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SEDIMENTARY
DYKES AND SILLS INTRUDED INTO THE MAIN COAL SEAM OF THE GRETA FORMATION IN THE
ABERDARE I.P.
MARTINI1, C. DIESSSEL2
AND K.H.R. MOELLE2
1Department
of Land Resource Science, University of Guelph, Guelph, Ontario, Canada.
The
Lower Permian Greta coal measures of the Sydney Basin, Australia, developed
under cold to cold-temperate climate, on terminal positions of large alluvial
fans. The fans derived from an eastern actively rising tectonic fold belt.
Numerous sedimentary dykes, sills and splay deposits occur in the main coal seam
near washout (channels), particularly in the Aberdare North and in adjacent
collieries. The sedimentary intrusions vary in size from a few centimetres to
several metres in thickness and tens of metre in length. Discrimination between
sandy splays and sills is difficult, at times, if it was not by the fact that
the sedimentary sills are seen shooting off vertical dykes, and, elsewhere, do
not parallel the coal ply, rather they cross at low angle and interrupt
otherwise continuous, thin clay partings. The clay partings occur in sets and
are reliable, widespread, stratigraphic horizons within. They are interpreted as
tonsteins: transformed ashfalls. The dykes and sills form at different locations, times, and
are related to different processes. The dykes intrude from the floor of the coal
seam, the roof and laterally from into sandy washouts. Some were intrude during
early burial stages and have been heavily folded by compaction. Others show very
little compaction, indicating a later intrusion stage. Several dykes have also
been subsequently faulted horizontally along the clay bands, as tectonic
stresses were released along those plastic zones. The dykes, particularly those
intruded form the floor of the seam, and sills occur more commonly and are
larger near the flanks of sandy washouts. The intrusions seldom occur where the
main coal seam is fully developed, and, in such cases, several relatively small
wedge shaped dykes protrude from the roof downward. A variety of processes may
have been responsible for the intrusions, there including (a) sands capable of
liquefaction, covered by a relatively impermeable peat and, seldom, thin clay
layer, (b) stream switching into thick peatlands, and rapid channel
sedimentation over peat and its water saturated "underclay- like"
sandy material, (c) sudden input of seismic energy, (d) formation of fractures
in the peat by either frost or/and by slumps along stream banks, (e) fracturing
of peat-coal due to differential burial compaction near the margins of sandy
washouts, (f) generation of gasses during diagenesis leading to fracturing of
host deposits and fluidization of the intruding materials. |
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