14.4 Evolution of Coal
14.4 Evolution of Coal
14.4.1 General Aspects of Coal Formation
In contrast to oil and natural gas, coal remains in its
primary position within a sedimentary body, but with
continued burial it undergoes processes similar to the
diagenesis and catagenesis of hydrocarbon source
rocks. Coals contain 75-100% and coaly shales 2575% dry organic matter. Coals originate from the
detritus of terrestrial plants forming peat in "mires".
Important prerequisites for the formation of substantial volumes of coal are:
(1) The evolution ofplant life on the continents.
(2) A relatively high plant productivity under favorable climatic conditions with sufficient nutrient supply.
(3) Very low input of detrital material into the peataccumulating basin.
(4) A depositional environment where plant remains
can be preserved.
In modem environments, the peat production rates
are highest in wet tropical areas (3-6 mmla) and medium in warm temperate climates (1-3 mmla). Peat
growth is favored by groundwater discharge rich in
nutrients. Dilution of peat by clastic material is limited in areas where the river systems develop stable
channels with levees (e.g. in anastomosing systems),
or the trunk-channel belts of rivers crossing large
alluvial plains are diverted from the centers of peat
formation. The latter scenario has been quoted for
the formation of widespread coal intervals in
intracontinental basins (Belt 1993). Another favorable situation may be given where the shorelines remain ± stable. Furthermore, the interplay between the
vertical growth rates of peat and the rate of accommodation space provided by subsidence and relative
change of the groundwater table (acting as baselevel) control the formation and preservation of coal
in ancient environments. It is important to note that
the potential growth rates of peat are much higher
than the normal rates of subsidence and thus also the
average subsidence rates of the coal-bearing strata.
Therefore, the common coal seems resulting from
one phase of peat growth cannot be interpreted alone
by subsidence. In these cases, additional accommodation space is required for the accumulation of peat.
This can be provided by several processes:
- Rapidly rising groundwater table (in continental
settings). This may result from rapid change to wetter
climate or fast aggrading alluvial plains creating
backswamp areas.
- Rapid relative sea-level rise causing a rise in the
water level of coastal swamps and shallow lakes including the groundwater table of neighboring areas.
723
This can also be accomplished by step-wise subsidence.
- Sediment load-driven increased subsidence in areas
of rapid sediment accumulation and associated substantial sediment compaction (e.g. in delta areas).
Under these circumstances, aided by tectonic subsidence, a kind of balance can be reached between optimal peat growth and necessary accommodation
space. If the rate of gaining accommodation space
exceeds peat growth, the peats become drowned and
finally buried under clastic sediments. With accommodation too low, the growing peat is exposed to the
air and cannot be preserved, or it is reworked.
In coastal areas, peat formation is therefore favored by relative sea-level rise, but this rise must not
be too slow or too rapid. Thus, the growth of peat
also depends on the frequency and amplitude of the
sea-level changes. It appears that 4th or 5th order
sea-level fluctuations of some amplitude and the time
span between the late LST and early HST (often the
late TST) within these variations are most appropriate for the formation and preservation of significant
coal beds (cf. Sects. 7.6.2 and 7.6.4). This is the reason why coal cycles or cyclothems are very common
in transitional environments.
The eoal facies within individual, 3-4 m thiek paralie
seams, formed during parasequenees, may evolve from
huminite-rieh bright eoal to dull clayey eoal rieh in
liptinite, alginite and pyrite. In these eases, an aeeeierated
rise in sea or base level outpaeed peat growth (Petersen et
al. 1998; see also Holz 1998).
The a\erage sedimentation rate of Permian eratonie
coal-bearing strata in Australia, eontaining 12-18% coal,
was about 50 mlMa, whereas in the proximal part ot the
Sidney foreland basin the rate was 500 m1Ma (6-8% eoal;
Hunt 1989). If the eoal seams were deposited at the same
rate as the coal-bearing strata, the sedimentation rate for
coal alone would have been 0.05 or 0.5 mm/a, respeetively.
Especially the value for the cratonic coal basin appears to
be too low compared with the modem peat production
rates. The substantial compaction of peat is probably not
sufficient to explain this discrepancy.
These findings imply that very thick peat accumulations, required for the generation of coal seams of
some tens of meters in thickness, must have grown
intermittently. In fact, such discontinuities have been
identified in thick coal beds (Shearer et al. 1994).
According to the depositional environment we
distinguish between:
- Limnic coals of shallow freshwater lakes and
swamps (cf. Fig. 10.13), and coals of fluvial p1ains;
cf. Sect. 2.2.3), and
- Paralic coals of brackish transitional environments
such as those observed in coastal and marine deltaic
areas (cf. Sect. 3.5.3).
