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Author Topic: Why did the Burgess shale form?  (Read 17541 times)

Offline Bass

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Why did the Burgess shale form?
« on: 19/02/2008 17:45:47 »
The Burgess Shale, which was deposited at the beginning of the Cambrian explosion, is unique for its weird fossils and preservation of soft-body parts (normally, only hard-body parts are preserved).  The mystery of the Burgess Shale may have been solved by geologists Sarah Gabbot and Jan Zalasiewicz at the University of Leicester.  After closely studying these rocks, they hypothesize that the Burgess Shale was deposited as a thick slurry of mud which poured down a steep underwater slope (marine mudslide).  The instant burial cut off decay, allowing the soft-body parts to be preserved.  Article in the Journal of the Geological Society.
« Last Edit: 12/06/2011 16:08:41 by chris »


 

Offline JimBob

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Re: Why did the Burgess shale form?
« Reply #1 on: 19/02/2008 18:45:29 »
I don't know about this. For one thing, I thought the Burgess shale was a shale formed in anoxic conditions. To me it is counterintuitive that anoxic muds would bury life forms dependent on oxigen since the muds would have been formed in the anoxic zone. How are the critters going to live to be buried?
 

Offline daveshorts

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Re: Why did the Burgess shale form?
« Reply #2 on: 02/03/2008 10:37:11 »
Unless you had an oxygen rich shallow sea by a river or volcano, then there was a sudden flood/eruption which dumped a huge amount of sediment, this would cut off the oxygen and everything would go temporily anoxic very quickly.
 

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Re: Why did the Burgess shale form?
« Reply #3 on: 02/03/2008 14:39:41 »
What were natural oxygen levels like in the Cumbrian period?

What were the normal processes of decay at that time?  Decay is primarily a process caused by one life form (normally microorganisms or small macroorganisms) feeding off the corpse of another life form - what organisms existed in the Cumbrian that could achieve this?
 

Offline Exodus

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Re: Why did the Burgess shale form?
« Reply #4 on: 02/03/2008 14:49:37 »
I don't see this as new research, scientists have offered that theory for years... Shales normally form in low energy conditions where fine particles are able to settle out, this is often deep ocean which often more anoxic (hence the dark colour of many shales.) There are many fossil specimens, some that we had at my university of trilobites that had been squashed and there was even signs of blood in the fossil. Does the article state about how their theory differs from previous ones or have they got new evidence? Is it the mud theory that is new?

With response to Jimbob ca you not have the situation where you have anoxic conditions just below the surface of the mud, like we see in ponds or bogs here?

If anyone is interested with the Burgess Shale, there is a really nice book published by Stephen J Gould in 1989 which talks about the creatures... obviously its a little dated now but its still a nice read.
 

Offline Exodus

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Re: Why did the Burgess shale form?
« Reply #5 on: 02/03/2008 14:53:29 »
Unless you had an oxygen rich shallow sea by a river or volcano, then there was a sudden flood/eruption which dumped a huge amount of sediment, this would cut off the oxygen and everything would go temporily anoxic very quickly.

That is a way you can cut off oxygen levels but...
But to produce shales you need a low energy environment which is not influenced by heavy currents or waves... these are found deeper in the oceans...
 

Offline JimBob

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Re: Why did the Burgess shale form?
« Reply #6 on: 04/03/2008 01:13:14 »
Yes, this is my view - a catastrophic mud slide seems very unlikely.
 

Offline Exodus

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Re: Why did the Burgess shale form?
« Reply #7 on: 06/03/2008 23:51:33 »
Yes, this is my view - a catastrophic mud slide seems very unlikely.

But how then do we explain this area with such high occurences of fossils, some of which show evidence of death through compaction... Could it not have been some form of turbidity current?
 

Offline JimBob

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Re: Why did the Burgess shale form?
« Reply #8 on: 07/03/2008 01:13:28 »
I guess that might be possible, but it does not seem very likely. The Burgess shale is all very fine grained. I would expect turbidities to have the "fining upward" size gradation that is essentially the definition of a turbidite. You don't see this in the Burgess.
 

