Re: Canadian Coal - depositional setting

From: Kevin Sharman <ksharman@pris.bc.ca>
Date: Sun Jan 25 2004 - 14:33:13 EST

----- Original Message -----
From: "Bill Payne" <bpayne15@juno.com>
To: <ksharman@pris.bc.ca>
Cc: <asa@calvin.edu>
Sent: Sunday, January 25, 2004 8:24 AM
Subject: Re: Canadian Coal - depositional setting

> > 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.

As you say, this is speculative. Speculation is a good start, but by itself
can't form the basis of a scientific argument. My understanding is that the
pre-flood world described in the Bible is rather dry, and "rather large land
areas producing vast areas of peat" are not mentioned, to my knowledge.

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.
>
> > 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?

Good point. Kalkreuth: "..strandplains have a shallow water table and
contain one of the most highly transmissive and laterally uniform
aquifers...Porosities through the nonindurated shoreface sands are typically
30% to 50%..The climate during deposition of Gates sediments was generally
humid to sub-humid, providing an ample meteoric source for groundwater
discharge. Discharge of meteoric groundwater dominates water table
circulation and a meteoric recharged groundwater system can maintain a very
large area having a shallow to emergent watr table that is ideal for plant
growth and peat preservation..The groundwater rises or maintains its
position as compaction of the peat mat takes place and the strandplain
subsides."

So, there is a contribution from expulsion of marine waters by compaction,
but the groundwater regime is dominated by meteoric (fresh) water, diluting
the sulphate content and allowing low ash coal to form.
>
> 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."].

The shales as we see them today are 100 to 200 meters thick.
>
> 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.

I explained (Jan. 23)"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." Kalkreuth explained that shale compaction was
dominant, but there was also tectonic subsidence. Compaction of the peat
happens too; thanks Bill.
>
> 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?
Yes, the peat mire would dry out. There is petrographic evidence in the
coal for repeated episodes of this.

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).

The rate does not need to be constant, it needs to be between the upper rate
(above which a lake would form) and the lower rate (below which the peat
emerges and growth stops).

> 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).

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?

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.
>
> 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?
There would be a series a few of distributaries feeding sediment, which was
redistributed long distances by the longshore drift (to the next
distributary).

Do you have any photos of a river channel cutting the
> coal?
In the intensively explored and mined area (20 km X 10 km), there are no
river channels cutting the basal coal. There is an area I can think of
where the basal coal thins to ~1 m, but I can't say if this is because it's
cut out by a channel. Outside of this, drillhole control is more sparse.

In plan view, were they generally straight or meandering? What
> were the sediment sizes and percentage of each size of the channel
> deposits?

See the Carmichael abstract. "The channels are mainly braided river types
with anastomosing or straight (non-braided) and meandering river channels
also present." See also the block diagram of the Gates which I posted a
link to. The core photos are most instructive.
>
> 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.

See the block diagram I posted a link to. It's a cartoon, but shows that
there was an elevated source area, then a broad (75 km wide) flat coastal
plain. The source area was many hundreds of km long (the rising Rocky
Mountains), and hundreds of km wide. The proximal deposits of the material
eroded from the source area were laid down at the foot of the mountains,
uphill from the coastal plain, because of the reduction of gradient. The
sediment that did make it over the swamps is represented by the interseam
strata.

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.

How's that for quick turnaround? :-)

Kevin
Received on Sun Jan 25 14:33:58 2004

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