Re: Canadian Coal - depositional setting

From: Bill Payne <bpayne15@juno.com>
Date: Fri Jan 23 2004 - 00:36:48 EST

On Mon, 19 Jan 2004 22:16:06 -0700 "Kevin Sharman" <ksharman@pris.bc.ca>
writes:

> Instead of trying to trip me,

Easy, Bill. Asking you to substantiate your position is not trying to
trip
you, in my opinion. If you feel I'm being too harsh, please let me know.

And in another post:

> You jump me for wanting to look at coal instead of the associated
rocks.

No Bill, I said (Jan. 16th): "Coal does not occur in isolation. One
must
look at the coal itself AND the context of it." Bill, I think you are
getting a tad defensive.

My apologies, Kevin, please forgive me. I was tired when writing and am
frustrated because it takes me so long to find time to respond. I need
to print out all of these posts and absorb what has been said, and of
course, I need to do more reading. You're doing a great job holding me
on track and forcing me to think about things that would normally not
occur to me, or as Scripture says: "Iron sharpens iron."

Given my work load, I probably won't do much better in the future, so let
me say I appreciate your bearing with me.

Now Kevin, let me ask a few quick question from your post of 1/19/04
under this same thread. I don't mean to evade the rest of your post but
if I wait until I can answer the complete post it'll be a while. I think
this will move along a little faster if I can rifle in with specific
questions/comments. I'm not trying to make work for you, but you seem to
be cross-checking these posts anyway so I doubt that much will be lost.
This is like a game of chess, except that we can't see all of the pieces.

[I wrote] > 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.
 
[KS wrote] In a word, subsidence. (80 feet above should read 80 METERS
to get an 8
meter seam with 10X compaction) Kalkreuth et al (1989) has a model: "Coal
seams formed on Lower Cretaceous wave-dominated strandplain sediments in
Western Canada are characterized by great lateral continuity, substantial
thicknesses, and relatively low ash and sulphur contents. The coals
formed
behind an active shoreline in areas undergoing subsidence due to shale
compaction and dewatering." and "The shoreface sandstones of the
strandplain began to subside almost immediately after their deposition
and,
as a result, it is only the immediate coastal sands which are at sea
level.
The marine muds in front of the wave-dominated delta were initially
deposited with high porosity, typically exceeding 50%."

I understand subsidence, but to work this subsidence would have to match
the accumulation of peat or the swamp would drown or dry out (unless
there was heavy rainfall year around). So you would need 80 meters of
subsidence at the rate of 1 to 2 mm of peat accumulation per year for
400,000 to 800,000 years to get an 8-meter seam of coal. Agreed?

What is the compaction ratio when you dewater a meter of marine mud with
50% porosity? Do you know the porosity of the shale formed from the mud?

What is the rate of dewatering of the marine mud when loaded with a
porous sand? What is the thickness of marine mud below the sand? Will
the rate of dewatering/compaction remain constant for 400,000 to 800,000
years?

At some point in the past you wrote: "Some of the coals in the lower
Gates were deposited in swamps which extended inland for at least 75 km
from the shoreline," and "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)."

If these seams in the lower Gates are continuous for 20,700 km^2, then
how did the sand and mud get from the inland, erosional source areas
across the swamps to the ocean where they were sorted and deposited? Are
there river-channel deposits that cut through the coal seams? Or do I
misunderstand the model?

Take care,

Bill

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Received on Fri Jan 23 00:48:30 2004

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