Otter Trail

Geological Formations at Oakhurst Hut

By Alan Goldsmith (MSc)


During the Footprint Hiking Club’s Otter Trail excursion during July 2003, the third night of the hike was as usual spent at the Oakhurst Huts at the mouth of the Lottering River. It was at this camp approximately 20m along the beach to the west that a superb geological exposure was discovered.

 

This part of the trail lies in the Table Mountain Subgroup, a geological unit comprising alternating sandstones and shale’s. About 250 Ma ago these rocks were deformed by extreme pressure and heat during a period which gave rise to the region known as the Cape Fold Belt. The following pictures display these rocks next to the Oakhurst Camp. A brief description is given below explaining the patterns and textures visible in the pictures.

 

When a rock unit is squashed it undergoes changes, which often manifest themselves as folds. At Oakhurst the rocks have undergone extreme folding, which has resulted in some spectacular exposures, which have been accentuated by two factors:

  • The alternating nature of the rocks, which results in different textures being displayed over very small distances, (centimeters).
  • The erosion / weathering caused by the action of the sea and accompanying spray which has created unique three dimensional exposures.

 

The photographs illustrate the different geometries that rocks take as a result of folding, depending on their mineral composition. In the photographs the most obvious units are the pale / light coloured bands of sandstone. Due to the competent nature of the sandstone, you will notice that as you trace the rock unit around the fold, the thickness remains relatively constant in a direction perpendicular to its boundary.

 

In contrast, the darker, less visible shale bands in between display changes in thickness between the hinge area and the limbs of the folds. The hinge is where the most acute angle is. In this case, the folds are relatively upright so the hinges are mainly pointing upward, or downward. The limbs can be described as the sides of the upright triangles. The shale bands are an excellent example of where, due to extreme pressures material is squeezed into the hinge areas, compared to the limbs.

 

Another textural feature resulting from the compositional differences between the sandstone and shale, is the development of cleavage. Although this cannot be seen in the photographs, it is very obvious in close field inspection. As the sandstone is mainly composed of equigranular quartz grains, there is not much potential for the grains to be reoriented into a preferred direction. This is in contrast to the shales which are largely composed of clay minerals that inherently have platy shapes and as a result, can become strongly oriented into a preferred direction, orthogonal to the maximum stress direction during the deformation.

 A final characteristic of the Oakhurst exposures is the fact that the folds appear to be pointing into the ground. If you imagine drawing a line along the same unit connecting points of maximum curvature, this is known as the fold axis. This line may be horizontal, vertical, or inclined at some angle. At Oakhurst the folds are clearly plunging to the west at a shallow angle. i.e. angled down in the same direction as the photographs have been taken.

 

If anybody is interested in learning more about similar exposures, or other sites of geological interest, please do not hesitate to contact me alang@sdtec.co.za

 


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