Chapter Eight
Structural Geology
It is evident that without uplift and subsidence related to tectonic and structural events there
would be no sediments. Sediments are derived from positive crustal elements and deposited in
negative elements. Petroleum traps occur as result of structural activity.
If a body is subjected to directed forces, it usually passes through three stages of deformation.
1) Elastic: at this stage, if the stress is withdrawn, the body returns to its original shape and
size.
2) Plastic: If the stress exceeds the elastic limit, the deformation is plastic. At this stage the
specimen only partially returns to its original shape even if the stress is removed.
3) rupture: When there is a continued increase in the stress, one or more fractures develop,
and the specimen eventually fails by rupture.
Most rocks at room temperatures and pressures fail by rupture before attaining a stage of
plastic deformation. But, at sufficiently high temperatures and confining pressures rocks can
reach plastic stage.
Under tension, the breaking strengths of rocks are far less, averaging only 5 percent to 10
percent of the breaking strength under compression.
Factors controlling behavior of materials
1) Confining pressure:
Cylinders of rock were prepared for testing rock deformation. A confining pressure was
applied on the cylinder side. Then a compressive (or tensile) stress is applied at the end of the
cylinders.
Such test indicate that rocks exhibiting very little plastic deformation at low confining
pressure (or near the surface of the earth) may be very plastic under high confining pressure.
Figure 2-11 illustrates the effect of confining pressure on the breaking strength of several
different rocks.
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2) Temperature
Changes in temperature modify the strength of rocks. Hot steel, for example, undergoes
plastic deformation much more readily that does cold steel. Figure 2-12 shows two tests run
on Yule Marble. To produce a given strain far less stress is necessary when the specimen is
hot that when it is cold. It is apparent that plastic deformation is far less common near the
surface of the earth, where the confining pressure and the temperature are low, that it is at
greater depths, where higher temperatures and greater confining pressure increase the
possibility of plastic deformation.
3) Time
Although geologic time is impossible to duplicate experimentally, it is possible from
experiments to make some deductions concerning the influence of time. Creep refers to the
slow continuous deformation with the passage of time. Limestone under atmospheric pressure
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and at room temperature has a strength of 2560 kg/cm . In a long–time experiment, Limestone
2
subjected to a compressive stress of 1400 kg/ cm (half the value of the strength) deforms
rapidly at first, them more slowly (Fig. 2-13). At the end of one day, it has been shortened
about 0.016 percent; after 10 days about 0.011 percent; after 100 days about 0.616 percent;
and after 400 days a little more than 0.019 percent.
4) Solutions
In Figure 2-17 dry specimen had shortened about 0.03 percent, but there was no further
detectable deformation even after 40days.
Specimen that contains some water (intermediate curve) had shortened 1 percent at the end of
30 days and 1.75 percent by the end of 36 day, when the load was released.
2
The strength of the dry alabaster is 480 kg/cm . Strength under similar conditions, but with
the specimen free to react with water, is estimated to be only 92 kg/ cm2.
Hence, strength of the wet specimen is much less than the strength of the dry specimen at the
same temperature.
Folds :
Fold Terminology
1)
Anticline: older rocks are in the center of fold (Fig. 3-4A and plate 8).
2)
Syncline: younger rocks are in the center of fold (Fig. 3-5A).
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3)
The limbs are the sides of fold.
Overturned fold: both limbs dip in the same direction (Fig. 3-6c and plate 9).
Symmetrical and asymmetrical fold: figure 3.6 A & B.
Plunge of folds: Plunging folds are easily recognized from their outcrop pattern. Figure 3-21
is a geologic map of non-plunging syncline. The beds on the opposite limbs strike parallel to
each other; they do not converge. Figures 3-23 and 3-24 are geological maps of plunging fold;
the beds on the opposite limbs strike toward each other and the formations converge. A
plunging fold is shown in plate 10.
Figures 3-25A and B show a geologic map of doubly plunging fold.
A dome is an anticlinal uplift that has no distinct trend (Fig.3-25C). A basin is a synclinal
depression that has no distinct trend (Fig. 3-25D).
Refolding: Folds may be refolded (Plate12 & Figure 3).
