re: length of Cambrian vs. Cretaceous

[cnedin@geology.adelaide.edu.au (Chris Nedin)]

>On Tony Fiorillo's comment:
>
>Well, actually, no. The Cretaceous was significantly longer.  The base of the
>Cambrian has been considerbly "younged" and is maximally recognized at
>558 MA, with some workers placing the "golden spike" in the Lena River
>section in Siberia, the proposed stratotype, as young as ca. 538 MA.
>THis is really screwing up the relative early-middle-late Cambrian
>concepts.  Nevertheless, the Cretaceous was significantly of longer
>duration--forget 570MA as the base of thje Cambrian.

The base of the Cambrian is probably <540 Ma, and certainly younger then
the Bowring et al. (1993) age of 544 Ma. This is because Bowring et al.
used zircons from breccias and conglomerates to obtain dates, i.e. reworked
material - their dates are too old. Bowring et al. relied on a Botomian
(mid-late Early Cambrian) age of 523 Ma from South Australia to tie in
their 544 figure, However, the 523 figure is being revised (due to
recalibration) and, whilst the new figure has not been released yet, lets
just say it 'aint getting any older! :-)

Somewhere between 535-540 Ma is about right, with the emphasis on the 535 end.

The Early Cambrian takes up the majority of this, probably stretching from
538 (to take David's number) to around 515 Ma. The Middle-Upper Cambrian
boundary is at roughly 505 Ma and the Cambrian-Ordovician boundary is
around 500 or 495 depending on who you listen to.


FYI this makes the Burgess Shale around 510-512 Myo (roughly)

Chris

Bowring, S. A. et al. (1993) Calibrating rates of Early Cambrian evolution.
Science, 261: 1293-1298

cnedin@geology.adelaide.edu.au,   nedin@ediacara.org
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Many say it was a mistake to come down from the trees, some say
the move out of the oceans was a bad idea. Me, I say the stiffening
of the notochord in the Cambrian was where it all went wrong,
it was all downhill from there.