RE: Age of base of Hell Creek Formation

["Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>]

> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Phillip Bigelow
>
> Lucas, S. G. and Sullivan, R.M., eds., 2006, Late Cretaceous vertebrates
> from the Western Interior. New Mexico Museum of Natural History and
> Science Bulletin 35.
>
> However, if I use Lucas et al.'s chrono numbers (page 15), then
> Schweitzer et al's (2005) "~70 ma" estimate for the base of the Hell
> Creek Formation would drop it down into the Edmontonian!  Yikes!
> Something is amiss.
>
> Any explanation?  Again, I ask:  Where did Schweitzer et al. get their
> numbers?

I know that the Montana State team has a long term project on the stratigraphy 
of the Hell Creek ongoing, so perhaps the date comes
from that work.

The most up-to-date work that I know of on the geochronology of the Hell Creek, 
though, is Hicks et al. 2002:

Hicks, J.F., K.R. Johnson, J.D. Obradovich, L. Tauxe & D. Clark. 2002. 
Magnetostratigraphy and geochronology of the Hell Creek and
basal Fort Union Formations of southwestern North Dakota and a recalibration of 
the Cretaceous-Tertiary Boundary, in J.H. Hartman,
K.R. Johnson & D.J. Nichols (eds.), The Hell Creek Formation and the 
Cretaceous-Tertiary boundary in the northern Great Plains: An
integrated continental record of the end of the Cretaceous. GSA Special Paper 
361: 35-55.

Their estimate places the total time range encompassed by the Hell Creek as 1.9 
Myr at most, and quite possibly as little as 1.05
Myr, with a preferred value of about ~1.36 Myr. They suggest that there is a 
substantial disconformity (of about 3.3 Myr duration)
within the underlying Fox Hills Fm.

So following this, even the lowermost T. rex specimens are only from about 67 
Ma (with 65.5 being the K/T boundary).

                Thomas R. Holtz, Jr.
        Senior Lecturer, Vertebrate Paleontology
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
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