What's a little fold between lone creeks?
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This Earth Cache is all about the exposed geological folds that
are visable at Lone Creek falls near Sabie.
The term fold is used in geology when one or more layers of rock
are bent or curved as a result of permanent deformation. These
folds may vary in sizes ranging from microscopic crinkles to
mountain sized folds. They can occur as singular folds or in
extensive fold trains of different sizes and on a variety of
scales.
Folds can be the result of various conditions of stress,
hydrostatic pressure, pore pressure and temperature. They can be
found in the full spectrum of metamorphic rock and even as primary
flow structures in some igneous rocks. When a set of folds is
distributed on a regional scale it is referred to as a fold belt,
which is a common feature of orogenic (the process by which
mountains are created) zones. Folds are often caused by the
shortening of existing layers above or below the fold zone, but may
also be formed as a result of displacement.
Fold Types
• Anticline: linear, strata normally dip away from axial
center, oldest strata in center.
• Syncline: linear, strata normally dip toward axial center,
youngest strata in center.
• Antiform: linear, strata dip away from axial center, age
unknown, or inverted.
• Synform: linear, strata dip toward axial centre, age
unknown, or inverted.
• Dome: nonlinear, strata dip away from center in all
directions, oldest strata in center.
• Basin: nonlinear, strata dip toward center in all
directions, youngest strata in center.
• Monocline: linear, strata dip in one direction between
horizontal layers on each side.
• Chevron: angular fold with straight limbs and small
hinges
• Recumbent: linear, fold axial plane oriented at low angle
resulting in overturned strata in one limb of the fold.
• Slump: typically monoclinal, result of differential
compaction or dissolution during sedimentation and
lithification.
• Ptygmatic: Folds are chaotic, random and disconnected.
Typical of sedimentary slump folding, migmatites and decollement
detachment zones.
• Parasitic: short wavelength folds formed within a larger
wavelength fold structure - normally associated with differences in
bed thickness.
• Disharmonic: Folds in adjacent layers with different
wavelengths and shapes.
Causes of folding
Folds appear on all scales, in all rock types, at all levels in the
crust and could be the result of a variety of causes.
Layer-parallel shortening
When a sequence of layered rocks is shortened parallel to its
layering, this deformation may be accommodated in a number of ways,
homogeneous shortening, reverse faulting or folding. The response
depends on the thickness of the mechanical layering and the
contrast in properties between the layers. If the layering does
begin to fold, the fold style is also dependent on these
properties. Isolated thick competent layers in a less competent
matrix control the folding and typically generate classic rounded
buckle folds accommodated by deformation in the matrix. In the case
of regular alternations of layers of contrasting properties, such
as sandstone-shale sequences, kink-bands, box-folds and chevron
folds are normally produced.
Fault-related folding
Many folds are directly related to faults, associate with their
propagation, displacement and the accommodation of strains between
neighboring faults.
Fault bend folding
Fault bend folds are caused by displacement along a non-planar
fault. In non-vertical faults, the hanging-wall deforms to
accommodate the mismatch across the fault as displacement
progresses. Fault bend folds occur in both extensional and thrust
faulting. In extension, listric faults form rollover anticlines in
their hanging walls. In thrusting, ramp anticlines are formed
whenever a thrust fault cuts up section from one detachment level
to another. Displacement over this higher-angle ramp generates the
folding.
Fault propagation folding
Fault propagation folds or tip-line folds are caused when
displacement occurs on an existing fault without further
propagation. In both reverse and normal faults this leads to
folding of the overlying sequence, often in the form of a
monocline.
Detachment folding
When a thrust fault continues to displace above a planar detachment
without further fault propagation, detachment folds may form,
typically of box-fold style. These generally occur above a good
detachment such as in the Jura mountains, where the detachment
occurs on middle Triassic evaporites.
Compaction
Folds can be generated in a younger sequence by differential
compaction over older structures such as fault blocks and
reefs.
Folding in shear zones
Shear zones that approximate to simple shear typically contain
minor asymmetric folds, with the direction of overturning
consistent with the overall shear sense. Some of these folds have
highly curved hinge lines and are referred to as sheath folds.
Folds in shear zones can be inherited, formed due to the
orientation of pre-shearing layering or formed due to instability
within the shear flow.
Sedimentary folding
Recently deposited sediments are normally mechanically weak and
prone to remobilisation.
Slump folding
When slumps form in poorly consolidated sediments they commonly
undergo folding, particularly at their leading edges, during their
emplacement. The asymmetry of the slump folds can be used to
determine paleoslope directions in sequences of sedimentary
rocks.
Dewatering
Rapid dewatering of sandy sediments, possibly triggered by seismic
activity can cause convolute bedding.
Igneous intrusion
The emplacement of igneous intrusions tends to deform the
surrounding country rock. In the case of high-level intrusions,
near the Earth's surface, this deformation is concentrated above
the intrusion and often takes the form of folding, as with the
upper surface of a laccolith.
Flow folding
The compliance of rock layers is referred to as competence: a
competent layer or bed of rock can withstand an applied load
without collapsing and is relatively strong, while an incompetent
layer is relatively weak. When rock behaves as a fluid, as in the
case of very weak rock such as rock salt, or any rock that is
buried deeply enough, they typically show flow folding (also called
passive folding, because little resistance is offered): the strata
appear shifted undistorted, assuming any shape impressed upon them
by surrounding more rigid rocks. The strata simply serve as markers
of the folding. Such folding is also a feature of many igneous
intrusions and glacier ice.
Sources: Wikipedia
To Log this Earth Cache you need to do the following:
1. Take a photo at GZ and upload with your log.
2. Answer the following questions:
a) How high do you guess the falls are?
b) What is the predominant fold type to the right of the falls,
Anticline or Syncline?
c) Please give me a short discription of the folds that you can see
and after reading the content above tell me what you think caused
these folds.
Please take note that if you go and do this cache before 7h30 in
the morning you will not have to pay a entrance fee, but if you are
still there or only arrive after that, there is a R10 per person
charge that is payable between 07h30 - 18h00.
NOTE: You may log your visit prior to approval, but e-mail
submissions that do not meet the above criteria will be deleted.
Additional Hints
(Decrypt)
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