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Author Topic: Geology of the Great Karoo  (Read 7742 times)

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Offline Pistonpete

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Re: Geology of the Great Karoo
« Reply #20 on: October 13, 2009, 06:37:30 am »
Good stuff & thanks for sharing your extensive knowledge :thumleft:
'Routine is the thief of time'
 

Offline DeepBass9

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Re: Geology of the Great Karoo
« Reply #21 on: October 13, 2009, 08:06:18 am »
+1  :thumleft:

Offline Ned_da_biker

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Re: Geology of the Great Karoo
« Reply #22 on: October 13, 2009, 08:41:11 am »
Gr8 stuff!

Was wondering how many geo's we have on this site?   :patch:
ome mistakes are too much fun to only make once. :-)
 

Offline DeepBass9

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Re: Geology of the Great Karoo
« Reply #23 on: October 13, 2009, 08:45:23 am »
At least 4 by my count.

Offline tok-tokkie

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Re: Geology of the Great Karoo
« Reply #24 on: October 13, 2009, 09:02:33 am »
Gr8 stuff!

Was wondering how many geo's we have on this site?   :patch:
I am not one.  That post is just a summary of the book I read (as stated early in the thread).
Pleased it is of interest to some here.
 

Offline eikeboom

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Re: Geology of the Great Karoo
« Reply #25 on: October 13, 2009, 11:38:01 pm »
Toktokkie, some interesting snippets (and sorry I have no photos of these!):
I've seen 2 fairly prominent dolerite dikes in the mountains around here, etched against the lighter coloured rocks of the sandstones or weathered cape granites:
The one is in the side of Lion's Head in the road cut as you drive (sorry ride  :ricky: ) over to Camps Bay from the city bowl, taking Kloof Rd if I remember correctly, just after the split on top.
The other one is in Helshoogte Pass on the way to Franschhoek, about halfway up to the top on the righthand side in the road cut. This one is only about 1 or 2 m wide I think, one can see how the intrusive magma has baked the surrounding rock.

The grey rock which is quarried all around the western cape for tar tops is metamorphosed shale, transformed from the heat of granitic magma which intruded from below and cooled and solidified as domes below the surface.

When you visit Whitestripes again have a look at the interesting variety of different granites along the west coast. Some contain large (slightly white, cream or pinkish) crystals of a certain mineral (magma has cooled down sloooowly), others are fine-grained (cooled down faster, less time for crystal growth before solidification). Sometimes you'll even see a huge black "rock" (ball size, pumpkin size, car size or even up to house size!) inside a granite boulder - these are chunks of the sedimentary rocks which broke off during the intrusion from below, and were assimilated into the semi-fluid magma and baked in place.

Did you know that probably all the clays we have around here are weathered (eroded and chemically broken down) granite? The buff and black coloured minerals have become clays and the quartz crystals have washed out and are still quarts (pebbles and sand). Our white sand come from eroded sandstones and quartz veins and weathered granites! (Otherwise some sand are broken pieces of shells and limestone).

Again on granites: in the Chapmans Peak road cuts you spectacularly cross at one stage the sharp boundary between the granite below and the sedimentary layers on top. Where there is no baking of the sand-and siltstones, the granite was first eroded down somewhat before the near horizontal layers were deposited on top from elsewhere.
Let's go into the mountains...there's likely to be peace and quiet
 

Offline eikeboom

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Re: Geology of the Great Karoo
« Reply #26 on: October 13, 2009, 11:45:07 pm »
Sh*t sorry this was not about the Great Karoo!

Outside Laingsburg see this evidence of the sedimentary processes you describe so interestingly:



If the rock layer is young enough there are sometimes animal or bird tracks to be seen on slabs like this, as if imprinted yesterday!
Let's go into the mountains...there's likely to be peace and quiet
 

Offline tok-tokkie

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Re: Geology of the Great Karoo
« Reply #27 on: October 14, 2009, 08:38:47 am »
Sh*t sorry this was not about the Great Karoo!

Outside Laingsburg see this evidence of the sedimentary processes you describe so interestingly:



If the rock layer is young enough there are sometimes animal or bird tracks to be seen on slabs like this, as if imprinted yesterday!
Where is that outside Laingsburg?  It is spectacular.
The granite intrusion is, of course, very well explained at the end of Sea Point ('Queens' surf spot).  You can see the metamorphosis of the shale in all its stages.
 

Offline eikeboom

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Re: Geology of the Great Karoo
« Reply #28 on: October 14, 2009, 07:49:39 pm »
The tilted slabs of ripples are right outside Laingsburg on the Beaufort West side, as you leave the outskirts of town, going uphill on the left-hand side.
Let's go into the mountains...there's likely to be peace and quiet
 

Offline Stofdonkie

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Re: Geology of the Great Karoo
« Reply #29 on: October 21, 2009, 09:17:11 am »
Arguably one of the best posts on this forum!

I love traveling with my eyes 'open' and the geology thing has started interesting me over the last year or so (after the plant, birds, history route).

Would love to put together a geology/history/ecology bike trip around the Karoo. Such an interesting place deserves much more attention than just 'gravel travel'.

Nice one tok-tokkie.
Carpe BM.
 

