Re: Canadian Coal - depositional setting

From: Kevin Sharman <ksharman@pris.bc.ca>
Date: Wed Jan 28 2004 - 19:47:36 EST

----- Original Message -----
From: "Bill Payne" <bpayne15@juno.com>
To: <ksharman@pris.bc.ca>
Cc: <asa@calvin.edu>
Sent: Tuesday, January 27, 2004 10:51 PM
Subject: Re: Canadian Coal - depositional setting

> On Mon, 26 Jan 2004 23:02:23 -0700 "Kevin Sharman" <ksharman@pris.bc.ca>
> writes:
>
> > 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.
>
> > 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.
>
> Two things:
> 1) You've got the clastic influx down at step 4, after the peat surface
> is smooth. What happens if a clastic influx hits the swamp after step 1
> and before step 2? Tree trunks should be preserved in the parting. The
> roots are dead so there will be no bioturbation from roots.

If parting mud is introduced into the swamp after it is flooded, but before
the trees fall over, then the trees will fall over eventually (their roots
will rot out), and mud will be mixed with the peat as the peat and trees
decompose. No parting, but a high ash zone in the coal, or discontinuous
lenses of mudstone in the coal (quite common in these coals).

> I would
> expect bioturbation by critters unless this is an oxygen-deficient
> environment. On 1/25/04 you said: "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)." So are these lake bottoms oxygen deficient, and if so, why?

Yes, because decomposition of the peat uses oxygen up, and stagnant water
doesn't get new oxygen input. The pH of swamps is too low for much in the
way of bioturbating animals.

> Are modern drowned swamps oxygen deficient?

Yes, bivalve has responded well to this in his post.

>
> 2) In step 5, if the peat swamp doesn't reestablish, then the parting
> isn't a parting, it becomes part of the roof (rock above the coal seam).
> If the swamp does reestablish, I assume we are past the shrub phase and
> trees will resume growing. And bioturbation (I think I'm re-re-repeating
> myself, but who am I to mention such a thing :-)) from the roots will
> destroy the parting. There, at last - CHECK! :-)

No, the trees will not resume growing, because they died when the swamp was
flooded. Pioneering vegetation must take over again. If the parting
material IS bioturbated by roots, so what? It will get mixed into the peat,
and a parting won't form. There will be a high ash zone in the resulting
coal. So parting formation is not a sure thing when you get a clastic
influx.
>
> > 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.
>
> Kevin, you knew I wouldn't blow by that. You say the sand is "fed by the
> distributary channels passing through the swamp." On 1/25/04 you said:
> "There would be a series a few of distributaries feeding sediment, which
> was redistributed long distances by the longshore drift (to the next
> distributary).
>
> I asked: "Do you have any photos of a river channel cutting the coal?
>
> You responded: "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."
>
> You admit that "there are no river channels cutting the basal coal" in
> the area most likely to see them - "the intensively explored and mined
> area." You mention an area of thin coal, but you don't know if it's a
> channel cut. Then you say "Outside of this, drillhole control is more
> sparse", which is another way of saying you have no observational
> evidence of channels in the areas where data is limited, and you have no
> observational evidence of channels in the areas where data is intensive.
> Kevin, you and Carmichael believe in distributaries because you
> intuitively know you must have them to prograde the shoreline, but you
> have never seen with your eyes what you say must be there for your model
> to work. I think this is called blind faith. In fact, what you are
> doing is more blatant - you are propounding channels feeding
> distributaries when the observations say there are none.
>
> You also said: "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.'" Excuse me, but WHAT channels? I sorry, I didn't mean to
> yell. But Carmichael is referring to channels that aren't there. From
> what I understand, there may be channels in the interseam sediments (big
> deal), but there are none in the swamp deposits, i.e., there are no
> intraseam channels. And without intraseam channels to transport sand
> across the swamp, you cannot prograde the shoreline while depositing
> peat.

Leckie (1986): "Northward bulges in the shorelines, interpreted as deltaic
lobes, occur..the deltaic lobes represent accumulations of sediment at the
shoreline fed by these rivers. The shape of these lobes suggests a series
of arcuate-type deltas..separate, stacked fluvial channel deposits are
present..the superposed lobes and fluvial deposits indicate that a major
fluvial depositional axis maintained its position during deposition of most
of the Fahler (equivalent to Gates - KS). .There is no evidence of major
lobe switching...Two major depocenters existed, one in the Bullmoose
Mountain-Mt. Spieker area, and the other west of the Alberta - British
Columbia border."

So, he's saying that there were only two major river systems feeding the
prograding shoreface sand. Coal swamps formed between and beside them.

As for the river channels themselves, we turn again to Carmichael:

"In the Five Cabin Creek area, type 1 channel deposits occur above nearshore
marine deposits of the Sheriff Member (the shoreface sand - KS)...type 1
channel deposits consist of fine- to coarse-grained pebbly sandstones and
conglomerates..the conglomerates are massive and clast supported, with a
sandstone matrix and are poorly sorted. Pebbles in the conglomerates are
rounded and 0.5 cm to 2 cm in diameter..some of the sandstones contain
mudstone pebbles and rip-up clasts. Coal spar is fairly common."

In other words, a fluvial channel, which has cut into the peat swamp at some
point as evidenced by coal spar. The basal coal is missing; the sandstones
and conglomerates rest directly on the shoreface sandstone. He notes rapid
lateral facies changes, characteristic of a fluvial environment.

The interseam sediments (above the basal coal) containing the channels
Carmichael refers to in his abstract represent the landward portions of a
series of prograding clastic wedges. Basinward, these have a shoreface
sand. Coal develops on top of these, just like it did on the shoreface
sand, and a few channels are preserved, just like the above example, to feed
the sediment to the ocean.

Now are you wishing you hadn't had your little outburst?

>
> Maybe I completely misunderstand the model, but hopefully you can see
> where I am getting hung up and help me past these points. Ummm, I feel
> better; not so dizzy anymore.
>
> > A whole series of environments, detailed in Carmichael's abstract. You
> have read it, haven't you?
>
> I guess you could do anything with "a whole series of environments."
> I'll get back to that later. We also need to talk about coal seams
> "draping" topography.

Think differential compaction of the sediments and peat beside the channel,
while the channel sandstones and conglomerates don't compact as much. The
result is seams "draped" over channels.

Kevin

Leckie, D. (1986) Rates, Controls, and Sand Body Geometries of
Transgressive-Regressive Cycles: Cretaceous Moosebar and Gates Formations,
British Columbia. AAPG Bulletin, v. 70, no. 5 (May 1986) p. 516-535.
>
> Later,
>
> Bill
>
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Received on Wed Jan 28 19:58:02 2004

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