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.
• 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.
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.
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.
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.
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.
Recently deposited sediments are normally mechanically weak and prone to remobilisation.
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.
Rapid dewatering of sandy sediments, possibly triggered by seismic activity can cause convolute bedding.
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.
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.
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.
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