Offline Laban

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Re: Geology of the Great Karoo
« Reply #30 on: October 21, 2009, 10:20:06 am »
Thanks for sharing TT!!
..."sometimes the people around you won't understand your journey, they don't need to, it's not for them"...Joubert Botha

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Offline tok-tokkie

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Re: Geology of the Great Karoo
« Reply #31 on: October 21, 2009, 11:12:38 am »

Would love to put together a geology/history/ecology bike trip around the Karoo. Such an interesting place deserves much more attention than just 'gravel travel'.

There is a book Circling the Great Karoo by Nicholas Yell with sub titla A Back-roads Journey through History on an Old Scrambler which is an old Yamaha TX250.  It is privately published - I got mine at Linex Yamaha Cape Town.  Nicely written & he did solo what you talk about.

--------------------------------------------------------------------------------------------------------

While posting here I would like to link in this picture I came across.  A trip in the Noordoewer region.


A dyke par excellance
 

Offline Stofdonkie

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Re: Geology of the Great Karoo
« Reply #32 on: October 21, 2009, 11:32:33 am »
I read his book. Very well done. He's a bit hard on the Boere in his historical recountings, but then, I might be a bit biased.

 ;D
Carpe BM.
 

Offline The Gentleman

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Re: Geology of the Great Karoo
« Reply #33 on: October 28, 2009, 06:40:06 pm »
Thanks for the great Post Tok-tokkie! I hope to take some of this in on the trip!

 

Online roxenz

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Re: Geology of the Great Karoo
« Reply #34 on: November 20, 2009, 09:19:30 am »
Oh very well done, Toktokkie! As a professional geologist I'd be very happy to accept you amongst our ranks as an Honourary Geologist!  :thumleft:  Perhaps Jules can make an appropriate sticker for you?  ;)

Slightly off-topic: the dyke at Sea Point and the outcrop in the bay mentioned by eikeboom is quite famous.  Even Darwin came to have a look at it.
 

Offline HVDM

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Re: Geology of the Great Karoo
« Reply #35 on: January 28, 2010, 09:43:33 am »
At least 4 by my count.

Another one here.

Tok-tokkie, if you have a chance, go look at the dolerite ring structures near Queenstown. Personally I find the Karoo boring, and prefer complex structures like the Limpopo Mobile Belt, the Natal metamorphic province etc.

There are also some interesting pipes in the Karoo in the western cape.
« Last Edit: January 28, 2010, 09:48:19 am by HVDM »
 

Offline Watty

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Re: Geology of the Great Karoo
« Reply #36 on: January 28, 2010, 01:37:47 pm »
At least 4 by my count.

Another one here.

Tok-tokkie, if you have a chance, go look at the dolerite ring structures near Queenstown. Personally I find the Karoo boring, and prefer complex structures like the Limpopo Mobile Belt, the Natal metamorphic province etc.

There are also some interesting pipes in the Karoo in the western cape.

Me too.  Graduated 1979 @ UPE.  Worked 10 years as a marine geologist researching the sedimentary processes of the South and East Cape estuaries.   No igneous and metamorphic stuff for me  :mwink:  Sand and mud can also be complex !
 

Offline HVDM

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Re: Geology of the Great Karoo
« Reply #37 on: January 28, 2010, 01:46:01 pm »
At least 4 by my count.

Another one here.

Tok-tokkie, if you have a chance, go look at the dolerite ring structures near Queenstown. Personally I find the Karoo boring, and prefer complex structures like the Limpopo Mobile Belt, the Natal metamorphic province etc.

There are also some interesting pipes in the Karoo in the western cape.

Me too.  Graduated 1979 @ UPE.  Worked 10 years as a marine geologist researching the sedimentary processes of the South and East Cape estuaries.   No igneous and metamorphic stuff for me  :mwink:  Sand and mud can also be complex !

Structure is where its at: bedding, cleavage, thrusts....

Sedimentology is OK, but have you ever seen mud engineers? Damn, the whole day looking at drillind mud, viscosities, densities, sediment load. They cant even drink a beer without a hole in the bottom of the mug, like a Marsh funnel :biggrin:
 

Offline Skinny

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Re: Geology of the Great Karoo
« Reply #38 on: January 28, 2010, 04:10:30 pm »
:biggrin:
 

Offline HVDM

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Re: Geology of the Great Karoo
« Reply #39 on: January 28, 2010, 07:44:42 pm »
Tok tokkie, going through pictures, I know the karoo of the eastern cap mostly. The dyke photo looks like its taken near queenstown area and looks to be part of upper Beaufort.

when you drive north from Grahamstown you see ecca shales. after you cross the Great Fish you start to move into Karoo. This lower part of the Karoo is slightly folded, so the rocks are tilted. In this area you get dykes. This is caused by the rising of the Cape mountains, which fratured the rocks, allowing dykes to intrude. north of bedford, adelaide Ft beaufort you get mountains with flat lying rocks. here you find mainly sills, where dolerite intruded between rock layers.

North towards queenstown and I think beaufort west, dolerites intruded in rings.

Do you see all the joints in the rocks? this was caused by stresses. In the south they are oriented 60 and 120 degree angles to each other, with two main directions. This was caused by rising of the Cape mountains. To the north they are 90 degrees to each other.

Most dykes if I recall are oriented ESE, since the direction of greatest  stress was NNE.

I'did work on the Karoo like 20 years back. But I prefer much more complex areas. The Karoo is young stuff  :biggrin: