Re: Canadian Coal - depositional setting

From: Kevin Sharman <ksharman@pris.bc.ca>
Date: Tue Jan 27 2004 - 01:02:23 EST

----- Original Message -----
From: "Bill Payne" <bpayne15@juno.com>
To: <ksharman@pris.bc.ca>
Cc: <asa@calvin.edu>
Sent: Monday, January 26, 2004 8:12 PM
Subject: Re: Canadian Coal - depositional setting

> > If you are not proposing picking up intact thick mats of peat, but
> rather
> > amalgamating loose collections and concentrating them into much smaller
> > areas, how do you keep them concentrated in a turbulent flood? As an
> > analogy, try dumping a bag of sawdust or peat moss on a lake or sea.
> The
> > material will spread out.
>
> You're right, I hadn't thought that through. I guess the mats would have
> to be bound together with roots until they started to settle out of
> suspension. Then getting thin mats to become "hundreds of feet thick"
> (my words) is problematic, as you pointed out.

I don't think even a rootbound mat could stay together for more than about 5
minutes after being ripped off the earth by a "massive wave which overtopped
the continents" and bobbing around while the entire earth is being
rearranged. I think you're coming around, Bill! :-)

> > > I thought decomposition was retarded by submergence and the supposed
> > > reducing environment of deposition. Glenn has said that tree trunks
> are
> > > preserved for at least, I think, hundreds of years when submerged.
> If
> > > so, the surface of the lake bottom would be populated with standing
> trees
> > > for centuries.
>
> > In anoxic waters, such as at the bottom of deep lakes or the ocean,
> there
> > will be no aerobic decomposition, and organic material will be
> preserved.
> > At the top of the peat when it is submerged, there is oxygen.
> Peatification
> > proceeds until the oxygen is used up, then anaerobic decomposition
> kicks in
> > (deeper in the peat profile).
>
> You didn't address my question. I am asking how you would prevent
> partings from forming on lake bottoms with standing trees. Partings seem
> to usually be thin (from millimeters in thickness on up) and laterally
> extensive. You need to explain how you would smooth out the lake bottom
> before any partings formed. I'm saying it would take centuries for
> waterlogged tree trunks to decay, and during this time no partings formed
> since partings don't contain standing tree trunks.

Sorry, let me clarify. What I'm saying is this:

1. water table rises (clean groundwater), drowning peat and stopping
growth.

2. trees topple over because their roots rot out

3. decomposition proceeds at the submerged peat surface with limited
oxygen, breaking down and gelifying peat

4. peat surface is now smooth. Clastic influx creates parting.

5. if conditions are right, the peat swamp re-establishes itself on top
of the parting.

The above process takes time, but we have lots of that in this model.
Petrographic evidence, too.

> > > In the meantime, until all of the trees had decomposed
> > > and the lake bottom was smooth, any partings would be interrupted by
> > > standing tree trunks. Since I don't think your partings include
> standing
> > > trees, what mechanism do you offer to keep turbid water out of the
> lakes
> > > until after the lake bottom is smooth?

You don't need to keep turbid water out of the swamp. As I mentioned
before, a clastic influx into the swamp while it's still growing won't make
a parting. Bioturbation by roots will incorporate the mineral matter into
the coal.

> >
> > As mentioned above, recharge of meteoric groundwater is the clean water
> > source. Turbid, sediment laden water is brought in by storm processes
> > breaking over river levees and introducing parting material into the
> swamp.
>
> I understand where parting material comes from. But if the swamp bottom
> is covered with trees and/or grass, the vegetation will baffle the flow
> of the turbid water, restricting the extent of the parting and
> interrupting the parting with vertical stems/trunks. I see a parting as
> a snapshot in time of the depositional surface, and that surface appears
> to be free of standing vegetation. It doesn't look like a recently
> flooded swamp or a swamp that received an overbank deposit of mud.
> Partings in the Pennsylvanian can cover hundreds of square miles. Are
> the partings in the Gates different?

Gates partings aren't as consistent as that. There is a lot of lateral
variability, although there are ones I've seen that are correlatable for 10
km.

>
> This is something I didn't understand. I thought the shoreline was
> prograding as the peat was being deposited, which is what I also gathered
> from the block diagram. But you're telling me that the shoreline only
> progrades when the interseam sediments bury the swamp, and a new swamp
> then forms on top of the new sediment??? Doesn't the block diagram show
> the peat riding up over the sand as they simultaneously transgress
> seaward? How can this occur if there is no additional sediment being
> introduced along the shoreline as the peat is being deposited?

Picture the whole package prograding as a unit. At a given time, the
shoreface sand is prograding, fed by the distributary channels passing
through the swamp. The swamp "follows" the shoreface as it progrades.
Meanwhile, at the mountain front, material is being eroded, some of which
covers the swamp to form the interseam rocks.
>
> > In the Gates, the proximal deposits have been removed by recent
> > (post-orogeny) erosion. In the case of the Mist Mountain Fm. of SE BC,
> the
> > overlying Elk Formation is a series of alluvial fan conglomerates and
> > sandstones with thin discontinuous coals. These represent proximal
> > deposits. The Cadomin Fm. of NE BC is another example of a proximal
> > alluvial fan deposit.
>
> What do you propose as an energy source to transport these proximal
> deposits over the swamp? Did the shoreline and swamp submerge and
> thereby increase the gradient below the proximal deposits? Or did the
> entire area subside?

Uplift of the source area. This is a double effect, because orogenic uplift
increases the rate of erosion, and it also causes subsidence due to
isostatic loading.

> I assume that these interseam deposits are not
> channeled, but are sheets of sediment that cover the entire area of the
> swamp (20,700 km^2) and beyond?

A whole series of environments, detailed in Carmichael's abstract. You have
read it, haven't you?
>
>
> Just kidding, but one of these days I'm going to make you think and that
> may take you more than a few minutes. :-)

Believe me, I'm thinking!

Kevin
Received on Tue Jan 27 03:51:48 2004

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