Re: Geological periods

[Stan Friesen <swf@ElSegundoCA.ATTGIS.COM>]

 From: sj@io.com (Steve Jackson)
 > 
 > So we agree on the end of the Cretaceous, but we're 5 million years
 > different on its beginning; we differ by 18 million years on the beginning
 > of the Jurassic; we differ by 20 million years on the beginning of the
 > Triassic.
 > 
 > Is there that much dispute, or has there been a systematic reorganization
 > of the periods, or what?
 > 
There are several factors at work here:

- There is about a 3-5% uncertainty in radiometric dating. This
  seems to account for about half of the variance. 

- There is often uncertainty as to the correct assignment of specific
  formations to geologic time units.  For instance some years ago
  many of the terrestrial formation from the southwest US were considered
  to be Latest Triassic, but have since been identified as Early
  Jurassic.  I suspect that this may have alot to do with the shift
  in the date of the beginning of the Jurassic.

- Different workers use different methods to resolve uncertainties.

swf@elsegundoca.attgis.com              sarima@netcom.com

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