Re: Canadian Coal - depositional setting

From: Kevin Sharman <ksharman@pris.bc.ca>
Date: Thu Jan 29 2004 - 19:33:11 EST

----- Original Message -----
From: "Bill Payne" <bpayne15@juno.com>
To: <ksharman@pris.bc.ca>
Cc: <asa@calvin.edu>
Sent: Wednesday, January 28, 2004 10:02 PM
Subject: Re: Canadian Coal - depositional setting

> On Wed, 28 Jan 2004 17:47:36 -0700 "Kevin Sharman" <ksharman@pris.bc.ca>
> writes:
>
> But you say you never see standing trees in partings? Are you saying the
> flooded stage _always_ lasts until the trees fall over?

Yes, in these coals, otherwise we would see standing trees.

>Also, trees are
> not continuous. If they do fall as you say, there still should be plenty
> of undisturbed area between the trees where the parting was not mixed
> with the peat. Are the "discontinuous lenses of mudstone" partings
> (parallel top and bottom) that are of limited extent

Yes.

> or true lenses
> (convex top and bottom).
>
> > 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.
>
> Parting destruction is a sure thing when you get roots.

Nope. Some tonstein layers in the South African Grootegeluk Formation coal measures contain 40% organic matter. It didn't fall from the sky with the ash. Also see below.

>Even if you have
> only a single stand of shrub roots penetrating a 5 mm mud layer on top of
> the peat and the rest of the vegetation grows on top of the single stand
> of shrubs, the initial stand of shrub roots and shrubs will have to be
> continuous across the mud layer to provide a base for the rest of the
> vegetation. That continuous layer of roots will bioturbate and destroy
> the continuity of the thin mud layer, making the formation of a parting
> impossible in a swamp. I still don't see that your model will explain a
> parting.

There is one of my pictures on Glenn's website showing a shale parting with roots in it. The roots didn't even hardly disturb the sedimentary layering, much less "bioturbate and destroy the continuity".

>
> [snip]
> > Now are you wishing you hadn't had your little outburst?
>
> Whatever it takes to get to the answer, Kevin.

Your choice of switching to a confrontational mode shows me that you are realizing that your argument is indefensible - you are willing to reject well-reasoned interpretations that fit the evidence in favor of your vague arm-waving, in an effort to keep your faith intact.

 

Instead of advancing coherent explanations of the depositional environment, you have ascribed interseam sediments and partings to: turbidites, well, maybe subaqeous deposits that may or may not be turbidites, well, I guess I can't explain tonsteins under a floating mat... This is vague arm-waving.

 

Your floating mat speculation is not the best fit for the observed data, and we're not even finished looking at all the data yet.

>
> There is an interesting abstract at
> http://gsa.confex.com/gsa/2003AM/finalprogramabstract_64603.htm which may
> bear on your explanation of sediment transport: "The presence of standing
> trees, still preserved in life position, in coal measures, according to
> Ager (1993), can show how rapidly they were buried in a rapid rush of
> sediment.

No, an example of rapid burial in a rapid rush of sediment is more like the example from the unit above the uppermost Gates seam which I presented previously, where only a few tree trunks are preserved in growth position, and the majority are lying flat at the bottom of an erosive contact, preferentially oriented.

>The proposal of an alternative genesis for the coal deposits
> considers Mutti et al.'s (1999) model, which links the turbidites from
> passive-margin basins to flows that originated in the continent during
> catastrophic floods. It is considered that, in human terms, almost every
> flood is catastrophic and the ones discussed by Mutti et al. (1999) have
> big proportions, in which energy liberated and tonnage of sediments are
> inconceivable with respect to Lyell's gradualism. Mutti et al. (1999),
> correlating turbidites and fluvial sediments as "fluvial-turbiditic
> sedimentation", consider catastrophic floods to be the main process in
> this kind of sedimentation. They studied many occurrences of humocky
> cross stratification (HCS), interpreting them as generating shelf lobes,
> associated with catastrophic flood-dominated deltas." (Begossi, Hower
> and Eble, An alternative proposal to coal genesis in the early Permian of
> the Parana Basin, Brazil. 2003 Seattle Annual Meeting, GSA) The abstract
> doesn't give the reference for Mutti et al.'s paper; if anyone finds it,
> please let us know the publication. The abstract also mentions another
> paper by Glasspool (2003), for which I would like to have the reference,
> along with Ager (1993).

Begossi and Favera (2002) in their paper 'Catastrophic floods as a possible cause of organic matter accumulation giving rise to coal, Paraná Basin, Brazil" describe a very special coal depositional environment that has nothing to do with the Gates coals, or any other economic coal for that matter (emphasis mine):

 

"Gondwana coals of the Rio Bonito Formation (Paraná Basin) in Southern Brazil have generally large ash yields, so they could be better called coaly siltstones than coal. In addition, hummocky cross stratification (HCS) was found in several coal beds of the Rio Bonito Formation throughout the basin. In this formation, the frequent and close relationship between facies involving rocks generated by subaqueous gravity flows (diamictites) and coal itself provides an excellent depositional model based on resedimentary processes acting during deposition, as well as a stratigraphic rearrangement of the present units.

 

Coal-forming organic sediments would come from trees plucked by the floods, as indicated by the wood logs floating in the diamictite, and reworking of previous peat accumulations. Every coal layer is covered generally by paleosoil siltstones, which represent colonization at the top of the catastrophic flood deposit, ending a sedimentary cycle.

Although the preferential mode of occurrence of HCS in shallow marine environments indicates a genesis attributed to storm action, other causes, such as catastrophic flooding, have been advanced. Mutti et al. [Mem. Sci. Geol. 48 (1996) 233] described flood-dominated deltaic systems with thick conglomerate, sandstone, and pelitic deposits, derived from small- to medium-scale fluvial systems and mountain-bordered drainage basins adjacent to the sea. In such settings, seaward sediment flow can increase dramatically when weather conditions can supply water in such amounts to produce catastrophic floods. Thick and laterally extensive sandstone lobes with HCS are the fundamental depositional elements of fan deltas and other river-dominated delta systems.

Diamictites and coal together could be a result from Jökullhlaups--an Icelandic term for glacial outburst flood--in case of catastrophic floods coming from a melting mountain glacier, similar to the Columbia River Valley Scablands (15,000 BP) and in modern Iceland examples."

This model is completely inapplicable to the Gates coals. It features very high ash coals, diamictite, and HCS in the seams, none of which are found in the Gates.

 

It's also inapplicable to your floating mat speculation - note the paleosoil siltstones covering every coal layer, and the requirement for a steep landmass close to the site of deposition, not your flooded world.

>
> > 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.
>
> So in this example you have thinner coal at the apex of the drape?

Yes.

>Do
> you ever see coal seams of constant thickness draped over lenses of sand?

No.
>
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Received on Thu Jan 29 19:35:51 2004

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