Bivane Dam is one of the newest dams in South Africa, having been completed in 2000. The dam is of arch construction and is located on the Bivane River, near Vryheid, KwaZulu-Natal, South Africa. Its primary purpose is for irrigation and domestic use.
Bivane Dam
This earthcache location is along the main access road to the resort (which is situated just upstream of the dam wall)
The focus of this earthcache is syncline and anticline folding in rocks.
Geological Folds
A fold is a wave-like geologic structure that forms when rocks deform by bending instead of breaking under compressional stress.
Anticlines are arch-shaped folds in which rock layers are upwardly convex. The oldest rock layers form the core of the fold, and outward from the core progressively younger rocks occur.
A syncline is the opposite type of fold, having downwardly convex layers with young rocks in the core. Folds typically occur in anticline-syncline pairs. The hinge is the point of maximum curvature in a fold. The limbs occur on either side of the fold hinge.
The imaginary surface bisecting the limbs of the fold is called the axial surface. The axial surface is called the axial plane in cases where the fold is symmetrical and the lines containing the points of maximum curvature of the folded layers, or hinge lines are coplanar.
Concentric folding preserves the thickness of each bed as measured perpendicular to original bedding. Similar folds have the same wave shape, but bed thickness changes throughout each layer, with thicker hinges and thinner limbs.
Folds form under varied conditions of stress, hydrostatic pressure, pore pressure, and temperature gradient, as evidenced by their presence in soft sediments, the full spectrum of metamorphic rocks, and even as primary flow structures in some igneous rocks. A set of folds distributed on a regional scale constitutes a fold belt, a common feature of orogenic zones. Folds are commonly formed by shortening of existing layers, but may also be formed as a result of displacement on a non-planar fault (fault bend fold), at the tip of a propagating fault (fault propagation fold), by differential compaction or due to the effects of a high-level igneous intrusion e.g. above a laccolith.
Fold Classification
Folds are classified on the basis of several geometric factors:
Tightness of folding
The tightness of folds can be described as open (limbs dip gently), tight (limbs dip steeply) or isoclinal (limbs are parallel).
Orientation of axial plane
The orientation of the axial plane relative to the horizontal together with the orientation of fold limbs allow subdivision into upright (axial plane vertical, limbs symmetric), overturned (axial plane moderately inclined, one limb overturned), or recumbent (axial plane near horizontal, one limb inverted).
Thickness of folded beds
Thickly-bedded, brittle units tend to form concentric folds with the bed thickness preserved normal to bedding surfaces. Thinly-bedded, clay-rich units have a tendency to develop a foliation parallel to the axial plane and form similar folds with the vertical distance between top and bottom of the unit preserved through the deformation.
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.
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.
Anticlines and Synclines
Domes and Basins
There are other fold types, but they are beyond the scope of this earthcache.
Earthcache Logging Tasks
In order to claim a find on this earthcache, please complete the tasks below and email me your answers to my geocaching.com profile.
Please do not use the message service!
Examine the sides of the cutting at the published coordinates - then answer as below:
- Task 1: Is there evidence of anticlines, synclines, or both?
- Task 2: In your opinion, is the folding tight or open? Please give reasons for your answer.
- Task 3: Would you say the folding is concentric or similar? Please give reasons for your answer.
- Task 4: In your opinion, what type of rock is this?
- Task 5: (optional). Please post a pic of you and your GPS at the published coordinates, (or at the dam).
References
http://www.geologypage.com/2015/12/geological-folds.html