Canadian Coal - depositional setting

From: Kevin Sharman <ksharman@pris.bc.ca>
Date: Wed Jan 14 2004 - 00:30:12 EST

Hi Bill,

Time for another coal discussion:

It is useful to look at the depositional setting of Gates Formation coals
for clues about the coals' origin. The basal seam in the Gates Formation
lies on a medium grained sandstone, as shown in the photos on Glenn's
website http://home.entouch.net/dmd/cancoal.htm

Kalkreuth et al (1989) discussed this: "Coal seams formed on Lower
Cretaceous wave-dominated strandplain sediments in Western Canada are
characterized by great lateral continuity, substantial thicknesses, and
relatively low ash and sulphur contents." and "Gates shorelines prograded
northwards as a series of sand and gravel wave-dominated deltas and
strandplains that extended as a sheet sand laterally along strike for at
least 230 km and downdip for up to 90 km."

Evidence for progradation is in the form of tonstein (volcanic ash bed)
correlations (Kilby, 1984) which showed that the top of the Gates Formation
is diachronous, with marine shales of the Hulcross Formation being deposited
in the north at the same time as Upper Gates non-marine sediments to the
south. As an aside, one of the bentonites in the Hulcross Fm. has been
dated (Ar40/Ar39 date of 107.1 Ma) and is used by Obradovich (1993) to
calibrate his widely used Cretaceous time scale. Another bentonite in the
Moosebar Formation below the Gates demonstrates that the lower contact is
diachronous as well. (Fans of Berthault's work take note - diachronous
lithostratigraphic contacts were not news even in 1984!)

Any floating mat hypothesis would have to explain how vegetation could be
deposited on a sand body that is prograding northwards over time. At any
given time, the shoreface sand existed as the top layer over only a small
part of the area, yet coal occurs directly overlying the shoreface sand over
its extent (230 km X 90 km = 20,700 km^2). The rate of progradation was
estimated by Leckie (1986) as 218 m to 437 m per 1000 years.

If you want to put the deposition of this sandstone into a biblical
timeframe, it would need to occur in a short time. The Phanerozoic
sedimentary sequence in this area is ~9000 m thick. Dividing this by 400
days (alleged duration of the Flood) gives an average rate of deposition of
22 meters/day. Using a thickness for the shoreface sand of 50 meters, this
would have to be deposited in a little over two days. Not a lot of time to
rain down ~80 meters of veg material to make that 8 meter seam, is it?

The sandstones are "clean" and "very well sorted", according to Carmichael
(p. 42). This kind of compositional and textural maturity is not a feature
of rapidly deposited sediment, but is found in modern shoreface sands, due
to reworking by wave action. The rate of progradation would have been 90 km
in 2 days! Trying to settle out roots vertically in a unit that is being
deposited this fast is a non-starter. Even in an old earth time frame,
settling roots out vertically in a shoreface sand environment doesn't work.
It would be too high energy - see the above quote about wave-dominated
deltas and strandplains.

There are also burrows in the sandstone facies, which could not have been
made in the short timeframe discussed above. Carmichael observed
Diplocraterion, Paleophycus, Ophiomorpha, Macaronichnus segregatus(?), and
Skolithos burrows in the shoreface sand facies. He also notes that "the top
30 cm to 1 meter of facies B (the upper portion of the shoreface sand) is
often strongly bioturbated. In most cases the bioturbation is caused by
roots."

Arguing that this sandstone was deposited more slowly than the average used
above doesn't help, either. The Cretaceous succession has repeated
shoreline progradations, so slowing down this one just means you must speed
up the rest to get the total amount of sediment allegedly deposited in the
Flood timeframe.

This depositional setting for the Gates coals (coal on shoreface sand) is
not unique. The coal-bearing Jurassic/Cretaceous Mist Mountain Formation of
southeastern BC also rests conformably on the Morrissey Formation, a
shoreface sand, as do the coals of the Cretaceous Mannville Formation of
central Alberta.

The interseam sediments consist of conglomerates, sandstones, and
siltstones. None of these units can be interpreted as turbidites, as has
been suggested for interseam sediments below floating mats. Carmichael
(1983) recognized a variety of fresh water depositional environments,
including stacked channel fills, proximal and distal splays, lagoonal
facies, lake deposits, etc. Channels and crevasse splays can be recognized
from isopachs of the conglomerate and sandstone bodies. Lake deposits
contain non-marine Unionid bivalves. None of the above rocks could be
deposited beneath an extensive marine floating mat, could they?

So Bill, please tie the foregoing information in with your floating mat
speculation.

Kevin

Carmichael, S. M. M. (1983): Sedimentology of the Lower Cretaceous Gates and
Moosebar Formations, Northeast Coalfields, British Columbia. Unpublished
Ph. D thesis, Department of Geological Sciences, University of British
Columbia, 1983.

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.

Kilby, W. E. (1984): Tonstein and Bentonite Correlations in Northeast
British Columbia (93O, P, I, 94A), Ministry of Energy, Mines, and Petroleum
Res., Geological Fieldwork 1984, Paper 1985-1, p. 257-278.

Leckie, D. (1986) Rates, Controls, and Sand Body Geometries of
Transgressive-Regressive Cycles: Cretaceous Moosebar and Gates Formations,
British Columbia. AAPG Bulletin, v. 70, no. 5 (May 1986) p. 516-535.

Obradovich, J.D. (1993), A Cretaceous Time Scale, in Caldwell, W.G.E., and
Kaufmann, E. G., eds., Evolution of the Western Interior Basin: Geological
Association of Canada, Special Paper 39, p. 379-396.
Received on Wed Jan 14 00:31:10 2004

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