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Re: VARIOUS SUBJECTS
Jaime A. Headden wrote:
> Josh Smith wrote:
> <The Yixian sediments are not limestones. They are siltstones and
> sandstones, with some really really minor, very cool evaporites. They
> vary in color from brown to yellow to white to grey.>
> The Sihetun site is like a layer cake of multicolored confetti, and
> if a layer popped out blue, I wouldn't be surprised.
Great bloody quote! I hope he lets me use that...that would be
excellent in a talk (devious grin).
> Now, on a related note, so far I've seen mention of three formations
> from the Sihetun site, one using a group alias (the Jehol Group) so
> I'd like to know if any of you dino-enthusiasts or geo-nuts out there
> have got a complete or fairly complete list of the geologic
> separations at that most fascinating place?
Whatever the rocks at Sihetun are called, I haven't seen anyone
argue for them to be pulled out of the Jehol Group and put somewhere
else, so I am confident that we have virtually 100% agreement that these
rocks fall within the Jehol Group. A geologic group is a group of rock
units (generally formations) which are collected together because they
all share some similar characteristic. The defining characteristic of
the Jehol Group is that all of the member formations are comprised of the
sedimentary and volcanic fill of these several Mesozoic foreland and rift
basins that we are so concerned about (they are all related in this way,
so to speak: they are all Mesozoic, terrestrial, basin-fill, volcanic, and
volcanoclastic rocks intercalated with lacustrine and fluvial sediments).
As for what to call the Sihetun rocks, well, as far as I know,
Sihetun falls completely within the Yixian Formation, which is the most
basal member of the Jehol Group. It sits unconformably upon Precambrian
marine basement rocks or on the sediments of the Middle Jurassic
Tuchengzi Formation. The Yixian has two principal lithologic "types."
The lower type is neutro-basic volcanics and volcanoclastics intercalated
with lacustrine and minor fluvial sediments. The upper part is
principally tuffs and other pyroclastics. Sihetun falls within the basal
Now, as far as what to call it. Whatever. It doesn't change the
lithology. We can call it the "formation previously known as the
Yixian," if you like. The Chen et al. (1998) description of
_Sinosauropteryx_ refers to the Jianshangou beds. In doing this, they
are referring to the sedimentary intercalation at Sihetun and describing
it as a member of the Yixian Formation. As far as I know, this is an
informal name and the Jianshangou is not a formal member of the Yixian.
I don't know if the stratigraphy gods have ruled on this or not. Doesn't
matter. It is a good way to discuss the basal intercalation of the Yixian.
As for this "Chaomidianzi Formation" that Ji et al. (1998)
talk about in the _Caudipteryx_ paper, it is a term that does not appear
in print anywhere nor is it in GEOREF. It was cited in that paper in a
"Professional Papers on Stratigraphy" article that is in press. I assume
from this (though we cannot be sure until that article comes out) that Ji
is feeling the pressure of those of us who are calling for a Cretaceous
age for Sihetun and is attempting to hold onto his Jurassic age by
dividing up the Yixian and creating a new formation; one that he can then
say is Jurassic in age, regardless of what we say the Yixian is. I would
suspect that his reasoning for this centers on the fact that up until the
samples that I collected in July and those that John Ostrom talked about last
year at GSA, there were no confident dates from Sihetun. Also, the
stratigraphic information is so bad in the Beipiao area that we are not
certain of the relative stratigraphic placement of the previous dating sites
to the Sihetun quarries.
Whatever. The National Geographic article indicates that
_Caudipteryx_ comes from Sihetun. We can also choose to believe that
_Sinosauropteryx_ also comes from Sihetun (Phil and I still don't
completely agree on its provenance). It doesn't really matter if Ji
chooses to divide up the Yixian and call Sihetun by a different name.
The samples I collected come from an andesite directly below the quarry
levels and a basalt directly above it (each within several meters of the
principal _Confuciusornis_ horizon). When these samples get back from
being dated and the zircons from some tuffs in the Beipiao area that I
wacked on come back...then it will just be a matter of convincing some
authors to forget about this ridiculous notion they have that the igneous
rocks at Sihetun are intrusives...
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