
• Economic resources
• structural evolution
• Serpentine diapirs
• sedimentary vents
• mud lumps

3. Diapir
 The word ‘Diapir’ is derived from a greek word meaning of ‘to pierce’.
 The words were first used to describe anticlinal folds in the carpathian
mountains with salt cores that overlying strata.
 The concept was originally confined to include all types of piercement,
including magmatic injection.
 But this expansion of the term destroys its usefulness. Consequently, the
term will here refer to the injection of any solid rocks, whether sedimentary,
igneous, or metamorphic.
 In general the body cuts across the adjacent rocks, although locally it
may be concordant. The rocks most commonly involved are evaporites (rock
salt, gypsum, anhydrite), shale, and serpentine.

4. “A diapir is a type of geologic intrusion in which a more
mobile and ductily deformable material is forced into brittle
overlying rocks”.

5. Injected rocks may range in physical properties from solid rock to liquids.
(1) solid rock, which may have a small percentage of pore space and pore liquid;
(2) solid rock that is thoroughly broken up and fractured, with some liquid in the
fractures;
(3) solid rock that has become mobile due to partial melting, a feature that generally
occurs only at considerable depth in metamorphic terranes;
(4) a loose aggregate of particles, buoyed up by gases or liquids that could be derived
from either magmatic or sedimentary sources; and
(5) liquid.
The first two categories are diapiric, the last two are not. A strong argument could
be made for classifying the third type as diapirism, but it will be considered separately
rather than in this chapter.

6. Evaporite Diapirs
 Most Evaporite Diapirs are composed chiefly of halite (rock salt), much less
commonly of anhydrite or gypsum.
 They occur either as the cores of domical structural features or as the cores of
anticlines. E.g : Phacolith
 They are unusually well known because of their great economic importance,
the chief product being petroleum, but also including sulfur and salt.
 Very little information comes from surface exposures; it is obtained chiefly
from drilling.
 The internal structure, however, may be studied in the great underground
chambers of salt mines.
 Geophysical methods, notably gravitational, seismic, and magnetic, contribute
significant data.

7.  The salt diapirs in the Gulf
Coast of the United States are
exceptionally well known; they are
commonly referred to as salt domes.
 Over 300 such domes are
known on the mainland, and an equal
number are probably present on the
continental shelf extending from
Louisiana to Yucatan. In Northwestern
Germany

8.  A distinction must be made by the piercing
core and the sediments dragged up with it. The
cores are either circular in plan or elongate.
 The former have long been known as
domes, and, more specifically as salt domes,
because the core is largely halite.
 The hills, which rise from a few feet to 40
feet above the surrounding lowlands—in
exceptional cases as much as 80 feet—cover an
area of more than a mile in diameter.
SHAPE

9.  The cores of most Gulf
Coast salt domes are
essentially circular in plan,
and characteristically range
in diameter from 1 1/2 to 2
miles, but some are as much
as five miles in diameter.
 In many of these salt
domes the walls of the core
dip steeply outward; the top
may be flat or domical.
 Some are symmetrical,
the walls dipping at the same
angle on all sides; others are
asymmetrical; in some the
wall on one or more sides
may dip inward. In some the
core overhangs or mushrooms
on several sides.

10. COMPOSITION
 In the Gulf Coast the core consists principally of rock salt (halite) with several
percent of anhydrite.
 Argillaceous and potash-rich beds are interbedded with the salt in the German
stocks.
 A cap rock (Fig. 14-3) is generally present in the Gulf Coast domes. Absent from
many domes, it may reach a maximum thickness of 1000 feet.
 The cap rock characteristically consists of limestone, gypsum, and anhydrite,
the limestone on top and the anhydrite on the bottom. Commercial deposits of
sulfur occur on some of the domes.

11. INTERNAL STRUCTURE
 The beds, a fraction of an inch to several feet thick, are displayed in different
shades of gray and white; the gray beds have a greater content of anhydrite. The folding
is very complex (Fig. 14-4).
 The bedding is vertical and is well displayed on the vertical walls of the mine
workings. But the ceilings show isoclinal, attenuated, refolded and faulted folds that
plunge vertically.

12. Fig. 14-4. Map to show folding of salt within a salt dome. Grand Saline salt dome, Texas.

13. SHALE SHEATH
 In some domes a shale or clay sheath partially encloses the salt .
 Differing greatly in size, they consist of finely divided argillaceous material that
was dragged up by the rising salt.
 Thick breccias locally surround the salt cores in some of the Rumanian salt
Diapirs.

