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evolution-digest Tuesday, February 9 1999 Volume 01 : Number 1290
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Date: Tue, 09 Feb 1999 20:45:43 +1100
From: Jonathan Clarke <jdac@alphalink.com.au>
Subject: Re: Early Cambrian explosion
Welcome back Bill
Bill Payne wrote (in part):
> You mentioned root casts above, and I assume you would say the root zone
> below a coal seam is a paleosol. I now realize that there are roots
> below most coal seams, and I believe that these roots grew in situ.
> However, the roots are poorly developed and the root zone is generally
> only a few inches deep. The paper by Bob Gastaldo, which I critiqued
> last year at your request, persausively argues that the Carboniferous
> trees were massive (up to maybe 30 meters tall) and required long
> stigmarian axial root systems for support. Bob failed to note that we
> don't observe these long root systems beneath coal seams.
>
> Where we do see long root systems, I believe that they may have been
> eroded from their original growth site, rafted by the flood to another
> location, and reburied in growth position, similar to the numerous
> vertical fossil tree trunks we observe (without roots).
>
> The shallow roots that we do observe beneath coal seams developed after
> the still living vegetation settled out of suspension. When it touched
> firm sediment, it began to root in an attempt to gain a foothold and
> begin a new forest. The fact that these roots are so shallow is evidence
> that the vegetation quickly died as it was buried by additional sediments
> in the flood. Therefore, we have transported (allochthonous) coal with
> in situ (autohthonous) roots.
>
> Comments?
>
I am glad that you have continued to reflect on these questions. It is great
that you recognise the evidence for roots beneath at least some coal
deposits.
Your proposed allochthonous model is to my mind very unlikely. It might be
just possible in rare instances, but to expect your model to explain all
rootlet horizons beneath all coal horizons is taking special pleading to an
extreme. It fails completely to explain laterally extensive rootlet horizons
beneath paleosols where there is no evidence what so ever for floating mats
of vegetation settling on the dunes, or even for them being submerged.
Occam's razor is an important component even in geology.
In Christ
Jonathan
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Date: Tue, 09 Feb 1999 20:46:06 +1100
From: Jonathan Clarke <jdac@alphalink.com.au>
Subject: Re: Flood Model [was Early Cambrian explosion]
Dear Karen
In your reply to Steve you made the following two statements. Various people
have responded to some others, I will just address these two.
Karen G. Jensen wrote:
> In many such reef structures the fossils are not really organically bound,
> but are suspended in the matrix. Capitan Reef, for example, has been
> recognized as a gigantic debris flow. Some reefs that show organic
> structure are oriented upside down. They were probably transported from
> where they grew -- which would suggest high-energy water movement. The
> reef problem is very interesting.
The reefs structures I am most familiar with are the Cambrian reefs of the
Wilkawillina, Moorowie, and Ajax Limestones of South Australia. Details of
these
have been written up in the literature y several authors, including myself.
These vary in size from about 1m in height to at least 180m. In many cases they
are capped by karst. Later reefs have then drown on the karst surface. the
relief on these surfaces can be up to 10m. the reef frameworks are constructed
by calcified microbes such as Renalcis and Epiphtyon. These are often (though
not always) growing on a substrate of Archaeocyaths. Internal cavities are
common, filled by both geopetal sediments and multiple rinds of fibrous cement.
The reefs are typically surrounded by proximal debris of reef material. They
are
not transported, but in place. However in several localities you can walk the
reef facies out into deeper water debris flows, black shales, and turbidites,
which thin away from the reefs. If you walk the other way you encounter rubbly
back reef lithologies, oncolitic (algal ball) limestones, and other indicators
of
very shallow water environments.
The other build ups I have seen are in the Devonian of Victoria. These are not
as well exposed, but their internal features and relationship to surrounding
sediments give every indication to them being in situ.
Who said that the Capitan reef is a debris flow? Has this been published? If
my
memory serves me right there are debris flow into the fore reef environment
there, and there may well be some into the back reef, of palaeo relief was
sufficient, but you need to be more specific. You are discussing a very large
and complex sequence of rocks in this statement!
> How do you get halite layers on the surface? They are usually not pure
> halite, and not very thick. You would have to evaporate many feet of sea
> water to get layers like we find in the column, and it wouldn't be pure
> halite -- or pure gypsum.
Thin halite layers are very common in salt lakes. In most it redissolves when
the
lakes flood, but not always. Lake McLeod has several m of permanant halite,
deposited by sewater seepage from the Indian Ocean. Halite is also forming at
the bottom of the Dead Sea. Thicker deposits are more difficult to get, but to
occur today. There is over 900 m of very pure halite in Salar de Atacama in
northern Chile. The halite is still forming, albeit slowly. Certainly there is
nothing like the saline giants being deposited today, but the mechanisms are
there and understood, it is just the scale that is different.
>
> First I have to ask you how a mile of turbidite deposits with
> graywake/shale packages
> could form slowly? If these are anything like the Great Valley Sequence
> (in NW California, which I studied during PhD research) they show little
> evidence of time between turbidites -- so little that there is hardly any
> bioturbation (but a few examples, showing that organisms were alive in the
> area, and some fossilized, during the deposition), and no signs of erosion
> between layers. To me, this (and other observations) speaks of rapid
> deposition of the turbidic sequence, which was later tilted (in the case of
> the GVS the tilting was part of seafloor accretion to W. North America).
> How much later? I don't know. Long enough to dewater and set the
> limestone well enough to keep its integrity (hours, days?) , but not so
> long that the layers were indurated so much that they would shatter upon
> tilting or folding (i.e., not months or years). There are some places with
> folded strata.
>
Have you published your research yet? It would be an interesting read.
Individual turbidites are of course deposited rapidly, but the lime between
depositional events is highly variable. If there are long periods between
individual events one would expect things like burrowing (providing the bottom
or
pore waters were not anoxic), erosion surfaces, even hardgrounds. Certainly the
turbidite successions I have seen contain these features. They can be very
subtle
though, and However you can't use the fact that the limestone beds were folded
and not brecciated as evidence of rapid deposition. All this is evidence of is
lack of induration as your correctly point out. Deep water limestones,
especially fine-grained ones, lithify very slowly. We know this from deep sea
cores. Of course, even indurated limestones can fold very nicely if the
conditions are right.
In Christ
Jonathan
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End of evolution-digest V1 #1290
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