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Message-ID: <36BF54F7.79C13A86@alphalink.com.au>
Date: Tue, 09 Feb 1999 08:19:51 +1100
From: Jonathan Clarke <jdac@alphalink.com.au>
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To: "Karen G. Jensen" <kjensen@calweb.com>
Subject: Re: Cambrian Explosion
References: <l03010d05b2e40185bac9@[207.211.81.101]>
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Dear Karen
Thank you for your welcome. I notice that your email was not circulated
to the
discussion group. Was this by accident (happens to me all the time!) or
because
you want to keep this private? If by accident we should forward this
correspondence to the discussion group.
Karen G. Jensen wrote:
> Dear Jonathan,
>
> Welcome! I feel honored that you came out of the lurker mode for this topic!
>
> Very interesting about the Cambrian-Neoproterozoic boundary in South
> Australia.
>
> You wrote:
> >There is nothing in the sediments of the Cambrian part of the succession
> >to indicate that they ... deposited by any fundamentally different
> >process (bar one) to the sediments of the Neoproterozoic part of the
> >succession. The exception is the presence of skeletal fossils in the
> >Cambrian.
>
> I believe it is that way in Canadian Rockies also (where one can hike up to
> the Burgess Shale -- I got to go up there in 1994!). But isn't designation
> of the Cambrian defined by the fossils?
Half your luck! I have wanted to go to Kicking Horse Pass ever since I
first
heard about it. I never quite made it to Ediacara, although have seen
the hills
in the background. They are no longer worth going to. Just before they
were
declared a fossil reserve, somebody went there will a bull dozer and
mined the
horizon. the Ediacara Member is widespread in the Flinders Ranges and
there are
plenty more fossils to be had. Not that I had any success! I did
manage to meet
the discoverer Reg Sprigg on a number of occasions though. Yes, the
Cambrian is
defined on the the identity of the fossils the strata contain. So are
all the
other Phanerozoic eras, but not the Precambrian ones, which are defined
radiometrically.
> >Karen, you are going have to be more specific: what evidence do you have
> >of "explosive sedimentation" in the Early Cambrian? What do you mean by
> >such a term?
>
> Thank you for calling me on my use of language here. I was contrasting two
> interpretation of the appearance of diverse forms in the Cambrian:
> extensive evolutionary diversification and extensive sedimentary burial.
> Perhaps extensive is a better word than explosive -- tho Genesis 7:11 says
> that the fountains of the great deep broke or burst, which sounds like it
> was probably explosive.
Yes, extensive, or abrupt, even massive, probably better convey the
sense of what
you are saying here.
> Also, I wasn't meaning to imply that this extensive (and/or explosive)
> sedimentation was only in the early Cambrian sediments, but that it
> extends through the Phanerozoic layers, which are just beginning in the
> basal Cambrian. Being in the western USA, I am especially aware of
> diagrams of the Colorado Plateau sediments, including the Grand Canyon
> series, which looks like a pile of blankets on top of the angular
> unconformity of tilted Precambrian layers beneath. That is a limited
> view, not taking into account the conformable sequences in Australia,
> Canada, and other places worldwide. I don't know the answers to all that.
It what way would you say the depositional processes of Phanerozoic
sediments
different from those of Proterozoic ones? Apart from the role played by
skeletal
and burrowing organisms that is.
> >You also said
> >....Clearly before the Cambrian Explosion event, but not at any specific
> >Precambrian horizon. And not with implicit faith in the methods of dating
> >Precambrian layers.
>
> In answer to a question of when Creation occurred, which implied that it
> should be visible at some geologic horizon (such as the pC-C boundary), I
> note that it would not be visible at any specific horizon, because
> burial/fossilization, which would be our visible evidence of life, is
> necessarily after origin, and not necessarily at origin.
>
> >What methods of "dating Precambrian layers" do you not have implicit faith
> >in?
>
> Well, really, all of them, because when the isotope ratios indicate dates
> that do not match the accepted timeframe (such as K-Ar ratios that "date"
> older than the accepted age of the earth), alternate explanations for the
> observed ratios are explained, and I would apply those explanations to the
> "acceptable" dates as well. It seems circular to me to accept isotope
> ratios as accurate only if they fit the expected date range.
Any numerical data set will have outliers. If I have 10 assays of the
same rock
which give a value for gold of 1 ppb and 1 that has a value of 1 ppm,
the high
value is an outlier. It is that which needs to be explained, and if
there is no
valid reason for it, rejected as analytical error. Of course if the 11
assays
had values all over the place between 1 ppb and 1 ppm then I would
either have to
say that there was extreme heterogeneity in the sample, or there was
something
suspicious about the lab. It is no different with radiometric dating.
If a
geologist rejects a K-Ar age of 800 Ma on a volcanic interbedded with
Cambrian
limestones, but accepts one of 520 Ma, it is because there is a
considerable body
of evidence that suggests that 800 Ma is too old for the Cambrian. It
is not
because someone has arbitrarily decided that the Cambrian should be
between
490-540 Ma. I have worked with perhaps a hundred or so radiometric
dates
(Ar/Ar, SHRIMP U-Pb, C14) and the results were fundamentally consistent.
Outliers were clearly outliers, the results were not a smorgasbord from
which I
chose the numbers I wanted.
> >Who are you implying has this faith?
>
> I believe that most of the people on this list have faith in the accuracy
> of radiometric dating.
I can't speak for them but based on my experience, I would accept it was
well.
of course, there are always exceptions, and I have poured scorn on some
dates in
my time as well.
> Does this clary my words better for you? I don't claim to have all the
> answers by any means.
Yes it does, thank you. Of course this then generates further
questions. I
don't claim all answers either. The fact that neither of us knows all
the answers does not mean that there are no answers, and that some are
already known by someone. That is why public discussion before a broad
audience is good. As for the others is by examining God's world and
God's
word that we learn the facts and by honest and humble discussion that we
refine
our ideas.
>
>
> Romans 11:33
>
Amen
> Karen
In Christ
Jonathan
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