Canadian Coal - more on depositional setting

Hi Bill,

Now that you have decided that the best explanation for vertical roots in
sandstone below J seam is a bout of opportunistic in situ growth (Feb 22):
"My best synthesis of these interpretations (putting aside my
preconceptions) of in situ and insufficient roots is to say that the
vertical roots represent an opportunistic growth, followed by flooding and
deposition of organics from a floating mat", we should look at some more
data.

Dale Leckie did his Ph.D. thesis on the Gates Formation about 20 km north of
the location where I took photos of the J seam with roots below it. Leckie
(1983) describes a conglomerate immediately below the coal section of the
Gates Formation. This conglomerate is 30 m thick and completely pinches out
within 0.5 km. "at one location the conglomerate overlies rooted
sandstone..this conglomerate is poorly sorted, and matrix supported with
clasts up to 6.5 cm. It contains large in situ vertical roots throughout."

"the top of the coarsening upward sandstone of cycle 2 (immediately below
the J seam equivalent - KS) is always rooted and/or overlain by "A" coal
seam..overlying "A" seam is 5.5 m to 20 m of thinly bedded carbonaceous
shales, siltstones, and very fine sandstones before the next major coal, "B"
seam..sediments are thin bedded (usually several centimeters but
occasionally up to 2 m); both coarsening and fining upwards trends
occur..Carbonaceous material occurs throughout as coal stringers, in situ
vertical roots, delicate plant impressions, and comminuted plant material."

Then above "B" seam he describes more conglomerates with roots: "The
conglomerate and sandstone at the south end of Mt. Spieker is 38 m thick; it
is bracketed above and below by coal seams (B seam below it and C seam above
it - KS) as well as rooted and carbonaceous intervals within." This
interval was cored, and "in situ vertical roots were found in sandstone of
this core and in the conglomerate of another. Grain size of the
conglomerate ranges from beds of well sorted granulestone to 10 m thick
intervals of poorly sorted conglomerate with clasts as large as 20 X 18 X 12
cm."

So we have a package of sediments over 70 meters thick with coarse
conglomerates, vertical roots and coal seams. Please explain. I hope you
don't want to ascribe the vertical roots in conglomerates to turbidity
currents. If you are trying the "opportunistic growth" angle for these
roots, and a floating mat idea for the coals, you have to switch back and
forth between these two mechanisms repeatedly, and have several long periods
of growth to account for multiple generations of vertical roots over a large
vertical distance.

How do you explain the 30 meter thick conglomerate with large in situ
vertical roots and the 38 meter thick conglomerate between B and C seams
with vertical roots? I'll give you a hint - these aren't turbidites.

How do you explain the package of shales, siltstones, and sandstones
overlying A seam, with roots, and coarsening upward and fining upward
trends? I'll give you another hint - these aren't turbidites either.

It's time to face up to the realization that the ad hoc Flood floating
mat/opportunistic growth combination cannot explain this data.

Kevin

Leckie, D.A. (1983) Sedimentology of the Moosebar and Gates Formations
(Lower Cretaceous), unpublished Ph. D. thesis, McMaster University, Ontario,
515 p.
Received on Thu Feb 26 08:55:26 2004