14.4 Evolution of Coal
14.4.1 General Aspects of Coal Formation
In contrast to oil and natural gas, coal remains in its
primary position within a sedimentary body, but with
continued burial it undergoes processes similar to the
diagenesis and catagenesis of hydrocarbon source
rocks. Coals contain 75-100% and coaly shales 2575% dry organic matter. Coals originate from the
detritus of terrestrial plants forming peat in "mires".
Important prerequisites for the formation of substantial volumes of coal are:
(1) The evolution ofplant life on the continents.
(2) A relatively high plant productivity under favorable climatic conditions with sufficient nutrient supply.
(3) Very low input of detrital material into the peataccumulating basin.
(4) A depositional environment where plant remains
can be preserved.
In modem environments, the peat production rates
are highest in wet tropical areas (3-6 mmla) and medium in warm temperate climates (1-3 mmla). Peat
growth is favored by groundwater discharge rich in
nutrients. Dilution of peat by clastic material is limited in areas where the river systems develop stable
channels with levees (e.g. in anastomosing systems),
or the trunk-channel belts of rivers crossing large
alluvial plains are diverted from the centers of peat
formation. The latter scenario has been quoted for
the formation of widespread coal intervals in
intracontinental basins (Belt 1993). Another favorable situation may be given where the shorelines remain ± stable. Furthermore, the interplay between the
vertical growth rates of peat and the rate of accommodation space provided by subsidence and relative
change of the groundwater table (acting as baselevel) control the formation and preservation of coal
in ancient environments. It is important to note that
the potential growth rates of peat are much higher
than the normal rates of subsidence and thus also the
average subsidence rates of the coal-bearing strata.
Therefore, the common coal seems resulting from
one phase of peat growth cannot be interpreted alone
by subsidence. In these cases, additional accommodation space is required for the accumulation of peat.
This can be provided by several processes:
- Rapidly rising groundwater table (in continental
settings). This may result from rapid change to wetter
climate or fast aggrading alluvial plains creating
backswamp areas.
- Rapid relative sea-level rise causing a rise in the
water level of coastal swamps and shallow lakes including the groundwater table of neighboring areas.
723
This can also be accomplished by step-wise subsidence.
- Sediment load-driven increased subsidence in areas
of rapid sediment accumulation and associated substantial sediment compaction (e.g. in delta areas).
Under these circumstances, aided by tectonic subsidence, a kind of balance can be reached between optimal peat growth and necessary accommodation
space. If the rate of gaining accommodation space
exceeds peat growth, the peats become drowned and
finally buried under clastic sediments. With accommodation too low, the growing peat is exposed to the
air and cannot be preserved, or it is reworked.
In coastal areas, peat formation is therefore favored by relative sea-level rise, but this rise must not
be too slow or too rapid. Thus, the growth of peat
also depends on the frequency and amplitude of the
sea-level changes. It appears that 4th or 5th order
sea-level fluctuations of some amplitude and the time
span between the late LST and early HST (often the
late TST) within these variations are most appropriate for the formation and preservation of significant
coal beds (cf. Sects. 7.6.2 and 7.6.4). This is the reason why coal cycles or cyclothems are very common
in transitional environments.
The eoal facies within individual, 3-4 m thiek paralie
seams, formed during parasequenees, may evolve from
huminite-rieh bright eoal to dull clayey eoal rieh in
liptinite, alginite and pyrite. In these eases, an aeeeierated
rise in sea or base level outpaeed peat growth (Petersen et
al. 1998; see also Holz 1998).
The a\erage sedimentation rate of Permian eratonie
coal-bearing strata in Australia, eontaining 12-18% coal,
was about 50 mlMa, whereas in the proximal part ot the
Sidney foreland basin the rate was 500 m1Ma (6-8% eoal;
Hunt 1989). If the eoal seams were deposited at the same
rate as the coal-bearing strata, the sedimentation rate for
coal alone would have been 0.05 or 0.5 mm/a, respeetively.
Especially the value for the cratonic coal basin appears to
be too low compared with the modem peat production
rates. The substantial compaction of peat is probably not
sufficient to explain this discrepancy.
These findings imply that very thick peat accumulations, required for the generation of coal seams of
some tens of meters in thickness, must have grown
intermittently. In fact, such discontinuities have been
identified in thick coal beds (Shearer et al. 1994).
According to the depositional environment we
distinguish between:
- Limnic coals of shallow freshwater lakes and
swamps (cf. Fig. 10.13), and coals of fluvial p1ains;
cf. Sect. 2.2.3), and
- Paralic coals of brackish transitional environments
such as those observed in coastal and marine deltaic
areas (cf. Sect. 3.5.3).