Re: Sulphur, Forams and Partings in Coal

  ----- Original Message -----
  From: bpayne15@juno.com
  To: ksharman@pris.bc.ca
  Cc: asa@calvin.edu
  Sent: Saturday, December 27, 2003 8:48 PM
  Subject: Sulphur, Forams and Partings in Coal

  On Mon, 22 Dec 2003 10:24:37 -0700 "Kevin and Birgit Sharman" <ksharman@pris.bc.ca> writes:

> It is generally accepted that coals with the original peat or roof
> strata formed in marine influenced environments have a higher sulphide
> sulphur content than those with fresh water influence (Holuszko et al, 1993,
> Stach et al, 1982). The mechanism involved is bacterial reduction of
> sulphate (supplied by seawater) to sulphide.

  How did the seawater get into the peat swamp?

  Two ways: 1) marine peats form in the intertidal zone, and seawater comes in with the tides, i.e. the marine grass peats of the Atlantic coast of the US, or mangrove peats.

  Or 2) a marine inundation during or after the deposition of the peat. This is responsible for a marine parting, which is common in the Carboniferous coals you deal with but rare in W. Canadian Cretaceous coals, or a larger scale marine transgression which ends coal development and deposites thicker marine strata.

> "Low ash and low sulphur coals were deposited as fresh water peats
> which were underlain and overlain by fresh-water clastic sediments in
> which limestones were absent." (Stach et al, 1982, Cecil et al, 1979a).

> Using the above as criteria, the low sulphur coals of the Gates Fm.
> could not have been deposited in a marine floating mat model. A fresh
> water floating mat model would have to account for the lateral extent of
> these coals (230 km X 90 km = 20,700 km2), deposition of the interbedded
> sediments (65 m to 90 meters thick), and the multiple seams (8 seams, 0.5
> meters to 10 meters thick). Proposing a zone of fresh water in contact with
> marine water leaves one with explaining why this would not be mixed. Even
> brackish water peats have high sulphur content.

  High rainfall could keep the salt water away from the peat mat.

  This is speculative at best, Bill. Please show a modern example of a high rainfall marine area with reduced (to below the "brackish" concentration) salinity at the sediment/water interface. If you are claiming that "things were different back then", please back this up with evidence.

  The extensive lateral continuity of the coal and interbedded sediments of constant thickness supports the floating mat model. Thick coal only requires a 10x thicker mat of peat - the mats must have been hundreds of feet thick.

  Please describe a modern example of an ecosystem which, when ripped up, could produce a peat mat hundreds of feet thick (actually you would need one thicker than that to produce the thicker seams in the geologic record). If you are claiming that "things were different back then", please back this up with evidence.

  Multiple seams only require an influx of sediment to separate the seams

  Please describe the mechanism for depositing this sediment beneath the mat.

> Roots are sometimes observed in
> the top of the parting in these coals, evidently originating from the coal
> above (Glenn has one of my photos on his coal page showing this, the one
> with the label "shale with roots" in the lower right corner).

  Are these "shrub" roots? Are you proposing coal-forming shrubs? If not, where are the big tree roots?

  As we discussed before, first shrubs, then trees. Tree roots do not have to penetrate vertically very far, and typically don't in a waterlogged environment.

> I agree that one would expect at least some trees to be sticking
> through the parting if it were deposited by an influx of water and sediment into
> the swamp (unless the influx of water knocks down the trees). I also
> agree that it is surprising that the lower contacts of the partings can be so
> sharp and planar if the mud is draped over an irregular surface like the top
> of a peat swamp.

  If the influx of water knocked the trees down, the tree roots should have been pulled up, destroying the flat top of the peat and resulting in pot holes filled with sediment. Since the partings are generally of a consistent thickness, it is more likely that the surface topography of the peat was preserved by the parting, i.e., there were no trees growing there.

   
  Carmichael (1983) in his thesis on the Gates Fm. describes the contact between the top of the D seam (the uppermost thick seam in the Gates) and the overlying unit, which he calls facies 1A. "lithologies consist of fine to granular sandstones and conglomerates.the base of facies 1A is abrupt or erosional.the basal surface is planar and locally channeled.groove marks and impressions of small wood fragments or large logs are common near the base and along the basal surface.vertical log impressions up to 2 meters high are present in coarse to granular sandstones at the base of facies 1A.. their vertical orientation and position directly above the D coal seam suggests that they represent tree trunks in growth position." (p. 136, 143). This facies is interpreted as an estuarine channel fill with tidal influence.

  This evidence supports the idea that most standing trees at the top of the original peat were flattened due to a high energy transgression of the sandstone/conglomerate of facies 1A, with some trees left standing. This depositional environment would not be conducive to vertical emplacement of waterlogged transported tree trunks like the Mt. St. Helens analogue.

  In the floating mat model, stumps and roots may or may not settle out with the other peat materials.

  They would have to settle out sometime, wouldn't they?

  In the in situ model, standing stems/trunks should always be preserved from the coal below, along with root mats from above.

  See above. The trees can be flattened, and the peat surface bevelled, by the influx of water and sediment. Also, as you mentioned, the top of the peat may not have had trees growing on it. Succession of the peat community from a forest swamp into open grass marshes has been proposed in the literature, aided by wildfire.

   
  Are you willing to follow the evidence where it leads? Of course, that's a two-edged sword that will cut me just as quickly as you. :-)

  Yes. The difficulty lies in us accepting one another's conclusions. While we like to think that we are open minded, we may not be able to admit fully if our position is untenable. The real test is convincing a disinterested third party. ("disinterested" is probably a good description of the average ASA list reader after reading some of our posts!)

  Sample preparation for spore analysis included maceration with concentrated HNO3 for 6 days. I suppose this aggressive acid would have destroyed any foram tests that may have been in the coal, so I don't think we can say that there are no forams in these coals. Do you agree?

  Yes.

  If so, is there another method of sample prep or sample analysis that would preserve the forams?

  Not sure.

  Incidentally, Kevin, I ordered a copy of Stach. I look forward to reading it, and thank you for the reference.

  Great! it's quite a good reference.

  Kevin
Received on Tue Dec 30 23:23:09 2003