Geology of the Great Karoo

[tok-tokkie]

Geology of the Great Karoo

I wrote this as another part of my report of our visit to Loxton but now I think it should be a thread by itself. http://trailrider.route42.co.za/index.php?topic=4208.0   I will break it up into sections as it would be a very long post all in one piece.  First I will give a summary of a very good book I have then I will post some photos I took around Loxton of the geology there.

One of the people Pat had organised to be on the trip was Judy Maguire from Prince Albert.  She is a palaeontologist (someone who studies fossilised plants & animals) and her husband is a geologist (he was away).  Judy knows a hell of a lot about the formation of the landscape and was able to tell us what the rocks were and how they came to be there.  I asked her for a recommendation of some books  to get as I have become interested in that aspect of the Great Karoo.  She recommended three books. One I already had on order, one I have been able to get and the third is out of print but I am looking for it in second hand shops.



I am going to summarise the geology of the Karoo based on the two books I have.  Be aware that that is the sum of my knowledge – I have just become interested in all aspects of the Karoo and this essay serves to be my learning tutorial on the subject – I find I remember things much better if I have to write my own description.  All of the illustrations come from those two books and I credit them at the end.


Geology map.  Notice that the Karoo is either pale khaki (the outside) or darker khaki (the center) and that Lesotho is also two coloured.  All four of those groups of rocks are part of the ‘Karoo Supergroup’ (a term used by geologists).  The violet is an igneous rock (one that came up from deep in the earth’s crust as hot molten rock), the other three are sedimentary rocks (ones that formed in water [or by wind] when sand and mud was washed down in rivers and settled out at the bottom of a swamp, lake or sea).  So, basically, the Karoo was underwater when it formed.  How was that when now it is miles from the sea?


This section drawing shows how the rocks are arranged vertically.  UNFORTUNATELY the light khaki band is now mid green or brown (Dyka & Ecca groups which were lumped together in the first map)

[Operator]

I like these posts of yours.

I had a geography teacher in school, that tickled my interest about these things.
I could not believe that a person can have such a natural passion for rocks and rockformations
and that attracted my interest.

Although I won't understand it all, I will follow it in detail. Thanks for the effort.

[tok-tokkie]

How did this all come about?


Time scale model

First of all a time scale we can relate to.  A figure of 550 million years ago is hard to visualise.  I will use a scale of 1 second = 1 year so:

1 year                                  = 1 sec                                       1 second
1 lifespan ~= 100 years                  = 1 minute  40 seconds                                 2 minutes      
Millennium = 1000 years               =  16 minutes 40 seconds                                       ¼ hour
1 million years                     =  57 days 20 minutes                                            2 months
1 billion years                    = 158 years 6 months 2 weeks  6 days                      150 years

In the last column I have given an approximate equivalent so you can get some feeling for the time span of 1 billion years.  If your life was just 2 minutes then, on that scale, 1 billion years would be 150 years.  I will convert ages to this scale in {} in what follows. I hope this helps you visualise how long the time periods are.


 Creationists.

I don’t wish to offend fundamental religious beliefs.  The Bible states that the earth has 4 corners and Paul Kruger (& many others) believed that meant the earth must be flat. Obviously no one can believe that anymore what with the Space Shuttle going round the earth as I write this.  The RC church accused Galileo of heresy when he claimed that the earth moves around the sun because their dogma was that the earth is the center of everything. Only in 1992 did they apologise for their mistaken accusation and they said ‘the theologians who attacked Galileo failed to understand that the Scripture was not literal when it came to a description of the physical world’.  If you substitute ‘age’ in place of ‘day’ in what is written in Genesis  about the creation of the world then there is no real conflict between what I write here & what appears there.  See http://en.wikipedia.org/wiki/Creation_according_to_Genesis


Continents Form

The world began about 4,5 billion years ago {700 years on our scale = incredibly long ago if we have less than 2 minutes}.  It was a very hot ball of liquids and gasses at first, the first rocks formed as it cooled about 4 billion years ago {600 years ago on our scale} See http://en.wikipedia.org/wiki/History_of_the_Earth  http://en.wikipedia.org/wiki/Geological_history_of_Earth , These rocks were like the crust (slag) that floats on lead when you melt it – literally, because the center of the earth is molten iron and nickel which is heavier than rocks.  The rock crust moves about the surface of the earth driven by the convection currents in the liquid core.  At times all the rock crust joins together as one ‘supercontinent’ then it splits apart into smaller continents which drift away from each other.  It seems that this happened several times but it is very difficult to know about any but the last one.  That was when Pangaea formed , it took 300 million years for this to happen and it was accomplished about 300 million years ago {50 years on our scale} . http://en.wikipedia.org/wiki/Pangaea  Then 180 million years ago {30 years on our scale where we have less than 2 minutes to live} Pangaea started to break up.  First Europe, Asia and North America broke away leaving the southern continents joined together as Gondwana.  There is a complication here because Gondwana had formed separately long before it became joined to the other bits to create Pangaea and then that broke up so Gondwana was again a separate supercontinent.  We are now dealing with this later life of Gondwana. http://en.wikipedia.org/wiki/Gondwana  One of the early contributors to our knowledge of Gondwana and plate tectonics (the movement of continents) was a south African, Alexander du Toit. http://en.wikipedia.org/wiki/Alexander_du_Toit


