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The age of the Daohugou "Formation"



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

The paper is short and concise, so I'll simply let it speak for itself:

"Since the name of the Daohugou Formation was not appropriately set up with a stratigraphic section, we would prefer to use the Daohugou Bed in this paper (Wang, 2000)."

"Some even went further to suggest that the Daohugou deposits could be correlated to the Middle Jurassic (Bathonian) Jiulongshan Formation (Ren et al., 2002)."
The age of that formation has been questioned, too (but only as a pers. comm. to the first author of the description of the Morrison salamander *Iridotriton*, and without a mention of what the actual age may be, other than "considerably younger"). It has yielded *Chunerpeton*, which was described as a cryptobranchid (not just -oid!) salamander... which may, rumor has it, not be correct either.


"The shale deposits obviously represent a later event than the ignimbrite; hence, the dating of the ignimbrite will provide a maximum age for the fossil-bearing sediments." The ignimbrite is 159.8 +- 0.8 Ma old. "Considering the vastly different isotope system behavior, this result is more or less concordant with the SHRIMP U-Pb zircon age" of 165.5 +- 1.5 Ma ago.

"The direct contact between the ignimbrite and the Daohugou sediments near the Daohugou village (119.22°E, 41.32°N) shows no baking of the sediments near the volcanic rocks, excluding the possibility of the ignimbrite having intruded later. Therefore, the overlying Daohugou Bed lake deposits clearly represent a geological event later than the ignimbrite."

"The Daohugou Biota is unambiguously different from that of the Jehol Biota despite the presence of the evidence of a close connection. As in the case of invertebrates and plants, the vertebrate assemblage of the Daohugou Biota shows more primitive appearance than the Jehol Biota. The Daohugou Biota is unambiguously different from that of the Jehol Biota despite the presence of the evidence of a close connection. As in the case of invertebrates and plants, the vertebrate assemblage of the Daohugou Biota shows more primitive appearance than the Jehol Biota."

"Despite the dating reported in this paper, the exact age of the Daohugou Bed in Inner Mongolia remains unsolved as there is yet no evidence showing how long the gap existed between the deposits and the volcanic base. It is, however, noteworthy that at the Daohugou locality, the fossil-bearing sediments are also found to overlie the Tuchengzi Formation in a few sites. The Tuchengzi Formation comprises gray, green to reddish sandstones, conglomerates and shales and is unconformably overlied by the Yixian Formation in many sites of Liaoning and its neighboring areas. Earlier (^40)Ar/(^30)Ar dating of the upper part of the Tuchengzi Formation in Beipiao, Liaoning Province gave a mean age of 139.4 +- 0.2 Ma (Swisher et al., 2002). Therefore, it appears that the Daohugou Bed is most likely younger than this age. Considering the abovementioned contacts and dating, as well as the primitive appearance of the Daohugou Biota compared to the typical Jehol Biota, we propose that the Daohugou Bed may represent the Early Cretaceous intermontaneous deposits between the Tuchengzi and the Yixian formations."

Compare this age for the Tuchengzi Fm (the home of *Chaoyangsaurus*), previously supposed to be Tithonian, with the Berriasian-Valanginian boundary which was http://www.stratigraphy.org/gssp.htm 140.2 +- 3.0 Ma ago.

In sum, the fossils of Daohugou could be from mid-late Valanginian to mid-late Barremian in age (remember that the lacustrine part of the Yixian Fm is around the Barremian-Aptian boundary, 125.0 +- 1.0 Ma ago http://www.stratigraphy.org/gssp.htm). Too bad.

There seems to be an incredible amount of middle Early Cretaceous in northeastern China...

Google finds the pdf.