Re: Geological periods

["King, Norm" <nking.ucs@smtp.usi.edu>]

Steve Jackson asked (01/22/96; 1:11p):

>I had thought that there was consensus about the dates used for
>Triassic/Jurassic/Cretaceous, but apparently the reference I've been 
using
>for my latest project is contradicted by some later material.

>Sattler's DINOSAURS OF NORTH AMERICA (1981) gives:
>        Triassic 190-225 mya
>        Jurassic 140-190 mya
>        Early Cretaceous 100-140 mya
>        Late Cretaceous 65-100 mya

>But, in trying to track down why one reference thinks Dilophosaurus 
might
>have been Late Triassic and another calls it Early Jurassic, I ran into
>this schedule; it's in Lessem's KINGS OF CREATION, and matches the one 
in
>the Dino Society Encyclopedia -

>        Triassic 208-245 mya
>        Jurassic 145-208 mya
>        Cretaceous 65-145 mya

[snip]

Stan Friesen's posting (1/22/96; I lost the time) did not address the 
date for the beginning of the Triassic.  The time scale published by 
Harland et al. in 1982 pegs the beginning of the Triassic at 248 Ma, and 
most current literature uses a figure between 250 and 245 Ma.  Do some 
people who are knowledgable about radiometric age-dating disagree 
significantly with the dates for Triassic and adjacnet strata in the 
Harland et al. scale?  Earlier time scales (your reference was 1981) 
often showed the Triassic beginning at 225 Ma.  But Harland's et al. 
scale shows the LATE Triassic (Carnian Epoch) beginning 231 Ma, and the 
oldest dinosaurs would be almost that old--I've seen 225 Mya most often.  
The older time scales, showing 225 Ma for the beginning of the Triassic, 
use about 205 Ma for the beginning of the Carnian. If I were you, I'd use 
the Harland et al. dates.

Reference:

Harland, W.B., A.V. Cox, P.G. Llewellan, C.A.G. Pickton, A.G. Smith, and 
R. Walters, 1982, _A Geologic Time Scale_:  Cambridge Univ. Press.



*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*-*
Norman R. King                                       tel:  (812) 464-1794
Department of Geosciences                            fax:  (812) 464-1960
University of Southern Indiana
8600 University Blvd.
Evansville, IN 47712                      e-mail:  nking.ucs@smtp.usi.edu