[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

RE: Age of Archaeoceratops



> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Tracy L. Ford
>
> The Cretaceous period is broken down (as all ages) to stages, and epochs.
> There are three epochs in the Cretaceous;

Actually, by most international standards of stratigraphy, there are only
two Cretaceous epochs: the Early Cretaceous Epoch and the Late Cretaceous
Epoch, defined before the immense length of the Cretaceous (in numerical
time) was known.

> The Neocomian is a
> Subepoch of the
> Early Cretaceous Epoch, it is further broken down into three stages;
> Berriasian, Valanginian, and the Hauterivian. The next Subepoch is the
> Gallic, which actually is in both the upper Early Cretaceous Epoch and the
> Lower Gulf Epoch (or upper Cretaceous, but I don't think anyone uses the
> Gulf Epoch, but if you have subepochs you have to have epochs, IMHO, it's
> like having subfamilies without familes, etc). The lower Gallic has the
> Barremian, Aptian and Albian (which is middle to late Early Cretaceous
> Epoch); the upper Gallic is the Cenomanian then the Turonian. The last
> subepoch is the Senonian: Coniacian, Stantonian, Campanian and finally the
> Maastrichtian.

Gulfian is a strictly North American term, as is Comanchean (North American
Early Cretaceous).  Neocomian, Gallic, and Senonian are strictly speaking
European units, but since the first and the last been used for a LONG time
in stratigraphy it is not at all uncommon to see them show up in literature
for non-European geology.  (Gallic was coined more recently, and is less
widespread).

While I think a triune division of the Cretaceous (Neocomian, Gallic, and
Senonian) makes more sense in so far as it is finer divisions of this vast
amount of time, by the International Commission on Stratigraphy of the
International Union of Geological Sciences does not regard the subepochs as
formal divisions, and only formally recognizes the Early/Late division (and,
of course, the stages).

> Some people just use the subepochs and not the stages. This may be because
> they haven't or can't get a more precise age to the formation they are
> working in.

Precisely (so to speak: after all, it's a lack of precision that's the
problem).

Most time divisions of the geologic record are based on marine microfossils
and macrofossils, and are occasionally correlated with the terrestrial realm
via pollen and the like.  As such, we have (in general) far less precision
fiting terrestrial units into the global geologic time scale than we do
marine sediments.  Sadly, of course, it is the terrestrial units that by and
large have the best dinosaur fossils!

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796