14. STRUCTURE OF SURROUNDING SEDIMENTARY ROCKS
 The sedimentary rocks surrounding the core are uplifted into a dome or anticline.
In some domes the bedding in the overlying sedimentary rocks appears to be parallel
to the contact with the salt; these have been called Nonpiercement domes.
 This feature is the salt was exposed to erosion prior to the deposition of the
sediments and such domes probably have cross-cutting contacts at depth.
 The sedimentary rocks on the top of the dome are commonly broken by normal
faults. They may be radial (Fig. 14-6A) or may belong to a more or less parallel system
in which one or more graben are conspicuous (Fig. 14-6B).
 Similar faults have been produced in experimental studies. The sedimentary rocks
flanking the core dip outward at various angles, and in many instances are broken by
faults.
 Drill records show that the core pierces the sedimentary rocks.

16. Fig. 14-6. Faulting on salt domes. Heavy black lines are faults; D is the downthrown side.
(A) Clay Creek salt dome, Texas; structure contours on top of cap rock; contour interval
500 feet.
(B) Conroe oil field, Texas; structure contours on top of main Conroe sand; contour
interval 100 feet.

17. • In some of the Rumanian diapirs the rock salt is exposed at the surface of
the earth, and in some of the Iranian domes the salt has literally flowed onto
the surface to form spectacular “glaciers” composed of rock salt.
ROCK GLACIERS

18. STRUCTURAL EVOLUTION
 A vast amount of precise information has accumulated on the structural evolution
of salt domes, both in America and abroad. Many, of the American salt domes have
been rising throughout Tertiary time.
 The evidence is primarily stratigraphic. An angular unconformity, such as that
illustrated in Fig. 14-7A, shows that considerable uplift occurred after the deposition
of formation a, but before the deposition of formation b. The salt rose up through
formation a, truncating the bedding and doming up the sediments.
 Erosion followed, removing many of the younger beds in formation a. Formation b
was subsequently deposited, and this was followed by renewed upward movement that
slightly domed formation b.
 In other instances a formation may become thinner over the top of the dome. This
indicates, as illustrated in Fig. 14-7B, that the dome was actively rising throughout the
deposition of formation d, but unconformities within formation d may be difficult to
detect.
 all the data are obtained from drill holes.

19. • Topographically expressed salt domes have probably been active in relatively
recent times. Moreover, if Pleistocene or Recent gravels on the dome are uplifted
relative to their position in the surrounding region, it is obvious that the salt has
been active during the Quaternary.

20. ORIGIN
 Salt diapirs result from the intrusion of solid halite into the surrounding sediments.
The salt is derived from some underlying source bed, usually thousands of feet thick.
 In the Gulf Coast the source is very probably the Louann Salt, of Jurassic, Triassic,
or Permian age, and as much as 5000 feet thick. In Germany the source is the Permian
Zechstein, which is as much as 3000 feet thick.
 The motivating force in the Gulf Coast results from the difference in density
between the salt and the overlying sediments. Rock salt has a relatively uniform
density about 2.2 g/cm3, but varies depending upon the amount of anhydrite and
temperature.
 Between the surface and a depth of 2000 feet, the average density of the
sediments is 1.9 to 2.2 g/cm3, but below a depth of 2000 feet the density of the
average sediment increases progressively to a value of 2.46 at a depth of 20,000 feet.

21. • Thus, below a depth of 2000 feet an unstable gravitational situation exists
and the salt tends to move upward in the same way that a lighter fluid rises through
heavier overlying fluid (Fig. 14.8)

22.  If a small anticlinal flexure exists on top of the original salt bed, upward movement
starts here, and salt is drained away from the surrounding region. Eventually, the salt bed
in the adjacent area may become so thin and constricted that further addition of salt is
impossible.
 The elastic limit (yield stress)9 of rock salt at 300°C and a confining pressure of 2000
bars is 100 kg/cm2. But even at temperatures as low as 35°C it shows creep phenomena
when the confining pressure is as low as 25 kg/cm2.
 Thus salt may flow at relatively low differential stress. Many estimates have been
made of the equivalent viscosity of rock salt, some based on laboratory experiments,
some based on the rate of convergence of openings in mines.

 The best estimates at room temperature range from 3.5 x 1016 to 4 x 1018 poises.
Increasing temperature lowers the viscosity. The plastic deformation within the individual
halite crystals takes place by gliding and dislocations along cube and dodecahedral planes

23. • The evolution of salt domes has been analyzed by computerized mathematical
models. Equations are prepared to calculate the manner in which the salt dome
grows.
• Many parameters are involved, such as the density of the salt and the
surrounding sediments, buoyancy in pounds per square inch (difference in density of
salt and sediment at different depths), temperature, ultimate strength, time, and
many others.
• Various values for these parameters may then be entered in the equations.
Obviously, such calculations would be a very time consuming process, in fact
prohibitive, if done by hand.
• But the computer handles a large number of substitutions in a short time. The
results can be printed out on paper, as in Fig. 14-9, or they can be viewed in a closed
circuit television. Figure 14-9B is a model of a salt dome that rose through shales
and sandstones.

