Green River and Ash layers

[redghost]

Hello:

If my memory serves me correctly, I believe there are multiple Lava/ash layers located in the Green River formation.

Have scientists done radiometric studies on these (I would assume so) and correlated the varve difference (years) between them to see if they
match?

Any pointers to studies on this would be greatly appreciated.

cheers

 

[thaumaturgy]

I can't find a lot of immediately accessible links, but there's this article:
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Eocene calibration of geomagnetic polarity time scale reevaluated: Evidence from the Green River Formation of Wyoming

Malka Machlus1, Sidney R. Hemming1, Paul E. Olsen1 and Nicholas Christie-Blick1

[SIZE=-1]1 Department of Earth and Environmental Sciences and Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA [/SIZE]

We reevaluate the Eocene geomagnetic polarity time scale on the basis of single-crystal 40Ar/39Ar ages for air-fall tuffs from the Wilkins Peak Member of the Green River Formation of Wyoming. Tuff 6 is dated as 49.1 ± 0.2 Ma, and tuff 3 is dated as 50.4 ± 0.3 Ma (maximum estimate). When combined with published magnetostratigraphic constraints, these age determinations suggest that the currently accepted age of chron C22r is 1.5–2.5 m.y. too old, which supports a significantly longer duration for the early Eocene, for the early Eocene climatic optimum, and the Wasatchian North American Land Mammal Age.

Geology; February 2004; v. 32; no. 2; p. 137-140; DOI: 10.1130/G20091.1 © 2004 Geological Society of America
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Apparently within the Green River formation there are some ashfalls from the Absaroka Volcanic Field and the San Juan Volcanic Field that provide "datable horizons" within the Green River which help constrain (along with other methods) to the Eocene. The Eocene in general spans about 22 million years from 56 to 34 million years ago. (LINKY)

 

[juvenissun]

Hello:

If my memory serves me correctly, I believe there are multiple Lava/ash layers located in the Green River formation.

Have scientists done radiometric studies on these (I would assume so) and correlated the varve difference (years) between them to see if they
match?

Any pointers to studies on this would be greatly appreciated.

cheers

I don't know how many lava layers in the basin. But it seems there is not many. And I believe ash layers there would not be good to give any eruption age any more.

 

[samiam]

Oh, you mean like this: http://home.entouch.net/dmd/age.htm

 

[thaumaturgy]

I don't know how many lava layers in the basin. But it seems there is not many. And I believe ash layers there would not be good to give any eruption age any more.

Huh??? What kind of radiometric dating are you talking about here, Juvenissun? What kind of elemental make-up are you hypothesizing?

Care to flesh that hypothesis out? Tell us what isotopes you think are used in dating what is likely felsic to intermediate ash samples. "Any more" indicates to me you think they would be too short lived to be of any value in dating these ash beds?

 

[thaumaturgy]

redghost,
I will respectfully ask that you verify most of Juvenissun's geology claims. While he will occasionally be correct on some points, many others he simply will make 'one-line' comments on, nearly free of details or commentary.

I will be honest enough to admit that while I do have several degrees in geology, I am not a specialist in radiometric dating or the Green River formation. However, I am more than willing to do the kind of legwork Juvenissun will not. In other words I am willing to look up a reference or two.

To that end Juvenissun's claims around the utility of radiometric dating in the Green River would appear to be somewhat moot based on the earlier link I posted.

In reviewing the link's abstract I note that they rely on 40Ar/39Ar which is a technique often used in dating felsic or intermediate igneous materials. The key being that many felsic (feldspar and silica-rich igneous rocks, including ash) contain a significant amount of potassium-40, which is, itself, a radioactive isotope that decays to Argon-40.

The details of the technique and it's advantages over K/Ar dating can be found HERE.

THIS LINK is also useful in explaining some of the details around Ar-Ar dating.

As for the utility of Ar-Ar dating to even relatively geologically young rocks, a report from UC Berkley has found that Ar-Ar can be used to date ridiculously young rocks by analyzing and pegging the date for the Vesuvius eruption of 79AD (LINKY)

In argon-argon dating, first developed at UC Berkeley in the 1960s, samples are irradiated with neutrons to convert potassium-40 to argon-39, which is normally not present in nature. The ratio of argon-40 to argon-39 gives a measure of the age of the sample less subject to experimental error.(ibid)

(NOTE: It is likely Juvenissun has me on "ignore" because I am an actual trained geologist and I usually don't deal kindly with his vague and veiled "commentary" on geologic topics. Juvenissun claims to be a geologist, but most of the geologists on this board are trying to figure out what his actual bona fides are. I will caution you to please double check all information provided by the geologists as well as folks like Juvenissun)

 

[redghost]

redghost,
As for the utility of Ar-Ar dating to even relatively geologically young rocks, a report from UC Berkley has found that Ar-Ar can be used to date ridiculously young rocks by analyzing and pegging the date for the Vesuvius eruption of 79AD

Thanks thaumaturgy...you actually anticipated my next question (What radiometric studies have been done on Vesuvius).

I found the info on the varve/tuffs on gmorton's site (thx samiam).

cheers

 

[juvenissun]

Oh, you mean like this: http://home.entouch.net/dmd/age.htm

If you are talking to me, then the answer is no.