Re: Canadian Coal - depositional setting

From: Bill Payne <bpayne15@juno.com>
Date: Mon Jan 19 2004 - 00:41:12 EST

On Fri, 16 Jan 2004 16:20:30 -0700 "Kevin Sharman" <ksharman@pris.bc.ca>
writes:

> > Do any of the tonsteins or bentonites cut through the coal?

> No, Kilby commented in another paper that following the tonsteins
> landward was hard because the fresh water depositional environment
reworked
> them (sorry, no reference at hand). There are no tonsteins that I
know
> of directly in the Gates seams.

Well Kevin, you say [above] that "There are no tonsteins that I know of
directly in the Gates seams", but you have said that there are other
partings, presumably in the Gates seams. Tonsteins (thin layers of
volcanic-ash) are a special class of partings, or layered impurities, in
this case within a coal seam. On Dec 30, 2003, you wrote:

{KS replies} As I've said before, thin widespread partings are hard to
explain with either model. I have pointed out that the top surface of a
floating mat deposited peat would not be smooth. The top surface of the
parting in your model would have soft sediment deformation features from
the next layer of veg material landing on it, wouldn't it? As far as I
know, we don’t see these in partings. Or are you contending that the
parting was lithified prior to depositing the next layer of veg material?

I would say that the top surface of a floating-mat-deposited peat would
be relatively smooth, at least smooth enough to allow a layer of clastics
or dust to settle out of water and form a blanket of impurities over the
peat deposit. With subsequent 10x + compaction as the peat is coalified,
the surface would ultimately be much flatter than it was when the parting
material settled over the peat. As to the lack of soft-sediment
deformation features, this is indirect evidence for a smooth depositional
surface on the peat - which fits the floating-mat model but works against
the swamp model with it's standing trees, shrubs and grasses.

Now, if the Gates coals were formed in a fresh-water swamp depositional
environment, and if the tonsteins were reworked and therefore not
preserved, how is it that you do have partings other than tonsteins
present in these coals? Why were not all of the partings reworked and
therefore not preserved?

> Bill, progradation is regression, not transgression. There is a
> relative lowering of sea level as you go upsection. I would give you
the
> benefit of the doubt and call your comment a typo, but I see you've
repeated it
> below.

You're very diplomatic, but it was no typo. I should have looked up
progradation (like I did diachronous), but I didn't think about it. I
did begin to wonder, while driving home from work Friday afternoon,
whether we were talking about trans- or regression of the shoreline. I
appreciate your patience in bringing me along - you are much better
grounded (no pun intended) in these topics than I am. If I had my
druthers and was independently wealthy I would quit my job and travel the
world studying coal and geology in general. Maybe our oilman friend
Glenn would fund me? :-)

> > > Any floating mat hypothesis would have to explain how vegetation
> > > could be deposited on a sand body that is prograding northwards
over time.
> > > At any given time, the shoreface sand existed as the top layer over
only a
> > > small part of the area, yet coal occurs directly overlying the
shoreface
> > > sand over its extent (230 km X 90 km = 20,700 km^2). The rate of
progradation
> > > was estimated by Leckie (1986) as 218 m to 437 m per 1000 years.

In the floating-mat model the rate of progradation, along with every
other time estimate, is up for grabs. I will need to understand more
about the strata before I attempt to answer your objection above. But in
a prograding environment where the land is emerging as the shoreline
retreats, the swamp model will have to explain how you can accumulate and
preserve 80 feet of peat above the base level of erosion.

Also, when the tree roots can no longer draw nutrients from the mineral
substrate (due to the thickness of the peat), the growth of vegetation is
stunted. How big were the trees at the top of the coal?

> I contend that this would not happen in a high energy environment like
a
> beach sand. Roots would be expected to be oriented ~horizontally. So
the
> beach sand model has predictive power based on observed modern
examples. The
> branching upward of some roots, I thought you backed off on this line
of argument on Jan. 2: (snip)
> "Therefore, I'll back off my statement that roots don't branch upward -
they usually
> don't. I suppose you and Glenn win that one. :-)"

 I said they _usually_ don't branch upward, but I think I also said that
the upward branching does look odd. The thin radiating roots could have
been bent out of their original orientation in the process of getting
re-buried, so I think that the branching upward feature may still be a
problem for you, although I won't be adamant that it is.

> This is explainable as a layer of finer material which acted as a
> micro-perched water table, so the roots didn't need to grow any
> further down. Seizing on this when dozens to hundreds of roots around
it
> don't terminate at a common plane sounds like selective use of data.

