Re: Canadian Coal - depositional setting

From: Bill Payne <bpayne15@juno.com>
Date: Sun Jan 25 2004 - 10:24:47 EST

Good evening, Kevin.

On Fri, 23 Jan 2004 21:09:57 -0700 "Kevin Sharman" <ksharman@pris.bc.ca>
writes:

> No.
> 80 m=80,000 mm
> 1 mm/year X 80,000 mm=80,000 years
> 2 mm/year X 80,000 mm=160,000 years

Don't ask me what I was thinking, but thank you for correcting my errors
and, in another post, yours as well!

> I hasten to point out that you would need similar time periods of
subsidence
> at those rates of peat growth to get your pre-flood vegetation mat
"hundreds
> of feet thick" (your words), and a veg mat hundreds of feet thick would
only
> build one thick seam.

I don't think so. If most of the surface of the land masses we have
today were thickly vegetated swamps and accumulated some lesser
thicknesses of peat, and then if that peat and swamps were
catastrophically uprooted to become floating mats, there would be enough
peat to form all of the coals that became part of the geologic column.
In this model long time periods are not required, rather large land areas
produce vast areas of peat which are later concentrated into much smaller
areas as they settle out of water, yielding thick coal seams.

I know this scenario is very hypothetical and no, I don't have any hard
numbers to apply, but I offer this sketch as the basis of the model I am
attempting to flesh out. In the YEC model the pre-Flood lands are
destroyed, so there is no way to verify what that surface looked like.

> Kalkreuth et al (1989): "Compaction of the muds was rapid during the
> initial stages of burial, there typically being a porosity reduction of
15%
> to 17% in the first 200 m (Hamilton, 1976). Compaction squeezed out
> interstitial water and the increased temperature and confining
pressure
> also released chemically bound water from clays and other hydrated
minerals.
> The expelled waters flowed updip and landwards and may have contributed
to
> recharging the strandplain water table (Galloway and Hobday, 1983).
The
> subsidence was largely a result of compaction of the underlying shales
by
> dewatering and clay particle rearrangement with a lesser tectonic
contribution."

Aren't these marine waters being expelled by compaction, and aren't the
coals low sulfur?

> So, the shales would end up with ~35% porosity, and less as they get
buried deeper.

> Now (after compaction and lithification), 100 to 200 meters. The
overlying
> shoreface sandstones are about 30 to 50 m thick, and would have
compacted
> also, although not as much, as would all the underlying sediments.

What is "100 to 200 meters"? The thickness of the zone of compaction
["there typically being a porosity reduction of 15% to 17% in the first
200 m (Hamilton, 1976)"] or the amount of subsidence due to compaction ["
Now (after compaction and lithification), 100 to 200 meters."].

> "It is critical to the model that the rate of subsidence matches the
rate of
> peat accumulation. If the rate of upward growth of the peat swamp is
> matched by subsidence due to shale compaction, then substantial
thicknesses
> of peat can accumulate. If the rate of subsidence exceeds the rate of
peat
> formation then the area will be submerged and a large, probably shallow
lake
> will form instead of a peat forming swamp."

I agree that rate of subsidence is absolutely critical in the swamp
model, and you have said that the rate is not constant: "Compaction of
the muds was rapid during the initial stages of burial, there typically
being a porosity reduction of 15% to 17% in the first 200 m...." I
hesitate to stick my neck out again with more math, but would 17% of 200
meters result in 200 m x 0.17 = 34 meters of vertical compaction
(subsidence)? If so, where do you get the other 80 m - 34 m = 46 meters
of subsidence to accommodate 80 meters of peat? If I may offer a
suggestion, I would expect the peat to be compressing as it accumulated,
so that would help your case some, but I don't know how much.

Also, if "Compaction of the muds was rapid during the initial stages of
burial," how long did the rapid compaction last, and was it too rapid for
a swamp to form? If it was not too rapid, then what happened when the
rate of compaction/subsidence slowed? Was peat now unable to accumulate
due to lack of subsidence? What is the time frame for 15 to 17%
compaction? It appears that you do not have enough space to accommodate
80 meters of peat, and you do not have a constant rate of subsidence that
would match the rate of peat accumulation for 40,000 to 80,000 years (at
the rate of peat accumulation of 1 to 2 mm/year).

> I would add that when a lake forms would be when a parting can be
> introduced, after peat growth has stopped temporarily due to
submergence and
> decomposition is underway.

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 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?

> Variations in the subsidence rate cause water table fluctuations. This
is
> thought to be a reason for the petrographic differences found in coal
> (vitrinite vs. inertinite). Coal petrology will be the subject of a
future
> post - maybe that's an incentive for you to catch up!

Right now I'm just trying to keep you from cutting my head off; I can't
even think about catching up. :-) Keep up the good work!

> Yes, there are river channels, but not many. Kalkreuth again: "The
wave
> dominated delta had only one or two active distributaries (Coleman,
1981)
> and consequently large areas were removed from active
> sedimentation....Sediment brought to the shoreline by the
distributaries was
> reworked and redistributed by wave action and longshore drift."

If the shoreface sand was 230 km long, where along this length were the
deltas? If at least one distributary was not at the upstream end of the
longshore drift, and if the longshore drift did not periodically reverse
itself, then what would have been the source for the sand upstream of the
distributaries? Do you have any photos of a river channel cutting the
coal? In plan view, were they generally straight or meandering? What
were the sediment sizes and percentage of each size of the channel
deposits?

> Instead of saying "Laterally continuous, extensive sheets of coal up to
12
> meters thick, sitting directly on littoral sandstones.." I suppose
Kalkreuth
> should have said "Laterally continuous (interrupted only by one or two
river
> systems supplying sediment to the ocean), extensive sheets of coal up
to 12
> meters thick, sitting directly on littoral sandstones..."

Transportation of the large volumes of sediment past the swamp is a
critical part of his model. What was the length of the source area? I'm
not sure of the orientation of the prograding shoreline, but let's say it
was 230 km north-south and prograding westward. Was the source area also
230 km north-south? What was the width of the source area, and the
approximate square km? If the source area was >230 km north to south
along the boundary with the swamp, how do you keep the sediments washing
down from the mountains in the source area from prograding out over the
swamp? Instead they seem to be funneled into one or two channels across
the swamp to the shoreline.

Bill

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Received on Sun Jan 25 10:33:36 2004

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