Geology Reference
In-Depth Information
“
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Several
cyclothems have been defined (Kosanke et al., 1960; Weller, 1931), but, in a top-down sequence,
they basically consist of shale, limestone, shale, coal, underclay, limestone, shale, and sandstone. The ideal
cyclothem is typically associated unstable shelf or interior basin conditions. It represents maximum alteration of
marine and nonmarine conditions, typical of western Illinois. The Southern Appalachian or Piedmont-type
cyclothem is characterized by dominant continental clastic sediments, well-developed coals, and few marine
beds. Alternating limestone and shale are representative of marine cyclothems, with thin sandstones and sub-
ordinate underclays and coals (Krumbein and Sloss, 1963).
ideal
Several mechanisms have been proposed for the formation of cyclothems. Diastrophic theories attribute them to
sinking basins and rising source areas. Climactic theories propose that glaciations produced sea level oscillations,
rainfall cycles, and variable erosion. Sedimentation theories attribute the formation of cyclothems to differential
deposition related to depth of H
2
O, strength of currents, distance from a river
s mouth, and compaction of
sediments. It is unlikely that a single mechanism operating on a limited time span can adequately explain the
development of multiple types of sedimentary cycle (Weller, 1931, 1964). In the Appalachian area, the sedimentary
rocks are similar to deltaic deposits. Coal, ironstones and limestones are the chemical deposits formed in
nonmarine, brackish, or marine environments. Episodes of detrital or clastic sedimentation interrupted the devel-
opment of chemical deposits. The detrital rocks have a finite horizontal limit and grade laterally into chemical
sediments (Ferm and Cavaroc, 1969).
'
Coalification
P
eat formation is considered the biochemical stage of coal formation, during which plant residues are partially
decomposed. The geochemical stage of coalification is a continuous and irreversible process that produces a rock
from the organic sediment. In the long term, coalification produces progressively higher rank coals (ASTM, 2005)
from lignite through subbituminous, high-volatile bituminous, medium-volatile bituminous, low-volatile bitumi-
nous to anthracite. Heat and pressure are the primary agents of coal metamorphism, rather than time (Figure 1.1.4).
Temperature and pressure increase as a function of depth; high temperature is also related to folding and faulting
and to the presence of igneous intrusions.
The first step in coalification is the removal of H
2
O due to the weight of overlying sediments (Figure 1.1.5). An
increase in the carbon concentration and a decrease in the hydrogen and oxygen concentrations are noted in higher
rank coals (Hessley et al., 1986).
Coal macromolecules are formed from altered biopolymers in plants (Hatcher and Clifford, 1997). Dehydroxyla-
tion, ether cleavage, and demethylation are proposed mechanisms by which brown coal and lignite are produced
from lignin. The removal of alkyl side chains and condensation reactions are assumed to account for increasing
aromatic character. Coalification to the bituminous rank involves the reduction in oxygen content
through
Time
Swamp
Pressure
Heat
Peat
Lignite
Coal
KGS
Figure 1.1.4. The transition from peat to hard coal, due primarily to heat at elevated pressures and extended
periods of time. From Kentucky Geological Survey, University of Kentucky, http://www.uky.edu/KGS/coal/, with
permission
.
