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tgamble · 01-21-2002, 05:49 AM

This sounds like a stupid question but I'm seeing a lot of conflicting numbers. 40 million years to about 70 million.

This one says 70 million but the time span adds up to 40!

<a href="http://www.humboldt.edu/~natmus/Exhibits/Life_time/Cambrian.web/" target="_blank">http://www.humboldt.edu/~natmus/Exhibits/Life_time/Cambrian.web/</a>

<a href="http://museum.gov.ns.ca/fossils/geol/camb.htm" target="_blank">http://museum.gov.ns.ca/fossils/geol/camb.htm</a>

<a href="http://www.ucmp.berkeley.edu/help/timeform.html" target="_blank">http://www.ucmp.berkeley.edu/help/timeform.html</a>

30 million years seems to be quite a difference. Is there disagreement or is someone just making a mistake?

John Solum · 01-21-2002, 06:27 AM

Hello,

The currently accepted start of the Cambrian is ~544 million years ago, but before that it was ~570. The age was changed from 570 to 544 Ma when better exposures of rock that could be radiometrically dated were found at the Cambrian/Precambrian boundary. In this case zircons from conglomerates and pyroclastic breccias in Siberia were dated using U/Pb and Pb/Pb yielding ages of 543.9 +- 0.24 Ma and 543.6 +- 0.5 Ma. Here's the reference:

Bowring, S. A., Grotzinger, J. P., Iaschsen, C. E., Knoll, A. H., Pelechaty, S. M., and Kolosov, P. Calibrating rates of Early Cambrian evolution. Science, vol. 261, pp. 1293-1298, 1993.

I guess it's taking a while for people to get up to date. I still see the 570 date on a lot of web sites and in quite a few intro geology books too.

Oolon Colluphid · 01-21-2002, 06:54 AM

Welcome, John! If you post stuff at that level, you're most welcome to come here as often as you wish!

Cheers, Oolon

tgamble · 01-21-2002, 09:18 AM

Quote:

Originally posted by John Solum:
<strong>Hello,

The currently accepted start of the Cambrian is ~544 million years ago, but before that it was ~570. The age was changed from 570 to 544 Ma when better exposures of rock that could be radiometrically dated were found at the Cambrian/Precambrian boundary.

/B]

How's the boundary identified?

Quote:

In this case zircons from conglomerates and pyroclastic breccias in Siberia were dated using U/Pb and Pb/Pb yielding ages of 543.9 +- 0.24 Ma and 543.6 +- 0.5 Ma. Here's the reference: Bowring, S. A., Grotzinger, J. P., Iaschsen, C. E., Knoll, A. H., Pelechaty, S. M., and Kolosov, P. Calibrating rates of Early Cambrian evolution. Science, vol. 261, pp. 1293-1298, 1993.

I guess it's taking a while for people to get up to date. I still see the 570 date on a lot of web sites and in quite a few intro geology books too.</strong>

Looks like Patricks been replaced.

Thanks for the response.

[ January 21, 2002: Message edited by: tgamble ]</p>

John Solum · 01-21-2002, 10:09 AM

Thanks to everyone for the warm welcome.

Quote:

How's the boundary identified?

The boundary's identified by the appearance of particular types of fossils. In this case the base of the Cambrian formation is marked by an unconformity and within the overlying rocks are assemblages of fossils that the authors correlated with index fossils from other well-known and previously described areas. The authors provide a description of 13 or so fossils that they found that indicate that the rocks are "basal Cambrian in age."

Quote:

Looks like Patricks been replaced. Thanks for the response.

LOL, you're welcome, but I certainly can't keep up with Patrick. I'm amazed at how quickly he can compose a long, well-thought-out post.

Quetzal · 01-21-2002, 12:24 PM

John: Most hearty welcome! Bienvenido to the 12th Circle of Hell (aka II Bar & Grill)!

ps418 · 01-21-2002, 01:18 PM

Howdy John,

Good to see you here!

PS- For those who dont know, John is the author of an excellent article on <a href="http://www-personal.umich.edu/~jsolum/yec/lewis.html" target="_blank">thrust faults.</a>

Patrick

01-21-2002, 05:49 AM	#1
tgamble Veteran Member Join Date: Sep 2000 Location: St. John's, Nfld. Canada Posts: 1,652	How long was the cambrian period? This sounds like a stupid question but I'm seeing a lot of conflicting numbers. 40 million years to about 70 million. This one says 70 million but the time span adds up to 40! <a href="http://www.humboldt.edu/~natmus/Exhibits/Life_time/Cambrian.web/" target="_blank">http://www.humboldt.edu/~natmus/Exhibits/Life_time/Cambrian.web/</a> <a href="http://museum.gov.ns.ca/fossils/geol/camb.htm" target="_blank">http://museum.gov.ns.ca/fossils/geol/camb.htm</a> <a href="http://www.ucmp.berkeley.edu/help/timeform.html" target="_blank">http://www.ucmp.berkeley.edu/help/timeform.html</a> 30 million years seems to be quite a difference. Is there disagreement or is someone just making a mistake?

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01-21-2002, 06:27 AM	#2
John Solum Regular Member Join Date: Jan 2002 Location: Ann Arbor, MI Posts: 139	Hello, The currently accepted start of the Cambrian is ~544 million years ago, but before that it was ~570. The age was changed from 570 to 544 Ma when better exposures of rock that could be radiometrically dated were found at the Cambrian/Precambrian boundary. In this case zircons from conglomerates and pyroclastic breccias in Siberia were dated using U/Pb and Pb/Pb yielding ages of 543.9 +- 0.24 Ma and 543.6 +- 0.5 Ma. Here's the reference: Bowring, S. A., Grotzinger, J. P., Iaschsen, C. E., Knoll, A. H., Pelechaty, S. M., and Kolosov, P. Calibrating rates of Early Cambrian evolution. Science, vol. 261, pp. 1293-1298, 1993. I guess it's taking a while for people to get up to date. I still see the 570 date on a lot of web sites and in quite a few intro geology books too.

01-21-2002, 06:54 AM	#3
Oolon Colluphid Contributor Join Date: Sep 2000 Location: Alibi: ego ipse hinc extermino Posts: 12,591	Welcome, John! If you post stuff at that level, you're most welcome to come here as often as you wish! Cheers, Oolon

01-21-2002, 12:24 PM	#6
Quetzal Senior Member Join Date: Nov 2001 Location: Ecuador Posts: 738	John: Most hearty welcome! Bienvenido to the 12th Circle of Hell (aka II Bar & Grill)!

01-21-2002, 01:18 PM	#7
ps418 Veteran Member Join Date: Mar 2001 Location: Louisville, KY, USA Posts: 1,840	Howdy John, Good to see you here! PS- For those who dont know, John is the author of an excellent article on <a href="http://www-personal.umich.edu/~jsolum/yec/lewis.html" target="_blank">thrust faults.</a> Patrick

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