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Liaoning age



> Mark Norell asked me to forward this to the list-

> Second, in response to Josh Smith's comment on my feelings about the
> age of the Liaoning beds.  To quote from our paper. The age of the
> fossil beds in western Liaoning is debated.  Certain faunal elements
> suggest a Late Jurassic age; radiometric work from several sites near
> Sihetun has suggested conflicting dates of 124.6 mybp or 147 mybp.
> The age of these beds is a complex problem and probably several ages
> are represented at different quarry sites.  No radiometric samples
> exist from the Fanzhangzi quarry (NGMC 91 site) and stratigraphic
> correlations are imprecise because the Fanzhangzi quarry is over 130
> kilometers from the Sihetun site. Much of the press took an average of
> this date. A lot more work needs to be done before the ages of each
> quarry are sorted out.  Personally, I could care less if  these beds
> are Late Jurassic or Early Cretaceous, however, if anyone has any
> objections to the paperLo, C-H. Chen, P.-J., Tsou, T.-Y., Sun, S.-S &
> Lee, C.-Y. 40Ar/39Ar laser single-grain and K-Ar dating of the Yixian
> Formation. NE China. in Jehol Biota (eds Chen, P-J. & Jin, F.) 328-340
> (1999). I would like to hear them.

Even though I think that the current age data for the Yixian Formation
overall are more indicative of a Barremian age than a Tithonian one, I
like Mark, don't care.  I AM interested in the answer, but that is more
because I hate not knowing things than caring about WHICH date is
important.  Lest I get put into the "Cretaceous" camp on this issue, let
me repeat that which answer we end up with is unimportant to me.  We
should be able to discuss a scientific problem openly in pursuit of the
answer without forming into factions and developing a huge emotional
stake concerning which answer is correct.  The anti dinosaur bird people
have certainly done this, to the point where I question their
objectivity.  I cannot see how that is advancing anything.  Honestly, I
think it would be more interesting if the sediments WERE Jurassic.
However, the available data, as I understand them, are weighted towards
the Early Cretaceous at this time.  Jerry Harris and I have spent a lot
of time over the last few years trying to sort this problem out to the
degree that can be done without a NEW, detailed structural analysis of
what has been called the Yixian Formation in numerous, separate,
depositional basins, and it seems like there are always problems with
the ages that point towards the Late Jurassic.  We have discussed this
in as much detail as space allows in the upcoming Ostrom volume (but as
Mark said, this is a complex problem and will need additional sorting
and me and Jerry are working on that), but to summarize very briefly:

The biostratigraphy of the Yixian Formation can probably be safely
described using the phrase "a bloody mess" (and we are dealing with the
"Yixian" in the classic formational sense (with all of the problems
associated with that...) because we as a community lack an understanding
of these lithologies in a sequence stratigraphic sense).  The lithology
of the newly proposed Chaomidianzi Formation is not dissimilar enough
from the Yixian to be separated out from it in any sense that the
International Commission on Stratigraphic Nomenclature will see as valid
and the new unit has received little support in print thus far (Swisher
et al., 1999), so we will continue to recognize only Yixian as the name
for these rocks pending further data.

Some of the Yixian biota has Late Jurassic affinities and this is
largely where the recent support for a Late Jurassic age for the Yixian
Formation has rested.  However, as we will discuss in the Ostrom volume,
many of the taxa used to support these Late Jurassic ages either have
long (or particularly poor) stratigraphic ranges globally, or are
endemic forms known only from small samples from the rocks in question.
Either case makes them poor biostratigraphic indicators.  This is the
case with virtually all of the Yixian tetrapods (expect perhaps
_Psittacosaurus_, which has problems of its own as a biostratigraphic
index, but which does support a Cretaceous, rather than Jurassic age).
To use _Sinosauropteryx_, for example, to support a Tithonian age for
the basins that contain it because it looks like a Tithonian form, is
fraught with problems when the sample size is in the single digits and
all of the specimens come from within the rocks in question themselves.
If a _T. rex_ were found in the lowermost Fort Union Formation, the cry
would not be that the Fort Union was Cretaceous, but rather that we now
had conclusive evidence of a Paleocene dinosaur.  When the entire Yixian
biota is examined, the valid biostratigraphic data are currently rather
strongly weighted towards the Lower Cretaceous rather than the Jurassic.

