ages of feathered dead stuff

[smithjb@sas.upenn.edu (Joshua Smith)]

Dinogeorge@aol.com wrote:
> 
> In a message dated 6/6/99 6:08:42 PM EST, sarima@ix.netcom.com writes:
> 
> << Is this solid enough to settle the controversy over the dating of these 
> faunas?
>  
>  Or is there still enough uncertainty to allow some to continue to claim a
>  Late Jurassic date for them? >>
> 
> Don't think there's much if any support remaining for a Late Jurassic date 
> for these faunas. These radiometric dates are >well< into Early Cretaceous, 
> by around 20M years.
> 

        OK. Here is the deal as of this week on the age disput of the 
Sihetun area and its associated dead stuff.  This is what I am willing to 
say on the record without pissing off co-authors, editors, embargos, etc.

        As we pretty much all know by now, there are two principal 
hypotheses offered for the age of the basal Jehol Group (referred as the 
basal Yixian Formation, but seperated into the Yixian Formation and the 
Chaomidianzi Formation by Ji et al., in press).  The Jehol Group is the 
collected units that have produced the Mesozoic fauna and flora that we 
are all so familiar with (e.g., the _Lycoptera_ fish fauna, the _Eostheria_ 
"bug" fauna, the dead stuff with feathers, all those damn birds, etc.).  
The group consists of the sedimentary and igneous fill of a number of 
foreland basins that began forming probably in the middle of the Jurassic 
in response to reactivation of the Tan-Lu Wrench Fault, a whopping 
strike-slip fault that runs from the central-south area of China all the 
way up into northeastern Liaoning.  The actual tectonics have to do with 
movements off various plates against one another, but that isn't too 
important.  The basic jist is that the fault began to move along a rough 
northeast, southwest line and created tensional stress in the region of 
Liaoning in a very rough northwest-southeast direction.  This of course 
caused all of the normal crustal thinning and extensional strain on the 
Achean and Paleozoic carbonates that made up the majority of the basement 
rocks in Liaoning during the Jurassic--the rocks failed in many places, 
normal faults were formed, the blocks cut by these faults began to 
subside, and these newly-formed basins began to fill with clastic sediments. 
As fault bounded basins are almost always associated with some sort of 
crustal thinning, the fractures didn't have to propagate too too far into 
the upper crust to begin intersecting mantle magmas.  Thus, we had a lot 
of rift-basin volcanism beginning to take place.  These were not often 
volcanoes in the normal sense but rather big-ass fissures that spewed up 
basaltic composition magmas.  As this generally occured with movement 
events on various faults (which also created sediment pulses into the 
basins), we ended up with the layer cake volcanics intercalated with 
fluvial and lacustrine sediments (at least in the beginning of these 
basins genesis, which is the time period we are concerned about).  This 
is a hard-core terrestrial system--it is an environment completely 
different from the Solnhofen Limstone that crops out in Bayern, which 
consists of the remnents of the estuaries that _Archaeopteryx_ was 
winging over.

The Liaoning basins have been the subject of study by the Chinese for 
close to a hundred years, long before the dinosaur community had any idea 
they were even there (this is because the upper formations of the Jehol 
Group are basically coal measures...).  Interestingly, the age of these 
rocks has been in dispute almost since the beginning of interest in this 
area.  Unfortunately, most of the evidence that was (and still is) 
offered up in support of any of the age hypotheses put forth (Middle Jr, 
Late Jr, Early K, or even Late Triassic at one point) has been 
biostratigraphic in nature.  The problem is, most of the biostratigraphy 
has been done using endemic forms known only from the basins in 
question.  It is therefore crap.  It violates at least two the main 
premises of biostratigraphy (the taxa used must be very abundant, tightly 
constrained temporally, and widely distributed geographically).  
Virtually all of the biostratigraphy done using vertebrates (most of the 
recent stuff especially) is invalid.  I cannot say anything more about 
this right now.  Wait a little while.

However, the sediments we are concerned with are sandwiched between 
igenous rocks.  It should be really simple to get radiometric ages from 
these and determine the absolute age of the basins, right?  Well, as with 
everything in geology, it is a rather complicated question.  As 
paleontologists, we tend to see a radiometric date and assume that it is 
gospel. Well, most of the dates done thusfar are based on Ar/Ar decay 
reactions or K/Ar decay reactions.  Argon is a great tool for 
geochronology, except that it is a rather unstable gas, geologically 
speaking.  The damn stuff tends to get driven off if the rocks in which 
it sits are reheated.  There are multiple layers of basalt in these 
Yixian lake sediments; the Sihetun quarry in particular is a 
basalt-siltstone sandwich.  I am highly skeptical of any age that is 
taken from anything other than the highest basalt in the section.  So, no 
matter what these dates are saying, if they are based on Ar reactions, 
take them with a grain or six of salt...

So, the take home message is this.  AS of right now, we still are unsure 
of these dates.  If the tuffs have been dated using single crystal 
methods on zircons, then they are probably on.  Otherwise, they have 
probably been altered.  The biostratigraphic evidence at Sihetun is 
ambiguous--the radiometric dates are not all in agreement either.  They 
do not ALL say Cretaceous.  There are Tithonian dates out there.

Also, there is still a huge amount of support for a Jurassic age by many 
of the people working out there.  True, much of this is based on 
biostratigraphy, but nonetheless, they are getting papers published...
There is a lot of political pressure on the Chinese scientists because of 
some of the synapomorphies that birds such as _Confuciusornis_ has (e.g., 
pygostyle, edentulus beak, fused rostrom, etc.).  If Sihetun truly is 
Jurassic (and personally, no matter what I think the data say, I think it 
would be much cooler if it were Jurassic...), then it really threatens 
_Archaeopteryx_ as the central figure in avian evolution during the 
Tithonian.   So, don't expect a huge about of objectivity in much of the 
work coming out of this place--it is a very complicated issue, both 
scientifically and politically.


 -- 
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Josh Smith
University of Pennsylvania
Department of Earth and Environmental Science
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