alluvial-fan coal

1. coal
Eman Mohamed Ali
Ahmed Yasser
Mohamed Elsayed

3. Coals in Relation to Delta Type
1. Fluvial-Dominated Deltas
 Substantial coal seams trending approximately parallel to
depositional dip are associated with bird foot-type or elongate
river-dominated deltas, accumulating along levees and in-filled
bays.
 Coals of lobated deltas accumulate during both the constructive
and destructive phases of deltaic sedimentation.
 During the constructive phase, the delta plain is traversed by
numerous distributary channels. Peat formation is restricted to the
inter-distributary tracts, which in many cases are of limited extent.
 The coals are characterized by "wants" corresponding to the
positions of the active distributary channels.

4.  The seaward edge of the abandoned delta lobe is attacked by waves,
creating transgression. barriers backed by shallow, brackish bays.
 Peat accumulates on the more slowly subsiding delta platform
landward of the maximum marine influence.
 As transgression progresses, the peats are likely to be subject to
marine influence, which impairs the chemical quality of the coal, at
least in its upper parts.

5. 2. Wave-dominated deltas
 Associated coals tend to
follow belts parallel to the
shoreline, but extend
perpendicularly through
abandoned channels and
inlets.
 The seams are interrupted in
places by wash over fans, and
contain wind-blown detrital
grains.
 Abandonment-phase seams
are likely to be moderately
strike-elongate.

6. 3. Tide-Dominated Deltas
 Coals related to tide-dominated delta systems are rare
 the luxuriant swamps that characterize a number of tropical tide-
dominated deltas. (Klang and Langat Rivers of tropical Malaysia provides
a good example)
 Mangrove swamps are extending seaward as the coast Delta-front
sands grade upward into tidal-channel and tidal-flat deposits, representing
a thick, upward-fining sequence.

7. Coal-Bearing Deltaic Facies
 Variations in thickness, lateral continuity, ash, sulfur and trace
elements are all determined in large part by the fluvio-deltaic sub-
environment.

8. Back-Barrier Coals
 Barriers form during delta progradation or a subsequent phase of
delta destruction are backed by shallow bays and lagoons, fill
with sediments supplied from land and sea and evolve into back-
barrier marsh.
 The resulting peats show a pronounced shore-parallel trend
except where they extend up in-filled reentrant bays, former inlets,
abandoned channel tracts.
 Many back-barrier coals are thin, discontinuous, have high
sulfur content, but some such as the Beckley Seam of West
Virginia and others in western North America are commercially
important. In some cases, only the upper parts are pyritic.

9. Marginal Delta-Plain Coals
o On river-dominated deltas, peats of the lower delta plain
accumulate on a platform of upward coarsening sediments which
include pro-delta and bay clays and silts overlain by, laterally
gradational into, mouth-bar and splay sands
o Delta-front sands of a prograding distributary mouth bar are
gradationally based, except in the immediate vicinity of distributary
channels.
o Lower delta-plain coals tend to be thin but persistent, particularly
in the paleo-slope direction, but seam continuity along strike is
limited by distributary sand axes.

11. The Lower/Upper Delta-Plain Transition
 The transition between (lower &upper) delta plains is laterally extensive
coals that are low in sulfur. tend to slightly elongated in the
depositional strike direction.
 The area of peat accumulation is less extensive in the upper delta plain, but
the environment is relatively stable.
 As a result, the coals are thick and lenticular with abrupt lateral changes
in seam characteristics. The coals are situated between scour-based
channel-fill sandstones which splay out in their upper parts, inter-fingering
with freshwater pond and back swamp facies.
Upper Delta-Plain Coals

12. Fluvial Coals
 Coal is commonly at the top of
upward-fining sequences
deposited by mixed-load rivers,
accumulating in back swamps
adjacent to migrating channel
tracts

13. Form
 On rapidly aggrading alluvial plains, channels sweeping across in one
direction and then the other may produce an intricate zigzag geometry
in cross section
 channels change position abruptly toward low-lying interfluves,
resulting in dip-oriented sands of lenticular cross section enclosed by
carbonaceous shales and coals
 Peat swamps form on the inclined surface of the well-defined levees, and
thicken toward the backs swamps separating the major fluvial axes
 The thickest coals tend to be between major sandstone lenses,
splitting and thinning as they ascend over the sandstones, but
thickening over the channel notch

14. Example
 Coals associated with Paleocene and Eocene fluvial
systems of the Powder River Basin of Wyoming and
Montana are commonly more persistent, with some
seams up to 30 ft (10m) thick extending along outcrop for
distances of 12 mi (20 km) with only moderate merging
and splitting.

15. Alluvial-Fan Coals
Coals are associated with
alluvial-fan and fan delta
systems in a variety of
intermontane and foredeep
plain settings.
Pull-apart basins and other
downfaulted segments of
strike-slip fault complexes
are particularly favorable
locations for coal-bearing
alluvial-fan systems.

16. • Idealized vertical sequence
characteristic of several Appalachian coal
basins, with a dip section showing
changes from the alluvial plain through
the delta plain to the barrier fringe, and its
interpretation according to the delta model
for that area.

17. Paleoenvironmental
Control Over Mineral
Matter in Coal
 Mineral matter in coal comprises
elements other than organic carbon,
hydrogen, nitrogen, oxygen, and
sulfur.
 It may consist of detrital grains such
as quartz that are washed or blown
into the peat-forming environment; it
may be introduced by ionic exchange
from interstitial waters; it may
comprise elements incorporated in the
vegetal precursors and released
during peatification; or it may be
chemically precipitated.
 the pH during peat formation is critical
in determining ash and sulfur content.
 In highly acidic environments mineral
matter in peat may be removed by
leaching.
 Peat degradation associated with less
acidic conditions concentrates mineral
matter in the peat.

18.  The coals beneath the marine units are characterized by an
anomalously high sulfur content.
 Trace-element content shows a similar increase where peats
were exposed to marine influence.
 Boron, titanium, vanadium, and zinc, for example, affect
industrial use.
 Some sulfides and other minerals are introduced by
solutions or form by alteration during the coalification
process.

19. Thank you