Canadian Coal - Petrology

From: Kevin Sharman <ksharman@pris.bc.ca>
Date: Sat Jan 31 2004 - 13:09:26 EST

Hi Bill,

It's time for a look at coal petrology as it relates to the formation of Gates coals and others. I am not going to start at the beginning on this subject, just make a few points. For a good summary, see http://www.newcastle.edu.au/discipline/geology/research/cfkd/undp.htm

 

Gates coals are rich in inertinite (Kalkreuth et al, 1989). Other Western Canadian coals are also inertinite-rich, such as the Jurassic/Cretaceous Mist Mountain Fm. coals of southeast BC, and the Gething Fm. coals of the Peace River Coalfield. Worldwide, Gondwana coals are generally higher in inertinite than Carboniferous coals.

 

The inertinite macerals (fusinite, semifusinite) are derived from oxidation of plant material, either as a result of drying conditions in the swamp (degradofusinite) or fire (pyrofusinite). This oxidation changes its optical properties and chemistry. Lamberson et al (1996) present evidence that the high inertinite content of the Gates coals is almost exclusively due to fires in the peat swamps. There is a cyclical change in the maceral content through the seam. Zones of inertinite rich material in the seams are interpreted to be fire horizons and evidence of lowered water table allowing drying and oxidation of the peat.

 

In a floating mat model, inertinite would not form after submarine deposition of "fresh" plant material, due to insufficient oxygen in the marine water. I have done work in the acid rock drainage field, and submerging pyrite-containing rock in even a shallow water cover reduces oxidation to near zero because of the reduced oxygen. The same would apply to fresh vegetation accumulated below a floating mat. In a comparison of brackish-marine and freshwater Florida peats, Cohen and Spackman (1977) found that fusinite was absent or occurred in traces in most marine peats. This is interpreted to be because the water cover prevents oxidation.

 

The only way inertinite could be found in the floating mat model is if it was formed in a swamp prior to the vegetation/peat mat being ripped up. Then as this mat debris is being deposited by "shedding", the inertinite rich zones would have to somehow re-segregate into layers. Of the maceral groups, inertinite is densest, followed by vitrinite. Liptinite (found in low quantities in Gates coals) is the lightest. So, if you propose segregation of macerals by density when they are shed from a floating mat, one would expect inertinite to be on the bottom, followed by vitrinite and liptinite. This is not what we observe. Gates coals exhibit a "dulling-upward" trend, where vitrinite percentages are highest at the bottom of the seam. Lamberson (1996): "Within the B seam, the variation in vitrinite/inertinite content is cyclic, with 9 cycles resolvable. Although the basal part of the seam is sheared, it is relatively bright, and is composed of 88% vitrinite by volume."

 

This observed pattern is common in seams of other coal basins as well. It fits with an in situ model of peat development - the area has to be wet to start a peat swamp (favoring vitrinite formation) but fluctuating water tables may occur during the life of the swamp, forming more or less inertinite. This observed pattern is not compatible with deposition from a floating mat.

 

Inertodetrinite is a coal maceral formed from broken up pieces of fusinite and semifusinite. Its presence has been used to imply transport of the peat material within the swamp (Diessel, 1982). Gates coals studied by Kalkreuth (1989) have an average IR ratio (fusinite + semifusinite/inertodetrinite) of 1.58. In other words, the IR ratio represents the ratio of intact inertinite to broken up inertinite. Diessel considered that IR ratios of <2 reflected hypautochthonous and allochthonous conditions in the swamp. A value of 1.58 reflects some transport, which Kalkreuth ascribed to "occasional flooding and storm events". Zones high in inertodetrinite are also high in mineral matter, lending support to this mechanism.

 

Stach (1982) states "unimportant rearrangements of plant remains or of peat take place repeatedly within a peat swamp during times of flooding. Consequently, "hypautochthonous" coals result and these are commonly characterized by a finely detrital composition, by a higher mineral matter content than truly autochthonous coals and by a pronounced micro-layering." He also states "Allochthonous coals are usually too rich in mineral matter for economic working."