I didn't notice a common plane for them to terminate in. If
sedimentation is relatively continuous and roots are being brought in
with the sand, then there wouldn't be a planar surface for the roots to
settle on, they would be continuously be incorporated into the sand,
i.e., they would overlap rather than having a common plane of
termination. If the plane of termination was the bottom of a
micro-perched water table, then the top of the micro-pearched would have
been above the bottom by the thickness of the perched water. Therefore,
why did the roots go all the way to the bottom of the perched zone, why
didn't they stop in the saturated zone above the aquitard?

> >and the odd, isolated tree root (if it's in situ, where are the
> > rest of the roots of the supposed root ball)

> We discussed this as being due to roots moving in and out of the
> plane of the rock face.

Just because "we discussed this" doesn't mean I accept what you said. If
we take the root ball of a tree and slice down through it, the slice
should cut more than one root longitudinally, and even if it didn't we
should see the cross-section of many more roots that were cut. I think
your best option here is to agree that this root floated in and was
buried vertically. You could agree that the tree root floated in and
still maintain that the other roots are in situ.

> I disagree. I contend that your vertically settling roots would not
happen
> in a high energy environment like a beach sand. Roots would be
expected to
> be oriented ~horizontally, not like those in the photos. You have
already
> said that the roots in the photos look identical to modern roots.

You seem to be hanging your hat on the vertical roots. For the sake of
argument I'll admit that I don't have a ready answer to explain vertical
roots in a high-energy sand environment. But, given the things that you
don't have a ready answer for, I'm not ready to concede this point.

> There is a 50 meter thick sandstone with burrows at most levels. You
tell
> me - can these animals burrow 50 meters upward in two days?

That would be 25 meters/day, or about 1 meter/hour, or less than 2
cm/minute. What's the problem?

> You answered it above - some areas below the seam have lots of roots,
some
> fewer. Carmichael's judgement was that there were strongly bioturbated
> zones. We can't argue with him, since we've mined out most of what
> he saw!

Carmichael's judgement was influenced by what he believed, and he
obviously believed that these deposits were in situ, hence his use of the
term "bioturbation". And yes we can argue with his interpretation of
bioturbation.

> > Are you proposing that after a single stand of shrubs, there was
enough
> > vegetation collected on the surface to prevent penetration of roots
into
> > the sand substrate?

> Yes.

Yes? You show us a photo of roots radiating from a shrub, which appear
to reach about 10 to 12 inches into the substrate. I find it difficult
to believe that a single stand of shrubs will provide a continuous mat of
peat at least a foot thick. Modern swamps accumulate only about 1 mm of
peat per year, yet you are proposing to accumulate 300 mm of peat in just
a few years. And you must accumulate a meter of shrub peat before the
trees begin to grow and sink their roots down as much as 1 meter.

> Back in March 1999 you and Jonathan Clarke discussed turbidites on
> ASA. See: http://www.calvin.edu/archive/asa/199903/0174.html in which
he defines the
> Bouma sequence (diagnostic criteria for turbidites), which, from the
post,
> you seemed to be unaware of at the time. This lack of knowledge of
criteria
> for turbidites did not prevent you from proposing them as a mechanism
for
> partings, however: (snip) Mar. 30, 1999) "I do see impurities in
coals. Because
> they are generally thin and widespread, I think they are the
> result of turbidity currents rather than traction currents."
>
> That was then and this is now, though. Do you see Bouma sequences
> (description) in the partings you are currently ascribing to turbidity
> currents (interpretation)? I asked you what mechanism you proposed
> to deposit the interseam sediment underneath a floating mat, and you
> have not had time to respond yet. Fair enough, but any response MUST
satisfy
> the descriptive data!

Turbidites can only transport the material available to them, and I still
say that turbidites can deposit interseam sediment beneath a floating
mat. If the available material is of a single size, then the turbidite
deposit will only contain material of that single size. My "lack of
knowledge of criteria for turbidites" does not invalidate the mechanism
as a possible explanation for the feature. Instead of trying to trip me,
let's see you explain partings within your swamp (with standing trees) or
marsh (with standing shrubs and grasses) models. Remember, any vertical
obstruction such as trees or shrubs, or even grasses, will act as a
baffle and rapidly slow the flow of turbid water. And also remember, in
a freshwater environment "that following the tonsteins landward was hard
because the fresh water depositional environment reworked them." That
same freshwater environment will also rework your partings.

Bill

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Received on Mon Jan 19 00:48:06 2004

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