Re: coal again!(1)

Jonathan Clarke (
Fri, 02 Apr 1999 07:06:28 +1000

Hi Bill

Bill Payne wrote:

> On Wed, 31 Mar 1999 19:25:09 +1000 Jonathan Clarke
> <> writes:
> >I have seen quite a few dropstones of all sorts, including examples I
> have
> >interpreted as glacial, volcanic projective, impact projectile, and
> vegetational rafting.
> I understand that dropstones are fairly common in eastern US
> Carboniferous coals. Of the options you offer above, I would think the
> only reasonable one in a coal seam would be vegetational rafting.

I would agree completely. How big are the pebbles? How common are they?

> >Schutz's study is largely discredited these days and has been superseded
> by
> >many studies which do show weathering in paleosols. Not only that, they
> show
> >many different weathering styles which reflect modern variations in
> >weathering and which are consonant with other palaeoclimatic indicators
> in
> >the sediments. A good place to start is Greg Retallack's book
> "Paleosols" (I
> >think).

Greg Retallack's book is actually called "Soils of the Past". It was
published in 1990 by Unwin Hyman. My mistake.

> I was aware of Schultz's study being discredited as you say. But, being
> open-minded as we are :-), would it be possible that the "paleosol
> weathering" were the result of secondary weathering after deposition?
> Coal seams generally carry more water than the sandstones/shales/clays
> they are associated with, so we would expect the clay aquitard beneath
> the coal aquifer to develop minor chemical degradation after deposition,
> due to the flow of water through the overlying coal. This expected
> chemical profile in the underclay beneath a coal seam might well mimic a
> soil-weathering profile.

This is a good and important question, and one that has to be carefully
considered when studying possible paleosols and weathering profiles. I have
certainly seen such subsurface weathering along aquitard. The have included
examples of both reduced and oxidised weathering.

A number of criteria for identifying paleosols are given by Retallack. They
included roots or root casts, soil stratification, and soil microstructure.
Peat related soils are histosols and are typically formed under waterlogged
and reducing conditions. Such soils are have poorly developed horizons and

With respect to alteration by moving groundwater, I'll contrast two
extremes. 1) where the water is oxidising, the other when it is reducing.
If oxidising it should produce oxidised alteration of the basal sediments
and the lower part of the coal. This would clearly be different in texture
and mineralogy from a paleosol produced under reduced conditions. 2) reduced
groundwater would produce relatively little alteration under the coal, and,
if this occurred very early in the burial history of the sediment (2a),
would indeed be difficult to tell from that formed by pedogenic processes
beneath peat. If the reduced fluids migrated later in the burial history
(2b) they would be at a higher temperature and one would expect higher
temperature minerals (illite-sericite rather than smectite-kaolinite, for
example). Any alteration caused by reduced waters moving along the interface
late in the history of the rock (2c), when it is near surface again, would
overprint earlier rock fabrics, and should be distinguishable from alteration
formed early on.

The really tricky one is 2b. So, if confronted by zone of alteration beneath
a coal I would look first of all for roots. Possible horizon development and
soil microstructures would be an added plus, but I would recognise that they
are an added bonus and not always present (or easy to document). If roots
were absent I would probably think of it as a later alteration. If I had
reason to suspect that the coal was transported (from coal lithologies
presumably, or the general lithologies association), I would look for
evidence that the roots were associated with the vegetation in the overlying
material. If there was no obvious connection in such a case I would start
considering the association to be fortuitous.

If you are masochistic, one could imagine all sorts of complications. Like
lignitic sediments. deposited over previously oxidised weathering
profiles. Or transported peat deposited over an pre-existing histosol
formed. Another one is where an original reduced paleosol has been oxidised
by later groundwater or near surface weathering. It should be possible to
discover what has happen in each case, although they would certainly be traps
for the unwary. However, unless one has evidence for such nasty things one
should not postulate them unnecessarily.

> This is one of probably several possible alternate interpretations that
> might explain the observations. Everyone seems to forget that Schultz's
> macroscopic field observations confirmed his belief that these coals were
> allochthonous.

I don't remember that aspect, but I'll have to read the Schultz paper again.

> I've very much enjoyed our discussion. I made a copy of Gastaldo's paper
> today, and will try to get to the Post Office with it tomorrow. If
> anyone else would like a copy, send me your name and address offline and
> I'll send one to you (as long as the demand isn't too great).
> Bill

God bless