Re: Canadian Coal - depositional setting

From: Bill Payne <bpayne15@juno.com>
Date: Thu Jan 29 2004 - 00:02:00 EST

On Wed, 28 Jan 2004 17:47:36 -0700 "Kevin Sharman" <ksharman@pris.bc.ca>
writes:

> 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).

But you say you never see standing trees in partings? Are you saying the
flooded stage _always_ lasts until the trees fall over? 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 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. 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.

[snip]
> Now are you wishing you hadn't had your little outburst?

Whatever it takes to get to the answer, Kevin. If you explained this
previously, and you may have, it didn't register with me. Now I
understand - thanks.

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. 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).

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

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Received on Thu Jan 29 00:34:32 2004

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