Recognition of folds
Direct observation
One or more of the following pieces of information are commonly used to deduce folds:
1) Plotting attitude of beds on a map
The plot of strike and dip of the bedding on a map show an anticline in figure 4-6A and a
syncline in figure 4-6B.
In a geologic map that has topographic contours, if the contact between two formations is
parallel to the contours, the strata are horizontal (Fig. 4-7A). If a contact maintains a uniform
trend, the strata are vertical (Fig. 4-7B).
The dip of the beds may be calculated, as is shown by Fig. 4-8. The bed drops 100 feet
vertically in 160 feet horizontally (line xy). The dip equals tan-1 (100/160). θ
2) Areal map pattern
In Figure 4-9 various alternate interpretations are possible if the strikes and dips, relative ages,
and topography are unknown. But with a few additional information a correct interpretation of
the map pattern can be achieved.
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3) Drilling
If some bed is sufficiently distinctive, either because of lithology or because of fossil content,
its altitude in various drill holes can be recorded and the structure determined. If drill cores
are obtained, the angle of dip of the bedding can be determined.
5) Geophysical methods
Various geophysical methods have been utilized to determine geological structure. The
principal methods may be classified as gravimetric, magnetic, seismic, and electrical.
Causes of folding
1) Tectonic processes
As the convection currents drag along the base of the crust (Fig 6-10), the overlying
sediments are forced to occupy a smaller area than formerly, which cause it to be folded.
2) Nontectonic processes
Differential compaction of sediments produces broad open folds that in many instances are of
economic importance in petroleum geology (Fig.6-19). If at some subsequent time the mud
compacts 20 percent, the highest sedimentary bed will sink 20 feet. Beyond the limits of the
hill, the highest sedimentary bed sinks 40 feet.
Faults
Some faults move blocks up or down, others move blocks laterally over one another.
Fault Terminology
Figure 250 illustrates the primary components observed in fault-related deformation.
Dip: the angle that a fault plane makes with the horizontal.
Strike: is represented by the line of intersection between the fault and horizontal surface.
Heave: horizontal movement component of a fault.
Throw: is the vertical movement component of the fault displacement
Slip: represented by the actual linear movement along a fault plane.
The hanging wall of a fault is located above the fault surface and bears upon it. The footwall
of a fault is beneath the fault.
Recognition of Faults
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The criteria for the recognition of faults may be considered under the following headings:
1) Discontinuity of structures
If strata suddenly end against different beds, a fault may be present (Plates 18 and 19). But,
discontinuity of structures is, in itself, not proof of faulting; the truncation of structures is also
typical of unconformities or intrusive contacts (plate 30).
2) Repetition and omission of strata
Figure9-1 is a geological map of a region of folded and faulted sedimentary rocks. A syncline
lies near the center of the map.
As one or more formations are missing, the lines FF and F’F’ must be the traces of faults.
3) features characteristic of fault planes
Fault plain had some distinctive features such as:
1)
Slickensides: are polished and striated surfaces that result from friction along the fault
plane.
2)
Drag: as is shown in Fig. 9-2B, because of friction the beds in the hanging wall are
dragged up, whereas the beds in the foot wall are dragged down.
Evaporite diapirs (Salt Domes)
Shape
They are either circular in plan or elongate
(Fig. 14-2). The salt core overhangs or
mushrooms on several sides. Many slat does extend to depths of several miles.
Composition
The core consists of halite with several percent of anhydrite (Fig. 14-3). The cap rock consists
of limestone, gypsum, and anhydrite.
Shale sheath
In some domes a shale or clay sheath partially encloses the salt (Fig. 14-5). This sheath was
caused by the rising salt.
Structure of surrounding sedimentary rocks
The sedimentary rocks surrounding the core are uplifeted into a dome or anticline.
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In some domes the bedding in the overlying sedimentary rocks appears to be parallel to the
contact with the salt, which is probably due to the fact that the salt was exposed to erosion
prior to the deposition of the sediments.
The sedimentary rocks on the top of dome are commonly broken by normal faults (Fig. 14-6).
Structural evolution
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 the slightly domed formation
b.
In other instances formation a 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.
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 (Fig.14-1). The
motivating force of the intrusion results from the difference in density between the salt and
the overlying sediments.
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