 

The majority of the zones within the Gates coal seams do not fit the above description - they are low in mineral matter and don't have pronounced microlayering. Your floating mat coals would be full of mineral matter due to mixing of peat and suspended sediment during deposition.

 

In a floating mat model which transported already-formed peat, one would expect that virtually all the inertinite would be in the form of intertodetrinite due to extreme transport and mechanical abrasion of the peat, especially if the peat is ripped off a landmass by "a massive wave which overtopped the continents" (your words) and deposited in a high energy environment such as a shoreface sand.

 

So, a floating mat coal would have either no inertinite, or virtually all the inertinite would be inertodetrinite, and there would have to be a mechanism to sort the inertinite into zones as they are found in the Gates coals.

 

Note that I didn't say "petrology shows that floating mat coals are impossible." I am saying that you need to propose a mechanism which is better than the in situ model for explaining the data from petrology .

 

Partings are a subject that is one of our points of disagreement. The model I proposed for parting formation (water table rises, drowns peat, any trees fall over, top of peat decomposes and gets smooth, clastic input makes parting) is supported by petrological evidence. As I mentioned, inertinite is formed from oxidation of the peat; if the peat is less oxidized, vitrinite will form from woody material. Vitrinite formation is favored by a high water table.

 

Coal petrologists have developed indices of the different coal macerals to help interpret the depositional environment of the swamp. These are useful but not foolproof, and the best interpretations are supported by sedimentologic and palynology data. The Gelification Index (GI) is the ratio of vitrinite to inertinite. The Groundwater Influence Index (GWI) is the ratio of gelified vitrinite and mineral matter to structured vitrinite. Higher values of these two indices are an indicator of wet conditions. In all the detailed petrographic seam sections I have seen in Gates coals, Upper Cretaceous/Paleocene coals, and the Carboniferous coal I looked at (Minto coalfield, New Brunswick), the coal just below a parting has high GI (high vitrinite) and GWI, indicating flooding, but low mineral matter, showing that it was "clean" water.

 

Gentzis et al (1989) advance an idea for the formation of thin partings in Paleocene sub-bituminous coal. "The composition of the partings indicates very poor peat preservation conditions, the pH of the swamp water was probably high (>4.5) which, in turn, stimulated extreme biological degradation and subsequent accumulation of plant-derived inorganics which were the precursors of the mineral component of most of the partings." This model takes gelification a few steps further, completely degrading the plant matter. The inherent ash (that found within the plant matter) of these coals is ~5%. This may explain thin partings that don't show obvious clastic origin (sedimentary structures, etc.)

Kevin

 

Cohen, A.D. and Spackman, W. (1977) Phytogenic Organic Sediments and Sedimentary Environments in the Everglades-mangrove complex of Florida, Palaeontographica, Abt. B, 162144 pp., Stuttgart.

 

Diessel, C.F.K. (1982): An Appraisal of Coal Facies Based on Maceral Characteristics, Australian Coal Geology, Part 2, v. 4, p. 474-484.

 

Gentzis, T., Goodarzi, F., and Stasiuk, L. (1989) Petrology and Depositional Environment of Upper Paleocene Coals From the Obed-Marsh Deposit, West-Central Alberta, in Advances in Western Canadian Coal Geoscience, Alberta Research Council Information Series No. 103.

 

Lamberson, M., Bustin, M., Kalkreuth, W. D. and Pratt, K. C. (1996): The formation of inertinite-rich peats in the mid-Cretaceous Gates Formation: implications for the interpretation of mid-Albian history of paleowildfire. Palaeogeography, Palaeoclimatology, Palaeoecology, v. 120, Issues 3-4, March 1996, p. 235-260.

 

Kalkreuth, W., D.A. Leckie, and Labonte, M. (1989): Gates Formation (Lower Cretaceous) Coals in Western Canada, A Sedimentological and Petrographic Study. Contributions to Canadian Coal Geoscience, GSC Paper 89-8.
Received on Sat Jan 31 13:10:35 2004

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