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And the age of the Daohugou Bed is...



Several weeks ago Mickey M. kindly sent me the paper by Gao & Ren (2006). It 
defends the Middle Jurassic age of the Daohugou Bed, mainly by pointing out 
weirdnesses in the He et al. (2004) paper that has been discussed here -- I'm 
quite embarrassed not to have noticed that, by definition, an ignimbrite* 
cannot intrude into a sediment as if it were a plutonite! -- and by noting that 
He et al. (2004) fail to include detailed geographic and stratigraphic 
information in their short paper.

* Latin: ignis = fire, imber = rain. Rock composed of volcanic ash. -- BTW, 
this doesn't seem to matter, because He et al. (2004) argue, unnecessarily, 
that the ignimbrite did not intrude.

However, Gao & Ren (2006) only cite the even shorter paper by He et al. (2005) 
in passing. That paper is a response to an earlier critique of He et al. 
(2004). It shows that a radiometric age from a tuff within the Daohugou Bed 
(164 to 165 Ma) is questionable because "arguably the same tuff" contains 
crystals that have highly diverse radiometric ages, up to over a billion years. 
It also contains a photo that is supposed to show the Daohugou Bed overlying 
(rather than underlying) the famous ignimbrite with the Middle-Late Jurassic 
boundary age; the line drawn between the two layers is a guesstimated 70° from 
horizontal...

Incidentally, Gao & Ren (2006) subtly misquote He et al. (2005) by omitting the 
"arguably" from the above quote. He et al. (2005) give the tuff layer a number, 
and another tuff layer another number.

Gao & Ren also try to argue with biostratigraphy; but insects are what was used 
to put the Yixian Fm into the Jurassic, and conchostracans are AFAIK still 
dangerous. They are of course right that salamanders, invoked by He et al. 
(2004), are rather useless for biostratigraphy.

Conclusion? I wonder how many Daohugou Beds there actually are. I wonder about 
the correlation. Maybe some of those beds are above and others below the 
ignimbrite. The only thing that everyone seems to agree on is that they all 
underlie the Yixian Fm -- but the Yixian Fm only starts in the Barremian. The 
Dabeigou Fm, which has a radiometric date from the Hauterivian-Barremian 
boundary (mentioned onlist this summer), is not mentioned in the entire 
discussion; on the other hand, Gao & Ren (2006) equate the Jiulongshan Fm with 
the Haifanggou Fm and the Lanqi Fm with the Tiaojishan Fm without explaining 
this or citing sources.

Gao K. & Ren D.: Radiometric Dating of Ignimbrite from Inner Mongolia Provides 
no Indication of a Post-Middle Jurassic Age for the Daohugou Beds, Acta 
Geologica Sinica English Edition 80(1), 42 -- 45 (February 2006)

He H., Wang X., Zhou Z., Zhu R., Jin F., Wang F., Ding X. and A. Boven: 
^40Ar/^39Ar dating of ignimbrite from Inner Mongolia, northeastern China, 
indicates a post-Middle Jurassic age for the overlying Daohugou Bed, 
Geophysical Research Letters 31, L20609, 4 pages (2004)

He H., Wang X., Zhou Z., Zhu R., Jin F., Wang F., Ding X. and A. Boven: Reply 
to comment by Liu and Liu on "^40Ar/^39Ar dating of ignimbrite from Inner 
Mongolia, northeastern China, indicates a post-Middle Jurassic age for the 
overlying Daohugou Bed", Geophysical Research Letters 32, L12315, 3 pages (30 
June 2005)

I can send the pdfs to everyone interested. The Geophysical Research Letters 
papers are also available on He's homepage, and the English edition of Acta 
Geologica Sinica is simply unavailable almost anywhere.
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