Hi James,
On Thu, 06 Jun 2002 11:27:45 -0500 "James Mahaffy" <Mahaffy@dordt.edu>
writes:
> Bill,
>
> Let me say that you are right in observing that often times coal has
> top and bottom contract that are very flat. Although there also coals
> that will sometimes grade into a dark roof shale.
Yes, Steve Austin showed us an example of that when I hooked up with him
and his class in his thesis area in KY several years ago. And he offered
a perfectly logical explanation.
Steve envisioned a floating mat of vegetation raining organics down on
the bottom, which became a coal seam. As the water was rising, the mat
continued to move landward with the retreating shoreline, and the area
which had been under the mat and receiving organics was gradually exposed
to sedimentation from the open water. This created a transitional zone
with the organics grading upward into fine-grained sediment which became
shale. The black shale got its color from the organics which mixed with
the sediment.
What's your explanation within the swamp model for this observation?
> Let me also say that the flat and very widespread
> nature of some of the beds in the Pennsylvanian of the midcontinent are
> interesting and may not fit nicely
> into some expectations we might have from temperate US vegetation.
>
> But:
>
> 1. Is the model of rafted peat any better for explaining a sharp
> contact between coal and the clastic [sand or mud] layers?
Yes. With a transgressive facies (rising water, landward movement of the
organic raft), as the raft moves laterally over a clastic bottom, debris
from the raft would begin to settle out and collect on the clastics, with
a sharp contact. Roots, which naturally absorb water, would tend to
settle out first, followed by the stems, trunks and leaves.
> You almost seem to
> assume that in the current geological models that we are assuming that
> the forest was buried upright. There are some examples of this, but
> usually the assumption is that the swamp dies out and that most likely
> you are burying rotten vegetation.
Think about this James. If you assume that the forest dies before being
buried (in order to get rid of the vertical trunks of the trees growing
in the swamp) then you must also assume that the forest dies each time
there is an overbank deposit of a parting. [For the non-geologists here,
a parting is a mud layer found within a coal seam. The assumption is
that the parting was the result of high water from a stream flooding the
surrounding low flat areas in the swamp, and covering the organics that
later became coal.] Otherwise, the parting would be interrupted by tree
trunks on a regular basis.
I have stood in a swamp to see how far I could see in any direction
without having my line of sight blocked by a tree. The absolute limit
was 300 feet. If modern swamps are in any way similar to ancient ones,
then partings should contain many, many tree stumps. For example, a
strip-mine highwall, or an underground-mine longwall of 300 feet in
length should contain AT LEAST one and likely several vertical tree
trunks in any parting. How many tree trunks have you seen standing in
the Blue Band? And what is the areal extent of the Blue Band [this is a
parting in the Herren Coal of Illinois]? A little more than 300 feet,
isn't it?
Furthermore, even if the swamp did die just before the parting was
formed, the top contact with the overlying coal would become gradational
as the swamp trees re-established and began penetrating the substrate
(parting). Bioturbation would destroy any planar structure.
> To check this out you should compare
> coal is an old lobe of the Mississippi delta or a modern swamp that is
> producing peat and would not in your model be produced by the flood.
> If the lithology is quite different you can build a better case. I
> don't think it is but it might but it would be worth checking out.
I'm not sure what you are saying I should look for here. I can tell you
that there are no planes in a modern swamp, and nothing is laterally
continuous for more than a few feet. I.e., things are totally different
in a swamp than the features we observe in coal.
> 2. You simply must show that you have read Cohen, Fisk and some of the
> others who have studied modern peat development. It makes your case
> much stronger to be aware of the literature.
I read GSA Special Paper 286 (Modern and Ancient Coal-Forming
Environments) from cover to cover. The first 7 papers present research
"on modern tropical peat and associated sedimentary environments in
Indonesia........The second objective of this volume....is to address the
origin of coal and coal-bearing strata." (p 1)
In the first papers, the contact between peat and the mineral substrate
is DEFINED as the layer where ash goes from >25% (substrate) to <25%
(peat)! That is to say, the contact between peat and mineral substrate
is completely gradational. And this gradational contact will not be made
sharp by compression of 3 to 10x (as peat becomes coal).
Bill
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