Re: Canadian Coal - depositional setting

From: Kevin Sharman <ksharman@pris.bc.ca>
Date: Fri Jan 23 2004 - 23:09:57 EST

----- Original Message -----
From: "Bill Payne" <bpayne15@juno.com>
To: <ksharman@pris.bc.ca>
Cc: <asa@calvin.edu>
Sent: Thursday, January 22, 2004 10:36 PM
Subject: Re: Canadian Coal - depositional setting

Hi Bill,

> [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?

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

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.
>
> 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?

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."

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

Kalkreuth: "the shoreface sandstones of the strandplain began to subside
almost immediately after their deposition."

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.
>Will
> the rate of dewatering/compaction remain constant for 400,000 to 800,000
> years?

"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 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.

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!
>
> 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?

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."

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..."

Kevin
Received on Fri Jan 23 23:11:44 2004

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