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Cutting edge

A cache by Sterreman and sy-chispa Send Message to Owner Message this owner
Hidden : 8/20/2009
Difficulty:
2 out of 5
Terrain:
1 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

The Karoo is a fascinating place! But you will not be able to appreciate it while speeding through - you need to stop and have closer look, particularly at its most interesting geology.

Let’s backtrack about 350 million years to when the then continents were happily moving around on the slightly plastic mantle of the Earth, each being carried by structures known as tectonic plates. They began to collide into one grand continent known as Pangea (meaning Entire Earth), with the southern part called Gondwana. Along the ridges where the future continents of Africa, South America and Antarctica were to rift roughly 220 million years later, mountain ranges were being pushed up, one of which we know today as the Cape Fold Belt which you, amongst others, traverse between here and Cape Town. These mountains, together with those now in Antarctica, drained their rivers inland, not out to sea. As a result, for hundreds of millions of years, water and its erosion products (mud, sand and rocks) washed down from the mountains, steadily poured into a great inland sea, the Karoo Basin. These sedimentary deposits would eventually reach a thickness of over 6 km at its southern edge.

At the position of this EarhCashe the inland sea was probably only a few tens of meters deep. Here sediments were being laid down at the mouth of a very large north-flowing river, part of the river delta, depositing mud onto the bottom of the sea. Occasionally, the slightly angled build-up, being very fluid, would slip as an underwater avalanche, sporadically causing the fine layers of laid down mud to be mixed together and the water was temporarily turbid. This produced alternating strata with thicknesses of less than a centimetre to tens of centimetres. Eventually, under the weight of later deposits, it all solidified into the layers of mudstone and turbidites that you can see in this magnificent road cutting today.

To make it even more interesting, at the time this was happening there were large volcanoes active along the region where South America was splitting away from South Africa, about 1000 km to the west, and the prevailing northwesterly winds were blowing the volcanic ash this way. The ash sifted down on the surface of the western side of the inland sea, sank to the bottom, and produced interleaved layers with the mud deposits. The turbidites we see, therefore have colours varying from the grey of pure mud through to yellow of pure ash.

Then, about 130 million years ago, the single large continent began to split up into Gondwana and Laurasia, then into various smaller pieces as they drifted apart. All of the Karoo deposits were originally laid down in horizontal strata, but later pressures, including from the splitting of the continents, caused folding of some of them. The term folding is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of plastic (i.e. permanent) deformation. In the process of continental drift, pressures build in various parts of the Earth's crust, and slowly, over thousands of years, the crust is transformed as layers of rock are forced to buckle, warp or bend.

Where you are standing now was about 2 km below the surface when all this happened. Since the break-up of Gondwana, southern Africa has been a rising continent, lifted up by vertical currents in the mantle beneath. We thus live on an eroding surface where all this material had been weathered away and transported by rivers out to sea. If you want to be at the level where the dinosaurs lived you will need to get in a helicopter and rise about a kilometer above the present surface. The Karoo around you here is roughly 230 million years old.

(Source: Prof Brian Warner
Department of Astronomy, University of Cape Town , South Africa AND
School of Physics and Astronomy, University of Southampton, UK.)

To claim “Found it” you must email me satisfactory responses to the following:
Any logs not accompanied by an email will be deleted.

  1. Find safe parking near the designated coordinates. Locate the formation where the greatest amount of folding took place and send me a picture of you/your party and your navigation device, showing this feature in the background. (Note: There are two sections of folding, you may choose either one.)
  2. Send me a close-up picture where you point out an example layer which formed during a long period of pure mud deposition.
  3. Send me a close-up picture where you point out an example layer which formed during a period of high volcanic activity.
  4. Although sedimentary layers always form horizontally, between the two folding features they are at a quite a steep angle. When and how could the strata have tipped over like this?
  5. Pace out the distance between the two folding features (i.e. the tipped section). Assuming a sedimentary rate of 20000years/metre, calculate roughly how many years it must have taken for this (tipped) section to be laid down.

Note: Do not post your pictures, replies or hints to this page, even if encrypted.

Additional Hints (Decrypt)

Ernq gur grkg pnershyyl, nyy gur nafjref ner va gurer. Rznvy lbhe cvpgherf gb jcx@fnnb.np.mn

Decryption Key

A|B|C|D|E|F|G|H|I|J|K|L|M
-------------------------
N|O|P|Q|R|S|T|U|V|W|X|Y|Z

(letter above equals below, and vice versa)



 

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