From: Jonathan Clarke <jdac@alphalink.com.au>
> Part of a bioherm where they can see soliatry and colonial rugose corals,
> tabulate corals, stromatoporoids, and assorted hash in a mud matrix.
> I as them to consider in what sort of environment such an assemblage
> is likely to form.
>
> A laterally equivalent succesion of bedded wackestones and packstones
> with a similar biota. I again ask them consider what sort of environment
> these might have formed in and ask them to say why it might be different
> to the previous outcrop.
>
> A siltstone succession interbedded with the limestones. I ask them to
> collect the fossils (brachipods, bivalves, gastropids, and crinoids) and
> then to suggest why the environment is different from the two limestone
> localities.
I requested from Johnathan further clarification of the relationships of
these three formations.
>The lower limestone is about 150 m thick, the dividing siltstone is about
> 50 m thick and continuous, and the upper limestone is up to 700 m thick.
> The upper limestone contains scattered bioherms, the largest probably
> 100 of so m long and 20-30 m thick. These thicknesses are from memory,
> but should be of the right order. The bioherms are lenticular
(plano-convex
> with the convex surface uppermost) and often standing as modern
> topographic highs. The sedimentology of the sequence has not been
> studied in any detail, although the palaeontology has, because of the
> diverse fauna (corals, brachipods, molluscs, conodonts and fish,
especially
> placoderms and lungfish). The whole succession overlies Silurian
> volcanics and is unconformably overlain by a late Devonian redbed
> sequence, also famous for its non-marine fish faunas. I can get you more
> precise references if you are interested.
It sound like the sedimentology of the sequence really does need to be done.
It should seem obvious that the Bioherms, wackestones and packstones were
high energy deposits, which would not be much of a problem from a
catstrophic viewpoint. Most gradualists would consider the siltstone to be
very slow deposition. The Catstrophist would look to a turbidite or tsunami
for it's deposition source. Tsunami depositions often have biological
remnants scattered through out the deposit. Turbidites flow below water
because of their great density. It is that high density that allows for
rapid deposition of whatever size sediment the flow is carrying.
> ... getting them to see that rocks
> are not ink blots into which we see what we will, but natural documents
> which can be read.
They are natural documents that can be read only if you have an interpretive
paradigm. The paradigm determines what will be understood.
> They also should get a sense of the vastness of geological time
Not hardly. Only if they have been brainwashed with gradualism.
> Two years ago there was one lady in tears because of the
> way in which her world view had been challenged by the discovery of the
> reality deep. Her faith survived, whether by denying geology or by
> changing her world picture, I do not know.
I cannot see what she would be in tears about, but for possibily not being
told that deposition of siltstone and shale need not take a long time as
shown by turbiditeds and tsunami.
(I am going to have to refrain from responding any further to this, not
because I would not want to, nor because I cannot, but simply because I only
have so much time and there are some more important things I need to get
done. I'll go back into Lurking mode for a while again. See you. It's
been fun.)
This archive was generated by hypermail 2b29 : Wed Feb 13 2002 - 15:29:54 EST