24. • Several features may be noted:
(1) The vertical exaggeration is nearly two times.
(2) The dome is cut off from the main part of the salt bed.
(3) Overhang began when the salt dome penetrated the
sandstone.
In Fig. 14-9A the top of the salt has moved up in a
series of spines. The average growth rate of the top of a
typical dome is somewhat less than one foot per
thousand years; a dome would take 10,000,000 to
20,000,000 years to develop if the rate were constant.

25.  Petroleum is trapped in the sediments that flank the core of rock salt, and in
some instances it has been found in the cap rock. Large quantities of sulfur have
been obtained from the cap rock of some salt domes.
 Sulfur has probably been derived from the anhydrite and gypsum normally
present in the cap rock, but there is no agreement concerning the details of the
process of formation.
 The rock salt in the core has also been exploited economically. Potash salts
have been extensively mined in German salt domes, where the potash salts occur
in strata that were deposited during the accumulation of the sediments.
ECONOMIC RESOURCES

26.  Waste Disposal
Salt is an impermeable rock that has the ability to flow and seal
fractures that might develop within it. For this reason, salt domes have been used as
disposal sites for hazardous waste.
Man-made caverns in salt domes have been used as repositories for oil field
drilling waste and other types of hazardous waste in the United States and other
countries. They have also been considered for high-level nuclear waste disposal, but no
site in the United States has received that type of waste.
 Underground Storage Reservoirs
Some of the mines developed in salt domes have been carefully
sealed and then used as storage sites for oil, natural gas, and hydrogen.
Salt domes in the United States and Russia also serve as national repositories
for government reserves of helium gas. Salt is the only type of rock that has a
permeability so low that it can hold the tiny helium atoms.

27. SERPENTINITE DIAPIRS
 Ultramafic rocks are common in some parts of orogenic belts. Although some are
unaltered dunite (composed of olivine), peridotite (composed of olivine and
pyroxene), and other similar rocks, they are commonly altered to serpentinite, a rock
composed of the mineral serpentine.
 These ultramafic rocks have been injected into the enclosing rocks.But pure
dunite would crystallize at about 1700°C and peridotite at a somewhat lower
temperature.
 Nevertheless, the enclosing rocks commonly, but not always, lack contact
metamorphism; this implies that the ultramafic rock was relatively cool at the time of
injection. Intense fracturing, shear planes, and slickensides in the serpentinite are
consistent with the conclusion that they were injected in the solid state.
 The low density of the serpentinite is a factor favoring solid emplacement. Unlike
the Gulf Coast salt domes, squeezing by horizontal compression is the major factor
involved in emplacement.

28.  The products of sedimentary volcanism, are associated with some petroleum
areas, notably India, Burma, Rumania, Malaysia, Trinidad, Venezuela, and the
Caucasus Mountain area.
 Cones, identical in all respects to volcanic cones except in composition, are
composed of mud. Although most of the cones are only 100 feet high, a few reach a
height of over 1000 feet.
 The motivating force behind the eruption is gas and steam, under high pressure,
derived from petroleum reservoirs.
 Vents that presumably served as feeders for sedimentary volcanoes are preserved
in Miocene strata in a band 80 miles long in southeast Texas.
 More resistant to erosion than the surrounding sedimentary rocks, these units are
circular to oval in plan, 200 to 1200 feet in diameter, and rise 10 to 125 feet above
their surroundings.
SEDIMENTARY VENTS

29.  Mud lumps are extensively developed in the Gulf of Mexico at the mouths of the
Mississippi River. They are, of course, ephemeral features that are rapidly destroyed by
erosion.
 Olf the mouth of South Pass there are at present about 50 islands of this type, as
well as 50 submerged lumps. The islands average about 200 feet in length, but the
largest is 1500 feet long. The islands rise at the most, a few tens of feet above sea level,
and wave-cut terraces are present.
 The lumps are composed of clay or silt. The strata involved are Late Pleistocene to
Recent, and are about 400 to 500 feet thick.
 since piercement is not involved, they are not diapirs.
MUD LUMPS

30.  They are the product of
“injection folding” that results from
deposition of an extra load of
sediments (Fig. 14-11); sediments are
squeezed out from beneath this extra
load, but piercement is not necessarily
involved.
 The cores of some of the mud
lumps, however, have pierced the
anticlines, and the clays are fractured,
brecciated, and faulted.