Offline Exodus

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Re: Why did the Burgess shale form?
« Reply #9 on: 10/03/2008 00:44:29 »
Argh, that is true... I've missed talking about geloegy like this. Do you work in a Geological field Jim?
 

Offline JimBob

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Re: Why did the Burgess shale form?
« Reply #10 on: 11/03/2008 17:18:26 »
Yep, I am a geologist and biologist. (Biology for paleontology post grad studies.) Wanted to be a field geologist for the USGS before being hit with arthritis my junior year. Ended up in the oil business and sedimentology, with a heavy emphasis on my favorite subject, structural geology. Never liked the hard rock stuff. That is Bass's forte.
 

Offline Exodus

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Re: Why did the Burgess shale form?
« Reply #11 on: 11/03/2008 22:08:24 »
Yep, I am a geologist and biologist. (Biology for paleontology post grad studies.) Wanted to be a field geologist for the USGS before being hit with arthritis my junior year. Ended up in the oil business and sedimentology, with a heavy emphasis on my favorite subject, structural geology. Never liked the hard rock stuff. That is Bass's forte.


i miss sedimentology and palaeo... i still flirt with the idea of looking at careers in Oil even though i'm teching now... wouldnt know where to start though.
 

Offline JimBob

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Re: Why did the Burgess shale form?
« Reply #12 on: 11/03/2008 23:19:09 »
In the near future I might have a lead for you.
 

Offline tedstruk

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Re: Why did the Burgess shale form?
« Reply #13 on: 13/03/2008 23:34:12 »
shales bear oil...
Suppose an oil trap like a tarpit area existed during the cumbrian and these guys were all trapped in a noxious "nothing survives in there" hole, later to fossilize in their entirety?

PS... Ed Conrad is awful quiet...
 

Offline JimBob

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Re: Why did the Burgess shale form?
« Reply #14 on: 14/03/2008 00:29:53 »
I don't know about this. For one thing, I thought the Burgess shale was a shale formed in anoxic conditions. To me it is counterintuitive that anoxic muds would bury life forms dependent on oxigen since the muds would have been formed in the anoxic zone. How are the critters going to live to be buried?

Anoxic means "without or lacking oxygen." The correct spelling is Cambrian.

There are almost non-oil bearing Cambrian rocks. The Ellenburger and Arbuckle limestones in Texas and Oklahoma (respectively) are Cambrian and produce vast amounts of gas but the source of the gas is from the overlying Ordovician shales.
« Last Edit: 14/03/2008 10:48:08 by JimBob »
 

Offline Bass

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Re: Why did the Burgess shale form?
« Reply #15 on: 14/03/2008 01:10:49 »
... the source of the gas is from the overlying Ordovician shales.


And all this time I thought it was from Burgess Longhorns ???
 

Offline JimBob

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Re: Why did the Burgess shale form?
« Reply #16 on: 14/03/2008 10:48:56 »
That shows what a hard-rocker knows - almost nothing.
 

Offline Bass

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Re: Why did the Burgess shale form?
« Reply #17 on: 14/03/2008 16:38:25 »
HEY..... I graduated magma cum laude!
 

Offline JimBob

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Re: Why did the Burgess shale form?
« Reply #18 on: 14/03/2008 20:54:47 »
For the rest of you - that is an old, very lame, geologist joke. (groan)
 

Offline Bass

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Re: Why did the Burgess shale form?
« Reply #19 on: 15/03/2008 04:21:45 »
almost as old and lame as the two of us !
 

Offline JimBob

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Re: Why did the Burgess shale form?
« Reply #20 on: 15/03/2008 15:02:33 »
What ya say there, pard? speak up, I can't hear ya
 

Offline Exodus

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Re: Why did the Burgess shale form?
« Reply #21 on: 16/03/2008 23:51:05 »
In the near future I might have a lead for you.

sounds exciting! :-)
 

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Re: Why did the Burgess shale form?
« Reply #21 on: 16/03/2008 23:51:05 »

 

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