Gondwana began to split up to form the southern continents as we now know them 130 million years ago {21 years ago on our scale}.


Note on this diagram the dates are for when Gondwana first formed before it became part of Pangaea (and it later separated when Pangaea broke apart).

[Trailrider]

Very interesting so far. I never realized that India was part of Gondwana looking at where it is now. I don't know India's geology but I guess it's Northern parts must be mountainous with possibly some volcanic activity?

[tok-tokkie]

Very interesting so far. I never realized that India was part of Gondwana looking at where it is now. I don't know India's geology but I guess it's Northern parts must be mountainous with possibly some volcanic activity?

Here is another picture which gives some of the evidence for the arrangement of Gondwana.


India is moving northwards & crashing into the Asia plate right now.  That is what has created the Himalayas.  I will post about the Cape Fold mountains next which were caused by a similar event in the very distant past.

[Trailrider]

The Himalayas

Suddenly my question seems very stupid

[tok-tokkie]

Cape Fold Mountains


About 330 million years ago {55 years ago on our scale where we live for a bit more than 1 minute} the Cape Fold Mountains formed.  There had been a sea, the Agulhas Sea, in which sedimentary sandstone had been deposited; blue in the diagram.  This sandstone got buckled and folded as it was forced up to form the Cape Fold Mountains.  These mountains caused a sea to form where the present Karoo is, as you can see in the diagram.


This diagram shows just the Cape Supegroup with the outline of present day South Africa for reference; understand that this was happening when Gondwana still existed before it broke up so there was no Africa like we have it today; the same applies to the maps which follow.  Originally the Groups would have been continuous but they became bucked and bent then the tops of the mountains got eroded away so just the stumps now show and they are in bits and pieces. The next diagram shows how it works.


The Groups get their names from the place where the rocks are particularly prominent.  Later on you will meet the Beaufort (Beaufort West) & Clarens  Formation – but I don’t know where the Dwyka & Ecca Groups got their names from.
These mountains created the Karoo Sea with a steep southern coastline and a gently sloping northern coastline.  Gondwana was then over the south pole and covered in thick ice which expanded and contracted with the seasons polishing the underlying rock and dumped the rock that it had frozen in it. (p197)  This dumped rock is the Dwyka group which is the oldest of the Karoo group. 


This picture (shown previously) shows what the Karoo looked like after the Karoo sea had filled up with sedimentary rocks.  The steep southern shore and the gently sloping northern shore can be seen. On this diagram you can see how the Ecca Group is continuous but it only comes to the surface around the edge.  That is what has happened with the Cape Fold Groups as shown on the previous diagram.


This is a simplified map of the Karoo rocks.  Note that the Karoo rocks cover 2/3 of South Africa.  The northern part is actually the Highveld.

Next I will describe the sequence during the silting up of the Karoo Sea.

[White Stripes]

Very cool, TT.

Some of my ancestors had a very traumatic experience back then. Some of them went on an extended bike ride and could not come back when this all happened. They are still pacing up and down in Australia.

[tok-tokkie]

Dwyka Group

300 – 260 million years ago.    Duration 40 million years.
{50 – 43 years ago on our scale.   Duration 7 years.}


This is the start of the Karoo rocks.  The Karoo was underwater between two ranges of mountains that had glaciers moving down into the Karoo sea.


The northern side of our Karoo was shallow then.  The Karoo sea would have been off to the right of the picture with the meltwater running down that way.


On the south side the present Swartberg  mountains were being ground down by glaciers which dropped the debris in the deep water this side.  If you look at the first diagram of today’s post & also yesterday’s post you will see that the Falkland Plateau was ramming up against the Cape Supergroup forming the Cape Fold Mountains.  As shown on today’s first diagram this mountain range was considerably wider than what we now have – when we get to the end of the sequence you will see that the Falkland Plateau pulled away stretching the surface creating the huge Agulhas Bank which lies out to sea from the Southern Cape; that was also part of the huge mountain chain that eroded to provide the sediment that formed the Karoo.  The mountains we see now are lower and much thinner than they were when the Karoo was being formed.  The sediment formed at this time is now called the Dwyka group and is the oldest (lowest) of the Karoo system; best seen south of Laingsburg.  It formed under the influence of ice with the northern part being thinner and different to the southern part.



This section is similar to the previous one. P & N (Port Elizabeth & Nelspruit) are shown on the earlier diagram.  The Dwyka Group is jaggered along the joint with the Cape Supergroup because it was formed before the mountains had finished growing so it got folded there along with the Cape mountains as they continued to be folded up.

[tok-tokkie]

Ecca Group

260 - 250 million years ago.   Duration 10 million years.
{43 – 42 years ago on our scale.   Duration 1 year.}


The next layer is called the Ecca group.  It is much thicker than the previous Dwyka group.  The ice had all melted by this time because Gondwana had moved away from the South Pole.


The Karoo Sea went right across what is present day South Africa east to west so the Ecca deposit was also in Kwazulu –Natal, Gauteng & Mpumalanga.


These are the swamp forests that created the Highveld coal deposits.  Plants had developed as the Cape Supergroup was being laid down 450 million years ago and were now well established. {75 years ago on our scale}. http://en.wikipedia.org/wiki/Evolution_of_plants   Big  animals only appeared 290 million years ago so the plants had it pretty well their own way initially.  http://en.wikipedia.org/wiki/Evolution_of_animals



The Swartberg must have been huge (like the present Himalayas?) judging by the thickness of the Ecca group of rocks.  The three sedimentary layers have a combined thickness of 12 000 meters, all of that carried by water off the original mountains http://en.wikipedia.org/wiki/Karoo_Supergroup  The Cape Fold mountains were still being forced upwards and growing so the Dwyka (lowest) Group rocks were now also being folded along the southern side.

[tok-tokkie]

Beaufort Group

250 - 240 million years ago on our scale.  Duration 10 million years.
{42 – 40 years ago on our scale.   Duration 2 years.}


By 250 million years ago the Karoo sea was filled with silt.  That had taken 50 million years {8 years on our time scale where we have slightly more than a minute to live}.  Now the Karoo sea was like the present Okavango everywhere (previously the swamps had been on the northern & Kwa-Zulu sides). The rivers were flowing out of the Cape Fold mountains along the south, there were no rivers coming in from the north.  It was getting drier.
There was some huge catastrophy 251 million years ago {42 years ago on our scale} when 96% of all species became extinct – including plants.  This is long before the dinosaurs developed – they died in a later (much smaller) mass extinction.  This event is evident in the Beaufort group as, presumably, the disappearance of most of the plants allowed erosion to speed up dramatically and so the rocks are made of bigger particles – sand as against silt.  Slowly vegetation recovered, but it takes millions of years for new plants to evolve to replace the extinct ones and the sedimentation changes slowly to record this.



In this diagram note that sand gets deposited along the rivers and silt and mud in the lagoons.  This gives rise to the different rock types in the Beaufort Group.

[tok-tokkie]

Stormberg  (includes Clarens formation)

240 – 183 million years ago.   Duration 57 million years.
{40 – 30 years ago on our scale.   Duration 10 years.}



The area continued to get more and more dry just as we now see in Botswana.  Now different rock gets formed as desert sand solidifies.  This is still a sedimentary rock even though water was not involved.  Today these rocks, the Clarens formation, form the base of Lesotho.  Over the Karoo this layer has been eroded away.

[tok-tokkie]

Drakensberg Group

183 – 181 million years ago.    Duration 2 million years
{30,5 – 30,2 years ago on our scale.    Duration 4 months.}

All the time the Karoo rocks had been forming up until now the Cape Mountains were still rising due to compression.  183 million years ago {30,5 years on our scale} this suddenly stopped.  Instead the earth at South Africa started to stretch and cracked open and huge amounts of lava flowed over the Karoo.  This was the time when South America broke away as Gondwana split up.


I think they have got the colours on the key for Dwyka & Ecca the wrong way round because Dwyka lies under Ecca.  The one I particularly want you to look at is the Karoo Dolerite intrusions.

The lava came out through cracks called dykes and flowed out sideways to form plates (sills) 10 to 20m thick.  This happened time and again so eventually the layer of lava was 1 600 m thick – the Drakensberg.  The sills could also form underground as the lava forced its way between layers of sedimentary rock of the Beaufort, Ecca and Dwyka groups.  This underground lava was much denser than the surface flows (I have rock climbed in the Drakensberg and that rock is very soft and unreliable to climb on); this denser lava rock is called dolerite.  The whole of this volcanic episode seems to have taken just 2 million years {4 months on our scale} bringing to an end the 100 million year period that it took to form the rocks of the Karoo {16 years on our scale}.  It started with an ice sheet, then glaciers running out of growing mountains into a sea, then deltas as rivers flowed from the still growing (and eroding) mountains into huge lagoon; then the lagoon silted up to form a huge swamp as the climate continued to get warmer and the Cape mountains continued to rise but also erode down, then a big desert to the north encroached and this sand became compacted into rock, then suddenly the earth cracked open and lava flowed across the surface and also as sills between layers of the sedimentary Karoo rocks.  Much of the lava and upper sedimentary rock has now been eroded away leaving us with the jiggered Drakensberg and wind blown sandstone of Golden Gate and the Karoo koppies.


Another diagram of how the dolerite sill and dykes were formed.


Now this diagram is important.  Look at the levels of the three diamond mines, Orapa, Jagersfontein & Kimberly.  Notice the vast amount of erosion that has taken place at Kimberly; well below the dolerite level.  The Tilite level is the first of the Karoo layers (Dwyka) and the Dolerite level is the last so you can see that much of the Karoo system has been removed at Kimberly.   All that material has been carried away by water or wind.  Look how much of the diamond pipe has been eroded – that is the source of the diamonds now being mined at Kleinzee, Oranjemund & at sea on the West Coast.

[tok-tokkie]

References



An absolutely wonderful book if you are interested in the geology of South Africa. It costs R234.  Published by Struik.   All the diagrams with figure numbers like ‘Figure 7.11’ come from it.  There are lots of photos to illustrate the book, not just close ups but many of the rocks in the countryside.  It has chapters on the palaeontology .  It covers the entire country – the geology of the Highveld is really complicated and some of the very oldest stones on earth are to be found near Barbeton.  I have purposely not included any of their photos and have given a pretty note like summary.  Their writing is brilliant – easy to read, not at all dry academic and also not talking down or trivialising the topic. I can’t recommend this book highly enough.


A good book. R224.  Published by Struik. The first map is from it with the routes covered by this book shown in blue.  It thus covers the country well.  Most of the illustrations WITHOUT ‘Figure 7.12’ type headings come from this book; most of them have a frame around the diagram.  The book has geological maps for each route in sections with interesting rock sites marked on the map and discussed in the text.  Nicely written.

[tok-tokkie]

Geology Pictures.


My interest in the geology of the Great Karoo was sparked on the trip we did to Loxton.  There I met Judy Maguire who knows quite a lot about it and she recommended two books to me which I got on our return to Cape Town.  The photos I took are thus from before I had even read those books.  This one shows a Karoo koppie.  The thing about a Karoo koppie is it has a flat top and the top edge is a band of rocks.  That top band is the igneous (melted rock forced up from deep down in the earth) dolerite.  The dolerite is more weather resistant than the sedimentary Karoo rocks so it acts as a roof or umbrella protecting the less resistant rock below.  The umbrella used to be much bigger but it has been eroded away so the koppies get smaller & smaller over the ages.


This photo shows the typical sedimentary rocks of the vlaktes of the Karoo.  Notice how it lies in a pavement of bands as the layers of mud consolidated into rock.  This is the type of rock that was used to build the corbelled houses.


The layers of sedimentary rock, probably mudstone, are not all the same thickness.  Notice how it breaks up into square sided pieces.

[tok-tokkie]

Dolerite








These photos show the igneous rock, dolerite, at the top and the sedimentary rock (Beaufort series probably) in the foreground.  Notice the difference in colour.  The dolerite is dark with a red bias whereas these sedimentary rocks are pale.  Notice also the shape of the stones.  The dolerite forms rounded rocks whereas the sedimentary rocks are square sided.  That dolerite originally was forced between the layers of the Beaufort Group when it was hot and soft forming a continuous solid layer as it cooled and solidified.  Now that layer has been exposed and broken up into these roub\nded rocks by the weathering action associated with erosion.  Slowly it will be broken right down to sand – the red sand that is everywhere to be seen in the Karoo.


This picture shows  the hard dolerite top layer which breaks up into smaller pieces that roll down the side of the koppie onto the vlaktes below.  There is a lot of iron in the dolerite which gives it that red colour so you can see the red areas where the dolerite is on the side of this koppie.


