Canadian coal - block diagram of depositional setting

From: Kevin Sharman <ksharman@pris.bc.ca>
Date: Wed Jan 21 2004 - 08:56:46 EST

Hi Bill,

The Earth Science Dept. of UBC (University of British Columbia) has a
website with an excellent block diagram of Gates Formation depositional
environments at http://www.science.ubc.ca/~eoswr/clastic/clastic.html This
is complete with hyperlinks to core photographs showing the sedimentary and
biogenic structures used to interpret the environment. A fantastic teaching
tool, and a good example of making complex data accessible and graphic, even
for the non-geologist.

As promised, here is the abstract of Carmichael's thesis. As expected, the
block diagram ties in well with what he's saying; I wouldn't be surprised if
he helped make it.

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 Unpublished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.

Abstract

The Lower Cretaceous Gates and Moosebar Formations in the Northeast
Coalfields of British Columbia comprise a 350-450 m thick interval of marine
and non-marine, coal bearing clastic sediments. The Moosebar Formation and
lowermost unit of the Gates Formation (the Torrens Member) consist of
between 2 and 4 coarsening-upward marine cycles. In the northern part of
the study area, non-marine sediments in the Gates Formation occur
interbedded with 3 major marine tongues (the Sherriff Member, Fahler C and
Babcock Member) which pinch out towards the south. In the study area south
of Duke Mountain, sediments in the Gates Formation above the Torrens Member
are entirely non-marine.

The coarsening-upward marine cycles in the Moosebar-lower Gates interval
were deposited mainly during regressions when the shorelines prograded
northwards. The cycles are comprised of up to 3 main facies interpreted as
offshore, transition zone and shoreface-beach (and locally distributary
channel) deposits. Thin transgressive deposits are present at the base of
the Sherriff Member. Lower Gates shorelines are interpreted as high energy
and wave-dominated. Shoreline trends for the Torrens, Sherriff, and Babcock
members are approximately E-W, and approximately NW-SE for the Fahler C.

Non-marine deposits in the Gates Formation are interpreted as forming in
lagoons, fluvial channels, and overbank environments within a coastal plain
setting. The channels are mainly braided river types with anastomosing or
straight (non-braided) and meandering river channels also present. The
rivers flowed towards the northwest, north, northeast, and east with
northeasterly directions most common. Up to three separate very coarse
fluvial conglomerates, interpreted as proximal braided river-alluvial fan
deposits are present in the south near Mt. Belcourt. Overbank sediments
were deposited in levee, crevasse splay, lacustrine, and well to poorly
drained swamp environments.

The upper Gates marine unit (Babcock Member) contains both transgressive and
regressive deposits. Three types of transgressive deposits are recognized:

  1.. Thick (maximum 40 m) estuary mouth (shoal retreat massif) sandstones
and conglomerates preserved in linear belts approximately 0.4 - 2 km wide
trending NW-SE and N-S.
  2.. A thin (maximum 90 cm) but laterally extensive marine lag.
  3.. Lagoon-intertidal deposits.

Upper Gates regressive deposits include shelf to shallow marine sandstones
overlain by estuarine subtidal channel and shoal deposits which are in turn
overlain locally by tidal flat and coastal plain deposits with thin coal
seams. Upper Gates shorelines appear to be strongly tidally influenced.

Thick (maximum 10 m), laterally extensive coal seams occur in the lower part
of the Gates Formation. Coal seams in the upper Gates are thin (generally <
1.0 m). With the exception of a few very thin seams, the Gates coals appear
to be entirely autochthonous and to have accumulated as peat in swamps in a
coastal plain depositional setting. Some of the coals in the lower Gates
were deposited in swamps which extended inland for at least 75 km from the
shoreline. The maximum coal development in the Gates Formation (28 m total
coal and 6 seams > 1.5 m thick) occurs in the Foothills south of Kinuseo
Creek, near the boundary between transitional marine and non-marine facies
belts.

Cross sections based on closely spaced borehole and outcrop sections
illustrate the occurrence and distribution of coal seams in the Duke,
Honeymoon, Babcock, Frame, and McConkey pits. Thinning and pinchout of coal
seams occurs adjacent to fluvial channel and splay deposits and near the
landward pinchout of the Sherriff Member. Draping of coal seams over
fluvial channel deposits causes rapid variations in interseam thickness
which may lead to correlation problems in the early stages of exploration
and affect the potential mineability of coal seams.

The main detrital components in the Gates sandstones are quartz and chert,
with siliceous rock fragments, clastic sedimentary and metasedimentary rock
fragments, igneous rock fragments and feldspar present in lesser amounts.
Mesozoic to Cambrian sedimentary rocks of the Rocky Mountain Front and Main
Ranges are interpreted as the principal source of the detrital components.

Kevin
Received on Wed Jan 21 08:57:55 2004

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