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Re: Age of Gobi formations



Two errors:

  Firstly, I mistakenly referred to Tim Donovon as Tim Williams. This was an
oversight as I was working on correspondence to both at the same time.

  Secondly, I questioned the use of continental basalts, and forgot of the
various examples of basaltic outflows in continental regions, which I was
reminded of privately. Examples of these basaltic outflows include the Deccan
traps, and my own local Snake River Plain, which is famous for the basaltic and
clastic ash in the region that was a fairly recent event and resulted in the
fertile valley floor and the famous Hagerman beds being formed. My appologies
for my crappy rememberance on this matter. However, checking with Osmólska's
comments (1997, Currie and Padian eds.) on the Nemegt and my other papers on
Nemegtian dinosaurs and mammals, the Nemegt is composed almost entirely of
varying sandstones, fining upwards, with intercalated mudstones and some clays
and gravels; if any basalts were found, these appear to not be mentioned by
Osmólska (1997), and I fear I must be curious about the origin of these dated
basalts, which led to my rather obvious skepticism. This is only further
enforced by reading of the commented phrase that Donovon uses to defend a middle
Maastrichtian age for the Nemegt (courtesy Pete Buchholz -- thanks):

  "Dating of the barungoyot svita is difficult: most researchers give a
Senonian, most likely Campanian age.  Kielan-Jawarowska (1968, 1970) lowered the
age to of the Djodokhta to Coniacian-Santonian on the basis of mammal remains
from Bayan Zag and other places.  However, molluscs, ostrocods, and turtles do
not allow an age greater than Santonian.  In addition K-Ar dates from basalts in
the middle and upper parts of the Svita (in the gorge Borzongiin Gobi and
others) are 78-80 MYR (Campanian).  Hence, we date the Baruungoyot Svita as
Santonian-Campanian (Shuvalov and Nikolaeva, 1985)." (Shuvalov, 2000:pg.263-264)

  This reference does not provide how the Nemegt can or must be Maastrichtian,
if the middle Barun Goyot/Djadokhta "suite" can be dated to 78-80 ma, the lower
Campanian, implying a most a middle Campanian age for the Barun Goyot, and a
Santonian--Campanian age for the Djadokhta, or a lower Campanian age for all
(very difficult dating stuff when you can only date the middle part of a
sequence and not the termini).

The above reference is:
  Shuvalov and Nikolaeva,1985.  [On the age and distribution of Cenozoic
terrestrial basalts in the south of Mongolia].  Vestnik Leningradskogo
Universiteta, Seriya 7, Geologiya, Geografiya 14:52-59. (in Russian)

Shuvalov is:
  Shuvalov, 2000.  The Cretaceous stratigraphy and paleobiogeography of
Mongolia; pg. 256-278 in Benton, M J, M A Shishkin, D M Unwin, and E N Kurochkin
(eds).  The Age of Dinosaurs in Russia and Mongolia.  Cambridge University
Press.

  Cheers,

  Jaime A. Headden

  Little steps are often the hardest to take. We are too used to making leaps in
the face of adversity, that a simple skip is so hard to do.  We should all learn
to walk soft, walk small, see the world around us rather than zoom by it.

  "Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)