A different view of the dolerite rocks on the side of a koppie.  See how they  get smaller and smaller the lower down the slope they move; that is the weathering over the ages breaking the rocks down.  The soil below the rocks is red – that is soil created by breaking the dolerite down into grains of sand.  The sand from the sedimentary rocks tends to be yellow – though I will show a photo of very dark coloured sedimentary layer soon.

[tok-tokkie]

This photo shows the spillway on the dam in front of the house we stayed in.  The base layer is sedimentary rock cracked into sharp edged blocks, on the left are rounded dolerite rocks that the farmer has brought in and used to form the embankment for the side of his garden.  In the background is the end of the dam wall with the very sharp rocks which are more of the sedimentary Karoo rock contrasting to the rounded dolerite on the left of the picture.






This is on top of a koppie with the dolerite rocks everywhere.  These rocks are relatively young as they are still quite angular – as they age and roll down the slope they become more rounded.  In the third photo you can see the solid dolerite layer that has yet to break up into loose rocks.


This photo shows where the layer of dolerite has been cut through by the padmakers.  Notice how large and massive the rock is below the old surface.  Notice too the different colour; the newly exposed rock has not ‘rusted’ yet to become the typical dark red colour.

[tok-tokkie]

Sedimentary rock floor with cracks.


Igneous dolerite with a piece of sedimentary rock for comparison.  The dolerite when broken is ‘crystalline’ whereas the fine mudstone sedimentary rock has no crystal structure to it; it is smooth even where broken.


Sedimentary rock on the vlaktes with a stream flowing through it.  Most of the surface of the Karoo is formed by the Beaufort Group and Loxton is on that Group.  This shows the process of erosion in action.  This very small stream has, over the ages, been able to break up and carry away the rock layer that originally would have been continuous right across the picture.


Different sedimentary rock which is in a much thinner layer and breaking up into smaller pieces.  The colour is greyer than the previous example.


This donga cuts through a much darker grey layer of sedimentary rock.  This rock breaks up into small pieces about as big as a marble.  It is like a layer of grey gravel.


This picture shows the sandy rock that formed where the river actually flows and the mudstone that formed in the lagoons and marshes.  In the previous photo you can see quite different layers of rock.

[tok-tokkie]

This is the Vis river.


The Vis and Sak are the two remaining rivers of the Great Karoo that flow northwards into Brandvlei & Grootvloer Pan.  The last remnants of the system that created the Beaufort Group of rocks that forms the surface of most of the present day Great Karoo.  I took the photo on the recent Wuppertal/Ouberg Pass trip we did.  Having read up about the geology of the Karoo and knowing this diagram I was absolutely delighted to come across this scene.  This scene was just as exciting to me as was the one from the top of the Ouberg pass.  But today things have moved on and the “Shallow lake” of the diagram is now the dried pan of Brandvlei, Grootvloer of Verneukpan.


A close up of the lovely Vis River valley.  Young valleys have steep sides, mature valleys are wide and flat like this and old rivers running across the vlaktes wiggle about like the Berg does downstream of opposite Hopefield.

[tok-tokkie]

In this Google image I have tried to emphasise the mountain ranges.  The diagram just above of the rivers and shallow lake when the Beaufort Group was laid down shows a massive mountain range to the south.  These mountains were a combination of the Cape Fold Mountains and the mountains of the Falkland Plateau.  The rivers are shown as starting in the mountains south of where the present coastline is.  When the Falkland Plateau broke away it seems to have also stretched out the Cape Fold mountains creating the Little Karoo between the now separated Outeniqua and Swartberg mountains and also to have created a big depression through which the N1 runs between the Swartberg and the escarpment of the Great Karoo (the Roggeveld, Komsberg & Nuweveld Berge).  It would be interesting to find out how Seweweekspoort, Meiringspoort and the Tradouw valley were cut through the mountains when the rivers reversed and started to flow south instead of northwards..


This is the escarpment at the Ouberg Pass.  Notice the bands of hard rock.


This diagram shows the intrusion of dolerite into the Beaufort Group as sills.  That is what you are seeing in the picture.  I have zoomed right in on the rocks in the picture and I am pretty certain I am right in what I am saying – I wish I had had a proper look at them when we were there.  The rock at the roadside was typical sharp edged Karoo sedimentary rock but I am not absolutely certain if the bands in this photo are dolerite or just hard layers of sandstone.


Because I have this picture on Flikr I include it here.  It shows the escarpment of the Great Karoo with the Ouberg Pass near Sutherland.  It shows how erosion works at eating back the plateau and carrying it away in a river (the Doring) to the sea where that soil will be deposited to create fresh sedimentary rocks.


More of the same, next pass up from Ouberg this time Gannaga Pass near Middelpos.