However, the age of the rocks determines the age of the fossils when it
is possible to get such data, so the radiometric ages of the Yixian
volcanics should be examined closely.  The Yixian sediments are largely
lacustrine siltstones, sandstones, and volcanoclastics.  These rocks are
interbedded with some really beautiful basalt units.  By and large, the
entire lower Yixian Formation is a repeated sequence of these
lithologies.  The Yixian sediments crop out in a series of extensional
basins that are mostly normal fault bounded and which cut into the
largely Archean aged basement across Liaoning and Hebei.  As far as we
can tell, the extensional regime in the Liaoning-Hebei area was related
to strike-slip motion on what is referred to as the Tan-Lu wrench fault,
although there is some debate in the literature as to whether or not it
is in fact a wrench fault.

The trends of these Mesozoic basins, which contain the Yixian sediments,
are all largely the same and contain (within pretty large error bars)
similar thicknesses of sediment and are largely interbedded lacustrine
sediments, volcanoclastics, and igneous rocks. The petrography of the
sediments from around Lingyuan is almost identical (provenance data
aside, of course, as the source rocks are different) to that of the
Sihetun area siltstones.  The feeder dikes that have been mapped as
supporting the Yixian basalts all have basically the same trends and all
of the rocks in these basins are deformed to the same degree.  There is
no evidence to suggest that there was more than one period of extension
occurring during "Yixian" time in the late Mesozoic.  Moreover, even
with the problems that I will discuss below, every single valid
radiometric date that has been produced from the Yixian Formation in any
basin supports the idea that there was only one period of extension
here, regardless of how long that extensional period was thought to have
lasted.  Additionally, there is absolutely no evidence that any of the
Yixian volcanoclastics or basalts were anything other than coeval with
basin activity.  To make these rocks much younger than the sediments
requires the creation of an entire tectonic regime for which we have no
evidence.  The long and the short of all of this is that it is most
parsimonious to presume that all of these basins arose from the same
extensional stresses and were part of the same tectonic regime.  This is
just as valid from a structural standpoint as putting feathers on
_Deinonychus_ because _Sinornithosaurus_ had them is from a
paleontological one.

So, the initiation of Yixian deposition appears to have occurred at
about the same time in both the Beipaio and Lingyuan areas.  So, when
did happen?  Well, this is on the periphery of my knowledge, but the
radiometric ages are not as conflicting as one might think at first
glance.  First, we can basically disregard K-Ar and Rb-Sr ages. These
are based on glauconite or biotite that are no longer considered
reliable for this type of work.  This information isn't new, it was in
text books 15 years ago (G. Faure, 1986) so I am curious as to why
people are still producing whole rock K-Ar ages from these rocks.
Additionally, unless the target mineral is sanidine, which I think only
Swisher et al. (1999) have done, Ar dates from these rocks are
problematic.  Reheating (such as occurs in rift basins due to sporadic,
but consistent volcanic activity) alters argon ratios in rocks and
inflates ages.  Sanidine is good because it retains Ar well, but it can
still lose it during reheating, as I understand it.  Thus, if anything,
as far as I understand it, the real age of the basal most part of the
Yixian Formation is probably slightly younger than the 122-124 Ma age
that Swisher et al. (1999).  What I think we really need are some U-Pb
dates from zircons, but the Swisher dates are really pretty good as far
as I understand.


Thus, regardless of the fine tuning (which Mark is completely right that
we really don't have pinned down yet) the most parsimonious view
supports a roughly coeval initiation of basin formation for all of
sources of Yixian sediment. The trend of the data currently suggest that
this occurred in the Cretaceous.  This is not to say that I think that
the Yixian in Lingyuan is necessarily 124 Ma, but the evidence out there
doesn't really support the idea that it is 20 million years older.



--
Josh Smith
Department of Earth and Environmental Science
University of Pennsylvania
471 Hayden Hall
240 South 33rd Street
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